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Tagus Floods '06 Workshop Project POCTI/CTA/39427/2001 ABSTRACTS BOOK 19-21 July, 2006 Lisbon Portugal www.workshoptejo.fc.ul.pt
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Page 1: Tagus Floods '06 Workshopworkshoptejo.fc.ul.pt/Tagus Floods_ABSTRACTS.pdf · Tagus Floods '06 Workshop Project POCTI/CTA/39427/2001 ABSTRACTS BOOK 19-21 July, 2006 Lisbon Portugal

Tagus Floods '06 Workshop

Project POCTI/CTA/39427/2001

ABSTRACTS BOOK

19-21 July, 2006 Lisbon Portugal

www.workshoptejo.fc.ul.pt

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Page 3: Tagus Floods '06 Workshopworkshoptejo.fc.ul.pt/Tagus Floods_ABSTRACTS.pdf · Tagus Floods '06 Workshop Project POCTI/CTA/39427/2001 ABSTRACTS BOOK 19-21 July, 2006 Lisbon Portugal

Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ORGANIZED BY Centre of Geology of the Faculty of Sciences of the University of Lisbon Department of Geology of the Faculty of Sciences of the University of Lisbon Centre of Geographical Studies of the University of Lisbon

ORGANIZING COMMITTEE (University of Lisbon)

M. Teresa M. Azevêdo

Catarina RamosElisabete Nunes

Cátia Sousa

Rita Caldas Pedro Fonseca André Fernandes

SCIENTIFIC COMMITTEE

Ana Ramos Pereira (Lisbon, Portugal)

M. Conceição Freitas (Lisbon, Portugal) César F. Andrade (Lisbon, Portugal)

Pedro P. e Cunha (Coimbra, Portugal) Diamantino Pereira (Braga, Portugal)

João Cabral (Lisbon, Portugal) João Rocha (Lisbon, Portugal)

M. Eugénia M. Palomares (Madrid, Spain) Gerardo Benito (Madrid, Spain) Suzanne Daveau (Lisbon, Portugal) Rui Taborda (Lisbon, Portugal) Mini Garzón (Madrid, Spain) Nuno Pimentel (Lisbon, Portugal) Paolo Mozzi (Padova, Italy)

SUPPORTED BY

Centro de Geologia Universidade de Lisboa

Fundação para a Ciência e a Tecnologia – Ministério da Ciência e do Ensino Superior

SPONSORED BY Câmara Municipal de Lisboa Caixa Geral dos Depósitos

Flor do Tejo Construções, Lda. Front cover – the postcard dates back to 1910 and the background photo was taken by J. Benoliel, in 1912.

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Contents

CONFERENCE TIMETABLE 3

GEOTARIF PROJECT PRESENTATION 9

The Geotarif Project AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &

PEREIRA, D. I. 11

KEY NOTE PRESENTATIONS 15

A hydrometeorological approach to the floods of Tajo River. Llasat, C. 17

Apprendre à vivre dans des zones inondables FAYE, J. 21As cheias fluviais em leitos aluvionares com variabilidade natural e alterada. Evidências no

Rio Tejo. ROCHA, J. S. & FERNANDES J. N. 25Radiotracers in flooding processes. SANCHEZ-CABEZA, J.A. 29O Paleolítico no Médio e Baixo Tejo. RAPOSO, L. 33

Paleoflood and historical flood records along the middle and upper Tagus river catchment: Climatic and flood hazard implications.

BENITO, G. 35

PRESENTATIONS 37 Scale invariant study of river discharges from the Tagus Basin LIMA, M.I.P., LIMA, J.L.M.P., RODRIGUES, R.R., BRANDÃO, C. & ROSÁRIO, A.S.S. 39The role of NAO extreme phases on the monthly discharges, and the volume and date of the

annual maxima flows (1950-2003) in the Tagus River. LÓPEZ-MORENO, J.I., VICENTE-SERRANO, S.M., BEGUERÍA, S. & GARCÍA-RUIZ, J.M. 42Usefulness of Paleo-flood information in flood quantile estimation PARIDA, B.P. 44The impact of the North Atlantic Oscillation on precipitation and water resources of the Iberian

Peninsula TRIGO, R. M. 48A Medição dos Caudais de Cheia no Rio Tejo. ROCHA, J.S., VAN ZELLER, M.E. & FERNANDES, J.N. 51

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Active tectonic structures in the Lower Tagus Valley: state of the art. CABRAL, J. 53Acções antrópicas na Bacia Hidrográfica do Tejo na época contemporânea: florestação,

correcção torrencial e controlo de cheias. FREITAS, J.G. 56As cheias históricas na região de Abrantes. ALMEIDA, M.C., AZEVÊDO, T.M. & PIMENTEL, N.L. 59Floods in the Czech Republic during the past millennium: data, fluctuations, impacts. BRÁZDIL, R. 62The Tagus River and its historical floods (Santarém, Portugal). AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &

PEREIRA, D. I. 64Palaeoenvironments of the Tagus valley during the last 15 ka: sedimentological, palynological

and micromorphological evidence of the Entrevalas SEV coring (Santarem, Portugal). AZEVÊDO, M.T., FAVARETTO, S., MIOLA, A., MOZZI, P., NICOSIA, C., NUNES, E. & SOSTIZZO, I. 68Sistemas meteorológicos de precipitação e as cheias nas bacias hidrográficas: Rio Tejo. SIMÕES, I.M.S.N.V. 71Possibilidades de aplicação da luminescência opticamente estimulada (OSL) na datação de

sedimentos do rio Tejo. MARTINS, A.A. & CUNHA, P.P. 72Geo-archaeological evidence of destructive floods during the settlement of Faial (Azores). MADEIRA J. & SILVEIRA, A.B. 76Geomorphological assessment of the Middle Tagus alluvial plain. PEREIRA, A. RAMOS; RAMOS, C.; AZEVÊDO, T. M.; NUNES, E.; FREITAS, M. C.; ANDRADE, C. &

PEREIRA, D.I. 79Characterization of the Tagus floodplain sediments: their importance in the search of channel

variations and human impacts over time. AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &

PEREIRA, D. I. 82Understanding lateglacial and Holocene environmental changes through the sedimentological

study of a core (Santarém region) FREITAS,, C., ANDRADE,, C. , AZEVEDO, T.M., PEREIRA, A. RAMOS, RAMOS, C., NUNES, E.&

PEREIRA, D.I. 85

POSTERS 91

Estimation of recent alluvial sedimentation rates, Upper Odra, Poland. CZAJKA, A. 93Peligros e impactos producidos por las modificaciones de origen antrópico durante la avenida

del arroyo Rivillas en 1997 (SW de España). MOYA-PALOMARES, M.E. & CENTENO, J.D. 95Desarrollo espacial y evolución de una flashflood a través de la información aportada por

depósitos efímeros ORTEGA, J.A. & GARZÓN, G. 98

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Conference Timetable

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ROOM 6.2.56

WEDNESDAY, 19 JULY, 2006

9.30H - 10.00H RECEPTION AND REGISTRATION

10.00H - 11.00H OPENING SESSION

11.00H - 11.30H COFFEE BREAK

11.30H - 12.00H THE GEOTARIF PROJECT

Teresa M. Azevedo, Catarina Ramos, Ana Ramos Pereira, Elisabete Nunes, Conceição Freitas, César Andrade & Diamantino I. Pereira

12.00H – 12.30H

KEYNOTE PRESENTATION A HYDROMETEOROLOGICAL APPROACH TO THE FLOODS OF TAGUS RIVER Maria-Carmen Llasat & Concepción Marcuello

12.30H – 12.40H DISCUSSION

12.40H – 14.00H LUNCH

14.00H – 14.30H KEYNOTE PRESENTATION APPRENDRE A VIVRE DANS DES ZONES INONDABLES Jacques Faye

14.30H – 14.40H DISCUSSION

ORAL PRESENTATIONS

14.40H – 14.55H SCALE INVARIANT STUDY OF RIVER DISCHARGES FROM THE TAGUS BASIN Maria Isabel P.de Lima, J.L.M.P. de Lima, R.R. Rodrigues, C. Brandão, A.S.S. Rosáriz

14.55H – 15.10H THE ROLE OF NAO EXTREME PHASES ON THE MONTHLY DISCHARGES, AND THE VOLUME AND DATE OF THE ANNUAL MAXIMA FLOWS (1950-2003) IN THE TAGUS RIVER Juan I. López-Moreno, Sergio M. Vicente-Serrano, Santiago Beguería, José M. García-Ruiz

15.10H – 15.25H USEFULNESS OF PALEO-FLOOD INFORMATION IN FLOOD QUANTILE ESTIMATION Bhagabat P. Parida

15.25H – 15.40H THE IMPACT OF THE NORTH ATLANTIC OSCILLATION ON PRECIPITATION AND WATER RESOURCES OF THE IBERIAN PENINSULARicardo M. Trigo

15.40H – 16.00H DISCUSSION

5

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

16.00H - 16.20H COFFEE BREAK

16.20H - 16.50H KEYNOTE PRESENTATIONL AS CHEIAS FLUVIAIS EM LEITOS ALUVIONARES COM VARIABILIDADE NATURAL E ALTERADA. EVIDÊNCIAS NO RIO TEJO

João Soromenho Rocha & João Nuno Fernandes

16.50H - 17.00H DISCUSSION

ORAL PRESENTATIONS

17.00H - 17.15H A MEDIÇÃO DE CAUDAIS DE CHEIA NO RIO TEJO João Soromenho Rocha, Maria Emília van Zeller e João Nuno Fernandes

17.15H - 17.30H ACTIVE TECTONIC STRUCTURES IN THE LOWER TAGUS VALLEY: STATE OF THE ART João Cabral

17.30H - 17.45H DISCUSSION

17.45H - 18.15H POSTER SESSION ESTIMATION OF RECENT ALLUVIAL SEDIMENTATION RATES, UPPER ODRA, POLAND Agnieszka Czajka PELIGROS E IMPACTOS PRODUCIDOS POR LAS MODIFICACIONES DE ORIGEN ANTRÓPICO DURANTE LA AVENIDA DEL ARROYO RIVILLAS EN 1997 (SW DE ESPAÑA) Maria Eugénia Moya-Palomares & Juan D. Centeno DESARROLLO ESPACIAL Y EVOLUCIÓN DE UNA FLASHFLOOD A TRAVÉS DE LA INFORMACIÓN APORTADA POR DEPÓSITOS EFÍMEROS José A. Ortega & Guilhermina Garzón GEOTARIF PROJECT POSTERS

18.30H ICE-BREAKER

THURSDAY, 20 JULY, 2006

8.30H - 19.00H FIELD TRIP TO THE MIDDLE AND LOWER TAGUS

6

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ROOM 6.2.56

FRIDAY, 21 JULY, 2006

9.30H - 10.00H KEYNOTE PRESENTATION RADIOTRACERS IN FLOODING PROCESSES Joan Albert Sanchez-Cabeza

10.00H - 10.15H DISCUSSION

ORAL PRESENTATIONS

10.15H - 10.30H ACÇÕES ANTRÓPICAS NA BACIA HIDROGRÁFICA DO TEJO NA ÉPOCA CONTEMPORÂNEA: FLORESTAÇÃO, CORRECÇÃO TORRENCIAL E CONTROLO DE CHEIAS Joana Gaspar de Freitas

10.30H - 10.45H AS CHEIAS HISTÓRICAS NA REGIÃO DE ABRANTES Mónica C. Almeida, Teresa M. Azevêdo & Nuno L. Pimentel

10.45H - 11.00H FLOODS IN THE CZECH REPUBLIC DURING THE PAST MILLENNIUM: DATA, FLUCTUATIONS, IMPACTS Rudolf Brázdil

11.00H - 11.15H THE TAGUS RIVER AND ITS HISTORICAL FLOODS (SANTARÉM, PORTUGAL) Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira

11.15 - 11.35H DISCUSSION

11.35H - 11.55H COFFEE BREAK

11.55H - 12.25H KEYNOTE PRESENTATION O PALEOLÍTICO NO MÉDIO E BAIXO TEJO Luís Raposo

12.25H - 12.35H DISCUSSION

12.35H - 14.00H LUNCH

14.00H - 14.30H KEYNOTE PRESENTATION PALAEOFLOOD AND HISTORICAL FLOOD RECORDS ALONG THE MIDDLE TAGUS RIVER CATCHMENT: CLIMATIC AND FLOOD HAZARD IMPLICATIONS Gerardo Benito

14.30H - 14.40H DISCUSSION

7

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ORAL PRESENTATIONS

14.40H - 14.55 PALAEOENVIRONMENTS OF THE TAGUS VALLEY DURING THE LAST 15 KA: SEDIMENTOLOGICAL, PALYNOLOGICAL AND MICROMORPHOLOGICAL EVIDENCE OF THE ENTRE-VALAS SEV CORING (SANTAREM, PORTUGAL) M. Teresa Azevêdo, S. Favaretto, A. Miola, Paolo Mozzi, C. Nicosia, Elisabete Nunes & I. Sostizzo

14.55H - 15.10 SISTEMAS METEOROLÓGICOS DE PRECIPITAÇÃO E AS CHEIAS NAS BACIAS HIDROGRÁFICAS: RIO TEJO Ilda Mª Sanfins Novo Villa Simões

15.10H - 15.25 POSSIBILIDADES DE APLICAÇÃO DA LUMINESCÊNCIA OPTICAMENTE ESTIMULADA (OSL) NA DATAÇÃO DE SEDIMENTOS DO RIO TEJO António Antunes Martins & Pedro Proença e Cunha

15.25H - 15.40 GEO-ARCHAEOLOGICAL EVIDENCE OF DESTRUCTIVE FLOODS DURING THE SETTLEMENT OF FAIAL (AZORES) José Madeira & António Brum da Silveira

15.40H - 16.00H DISCUSSION

16.00H - 16.20H COFFEE BREAK

16.20H - 16.35H HUMAN INTERVENTIONS IN TAGUS RIVER CHANGES Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira

16.35H - 16.50 GEOMORPHOLOGICAL ASSESSMENT OF THE MIDDLE TAGUS ALLUVIAL PLAIN Ana Ramos Pereira, Catarina Ramos, Teresa M. Azevêdo, Elisabete Nunes, Conceição Freitas, César Andrade & Diamantino I. Pereira

16.50H - 17.05H CHARACTERIZATION OF THE TAGUS FLOODPLAIN SEDIMENTS: THEIR IMPORTANCE IN THE SEARCH OF CHANNEL VARIATIONS AND HUMAN IMPACTS OVER TIME Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira

17.05H - 17.20H UNDERSTANDING LATEGLACIAL AND HOLOCENE ENVIRONMENTAL CHANGES THROUGH THE SEDIMENTOLOGICAL STUDY OF A CORE (SANTARÉM REGION) Maria da Conceição Freitas, César Andrade , Teresa M. Azevedo, Ana Ramos Pereira, Catarina Ramos, Elisabete Nunes & Diamantino I. Pereira

17.20H - 17.45H DISCUSSION

20.00H CLOSING DINNER

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

GEOTARIF Project

9

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

THE GEOTARIF PROJECT

TERESA M. AZEVÊDO1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, ELISABETE NUNES1,CONCEIÇÃO FREITAS1,

CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3

1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016

Lisboa, Portugal

e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal

e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal

e-mail: [email protected]

Middle age authors described the Tagus as a second Nile, due to the extraordinary fertility of

its “lezíria” (from al-jazira, which means “the island” in Arabic language). There are several

references of the human occupation of Tagus Valley before the Portuguese nationality,

especially Arabic and Roman occupations. Archaeological excavations in this region confirm the

great importance of all this area in proto-historic times. However Tagus floods were since

always, the most catastrophic events, killing cattle and people, destroying homes and

agricultural lands. Even today, floods isolate populations for weeks, cause roads and railway

cuttings and great economic losses. They are referred since the beginning of the Portuguese

nationality and an extensive literature review, including works by archaeologists, historians and

military civil engineers, shows how they have always been a main concern of Portuguese

governors, since the XII century. Since 70’s these events were highly diminished by the

construction of dams in the Spanish and Portuguese Tagus Basin.

Following a first very grateful experience with the Praxis XXI Project "An Interdisciplinary

Approach to Flood Risk Evaluation - Hydrology, Geomorphology and Sedimentology of the

Douro, Tejo and Sado Basins" in which were developed for the first time in Portugal, at

academic level, techniques and methodologies not previously used, the research team focused

on the effectiveness of a follow-up Project that would use hydrological, geomorphological,

sedimentological and historical study of the Tagus floods.

Main Goals of the GEOTARIF Project

– Historical research of the Portuguese Tagus flood, namely relationship between the

several historic flood episodes and the artificialization of the river and comparative analyses of

maps since 1760.

11

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

– Geomorphological research based on XX century charts and field survey in order to

evaluate alluvial plain mesomorphology, the old positions of the channels and other previous

morphological units as well as the last 500 years dynamics.

– Sedimentological research for the evaluation of the textural parameters of cored and

present-day analogues to evaluate hydrological energy changes in the flows.

– Geochemical analysis to evaluate, through the variation of peak concentrations, the

degree of pollution in the floodplain sediments.

– Dating of samples using 14C, 210Pb and 137Cs and evaluation of sedimentation rates.

To collect all the historical data about the Tagus river valley the following tasks were carried

out: (i) historical research in the main Archives (Distrital de Santarém, Torre do Tombo, MOP,

National Library) on engineering works performed on the Tagus in historic times, and in

Eclesiastic Archives about past floods; (ii) compilation and analysis of historical cartography of

Middle Tagus basin with comparison between all the available maps since the oldest till the

present ones to evaluate the changes in width and direction of the channels and migration of

the bars.

Morphological elements of the alluvial plain were defined based on 5m contour lines and

numerous elevation points of several charts of the Portuguese topographical charts. A model of

georeferenced elevation points and every meter contour lines of the alluvial plain was built,

allowing the construction of a DEM model. This methodology allowed to select in the field, the

best sites for manual and mechanical drilling and to accomplished the geomorphological

boreholes framework as well as to collect the most significant superficial samples in order to

compare than with core samples.

Several drills were opened to obtain undisturbed sedimentary sequences for

sedimentological, geochemical studies and dating, in order to reconstruct the past

environmental changes of the study area, focusing on the identification, characterization and

frequency assessment of floods

This work included test coring with light equipment (gauge and Edelman augers, Van der

Staay suction corer) completed by opening of small trenches for macroscopic evaluation of the

local lithostratigraphy. This consists essentially in boreholing the soft sedimentary column using

heavy pressure driven coring equipment and casing to obtain an undisturbed and continuous

set of cores at each target location. All coring locations were coordinated and connected to the

Portuguese geodetical Datum by the same group using adequate tacheometer procedures.

The sedimentological analysis enables to define the transport and deposition of sediments

during the Holocene in order to evaluate the fluvial dynamics changes of the Tagus River.

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Sediments were described for colour, texture and mineralogy (granulometric, morphoscopic

and heavy mineral study of the sandy fraction, and the mineralogical study of clays), fossil

contents, structures and facies variation. The correlation between statistic parameters of the

present analogues and the core samples allowed identify the correspondent environments,

avulsion episodes and progressive changes of the channel.

One of the most important components was the geochemical study of the variation of the

heavy metal concentration, only studied in Portugal for the river waters or for the estuarine

sediments but very few for the floodplain ones. This study, which started in the prior project,

continued due the given precious information about the influence of human activity on the heavy

metal contamination through times. In the floodplain fine sediments, heavy metals supplied by

mineral exploitation, industrial development and agricultural fertilizers can concentrate in high

levels. The evaluation of the heavy metal concentration along the drill profiles allowed

evaluate the degree of pollution over time and the 14C, isotopic and polinic datings of the main

variation levels allowed to interpret them in terms of historical events.

Radionuclides are also disseminated in the floodplain sediments. The radionuclides 137CS

and 210Pb evaluation for the first meters of the same drill profiles of the Tagus floodplain

sediments also contributed to this original scope in the sense that it was possible to get

knowledge of the contamination of soil profiles of Tagus floodplain and evaluate the recent

sedimentation rate. The dating of these levels allowed establishes the most probable origin for

the peaks and the eventual degree of contamination by the radionuclides.

The the palynological results of fourteen samples from the Entrevales sequence enabled to

consider the existence of three zones completely sterile in pollen content, suggesting to divide

the record into two main parts: for the first half of the Holocene (from bottom till 8m deep),

pollen, spore and NPP records suggest the existence, under mediterranean climatic conditions,

of a freshwater shallow basin which dried out in summer. Locally the aquatic community,

dominated by Isoetes, indicates an acidic and oligotrophic environment, but some foraminifers

could reveal that, at least sporadicly, marine or brackish water could reached the basin. In the

upper part of the sequence palynological results seem to indicate deforestation and heathland

expansion starting during the Bronze Age.

So, this Project aims at a new perspective, in the sense it correlates all the geological and

morphological data with historical and archaeological events having positive impacts in ethic,

social and environmental terms in the sense it allowed support adequate policies of

environmental management and risk/hazard assessment. The results achieve within the Project

was possible only by the interdisciplinary team work

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

The diffusion of results was undertaken by:

-Technical progress reports and final report to the FCT;

-Oral and poster presentations at national and international scientific meetings;

-A workshop in the 3rd year for the presentation, diffusion and discussion of the results with

other researchers of the same scientific areas.

-Publication of the results, models and conclusions on scientific (refereed) journals or revues;

-Reports to local authorities, of the most relevant conclusions on flood risk, soil contamination

by high heavy metal levels and by radionuclides pollution.

The implementation of the “Núcleo de Estudos do Tejo” (NET) in the Department of Geology

of the FCUL, including a Tagus Database, which allows to insert all the works concerning the

Tagus river, in so different scientific areas as history, geology, physical geography,

archaeology, cartography, pedology, agriculture, palinology, geochemistry, geochronology,

hydraulic engineer, literature etc.

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Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Key Notes

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A HYDROMETEOROLOGICAL APPROACH TO THE FLOODS OF TAGUS RIVER

MARIA-CARMEN LLASAT1 AND CONCEPCIÓN MARCUELLO2

(1) Department of Astronomy and Meteorology, University of Barcelona, Avda. Diagonal 647, 08028-Barcelona, Spain

e-mail: [email protected](2) Centro de Estudios Hidrográficos del CEDEX, Paseo Bajo Virgen del Puerto, 3, 28005-Madrid, Spain

e-mail: [email protected]

Introduction Between 1950 and 1999 there were a total of over 2,200 deaths in Spain (of which 1,400

were in Catalonia) and material damage amounting to over €301,000,000 a year as a

consequence of floods. Usually the most affected area is the Mediterranean one, where heavy

rainfalls and floods are frequent during the autumn season. For instance, in Catalonia

(approximately 35000 km2), the highest number of floods corresponds to the river Ter in Girona,

with a figure of 177 for the period 1322-2002, while the river Llobregat shows 171 floods at its

mouth, for the period 1315-2002, and the river Segre, only 51 floods for the period 1306-2002

(Llasat et al, 2005). However, although in the centre and west part of the Iberian Peninsula

floods are not so important, their role and impact are not insignificant. Then, more than 150

floods have been recorded in the Tagus basin (approximately 55800 km2) between the years

849 and 1985 A.D. Some places, like Aranjuez, have been affected by more than 50 floods in

the last 500 years, 60% of them were recorded between the months of January and March,

while Talavera de la Reina has recorded for the same period, 19 floods with a 50% of them

between December and January. Besides the different season of the year during which floods

are produced, Mediterranean floods have different features that those ones of the Tagus basin,

being one of them the size of the catchments, that is only comparable to this one of the Ebro

River.

The objective of this contribution is to analyse the main hydrometeorological features of the

floods recorded in the Tagus basin and comparing them with these ones of the Mediterranean

catchments, for which more meteorological analysis and information is available. Besides this,

and taking into account the climate variability, a reflexion on the rainfall temporal distribution

and its effect in floods evolution is done.

A floods classification A distinction between four kinds of floods in the Iberian Peninsula can be made:

1) Short-lived events (less than 3 hours) of very intense precipitation (peaks of rainfall rate

above 3 mm.min-1) but for which the total amount of rainfall is not usually very high (less than

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100 mm). They appear during summer and early autumn and produce “local flash-floods” in

short water courses (5-50km2) with considerable slope and characterised by torrential regime

and, sometimes, non-permanent flows.

2) Episodes of heavy rain sustained for several hours that can produce catastrophic floods

due to the daily rainfall amount (usually more than 100 mm) or widespread extension (usually

more than 2000 km2) very high above-normal levels. It is possible to distinguish between two

subclasses. Type 2a usually lasts less than 24 hours and the maximum precipitation is usually

recorded in less than 6 hours, with accumulated rainfall of nearly 200 mm. They can produce

catastrophic flash floods in rivers of the previous type or rivers with modest basins and flows

(50-2000 km2). Type 2b lasts more than 24 hours but generally less than 5 days. Although

accumulated rainfall usually has values between 200 and 400 mm, values of more than 800 mm

are possible in some Spanish regions.

3. Episodes of long duration with weak average pluviometric intensity values, while there

may be peaks of high intensity. Total precipitation can be above 200 mm. If floods occur, they

are usually in rivers of high-mountain (>2000 m) and pluvial-snowmelt origin with large basins

and perennial flows (>2000 km2).

4. Flood events mainly produced by snow melting. It should be noted that the melting of

snow around the mountainous headwaters of the rivers only brings regular and foreseeable

rises during the spring, and only occasionally gives rise to overbank flows when accompanied

by heavy rainfall or moderate but continuous rainfall.

Other kinds of floods not considered in the above classification are those produced by the

rising levels of phreatic water, ice breaking, dam’s bursts and sea waves.

Main features of floods recorded in the Tagus basin They are usually produced in winter (mainly during December and January months) by long

rainfalls over the entire region with some convection embedded in stratiform systems, and

would be considered of type 3. This convection can be responsible of daily values above 140

mm, like the event recorded in January 1996. These precipitation systems are mainly developed

as a consequence of the pass of Atlantic fronts, usually associated to deep depressions. But

information about recent events shows that heavy rainfalls are also produced in summer and

autumn and that can have similar features to those events related with catastrophic floods

produced in Mediterranean region (Rigo and Llasat, 2004). Summer floods are usually

produced by convective events that can affect an extended area and that usually are produced

by perturbations that affect the region between 1 and 3 days (i.e. August 1987), but that can last

more than 5 days (i.e. July 1987). In these cases daily rainfall is usually less than 80 mm. They

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would be of type 1. On the contrary, floods produced in autumn are related with convective

events developed in the framework of generalized stratiform rainfalls that last more than 10

days and affect the entire region (i.e. November 1989). Sometimes they can be considered like

floods of type 2. Cumulated daily rainfalls can overpass 100 mm and total rainfalls more than

200 mm, although their never achieve so high values than in the Mediterranean region.

Rainfall and floods evolution

The analysis of areal and punctual rainfall for the Duero and Tagus basins do not show any

common significant trend for the last century (Barrera and Llasat, 2004; Llasat and Quintas,

2003), not for annual values neither for winter values. Trends on flood frequency are not clear,

but a high sensitivity to the climatic variability is founded when the last millennium has been

analysed (Benito et al, 2003) and some anomalies common to the Mediterranean rivers have

been founded. Then, the better comprehension of the meteorological situations responsible of

those floods and its evolution in future scenarios could add to diagnose possible future

changes.

References Barrera, A. and M.C. Llasat, 2004. Evolución regional de la precipitación en España en los últimos 100

años. Ingeniería Civil, 135, 105-114

Benito, G., A. Díez-Herrero and M. Fernández de Villalta, 2003. Magnitude and frequency of flooding in

the basin (Central Spain) over the last millenium. Climatic Change, 58, 171-192.

Llasat, M. C., and Quintas, L., 2004. Stationarity of monthly rainfall series, since the middle of the 19th

century. Application to the case of Peninsular Spain, Natural Hazards, 31, 613-622, 2004.

Llasat, M.C., Barriendos, M., Barrera, A. & Rigo, T. 2005. Floods in Catalonia (NE Spain) since the 14th

century. Dans Benito, G, Ouarda, T.B.M.J., Bárdossy, A (Eds.) Palaeofloods, hystorical data & climate

variability: Applications in flood risk assessment, Journal of Hydrology 313 (1-2), 32-47.

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APPRENDRE A VIVRE DANS DES ZONES INONDABLES

JACQUES FAYE Direction de la prévention des pollutions et des risques. Ministère de l'Écologie et du Développement Durable,

France

e-mail: [email protected]

Apprendre sauve Pour une éducation à la prévention des risques majeurs

Depuis juin 2003, la Stratégie nationale pour le développement durable intègre l’éducation

aux risques majeurs dans l’éducation à l’environnement pour un développement durable.

Des concepts précis Chaque jour, la vulnérabilité globale augmente progressivement et se différencie en fonction

du niveau de développement des territoires. Or, on ne peut empêcher les séismes ni contrôler

les tempêtes.

Aux images de catastrophes, au sentiment de fatalité ou de peur, nous voulons répondre par

une éducation à la prévention qui permette à chacun de prendre des précautions, d’anticiper les

évènements, d’être plus lucide et confiant en lui et ses proches. Cette éducation est à la croisée

de l’éducation à l’environnement et de l’éducation à la sécurité. Avec l’éducation à la

citoyenneté, à la santé, à la sécurité sur la route, à la maison ou au travail…. Elle contribue à

une éducation à la responsabilité individuelle. Elle permet de comprendre les interactions

existantes entre l’homme et son milieu environnant, car, s’il est nécessaire de protéger

l’environnement, il nous faut aussi le prendre en compte pour nous protéger. L’objectif est de

susciter des comportements réfléchis, adaptés et solidaires dans la perspective d’un

développement durable des territoires et des sociétés.

Il convient de décrire l’aléa d’origine naturelle ou anthropique et de l’analyser à différentes

échelles d’espace et de temps, par rapport à des enjeux humains, économiques,

environnementaux et patrimoniaux afin de réduire leur vulnérabilité, ce qu’on nomme mitigation

ou résilience.

Pour une éducation préventive Aujourd’hui, la société française refuse de considérer les catastrophes comme une fatalité et

devient de plus en plus exigeante à l’égard de ceux qui ont mission d’assurer notre sécurité. On

constate cependant une évolution du citoyen de ne pas s’en remettre uniquement à l’État et de

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participer à sa propre sauvegarde, à celle de ses proches et de ses biens, dans la mesure où il

a connaissance des mesures et des comportements d’urgence appropriés. C’est dans cette

optique que l’éducation à la prévention des risques majeurs est affirmée, depuis plusieurs

années, comme l’une des finalités de l’école. Comme l’éducation à la sécurité routière, les

programmes scolaires intègrent les risques majeurs dès le premier degré, avant que l’essentiel

ne soit abordé au collège en classes de 5ème et de 4ème, puis de nouveau au lycée, en 2nde, en

croisant les disciplines et favorisant des dispositifs pédagogiques comme les Travaux

Personnels Encadrés pour ne citer qu’un seul exemple.

À ce titre, la Direction de la prévention des pollutions et des risques du Ministère de

l’écologie et du développement durable [MEDD] anime depuis une douzaine d’années, un

réseau de formateurs Risques Majeurs éducation [RMé] pour inciter, ou venir en appui, à des

projets éducatifs dans le cadre scolaire ou périscolaire. Plus de quatre cents agents de

l’Éducation nationale, mais aussi d’autres ministères comme l’Équipement, l’Intérieur ou

l’Agriculture, interviennent auprès des enseignants, des chefs d’établissement ou des directeurs

de centres de loisirs qui les sollicitent. À la demande du MEDD, ils se sont rassemblés en 1998

au sein d’un Institut français des formateurs risques majeurs et protection de l’environnement

(IFFO-RME). D’autres structures comme la Gendarmerie nationale avec ses formateurs relais

enquête environnement (FREE), les secrétariats permanents pour la prévention des pollutions

industrielles (SPPPI), les Centres Permanents d’Initiatives pour l’Environnement ou des

établissements publics interviennent aussi dans le cadre de cette éducation préventive.

Pédagogie et exemplarité Si l’École doit être pour l’élève, le lieu privilégié de l’apprentissage à cette prévention,

l’établissement qui l’abrite doit éviter d’être particulièrement vulnérable. Or, si la tempête de

Noël 1999 a révélé la fragilité de certains bâtiments, comme le souligne l’Observatoire National

de la Sécurité des Etablissements scolaires et d’Enseignement Supérieur, c’est surtout la

grande impréparation de la communauté scolaire à ce type de situation qui suscite des

inquiétudes. Que doit faire un chef d’établissement ou un directeur d’école en cas de danger ou

d’alerte? Comment éviter toute panique due à l’ignorance des enseignants et des élèves alors

que les consignes demandent aux parents de ne pas aller chercher leurs enfants ?

La mise en place du Plan Particulier de Mise en Sûreté se conçoit dans un cadre élargi d’éco

responsabilité des établissements d’enseignement. Son élaboration implique aussi la

communauté gestionnaire : commune, conseil général ou régional, pour la mise en œuvre des

règles de mise en sûreté au niveau de l’aménagement, de l’équipement et de la signalétique

comme elle le fait pour l’incendie et l’accessibilité. En décembre 2000, à Sofia (Bulgarie), à

l’initiative de l’Accord Partiel Ouvert « risques majeurs » du Conseil de l’Europe, les

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représentants des différentes délégations avaient déjà retenus dix points essentiels : identifier

les risques prévisibles, évaluer la vulnérabilité des bâtiments et des installations, engager les

travaux indispensables à la sécurité des personnes, former les différentes personnes

responsables, sensibiliser les personnels, les élèves et les parents aux conduites à tenir,

organiser la communauté éducative, répondre aux besoins des minorités ou des personnes

handicapées de l’établissement, afficher les consignes et les évaluer régulièrement par des

simulations et intégrer l’éducation préventive dans le projet pédagogique de l’établissement.

Une pédagogie ludique et différenciée L’éducation à la prévention des risques majeurs se conçoit au sein de l’école mais

également en dehors, d’autant que le temps périscolaire tend à devenir aussi important que le

temps scolaire et que l’École a d’autres missions. C’est pourquoi il convient de développer des

outils adaptés à une éducation associée aux sports et aux loisirs : le jeu est ainsi un vecteur

essentiel du message. Plusieurs prototypes ont été élaborés : du simple jeu comme Cata’stop à

des jeux de rôle plus complexes requérant l’intervention d’un éducateur comme Rivermed,

consacré aux inondations en régions méditerranéennes, Les activités sportives ou de pleine

nature sont également l’occasion de former la jeunesse aux risques et à l’apprentissage des

moyens de prévention et d’autoprotection comme le montre la démarche ballon vert.

Une culture de prévention généralisée et cohérente Des réflexions sont menées par le Ministère de l’Intérieur, Direction de la défense et de la

sécurité civiles [DDSC], sur la sensibilisation aux exigences de sécurité civile ainsi qu’à celles

menées dans le cadre de l’Accord partiel ouvert sur les risques majeurs du Conseil de l’Europe

et celles de la Direction Générale Environnement [DG XI] de l’Union Européenne sur la sécurité

des enfants en Europe.

L’éducation préventive intégrant tous les risques (routier, accidents domestiques, incendie…)

a sa place dans la formation citoyenne. Elle doit être généralisée mais aussi se structurer,

s’organiser de manière cohérente, se donner des éléments de référence et concevoir des outils

d’évaluation pour être relayée par l’institution scolaire, les collectivités territoriales, la société

civile.

L’atout Internet et des ressources pédagogiques L’émergence des nouvelles technologies de communication et d’information est un atout

extraordinaire car elles permettent de répondre aux exigences de transparence et de

démocratie en facilitant l’accès à l’information environnementale. A ce titre, la Direction de la

prévention des pollutions et des risques [DPPR] a créé www.prim.net comme site portail pour

l’accès à l’information préventive, qu’elle ait pour origine l’État, les collectivités territoriales, les

établissements publics ou privés. Actualisé en permanence, il permet d’avoir un panorama

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complet des sources d’information possibles, de profiter d’une veille sur les sites dédiés .

L’usage d’Internet correspond de plus à la mondialisation des compétences et des actions

nécessaires à la prévention des risques. Il permet d’offrir à chacun le contenu pédagogique

nécessaire à son autoprotection : sorte de cartable virtuel très utile pour l’éducation à distance

et à la Francophonie.

Une nouvelle stratégie internationale La Stratégie Internationale de Prévention des Catastrophes a pour objet de développer la

résistance des communautés face aux catastrophes en les sensibilisant à l’importance de la

gestion des risques comme volet à part entière au service du développement durable-,

atténuant ainsi les pertes au plan humain, socio-économique, environnemental et culturel

causées par les catastrophes naturelles ou technologiques. L’éducation a un rôle essentiel à

jouer pour que cet objectif soit atteint. En janvier 2005, 168 gouvernements, réunis à Kobe,

Hyogo (Japon) à l’occasion de la Conférence mondiale sur la prévention des catastrophes, ont

adopté un plan décennal pour rendre le monde plus sûr face aux catastrophes naturelles. Le

« Cadre d’action de Hyogo pour 2005-2015: Pour des nations et des collectivités résilientes

face aux catastrophes » définit les lignes d'action, fixe les priorités et détermine les moyens

pratiques permettant de réduire considérablement, d’ici 2015, les sinistres liés aux

catastrophes. Dans ce contexte l’UNESCO mobilise une alliance mondiale susceptible de

promouvoir coopération et partenariats pour informer, sensibiliser et éduquer la jeunesse afin

que les collectivités soient mieux préparées et plus fortes. L’Accord partiel ouvert Risques

Majeurs du Conseil de l’Europe participe à cette même dynamique qui doit être visible par

l’inscription de l’autoprotection dans les droits de l’enfant, la mise en place d’un label de

reconnaissance pour les établissements et sa promotion à l’échelle internationale.

Pour un monde plus sûr Sans sous-estimer l’évolution climatique, la grande vulnérabilité de nos sociétés reste la

raison principale des désastres. Les années passées montrent l’intérêt et l’efficacité de la

prévention pour nous protéger et poser les bases d’un aménagement durable des territoires.

L’éducation à la prévention des risques majeurs doit nous apprendre à anticiper les

phénomènes extrêmes ou exceptionnels, à ne plus les baptiser catastrophes, à ne plus les tenir

pour des manifestations de la fatalité. Chacun pourra ainsi apprendre à les observer avec

vigilance et à réduire leurs conséquences.

Les orientations de l’Education nationale, le code de l’environnement, la loi de modernisation

de la sécurité civile par son article 5 et la volonté du Gouvernement français, exprimée par une

mission confiée au professeur Michel RICARD, constituent, pour la France, un cadre

dynamique pour l’éducation à la prévention des risques majeurs.

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AS CHEIAS FLUVIAIS EM LEITOS ALUVIONARES COM VARIABILIDADE NATURAL E ALTERADA. EVIDÊNCIAS NO RIO TEJO.

JOÃO SOROMENHO ROCHA & JOÃO NUNO FERNANDES Laboratório Nacional de Engenharia Civil, Av. Brasil, 101, 1700-066 Lisboa, Portugal

e-mail: [email protected], [email protected]

Os estudos hidráulicos de regolfo, que permitem a definição das zonas inundáveis têm

definir adequadamente a grande complexidade dos leitos de cheia, onde se destacam as obras

de defesa, diques. Conhecidos os dados de base, devem ser calibrados e validados os

cálculos efectuados por modelos numéricos, os quais incluem entre outros os coeficientes de

rugosidade dos leitos (Brunner, 2002).

Na determinação das zonas inundáveis é importante a definição da geometria dos perfis do

rio. A topografia fora do leito principal pode ser bem avaliada utilizando as Cartas Militares de

Portugal à escala 1:25 000. Num leito aluvionar é importante avaliar a variação do leito

principal. Essa variação não é perceptível facilmente em levantamentos com pequenos

intervalos de tempo mas já é perceptível quando se comparam levantamentos com intervalos

de vários anos (HP, 1977).

Na análise da evolução morfológica do leito principal do rio Tejo foram também verificadas

as variações dos talvegues que ocorreram nas três secções das estações hidrométricas. Desta

forma, verificou-se que a variação total da tendência do talvegue ocorrida em 15 anos em

Almourol foi de 1 m, a mesma que em Ómnias em 13 anos. No Tramagal não houve uma

tendência para a variação do talvegue. Esta análise permitiu também verificar as grandes

variações sazonais e interanuais da cota do talvegue, bem como a rápida redução da cota do

talvegue para caudais elevados, e a “recuperação” dessa cota mais lenta com caudais mais

reduzidos (Rocha et al, 2005). Verificou-se que as variações anuais, que podem atingir valores

até 3 m, são superiores à variação das tendências a médio prazo.

A avaliação dos erros de cálculo numérico pode ser feito com o levantamento de marcas de

cheia. No rio Tejo foi feito esse levantamento com equipamento de GPS. Por recurso aos

dados disponíveis de caudais foi possível, para algumas datas, conhecer o caudal escoado.

Tendo em conta a localização da marca, o caudal e a altura de água fez-se a comparação com

o perfil de cálculo mais próximo. Desta forma, foram levantadas 64 marcas de cheia ao longo

do troço, das quais é conhecido o caudal em 26.

Relativamente a esta análise, é necessário ressalvar que são comparadas as alturas de

água calculadas, com a batimetria de 1998, com as alturas de água de diferentes datas.

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Existem diversos factores que deverão ser tidos em conta nesta comparação, como a distância

da marca de cheia ao leito principal do rio, a dificuldade de obtenção de caudais exactos para

os anos ou datas presentes nas marcas de cheia e a exactidão de algumas das marcas de

cheia. Além disso, importa referir que a comparação é feita com o perfil de cálculo mais

próximo, havendo que considerar os erros associados.

A análise realizada permitiu concluir que do cálculo hidráulico resultam, maioritariamente,

valores superiores às marcas de cheia. Concretamente, das 26 marcas de cheia, em 21 o

estudo hidráulico forneceu valores superiores e em 5 forneceu valores inferiores. As diferenças

entre o cálculo e a marca de cheia são maiores quanto maior o caudal escoado. A média das

diferenças encontradas é de 0,95 m. Existe a possibilidade de o coeficiente de Manning, n,

estar estimado por excesso no leito de cheia e de não terem sido tidas em conta as variações

do leito aluvionar.

A análise teórica da morfologia fluvial deve considerar uma grande variedade de escalas de

tempo e de espaço (de Vriend, 1997). O movimento de um grão de areia deve ser analisado

com uma escala de tempo correspondente à da turbulência e com uma escala espacial da

ordem dos milímetros.

A análise da forma da secção transversal exige uma escala temporal dos dias (em ocasiões

de cheia) ou dos anos (para a estiagem) e uma escala espacial das centenas de metros.

Por sua vez, para a análise da evolução do perfil longitudinal, com uma escala espacial das

centenas de quilómetros, deve ser considerada uma escala temporal dos anos até aos séculos.

No limite a escala geológica também controla a evolução do perfil longitudinal. Para a análise

da evolução do leito do rio Tejo e da sua influência nas cheias e inundações, as duas escalas

intermédias são as mais importantes.

Foi também determinada a relação entre os caudais sólidos presentes no rio Tejo. Nesta

análise foram tidas em conta as parcelas provenientes de montante e dos afluentes, bem como

a capacidade de transporte ao longo do rio. Assim, sempre que a capacidade de transporte

exceder a estimativa da afluência de material sólido existe uma tendência para a erosão e

consequente escavação do fundo do rio. No caso contrário se o material afluente for superior à

quantidade de material que possa ser transportado haverá lugar a assoreamento.

Para além da variação natural do leito aluvionar do rio Tejo, há também variações que

derivam da actividade humana. Estas são a construção de grandes albufeiras (Castelo do

Bode, Alcântara, …), a diminuição de zonas florestais e a extracção de areias (DRA-LVT,

2000).

Esta última actividade é a que tem maior impacto na variação do leito aluvionar do rio Tejo.

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A avaliação teórica do material sólido afluente ao vale do rio Tejo está sujeita a elevada

incerteza, muito maior do que a da avaliação dos volumes de extracção. A menor incerteza na

avaliação é da variação do fundo aluvionar. Por esse motivo, o parâmetro dominante na

avaliação do estado da linha de água é a variação do fundo aluvionar.

A análise dos valores de extracção e de variação do fundo permite concluir da existência de

uma actividade de desassoreamento que, considerando o balanço de extracção da ordem dos

– 1,6.106 m3, equivale a uma descida anual de cerca de 5 cm por ano. No entanto, o

desassoreamento não é uniforme, sendo por esse motivo importante verificar a sua distribuição

ao longo do rio Tejo.

Verifica-se que o desassoreamento do rio Tejo ainda se encontra, na generalidade, numa

gama de valores que é menor do que a variação natural do fundo aluvionar do rio Tejo, na

relação de cerca de 1 para 2, isto é, a descida média do leito do rio de 1 m é inferior à variação

média interanual do leito médio, da ordem dos 2 m.

Referências Bibliográficas Brunner, G.W., 2002. HEC-RAS River Analysis System Hydraulic Reference Manual (Version 3.1 ed.) –

http://www.hec.usace.army.mil/software/hec-ras/ USACE, Davis, EUA

DRA-LVT, 2000. Extracção de inertes no rio Tejo. Estudo de Incidências Ambientais. Relatório Síntese.

Direcção Regional do Ambiente de Lisboa e Vale do Tejo. Ministério do Ambiente e do Ordenamento

do Território. Lisboa. 48 páginas e Anexos.

Hidrotécnica Portuguesa, 1977. Regularização do rio Tejo – Plano Geral. Anexo 1 - Estudo hidráulico do

rio Tejo. Ministério das Obras Públicas, Direcção-Geral dos Recursos e Aproveitamentos Hidráulicos,

Lisboa.

Rocha, J. Fernandes, J., Ferreira, G., Ferreira, T., 2005. Plano Específico de gestão de extracção de

Inertes no Domínio Hídrico do rio Tejo. Relatório 396/05-NRE, Laboratório Nacional de Engenharia

Civil.

de Vriend, H.J., 1997. River morphology: A manifestation of nonlinear dynamics. In Managing water:

Coping with scarcity and abundance. Proceedings of Theme A, Water for a changing global

community, XXVII IAHR Congress and ASCE, San Francisco, pp. 10-15, August.

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RADIOTRACERS FOR DATING FLOOD PROCESSES

JOAN-ALBERT SANCHEZ-CABEZA IAEA Marine Environment Laboratories, 4 Quai Antoine 1er, MC-98000 Monaco

e-mail: [email protected]

Environmental radiotracers have been proved to be a very useful tool with a large variety of

applications such as the study of the distribution and migration of pollution in the environment,

and radionuclide inputs and budgets in the terrestrial, freshwater and marine environments. In

particular, atmospheric radiotracers such as 137Cs have become common tools for the

quantification of soil erosion processes (Zapata, 2002). 137Cs is a man-made radionuclide injected into the atmosphere by nuclear weapons tests

which occurred mainly in the late 1950s and early 1960s and subsequently deposited on the

Earth’s surface. The 137Cs levels changed dramatically in Europe due to the Chernobyl accident,

when large amounts of 137Cs were released, dispersed and deposited in the environment in a

very heterogeneous pattern, closely following rainfall during the passage of the polluted plume.

The atmospheric concentrations of 137Cs follow a well-known time trend, with an offset in 1954,

a maximum in 1963 and, in Europe, a second maximum in 1986 (UNSCEAR, 2000). These

signatures can be used as environmental radio-chronometers in a variety of ways. Dried and

homogenised samples of a few tens of grams are measured by low-background high-resolution

gamma spectrometry. 210Pb is a member of the 238U decay series. 222Rn (T1/2= 3.8 d) exhales from the continental

crust to the lower troposphere and is dispersed until it decays into 210Pb, which attaches to

aerosols and is deposited on the Earth’s surface (excess 210Pb). On the other hand, 210Pb

formed in situ in soils and in equilibrium with its parent radionuclide 226Ra (T1/2 = 1601 yr) is

referred to as base (or supported) 210Pb. In general, the governing mechanism in the deposition

of atmospheric substances to soils is wet deposition. Although 210Pb can be measured by

gamma spectrometry, when the sample size is small (100s of mg) or the calibration of gamma

spectrometers is difficult, it can also be measured through its daughter radionuclide 210Po by

low-background high-resolution alpha spectrometry after total digestion using an analytical

microwave oven (Sanchez-Cabeza et al., 1998). 14C (T1/2 = 5730 yr) is also used in many environmental studies because absolute

chronologies can be obtained when proper samples are collected. Nowadays it is commonly

determined in very small samples (10s of mg of C) by graphite synthesis and subsequent

Accelerator Mass Spectrometry. Its use as an absolute radio-chronometer is limited by bomb-

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fallout in the XX century, although its use can be complementary for shorter lived radionuclides

such as 137Cs and 210Pb.

The depth distributions of 137Cs and excess 210Pb (210Pbex) in undisturbed soil cores usually

decrease almost exponentially. Some may also exhibit sub-surface concentration maxima of

varying widths. The depth distribution of a fallout radionuclide is the result of many factors, such

as soil properties, overlying vegetation and hydrology. Processes such as sorption, leaching or

mass transport through macropores or by bioturbation, can also drastically affect the radiotracer

profile. It has been shown that soil cores show 210Pb atmospheric fluxes which correlate very

well with mean annual rainfall in large regions such as N Spain, and this can also be the case

for 137Cs but only at the basin scale.

Undisturbed sediments with constant sediment accumulation show an exponential decrease

of 210Pbex concentrations due to radioactive decay that can be used to date the sediment

horizons. When sediment accumulation is not constant and/or sediment mixing occurs, more

complex models can be used to determine accumulation rates. 137Cs profiles can also be used

to confirm the dates obtained from 210Pb dating. If the sediment core is long enough, it is also

useful to use 14C to confirm mean accumulation rates or to determine changes of accumulation

over longer time periods.

However, during flood events, a large amount of matter (and therefore radionuclides

attached to particles) is remobilised and the dating hypotheses are no longer valid. In

radionuclide profiles it is not unusual to find lower activities due to signal dilution with older

catchment soils and, if the sampling and analytical resolution is good enough, a layer of

homogenous concentration can be observed and dated from the upper and lower profile

sections. Intensity of the flood can also be deduced from the width of the flood layer. This

technique has been used in some environments such as reservoirs, floodplains and river

prodeltas.

Various strategies are used to date flood events. First, 137Cs is an excellent time marker of

floods after 1954. 210Pb high resolution sedimentary records, together with grain size, and

possibly some characteristic metals, can be a powerful technique to date floods which occurred

over the last 100 years. 14C signals during older floods should not be dissimilar to that of other

radiotracers and absolute dating of events can be obtained from a high-resolution age model.

Finally, it is suggested that a full characterisation of flood events would need an extensive

knowledge of radiotracer distributions in a particular catchment, such as the studies carried out

in modern erosion research.

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References Sanchez-Cabeza, J.A., Masqué, P., Ani-Ragolta, I., 1998. 210Pb and 210Po analysis in sediments and soils

by microwave acid digestion. J. Radioanal. Nucl. Chem., 227/1-2, 19-22.

UNSCEAR, 2000. Sources and Effects of Ionizing Radiation, United Nations Publications, New York.

Zapata, F., 2002. Handbook for the Assessment of Soil Erosion and Sedimentation Using Environmental

Radionuclides, Kluwer.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

O PALEOLÍTICO NO MÉDIO E BAIXO TEJO

LUÍS RAPOSO Director do Museu Nacional de Arqueologia. Professor convidado do Departamento de História da Faculdade de

Letras da Universidade de Lisboa

e-mail: [email protected]

Apresentam-se os principais locais e zonas de ocupação humana no vale do Tejo, em

território português, de acordo com três critérios de observação, que se procuram cruzar entre

si: (a) história das investigações; (b) situação geográfica; (c) cronologia e inserção cultural.

Relativamente ao primeiro aspecto, salientam-se as descobertas feitas no início do século

XX nos arredores de Lisboa (especialmente na chamada “várzea de Loures” e zonas

limítrofes); os importantes trabalhos de Georges Zbyszewski e Henri Breuil realizados

principalmente na região de Alpiarça; e as investigações conduzidas nas últimas décadas tanto

no vale do rio principal (região de Ródão), como em vales subsidiários (Zêzere, Trancão, Rio

Maior, Almonda).

Quanto ao segundo aspecto, estabelecem-se os contrastes existentes entre o Médio Tejo e

o Baixo Tejo, e, dentro deste, entre o Tejo aluvial e o Tejo estuarino.

Finalmente, quanto ao terceiro aspecto, apresentar-se-ão os principais locais do Paleolítico

do Vale do Tejo, nos seus sucessivos momentos evolutivos, com referência especial à

economia das matérias-primas, às práticas de subsistência e às territorialidades. Serão ainda

sugeridas e discutidas hipóteses de trabalho que sugerem a forte interacção existente entre

cultura e ambiente na caracterização da ocupação humana paleolítica do Vale do Tejo.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

PALAEOFLOOD AND HISTORICAL FLOOD RECORDS ALONG THE MIDDLE TAGUS RIVER CATCHMENT: CLIMATIC AND FLOOD HAZARD IMPLICATIONS

GERARDO BENITO CSIC, Centro de Ciencias Medioambientales, Serrano 115 bis, 28006 Madrid, Espanha

e-mail: [email protected]

Information on hydrologic variability and extreme floods of the Tagus River have been

completed using both, palaeoflood hydrology (a new and developing branch of hydrology and

geomorphology based on geologic indicators) and historical information (based on documents

and chronicles). Slackwater flood deposits along the Tagus River (at Puente del Arzobispo and

Alcántara) provide an excellent record of flood events for the last 10,000 years. For this period,

more than 80 individual flood events have been recorded and their minimum peak discharge

have been estimated. The palaeoflood record shows that extreme floods are not randomly

spaced in time but there are clusters of flood events at during specific periods. High-magnitude

floods occurred on the Tagus river from 9440 to 9210 14C yr BP (8540-8110 BC), 8500 to 8000 14C yr BP (7500-7000 BC), ∼6750 14C yr BP (∼5000 BC), 1200 to 950 14C yr BP (AD 785-1205),

∼410 14C yr BP (AD 1450-1500), and 170 to 80 14C yr BP (AD 1670-1950). The palaeoflood

record over the last 1000 years was supported with documentary flood data collected at

Aranjuez, Toledo, Talavera de la Reina and Alcántara. High flood frequencies were registered

in the periods: AD 1160-1210 (3%), 1540-1640 (11%; peak at 1590-1610), 1730-1760 (5%),

1780-1810 (4%), 1870-1900 (19%), 1930-1950 (17%) and 1960-1980 (12%). According to the

present flood-producing mechanisms, these periods with increased flood magnitude and/or

frequency in the Tagus River are strongly related to increased moisture influx and winter

precipitation in the Iberian Peninsula, especially in the western zone. The sedimentary and

documentary record of water surface elevation reached by past floods enabled the calculation

of robust palaeodischarge estimates for floods that occurred during recent centuries or

millennia. The frequency and magnitude of these past flood records are discussed on the

context of flood risk assessment and on long-term flood-climate relationships.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Presentations

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

SCALE INVARIANT STUDY OF RIVER DISCHARGES FROM THE TAGUS BASIN

MARIA ISABEL PEDROSO DE LIMA1,2, J.L.M.P. DE LIMA1,3, R.R. RODRIGUES4, C. BRANDÃO4, A.S.S. ROSÁRIO4, 5

1Institute of Marine Research - Coimbra Interdisciplinary Centre, Portugal 2Forestry Department, ESAC/Polytechnic Institute of Coimbra, ESAC/IPC, Bencanta, 3040-316 Coimbra, Portugal

e-mail: [email protected] Department of Civil Engineering, University of Coimbra, Portugal

4Institute for Water, Portugal 5Research fellow

River discharge is known to be dependent on climatic regimes (namely the rainfall input) and

complex rainfall-runoff processes occurring at a variety of scales in time and across the

catchment’s basin. As a consequence, this highly non-linear hydrologic process exhibits wide

variability over a broad range of time and space scales. This variability is an important issue in

many studies and areas of research (e.g. hydrology, hydraulics, water resources, land planning,

and urban development). It involves a large dynamic range, which in certain cases leads to

catastrophic events related, for example, to both flood and drought situations.

It is thus important to analyse runoff with methods that have the potential to assess the full

range of river flow fluctuations. Multifractal theory (e.g. Schertzer and Lovejoy, 1989, 1991) can

play an important role in such studies. Moreover, multifractal methods are innovative for the

analysis of extremes, offering practical tools for assessing their probability of occurrence.

Multifractal theory predicts the existence of heavy tails (i.e., power-law tails) in the flow rate

probability distributions. This statistical behaviour may indicate that, in some cases, the

probability of exceeding certain events is greater than the probability predicted by more

"conventional" models (e.g. Gumbel distribution). Runoff models based on non-scaling type

processes (e.g. Auto Regressive Moving Average type processes) usually involve only weak

variability (e.g. exponential probability tails). In this framework, two or more different

distributions are often necessary to fit different regimes such as the “regular” and the “extreme”

events. Multifractal theory offers a single framework to deal with these different regimes.

The approach to the study of river flows based on multifractal theory has been reported in

only a few studies, e.g. Ladoy et al. (1991), Tessier et al. (1996), Pandey et al. (1998).

Therefore, the applicability of multifractal theory to runoff processes has still not been fully

explored.

This work reports results of scale invariant and multifractal analyses of river discharges from

the Tagus basin, concentrating on the statistics of extreme events. Monthly and daily data are

analysed. The time span of the records is up to several decades. The longest river flow

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

discharge time series studied is from Vila Velha de Ródão, in the river Tagus, for a period of 74

years; the corresponding drainage basin is 59247 km2.

The data are from different hydrometric stations located in the Tagus drainage basin; these

stations are located in the Tagus River and in some of the tributaries. The drainage basins have

different geometric, geological and climatic characteristics, as well as land use. The areas of the

basins are comprehended between less than 100 km2 and almost 65000 km2. The Tagus

drainage basin covers an area of about 80629 km2, being 24800 km2 (29.8%) in Portugal. The

basin has a general orientation ENE to WSW; its length is approximately 700 km and the

average width is around 120 km. The Tagus River is 1100 km long; 230 km run in Portugal and

43 km in the border with Spain. In Portugal, the main tributaries are, in the left bank, the river

Sorraia, with a drainage basin of 7555 km2, and in the right bank (northern sector) the river

Zêzere, with 5080 km2. The annual average flow discharge from the river Zêzere is about 3290

hm3/year, whereas in the river Sorraia is about 1185 hm3/year.

Results confirm that the temporal structure of river discharge exhibits scale invariant

behaviour. Thus, discharges Qλ observed at scales of resolution λ-1 respect the multiple scaling

behaviour of the probability distributions: , where c(γ) is the codimension

function and γ is the order of singularity of the discharge (e.g. Schertzer and Lovejoy, 1987).

Moreover, the statistics of extreme events can be described by a power-law, ,

where s is a sufficiently large discharge-threshold; this behaviour indicates divergence of

statistical moments above a certain critical order q

)()Pr( γγλ λλ cQ −≈≥

DqssQ −≈≥ )Pr( λ

D (fig. 1). This value is rather low in certain

cases, indicating violent river regimes. The magnitude of this moment was observed to vary with

the river course analysed within the Tagus basin.

Fig. 1 - Empirical probability distributions of daily river

discharge, for the data from Vila Velha de Ródão.

Discharge is expressed in m3/s.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

This study will pursue investigating the relations between the statistical behaviour of

extremes and the physical characteristics of the drainage basins as well as the statistics of the

rainfall input.

This work has been carried out under the research project POCTI/ECM/48619/2002, funded

by the Portuguese Foundation for Science and Technology and FEDER.

References Ladoy, P., S. Lovejoy and D. Schertzer, 1991. Extreme variability of climatological data: scaling and

intermittency. In: Schertzer, D., and S. Lovejoy (eds.), Non-linear variability in Geophysics: scaling and

fractals. Kluwer Academic Publ., 241-250.

Pandey, G., S. Lovejoy and D. Schertzer, 1998. Multifractal analysis of daily river flows including extremes for

basins of five to two million square kilometres, one day to 75 years. J. Hydrology, 208(1-2): 62-81.

Schertzer, D., and S. Lovejoy, 1987. Physical modeling and analysis of rain and clouds by anisotropic scaling

multiplicative processes. Journal of Geophysical Research, 92(D8), 9693-9714.

Schertzer, D., and S. Lovejoy, 1989. Nonlinear variability in Geophysics: multifractal simulations and analysis.

In: Pietronero, L. (ed.), Fractals’ physical origin and properties. Plenum Press, 49-79.

Schertzer, D., and S. Lovejoy (eds.), 1991. Non-linear variability in Geophysics: scaling and fractals. Kluwer

Academic Publ., 318 p.

Tessier, Y., S. Lovejoy, P. Hubert, D. Schertzer and S. Pecknold, 1996. Multifractal analysis and modeling of

rainfall and river flows and scaling, causal transfer functions. J. Geophysical Res., 101: 26427-26440.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

THE ROLE OF NAO EXTREME PHASES ON THE MONTHLY DISCHARGES, AND THE VOLUME AND DATE OF THE ANNUAL MAXIMA FLOWS (1950-2003) IN THE TAGUS

RIVER

JUAN I. LÓPEZ-MORENO1,2,*, SERGIO M. VICENTE-SERRANO2, SANTIAGO BEGUERÍA2, JOSÉ M. GARCÍA-RUIZ2,*

1Dpt. Geosciences. University of Fribourg. Perolles, CH-1700 Switzerland *e-mail: [email protected]

2Instituto Pirenaico de Ecología, CSIC (Spanish Research Council), Campus de Aula Dei, P.O. Box 202, Zaragoza,

Spain *e-mail: [email protected]

The North Atlantic Oscillation (NAO) is one of the most important atmospheric circulation

patterns in Europe, which determines noticeably the climate in the Tagus basin (Trigo et al.,

2004). Positive phases of the NAO are related to high Sea Level Pressures (SLP) and low

precipitation. Negative phases show the opposite pattern. The NAO is better recognised during

winter, season in which its climate effects are more important.

On the other hand, water resources in the Tagus basin are characterised by a high temporal

variability, being the drought periods and floods highly frequent (Benito et al., 2003). In the

Tagus basin, drought periods usually cause important conflicts between users; whereas

frequent floods may be considered as a risk and cause important economic losses (Ramos and

Reis, 2001).

Using river discharges (monthly and annual maxima flows) and reservoir storages, from the

Confederación Hidrográfica del Tajo (Spain) (http://www.chtajo.es/redes/cantidad.htm) and the

Sistema Nacional de Informaçao de Recursos Hidricos (Portugal) (http://snirh.inag.pt/), we have

analysed the role of the extreme phases of the winter NAO on the water resources and flows in

the Tajo’s basin between 1950 and 2003.

We show that river discharges are highly determined by the winter NAO extreme phases.

Negative NAO phases direct positive anomalies in the river discharges whereas the positive

NAO phases produce negative anomalies. Moreover, these anomalies are even identified

several months after the winter, during the summer and the fall. Also some differences exist in

this response between areas as a function of the location and the river characteristics

(regulation, average flows, etc.). There is a delay in the response of the river discharges to the

NAO extreme phases; nevertheless the impacts are better recognised towards the East. Thus,

in the headwaters, significant anomalies are even identified seven months after the NAO

extreme phase.

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There is not a lineal relationship between the NAO and the river discharges of the Tagus

basin since the response to the negative phases is slower than positive phases but the former

shows anomalies maintained during a higher number of months.

Annual maxima flows are also highly affected by the NAO extreme phases. During the

negative phases, the volume of the maxima flows is higher and the annual maxima flows are

mainly recorded in winter. On the contrary, during the positive phases, the volume of the annual

maxima is lower and the maximum annual flow is dominantly recorded in autumn.

References Benito, G., Diez-Herrero, A. and de Villalta, M.F., 2003. Magnitude and frequency of flooding in the

Tagus basin (Central Spain) over the last millennium. Climatic Change. 58: 171-192.

Ramos, C. and Reis, E., 2001. The floods in the south of Portugal in different kinds of drainage basins.

Finisterra, 71: 61-82.

Trigo, R.M., Pozo-Vázquez, D., Osborn, T.J., Castro-Díez, Y., Gámiz-Fortis, S., Esteban-Parra, M.J.,

2004. North Atlantic Oscillation influenceon precipitation, river flor and water resources in the Iberian

Peninsula. International Journal of Climatology, 24: 925-944.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

USEFULNESS OF PALEO-FLOOD INFORMATION IN FLOOD QUANTILE ESTIMATION

BHAGABAT .P. PARIDA Dept. of Environmental Science, University of Botswana, P/Bag UB 00704, Gaborone, Botswana.

e-mail: [email protected]

Collapse of the climate negotiations in the recent past have freed the industrialised nations

from any restrictions in their greenhouse gas emissions, as a result of which the world is likely

to see a substantial increase in CO2 emissions, the major contributor to the risk of global climate

change including rise in temperature (Raskin and Kemp-Benedict, 2004). It is well known that

resulting general rise in global temperature will accelerate the hydrologic cycle involving

changes in precipitation hence in the runoff, which may ultimately result in increased frequency

of many weather related disasters including floods and have been estimated at some 12000

disasters since 1980. As reported by Office of Disaster Assistance of the USAID, floods alone

had the highest profile affecting some 275 million people between the decade of 1980-1990 and

attained highest number of declarations (165) in the subsequent decade i.e. 1990-2000, which

in 2002 alone resulted in 66% of the total losses with some 600 events from out of a total of 700

natural disasters. Even recently, in 2004, weather related disasters caused nearly $105 billion

losses (in 2003 dollars) – which was almost twice the total in 2003. Even looking at the

disasters due to floods, it not only had the highest numbers of events but also has been

responsible in affecting the highest number people compared to any other disaster. For

example, the number of flood events rose from 107 in 2004 affecting about 74 million people to

168 in 2005 affecting nearly 117 million people (EM-DAT, 2006).

Frederick and Major (1997) in yet another study have reported that not only the timing and

magnitude of the global temperature changes are uncertain, but less is known about the climate

change and their impacts at basin and watershed levels. General Circulation Models (GCMs)

the principal tools relating changes in atmospheric chemistry to changes in climatic variables

such as temperature and precipitation; though project that a 1.5 to 4.5 ºC rise in global mean

temperature would increase the global mean precipitation about 3 to 15%, do not provide the

requisite degree of region specific information. It is projected that, in areas with increased

precipitation, higher evaporation rates may lead to reduced runoff. So, hydrological

uncertainties attributed to changing atmospheric chemistry are likely to persist in the

foreseeable future.

Again, it has been observed that climate change predictions are primarily based on

greenhouse gas emissions while the impact of land use has been a neglected factor in such

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studies. In a recent study Feddema et al., (2005) indicated that consideration of the land use

land cover changes have shown significant changes with opposite signs in the A2 scenario

compared to the ones issued by Inter Governmental Panel on Climate Change (IPCC,2001),

and may lead to far reaching consequences. This is particularly important looking at the world

wide rapid rate of urbanisation leading to significant changes in land use. For example, during

the year 2000, the global population changed from less than 50% urban to more than 50%

urban. It is estimated that by 2010, urban population will account to more than 60% of the world

population, threatening a significant change in the land use in terms of human settlements and

industries. It is therefore quite likely that one may experience larger and more frequent floods.

Managing such events though may remain more human oriented, yet providing tangible

solutions still remain scientific.

In flood estimation procedure, generally emphasis on two aspects is given viz. on the

appropriate choice of a statistical model and a robust method of parameter estimation, such that

both descriptive and predictive aspects are well covered (Cunnane, 1987). While this is true, the

limitation of the historic data is often overlooked and any peculiarity in the data are analysed by

hypothesising an elaborate statistical model. For example, the present-day analysis a priori

assumes the data to be perfectly correct and free from any measurement error. Related studies

by Potter and Walker (1981) have shown that the effect of modelling error because of this is far

more damaging than the sampling error. Even, another related study by Hosking and Wallis

(1986) have shown that measurement error can result in yielding less accurate design flood

estimates even with the use of Probability Weighted Moments (PWM) method of parameter

estimation. This particularly is more relevant when it is well accepted that the method of PWM

(Greenwood et. al., 1979; Hosking, 1986) or even the method of L-Moments (Hosking, 1990)

produce efficient and robust estimators while dealing with small samples or samples containing

outliers or even in a case where a wrong choice of the parent distribution has been made

(Wallis and Wood, 1985). It is also in general agreement that a three-parameter distribution by

and large is acceptable for modelling the flood data as it is capable of taking into account the

skew properties of the flood data adequately and yield quantiles with least mean squared error.

Studies by Cong and Xu (1987) and Parida (2002) showed that measurement error can change

the skew properties of the sample considerably and may suggest a different distribution other

than the parent. To overcome this, inclusion of paleoflood information wherever available has

been highly recommended (Hosking and Wallis, 1986; Stedinger and Cohn, 1986; Parida,

2000). Even they emphasize on its usefulness when the number of parameters to be estimated

are more as in case of three parameter distribution compared to a two parameter one.

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Yet another dimension to the problem of flood estimation is to obtain reliable design flood

estimates when the catchments are ungauged or when one needs to develop a nation-wide

uniform flood estimation procedure. For these, Regional Flood Frequency Analysis (RFFA)

methods in various forms have been in advocated, which besides being able to provide

meaningful solutions to the above problems, have been found to improve the at-site flood

estimates within the region (Cunnane, 1988). It is particularly so, as the regional estimators are

least affected by most of the data related problems mentioned earlier. It can be therefore be

said that reliable estimates of flood quantiles can be obtained through RFFA together with

consideration of Paleoflood information, as it allows the series to get the experience of what the

stream has experienced. In a simulation study Parida (2000) has shown that incorporation of

paleoflood information infested with as high as 30% measurement error to a series of

observations infested with 10% yield more reliable quantiles than from the same sample having

no measurement error. But what is not clearly known is the extent of usefulness in inclusion of

such paleo flood events on the at-site and regional quantile estimates, particularly when a

region is delineated using critical attributes such as the climate and land use responsible for

generation of such extreme floods. Therefore, a case study using real life data from the 3-d

hydro-meteorological region in India has been undertaken, where at two out of some 17 gauged

sites evidence of such extraordinary floods events had been observed. Quantiles using PWM

procedure with the inclusion of such extra-ordinary data with recent observations (i.e. the case

of censored sample) were undertaken. It was found that the quantile estimates though did

improve considerably particularly at sites where these extraordinary events were available, did

not change the regional growth curve characteristics for the study region.

References: Cong, S., and Xu, Y., 1987. Effects of Discharge Measurement Error on the Results of Flood Frequency

Analysis, Application of Frequency and Risk in Water Resources, V.P. Singh, Ed, 175 – 190.

Cunnane, C., 1987. Review of Statistical Models for Flood Frequency Estimation, Hydrologic Frequency

Modeling, D. Reidel Pub. Co., Holland, V.P. Singh, Ed., 49 – 95.

Cunnane, C., 1988. Methods and Merits of Regional Flood Frequency Analysis, J. Hydrology, 100, 269 –

290.

EM-DAT, 2006. The OFDA/CRED International Disaster Database, University Ctholique de Louvain,

Brussels, Belgium.

Feddema, J.J., Oleson, K.W., Bonan, G.B., Mearns, L.O., Buja, L.E., Meehl, G.A. and Washington, W.M.,

2005. The importance of Land-Cover in Simulating Future Climates, Science, 310 (5754), 1674-1678.

Frederick, K.D. and Major, D.C., 1997. Climate Change and Water Resources, J. Climate Change, 37,

7-23.

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Greenwood, J.A., Landwehr, J.M., Matalas, N.C., and Wallis, J.R., 1979. Probability Weighted Moments:

Definitions and Relation to Parameters of Several Distributions Expressible in Inverse Form, Water

Resour. Res., 15(5), 1049 – 1054.

Hosking, J.R.M., 1990. L-Moments: Analysis and Estimation of Distribution Using Linear Combination of

Order Statistics, J. Royal Statistical Society –B, 52 (1), 105 –124.

Hosking, J.R.M., and Wallis, J.R., 1986. The Value of Historical Flood Data in Flood Frequency Analysis,

Water Resour., Res., 22(4), 543 – 550.

IPCC , 2001. Climate Change: Impacts, Adaptations and Vulnerability, Inter governmental Panel on

Climate Change, Cambridge University Press, p 1031.

Parida, B.P., 2000. Worth of Extreme Flood Events in Site Specific Flood Frequency Analysis,

Proceedings International Conference on Integrated Water Resources Management for Sustainable

Development, New Delhi, India, 823 – 832.

Parida, B.P., 2002. Effect of Measurement Error on Flood Estimation, Proceedings International

Conference on Flood Estimation, Berne, Switzerland , CHR Report II-17, 775-778.

Potter, K.W., and Walker, J.F., 1981. A Model of Discontinuous Measurement Error and its Effect on the

Probability Distribution of Flood Discharge Measurement, Water Resour. Res., 17(5), 1505 – 1509.

Raskin, P. and Kemp-Benedict, E., 2004. Background Paper for UNEP’s Third Global Environment

Outlook Report (GEO-3), UNEP, Kenya.

Stedinger, J.R., and Cohn, T.A., 1986. Flood Frequency Analysis with Historical and Paleoflood

Information, Water Resour. Res., 22(5), 785 – 793.

Wallis, J.R., and Wood, E.F., 1985. Relative Accuracy of L-P III Procedures, J. Hydraul. Div. ASCE,

111(7), 1043 – 1056. (see also discussion and reply 113(9), 1205 – 1214.)

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

THE IMPACT OF THE NORTH ATLANTIC OSCILLATION ON PRECIPITATION AND WATER RESOURCES OF THE IBERIAN PENINSULA

RICARDO M. TRIGO Centro de Geografia da Universidade de Lisboa, University of Lisbon, Portugal

e-mail: [email protected]

The Iberian Peninsula area is characterised by large values of inter-annual variability of

precipitation. As a consequence river flow is also characterised by large disparities between wet

and dry years, especially in southern Iberia. This situation portrays a major problem in water

resources management in general, and production of hydroelectricity in particular. In Portugal

the hydroelectric production represents (on an average year of precipitation) one third of the total

Portuguese electricity production but its absolute value can vary by a factor of almost three

between wet (16 TWh) and dry (6 TWh) years.

Only recently it has been recognized that the strong control exerted by NAO on precipitation

over the Mediterranean basin could be directly reflected on the seasonal streamflow of rivers

across the region (Trigo et al., 2004). In fact, recent works have proved that this is the case for

the Middle East rivers Tigris and Euphrates (Cullen and deMenocal, 2000) and the large central

European river Danube (Stanev and Peneva, 2002; Rimbu et al., 2002). Here we intend to show

that such influence on river flow regimes is extensive to the Iberian rivers, and that the

magnitude of such influence is even larger for Iberian rivers than it is the case with the Danube,

Tigris or Euphrates.

River flow data from both Douro (at Pocinho) and Tejo (at Fratel) spans between 1922 and

1997. The impact on the hydrological cycle (from Oct. to Sep.) of years characterized by winters

with large positive and negative NAO index anomalies is shown in Fig. 1a for Tejo River. For

river Douro (not shown) these differences are significant (at the 5% significance level) only

between January and April while for river Tejo they are consistently significant between January

and September. This reflects the fact that river Tejo basin is located in central Iberia, a region

more affected by the impact of the NAO in the precipitation field (Trigo et al., 2002, 2004).

We have computed the correlation coefficients between winter river flow (DJFM) and

contemporaneous winter NAO index. We have also computed the lagged correlation between

the NAO index for DJF and the river flow for JFM. Furthermore, we have computed correlation

coefficients for shorter periods, namely between 1922 and 1971 and between 1972 and 1997.

Two important conclusions can be drawn:

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1) The magnitude of all lagged correlation coefficients is consistently higher than the

corresponding non-lagged ones. This fact is particularly relevant because it increases the

potential for using these empirical relationships in effective forecasting tools.

2) There is a major increment in the magnitude of correlation coefficient values between the

first sub-period and the second sub-period. This is particularly impressive for the Douro River

where it increases between -0.28 and -0.62. Fig. 1b shows the inter-annual variability of river

Tejo against the lagged winter NAO index (multiplied by –1 to facilitate analysis).

These results show that the large inter-annual variability of these two rivers flow is largely

modulated by the NAO phenomena. Such modulation, associated with the recent positive trend

of the NAO index, might implicate a significant decrease of the available flow. This reduction can

represent an important hazard for the two Iberian economies due to its negative impact in

agricultural yield and hydroelectric power production.

Fig. 1 - (a) Monthly river flow of river Tejo at Fratel for winters with NAO index > 0.5 (dotted curve), for

winters with NAO index > -0.5 (dashed curve) and for the average winter (solid line), (b) Interannual

variability of the mean winter (JFM) river flow (solid curve) and the lagged winter (DJF) NAO index,

multiplied by –1 to facilitate analysis (dashed curve); both curves have been normalised and so are

dimensionless.

References Cullen, H. M. and P.B. de Menocal, 2000. North Atlantic Influence on Tigris-Euphrates Streamflow. Int. J.

Climatol., 20, 853-863.

Rimbu, N., C. Boroneant, B. Carmen and D. Mihai, 2002. Decadal variability of the Danube river flow in

the lower basin and its relation with the North Atlantic Oscillation. Int. J. Climatol., 22, 1169-1179.

Stanev, E.V. and L.P. Elissaveta, 2002. Regional sea level response to global climatic change: Black sea

examples. Global and Planetary Changes, 32, 33-47.

Trigo, R.M., T.J. Osborn and J. Corte-Real, 2001. The North Atlantic Oscillation influence on Europe:

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climate impacts and associated physical mechasims. Climate Research, 20, 9-17. 10.

Trigo R.M., Pozo-Vazquez D., Osborn T.J, Castro-Diez Y., Gámis-Fortis S., Esteban-Parra M.J., 2003.

North Atlantic Oscillation influence on precipitation, river flow and water resources in the Iberian

Peninsula. International Journal of Climatology, 24, 925-944.

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A MEDIÇÃO DE CAUDAIS DE CHEIA NO RIO TEJO

JOÃO SOROMENHO ROCHA1, MARIA EMÍLIA VAN ZELLER2 E JOÃO NUNO FERNANDES1

1Laboratório Nacional de Engenharia Civil

e-mail: [email protected], [email protected]ão de Coordenação e Desenvolvimento Regional de Lisboa e Vale do Tejo

Av. Brasil, 101, 1700-066 Lisboa, Portugal

e-mail: [email protected]

A medição de caudais no rio Tejo é feita em3 estações hidrométricas, Tramagal, Almourol e

Ómnias.

A estação do Tramagal fica situada próxima da localidade com o mesmo nome, a cerca de

100 km de Vila Franca de Xira. Para montante, o afluente mais próximo é Rio Torto.

A estação de Almourol fica situada a montante do castelo de Almourol, a cerca de 85 km de

Vila Franca de Xira. Para montante, o afluente mais próximo é o Rio Zêzere.

A estação de Ómnias fica situada a jusante de Santarém, a cerca de 45 km de Vila Franca

de Xira. O afluente mais próximo desta estação, para montante, é o Rio Alviela.

De forma a determinar as curvas de vazão destas estações, ao longo do tempo, foram

sendo efectuados levantamentos batimétricos pela DRAOTLVT permitindo, ao mesmo tempo, o

conhecimento da evolução morfológica nestas secções. Além disso, dadas estarem disponíveis

as séries de caudais para estas estações é possível verificar a variação do talvegue e do leito

médio com o regime de caudais.

As séries de caudais disponíveis com interesse para o estudo das cheias são:

i) séries de medições do caudal instantâneo efectuadas na altura do levantamento;

ii) séries de caudais médios diários disponíveis no SNIRH,

iii) nível hidrométrico instantâneo da rede hidrométrica automática (que, com o

conhecimento das curvas de vazão permite o conhecimento do caudal instantâneo).

Com os dados disponíveis foi possível determinar as variações do talvegue ao longo dos

períodos de tempo disponíveis para cada estação (desde 1990/91 a 2004/05 para as estações

de Tramagal e Almourol e 1986/87 a 1999/00 para a estação de Ómnias). Nas séries de

caudais disponíveis são conhecidos os caudais instantâneos medidos na altura do

levantamento para todas as estações. A rede automática foi iniciada em 200l. Os caudais

médios diários publicados pelo SNIRH terminam em 1990, cobrindo assim uma parte da série

de batimetrias da estação de Ómnias (com períodos coincidentes).

Relativamente à variação do talvegue nas estações hidrométricas verifica-se que:

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− Existe uma grande variabilidade sazonal e inter-anual da cota do talvegue, chegando a

verificar-se variações de 2,5 m em curtos períodos de tempo;

− A relação indirecta entre a cota do talvegue e do caudal, um caudal elevado provoca

uma redução acentuada da cota do talvegue, a “recuperação” para a cota original é mais lenta,

− No Tramagal não houve uma tendência de variação significativa no período de

observação (cerca de 15 anos);

− Em Almourol observou-se uma tendência para o abaixamento, traduzido em cerca de

1 m no período de observação (cerca de 15 anos);

− Em Ómnias observou-se uma tendência para o abaixamento do talvegue de cerca de

1 m no período de observação (cerca de 13 anos).

A análise da variação do leito médio encontra-se dependente da digitalização dos dados. No

futuro poder-se-á ainda tentar recuperar dados mais antigos para estender para datas mais

recuadas a avaliação da variação do fundo aluvionar do rio Tejo.

Referências Bibliográficas Hidrotécnica Portuguesa, 1977. Regularização do rio Tejo – Plano Geral. Anexo 1 - Estudo hidráulico do

rio Tejo. Ministério das Obras Públicas, Direcção-Geral dos Recursos e Aproveitamentos Hidráulicos,

Lisboa.

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ACTIVE TECTONIC STRUCTURES IN THE LOWER TAGUS VALLEY: STATE OF THE ART

JOÃO CABRAL1

1 Fac. Ciências de Lisboa, Dep. Geologia, LATTEX / IDL, Ed. C6, 2º Piso, Campo Grande, 1749–016 Lisboa,

Portugal

e-mail: [email protected]

Geological Setting The Lower Tagus Valley (LTV, central-western zone of mainland Portugal) is sited in the

Lower Tagus Basin (LTB), a NE-SW elongated tectonic depression comprising up to nearly

2000m of Cenozoic sediments, located east of the Mesozoic Lusitanian Basin. It was formed

mainly in the Neogene as a compressive foredeep related to tectonic inversion of the Lusitanian

Basin, due to a NW-SE Miocene compression (Ribeiro et al., 1990, Rasmussen et al., 1998;

Carvalho, 2003). There is structural inheritance from the underlying Mesozoic and Paleozoic

basement through the compressive reactivation of previously extensional structures of the

Lusitanian Basin, in a relatively complex fault pattern that makes up the Lower Tagus Valley

fault system.

Data from the surface geology, from oil exploration wells drilled in the LTV area and from

seismic reflection profiles that cross the LTB show that the lowermost Cenozoic deposits, of

Eocene-Oligocene age, are of relatively uniform thickness (200-400 m), whereas the

subsequent Neogene sediments are much thicker and variable. More than 800 m of

predominantly siliciclastic continental and interfingered shallow marine Miocene sediments have

been preserved in Neogene depocenters. Fluvial feldspathic sands and gravels of Pliocene age

unconformably overlie the Miocene deposits.

The culminating surface of the Pliocene sedimentation is still preserved in a few areas of the

LTB, reaching a height of 100-120 m in the LTV (Martins, 1999). Stepped Quaternary terrace

deposits of the Tagus River and its tributaries occur along their valleys providing evidence of

reversal of the regional subsidence to uplifting in most of the LTV area, which generated the

entrenchment of the fluvial network since the Lower Pleistocene (Cabral, 1995).

The distribution of the Tertiary sediments in the LTB is asymmetric. The Miocene rocks

outcrop predominantly in the western side of the Tagus valley, while the Pliocene deposits

dominate at the eastern side. This is an effect of differential subsidence between the two areas

during deposition of the Pliocene fluvial sediments, leading to a smaller thickness of these

sediments at the west, where they were mostly removed by subsequent erosion in the

Quaternary. The evolution from a drainage network flowing on a vast alluvial plain in the Upper

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Pliocene to an entrenched fluvial system in the Quaternary, as a result of relative lowering of

base level, was apparently controlled by the same NNE-SSW regional structure, which settled

the lower reach of the Tagus river and so controlled the distribution of most of the Quaternary

sediments in the area.

Regional seismicity and active faults in the LTV The LTV has been subjected to earthquakes that caused severe damage and many

casualties, making seismic hazard evaluation a major regional issue. The regional seismicity

includes large distant events, as the 1755 “Lisbon earthquake” (M ≥ 8.5), generated in the

Eurasia-Africa plate boundary zone (Martins and Mendes Victor, 1990, Zitellini et al., 1999,

2001; Baptista et al., 2003; Gracia et al., 2003), and local intraplate earthquakes with M=6-7, as

in 1344, 1531 and 1909 (Martins and Mendes Victor, 1990; Teves-Costa et al., 1999, and

others). Since 1909, seismic activity has been low and diffuse, difficult to correlate with the

surface geology.

The regional earthquake activity indicates the presence of active seismogenic faults in the

area that represent a serious threat for the densely populated region. However, major

uncertainties concerning the seismic sources persist due to the low slip rates and the presence

of a thick sedimentary cover that conceals the faults in the underlying basement. In fact, most of

the active faults in the LTV region are hidden or partially hidden under the Tertiary and

Quaternary alluvial fill of the LTB. Evidence of surface faulting in the Quaternary is scarce,

although for a few structures there is geomorphic and outcrop evidence of post- Upper Pliocene

faulting, as for the Azambuja fault (Cabral et al., 2004). The predominance of hidden faults

requires the search for sub-surface data, mainly through geophysical methods (seismic

reflection, gravimetric, geoelectric) and from boreholes (mostly drilled for the oil industry or

water exploitation).

Not withstanding the existence of a macro-scale, NNE-SSW trending, wide fault zone in the

basement, no evidence was found for the presence of such a continuous structure at the upper

crustal levels that might correspond to a so-called Lower Tagus Valley Fault, often referenced in

the bibliography. The sub-surface data, together with the surface geology, reveal structural

complexity characterized by a segmented regional fault system, with NNE-SSW en echelon

faults presenting Neogene reverse offsets, linked by WNW-ESE faults that accommodate large

vertical offsets, probably working as transfer zones.

Seismogenic potential of the active faults Evidence of Quaternary tectonic activity was found for some of these structures. A few

estimates of low slip rates for regional active structures have been obtained (0.1-0.05mm/yr).

The faults range in length from 10 to 40 km, being capable of individually generating maximum

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earthquakes of magnitude 6.2 to 6.9. These fall in the magnitude range estimated for the

historical seismicity, suggesting that the historical earthquakes represent “maximum

earthquakes”. However, the historical earthquakes in LTV deviate from the Gutenberg-Richter

recurrence law obtained for the instrumental seismicity, suggesting the interaction between

adjacent faults which produces clustering of earthquakes and reduces recurrence time.

References Cabral, J., 1995, Neotectónica em Portugal Continental. Memórias do Instituto Geológico e Mineiro,

Memória 31, Lisbon, 265 pp.

Cabral, J., Ribeiro P., Figueiredo P., Pimentel N., Martins A. (2004). The Azambuja fault: An active

structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal).

Journal of Seismology, 8, pp. 347–362.

Carvalho, J., 2003. Sísmica de alta resolução aplicada à prospecção, geotecnia e risco sísmico. PhD

Thesis, University of Lisbon.

Gràcia, E., Dañobeitia, J., Vergés, J., the PARSIFAL Team (N. Zitellini, M. Rovere, D. Accetella, A.

Ribeiro, J. Cabral, L. Matias, R. Bartolomé, M. Farrán, D. Casas, A. Maldonado, A. Pazos, D.

Córdoba, and X. Roset), 2003. Mapping active faults offshore Portugal (36ºN-38ºN): Implications for

seismic hazard assessment along the southwest Iberian margin. Geology 31, 83-86.

Martins, A. A. (1999): Caracterização Morfotectónica e Morfosedimentar da Bacia do Baixo Tejo

(Pliocénico e Quaternário). Dissertação de doutoramento, Universidade de Évora, Évora, 500 pp.

Martins, I., Mendes-Víctor, L. A., 1990. Contribuição para o Estudo da Sismicidade de Portugal

Continental. Instituto Geofísico Infante D. Luís, Publicação 18, University of Lisbon.

Rasmussen, Erik S., Lomholt, S., Anderson, C., Vejbaek, O. V., 1998. Aspects of the structural evolution

of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal. Tectonophysics

300, 199-225.

Ribeiro, A., Kullberg, M. C., Kullberg, J. C., Manupella, G., Phipps, S., 1990. A review of Alpine tectonics

in Portugal: Foreland detachment in basement and cover rocks. Tectonophysics 184, 357-366.

Teves-Costa, P., Rio, I., Marreiros, C., Ribeiro, R., Borges, J. F., 1999. Source parameters of old

earthquakes: Semi-automatic digitalization of analog records and seismic moment assessment.

Natural Hazards 19, 205-220.

Zitellini, N., Chierici F., Sartori, R., Torelli, L., 1999. The tectonic source of the 1755 Lisbon earthquake

and tsunami. Annali di Geofisica 42, 49-55.

Zitellini, N., Mendes Victor, L. A., Cordoba, D., Danobeitia, J., Nicolich, R., Pellis, G., Ribeiro, A., Sartori,

R., Torelli, L., Bartolome, R., Bortoluzzi, G., Calafato, A., Carrilho, F., Casoni, L., Chierici, F., Corela,

C., Correggiari, A., Della Vedova, B., Gracia, E., Jornet, P., Landuzzi, M., Ligi, M., Magagnoli, A.,

Marozzi, G., Matias, L., Penitenti, D., Rodriguez, P., Rovere, M., Terrinha, P., Vigliotti, Ln, Zahinos

Ruiz, A., 2001. The 1755 Lisbon earthquake and Tsunami: localization and investigation of the

tectonic source. EOS, Transactions, Am. Geophys. Union 82, 290-1.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ACÇÕES ANTRÓPICAS NA BACIA HIDROGRÁFICA DO TEJO NA ÉPOCA CONTEMPORÂNEA: FLORESTAÇÃO, CORRECÇÃO TORRENCIAL E CONTROLO DE

CHEIAS

JOANA GASPAR DE FREITAS Faculdade de Letras de Lisboa/ R. Prof. Prado Coelho, n.º 40, 6.º Dto 1600-656 Lisboa

e-mail: [email protected]

Introdução «O Tejo é como um cavalo selvagem, indómito, impetuoso, cheio de vida, destruidor.

Domesticado, pode ser, na Península Ibérica, um servidor leal, paciente, utilitário, produtor do

bem-estar das populações ribeirinhas e valioso auxiliar nas economias nacionais» (Correia,

1967).

A história das acções antrópicas na bacia hidrográfica do Tejo nos últimos dois séculos

caracteriza-se pelas tentativas do homem para dominar o rio com o intuito de mitigar a acção

destruidora das grandes cheias e de explorar o potencial económico das suas águas, através

das múltiplas actividades que ali se podem desenvolver. As relações entre o homem e o rio

remontam a tempos pré-históricos, mas é sobretudo a partir do século XIX que se assiste a

uma intensificação do labor humano sobre a bacia hidrográfica do Tejo. Numa época em que

os progressos da ciência e da técnica permitiram o aumento significativo da capacidade de

intervenção e transformação do meio, os trabalhos levados a cabo tiveram reflexos profundos

na configuração do rio.

Resultados e Discussão O interesse das autoridades estatais pelo Tejo girou sempre em torno do problema das

cheias e da utilidade económica das suas águas. Até ao século XX, a riqueza deste rio estava

nas férteis lezírias das suas margens e na sua navegabilidade. Por isso, os projectos e

trabalhos feitos nesse período visavam sobretudo proteger os campos da acção das

inundações mais violentas, promover o enxugo e drenagem de paúis e garantir a navegação

até ao interior do país, permitindo a circulação de pessoas e bens entre a capital e as Beiras

(Gaspar, 1970; Azevedo, 2004). Depois, outras funcionalidades foram encontradas para as

águas do rio: a produção de energia hidro-eléctrica, a rega e o aproveitamento industrial e

turístico. A tecnologia disponível e uma maior capacidade financeira permitiram a realização de

grandes obras de engenharia, que vieram solucionar (em parte) a velha preocupação das

cheias.

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Vários estudos técnicos foram realizados com o objectivo de identificar as causas dos danos

provocados nos campos adjacentes e de apresentar soluções para minimizar os estragos

(Cabral, 1790; Relatório acerca da arborização, 1868; Loureiro, 1911). Já, em finais do século

XVIII, se considerava que a medida mais eficaz passava pela regularização do curso do rio,

impedindo o seu assoreamento e promovendo a fixação das margens (Cabral, 1790). Na

legislação de Oitocentos está presente a preocupação com a arborização das vertentes da

bacia hidrográfica para travar a sua erosão (Diário da Câmara dos Senhores Deputados, 19-

10-1844, 18-02-1881, 28-04-1887). A partir de 1901, a correcção torrencial passou a ser uma

das incumbências dos Serviços Florestais (Diário da Câmara dos Senhores Deputados, 31-12-

1901), trabalho cuja importância foi reconhecida pelo Estado Novo, que o incrementou através

do Plano de Povoamento Florestal (1939) e dos Planos de Fomento (Diário das Sessões da

Assembleia Nacional, 21-11-1952, 13-04-1958, 07-11-1867). Assim, foram identificados os

afluentes que mais contribuíam para o aumento do caudal sólido das cheias do Tejo e parte

dos seus trechos foi submetida ao regime florestal (Diário da Câmara dos Deputados, 17-04-

1919; Plano de Povoamento Florestal, 1939; Diário das Sessões da Assembleia Nacional, 08-

09-1958, 19-07-1965; Rego, 2001). Verifica-se que o maior esforço de arborização foi feito nas

bacias dos afluentes e não no rio principal, por se considerar que o regime de águas do Tejo

seria sempre determinado pelo que se fizesse em Espanha, sendo pois preferível investir nos

cursos de água que nasciam em Portugal (Plano de Povoamento, 1939, p. 159-160). Depois

dos anos 50, mesmo com a construção de barragens na bacia do Tejo e com a prossecução

das obras de correcção torrencial, a arborização continuou a ser fundamental para consolidar a

eficácia destes trabalhos (Andrada, 1982).

O problema das cheias não foi ainda hoje ultrapassado. Durante os séculos XIX e XX são

recorrentes as queixas dos proprietários ribeirinhos e é prolixa a legislação referente à

arborização e à reparação de diques e valas (Diário da Câmara dos Senhores Deputados, 03-

02-1823, 19-10-1944, 18-02-1881, 28-04-1887, 08-03-1902; Diário das Sessões da Assembleia

Nacional, 22-04-1938), o revela não só a dificuldade em fazer aplicar a lei, mas também a

incapacidade técnica para controlar as cheias (Diário das Sessões da Assembleia Nacional,

18-01-1956, 08-01-1964). A falta de meios pecuniários e a impossibilidade de prosseguir com

os trabalhos de forma racional, metódica e continuada, recorrendo-se sobretudo a intervenções

pontuais e de emergência, ajudam a explicar parte da questão (Diário das Sessões da

Assembleia Nacional, 11-01-1940; Diário da Assembleia da República, 12-03-1981).

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Conclusão Dado que a acção humana sobre a bacia hidrográfica do Tejo tem muitos séculos de história

documentada, conhecer de que forma e com que objectivos tentou o homem modificar as

características do rio e seus afluentes pode ser significativo para entender a dinâmica actual

desses cursos fluviais e determinante no planeamento de futuras intervenções.

Fontes e Bibliografia Andrada, Eduardo Campos, 1982. 80 anos de actividade de correcção torrencial – hidráulica florestal

(1901-1974), s.l., Direcção Geral das Florestas.

Azevêdo, M.ª Teresa, 2004. As mudanças de percurso do Tejo nos tempos modernos. Causas naturais e

antrópicas. Evolução geohistórica do litoral português e fenómenos correlativos. Actas do Colóquio,

Lisboa, Universidade Aberta.

Cabral, Estevão Dias, 1790. Memória sobre os danos causados pelo Tejo nas suas ribanceiras.

Memórias Económicas para o Adiantamento da Agricultura, das Artes e da Indústria em Portugal e

suas Conquistas, tomo II, Lisboa, Oficina da Academia das Ciências.

Correia, José de Araújo, 1967. O Tejo. Lisboa, Verbo.

Diário da Assembleia da República, 1976-1990

Diário da Câmara dos Deputados, 1911-1926

Diário da Câmara dos Senhores Deputados da Nação Portuguesa, 1820-1910

Diário do Governo, 1821-1970

Diário das Sessões da Assembleia Nacional, 1935-1974

Gaspar, Jorge, 1970. Os portos fluviais do Tejo. Separata da Revista Finisterra. Revista Portuguesa de

Geografia, vol. V-10, Lisboa.

Loureiro, Adolfo, 1911. “O rio Tejo e a sua navegação”, Trabalhos da Academia das Ciências de

Portugal, 1.ª série, tomo II, 1.ª parte, Lisboa, Academia das Ciências.

Plano de Povoamento Florestal, 1939. Relatório, proposta de lei, parecer da Câmara Corporativa e lei n.º

1971, publicada no «Diário do Governo», n.º 136, 1.ª série, de 15 de Junho de 1938. Lisboa,

Imprensa Nacional.

Rego, Francisco Castro, 2001. Florestas Públicas, s.l., Direcção-Geral das Florestas.

Relatório acerca da arborização geral do país apresentado a sua Ex.ª o Ministro das Obras Públicas,

Comércio e Indústria em resposta aos quesitos do artigo 1.º do decreto de 21 de Setembro de 1867,

1868. Lisboa, Imprensa Nacional.

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AS CHEIAS HISTÓRICAS NA REGIÃO DE ABRANTES

MÓNICA C. ALMEIDA1, TERESA M. AZEVÊDO2 & NUNO L. PIMENTEL2

1Mestrado C.T.V., Fac. Ciências da Univ. de Lisboa

e-mail: [email protected] e Dep. Geologia da Fac. Ciências da Univ. de Lisboa

e-mail: [email protected], [email protected]

Contexto regional As inundações do Tejo são quase tão antigas quanto o próprio rio, sendo a principal razão

da fertilidade e importância dos terrenos da lezíria, mas também a causa de destruição de

numerosos bens. O fenómeno ocorre com alguma frequência, devido às características da

bacia hidrográfica que tornam quase impossível confinar as águas do rio no seu leito ordinário,

principalmente durante os Invernos mais chuvosos (Ramos, 2000).

Durante dias, por vezes semanas, a área de Abrantes mais próxima do rio, cobre-se de

água, mágoa e inquietação, com várias povoações isoladas, onde apenas se chega de barco.

Isto ocorre nomeadamente devido à morfologia do vale, às características das litologias

aflorantes, e devido a certas acções do Homem. A montante de Abrantes os terrenos do

Maciço Antigo, rochas resistentes e impermeáveis, favorecem o escoamento superficial e, por

consequência, as inundações. Por outro lado, a crescente urbanização dos vales inundáveis na

região de Abrantes, que sempre constituíram um pólo de atracção para o desenvolvimento da

habitação, têm contribuído para a impermeabilização crescente do solo, que durante a

ocorrência de inundações leva a água a atingir níveis elevados. A inundação de povoações

como Rossio ao Sul do Tejo, na margem esquerda, e Rio de Moinhos, na margem direita, está

sobretudo relacionada com a baixa cota a que estas povoações se encontram, o que em

conjugação com a morfologia do vale, vai facultar o espraiamento das águas e a consequente

inundação. Outro aspecto de igual relevância para a ocorrência de cheias, é o assoreamento

do rio Tejo, que na região de Abrantes, ocorre devido, em grande parte, à desflorestação e/ou

alteração do coberto vegetal existente (Madeira, 2001).

O registo histórico documental das cheias na Região de Abrantes

É a partir do século XIX que começam a surgir com maior frequência os relatos, bem

documentados em jornais (locais e nacionais) e em revistas da época, sobre cheias e

catástrofes a elas associadas. São numerosas as notícias acerca das tumultuosas invasões do

rio Tejo na região de Abrantes, reportadas no jornal “Correio de Abrantes” (1941, 1978, 1979,

1989), desde 1884, de que se recordam algumas mais importantes.

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A cheia de 1876, conhecida como a maior cheia do Tejo de que há memória, registou na

escala hidrométrica do Rodão 25,40 m e atingiu quase 10000 m3/s de água em Tancos (fig. 1).

A repercussão da cheia em Ródão na região de Abrantes (cerca de 70 km a juzante) verifica-se

logo após 3 horas (Rodrigues et al., 2003).

Fig.1 – Cheia histórica de 1876, em Vila

Velha de Ródão e Tancos (4)

Já no séc. XX, na década de 40 (1940, 1941 e 1947) ocorreram várias inundações

dramáticas e que ocasionaram estragos consideráveis. A 2 de Fevereiro de 1941, a escala

hidrométrica do Rodão chegou aos 22,60 m e o Tejo subiu a pontos julgados inacessíveis,

como por exemplo, primeiros andares de casas do Rossio a Sul do Tejo. O Tejo teve então a

maior cheia que as gerações de então tinham visto, quase atingindo os níveis da de 1876.

Inundou, entre outros, o Rossio a Sul do Tejo (cota 32 m), a povoação de Rio de Moinhos

(35 m), a estação ferroviária de Abrantes (35 m), Arrifana (29 m), a estrada para o Tramagal

(30 m), a igreja e o cemitério de S. Miguel do Rio Torto (35 m), etc., para cotas de estiagem

próximas de 20 metros. A linha ferroviária ente Tramagal e Rossio a Sul do Tejo (30 m) esteve

também ela coberta de água, nas pontes sobre o Tejo os pilares ficaram cobertos quase até ao

tabuleiro, com mais de 1500 casas abandonadas precipitadamente numa área inundada

equivalente a 50 km2, “enfim, um horror.” A notícia faz, ainda, referência aos níveis de cheia

atingido em várias casas do Rossio.

Após alguns anos de perfeita acalmia, segundo os registos da época, em Março de 1978,

violentas tempestades originaram cheias de incalculáveis prejuízos em várias zonas da região.

No Rossio, estabelecimentos comerciais e habitações foram inundados, destruindo tudo o que

havia dentro, gado e aves foram arrastados pela corrente de cheia, conduzindo a prejuízos

elevados. Viveu-se uma completa desolação nas ruas do Rossio, onde a forte corrente das

águas arrancou calçadas, destruiu esgotos, derrubou muros, etc.

Apenas um ano depois, em Fevereiro de 1979, pluviosidade que ultrapassou o normal das

décadas anteriores, assolou de novo a região de Abrantes. O nível das águas foi subindo, até

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

ultrapassar o máximo de 1941, que era o maior desde há longa data, provocando a maior cheia

do séc. XX. Desde Alferrarede até Constância, com passagem por Rio de Moinhos, “tudo foi

água” (fig. 2). Segundo as crónicas de então, estas cheias foram, em parte, também

provocadas pelas descargas da barragem de Alcântara em Espanha.

Fig. 2 - Cheia em Constância, 1979 (Soares,

M.M., 1995)

Fig. 3 – Cheia em Constância, 1989

(www2.snbpc.pt)

As chuvas intensas que se verificaram no mês de Dezembro de 1989, provocaram “como há

muito se temia”, enormes cheias no vale do Tejo. O Rossio ao Sul do Tejo e Rio de Moinhos

foram muito atingidos, alcançando o rio um nível próximo do de 1979. Também a vila de

Constância foi muito afectada pelas águas, tendo a sua parte mais baixa estado submersa

(fig. 3). Não houve vítimas a lamentar, mas os danos materiais foram colossais e muitas

famílias ficaram desalojadas.

Mais recentemente, a magnitude da cheia histórica de 1876 foi ultrapassada nos trágicos

episódios de Novembro de 1997, que provocaram a morte de 11 pessoas em Portugal

(Rodrigues et al., 2003).

Referências Bibliográficas Correio de Abrantes., 1941, 1978, 1979, 1989. Edição Minerva, LDA.

Madeira, M.C., 2001. Condições Geomorfológicas, Climáticas e Antrópicas das Inundações do Rio

Tejo em Abrantes. Diss. de Mestrado, Faculdade de Letras da Universidade de Coimbra, 104 p.

Ramos, C., 2000. Cheias e Impacte Ambiental. Seminário de Geologia do Ambiente, Departamento

de Ciências da Terra, Universidade do Minho, Braga. 2 p.

Rodrigues, R., Brandão C., Costa J.P., 2003. Breve Nota sobre as Cheias no Tejo e o seu Sistema de

Vigilância e Alerta. Direcção dos Serviços de Recursos Hídricos, Instituto da Água.

Soares, M.M., 1995. Navegando no Tejo. Comissão de Coordenação da Região de Lisboa e Vale do

Tejo, 161p.

http://www2.snbpc.pt/portal/page?_pageid=35,46761,35_46771:36_35022&_dad=portal&_schema=P

ORTAL

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

FLOODS IN THE CZECH REPUBLIC DURING THE PAST MILLENNIUM: DATA, FLUCTUATIONS, IMPACTS

RUDOLF BRÁZDIL Institute of Geography, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic

e-mail: [email protected]

This work presents data on floods in the Czech Republic, based on historical documentary

evidence since the 12th century and measured hydrological data since the 19th century, in

order to detect long-term fluctuations of floods and their impacts. Floods in the Czech Republic

during the instrumental period were studied for the following rivers and hydrological stations: the

Elbe – Děčín (1851–2003), the Vltava – Prague (1825–2003), the Ohře – Louny (1884–2003),

the Odra – Bohumín (1896–2003) and the Morava – Kroměříž (1881–2003). Different data

sources were used for the study of floods in the pre-instrumental period: narrative written

sources, visual daily weather records, personal correspondence, special prints, official

economic records, newspapers, pictorial documentation, stall-keepers’ and market songs,

scientific papers and communications, and finally epigraphic sources (mainly flood water-

marks). The first credible report on a flood in Bohemia from sources of a narrative written

character relates to September 1118 and comes from the chronicle of Canon Kosmas of Prague

(Brázdil et al., 2005). In the period of systematic hydrological observations (i.e. since about the mid-19th century)

in the Czech Republic, the total number of floods, as well as their extremities expressed by the

N-year return period of maximum peak discharges, has been going down. This decline can be

ascribed above all to the reduction of frequencies of occurrence of floods of the winter synoptic

type (related to snow melting and ice damming, accompanied by rain), due to global warming,

through which, in consequence of the later onset of winters and lower accumulation of water in

snow cover, the number of floods has gone down, mainly in the months of February to April.

Documentary data on floods in the Czech Lands make it possible to extend the information

base about floods to far before the beginning of systematic hydrological observations, in fact

several centuries back. Work with this data requires, however, a critical analysis of the

employed sources, which is unfortunately lacking in most of the older papers in the Czech

Republic, as well as some of the newer, published overviews of historical floods. Critically

evaluated information can then be used to study the frequencies of occurrence for the floods,

their seasonality, synoptic typology and impacts. Through this, it becomes possible to document

disastrous floods in the Czech Lands for the last eight to nine centuries, with a relatively high

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degree of reliability. Synthesis of floods based on instrumental data and documentary sources

shows long-term trends with a maximum of flood events during the 19th century and the second

part of the 16th century. From this analysis it is evident that, although several destructive floods

occurred in the 20th century, it can be considered as a very favourable hundred year period

(with the exception of the flood in July 1997).

The most disastrous well-documented floods affected the territory of the Czech Republic in

September 1118, January–February 1342, July 1432, August 1501, March and August 1598,

February 1655, June 1675, February 1784, February 1799, March 1845, February 1862, May

1872, September 1890, July 1897, July 1903, August–September 1938, July 1997 and August

2002. From the flood in May 1872 to the present, i.e. in the period subject to global warming,

they have been exclusively floods of the summer synoptic type (i.e. generated by continuous

heavy rains). The accumulation of floods of the winter synoptic type in February 1784, February

1799, March 1845 and February 1862 falls, from the climatological point of view, into the last

part of what is known as the Little Ice Age.

Information available until now, has not yet made it possible to give a definite answer to the

question of “to what extent the most recent floods – 1997, 1998, 2002 and 2006, which came

after a relatively very quiet period – are a possible consequence of the anthropogenically

conditioned process of global warming”, and “whether in the near coming years similar disasters

will occur with the same or greater intensity and frequency”.

Flood events today, in comparison with historical floods of the same extremity, have more

disastrous consequences than in the past, a fact closely linked to the increasingly more

complex infrastructure of human society and with a growing degree of anthropogenic

remodelling of the cultural landscape. Floods, as in the past, will also rise in the future whenever

meteorological, physico-geographical and anthropogenic conditions are favourable to them. The

problems of anti-flood protection may become still more pressing in the future than at present, if

assumptions of continuing anthropogenically-conditioned climatic change projected into more

marked fluctuation of the runoff process and flood activity are confirmed.

Acknowledgement This study was financially supported by the research project MŠM0021622412

(INCHEMBIOL).

References Brázdil, R., Dobrovolný, P., Elleder, L., Kakos, V., Kotyza, O., Květoň, V., Macková, J., Müller, M., Štekl,

J., Tolasz, R., Valášek, H. (2005): Historical and Recent Floods in the Czech Republic. Masaryk

University, Czech Hydrometeorological Institute, Brno, Praha, 370 pp.

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THE TAGUS RIVER AND ITS HISTORICAL FLOODS (SANTARÉM, PORTUGAL)

TERESA M. AZEVÊDO1, ELISABETE NUNES1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, CONCEIÇÃO FREITAS1,

CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3

1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016

Lisboa, Portugal

e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal

e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal

e-mail: [email protected]

Since written information exists, river floods were recorded in local and regional archives in

the entire world. These are specially constituted by documents in churches about pilgrimages to

ask for rain, people perished in floods, chronicles of the kings, documents in libraries,

autarchies, old magazines, newspapers, etc. For thousands of years ago, countries like Egypt,

have been keeping records from the Nile floods. Other countries started the records too late or

lost the older ones due to natural catastrophes, e.g., fires or wars.

This is the case of Portugal. Being the Tagus one of the biggest Iberian rivers, the

information about historical floods was expected to be abundant. However, most of the old

archives were lost during invasions, fires, wars and the big earthquake of 1755. Therefore,

because of these reasons, there are so few records about Tagus floods.

The Tagus floods were however referred in many old historical texts mainly due to the very

good nature of the lands left back by the floods. Several Arabic references and even the

description presented by the English crusader Osberno, engaged for the Lisbon request to the

Moors in 1147, tell us the magnificent properties of the Tagus lands at those times.

The research about historical floods is based on cartographic documents, old written

documents, newspapers, magazines and photos. The field survey and the data gauge are the

main documents used for the study of present floods.

One of the goals of this historical research was to establish a flood magnitude scale

(Azevêdo et al., 2004) and a flood risk map using data from the documents analysed (Nunes,

2001). Another important component was the comparative cartographic analysis of all the maps

since 1866 to the present day, at four different scales.

An example of the research is the artificial change of the Tagus channel, described in the

documents archived in “Torre do Tombo”, which gave place to several papers (Dias, 1984;

Azevêdo & Nunes, 2000, Azevêdo, 2001, Azevêdo et al., 2003, Azevêdo et al., 2004). This

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amazing case is the one in which the Prince D. Luis, brother to the king of Portugal D. João III,

requests permission to change the Tagus’ course between Tancos and Chamusca, due to the

river sand deposition that every flood ruined its cultivations causing big damages.

The Tagus was then pushed 1 km to the north in a 10 km long

section and a new, perfectly straight channel was opened (fig. 1).

However, the river was not stable in this new course. It began to

meander and eroded an area of 30 km2 in only 15 years, causing great

damages, now to the Order of Christ monks, located in Quinta da

Cardiga. Nowadays, it is visible that the Tagus’ main channel runs right

next to the walls surrounding the farm.

Fig. 1 – Old scroll

showing the 3 different

courses of the Tagus

in the XVIIIth Century

(in A.N.T.T, undated).

Another major change to the Tagus’ main channel was ordered by

king D. João V two centuries later, in the downstream segment between

Valada and Vila Franca de Xira. His intention was to improve the river’s

navigability and so the channel was once again shifted, widened,

deepened and the existing five channels were joined together forming

the present-day one.

Other important sources of information are inquiries to the population, inscriptions and flood

marks. Floss can also be used to estimate water levels from recent floods when there aren’t any

other elements available. Inscriptions and marks can be preserved for centuries on gates,

bridges, and houses or can be expressly built to record a particular flood. The old and

centennial flood marks drawn on the outside walls of the houses, where the inhabitants wrote

the water levels and dates, have recently been substituted by glazed tiles only with levels,

having all the dates disappeared. This is an excellent example of the degree of knowledge of

the people directing water governmental institutions.

However, the old flood marks face a great problem, that is, they have to be renovated

every now and then (the case of the renovated marks in Reguengo do Alviela) and by doing

so, we can never be 100% certain if they are still representing the original position.

One of the issues with flood marks is its meaning.

(i) Taking into account that the big Tagus floods take place when the river waters surpass 7

m high in Santarém bridge, 50 large floods were registered (Almeida D’Eça, in Sobrinho, 1980)

between 1852 and 1996. In the same period, the flood marks surveyed in 20 places in the

floodplain, registered 19 floods. Hence, the flood marks’ method gives a trial of 38% of the total

big floods that occurred in the Tagus floodplain.

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(ii) The flood marks evaluation shows great flood

events irregularity, with decades of no big floods and

others with 2, 3 or 4 large floods (table 1).

(iii) Flood marks also show that the months with a

higher flood risk are December, January, February

and March (table 1).

(iv) Along the studied period, the highest

magnitude flood event occurred in February 1979. If

the two places (of the 20) with more flood marks

events registered (Quinta do Monteiro e Reguengo do

Alviela) are considered, it can be seen that the flood

marks’ method gives that information precisely.

(v) Finally, the flood marks of the February 1979

event are the best represented along the floodplain (in

18 of the 20 places).

Another aspect related to floods frequency is its

influence on regional architecture: the riverside

villages of Caneiras and Aldeia da Palhota are very

curious examples of how the local inhabitants are

familiarised with floods. Their houses are pile dwelling buildings, built circa 5 m above the

normal water level.

Table 1 - Tagus floods referenced by flood marks

Year Month Day Number of Places

1816 12 6 1

1895 2 17 17

1896 12 7 1

1909 12 23 2

1912 1

1919 1

1936 3 16 2

1940 1 3 3

1941 1 25-27 5

1946 1 25 2

1947 3 5 1

1963 1

1964 1

1969 1

1970 1 13 1

1978 3 4 5

1979 2 1-11 18

1986 2 19 1

1989 12 23-27 3

1996 1 09 1

1999 2 1

2001 1

Bibliography A.N.T.T., undated. Conventos de Tomar, Ordem de Cristo, Maço 30 (vermelho), nos 1, 2 e 4, fl. 21.

Azevêdo, M. Teresa M., 2001. A utilização dos dados históricos no estudo das cheias do Tejo. Estudos

do Quaternário, 4, APEQ; 69-77. Porto.

Azevêdo, T.M. & Nunes, E.V., 2000. A acção antrópica na mudança da tipologia do Tejo ao longo do

tempo. Actas. In: II Jornadas do Quaternário da APEQ. Porto.

Azevêdo, T.M.; Nunes, E.; Ramos, C. & Ramos Pereira, A., 2003. Historical floods of the Tagus River in

the Santarém region, Portugal. Abstracts. Ciências da Terra, Vol. Especial, Nº V. VI Congresso

Nacional de Geologia, Univ. Nova Lisboa, Costa da Caparica, p. 92 (CD-Rom Ed., p. H10-H13).

Azevêdo, T.M.; Nunes, E. & Ramos, C., 2004. Some morphological aspects and hydrological

characterization of the Tagus floods in the Santarém region, Portugal. Natural Hazards, 31. Kluwer

Academic Publishers, Netherlands. p. 587-601.

Nunes, E., 2001. Estudo Multidisciplinar da Variabilidade Temporal e Espacial do Tejo na Região de

Santarém. Tese de mestrado (não publ.). Univ. de Lisboa, 209 pp.

66

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

Dias, J.J. Alves, 1984. Uma grande obra de engenharia em meados do século XVI. A mudança do rio

Tejo. Rev. Nova História, Ed. Estampa, Lisboa.

Pereira, A. Ramos; Ramos, C.; Azevêdo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C. & Pereira, D.I.

2006. Geomorphological assessment of the Middle Tagus alluvial plain. Abstracts. Tagus Floods ’06

Workshop, Lisbon, Portugal.

Sobrinho, A.S., 1980. Os temporais de Fevereiro de 1979 no Ribatejo e região de Lisboa. Finisterra, vol.

XV, 29, 85-93.

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

PALAEOENVIRONMENTS OF THE TAGUS VALLEY DURING THE LAST 15 KA: SEDIMENTOLOGICAL, PALYNOLOGICAL AND MICROMORPHOLOGICAL EVIDENCE OF

THE ENTRE VALAS SEV CORING (SANTARÉM, PORTUGAL)

M. TERESA AZEVÊDO1, S. FAVARETTO2, A. MIOLA2, PAOLO MOZZI3, C. NICOSIA4, ELISABETE NUNES1 &

I. SOSTIZZO2

1 Centro de Geologia, Universidade de Lisboa, Edifício C6, 3º Piso; Campo Grande, 1749-016 Lisboa, Portugal

e-mail: [email protected], [email protected] Dipartimento di Biologia, Università degli Studi di Padova, via U. Bassi 58/B, 35121 Padova, Italy

e-mail: [email protected], [email protected], [email protected] Dipartimento di Geografia, Università degli Studi di Padova, via del Santo 26, 35123 Padova, Italy

e-mail: [email protected] Department of Geology and Soil Science, University of Gent, Krijgslaan 281 S8 9000 Gent, Belgium

The 20 m deep coring object of this study was drilled in the Tagus River alluvial plain in

Entre Valas, few kilometres upstream of Santarem on the northern side of the valley, less than 1

km from the valley margin (SEV is the acronym for the site Santarém-Entre Valas). The

lowermost portion between 20 and ca. 15 m is mainly sandy, locally with pebbles. The

sediments above are mostly silty-clay with an increase in sand in the top 3 m. The radiocarbon

dating of carbonaceous organic material (peat, wood) at different depths gave the following

ages: 18,109 cal BP at 16.7 m; 11,219 cal BP at 14.7 m; 8097 cal BP at 12.3 m; 6960 cal BP at

10.7 m; 3550 cal BP at 6.5 m; 3086 cal BP at 4.5 m; 816 cal BP at 1.0 m. Grain size analyses

have been performed in the whole coring, with sample spacing of some decimetres. Pollen and

non-pollen-palinomorph (NPP) analyses have been carried out on 22 samples from different

depths between 15.5 and 3.9 m. Pollen and spore identification was based on Moore et al.

(1991), Reille (1992, 1998) and the modern reference collection at the Department of Biology,

University of Padova. NPPs identification and nomenclature follow van Geel (1978), van Geel et

al. (1980-81), van Geel et al. (1983), van Geel et al., (1989). Micromorphology has been carried

on 11 samples in the 13.5–3.0 m interval. Thin section description follows Bullock et al. (1985)

and Stoops (2003).

Relatively coarse, fluvial channel sediments were deposited during the Late Glacial, followed

by finer overbank deposits of Holocene age. Some macroscopic evidence of pedogenesis have

been described in the coring at depths of ca. 3-4 m, but no specific indicators of pedogenesis

have been observed in thin section. The abundant calcite nodules are generally ascribed to the

gradual infilling of microvoids under vadose circumstances and are not necessarily related to

soil forming processes. At the bottom of the coring (15.5-13.7 m) palynomorphs are absent, in

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

the middle part (13.7-10.0 m) there is the highest palynomorph concentration and at the top

(10.0-3.9 m) there is a poor concentration and the worst preservation of the pollen grains.

Crossing the palynological and micromorphological data, the Holocene sedimentary series

corresponding to the top 15 m of the coring can be split in two main sequences, with the

boundary at a depth of ca. 8 m. The presence of Quercus ilex, Juniperus type and Arbutus

pollen types, together with some pollen grains of Vitis vinifera and Olea europea, shows that the

lower sequence formed under mediterranean climatic conditions. The aquatic plant community

was apparently dominated by Isoetes, indicative of freshwater condition and of shallow ponds

which seasonally dried out. The presence of foraminifera, observed during pollen analysis,

indicates possible inputs of marine water. The abundant pyrite, often oxidized to form sulphuric

minerals such as jarosite in association with gypsum, observed in thin section, may also be

explained in terms of an occasionally brackish underground water table. Even though the

vegetation was not that of a coastal salt marsh, the backswamp ponds in the floodplain could

probably receive marine water. Such events were presumably related to the interplay between

the floodings of the Tagus and the remounting tides. Sea surges and consequent spillout of sea

water in the plain may have happened during storms with strong western winds from the Atlantic

Ocean and/or exceptionally high tides. These processes could take place in the floodplain of a

deltaic/estuarine system.

Pollen analysis suggests that mediterranean climatic conditions persisted in the upper

sequence. It also gives some indication of deforestation and heathland expansion which, in the

region, are known to start significantly from the Bronze Age onwards. Similarly,

micromorphology indicates the presence of soil relicts of former topsoil horizons, which have

been eroded and re-deposited, at a depth of 3.5 m. The groundmass also contains finely

comminuted charcoal and charred organic matter fragments. The existence of a phase of

topsoil erosion, which could be man- or natural fire-induced, should be carefully taken into

account. Sedimentation took place by overbank deposition of the Tagus River, with no evidence

of coastal influx. Further hypotheses on vegetation development and local environments for this

part of the sequence cannot be formulated because the pollen grains were generally

deteriorated.

The transition from a deltaic/estuarine situation to an alluvial plain indicates a more inland

position of the coastline in the Middle Holocene, followed by a regression. The maximum

transgression was reached at the culmination of the post-glacial eustatic sea-level rise, while

the regression has been due to the basinward progradation of the alluvial wedge of the Tagus

fluvial system during the Upper Holocene sea highstand. The palaeoenvironmental evolution of

the Tagus valley during the last 15 ka may serve as an analogue of the sedimentary response

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of the Tagus system to past glacio-eustatic sea level rise. This may help to better understand

the Pleistocene sequences of the Tagus alluvial terraces which are present on the southern

side of the valley (Zbyszewski, 1946, 1958; Mozzi et al., 2000).

References Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., 1985. Handbook for soil thin section

description. Waine Research, Wolverhampton, United Kingdom.

Faegri, K., Iversen, J., 1989. Textbook of pollen analysis, IV Ed., John Wiley and Sons, London.

Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis. II Ed., Blackwell. Oxford, Great Britain.

Mozzi, P., Azevedo, M.T., Nunes, E., Raposo, L., 2000. Middle terrace deposits of the Tagus river in

Alpiarça, Portugal, in relation to early human occupation. Quaternary Research, 54 (3).

Reille, M., 1992. Pollen et Spores d’Europe et d’Afrique du Nord. Laboratoire de Botanique Historique et

Palynologie. Marseille, France.

Reille, M., 1998. Pollen et Spores d’Europe et d’Afrique du Nord. Supplement 2. Laboratoire de

Botanique Historique et Palynologie. Marseille, France.

Stoops, G., 2003. Guidelines for analysis and description of soil and regolith thin sections. Soil Science

Society of America, Inc. Madison WI, U.S.A.

van Geel, B., 1978. A palaeoecological study of Holocene peat bog sections in Germany and the

Netherlands, based on the analysis of pollen, spores and macro- and microscopic remains of fungi,

algae, cormophytes and animals. Rev. Palaeobot. Palynol., 25, 1-120.

van Geel, B., Bohncke, S.J.P., Dee, H., 1980-1981. A palaeoecological study od Upper Late Glacial and

Holocene sequence from “De Borchert”, The Netherlands. Rev. Palaeobot. Palynol. 31, 367-448.

van Geel, B., Hallewas, D.P., Pals, J.P., 1983. A Late Holocene deposit under the Westfriese Zeedijk

near Enkhuizen (Prov. of N-Holland, The Netherlands): palaeoecological and archaeological aspects.

Rev. Palaeobot. Palynol. 38, 269-335.

van Geel, B., Coope, G.R., van der Hammen, T., 1989. Palaeoecology and stratigraphy of the Lateglacial

type section at Usselo (The Netherlands). Rev. Palaeobot. Palynol. 60, 25-129.

Zbyszewski, G., 1946. Etude géologique de la region d'Alpiarça. Comun. Serv. Geol. Portugal, XXVII, 145-268.

Zbyszewski, G., 1958. Le Quaternaire de Portugal. Boletim da Sociedade Geològica de Portugal, 12, 1-

227.

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SISTEMAS METEOROLÓGICOS DE PRECIPITAÇÃO E AS CHEIAS NAS BACIAS HIDROGRÁFICAS: RIO TEJO

ILDA Mª SANFINS NOVO VILLA SIMÕES Instituto de Meteorologia, Rua C do Aeroporto, 1700 Lisboa

e-mail: [email protected]

Em Portugal Continental, as cheias nas bacias hidrográficas estão associadas às situações

meteorológicas de fluxo perturbado de oeste persistente que origina períodos prolongados de

precipitação e que geralmente afectam quase todas as bacias hidrográficas do território.

Sistemas depressionários e passagem de sistemas frontais de forte actividade, podem originar,

também, situações de cheias em uma ou mais bacias hidrográficas.

Apresentam-se algumas situações meteorológicas que exemplificam os principais sistemas

meteorológicos de precipitação e que foram casos de cheias na bacia hidrográfica do rio Tejo.

Refere-se a importância da Vigilância Meteorológica e da Previsão do Tempo a curto e

médio prazo na previsão de cheias e no apoio à tomada de decisão em situação de cheias

assim como a importância das previsões a longo prazo (10 dias, mensal e trimestral) na gestão

dos recursos hídrícos.

Referências Bibliográficas Godinho, S. Contribuição para o Estudo da Cheia da região de Lisboa de 25-26 de Novembro de 1967.

Instituo de Meteorologia, Lisboa.

Godinho, S. Aspectos Meteorológicos das Inundações de Novembro de 1983. Instituto de Meteorologia,

Lisboa.

Malheiro, A.C., Ferreira, I.B., Freire, J.A., 1984. Análise à Escala Sinóptica da Situação Meteorológica

que condicionou o estado do tempo em Portugal Continental nos dias 18 a 21 de Novembro de 1983.

Instituto de Meteorologia, Lisboa.

Meteorological Office College, 1997. Source Book of the Forecaster's Reference Book. The

Meteorological Office, Bracknell.

Miranda, P.M.A., 2001. Meteorologia e Ambiente: Fundamentos de Meteorologia, Clima e Ambiente

Atmosaférico. Universidade Aberta, Portugal.

Condições Meteorológicas nos meses: Dezembro de 2000, Janeiro, Fevereiro, Março, Novembro e

Dezembro de 2001, Janeiro, Feveiro, Março, Novembro e Dezembro de 2002. Publicação mensal do

Instituto de Meteorologia, Departamento de Vigilância Meteorológica, Divisão da Análise e Previsão

do Tempo.

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POSSIBILIDADES DE APLICAÇÃO DA LUMINESCÊNCIA OPTICAMENTE ESTIMULADA (OSL) NA DATAÇÃO DE SEDIMENTOS DO RIO TEJO

ANTÓNIO ANTUNES MARTINS1 & PEDRO PROENÇA E CUNHA2

1Depart. de Geociências da Univ. Évora, Rua Romão Ramalho, 59, 7002-554 Évora

e-mail: [email protected] de Geociências, Depart. de Ciências da Terra da Univ. Coimbra, 3000-272 Coimbra

e-mail: [email protected]

Nas técnicas de datação de sedimentos por luminescência, a idade de uma amostra obtém-

se, no essencial, dividindo a paleodose pela taxa de radiação ambiental (dose-rate). A

paleodose é a energia (electrões) acumulada em defeitos (traps) do edifício cristalino de

minerais, desde a última vez em que estes foram expostos à luz solar. Os minerais

normalmente utilizados na OSL são o quartzo (Qz) e o feldspato potássico (Fk). A exposição à

luz descarrega a energia latente anterior, processo designado por bleaching. Nessa altura é

como se o “relógio” da contagem do tempo fosse colocado a zero. Logo que os grãos são

enterrados começam a receber radiação ionizante proveniente de elementos radioactivos (ex.

T, U e 40K) existentes nos sedimentos e também a dos raios cósmicos. A energia acumulada

pode ser medida em laboratório (obtendo-se a dose equivalente - De). É portanto crucial que a

energia latente da amostra de sedimento seja colocada próximo de zero antes deste ser

enterrado; caso contrário, a idade da amostra ficará sobrestimada. A sobrestimação da idade

resultante do incompleto bleaching pode ser significativa em sedimentos do Holocénico, mas

tem pouca relevância em sedimentos do Plistocénico, pois nestes a energia presente na altura

do enterramento é muito pequena relativamente à acumulada após a última exposição à luz.

O sinal OSL resultante da estimulação pela luz é mais facilmente “limpo” (bleached) do que

o sinal luminescente resultante da estimulação pela temperatura (termoluminescência - TL),

sendo essa uma das principais vantagens da OSL relativamente à TL. Na medição de grãos de

Qz usando o protocolo SAR (Murray & Wintle, 2000), a OSL permite datar sedimentos até

cerca de 150.000 anos, caso a taxa de radiação ambiental não seja elevada; idades da ordem

da dezena de anos podem ser obtidas usando técnicas apropriadas (Huntley & Lian, 1999;

Wallinga, 2002). A principal limitação do quartzo nas datações por OSL é a saturação dos

grãos que ocorre à volta de 300 Gray (Gy)1. Arenitos e lutitos do Holocénico podem ser

também datados com precisão por esta técnica e, consequentemente, servir para determinar a

crononogia de eventos geológicos recentes, tais como as cheias fluviais. O cálculo de taxas de 1 Gray é uma unidade no Sistema Internacional de medidas, que representa a quantidade de energia de radiação ionizante absorvida (ou dose) por unidade de massa. 1Gy = 1 J/kg

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sedimentação pode também ser efectuado, desde que se disponha de uma coluna

estratigráfica com suficiente constrangimento cronológico. Também se podem usar sedimentos

de idade já conhecida (ex: depósitos do tsunami de 1755 ou da cheia de 1909 do rio Tejo) para

aferir o processo de medição laboratorial ou determinar a importância do incompleto bleaching

nos ambientes com transporte aquoso de elevada turbulência. No processo de averiguação do

incompleto bleaching, normalmente reduz-se o número de grãos em cada aliquot para evitar

valores médios, chegando-se a fazer medições grão a grão da De. Uma distribuição muito

heterogénea da De ou a existência de alguns aliqutos com valores muito superiores aos

valores mais frequentes da população são indicadores de incompleto bleaching (Murray et al.,

2002). A presença de sinal latente anterior à deposição tem de ser tomada em conta no cálculo

das idades dos depósitos modernos.

Como exemplo, apresentam-se duas medições da De em grãos de Qz, feitas em

sedimentos holocénicos da planície aluvial do Tejo, na depressão de Montalvo. O enchimento

holocénico tem aqui 9 m de espessura visível. A amostra 052239 foi colhida a 1m do topo do

enchimento aluvial e a amostra 052238 4 m abaixo daquela. Relativamente à amostra 052238

(Fig. 1), parece óbvio que os aliquots com dose equivalente superior a 1 Gy correspondem a

situações de incompleto bleaching. Na amostra 052239 existem muito menos aliquots com

incompleto bleaching (apenas dois com 1,3 Gy). Nas duas amostras, aliquots com mais de 1

Gy muito provavelmente devem ser portadores de energia latente, anterior ao último

enterramento do sedimento; por isso, não foram considerados no cálculo das idades.

Obtiveram-se valores de 117 e 114 anos, que concordam com a posição estratigráfica das

amostras, mas com um intervalo de tempo demasiado curto para explicar os 4 m de espessura

de aluviões que separam as duas amostras. Os valores da radiação ambiental revelaram-se

altos (3,44 e 4,03 Gy/ano), mas não são impeditivos na determinação das idades destes

sedimentos modernos.

Casal Montalvo 052238 QZ

0

5

10

15

20

25

0,0 0,3 0,5 0,8 1,0 1,3 1,5 1,8 2,0 2,3 2,5 2,8 3,0 3,3 3,5 3,8 4,0 4,3 4,5 4,8 5,0 5,3 5,5

Gy

Média= 0,79 ± 0,13 Gy σ = 0,98n = 61

Casal de Montalvo 052239 QZ

0

5

10

15

20

25

0,0 0,3 0,5 0,8 1,0 1,3 1,5 1,8 2,0 2,3 2,5

Gy

M édia = 0,51± 0,03 Gyσ = 0,19n = 32

Fig. 1 – Histogramas relativos aos valores de De medidas em aliquots com grãos de Qz de duas

amostras da planície aluvial do médio Tejo português, do alvéolo tectónico de Montalvo. Na amostra

052238 foram medidos 61 aliquots e na 052239 medidos 32.

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Sedimentos do Holocénico podem ser datados utilizando o Qz, mas o incompleto bleaching

poderá ter um peso relativo importante no cálculo da De, sobrestimando as idades. As

datações OSL em Qz, apesar de precisas, revelaram-se praticamente inviáveis para datar

terraços do rio Tejo, cujos sedimentos possuem altas doses de radiação ambiental (geralmente

3 a 7 Gy/ka). Julga-se que as elevadas taxas de radiação ambiental possam estar relacionadas

com substratos graníticos existentes na bacia hidrográfica do rio Tejo, quer do lado espanhol,

quer do lado português. Aponta-se como exemplo o batólito de Nisa, com concentrações de

Urânio. Para os terraços, a datação OSL em Fk surge como alternativa; com este mineral pode

datar-se até 1 milhão de anos (Wallinga, 2002). Contudo, esta metodologia é mais complexa,

morosa e as idades obtidas precisam de ser corrigidas relativamente à perda de energia

(anomalous fading) que ocorre nos grãos de Fk ao longo do tempo (Wintle, 1973) e mudanças

de sensibilidade nos defeitos do cristal (Wallinga, 2002). Os dois fenómenos são ainda mal

compreendidos e as correcções são problemáticas. Apesar destas dificuldades, foi possível

obter datações OSL em Kf nos três níveis inferiores terraços do rio Tejo (sectores de Ródão e

Abrantes). Aqui existem cinco níveis geomorfológicos de terraços tendo-se alcançando idades

mínimas da ordem dos 200 mil anos para o terceiro nível de terraço (Martins & Cunha, 2006;

Cunha et al., submetido para publ.). Devido às características dos sedimentos não foi possível

obter qualquer idade, para os dois níveis superiores de terraço.

Projecto POCTI/CTE-GEX/58120/2004, da Fundação para a Ciência e a Tecnologia e co-financiado pelo FEDER

Referências bibliográficas: Cunha, P. P., Martins A. A., Huot, S., Murray A., Raposo, L., submetido para publ. Dating the Tejo River

lower terraces (Ródão, Portugal) to assess the role of tectonics and uplift. In: Impact of Active

Tectonics and Uplift on Fluvial Landscapes and River Valley Development (Eds P.G. Silva, F.A.

Audemard & A.E. Mather). Geomorphology, Special Issue).

Huntley, D.J., Lian, O.B., 1999. Using optical dating to determine when a sediment was last exposed to

sunlight. in: D.S. Lemmen and R.E. Vance (eds.), Holocene Climate and Environmental Change in the

Palliser Triangle: A Geoscientific Context for Evaluating the Impacts of Climate Change on the

Southern Canadian Prairies: Geological Survey of Canada Bulletin 534, 211-222.

Murray, A. S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of

sedimentary quartz: a status review. Geochronometria 21, 1-16.

Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot

regenerative-dose protocol. Radiaton Measurements 32: 57-73.

74

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Martins, A. A. & Cunha, P.P., 2006. Vantagens e limitações da luminescência opticamente estimulada na

datação de terraços do rio Tejo, sua importância na determinação da taxa de incisão fluvial. VII

Congresso Nacional de Geologia (Resumos), Univ. Évora.

Wallinga, J. 2002. Optically stimulated luminescence dating of fluvial deposits: a review. Boreas, 303-322.

Wintle, A.G. 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143-144.

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GEO-ARCHAEOLOGICAL EVIDENCE OF DESTRUCTIVE FLOODS DURING THE SETTLEMENT OF FAIAL (AZORES)

JOSÉ MADEIRA & ANTÓNIO BRUM DA SILVEIRA Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, LATTEX – Laboratório de

Tectonofísica e Tectónica Experimental e IDL – Instituto D. Luís

Edifício C6, Campo Grande, 1749-016 Lisboa, PORTUGAL

e-mail: [email protected], [email protected]

Four different regions characterize the morphology of the island of Faial, in the Azores

archipelago: the “Caldeira Volcano”, the “Graben of Pedro Miguel”, the “Horta-Flamengos

Platform”, and the “Capelo Peninsula” (Fig. 1A; Madeira, 1998).

The Horta-Flamengos Platform is located on the SE of Faial, around the town of Horta, and

includes the localities of Feteira, Granja, Flamengos, Farrobo and Conceição. It is a low-lying

region crossed by the Flamengos creek that drains part of the upper southeast slope of the

central volcano. The altitudes at which this flat area stands range from sea level to

approximately 140 m, with scattered scoria cones rising to altitudes up to 257 m.

Around Flamengos the region is almost horizontal (< 1º). This was a former depression

limited by the slopes of the Caldeira Volcano, to the west, the reverse of the Espalamaca fault

scarp (Graben of Pedro Miguel), to the north, and closed to the south and east by the

construction of a set of late Pleistocene scoria cones (Lameiro Grande, Quinta de S. Lourenço

and Conceição); this depression was gradually filled by lava flows, Holocene plinian and

phreato-plinian pyroclastic deposits from the Caldeira Volcano, and alluvium. Thus, the flat area

of Flamengos is an aggradation surface, ranging in altitude from 140 m to around 85 m, in

which 2 (or 3) river terraces can be recognised.

The alluvial deposits did not outcrop until 1992, when two roads were enlarged and

straightened.

In Cruz do Bravo, the new road cuts revealed a thickness of at least 2 m of alluvial deposits

below the present soil. Near by, the terrace surface, at an altitude of 140 m, drops to a lower

terrace level at altitudes of 125-120 m.

Entering Flamengos from Quinta de S. Lourenço, a road cut exposed a sequence of deposits

from the lower terrace of Flamengos creek. The succession is composed of two alluvial deposits

separated by a pyroclastic intercalation, and covered by a flood deposit (Fig. 1B):

1 – alluvial deposit (> 2 m), composed of lenses of well-rounded pebbles, millimetre to

decimetre size, without matrix, showing imbrication, intercalated in sand containing small yellow

pumice clasts, and presenting horizontal or cross-stratification. A soil (40 cm) developed on this

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unit; 2 – Phreato-magmatic pyroclastic deposit (85 cm), resting on a erosive surface; it is

composed of stratified grey trachyte pumice and ash, forming centimetre thick layers; 3 –alluvial

deposit (80 cm), similar to level 1, but presenting a fresher aspect; 4 – flood deposit, erosive on

the underlying units; it is a clay matrix massive deposit containing scattered pebbles and

cobbles, bits of coal, bones and teeth of mammals, fragments of tiles and pavement bricks,

earthenware (bowls and pots) and glazed pottery; 5 – present soil (40 cm).

Fig. 1: (A) – Digital terrain model for Faial, with the indication of phisiographic regions and localities

referred in the text; CV – Caldeira Volcano; GPM – Graben of Pedro Miguel; CP – Capelo Peninsula;

HFP – Horta-Flamengos Platform; 1 – Horta; 2 – Conceição; 3 – Farrobo; 4 – Quinta de S. Lourenço; 5 –

Flamengos; 6 – Cruz do Bravo; 7 – Granja; 8 – Feteira; 9 – Porto Pim; 10 – Praia do Almoxarife. (B) –

Log of the Flamengos section: 1 and 3 – river terraces; 2 – phreato-plinian pyroclastic layer; 4 – flood

deposit. Rectangles indicate samples dated by radiocarbon.

The presence of remains of mammals alone would indicate an event post dating 1432, when

cattle was introduced in the islands in order to reproduce and help the survival of the first

settlers; the association with pottery indicates a post settlement (circa 1466) occurrence.

However, even the oldest of the Flamengos inhabitants had no memory, not even from oral

tradition, of floods reaching the village altitude; effectively the Flamengos creek incised a narrow

gorge, some 5 metres deep, within which there are some old houses.

To find references to such destroying events one must search in the ancient chronicles,

where descriptions of destructive floods exist. Monte Alverne (circa 1700) refers that after the

initial settlement in Praia do Almoxarife, where they were subject to the attack of pirates and

privateers, the settlers moved to another place inland. This place was later called Flamengos

because there were many Flemish among the first settlers. The description mentions, however,

that they were frustrated because all they built was destroyed by the floods of the creek, fact

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that made them move once more to the place of Porto Pim, which would became the nucleus of

the present city of Horta.

Radiocarbon dating of the paleosol developed on level 1 and coal fragments from the

inundation deposit yielded conventional radiocarbon ages of 1860 ± 70 BP (sample FA1A;

Madeira et al., 1995) and 380 ± 80 BP (sample FA1B), respectively. Calibration by

dendrochronology of the latter indicates a central date of 1480 cal AD (with the following

intervals: 1440-1641 cal AD within a 68% probability and 1409-1666 cal AD within a 95%

probability). Considering that the settlement of Faial begun in 1466 the radiocarbon dates are

convincing and agree with flood descriptions during the initial stages of settlement.

Another deposit of this type, which also contained ancient glass and pottery, was observed

on the south slope of Monte das Moças, in Horta.

These facts have implications on the assessment of natural hazards in the area of Horta and

Flamengos, and show that although large floods did not occur in recent times, the potential for

destructive events exists and must be considered.

ACKNOWLEDGEMENTS This study is a contribution from research project GEODYN (LATTEX, POCTI-ISFL-5-32).

REFERENCES Madeira, J., 1998. Estudos de Neotectónica nas ilhas do Faial, Pico e S. Jorge: uma contribuição para o

conhecimento geodinâmico da junção tripla dos Açores. PhD Thesis, Lisbon University: 481 p.

Madeira, J.; Monge Soares, A. M.; Brum da Silveira, A. & Serralheiro, A., 1995. Radiocarbon dating

recent volcanic activity on Faial island, Azores. Radiocarbon 37(2), 139-147.

Monte Alverne, A., ca. 1700. Crónicas da Província de S. João Evangelista das ilhas dos Açores ...

Edição do Instituto Cultural de Ponta Delgada, 2ª edição, 1988, 3º vol.: 336 p.

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GEOMORPHOLOGICAL ASSESSMENT OF THE MIDDLE TAGUS ALLUVIAL PLAIN

ANA RAMOS PEREIRA1, CATARINA RAMOS1, TERESA M. AZEVEDO2, ELISABETE NUNES2, CONCEIÇÃO FREITAS2,

CÉSAR ANDRADE2 & DIAMANTINO I. PEREIRA3

1Centro de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal

e-mail: [email protected], [email protected] de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016

Lisboa, Portugal

e-mail: [email protected], [email protected], [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal

e-mail: [email protected]

The goal of geomorphological research within the Geotarif Project has been the recognition

of the alluvial landforms of the Middle Tagus alluvial plain.

The research focus on the Middle Tagus, between Tancos and Vila Franca de Xira / Benfica

do Ribatejo, where the River incises the sediments of its Tertiary Basin, the channel changes

direction and the valley widens rapidly leading to sediment deposition on a 2 to 13 km wide

alluvial plain, bounded by Pleistocene terraces. Despite being the largest Portuguese alluvial

plain, few geomorphological studies have been done.

Based on georeferenced topographic maps, contour lines each one-meter have been drawn.

The automatic interpolation was blind to the fluvial morphology, so hand-drawing interpretation

had to be done, using 5 m contour lines and numerous elevation points. This arduous work

allowed the definition of the alluvial landforms (several generations of natural levees and bars,

inactive channels, floodbasins and yazoo rivers) and the construction of a digital elevation

model (DEM) of the floodplain in the studied area with ArcView and ArcGis.

As one of the aims of the Geotarif project was the identification of different fluvial sediments

in the cores, each landform (present day and inactive) was sampled. The results from the study

were used to compare their textural parameters with those of the core samples (fig. 1). This

methodology leads to the identification, in subsurface sequences, of episodes of channel

avulsion and channel shifting.

In the alluvial plain, the middle reach of the present Tagus River is defined as a single

bedload channel with alternate bars, according to the classification of Miall (1996).

The channel has a low sinuosity (ρ = 1.05), but in this reach it shows a large variability: just

downstream Tancos it describes a large meander (ρ = 1.5) evolving to an almost straight reach

before the next bend.

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The channel width (W) varies between 270 and 590 m, its depth (D) ranges between 3.58

and 6.3 and the mean W/D ratio is 97. Therefore, since this reach of the Tagus shows: (i) highly

variable sinuosity, (ii) high variable W/D ratio, (iii) a very low channel slope (0.0006) and (iv) a

sand dominant bed load, according to Rosgen’s classification of natural rivers (Rosgen 1994

and 1996), it is an anastomosing river (DA5 in Rosgen classification). The present situation (a

single channel, however, is the result of multiple human interventions, known to have happened

since the Romans.

The main in-channel

bars are longitudinal

alternate bars close to the

banks (marginal bars) or

located in the middle of the

channel, reaching 1 km in

length and 200 in width.

The Tagus floodplain,

between Tancos and

Benfica do Ribatejo, a

down-valley distance of

47 km, varies in height from 22 m upstream to 7 m downstream, with a 0.31m/km-1

(approximately half of the channel slope). The floodplain shows a large lateral variation in width

of between 2 and 13 km. The former and present natural levees and crevasse splays reach 1 to

4 m above the average elevation of the floodplain and the floodbasins and abandoned channels

are 1 to 5 m below. The spatial distribution of these elements can be used to define the former

positions of the Tagus channels.

Fig. 1 – Tagus alluvial plain superficial deposits

One of the most impressive examples of the anthropogenic intervention in the foodplain can

be seen near Tancos (Azevêdo et al., 2006a). Initially, the river flowed near the southeastern

limit of the floodplain, close to Arrepiado and Carregueira terraces, adapting its course to the

NNE-SSW lower Tagus fault (fig. 2). However, in 1550 the course of the Tagus River was

changed to a straight channel in this area. The river, however, was not stable in this location

and began migrating to the northwest, adopting two new courses before stabilizing in the

present one – the previous floodbasin, in a lower position. Historical documents report that in

about a sixty-year period the channel shifted from the artificial position to the present-day one.

All three successive avulsions (to WNW), still recognizable in the field, built a major point bar

where abandoned channels and natural levees can be seen.

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Fig. 2 – Cross section of the alluvial plain between

the present-day channel and Chamusca (Arrepiado)

and the digital elevation model (DEM) in the same

area. Key: T1 – the original channel, T2 – artificial

channel, T3 and T4 – former channel positions

before the present-day channel.

References Pereira, A. Ramos; Ramos, C.; Reis, E.; Azevedo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C., 2002. A

dinâmica da planície aluvial do Baixo Tejo no Holocénico recente: aplicação de métodos de análise

geomorfológica e sedimentológica. Abstracts. 1º Seminário de Geomorfologia. Dinâmica

Geomorfológica. Progressos recentes na investigação e aplicações ao ordenamento do território.

Associação Portuguesa de Geomorfólogos, vol. I, Lisboa, p. 66-76.

Ramos, C.; Reis, E.; Pereira, A. Ramos; Azevedo, T.M.; Nunes, E.; Freitas, M.C.; Andrade, C., 2002.

Late Holocene evolution of the Lower Tagus alluvial plain and heavy metals content: Preliminary

results. In J.M. García-Ruiz, J.A.A.Jones & J.Arnáez, eds. Environmental change and water

sustainability. Zaragoza, p.167-182.

Azevêdo, T.M., Pereira, A. Ramos, Ramos, C., Nunes, E., Freitas, M. C., Andrade, C. & Pereira, D.I.,

(accepted). Floodplain sediments of the Tagus River, Portugal: avulsions and channel shifting

assessment and human impact over time, Sedimentology, Special Number dedicated to Peter Friend.

Ramos, C., Pereira, A. Ramos, Azevêdo, M.T. & Nunes, E., 2006. Dinâmica fluvial: o caso das mudanças

do traçado do Tejo na sua planície aluvial, II Congresso da Associação Portuguesa de

Geomorfólogos. Ciência e Sociedade – Associação Portuguesa de Geomorfólogos, vol. II, p.41-48.

Pereira, A. Ramos, Ramos, C., Azevêdo, M.T., & Nunes, E., 2004. Geomorphological and textural

analysis as a tool to evaluate the migration of Tagus river channel during the late Holocene (Portugal).

Geophysical Research Abstracts, Vol. 6, 04749 (EGU 04-A), European Geosciences Union, Nice.

Azevêdo, T.M., Nunes, E., Ramos, C., Pereira, A. Ramos, Freitas, M. C., Andrade, C. & Pereira, D.I.,

2006a). The Tagus River and its historical floods (Santarém, Portugal). Abstracts. Tagus Floods ’06

Workshop, Lisbon, Portugal.

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CHARACTERIZATION OF THE TAGUS FLOODPLAIN SEDIMENTS: THEIR IMPORTANCE IN THE SEARCH OF CHANNEL VARIATIONS AND HUMAN IMPACTS OVER TIME

TERESA M. AZEVEDO1, ELISABETE NUNES1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, CONCEIÇÃO FREITAS1,

CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3

1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016

Lisboa, Portugal

e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal

e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal

e-mail: [email protected]

The Tagus floods’ research implied a multidisciplinary

work using a diversity of methods such as

sedimentological, geochemical and geochronological

analyses, as well as geomorphological and hydrological

studies, performed in order to characterize the flood

sediments and the dynamics of the river during the

Holocene.

Fig. 1 – Simplified geology of the Portuguese

Tagus basin (modified after Daveau, 1970) and

digital elevation model (DEM). Key: 1 – dune

and coastal sands; 2 – recent alluvium; 3 –

Pleistocene terraces; 4 – Pleistocene Cano

limestone; 5 – Plio-Pleistocene; 6 – Tagus right

margin Miocene lacustrine limestones; 7 –

Miocene detrital series including the Palaeogene

in its eastern part; 8 – Palaeogene; 9 –

Mesozoic Lusitanian basin; 10 – basement; 11 –

location of the DEM of the studied floodplain and

the morphological position of cores and surface

samples (Pereira et al., 2004).

Following a preliminary regional sedimentological and

stratigraphic survey, in order to study the river’s

behaviour during the Holocene, four cores (FB – Fonte

Bela, QB – Quinta da Boavista, SEV – Santarém Entre

Valas and G – Goucharia), ranging in depth from 3.70 to

8.04 m were obtained from the floodplain (fig. 1) and 232

samples were analysed.

The SEV core site, 6 km north of Santarém and 5 km

from the main channel, was selected due to its distal

geomorphological position in the floodplain and its

frequency of flooding, i.e., it is inundated mainly during

major floods. Although this core reached 19.45 m, only

the first 8.04 m were selected in order to compare with

the other three cores.

Core QB was located in the lowest part of the

embankment in the main channel. Core site FB was

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selected due to its position in the medial zone of the floodplain; the core, totalling 7.4 m, is

composed of two different parts: an upper 2.5 m thick section corresponding to an existing ditch

in agricultural land and a lower 4.9 m section below the base of the ditch. Core G was selected

due to its position on the left bank, opposite to the SEV core site, is 5 km from the main channel

and frequently flooded.

A detailed textural analysis of the core samples and present analogues (bars, natural levees,

crevasse splay deposits, flood basin and abandoned channels) (Pereira et al., 2006b) was

emphasized due to the lack of preservation of sedimentary structures in the cores (fig. 1). Both

the present analogues and core sediments were well discriminated using mean diameter versus

standard deviation and average mean diameter versus average mud percentage graphical

correlations. The textural parameters defined (sand:mud ratio, mean, standard deviation,

skewness) and particularly the interparameter correlations together with twelve 14C numerical

ages of organic matter obtained, allowed the evaluation of:

(i) four different domains to be defined: the present day channel, with a low content of mud (<

5%) and the coarsest material (< 1 φ, medium sand); channel bars and natural levees from

previous channel-belts, with mud contents from 15 to 30% and a mean grain size range from

2.5 to 3.5 φ (fine-very fine sand); the floodplain itself, with 40 to 60% mud (3.5 to 5.5 φ, very fine

sand to coarse silt) and the more distal environment (flood basin and floodplain channel fill) with

the highest content of mud (> 85%) and a mean grain size ranging from 6.5 to 7.5 φ (fine to very

fine silt) and,

(ii) the chronological evolution of the different energetic environments of the floodplain for the

last 4 kyr.

These approaches permitted the determination of sedimentation rates for the different alluvial

plain environments. The highest sedimentation rates occurred in the flood basin and channel fill

domains, with values ranging from 2.23 mm yr-1 to 4.72 mm yr-1 and the lowest in the channel

(0.27 mm yr-1). Values from 0.81 to 1.60 mm yr-1 were recorded in sedimentary environments

proximal to the channel, where several crevasse splay episodes have been recognized. In the

period common to the four cores, i.e., the last 4000 years, the sedimentation rates decreased to

the present.

Bibliography Azevêdo, T.M., 2001. A utilização dos dados históricos no estudo das cheias do Tejo. Estudos do

Quaternário, 4, 69-77.

Azevêdo, T.M., 2004. As mudanças de percurso do Tejo nos tempos modernos. Causas naturais e

antrópicas. In: Evolução Geohistórica do Litoral Português e Fenómenos Correlativos. Geologia,

83

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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

História, Arqueologia e Climatologia. Eds António A. Tavares, Maria José. F. Tavares and João Luís

Cardoso, Universidade Aberta, Lisbon. 517-567.

Azevêdo, T. M., Nunes, E. and Ramos, C., 2004. Some morphological aspects and hydrological

characterization of the Tagus floods in the Santarém region, Portugal. Natural Hazards, 31, 587-601.

Pereira, A. R., Ramos, C., Reis, E., Azevêdo, T. M., Nunes, E., Freitas, M. C. And Andrade, C., 2002. A

dinâmica da planície aluvial do Baixo Tejo no Holocénico recente: aplicação de métodos de análise

geomorfológica e sedimentológica. Pub. Assoc. Port. Geom., I, 67-76.

Pereira, A. R., Ramos, C., Azevêdo, M. T. and Nunes, E., 2004. Geomorphological and textural analysis

as a tool to evaluate the migration of Tagus river channel during the late Holocene (Portugal).

Geophys. Res. Abstr., 6, 04749, European Geosciences Union, Nice.

Pereira, A. Ramos; Ramos, C.; Azevedo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C. & Pereira, D.I.

2006. Geomorphological assessment of the Middle Tagus alluvial plain. Abstracts. Tagus Floods ’06

Workshop, Lisbon, Portugal.

Ramos, C. and Reis, E., 2002. Floods in southern Portugal: their physical and human causes, impacts

and human response. Mitigation and Adaptation Strategies for Global Change, 7, 267-284.

Ramos, C., Reis, E., Pereira, A. R., Azevêdo, T. M., Nunes, E., Freitas, M. C. and Andrade, C., 2002.

Late Holocene evolution of the Lower Tagus alluvial plain and heavy metals content: preliminary

results. Environmental Change and Water Sustainability (Eds. J. M. García-Ruiz, J. A. A. Jones and J.

Arnáez), 167-182, Ins. Pirenaico de Ecología, Zaragoza.

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UNDERSTANDING LATEGLACIAL AND HOLOCENE ENVIRONMENTAL CHANGES THROUGH THE SEDIMENTOLOGICAL STUDY OF A CORE (SANTARÉM REGION)

MARIA DA CONCEIÇÃO FREITAS1, CÉSAR ANDRADE1 , TERESA M. AZEVEDO1, ANA RAMOS PEREIRA2, CATARINA

RAMOS2, ELISABETE NUNES1 & DIAMANTINO I. PEREIRA3

1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016

Lisboa, Portugal

e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal

e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal

e-mail: [email protected]

Under the Geotarif Project, the paleoenvironmental evolution of the Middle Tagus during the

Lateglacial and Holocene has been addressed. For this purpose, a deep core was drilled in

Santarém Entre Valas (SEV). This paper aims to report preliminary results obtained from the

sedimentological study of the more inclusive SEV sedimentary column.

The SEV core was retrieved from the Tagus alluvial plain (8.60 m above MSL) using Shelby

samplers driven by hydraulic pressure down to 19.4 m below surface, where a coarse gravel

unit, interpreted as Pleistocene in age, was found. The core was sub-sampled for each 10 cm

interval and studied for grain-size, organic matter and CaCO3 contents and geochemistry; seven

radiocarbon dates were obtained from selected levels to provide time boundaries. This

radiocarbon dating of carbonaceous organic material (peat, wood), at different depths, gave the

following ages: 18,109 cal BP at 16.7 m; 11,219 cal BP at 14.7 m; 8097 cal BP at 12.3 m; 6960

cal BP at 10.7 m; 3550 cal BP at 6.5 m; 3086 cal BP at 4.5 m; 816 cal BP at 1.0 m.

The sediments cored are essentially detrital and of terrestrial provenance. The results of the

sedimentological analysis of the first 8.04 m of the SEV core, of three other cores, the

comparison with present day analogues and the definition of avulsions events and shifting

episodes were presented in Ramos et al. (2006) and Azevêdo et al. (accepted). On the basis of

textural and compositional attributes the core has been divided in three main units (figs. 1 and

2).

Unit I – 19.4 to 13.5 m (> 18100 to circa 9900 cal BP) – consists of sandy mud to muddy

sand, with mean diameter in the range of fine sand to medium silt, usually poorly sorted and

positively skewed. This unit is free of organic matter and shows in its lower section the highest

values of CaCO3 and total Ca found throughout the core, although no shell remains were

observed. The concentration in Br is bellow detection limit, and U is present in the range

2-5 ppm. The La/Sm decreases up-unit but remaining in the highest values of the whole core.

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Bearing in mind existing sea level data, this unit is contemporaneous of a rapid rise in mean

sea-level (-100 to -20 m), which, however, must have remained at least 20 m below the

topographical surface at that time. This suggests a higher gradient of the paleo-river system

than today and sediments found in this unit must have been deposited marginally to a main

channel; in general sedimentation must have been more energetic than in later stages and

confined to a valley that probably was narrower, single-channel and more incised. This

interpretation is consistent with absence of organic material and dominant terrestrial

provenance, as suggested by CaCO3 and Br, the former eventually associating with

remobilization of particles consisting in carbonate-cemented aggregates remobilized from

nearby Miocene outcrops; the consistent change in La/Sm found up-unit suggests congruent

change in relative influence of distinct sources. We hypothesise that the coarser sedimentation

episode found circa 15 m, may represent a reactivation associated with Younger Dryas low-

stand.

18109 Cal BP

11219 Cal BP

8097 Cal BP

6960 Cal BP

3550 Cal BP

3086 Cal BP

816 Cal BP

0 20 40 60 80 100

20

18

16

14

12

10

8

6

4

2

0

0 10 20 30 0 1 2 30 2 4 6 8 10 12 0 1 2 3 4 5 -4 -2 0 2 4

mud (%) Mean grain size (φ) Sd (φ) Sk (φ)

Dep

th (m

)

CaCo3 (%) Organic matter (%)

Uni

t IU

nit I

I B

Uni

t III

AA

B

Fig. 1 – Mud, CaCO3 and organic matter content, textural parameters, unit division and dated horizons

of Santarém Entre Valas (SEV) core.

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Unit II – 13.5 to 2.9 m (circa 9900 to circa 2000 cal BP) – sediments of this unit contrast

markedly in texture with underlying materials and the contact with unit I is sharp. Two sub-units

have been considered:

Sub-unit IIA – 13.5 to 8.0 m (circa 9900 to circa 5100 cal BP) – is essentially made of mud

and slightly sandy mud, mean grain size falling in the range of clay and the size distribution

being negatively skewed. Sorting is better in the lower half of the sub-unit, where the sediment

is mud and decreases upward with the addition of secondary coarser fractions. Small amounts

of organic matter were found, the higher values in the basal section and decreasing further

upward. Br and U are higher than in Unit I suggesting marine influence and low oxygenation

and the vertical profiles show relative enrichment in the lower half (below 10 m) and smaller

concentrations above; the La/Sm is lower than in unit I and relatively constant, indicating

unvarying sources. Favaretto et al. (2006) and Azevêdo et al. (2006b) refer the presence of

foraminifera in this unit up to 10 m indicating limited marine-brackish water influence that may

have persisted until 6900 cal BP.

0 20 40 60 80 100

20

18

16

14

12

10

8

6

4

2

0

0 4 8 12 4.8 5.2 5.6 6 6.4 0 2 4 6 8 10Br (ppm) Ca (%) La/Sm U (ppm)

Uni

t IU

nit I

I B

Uni

t III

AA

B

Dep

th (m

)

Fig. 2 – Vertical profiles of some geochemical parameters and ratios of Santarém Entre Valas (SEV)

core.

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This date is in agreement with the age range accepted for the Holocene transgressive

maximum in the western Portuguese coast. We interpret sub-unit II A as deposited in a paralic

shallow and low energy environment, essentially dominated by fresh-water with occasional

marine incursions that allowed accumulation of some organic matter and eventually favoured

development of anoxia. The study of pollen, spore and non-pollen palynomorphs (Azevêdo et

al., 2006) indicates the occurrence, under Mediterranean climate, of such an acidic, oligotrophic

freshwater environment which could dry out in the summer. This interpretation is in agreement

with data indicating the marine base level having reached an elevation slightly below present-

day level circa 5500 BP; this would account for the abrupt decrease in energy of this location

that must have shifted into a distal estuarine environment with a week signal of marine

influence.

Sub-unit IIB – 8.0 to 2.9 m (circa 5100 to circa 2000 cal BP) – this sub-unit is quite uniform

and homogeneous, and just slightly coarser and more sandy than sub-unit II A. Organic matter

is vestigial, carbonate (again not associated with bioclasts and probably of terrestrial and local

source) and Ca increase upwards, while Br decreases markedly. U remains fairly constant and

La/Sm shows a similar pattern, yet with higher values than in the sub-unit II A. These data,

together with the absence of foraminifera tests, point to low energy, freshwater fluvial

sedimentation, related with an aggrading floodplain. Comparison with present-day analogues

suggests strong resemblance with a floodbasin environment (Azevêdo et al., accepted).

Unit III – 2.9 cm to surface (circa 2000 cal BP until present) – this unit comprehend two sub-

units.

Sub-unit III A – 2.9 to 0.9 cm (circa 2000 to circa 700 cal BP) – in this sub-unit the mud

content drops drastically and sediment coarsens, being essentially slightly muddy sand to

muddy sand. Organic matter is vestigial, carbonate content decreases and Br values are under

detection limits. This reflects a period of higher energy sedimentation, sharing similarities with

the present day natural levees that may correlate with a channel approach to the coring site

(avulsion episodes).

Sub-unit III B – 0.9 cm to surface (circa 700 cal BP until present) – the uppermost unit is

essentially muddy, showing strong oscillations in the sand content. The relevant differences in

composition associate with Br and Ca, which significantly increase. These materials correspond

to the present-day floodplain environment, located far from any marine influence. We

hypothesise an anthropic origin for Br, eventually related with the wide spreading of pesticides

and atmospheric fallout after combustion of fuel; both sources reduced after the 1980’ and this

may explain the reduction noticed in the topmost sediment layers.

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In summary, the sediments cored record a three-staged paleoenvironmental evolution of a

section of the Tagus floodplain since the Late Pleistocene, which are marked by distinct

compositional and textural breaks. The whole sequence represents a sedimentary cycle, with a

transgressive phase (until 6900 cal BP) followed by a forced regressive one.

References Azevêdo, M.T., Favaretto, S., Miola, A., Mozzi, P., Nicosia, C., Nunes, E. & Sostizzo, I., 2006b).

Palaeoenvironments of the Tagus valley during the last 15 ka: sedimentological, palynological and

micromorphological evidence of the Entrevalas SEV coring (Santarem, Portugal). Abstracts. Tagus

Floods ’06 Workshop, Lisbon, Portugal.

Azevêdo, T.M., Pereira, A. Ramos, Ramos, C., Nunes, E., Freitas, M. C., Andrade, C. & Pereira, D.I.,

(accepted). Floodplain sediments of the Tagus River, Portugal: avulsions and channel shifting

assessment and human impact over time, Sedimentology, Special Number dedicated to Peter Friend.

Favaretto, S., Sostizzo, I. & Miola, A. (2006). Pollen and NPP analyses of the Entre Valas sequence

(Santarém, Portugal), Unplubished Technical Report, Geotarif Project, 4 p.

Ramos, C., Pereira, A. Ramos, Azevêdo, M.T. & Nunes, E., 2006. Dinâmica fluvial: as mudanças de

traçado do Tejo na sua planície aluvial. Pub. Assoc. Port. Geom., 3, 41-48.

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POSTERS

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ESTIMATION OF RECENT ALLUVIAL SEDIMENTATION RATES, UPPER ODRA, POLAND

AGNIESZKA CZAJKA University of Silesia, Faculty of Earth Sciences, Bedzinska 60, 41-200 Sosnowiec, Poland

e-mail: [email protected]

The Upper Odra River is a sub-mountainous river of ground-snow-rain regime. It flows in the

temperate climatic zone. Before the regulation the Upper Odra (drainage area of 4666 km2,

mean flow of 41,5 m3/s) used to be a meandering, sand-bed river. As an effect of the training

works, agricultural, urban and industry development, the natural processes of erosion and

sedimentation were interrupted in the 19th century (Klimek, 1999). The embankment

constructions narrowed the width of natural floodplain to about 200 m and changed the natural

way of alluvial sedimentation. The construction of groins in 19th century resulted in a decrease

of the channel width from over 150 m before the rectification to approximately 50 – 70 m after

the training works. Nowadays, the Upper Odra channel sinuosity reaches from 1.3 to 2.3, while

the valley floor slope reaches 0,3 m/km. Construction of regular arrays of groins along the

Upper Odra channel created a new environment for sediment deposition. They keep the main

flow away from the river banks but also result in deposition of material adjacent to the groin

structures. This paper describes the rate of sediment accumulation within the channelised reach

of the Upper Odra River, in the lee-side of the groins.

The summer floods occur in the Upper Odra almost every year, promoting an opportunity to

overbank deposition (Ciszewski, 2002). The inter-groin infill is a significant sink for material

transported in the Upper Odra channel. Sediment accretion rates were derived using artefacts

as an alluvia age indicator. In the last 40 years, the average rate of sediment accretion has

been 5 cm per year, while shortly after channel stabilization(the rates ranged from 1.2 to 2.8 cm

per year. During large floods, up to 30 cm per event have been deposited (Czajka, 2005). Three different age indicators were used to calculate rates of vertical accretion since

approximately 1750: coal dust, plastic artefacts and 137Cs. The coal dust and the plastic objects

are very common in the youngest sediments. The coal dust can be used to estimate rates of

sedimentation on the Odra River since about 1750 when the first coal mines within Upper Odra

catchment started to work. The artificial radioactive element, 137Cs was used as an age indicator

for the sediments deposited after the year of 1964 (detectable caesium-137 began in 1954 and

the first peak appeared in 1958/1959. The second peak occurred at 1962-1964).

By determining the depth of the strata containing artefacts, more detailed age estimates were

obtained Rates of sediment accumulation per year were defined in thickness (cm a-1) and

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can be expressed as

calculated by dividing the thickness of sediments deposited above the stratum containing the

dated artefact by the number of years since the artefact deposition. The rate of sedimentation

)/(

ASPAs YYSR −=

where RS = average rate of sedimentation (cm a -1), SA = depth of stratum containing artefact of

known age (cm), YP = year of deposition of top layer of the unit and YSA = year of artefact

deposition. The average rate of the accretion of overbank sediments in the zone adjacent to the

river channel reaches1.7–6 cm/year.

Synthetic artefacts in the alluvia provide evidence of increasing rates of vertical accretion

during the last 50 years. These results correlate with data concerning the sediment budget

within the investigated sectors of the Upper Odra. On average, 195.000 tons of suspended

material transported by the Upper Odra has been yearly trapped as overbank deposits within

the investigated sections.

References Ciszewski D., 2002. Heavy metals in vertical profiles of the middle Odra River overbank sediments:

evidence for pollution changes, Water, Air, and Soil Pollution 143, 81-98

Czajka A., 2005. Accumulation of sediments within the channelized reach of the Upper Odra River,

Poland. In: Ramon J. Batalla, Celso Garcia (ed.), Geomorphological Processes and Human Impacts in

River Basins IAHS Publ. 299, 2005. str. 191-196

Klimek K., 1999. A 1000 year alluvial sequence as an indicator of catchment/floodplain interaction: the

Ruda valley, sub-Carpathians, Poland. In: Fluvial Processes and Environmental Change (ed. by

Brown A. G., Quine T. A.) Wiley & Sons Ltd.

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PELIGROS E IMPACTOS PRODUCIDOS POR LAS MODIFICACIONES DE ORIGEN ANTRÓPICO DURANTE LA AVENIDA DEL ARROYO RIVILLAS EN 1997 (SW DE ESPAÑA)

MARIA EUGÉNIA MOYA-PALOMARES 1 & JUAN D. CENTENO 2

1Departamento de Geología. Facultad de Ciencias. Universidad de Alcalá. Carretera Madrid-Barcelona, Km. 33,600.

Alcalá de Henares, Madrid, (España).

e-mail: [email protected] de Geodinámica. Facultad de Geología. Universidad Complutense de Madrid (España)

Introducción

Las avenidas relámpago o flash floods constituyen en la actualidad el peligro natural más

importante que se produce en redes secundarias de drenaje de la Península Ibérica. Las

avenidas relámpago causan una media de 20 muertos al año y perdidas materiales superiores

a los 210 millones de euros anuales. Muchas de estas consecuencias podrían ser evitadas con

una correcta ordenación territorial en consonancia con el conocimiento de los peligros de las

áreas implicadas. Un claro ejemplo de magnificación de un evento natural por modificaciones

antropicas lo constituyeron las inundaciones ocurridas en Noviembre de 1997 en el arroyo

Rivillas.

El arroyo Rivillas y la tormenta del mes de noviembre de 1997 El arroyo Rivillas es un afluente temporal del río Guadiana cuyo cauce permanece seco gran

parte del año. Presenta una cuenca de 321 km2 rellena de materiales cenozoicos (arenas y

arcillas fundamentalmente) que otorgan a la cuenca una permeabilidad baja. En la primera

semana del mes de noviembre de 1997 una borrasca extratropical atravesó la mencionada

cuenca con dirección SW originando una ciclogénesis explosiva (Elizaga y Martín 2001). Las

precipitaciones acumuladas en 24 horas superaron los 120 l/m2 en la mayoría de las estaciones

pluviométricas de la región (Núñez y del Pino, 2001) acompañadas de rachas de viento

superiores a los 100 Km/h. Las intensas precipitaciones en un corto espacio temporal (días 5-6

de noviembre de 1997) provocaron una importante crecida en el río Guadiana y el

desbordamiento de algunos tributarios de mencionado río como el Rivillas. El estudio de las

formas erosivas y sedimentarias permitió reconstruir el proceso. En este sentido, pueden

consultarse los trabajos de Moya-Palomares et al. (1998, 2006) y Ortega et al. (1998).

La avenida catastrófica y el papel jugado por las modificaciones antrópicas sobre los efectos del flujo

Debido a las actuaciones humanas en el valle y a parcial canalización del arroyo, el Rivillas

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presenta una baja sinuosidad (casi se podría decir que es un canal rectilíneo y constreñido por

los leeves artificiales de sus márgenes) con un flujo discontinuo y efímero durante gran parte

del año. Con un incremento de flujo las aguas tienden a migrar sobre la llanura aluvial,

rompiendo los leeves y trazando meandros como ocurrió en los primeros estadios de la

avenida de 1997. Cuando se produce el pico de la crecida, la llanura de inundación se

comporta como si de un fondo de canal aluvial se tratase. En condiciones naturales este

proceso no tendría mayor importancia pero la intervención humana hizo que dos barrios de la

ciudad de Badajoz se vieran convertidos en una trampa mortal para sus habitantes.

A lo largo del Rivillas se han identificado numerosas prácticas agrícolas y canalizaciones

sistemáticas del cauce, edificaciones, caminos sobre la llanura de inundación que

incrementaron los efectos del flujo. Una de las causas de la magnificación del evento fue sin

duda, la gran cantidad de material trasportado convirtiendo el flujo en un mudflow. La

sustitución de las viejas dehesas por pastizales y viñedos redujeron la tasa de infiltración e

incrementaron la cantidad de material sedimentario transportado. Esta cantidad de material se

acrecentó por el elevado número de construcciones que actuaron a modo de represas sobre la

llanura aluvial. Cuando estas presas fueron destruidas y sobrepasadas por las aguas, el flujo

adquirió “picos” de concentración energética.En Badajoz, la ocupación por edificaciones hasta

el borde del canal produjo un aumento de la altura de las aguas. Además, la estructura de

canalización en la confluencia de los arroyos Rivillas y Calamón que cubre parte de los ojos del

puente de la N-5 se convirtió en un obstáculo de salida de las aguas hacia el río Guadiana.

Agradecimientos Este trabajo ha sido financiado por el proyecto de Investigación PI2005/055 de la

Universidad de Alcalá.

Referencias Elizaga, F. y Martín, F., 2001. Estudio diagnóstico de la ciclogénesis explosiva del 5-6 de noviembre de

1997. V simposio Nacional de predicción. INM. Madrid 20-23 Noviembre de 2001. En CD.

Moya, M.E., Garzón, G., Ortega, J.A. & Centeno J.D., 1998. Estructuras sedimentarias resultado del

desbordamiento del arrollo Rivillas, Badajoz (España). Noviembre de 1997. Actas do V Congresso

Nacional de Geología. Tomo 84, Fascículo 1. Lisboa. pp. 209-212

Moya-Palomares, M.E. & Centeno, J.D., 2006. Morphologycal analyses of the flash floods in ouadis and

torrents with lower declive, the case of the Rivillas River (SW of Spain). 3rd Conference of the

Association of African Women Geoscientists. Women, Geosciences and Development, El Jadida.

Marroc. pp. 118-120.

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Nuñez, M. y Del Pino, J.D., 2001. Ciclogénesis Atlántica del 5 y 6 de noviembre de 1997, análisis

fenomenológico y mesoescala. V simposio Nacional de predicción. INM. Madrid 20-23 Noviembre de

2001. En CD.

Ortega, J.A., Garzón, G., Moya, M.E., 1998. Parámetros climáticos y ambientales de la avenida del

arroyo Rivillas en Noviembre de 1997. V Reunión nacional de Geomorfología. Granada. pp. 237-246.

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DESARROLLO ESPACIAL Y EVOLUCIÓN DE UNA FLASHFLOOD A TRAVÉS DE LA INFORMACIÓN APORTADA POR DEPÓSITOS EFÍMEROS

JOSÉ A. ORTEGA1, GUILHERMINA GARZÓN2

1 Depto. Medio Ambiente, E. S. Politécnica, Universidad Europea de Madrid, Madrid

e-mail: [email protected] Depto. Geodinámica, Facultad de Geología, UCM, 28040 Madrid

e-mail: [email protected]

Los estudios de delimitación de áreas inundables se centran cada vez mas en cálculos de

modelos numéricos, aún sabiendo las limitaciones de estos métodos debido no solo a la poca

calidad general de los datos hidrológicos de partida si no también a la propia naturaleza

cambiante del sistema aluvial. Frente a ello la cartografía de áreas inundables debería basarse

lo más posible en datos directos de campo, con los que puedan contrastarse los modelos

numéricos teóricos. Los métodos geológicos presentan (Costa y Baker, 1985) el principal

problema de la correlación no univoca entre formas y magnitud de la avenida, pero constituyen

elementos tangibles reales de la dinámica y del flujo de desbordamiento. La dificultad de

correlación se basa en la propia variabilidad de los elementos naturales, pero especialmente

por la interferencia con obras y obstáculos antrópicos, por lo que la información de los efectos

de una avenida resulta una información básica para el estudio de la dinámica e insustituible a

nivel de referencia para cualquier análisis hidrológico posterior. Por ello el esfuerzo de los

investigadores debe centrarse en recoger la máxima información inmediata posible después de

un evento catastrófico (Williams y Costa, 1987).

En el caso de la avenida del Arroyo Rivillas de noviembre de 1997 en Badajoz, el

levantamiento cartográfico inmediato a la inundación, ha servido para la elaboración del modelo

hidráulico, basado fundamentalmente en las marcas de altura del agua y sus depósitos (Ortega

y Garzón, 2005). Éstos se han clasificado de acuerdo con su ambiente genético y según su

localización respecto a la crecida, aspectos hidrológicos e hidráulicos, tamaño de grano y

energía de formación, naturaleza sedimentaria o erosiva y tamaño de la forma. El resultado es

un conjunto de 18 formas, la mayoría de ellas previamente descritas en la literatura, pero no

asociadas en su conjunto tanto con el resto de las morfologías, como a una cartografía a

escala 1:4.500.

A su vez, la relación de estos depósitos con el modelo hidráulico ha permitido establecer

una clasificación de rangos de estabilidad para las formas resultantes. Se ha obtenido el caudal

máximo de la crecida en tres de los tramos analizados (elegidos por su gran variedad de

formas y que son representativos puesto que se distribuyen a lo largo de todo el perfil

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longitudinal) a partir de la modelización mediante HEC-RAS y así se han obtenido resultados

sobre variables importantes en la estabilidad de las formas como son: profundidad de la lámina

de agua, velocidad, esfuerzo de cizalla y potencia fluvial. Empleando la cartografía

morfosedimentaria obtenemos unas relaciones entre elementos y los rangos de estabilidad

bajo los cuales las formas no quedan destruidas. Estos resultados son orientativos, puesto que

el HEC-RAS es un método hidráulico de cálculo unidimensional, que solo permite establecer

tres valores medios para cada sección (margen derecha e izquierda, y canal) por ello los

campos y condiciones obtenidas no pueden ser consideradas como las originales para su

formación. Aún así, estimamos que el trabajo base puede ser muy útil en investigaciones

futuras mejorando el modelo hidráulico, que debería ser 3D en vez de unidimensional.

La interferencia antrópica a través de cambios de uso del suelo ha modificado el aporte de

sedimentos originando una avenida relámpago más catastrófica. Aún así, las formas erosivas

dominan sobre las sedimentarias por lo que el balance sedimentario es negativo. Los

resultados preliminares obtenidos muestran que la profundidad no parece ser un factor

limitante en la permanencia de las formas posteriormente a la crecida. Algunas formas tienen

rangos muy grandes que llegan hasta los 140cm en las barras longitudinales arenosas y 150

en las barras de derrame. En ambas el umbral superior de calado es bastante elevado lo que

muestra la gran resistencia de la forma a ser erosionada por los caudales de descenso. Los

abanicos aluviales que llegan a la llanura de inundación permanecen estables al sumergirse en

profundidades bajas no superior al metro. Las formas erosivas muestran los valores más altos

de profundidad, entre 100 y 150cm para surcos, mientras que los canales antiguos encajados

tienen valores de calado máximo de 170cm.

La velocidad sí se revela como un factor más determinante en la aparición de formas

sedimentarias o formas erosivas. El umbral entre ambas lo hemos situado alrededor de 1m/s,

aunque se trata de un límite difuso. Las formas de erosión son las que requieren los mayores

valores de velocidad (no indicativa del momento de formación). Los surcos o acanaladuras se

dan en zonas de alta velocidad (superan 1m/s hasta 1,67 m/s), valor también superado por los

canales antiguos encajados. En relación con el esfuerzo de cizalla existe una clara delimitación

sobre la llanura entre las formas de depósito y las formas de erosión. En las morfologías

erosivas el valor del esfuerzo de cizalla medio se incrementa con cifras que alcanzan los 93

N/m2 de valor máximo para los surcos, siendo normales valores superiores a 30 N/m2. La

potencia del canal, tiene una distribución similar entre formas de erosión y sedimentación.

Como conclusiones más relevantes obtenemos que las efímeras formas de erosión y

depósito generadas tras una flashflood constituyen una valiosísima fuente de información para

conocer el desarrollo de la misma. Los eventos energéticos, mostrados en erosiones y

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Page 104: Tagus Floods '06 Workshopworkshoptejo.fc.ul.pt/Tagus Floods_ABSTRACTS.pdf · Tagus Floods '06 Workshop Project POCTI/CTA/39427/2001 ABSTRACTS BOOK 19-21 July, 2006 Lisbon Portugal

Abstracts

Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006

depósitos de material grueso pueden llegar a intercalarse con otros de menor energía, buena

muestra de los pulsos seguidos por la crecida de los cuales tenemos información directa de

testigos presenciales. La relación abanico-depósito y erosión en la llanura de inundación es

también una buena fuente de información sobre la llegada de caudales por parte de afluentes,

y también sobre el momento temporal en que esta se produce. Encontramos dos tipologías

muy diferentes en cuanto a esta ocurrencia temporal. Los derrames guardan una especial

relación con las actuaciones antrópicas (Moya et al. 1998 a y b; Ortega et al. 2004) y la

evolución de las modificaciones en el canal. Con una sencilla modelización, en el mapa

aparecen las zonas más sensibles a presentar este tipo de formas, como se ha comprobado

posteriormente en campo. Además son el origen del sedimento en la mayoría de las

macroformas sobre la llanura.

Sin duda los cambios humanos determinan el riesgo posterior, esto se ha podido comprobar

mediante estudio de las fotografías aéreas, localizando los puntos más conflictivos, que suelen

coincidir con zonas de erosión, y por tanto de alta energía en casi todas las fases de la crecida.

Un adecuado registro de formas, acompañado de un levantamiento topográfico y modelización

hidrológica-hidráulica permite establecer por tanto un gran número de características, y ayuda

a entender mejor el funcionamiento de las flashfloods.

BIBLIOGRAFIA Costa, J.E. y Baker, V.R., 1995. Surficial Geology. Techbooks, 498 p.

Ortega, J.A., Garzón, G. y Muñoz Barco, P., 2004. Incidencia de los acondicionamientos en pequeños

cauces y de los cambios de uso del suelo en las inundaciones de Badajoz de 1997. En: Cachón de

Mesa, J. y López-Piñeiro Pérez T. (eds). Congreso de Restauración de Ríos y Humedales. CEDEX.

Madrid, pp. 207-218.

Moya, M.E., Garzón, G., Ortega, J.A., 1998a. Depósitos de la avenida del Arroyo Rivillas, Badajoz.

Noviembre de 1997. V Reunión Nacional de Geomorfología. Granada. pp 229-236

Moya, M.E., Garzón, G., Ortega, J.A. y Centeno, J.D., 1998b. Estructuras sedimentarias resultado del

desbordamiento del Arroyo Rivillas, Badajoz (España), Noviembre de 1997. Congreso Nacional de

Geología. Lisboa. Sociedade Geológica de Portugal, 84-1. pp. A-209-212.

Ortega, J.A. y Garzón, G., 2005. Ephemeral deposit analysis from a riverine flashflood:

morphosedimentary evolution and significance in flood interpretation. VI Internacional Conference on

Geomorphology. Zaragoza, p. 93.

Williams, G.P. y Costa, J.E., 1987. Geomorphic Measurements after a Flood. In: Baker, V.R., Kochel,

R.C., Patton, P.C., (eds). Flood Geomorphology. Wiley, N. York. pp. 65-77.

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