SOES 6030 Advanced Independent Oceanography Research Project
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ALICE LEFEBVRE 2004-2005
UNIVERSITY OF SOUTHAMPTON SCHOOL OF OCEAN AND EARTH SCIENCES
UNIVERSITÉ BORDEAUX 1
UFR DES SCIENCES DE LA TERRE ET DE LA MER
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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Contents List of Figures iii List of Tables iv List of Plates v List of Annexes v 1. Introduction 1
1.1. General introduction 1 1.2. Marine aggregates extraction 1 1.2. Studied area 8 1.3. Objectives 12
2. Theoretical background 13 2.1. Sediment mobility 13
2.1.1. Introduction 13 2.1.2. Currents 14 a) Generalities 14 b) Current-induced bed shear-stress 15 2.1.3. Waves 16 a) Generalities 16 b) Wave-induced bed shear-stress 18 2.1.4. Combined waves and currents 20 2.1.5. Threshold of motion 21
2.2. Suspended sediment concentration 23 2.3. Grain size analysis 26
3. Materiel and Methods 28
3.1. Instrumentation 28 3.1.1. Electro-Magnetic Current Meter 28 3.1.2. Pressure sensor 29 3.1.3. Optical Backscatter Sensor 29 3.1.4. Wavelog 30
3.2 Field experiment 31 3.2.1. Deployment 31 3.2.2. Sediment samples and meteorological conditions 33
3.3. Calibration 33 3.4. Grain size analysis 35
3.4.1. Sieving and weighing 35 3.4.2. Settling tower 36 3.4.3 Coulter Counter 37 3.4.4. Total Particle Size Distribution 38
3.5. High frequency data processing 38 3.5.1. Pressure 38 3.5.2 U and V components of the flow 39 3.5.3. Turbidity 40
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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4. Results 42 4.1. Sediment characteristics 42
4.1.1. Video 42 4.1.2. Seabed sediment 43 4.1.3. Bottles sediment 46
4.2. General conditions during the experiment 48 4.2.1. Meteorological conditions and waves 48 4.2.2. Water level variations 51 4.2.3. Currents 53 a) Comparison of methods 53 b) Currents description and analysis 54 4.2.4. Suspended sediment concentration 56
4.3. Comparison of data at the three stations 57 4.3.1. Currents 57 4.3.2. Waves 58 a) Wave height and period 58
b) Directional wave spectra 59 c) High frequency surface elevation 60 4.3.3. Suspended sediment concentration 62
4.4. Sediment mobility 64 4.4.1. Current-only bed shear stress 65 4.4.2. Wave orbital velocity 65
a) Amplitude of the wave orbital velocity under the crest and under the trough 67 b) Comparison of theoretical and measured wave orbital velocity 68 c) Differences between the stations 68
4.4.3. Wave-only bed shear-stress 69 4.4.4. Total bed shear-stress 70 4.4.5. Fraction of sediment on motion 72
4.5. Suspended sediment concentration 74 5. Interpretation 77
5.1. Hydrodynamics effects 77 5.2. Effects on sedimentation 78
6. Conclusions 82 Bibliography 84 Annexes 89
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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List of Figures Figure 1.1: The two most commonly used methods for marine aggregates extraction.
A. Anchor hopper dredging, B. Trailor suction dredging. 3 Figure 1.2: Possible biological impacts of marine aggregate extraction. 5 Figure 1.3: Chain of possible effects caused by changing the bathymetry. 6 Figure 1.4: Reduction of current and wave velocity by increasing the water depth
in dredged area. 7 Figure 1.5: Zone of study. 9 Figure 1.6: Generalized sediment distribution map for the whole of Tromper
Wiek Bay and sites of marine aggregate extraction. 10 Figure 1.7: Side Scan Sonar from Tromper Wiek Bay. 11 Figure 2.1: The velocity profile for steady current flow over a bed showing
current shear (length of arrow proportional to velocity) in the boundary layer. 13 Figure 2.2: Types of surface waves, showing the relationship between
wave frequency and period, the nature of the forces that cause them, and the relative amounts of energy in each type of wave. 17
Figure 2.3: Airy waves showing the particle orbits at various depths below the surface. (a) In deep water, the particle or orbits are circular and their radius decays exponentially with depth. (b) With a depth of L/2, the orbits, including those of the surface particles have become elliptical. 18
Figure 2.4: Schematic diagram of non linear interaction of current-only (τc) and wave-only (τw) bed shear-stresses. 20
Figure 2.5: Threshold of motion of sediments beneath waves and/ or currents. 22 Figure 2.6: Wentworth grain-size classification together with the range
of various analysis techniques. 26 Figure 2.7: Ternary diagram for mixtures of clay, sand and gravel. 27 Figure 3.1: Schema of the ABLs. 28 Figure 3.2: Site of deployment of the three ABLs. The bathymetry is
given in meters, coordinates in UTM system. 31 Figure 3.3: General arrangement of settling tower. 34 Figure 3.4: Calibration curve 36 Figure 3.5: a. Separation of wave and current components from the total
high frequency files by applying a filter. b. Wave components which let determine wave significant and maximal orbital velocity under crest and trough and wave direction. c. Current components which let determine mean current speed and direction for each burst. 40
Figure 4.1: Frequency histogram and cumulative frequency curve
representing the grain size distribution at each sampling station. The median diameter of the sediments found in the bottles is also shown. 45
Figure 4.2: Frequency histogram and cumulative frequency curve representing the grain size distribution of the sediment found in the bottles at each sampling station. 47
Figure 4.3: Pressure and wind at Cape Arkona (the direction indicates where the wind comes from); wave height, direction (where it propogates towards) and period (Tz), current (where it goes) and depth at the station V1 during the experiment (HL = High water Level; LL = Low water Level). 49
Figure 4.4: Comparison of current speed and direction given in tidestat files and calculated from puvt files; example for station V1. 54
Figure 4.5: Schematic representation of the currents at station V1 and wind
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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at Cape Arkona before, at the beginning, at the end and after the storm. The size of the arrows gives an indication of the speed of wind and currents. 55
Figure 4.6: Currents at the three stations during the experiment. 57 Figure 4.7: Wave height and period for the three stations during the experiment. 58 Figure 4.8: Frequency, direction and spectral power density estimated using
EMEP method. 59 Figure 4.9: Surface elevation during the burst 73 (20/10, storm conditions)
showing the waves for the three stations. 61 Figure 4.10: Suspended sediment concentration (SSC) during the experiment
for the three stations. 62 Figure 4.11: Current-induced bed-shear stress (τc) and threshold bed
shear-stress (τcr); example for the station V1 for a roughness length z0 of 0.0003m (sand/gravel) and 0.006m (rippled sand). 64
Figure 4.12: Significant and maximum orbital velocity under crest and trough at station V1. 65
Figure 4.13: Difference of amplitude of wave orbital velocity under the crest and under the trough at station V1. 66
Figure 4.14: Significant measured and calculated orbital velocity during the experiment at station V1. 67
Figure 4.15: Significant wave orbital velocity for the three stations during the experiment. 68
Figure 4.16: Wave bed shear-stress (τw) calculated for a roughness length (z0) of 0.0003m (sand/gravel) and 0.006m (rippled sand) at stations V1 and V2 and a roughness length of 0.006m at station V3. 69
Figure 4.17: Wave-induced bed shear stress, (τw), maximum bed shear-stress for combined flow (τmax) and threshold bed shear stress (τcr) during the experiment; example for the station V1. 70
Figure 4.18: Relative importance of the wave-induced bed shear-stress on the maximum bed shear-stress during the experiment for the three stations. 71
Figure 4.19: Wave bed shear-stress and threshold of motion for different quartile at the three stations during the experiment. 73
Figure 4.20: Suspended sediment concentration (SSC) calculated at the height (z) of each OBS for the three stations. 75
Figure 5.1: Measured SSC at the three stations during the experiment, time
of sediment movement calculated from the bed shear-stress and time of sediment suspension at the height of the sensor from the calculated SSC. 79
Figure 5.2: Schematic representation of the sequence of events during the storm. 80 List of Tables Table 3.1: Presentation of puvt files. 30 Table 3.2: Presentation of Tidestat (a) and Wavestat files (b). 30 Table 4.1: Percentage in weight of the different fractions for each sample. 43 Table 4.2: Summary of statistical parameters (phi units) of the particle size
distribution of the seabed sediments for the three stations and their description according to McManus (1988). 44
Table 4.3: Summary of statistical parameters (phi units) of the particle size distribution of the sediments found in the bottles for the three stations and their description according to McManus (1988); the relative weight of sediment found at each station compared with the total amount of sediment found in all three bottles is also indicated. 46
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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List of Plates Plate 3.1: An ABL ready for deployment from the side of the vessel. 36 Plate 4.1: Pictures from the video showing the seabed. 42 List of Annexes Annexe 1: Model to illustrate the formation of relict sand and gravel,
example offshore south-eastern Britain 89 Annexe 2: Statistical measures of grain size parameters and descriptive
terms applied to parameter values. 90 Annexe 3: Sketch showing the Faraday effect, which forms
the basis of the electromagnetic current meter. 90 Annexe 4: Sketch showing the principle of capacitive pressure sensors
which use a thin diaphragm, quartz or silicon, as one plate of a capacitor. 91 Annexe 5: Constant pressure contours beneath a 100m wave. Water wave is 100m. 91 Annexe 6: A selection of information from the Beaufort Wind scale. 92 Annexe 7: Nomogram of deepwater significant wave prediction curves of
wind speed, fetch length and wind duration. 93 Annexe 8: Density and kinematic viscosity of water in function of temperature
and salinity. 94
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
Notations A orbital amplitude of wave motion (m) b Rouse number or suspension parameter Ca reference concentration at the height za (m) C(z) concentration at height z CD drag coefficient C0 concentration at the seabed d50 median grain diameter (m)
*D dimensionless grain size diameter
wf wave friction factor g acceleration due to gravity (ms-2) h water depth (m) H wave height (m) k wave number (=2π/L) Ks eddy diffusivity of sediment. l decay length scale L wavelength (m) U depth-averaged current velocity (ms-1) Uws significant wave orbital velocity (ms-1) Uwmax maximum wave orbital velocity (ms-1) U(z) velocity at depth z (ms-1) s ratio of sediment and water densities (= ρs / ρ) T period (s)
sw settling velocity (ms-1) z height of the sensor (m) za reference height near the seabed, at which reference concentration Ca is
calculated (m) z0 roughness length (m)
r∆ ripple height (m) θcr threshold Shields parameter θr modified Shields parameter. θw skin friction Shield parameter κ Von Karman constant = 0.40 λr wavelength of ripples (m) ν kinematic viscosity of water (m2s-1) ρ water density (kgm-3) ρs sediment density (kgm-3) τc current-only bed shear stress (Nm-2) τm mean shear-stress under combined waves and currents (Nm-2) τmax maximum bed shear-stress under combined waves and currents (Nm-2) τw wave-only bed shear stress (Nm-2) ø angle between current direction and direction of wave travel Φ grain diameter in phi units ψ mobility number
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
ACKNOWLEDGEMENTS
I would like to express my gratitude to Pr Michael B. Collins for giving me the
possibility to work on this project and for his continual support throughout the
study. I would like to extend my appreciation to Dr Erwan Garel for his constant
guidance with the data analysis and helpful comments and suggestions during the
writing of the report. I also would like to thank Pr Patrice Castaing, from my
University of Bordeaux 1, for his encouragement and support throughout the year.
Special thanks are extended to Clara for the long time she spent correcting my
English, and to Susannah for encouraging me to come to England.
Chapter 1: Introduction
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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CHAPTER 1: INTRODUCTION
1.1. General introduction
An ever increasing number of natural resources are being used to satisfy our day-to-
day needs. Terrestrial settings have traditionally been exploited to a greater extent
than the oceans due to relatively easier access. As a consequence of progress in
exploration and exploitation techniques, the ocean floor has become an important
source of materials, like for example, gas, oil and aggregates. However, the effects
of the exploitation of the marine seabed are not clearly defined. Depending on the
type of material extracted, the site location and the prevailing physical conditions;
exploitation effects may be insignificant or may disturb the marine ecosystem to
varying extents and lead to coastal erosion. The exploitation of the ocean floor is
increasing and there is a real need to quantify the effects that it causes. This study
will investigate the physical effects of marine aggregate extraction upon waves,
currents and suspended sediment concentrations in the Tromper Wiek Bay, located
in the western Baltic Sea.
1.2. Marine aggregate extraction
Aggregates are sand, gravel or crushed solid rock and are used in the construction
industry for purposes such as concrete, mortar and asphalt manufacture. For
example, to build one house requires 50 or 60 tonnes of aggregates and each mile of
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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motorway uses 200,000 tonnes (BMAPA, 1995). Aggregates are also used in beach
replenishment schemes and for fill-in coastal reclamation projects.
Aggregate demand in Europe has traditionally been met by land-based quarries and
pits, but in recent years offshore sources have made an increasingly important
contribution (Harrisson, 2003). The reasons for this are an increased awareness of
the environmental and social conflicts of terrestrial mineral extraction, increasing
legal restrictions for the exploitation of terrestrial resources, progress in extraction
techniques which facilitate the exploitation of marine sand as well as the advantages
of marine sediments with respect to quality, availability and ease of transport and
delivery (HELCOM, 1999). For instance, in 1992, the total production of marine
aggregates in the UK was 20.6 million tonnes (Meakins et al., 1999), which
represents around 18% of the UK�s total aggregate consumption (Selsby, 1992).
Sand and gravel deposits can be either relict or modern (Dyer and Huntley, 1999).
Relict deposits were formed during periods of post-glacial sea level rise. During
glacial times, the sea level was lower than at present and sand and gravel were
deposited by rivers that poured out onto the dry shelf. In the warmer interglacial
stages that followed, these deposits were re-worked during rising and high sea level
(BMAPA, 1995, Annexe 1). Modern deposits, on the other hand, have been deposited
and are controlled by modern hydrodynamic and sedimentological regimes. Sand
deposits can be either relict or modern, whereas gravel deposits can be only relict,
therefore constitute a non-renewable resource (Dyer and Huntley, 1999).
The extraction of aggregates from the seafloor is carried out by dredging. There are
two main types of dredging techniques: anchor dredging and trailer dredging
Chapter 1: Introduction
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(Figure 1.1). Their use is dependent on the type of deposit located (HELCOM,
1999). Anchor dredging involves a ship anchoring over a deep deposit with its pipe
drawing up the sand and gravel; trailer dredging requires the ship to drag its pipe
along the seabed sucking up material from more evenly distributed deposits. A large
dredger can load 5,000 tonnes of sand and gravel in around 3 hours (BMAPA,
1995).
For both techniques, the aggregates and water are piped aboard into the ship�s
hopper. As the hopper fills up the aggregates displace the water, which overflows
back into the sea, carrying with it fine suspended material which forms a turbidity
plume in the wake of the ship (Nakata et al., 1989). On some dredgers, screening of
the aggregates is carried out in order to maintain a specific sand to pebble ratio,
excess sand is returned to the seabed, which also generates a plume (Hitchcock and
Drucker, 1996).
The production of aggregates is essential to economic growth and the improvement
of living standards. The demand is constantly increasing; aggregates are now a
Figure 1.1: The two most commonly used methods for marine aggregates extraction. A.Anchor hopper dredging, B. Trailor suction dredging (from HELCOM, 1999)
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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strategic resource. However, marine aggregate extraction can have important
impacts on the environment and marine life.
The effects of marine aggregate extraction which have previously been studied
include (EUMARSAND, 2004):
• changes in seabed elevation which may alter inner shelf flows, enhance the
wave energy reaching the coast and therefore increase coastal erosion and
retreat,
• harmful effects on fauna, flora and water quality in the area of mining
including the destruction of benthic habitats and species, such as fish and
shelfish populations, the formation of turbid plumes of fine-grained sediment
during extraction which may affect the benthic ecology in a large area around
the extraction site and the creation of large depressions on the seabed
(depending upon extraction method) where anoxic conditions may develop,
• disturbance of cultural heritage sites e.g. shipwrecks of archaeological
interest.
• conflicts of interests between marine aggregates industry and other sea-bed
users such as fisheries, shipping, the oil industry and offshore wind farms.
The biological short term as well as the long term effects of marine aggregates
extraction upon the benthic community have been well studied in the English
Channel (Boyd and Rees, 2003, Boyd et al., 2003, 2004, Desprez 2000), in the
North Sea (Kenny and Rees, 1994, 1996), in the Baltic Sea (ICES/ACME, 1997,
Graca et al., 2004), and on the US coasts (Oliver, 1973, Drucker, 1995, Diaz et al.,
Chapter 1: Introduction
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2004). These studies gave variable results with no definite indications of the extent
and duration of the long term impacts.
The possible effects of marine aggregate extraction upon the benthic fauna are
summarised in Figure 1.2.
The principal physical effect of marine aggregate extraction is to change the
bathymetry of the seabed, which will in turn influence hydrographical conditions
and sediment transport (IADC, 1997, Figure 1.3). The effects on the physical
environment caused by alteration of the bathymetry depend on the existing
bathymetry, the shape and location of the dredged area relative to the wave and
current direction, the hydrographic conditions (tide, waves, currents) low or high
energy and the sedimentary regime (silt, sand, rock), sediment transport and
sedimentation rates.
Increased concentration of
suspended material
Reduced light penetration
Settling of larvae
influenced
Reduced growth of bottom vegetation
Covering of fauna/ mussels
Excavation of habitats for flora/
fauna
Figure 1.2: Possible biological impacts of marine aggregate extraction (from IADC, 1997)
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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Marine aggregate extraction may have a number of physical impacts:
- deepening the natural pathways of the water allows the energy to be
transported with less friction from the bottom, and then reduces the
current or wave impact in the dredged area compared with outside the
pit (Graca et al., 2004, Kleinhans et al., 2004, Figure 1.4),
- the reduced current or wave action at the seabed can increase
sedimentation inside the dredged area (Kenny and Rees, 1996, Desprez,
2000); this may produce a lack of sediment available to the coast and
lead to coastal erosion.
- the reduced friction at the seabed can also enhance wave or tidal energy
(Maa et al., 2004) and then increase coastal erosion.
Figure 1.3: Chain of possible effects caused by changing the bathymetry (from IADC, 1997).
Chapter 1: Introduction
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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The time scales of recovery of the pits are not clearly defined (Boers, 2004); it has
been found that this depends profoundly on the position of the extraction site
relative to the seaward limit of the shoreface. Locating extraction sites well beyond
this limit implies a slow regeneration due to the fact that the threshold of re-
mobilisation of the ambient sediment is surpassed only during extreme events.
Meanwhile, extraction on the shoreface means that pits can recover quite fast but
negative impacts on the coastal sediment budget cannot be ignored (Diesing et al.,
2004). In certain sites, the presence of weathered dredged tracks or pits can be
detected 10 years after the extraction (Boyd et al., 2004). In other sites the time
scales of regeneration are assessed to be in the order of decades (Diesing et al.,
2004).
Figure 1.4: Reduction of current and wave velocity by increasing the water depth in dredgedarea (from IADC, 1997).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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1.3. Studied area
The Baltic Sea is an almost enclosed sea with very restricted access to the North
Sea through the Kattegat. As a consequence the tides are rather small, a few cm at
the most in most places. (Kantha et al., 2005). Furthermore they have a mixed
nature, with diurnal or semi-diurnal components dominant depending on exact
location. The drainage basin of the Baltic Sea is located entirely within a humid
climate setting. This results in a freshwater surplus, with river discharges as the
main contributor (Klein, 2003).
In the Western Baltic Sea, the Pomeranian Bight is a typical coastal basin,
extending from Cape Arkona (Rügen Island) in the west to Poland in the east
(Figure 1.5); it is bordered to the north by a 20m depth contour and encompasses a
200km long coastline (Schwarzer et al., 2003). Pomeranian Bight is characterised
by the largest freshwater discharge into the Western Baltic and therefore the salinity
is low, around 8 (Lass et al., 2001). The dynamical regime of Pomeranian Bight is
governed by a locally wind-driven Ekman current and a compensating bottom
current, as well as by coastal jets (Lass et al., 2001).
The Tromper Wiek is a semi-enclosed bay on the northern coast of Rügen Island.
The circulation in the Tromper Wiek Bay is highly variable. Currents are primarily
influenced by wind as well as water level variations due to wind surges and seiches
(Klein, 2003).
Three periods with different prevailing wind directions occur annually: dominant
easterly winds from February to May, westerly winds from June to September and
westerly to southwesterly winds from October to January (Schwarzer et al., 2003).
Chapter 1: Introduction
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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Tromper Wiek's inner coastline is exposed only to wave approach from Northeast to
East and the fetch, limited by Bornholm Island and Sweden in the North, has a
maximum of approximately 90km (Schwarzer et al., 2003). Therefore, Easterly
winds produce high wave energy input transferred into high sediment movement
whereas westerly winds result in low energy input and low sediment movement
(Schwarzer and Diesing, 2001).
Figure 1.5: Zone of study.
Germany Poland
Sweden
Rügen
Tromper Wiek
Bornholm
35 km
North Sea
Baltic Sea
N
Cape Arkona
Pomeranian Bight
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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Tromper Wiek Bay has been a site of sand and gravel extraction for many years
(Figure 1.6).
The seafloor of the sand extraction site is characterised by marine fine sands in
water depths of between 14 and 21m (Albrechts, 1997). Sand has been extracted
twice (151 000 m3 in 1989 and 104 000m3 in 2000) by trailer suction dredging in a
water depth of around 11m causing relatively shallow (< 1m) furrows of several
hundreds of metres in length and less than 10m in width (Diesing et al., 2004).
The gravel extraction site in Tromper Wiek is situated in water depths of between 9
and 14m. There, the seafloor is covered by sandy gravel forming prominent NE-
SW-trending ridges, which are interpreted as being the remains of a drowned beach
ridge system dating back to the Pleistocene (Schwarzer et al., 2000).
Site of gravel extraction
Site of sand extraction
Figure 1.6: Generalized sediment distribution map for the whole of Tromper Wiek Bay (from Albrechts, 1997) and sites of marine aggregate extraction (after Diesing et al., 2004)
Chapter 1: Introduction
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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The gravel deposits are extracted by anchor hopper dredging, which results in the
formation of extraction pits 5 to 50m in diameter and up to 7m in depth (Figure
1.7.).
After extraction, the material is screened on board, i.e. sediments with grain sizes
<2mm are sorted out and spilt back into the sea. The spilt material settles down to
the area of extraction, creating recognisable sediment distribution patterns on the
seafloor (Diesing et al., 2004). Between 1988 and 2000, approximately 460,000 m3
of sediment were extracted of which half the volume was spilled back into the sea.
From repeated sidescan sonar surveys, Diesing et al. (2004) found that the pits do
not re-fill completely, but remain stable for at least several years. However, the
pattern of spilt sands shows rapid changes. Spilt sands are re-mobilised, especially
during late winter and early spring when easterly winds produce high waves within
Tromper Wiek Bay. The re-mobilised sands partly re-fill the pits as could be proved
by cores obtained from the seafloor inside the pits. Diesing et al. (2004) estimated
the appropriate time scales for regeneration to be in the order of years, at least.
Figure 1.7: Side Scan Sonar from Tromper Wiek bay (Ramso, 2004, unpublished).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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1.4. Objectives
This research will investigate the physical impact of an isolated gravel-pit in the
North-Western part of the Tromper Wiek Bay. The study will be based on the
analysis of waves, currents, water level and suspended sediment concentration high
frequency measurements inside and outside the crater during a 4days experiment.
The main objectives of this study are:
- to identify any hydrodynamic effects of the presence of the pit i.e. morphological
influence on currents and wave propagation; and
- to evaluate whether the crater can act as a trap for fine sediments.
Chapter 2: Theoretical Background
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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CHAPTER 2: THEORETICAL BACKGROUND
2.1. Sediment mobility
2.1.1. Introduction
The flow of a current or waves in the sea is usually accompanied by the formation
of a turbulent boundary layer adjacent to the seabed. This is a region of frictionally
retarded flow which is characterised by a spatial and temporal randomness of the
velocity field and through which the horizontal mean flow adjusts from zero at the
bed to its maximum value away from the bed in the free-stream (Heathershaw,
1988, Figure 2.1). Throughout this layer turbulent energy levels and shear stresses
also change, decreasing from maximum values near the bed, to zero at the outer
edge of the boundary layer. The bed shear-stress is defined as the frictional force
exerted on a unit area of the seabed by the current flowing over it.
Increasing current
velocity U
Incr
easi
ng h
eigh
t abo
ve th
e be
d z.
boundary layer
Figure 2.1 The velocity profile for steady current flow over a bed showing current shear (length of arrow proportional to velocity) in the boundary layer (after Open University, 2000).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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The total bed shear-stress (τ0) acting on the bed is made up of contributions from
(Soulsby, 1997):
• the skin friction τ0s produced by (and acting upon) the sediment grains
• the form drag τ0f produced by the pressure field associated with the flow
over ripples and/or larger features on the bed
• a sediment-transport contribution τ0t caused by momentum transfer to
mobilise the grains.
Only the skin friction contribution acts directly on the sediment grains, and it is
therefore this contribution which is used to calculate the threshold of motion, the
bedload transport, and the reference concentration or pick up rate for grains in
suspension. On the other hand, it is the total bed shear-stress that corresponds to the
overall resistance of the flow and determines the turbulence intensities which
influence the diffusion of suspended sediment to higher levels in the water column
(Soulsby, 1997).
For simplicity the subscript 's' is omitted from the skin friction bed shear-stress and
only the others contributions will have the subscript.
2.1.2. Currents
a) Generalities
Currents in the sea may be caused by tidal motions, wind-stress, atmospheric
pressure gradients, wave-induced forces, river out-flow, large-scale quasi-steady
water surface slopes and horizontal density gradients associated with oceanic
circulation (Soulsby, 1997). In the nearshore region, wave-induced (longshore)
Chapter 2: Theoretical Background
Alice Lefebvre University of Southampton School of Ocean and Earth Science
15
currents are dominant, whereas further offshore a combination of tidal and
meteorological forcing (including storm surges) dominates.
Strom surges are changes in water level generated by atmospheric forcing;
specifically by the drag of the wind on the surface and by variations in the surface
atmospheric pressure associated with storms (Flather, 2001). They last for periods
ranging from a few hours to 2 or 3 days and have large spatial scales compared with
the water depth. They can raise or lower the water level in extreme cases by several
metres. Both pressure and wind effects are present in all storm surges, but their
relative importance varies with location. Wind forcing is most important in shallow
waters whereas pressure (diminution of atmospheric pressure produces an increase
in depth) dominates in the deep ocean.
Associated storm surge currents, superimposed on tidal and wave-generated flows
can contribute to extremes in current and bed stress.
b) Current-induced bed shear-stress
The depth-averaged current velocity U (ms-1) can be calculated from a single
measurement in the water column using the empirical formula of Soulsby (1990):
)(32.0 71
zUz
hU
= for 0 < z < 0.5h (1)
where h is water depth and U(z) is the velocity at the depth z.
The current skin friction bed shear-stress τc is related to the depth-averaged current
speed U through the drag coefficient CD by the quadratic friction law
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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2UCDc ρτ = (2)
where ρ is the density of water and CD is the drag coefficient which can be
calculated using the log profile expression:
2
0ln1
+=
hz
CDκ (3)
where κ is the Von Karman constant (0.40) and z0 is roughness length, which, in the
absence of simultaneous measurements at different levels during the experiment,
can be estimated from the mean values of z0 for different bottom type given by
Soulsby (1997).
2.1.3. Waves
a) Generalities
Waves play a major role in stirring up sediments from the seabed, as well as giving
rise to steady current motions such as longshore currents, undertow, and mass-
transport velocities, which transport the sediments. Waves are classified according
to their period (Figure 2.2). Tides belong to the long period wave band; they play an
indirect role in sediment movement by creating tidal current. Lower periods waves,
called surface waves, may be generated either as a locally-generated sea (wave-sea)
due to the effect of local winds blowing over the sea for a certain distance (the
fetch), and time (duration), or as swell, which results from distant storms and
usually has a longer period and less spread in period and direction than a locally-
generated sea (Soulsby, 1997).
Chapter 2: Theoretical Background
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For small disturbances of the sea surface, the waves, according to the wave linear
theory, can be represented by a freely propagating, long-crested, sinusoidal wave
train (Tucker and Pitt, 2001). The orbits are closed (i.e. waves do not produce a net
displacement), circular in deep water and elliptical in shallow water (Figure 2.3).
From the wave linear theory, the amplitude of the wave orbital velocity can be
derived from the wave height (H), the period (T), the wave length (L) and the water
depth (h) by using the relation:
)sinh(khTHU w
π= (4)
where k is the wave number (=2π/L)
This applies to waves whose steepness (height/wavelength) is very small, in which
case the magnitude of Uw is the same under the trough and the crest (Soulsby,
1997). The orbital velocity beneath the wave crest is in the same direction as the
wave is travelling, and under the wave trough it is in the opposite direction. Then an
Figure 2.2: Types of surface waves, showing the relationship between wave frequency andperiod, the nature of the forces that cause them, and the relative amounts of energy ineach type of wave (from Knauss, 1997).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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asymmetry of velocities beneath the wave and the trough is a source of net transport
in the direction of greater orbital velocity (Soulsby, 1997). In practise the waves of
most interest for sediment transport will have a larger steepness, i.e. the amplitude
of orbital velocity will be different under the crest and the trough. A variety of non-
linear wave theories are available to deal with steep waves (Stokes 2nd-5th order
solutions, cnoidal theories, stream-function theory, see Tucker and Pitt, 2001).
b) Wave-induced bed shear-stress
Frictional effects near the bed produce an oscillatory boundary layer within which
the wave orbital velocity amplitudes increase rapidly with height from zero at the
bed to a value Uw at the top of the boundary layer. In the absence of a current the
turbulence is confined within the boundary layer, which for waves is only a few
millimetres or centimetres thick in contrast to the boundary layer of a steady current
Figure 2.3: Airy waves showing the particle orbits at various depths below the surface. (a) In deep water, the particle or orbits are circular and their radius decays exponentially with depth. (b) With a depth of L/2, the orbits, including those of the surface particles have become elliptical (from Tucker and Pitt, 2001).
Chapter 2: Theoretical Background
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19
in which it can be metres or tens of metres thick. This has the effect of producing a
much larger velocity shear in the wave boundary layer, which in turn causes the bed
shear-stress produced by a wave with orbital velocity Uw to be much larger than that
produced by a steady current with an equal depth-averaged speed U (Soulsby,
1997).
The amplitude τw of the wave's oscillatory bed shear-stress is usually obtained from
the bottom orbital velocity Uw of the waves via the wave friction factor wf , using
the quadratic friction law for waves
2
21
www Ufρτ = (5)
Several equations have been proposed to calculate the wave friction factor. It is
dependant on whether the flow is laminar, smooth turbulent or rough turbulent.
Grant and Madsen (1979) proposed the following expressions valid under waves
only or combined waves and currents:
1057.0=wf for 3
0
10<zA (6a)
0316.0=wf for 4
0
3 1010 <<zA (6b)
0135.0=wf for 5
0
4 1010 <<zA (6c)
00690.0=wf for 5
0
10>zA (6d)
where A is the orbital amplitude of wave motion at the bed
π2TU
A w= (7)
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2.1.4. Combined waves and currents
In most parts of coastal and shelf seas, both waves and currents play an important
role in sediment dynamics. The interaction of the two may affect wave
characteristics (by modification of the phase speed and wavelength of waves by the
current), current velocity (by generation of currents by the waves) and bed shear-
stress (Van Rijn, 1993).
Because of the non-linear interaction of the wave and current boundary layers, the
bed shear-stresses beneath combined flows are enhanced beyond the values which
would result from a simple linear addition of the wave-only and current-only
stresses (Soulsby, 1997, Figure 2.4).
Several models have been proposed to calculate the mean and maximum bed shear-
stress during a wave cycle (Grant and Madsen, 1979, Fredsøe, 1984, Huynh-Tanh
and Temperville, 1991, Davies et al., 1988). Using a data-based method Soulsby
(1995) deduced the simple equations
τc
τw
τmax
τm
(a) (b)
(c)
ø
Figure 2.4: Schematic diagram of non linear interaction of current-only (τc) and wave-only (τw) bed shear-stresses (from Soulsby et al., 1993).
Chapter 2: Theoretical Background
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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+
+=2.3
2.11wc
wcm ττ
τττ (9)
[ ] 2/122max )sin()cos( φτφτττ wwm ++= (10)
where τm and τmax are the mean and maximum bed shear-stress during a wave cycle,
τc is the current-only bed shear-stress, τw is the wave-only bed shear-stress and ø is
the angle between current direction and direction of wave travel.
τmax is used to determine the threshold of motion and entrainment rate of sediments,
and τm to determine sediment diffusion (Soulsby, 1997).
2.1.5. Threshold bed shear-stress
During very slow flows over a sand bed the sand remains immobile. If the flow is
slowly increased, a velocity is reached at which a few grains begin to move. This is
called the threshold of motion or incipient motion (Soulsby, 1997); a similar
process occurs beneath waves. Shields (1936) investigated the threshold of motion
in terms of the ratio of the force exerted by the bed shear-stress acting to move a
grain on the bed, to the submerged weight of the grain counteracting this. The
threshold Shields parameter θcr is defined as
dg s
crcr )( ρρ
τθ−
= (11)
where τcr is the threshold bed shear-stress, ρs is the sediment density, ρ is the water
density, d is the grain diameter and g is the acceleration due to gravity.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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It can be plotted against the dimensionless grain size diameter *D (Figure 2.5)
given by
dsgD3/1
2*)1(
−=
ν (12)
where ν is the kinematic viscosity of water and s is the ratio of density (= ρs / ρ).
Soulsby and Whitehouse (1997) proposed an equation to calculate the threshold
Shields parameter θcr from the dimensionless grain size *D
(13)
The threshold bed shear-stress can next be calculated using equation (11).
Figure 2.5: Threshold of motion of sediments beneath waves and/ or currents (fromSoulsby, 1997).
))020.0exp(1(055.02.11
30.0*
*
DDcr −−+
+=θ
Chapter 2: Theoretical Background
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2.2. Suspended sediment concentration
For current speeds or wave conditions significantly above the threshold of motion,
sand is entrained off the bed and into suspension, where it is carried at the same
speed as the current. For grains to remain in suspension, their settling velocity must
be smaller than the upward turbulent component of velocity. The settling velocity
sw of sand grains is determined by their diameter and density, and the viscosity of
the water. Soulsby (1997) proposed the following formula:
[ ]36.10)049.136.10( 2/13*
2 −+= Dd
wsν (12)
where ν is the kinematic viscosity of water, *D is the dimensionless grain size
diameter and d is the grain diameter.
In a sand suspension the settling of the grains towards the bed is counterbalanced by
diffusion of sand upwards due to the turbulent water motions near the bed. The
equation governing this balance is
dzdCKCw ss −= (13)
where C is the volume concentration of sediment at height z and Ks is the eddy
diffusivity of sediment.
If the eddy diffusivity is assumed to increase linearly with height above the bed, the
corresponding concentration profile is the power-law profile:
b
aa z
zCzC−
=)( (14)
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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where C(z) is the concentration at the height z above the bed, Ca is the reference
concentration at the height za and b is the Rouse number or suspension parameter
which is defined as
κ*uw
b s= (15)
where κ is the Von Karman constant and *u is the total friction velocity
( 2/10 )/( ρτ= ).
If the eddy diffusivity is assumed to vary parabolically with height, the Rouse
profile is obtained:
b
a
aa zh
zhzzCzC
−
−−
=)( (16)
where h is the water depth.
If the eddy diffusivity is assumed to be constant with height, which is the case
under waves for a rippled bed, the concentration profile is given by
lzeCzC /0)( −= (17)
where C0 is the concentration at the seabed and l is the decay length scale.
Various expressions have been given for l and C0, of which one of the most widely
used is that of Nielsen (1992) for rippled bed:
rs
w
wU
l ∆= 075.0 for 18<s
w
wU
(18a)
rl ∆= 4.1 for 18≥s
w
wU
(18b)
Chapter 2: Theoretical Background
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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30 005.0 rC θ= (19)
where r∆ is the ripple height and rθ is the modified Shields parameter which is
given by
250 )/1()( rrs
wr dg λπρρ
τθ∆−−
= (20)
The ripple height r∆ can be calculated using the expression proposed by Nielsen
(1992) valid for 156<ψ and 831.0<wθ (if these values are exceeded the model
predicts the wash out of ripples):
Ar )022.0275.0( 5.0ψ−=∆ (21)
where A is the orbital amplitude of wave motion at the bed (equation 7) and ψ is
the mobility number defined as
50
2
)1( dsgU w
−=ψ (22)
where Uw is the wave orbital velocity, g is the acceleration due to gravity, s is the
ratio of sediment and water densities and d50 is the median grain diameter.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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2.3. Grain size analysis
Sediment grains are classified according to their diameter into mud (which
comprises clays and silts), sands and gravel (which comprises granules, pebbles,
cobbles and boulders). Grains size can be expressed either in millimetres or phi
units, which are related by the expression:
d = 2 �Φ (23)
where d is the grain diameter in millimetre and Φ the grain diameter in phi units.
The most commonly used classification is the Wentworth scale (Soulsby, 1997,
Figure 2.6).
Different techniques are available to analyse the sediment samples depending on the
sample grain size (Figure 2.6).
Figure 2.6: Wentworth grain-size classification together with the range of variousanalysis techniques (from Heathershaw, 1988).
Chapter 2: Theoretical Background
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The proportions of the different fractions allow one to classify the sediment samples
on a ternary diagram (Figure 2.7)
The grain-size distribution is usually presented as a cumulative curve showing the
percentage by mass of grains smaller than d, versus d. Statistics are used to
characterise grain-size distributions (Annexe 2). The most commonly used
parameters are the median diameter d50 which is the diameter for which half of the
grain is finer, the sorting which is a measure of the spread about the average, the
skewness which indicates the preferential spread and the kurtosis which evaluates
the peakedness of the distribution (McManus, 1988).
Figure 2.7: Ternary diagram for mixtures of clay, sand and gravel (from Dyer, 1986).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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CHAPTER 3: MATERIEL AND METHODS
3.1. Instrumentation
The data were collected using three Autonomous Benthic Landers (ABLs, Figure
3.1), each of them held up on a frame and equipped with an electromagnetic current
meter (EMCM), a pressure sensor and an Optical Backscatter Sensor (OBS). The
ABLs operated autonomously taking measurements at regular intervals called bursts.
3.1.2. Electro-Magnetic Current Meter
A current meter permits the monitoring of the instantaneous (mean and fluctuating
parts) of the water motion in two directions (Voulgaris, 1992), therefore it estimates
the speed and direction of water moving relative to the instrument. In
electromagnetic current meters an alternating current (ac) or switched direct current
Figure 3.1: Schema of the ABLs.
Material and Methods
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(dc) magnetic field is imposed on the surrounding seawater using a coil buried in
the sensing head, and measurements of the potential gradients arising from the
Faraday effect are made using orthogonally mounted pairs of electrodes (Shercliff,
1962, Annexe 3). Unlike mechanical current meters, electromagnetic instruments
have no zero velocity thresholds (Collar and Griffiths, 2001) and are thus utilisable
for very slow flows.
3.1.2. Pressure sensor
The principle of the pressure sensor is that the pressure at a fixed point under a
wave system fluctuates in phase with waves (Tucker and Pitt, 2001). The pressure
sensor incorporated on the Valeport is based on the principle that changes in water
pressure cause changes in frequency of a resonant circuit using a silicon crystal
(Williams, 2005, Annexe 4).
3.1.3. Optical Backscatter Sensor
The OBS manufactured by D&A Instruments, was developed at the University of
Washington for monitoring suspended sediment concentrations (SSC) in the surf
zone (Downing et al., 1981) and has proved an excellent tool for suspended
sediment studies due to his high frequency response, relative insensitivity to
bubbles, approximate linear response to concentration and small size causing
minimal disruption to transporting flow (Kineke and Sternberg, 1992). There are
many advantages to using an OBS for measuring SSC rather than water sampling
methods, such as continuous monitoring, real time display and simultaneous
measurements with flow meters and water properties sensors.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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The OBS is a miniature nephelometer that measures scattering of infrared radiation
by suspended particles. Scattering is highly influenced by both the number of
particles and particle size (Kineke and Sternberg, 1992, Xu, 1997). Hence, most
optical sensors cannot be used successfully in highly turbid waters and are
extremely susceptible to the effects of particle size (Baker and Lavel, 1984).
3.1.4. Wavelog
The high frequency data were downloaded by a software package called Wavelog
and regrouped in puvt (pressure, U, V and turbidity) files (Table 3.1), which for
each burst give the date and time, the pressure (dBar), the U and V components of
the flow (ms-1) and the turbidity (V).
Just after their recording, the high frequency data were processed by Wavelog.
After correcting the pressure attenuation with depth and detrending the pressure
a.
b.
Table 3.2: Presentation of Tidestat (a) and Wavestat files (b).
Table 3.1: Presentation of puvt files.
Material and Methods
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burst data, Wavelog applies a spectral analysis to give the tide slope, mean depth
and statistics wave parameters, i.e. significant wave height (Hs), significant and
zero-crossing period (Tz and Tp), total energy and spectral data for each burst.
All the results are given in statistics files called tidestat and wavestat (Table 3.2).
3.2 Field experiment
3.2.1. Deployment
Hydrodynamics measurements were collected during the cruise of the Research
Vessel "ALKOR" (16 to 25/10/2004) as part of the research training network
EUMARSAND.
N
Figure 3.2: Site of deployment of the three ABLs. The bathymetry is given in meters,coordinates in UTM system.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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Three ABLs were deployed in the north-western part of the Tromper Wiek Bay in
the gravel dredging zone (Figure 3.2), one in the middle of an isolated 3m-deep
crater (station V3) and the two others on its edge (stations V1 and V2), at a water
depth of ~12m, between the 19th and 23rd of October 2004.
Bursts were taken every 30min lasting 8min 32sec at a frequency of 4Hz throughout
the study period, allowing the recording of 184 bursts at each station. The ABLs
were logged at the same moment therefore the bursts are taken at the same time at
the three stations.
This location allowed us to have two references stations (stations V1 and V2) and
one inside the area of gravel-extraction (station V3)which allows the
characterisation of the hydrodynamic conditions and turbidity inside and outside the
pit
Trigger
OBS
EMCM head
Weights
Pressure sensor
Bottle
Frame
Plate 3.1: An ABL ready for deployment from the side of the vessel.
Material and Methods
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The sensor height was 0.5m for stations V1 and V3 and 0.6m for station V2, the
OBS is situated 5cm lower (Plate 3.1). Three divers controlled the tilting and GPS
positioning of the three ABLs.
Wind data and atmospheric pressure were recorded throughout the experiment at
Cape Arkona (Figure 1.5). The data constitute 10min values recorded every 60min.
3.2.2. Sediment samples
Surface sediment samples were collected by divers at each mooring location.
Moreover, bottles were left on the frames that support the instruments during the
three days of the experiment allowing them to collect the sediments in suspension.
The bottles were situated approximately at the same height as the OBS (Plate 3.1).
A video was also recorded during the ABLs' recoveries which give the possibility
of visualising the sea floor and analysing the features present.
3.3. Calibration
The OBS measures the turbidity (in Volts), that has to be converted to Suspended
Sediment Concentration (gl-1) with a calibration curve. The particle size strongly
influences the scattering therefore the measurement of turbidity. As a result,
sediment from the field had to be used to calibrate the OBS (Sternberg et al, 1991,
Downing and Beach, 1981). Different techniques are proposed for the calibration,
such as the use of the Laser In-Situ Scattering and Transmissometer deployed
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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simultaneously to the OBS to measure the size distribution of the suspended
sediments (Agrawal and Pottsmith, 1994), the use of bottom sediment in a
calibration tank (Green and Boon, 1993), or calculation of the in situ calibration
coefficient by modelling grain-size distribution of suspended sediments (Xu, 1997).
In this study, we have chosen to do the calibration using a bucket filled with water
and field sediments as recommended by the manufacturers (D&A Instruments,
1988). Grain-size distribution of the suspended sediment may be different to that of
the bottom sediment, and the use of sediment found in the water column or bottom
sediment will change the results of the calibration (Kineke et al., 1989, Sternberg et
al., 1991). That is why we preferred the utilisation of sediments found in the bottles,
i.e. sediment in suspension at the height of the sensor during the experiment, to
those taken at the seabed.
Sediments were put in a bucket and OBS measurements were taken at the same
time as water samples, the latter were next filtered (2µm filters, dry weight
established prior to calibration). Filtered samples were then dried and weighed and,
hence, the suspended sediment concentration calculated for each sample.
Figure 3.3: OBS calibration curve.
y = 110.36x2 + 16.275xR2 = 0.8239
0
10
20
30
40
50
60
70
80
90
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9turbidity (V)
SSC
(mg/
l)
Material and Methods
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A calibration curve was drawn using the results (Figure3.3). Voulgaris (1992) found
that a polynomial curve gives a better correlation that a linear form when the
calibration is done with the bucket method and storm experiment conditions; in the
present study, we found the same results: a polynomial curve gives a better
correlation coefficient (0.82) than a linear form (0.73).
3.4. Grain size analysis
3.4.1. Sieving and weighing
To analyse the sediments taken at the experiment site, we began by sieving them in
order to separate them according to their grain diameter Ø into mud (Ø<63µm),
sand (63µm< Ø <2mm) and gravel (Ø > 2mm) fractions.
The mud fraction was put in a measuring cylinder and filled up to 1L with water.
After stirring the water and the mud for 2min and waiting 20sec, a sample of 20ml
was taken at 20cm height and put in a Petri dish previously weighed when dry. The
sand and gravel were put into boxes.
All the samples were then dried during one night in the oven at 60°C and weighed.
The gravel fraction was dry sieved by mechanical shaking for 15min with a series
of standard test sieves with successively smaller mesh sizes decresing by 0.25 phi
each time, from �1 phi (2mm) to �4 phi (16mm).
The sediment collected into the bottles was dried and weighed.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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3.4.2. Settling tower
To determine the mass frequency distribution of sand particle size, we used a
settling tower. The principle of this is that the force of resistance of a spherical
particle moving throw a fluid depends on the diameter and relative velocity of the
particle, and on the density and viscosity of the fluid. The assumption is made that
particles settle out individually, this is true for sand-size particles (Syvitsky et al.,
1991).
The settling tower (Figure 3.4) consists of a 2m long perspex tube with an external
diameter of 20cm. The tube was filled with fresh water at a temperature of 16°C
maintained throughout experiments. It is essential to maintain a constant
Figure 3.4: General arrangement of settling tower (from Rigler et al., 1981).
Material and Methods
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temperature since the settling velocity depends on the density of water, which is a
function of its temperature. A perspex pan (15cm in diameter) is used for sample
collection, the assumption is made that statistically equivalent proportions of all
size ranges within the sample are capable of passing between the pan and tube wall
during a set of readings (Rigler et al., 1981). The pan is suspended from a Sartorius
electromagnetic balance. The start of the recording sequence is set by an external
trigger, operated by the sediment release mechanism as the sample just comes into
contact with the water surface. The balance logs the weight every 6 Hz, this then
provided a raw file containing the parameters, which are weight (cumulative weight
of the sediment), distance (length of the column) and time.
Conversion of settling velocities to grain size was carried out using a Matlab
program written by Urs Neumeier (2003).
3.4.3 Coulter Counter
The mud fraction as well as the sediment found in the bottles were analysed using a
Coulter Counter LS 130 Laser Diffraction Size Analyser, measuring grain size
distribution from 0.4 to 1000µm. The principle of the Coulter Counter is that
particles of a given size diffract light through a given angle (Wen and Duzgoren-
Aydin, 2002). The angle increases with decreased particle diameter size. A parallel
beam of monochromatic light passes through a suspension contained in a sample
cell, the diffracted light is focused onto a detector. The detector measures the
distribution of scattered light in term of density. A lens focuses on the undiffracted
light to a point at the centre and leaves only the surrounding patterns, which do not
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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vary with particle movement. A computer controls every function of the instrument
and gives the PSD.
3.4.4. Total Particle Size Distribution (PSD)
The results of the Coulter Counter, the Settling Tower and the dry sieving were
amalgamated with respect to the relative weight of each fraction in order to have the
complete PSD. Statistic parameters were used to describe the PSD (Annexe 2).
3.5. PUVT files treatment
The high frequency data (puvt files) were processed using Matlab.
3.5.1. Pressure
The pressure measured by the pressure sensor is the sum of the atmospheric
pressure and the pressure of the water column over the sensor. Thus we first
subtracted the atmospheric pressure measured at Cape Arkona from the raw
pressure high frequency measurements.
The pressure disturbance created by waves decays with depth according to the wave
amplitude (and length) to water depth ratio and is a strong function of the wave
period (Pajala, 2002, Annexe 5). The pressure measurements then have to be
corrected to the pressure attenuation with depth in order not to underestimate the
smaller waves. This was done using a Matlab function written by Urs Neumier
(2005) and based on principles described by Tucker and Pitt (2001).
Material and Methods
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The data are then detrended, i.e. corrected for the tide using a Matlab routine, and
the mean pressure and the sensor height are added in order to obtain the total depth
in metres.
3.5.2 U and V components of the flow
The U and V components of the flow were corrected for the sensor heading
(direction of sensor relative to North) to give north and east components.
The 2 dimensional components of the flow represent the high frequency variations
of a water particle, this includes the displacement induced by waves and that
induced by currents.
The Fourrier analysis is a method which allows one to distinguish one frequency
from another in measurement data. This analysis can take a time series of
observations and transform it into its fundamental periods (Young, 1999). Using
such a method, the U and V components of the flow can be approximated by the
linear superposition of sinusoidal forms. Each component is characterised by its
frequency and energy.
By applying a filter on the spectral density of the U and V components of each burst,
we separate wave and current components. The applied filter separates frequencies
smaller than 0.083Hz (i.e. periods greater than 12sec), considered as current
components, from frequencies greater than 0.083Hz, considered as wave
components (Figure 3.5.a).
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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For each burst, the current speed is computed as the average of the current
components and the current direction is calculated from the angle of the line joining
the origin, i.e. sensor position, to the average value of current speed, and the
ordinate axis, i.e. north direction (Figure 3.5.c).
The wave components were analysed by adapting the Matlab function
Zero_crossing written by Urs Neumier (2005). The wave direction was calculated
by applying a linear regression to the data. A rotation was applied on the data so
that the x-axis was in the direction of wave travel. The zero crossing was calculated
and allowed us to compute the wave orbital velocities under the crest and the trough
as the maximum and minimum values between a downward crossing. Then, the
maximum and significant wave orbital velocities under the crest and the trough
were calculated for each burst. The significant wave orbital velocity was taken as
the mean of the 1/3 biggest orbital velocities.
3.5.3. Turbidity
The high frequency turbidity data were controlled and anomalous values (abrupt
and short increase to very high values) attributed to presence of fishes, algae or
sampling errors were removed. The turbidity was next converted in to suspended
sediment concentration using the calibration curve (Figure 3.3).
Material and Methods
Alice Lefebvre University of Southampton School of Ocean and Earth Science
41
Figure 3.5: a. Separation of wave and current components from the total high frequencyfiles by applying a filter. b. Wave components which let determine wave significant andmaximal orbital velocity under crest and trough and wave direction. c. Currentcomponents which let determine mean current speed and direction for each burst.
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Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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CHAPTER 4: RESULTS
4.1. Sediment characteristic
4.1.1. Video
On the video, one can see the patchy nature of the gravel deposits. Gravel
sometimes covers the entire seabed (Plate 4.1c.), or is in patches surrounded by
sand (Plate 4.1a. and d.) as described by Albrechts (1997). We also see that the size
of the gravel can be quite significant (Plate 4.1b.).
We observed that the seabed is sandy with some gravel patches at stations V1 and
V2 (Plate 4.1a. and b.), and devoid of gravel at station V3, inside the pit. The video
also indicates that the sand portions of the seabed are rippled.
Plate 4.1: Pictures from the video showing the seabed.
a. b.
c. d.
Cobble
Sand
Gravel
Sand
Gravel covered
with mussels
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
43
4.1.2. Seabed sediment
The total particle size distribution (PSD) of the sediment taken from the seabed is
presented in Figure 4.1, its characterising parameters are summarised in Tables 4.1
and 4.2.
In all the samples, the mud fraction represents less than 1% of the total weight
(Table 4.1).
The sample of station V1 contains gravelly sand sediments, essentially composed of
sand with approximately 6% gravel in which we note the presence of some small
mussel shells. The median grain diameter is 0.5mm, at the limit between the coarse
and medium sand; the distribution is unimodal with a dominant mode of 1.3 phi
(0.4mm), it is moderately sorted positively skewed, i.e. the preferential spread is
towards coarser grain (Figure 4.2) and leptokurtic.
The sample from station V2 is a sandy gravel, mainly composed of gravel; this is
principally due to the presence of a cobble (7cm long, 4cm large). We have seen the
patchy nature of the gravel deposits and we found that this boulder represents more
than 50% of the weight of the sample. Taking this cobble into account when
calculating the particle size distribution would have greatly under-represented the
sand fraction (which covers a large fraction of the seabed surface). It was therefore
Table 4.1: Percentage in weight of the different fractions for each sample.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
44
decided to remove it from the PSD, thus the percentage of the different fractions has
been re-calculated without it (V2�). This does not change the classification of 'sandy
gravel' found at station V2 (Table 3.1). The sample is thereafter composed of about
70% sand and 30% gravel. We note that an important part of the gravel fraction is
composed of mussel shells of all sizes, which represent around 5% of the total
weight of the sample. The median grain diameter is 0.5mm, as for station V1. The
distribution is bimodal in the sand fraction with a dominant mode of 1.6phi
(0.34mm) and a secondary mode, coarser than the dominant one, of 0.8phi
(0.57mm); it is moderately sorted, very positively skewed and platykurtic.
Contrary to the other samples, the sample of station V3, which was situated in the
dredged zone, does not contain gravel and is composed of more than 99% sand. The
median diameter of 0.34mm, classified as medium sand, is the finest of the three
stations. The distribution is bimodal, with a dominant mode of 1.7phi (0.31mm) and
a secondary mode of 1.1phi (0.48mm); it is mesokurtic and as opposed to the other
samples, this one is well-sorted and symmetrical.
Table 4.2: Summary of statistical parameters (phi units) of the particle size distribution of the seabed sediments for the three stations and their description according to McManus (1988).
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
45
Figure 4.1: Frequency histogram and cumulative frequency curve representing the grain sizedistribution at each sampling station. The median diameter of the sediments found in the bottles (d50 bottle) is also shown.
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Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
46
4.1.3. Bottles sediment
Figure 4.2 presents the PSD of the sediment collected in the bottle left on the frame
supporting the instruments and Table 4.3 summarises the statistical parameters.
The sediment of bottle V3 represents more than 50% of the total weight of
sediments collecting in the bottles of the three stations during the experiment (Table
3.3, relative weight). This means that substantially more sediment was in
suspension at station V3 than at the two other stations.
The median diameter indicates that it is the finer fraction of the PSD (Figure 4.1)
which is put in suspension at the height of the sensor, as expected (Soulsby, 1997).
It was found that the sample from station V1 has the largest median diameter, at
28phi (102.9µm), which is classified as very fine sand. The sediment from station
V2 has the smallest median grain diameter, at 4.17phi (55.6µm), this size is
classified as coarse silt. This dissimilarity can be explained by the fact that the
sample from station V2 has a lower dominant mode than does the station V1 sample
(Figure 4.1) therefore the suspended sediment is composed of finer particles at
station V2 and coarser ones at station V1.
However, we found that the median diameter of sediment particles from the station
Table 4.3: Summary of statistical parameters (phi units) of the particle size distribution of thesediments found in the bottles for the three stations and their description according toMcManus (1988); the relative weight of sediment found at each station compared with the totalamount of sediment found in all three bottles is also indicated.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
47
V3 bottle, which has a value of 3.86phi (68.9µm, very fine sand) is in between
those of the bottles of stations V1 and V2, whereas the median diameter of the total
PSD of seabed sediments is the finest of the three samples at station V3.
For all stations, the samples were found to be poorly sorted, symmetrical and
leptokurtic.
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Figure 4.2: Frequency histogram and cumulative frequency curve representing the grain size distribution of the sediment found in the bottles at each sampling station.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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4.2. General conditions during the experiment
4.2.1. Meteorological conditions and waves.
Meteorological conditions and wave height, direction and period variations
throughout the 4-day experiment can be divided into five time periods (�phases�
hereafter) ranging from calm conditions to the beginning, development and end of a
storm.
During Phase 1, from the beginning of the experiment on the 19th of October at
13:00, until the 20th of October at 6:00am, the wind speed is low (2-7ms-1), the
wave height is small (~10cm), the wave period is quite variable with a relatively
low average of 4sec. The pressure is high which indicates fine weather conditions
(Buckley et al., 2004).
On the 20th of October from 6:00am to 14:00, during Phase 2, the wind blows from
the SSE, resulting in a fetch of approximately 5km in the Tromper Wiek Bay and
70km in the Pomeranian Bight. The wind speed increases, but not enough to cause
an increase in wave height. The pressure begins to decrease during Phase 2.
During Phase 3, on the 20th of October at 14:00, the beginning of the storm is
apparent. From 14:00 to 23:00, the wind continues to blow from the SSE with a
maximum speed of 15ms-1, which is classified as a moderate breeze on the Beaufort
wind scale (Annexe 5). The wave height increases, reaching a maximum value of
1.3m at the end of Phase 3 and the wave period increases at the same time as the
wave height.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
49
Figure 4.3: Pressure and wind at Cape Arkona (the direction indicates where the wind comesfrom); wave height, direction (where it propogates towards) and period (Tz), current (where itgoes) and depth at the station V1 during the experiment (HL = High water Level; LL = Lowwater Level).
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Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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After 23:00 (Phase 4), the wind direction progressively changes from a SSE to a
SW source, i.e. wind blowing from the land. The fetch is greatly reduced (about
2.5km, Figure 1.5) and the wave height progressively decreases until the end of the
storm on the 21st of October at 16:00.
Due to the semi-enclosed nature of the bay, the wave height in Tromper Wiek Bay
is strongly influenced by the wind direction (Klein, 2003). Only if the wind comes
from the east can the wave height increase. A diminution in wave height is
effectively observed since the wind blows from the west. The pressure reaches its
minimum value of 995 dBar during Phase 4, which indicates cloudy or rainy
weather (Buckley et al., 2004).
Next, during Phase 5, the pressure is high, i.e. the weather improves, and the wave
conditions are calm, despite the wind being classified as a fresh breeze on the
Beaufort wind scale, due to the limited fetch.
The wave direction always falls between 250 and 280° during non-storm conditions
and equals 285° throughout the entire storm period (Figure 4.3).
The low wave period (4-5sec) observed during the whole experiment indicates that
the waves are locally-generated (periods of wind-waves = 1-10sec) and do not
constitute swell, which have a much longer period (10s or more, Melville, 2001).
The predicted wave height and period for a wind blowing at 10ms-1 on a fetch on
the whole Pomeranian Bight (70km) in deep water is of 1.3m of 5.5sec respectively
(Annexe 6). However, if a scenario with only the fetch on the Tromper Wiek Bay
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
51
taken into account (5km), a wave height of 0.35m and a period of 2.3s are
predicted. This is, nevertheless, an approximation since the water depth in the
Pomeranian Bight is shallower than 20m and the wind speed varies with time.
However, it does show that the waves recorded during the experiment were
certainly initiated in the Pomeranian Bight and travel to the Tromper Wiek Bay so
cannot be generated in the Tromper Wiek Bay. The wave direction data lends
support to this hypothesis since the waves are coming from the ESE, i.e. from the
Pomeranian Bight.
4.2.2. Water level variations
Water level oscillations with amplitude of about 15cm were observed (Figure 4.3).
These oscillations can be due to tides. The diurnal component of the tide is
dominant in the Tromper Wiek Bay but the semi-diurnal component is not
negligible (M2 + K2 / (S1 + K1) = 0.75, Kantha et al., 2005). However, the period
of these variations is variable (17h, 20h and 15h between two successive low water
levels, Figure 4.3). It is likely that the tides, which are of mixed nature, are
deformed by the local bathymetry and are subjected to water level variations caused
by the wind. Therefore the observed oscillation does not have the same period as
would have produced the tide alone. On the other hand, no variations occur during
the storm but we observe a decrease in the water level during the whole storm
period. This may be due to a storm surge, with wind blowing from the East pushing
the water out of the Tromper Wiek Bay and the whole Pomeranian Bight, thereby
inducing a diminution of the water level. This can have masked the tidal
oscillations.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
52
However, as we know that the water level variations in the Tromper Wiek Bay are
largely influenced by wind surges and seiches (Klein, 2003) we can also envisage
that these oscillations are not due to tides but to a seiche. A seiche is a slow
oscillation of the water about one or several axes (nodes) which can happen in lakes
or basins (Knauss, 1997). There are a variety of ways in which a seiche may be
excited in a natural body of water; one of the most common is the passage of a
storm in which the wind pushes the water level up at the downward end of the
basin. When the wind dies down and the wind stress is removed, the water runs
downslope and the lake surface begin to oscillate. The period T of the oscillation in
a bay is:
ghnlT 4= (19)
where l is the length of the basin, g is the acceleration due to the gravity, h is the
water depth and n (= 1, 3, 5�) is the number of the node.
However, it proves difficult to estimate the length of the basin and the number of
the node. We have seen that during the storm a surge may have been produce by the
wind blowing from the land. The induced decrease of the water level can next
initiate or be super-imposed on the previous oscillations. A basin length of 100km
(from the coast of the Pomerianian Bight to Sweden), a water depth of 70m and
assuming one node is present lead to a seiche period of 4h. Therefore, if the
oscillations recorded are produced by a seiche, it must have been initiated on a
bigger scale than that of the basin from the Pomeranian Bight to Sweden.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
53
We also observed smaller oscillations with a period of 1h and maximum amplitude
5cm. The 1h period recorded may well not be the �real� period of the phenomenon
but is the smallest oscillation period that can be recorded because bursts are taken
every 30min. These oscillations may be due to a seiche in the Tromper Wiek Bay.
The length of the bay from the coast to the depth 20m is about 5.5km, the mean
depth in the bay is estimated to be 13m. Assuming that there is only one node, we
find a period of 32min, which is half the observed period of 1h. Therefore, it does
not seem plausible that these oscillations were caused by a seiche on the scale of
Tromper Wiek Bay.
It is highly possible that the water level variations recorded during the experiment
are in fact a combination of all of these factors, tidal variations, storm surges and
seiches, all oscillations being influenced by the local topography.
4.2.3. Currents
a) Comparison of methods.
We begin by comparing the results given by Wavelog in the statistical files with the
results of the analysis of the high frequency files (Figure 4.4). The results are very
similar with a correlation coefficient of 0.95 for current direction and 0.81 for
current speed for station V1 (approximately the same is found for the other
stations). The slight difference is due to the fact that Wavelog does not separate the
wave and current components to calculate the current characteristics.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
54
Consequently, because the data calculated from the high frequency files are filtered
from the wave component, they better estimate the current characteristics and will
be used for the rest of the study.
b) Currents description and analysis
Before the storm, currents at station V1 at the height of the sensor (0.5m of the
seabed, ~11.5m depth) are very weak (~0.02ms-1), towards the South when the
water level increases and towards the North when it decreases (Figure 4.3).
Therefore these currents are certainly linked to the water level variations, be they of
tidal or other origin. At the beginning of the storm, the currents are slightly stronger
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dire
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from tidestat files
from puvt files
Figure 4.4: Comparison of current speed and direction given in tidestat files and calculatedfrom puvt files; example for station V1.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
55
(~0.05ms-1) and flow towards the NE. At this time, the wind blows from the SSE
which certainly induces a circulation of water inside the bay, pushing the water to
the N. The currents are recorded at the northwestern part the bay and indicate that at
this point the currents flow offshore (Figure 4.5).
During the storm the currents progressively turn and at the end of the storm flow
towards the NW. At this time, the wind is blowing from the SW, pushing the water
out of the bay, the circulation inside the bay induces the currents recorded at the
ABLs position and depth to flows towards the North.
After the storm, the currents flow towards the SSW whilst the wind is still blowing
from the SW. Therefore, these currents are almost certainly compensating bottom
2.5 km
NCape Arkona
2.5 km
NCape Arkona
2.5 km
NCape Arkona
2.5 km
NCape Arkona
Wind
Currents
Before the storm Beginning of the storm
End of the storm After the storm
Figure 4.5: Schematic representation of the currents at station V1 and wind at Cape Arkonabefore, at the beginning, at the end and after the storm. The size of the arrows gives anindication of the speed of wind and currents.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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currents, frequently observed in the Pomeranian Bight (Lass et al., 2001), which
compensate for the water that the wind pushes away.
4.2.4. Suspended Sediment Concentration
The suspended sediment concentration measured at the height of the sensor shows a
relatively constant value of 0.4mgl-1 during non-storm conditions (Figure 4.3). This
represents the background value which is the concentration during non-event
periods.
The SSC at station V1 begins to increase at 18:00, 4 hours after the beginning of the
storm when the wave height is 0.65m. SSC then increases significantly at 21:00
(wave height 0.85m); it reaches a maximum value of 8.2mgl-1 on the 21st at 3:00am
(wave height 1m). It decreases quite rapidly to a value of 1mgl-1 at 8:30am (wave
height 0.45m) and then decreases very slowly to the background value by 21:30, 5
hours and thirty minutes after the end of the storm.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
57
4.3. Comparison of data at the stations
4.3.1. Currents
Currents at station V2 exhibit similar variations to what was seen at station V1
(Figure 4.6) although they are slightly stronger (~20%) at station V2. Therefore
conclusions made for station V1 are also applicable to station V2.
Currents at station V3 are found to be quite different. Their direction is nearly
always northward to northwestward. Their speed is always slower (~50%) than at
stations V1 and V2. Before and after the storm, current speed for station V1 is very
low (around 0.01 ms-1) with very small variations and during the storm, it only
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Figure 4.6: Currents at the three stations during the experiment.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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reaches a value of 0.03ms-1.
Therefore we observed that the currents speed is reduced inside the gravel-dredged
pit compared with outside, as expected since the water depth is greater.
4.3.2. Waves
a) Wave height and period
The variations of wave height and period are very similar for the three stations
(Figure 4.7). However, we found that the wave height is approximately 10% larger
and the period 4% higher at station V3 than at stations V1 and V2.
This finding has no physical explanation since the wave theory predicts a decrease
of wave height with increasing water depth. This could be due to an offset of the
records caused by slightly different calibrations of the three instruments.
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Tz
(s)
Figure 4.7: Wave height (Hs) and period (Tz) at the three stations during the experiment.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
59
b) Directional wave spectra
The estimation of directional wave spectra was calculated using 'Diwasp', a toolbox
of Matlab functions. Directional wave spectra, calculated and plotted for bursts
recorded before and during the storm at stations V1 and V3 (V2 is very similar to
V1), show differences in the wave energy in storm and non-storm conditions.
In non storm conditions (Figure 4.7a. and b., example of burst 30 at stations V1 and
Figure 4.8: Frequency, direction and spectral power density estimated using EMEPmethod. a. station V1 burst 30; b. station V3 burst 30 (calm conditions); c. station V1 burst 75; d. station V3 burst 75 (storm conditions).
Figure 4.9: Wave height and period for the three stations during the experiment. a. b.
c. d.
V1 V3 Non-storm Non-storm
Storm Storm
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
60
V3), we observe that the energy is quite low (with the range of 10-3m2sdeg-1) and
distributed over a large band of directions and periods with an energy peak at
180°N and a period of 5sec.
During the storm (Figure 4.8c. and d., example of burst 75 at stations V1 and V3)
the maximum wave energy is much higher (~100 times higher) and concentrated
upon a narrower band of direction and period (~150-200°) than in non-storm
conditions (~120-270°), even though the peak direction and period are still almost
identical (180°-5sec).
These plots also show energy differences between the reference station V1 and the
station inside the dredged-pit, V3. In non-storm conditions, the maximum wave
energy at station V3 is higher (~50%) and is distributed over a larger range of
direction. It is observed during the entire storm period that the maximum wave
energy at station V3 is higher than at station V1 but is distributed over a much
narrower range of period and direction (e.g. during burst 75, at station V1 the
period, direction and peak energy are respectively 3.5-6.6sec, 150-200° and
0.4m2s/deg and at station V3 are 4.2-6.6sec, 170-190° and 1.4 m2s/deg).
c) High frequency surface elevation
The surface elevation variations of each burst allow one to actually see the waves as
they pass over the sensor (Figure 4.9). Each single wave is clearly identifiable as
well as the waves groups. We can see the progressions of the waves and wave
groups over the three sensors; the waves travel from east to west and are first
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
61
recorded at station V2, next at station V1 and finally at station V3. However, it is
important to remember that the stations are not exactly aligned in the direction of
wave propagation (Figure 3.3). Therefore slight variations between the stations are
likely due to lateral variations of the waves.
22:40 22:4510.5
11
11.5
12
12.5
13
dept
h (m
)
V2
22:40 22:4513.5
14
14.5
15
15.5
16
dept
h (m
)
V3
22:40 22:4510.5
11
11.5
12
12.5
13
dept
h (m
)
V1
Figure 4.9: Surface elevation during Burst 73 (20/10, storm conditions) showing the wavesfor the three stations.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
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4.3.3. Suspended Sediment Concentrations
The suspended sediment concentrations measured during the experiment at the
three stations is shown in Figure 4.10. We observe that the background value is the
same for the three stations (~0.4mgl-1).
The SSC at station V2 is found to be very similar to that at station V1. On the
contrary, SSC at station V3 is very dissimilar to those at the other two stations.
During the storm, SSC at station V3 is almost always greater than SSC at stations
V1 and V2. SSC at station V3 then begins to significantly increase at 19:30, i.e. an
hour and a half before the SSC at stations V1 and V2 begin, in turn, to considerably
increase. Subsequently, the SSC at station V3 is observed to be consistently higher
than the SSC at stations V1 and V2 for the entire storm period.
The maximum SSC value at station V3 is 20.3mgl-1, measured on the 21st October
at 1:00, at which time SSC at stations V1 and V2 is equal to 3mgl-1 and 2.8mgl-1
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2
4
6
8
10
12
14
16
18
20
SS
C (
mgl
-1)
V1
V2
V3
Storm
Figure 4.10: Suspended sediment concentration (SSC) during the experiment for the threestations.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
63
respectively. At the end of the storm, when SSC at stations V1 and V2 decrease
rapidly to a value of 1mgl-1, we note that the SSC at station V3 reaches the same
value simultaneously (at 8:30am) and from then on has the same value as do the
other two stations.
Therefore, it was observed that SSC at station V3, as compared to SSC at stations
V1 and V2:
- has the same background value,
- begins to increase before,
- is higher during the entire storm period and
- decreases simultaneously.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
64
4.4. Sediment mobility
4.4.1. Current only bed shear-stress
The skin friction bed shear-stress due to the current only (equation (2)) was
calculated using the results from the high frequency data and compared with the
threshold of movement (equation (11)). The temperature during the experiment was
5°C and the typical salinity in the Pommerian Bight for the month of October is 8
(Lass et al., 2001); therefore the water density ρ was taken as 1008kgm-3 and the
kinematic viscosity of water ν as 1.5x10-6m2s-1 (Annexe 6).
Soulsby (1983) proposes values of bed roughness length equal to 0.006m for
rippled sand and to 0.0003m for seabed composed of a sand and gravel mixture. We
have seen that the sediment sampled from station V3 was mainly composed of sand
and the video has shown that the bed is rippled; therefore the roughness length is
taken as 0.006m. Sediment samples from stations V1 and V2 are composed of sand
and gravel, so a value of 0.0003m can be used. However we have seen that large
portions of the seabed around stations V1 and V2 are composed of rippled sand,
thus a roughness length equal to 0.006m was also tested for these stations.
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0.05
0.1
0.15
0.2
0.25
0.3
τ (N
m-2
)
τ c (z
0 rippled sand)
τ c (z
0 sand/gravel)
τ critic
Storm
Figure 4.10: Current-induced bed shear-stress (τc) and threshold bed shear-stress (τcr); example for the station V1 for a roughness length z0 of 0.0003m (sand/gravel) and 0.006m (rippled sand).
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
65
The results show that the current induces a very weak bed shear-stress which is
incapable of putting the sediment into movement at any of the stations as well as for
all of the bed roughness lengths used (Figure 4.11, example at station V1, results
similar are found at the two others stations).
4.4.2. Wave orbital velocity
a) Amplitude of the wave orbital velocity under the crest and under the trough
The significant and maximum wave orbital velocity under the crest (Uwcrest) and
the trough (Uwtrough) calculated from the high frequency data at station V1 are
shown in Figure 4.12. The results are similar at the two others stations. It is
observed that the amplitudes of orbital velocity under the crest and the trough are
very similar; the difference between them is found to be very low (Figure 4.13).
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0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
ms-1
Uwsig crest
Uwsig troughUwmax crest
Uwmax trough
Storm
Figure 4.11: Significant and maximum amplitude of orbital velocity under crest and troughat station V1.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
66
The amplitude difference almost equals zero during non-storm conditions. It
increases during the storm and varies from -20 to 20% of Uwcrest for the maximum
orbital velocity and from -6 to 6% for the significant orbital velocity. The variations
at the three stations have almost the same amplitude but different signs. The
average of the difference of orbital velocity under the crest and the trough over the
whole experiment is between -1 and 1% of Uwcrest for all the stations.
Thus we found that the waves are highly symmetrical. Indeed, the steepness
(height/wavelength) is very small throughout the experiment (~0.01).
As a result, the crest orbital velocity alone will be used for the remainder of the
study since the difference between the crest and trough orbital velocities is not
significant. Results derived from the trough orbital velocity were calculated in order
to verify this assumption and were found to be the same as those calculated using
the crest orbital velocity.
20/10 21/10 22/10 23/10
0
0.05
0.1
ms-1
significant
maximum
Storm
Figure 4.12: Difference of amplitude of wave orbital velocity under the crest and underthe trough at station V1.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
67
b) Comparison of theoretical and measured wave orbital velocity
Using the significant wave height given in the statistical files, we calculated the
theoretical significant wave orbital velocity from the linear wave theory (equation
(4)) and compared it to the significant orbital velocity calculated from the high
frequency data (Figure 4.14).
The calculated orbital velocities underestimate by half the measured wave orbital
velocities for the low values, i.e. in non-storm conditions (Uw ≈ 0.025ms-1).
However, they accurately predict the measured Uw for the larger values (for Uw >
0.15 ms-1 the calculated orbital velocity is between 0 and 10% higher than the
measured one). It should be noted that the measured orbital velocities show fewer
variations from one measurement to another than the calculated orbital velocities.
The variations of the latter are found to be essentially due to variations in the wave
height (which are large) and are not attributable to fluctuations of the wave period,
which was found to be relatively constant throughout the storm.
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0.05
0.1
0.15
0.2
0.25
orbi
tal v
eloc
ity (
ms-1
)
Uwsig-crest
Uwsig calculated
Storm
Figure 4.13: Significant measured and calculated amplitudes of orbital velocity during the experiment at station V1.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
68
However, these results show that the difference between measured and calculated
amplitudes of orbital velocity is very small; hence the wave linear theory is
applicable to the waves recorded during this experiment.
c) Differences between the stations
The significant and maximum wave orbital velocities for the three stations are
presented in Figure 4.15.
Wave orbital velocities variations are the same at the three stations; however the
orbital velocity amplitude is smaller at station V3 than at stations V1 and V2,
especially during the storm (~0.05ms-1 smaller). This is effectively what is
predicted by the wave linear theory since the water depth is greater at station V3,
inside the crater, than at the reference stations V1 and V2.
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0.05
0.1
0.15
0.2
0.25
Uw
(ms-1
)
V1
V2
V3
Storm
Figure 4.14: Significant wave orbital velocity for the three stations during the experiment.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
69
4.4.3. Wave-only bed shear-stress
The skin friction bed shear-stress for the waves (equation (5)) was calculated using
the crest orbital velocity calculated from the high frequency data and compared
with the threshold of movement (Figure 4.16).
Figure 4.15: Wave bed shear-stress (τw) calculated for a roughness length (z0) of 0.0003m (sand/gravel) and 0.006m (rippled sand) at stations V1 and V2 and a roughness length of0.006m at station V3.
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0.5
1
1.5
2
2.5
3
3.5
τ (N
m-2
)
τ w
τ critic
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1
2
3
τ (N
m-2
)
τ w (z0 rippled sand)
τ w (z0 sand/gravel)
τ critic
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1
2
3
τ (N
m-2
)
τ w (z0 rippled sand)
τ w (z0 sand/gravel)
τ critic
V1
V2
V3
Storm
Threshold exceeded
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
70
We notice that a roughness length of 0.0003m or 0.006m used to calculate the wave
bed shear-stress at stations V1 and V2 does not alter the result due to the fact that
the wave friction factor, fw, does not change. Indeed in this model the wave friction
factor value is more sensitive to the wave orbital velocity value than to the
roughness length.
We can see that the wave bed shear-stress calculated for station V3 is nearly half
the value of those of stations V1 and V2. This is because the wave orbital velocity
is smaller at station V3. As a consequence the critic bed shear-stress is reached at a
different moment at stations V1 and V2, and V3 (Figure 4.16.
The threshold of movement is exceeded from 17:30 on the 20th to 14:30 on the 21st
at stations V1 and V2 and from 18:00 on the 20th to 13:00 on the 21st for station V3,
i.e. the sediment will be put into motion half an hour later and will stop moving an
hour and a half earlier at station V3 than at stations V1 and V2.
4.4.4. Total bed shear-stress
The maximum bed shear-stress under combined waves and currents (equation 10) is
found to be almost equal to the wave-induced bed shear-stress (Figure 4.17). The
20/10 21/10 22/10 23/100
0.5
1
1.5
2
2.5
3
3.5
τ (N
m-2
)
τ w
τ max
τ m
τ critic
Storm
Threshold exceeded
Figure 4.16: Wave-induced bed shear stress, (τw), maximum bed shear stress for combinedflow (τmax) and threshold bed shear stress (τcr) during the experiment at station V1.
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
71
relative importance of the wave bed shear-stress on the maximum wave shear-stress
(τw /τmax) was found to equal 100% for the three stations during almost the entire
period during which the threshold was exceeded (Figure 4.18). The current
component of the bed shear-stress is found to be important only when the threshold
is not exceeded. We note that the relative strength of the wave bed shear-stress is
sometimes higher than 100%; this is due to the different directions of the waves and
the current which may act to reduce the wave-induced bed shear-stress.
The mean bed shear-stress during a wave cycle almost always equals zero (Figure
4.17), i.e. the current does not diffuse the sediment upwards.
20/10 21/10 22/10 23/100
20
40
60
80
100
τ w
/ τ
max
%
20/10 21/10 22/10 23/100
50
100
τ w
/ τ
max
%
20/10 21/10 22/10 23/100
50
100
τ w
/ τ
max
%
V1
V2
V3
Storm
Threshold exceeded
Figure 4.17: Relative importance of the wave-induced bed shear-stress on the maximumbed shear-stress during the experiment for the three stations.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
72
Waves can effectively put the sediment into motion during the storm; however we
have seen that they are very symmetrical and so will induce a relatively small
amount of sediment transport. This is why the currents, even if they are too low in
magnitude to induce sediment movement or diffuse the sediment upwards
themselves, can play a transporting role once the sediment has been put into
suspension by the waves.
4.4.5. Fraction of sediment in motion
In order to evaluate which fraction of seabed sediment is put into motion during the
storm, we calculated the threshold of movement for the different fractions present in
each seabed sample and compared them with the wave bed shear-stress (Figure
4.19).
At station V1 all the sediment is put into motion but the coarser fraction is in
motion only when the bed shear-stress is maximal.
At station V2, not all of the sediment is put into motion. Sediment coarser than
~4mm (d84) will not move even at the maximum bed shear-stress, i.e. in the gravel
fraction, only the granules (2mm< Ø < 4mm) will move.
We see that all the sediments are put in motion almost simultaneously at station V3
because the seabed sample is entirely composed of sand.
These results indicate that the entire sand fraction is put into motion at all three
stations during the storm, the granule fraction moves when the bed shear-stress is
strong enough (τw > ~1.5 Nm-2) and sediment coarser than this does not move under
any circumstances during the experimental period. However, it is important to
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
73
remember that this indicates which sediment fractions are put into motion but does
not indicate whether the mode of transport is as bedload or in suspension. Indeed,
only the finest part of the sediment in motion can be transported in suspension, the
coarser fraction will move as bedload.
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1
2
3
4
bed
shea
r st
ress
(N
m-2
)
τw
d5 = 0.28224mmd16 = 0.34869mmd25 = 0.38689mm
d50 = 0.5mm
d75 = 0.80107mmd84 = 1mmd95 = 2.4623mm
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2
4
6
8
10
bed
shea
r st
ress
(N
m-2
)
τw
d5 = 0.23326mmd16 = 0.29525mm
d25 = 0.32988mm
d50 = 0.5mm
d75 = 2.4623mmd84 = 4.5948mm
d95 = 9.8492mm
20/10 21/10 22/10 23/100
0.5
1
1.5
2
2.5
bed
shea
r st
ress
(N
m-2
)
τw
d5 = 0.19278mmd16 = 0.24571mm
d25 = 0.26794mm
d50 = 0.34151mm
d75 = 0.41466mmd84 = 0.46329mm
d95 = 0.54525mm
V1
V2
V3
Figure 4.18: Wave bed shear-stress and threshold of motion for different quartile at the three stations during the experiment.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
74
4.5. Suspended sediment concentration
We have demonstrated that sediment movement is entirely due to wave action and
that the current does not diffuse the sediment upwards, therefore the suspended
sediment concentration was calculated using equation (17) valid under waves for a
rippled bed (Soulsby, 1997).
The median grain diameter of the seabed sample was used to calculate the mobility
number and the different Shield parameters. This is due to the fact that bed shear-
stress, ripples length and height, as well as mobility number are functions of the
total sediment distribution at the seabed. On the other hand, the median grain
diameter of the sediment found in the bottles was used to calculate the settling
velocity (equation (13)) because it allowed us to know the size of sediment which is
in suspension at this height. Sediment in the bottles was found to be a mixture of
mud and sand; the median grain diameter is classified as very fine sand at stations
V1 and V3 and coarse silt at station V2. We decided to use equation (13), valid for
sand grain sizes for the three samples since the median grain diameter is just at the
boundary between sand and mud and certainly exhibited non-cohesive sediment
behaviour rather than cohesive sediment behaviour.
For all stations, the calculated SSC does not predict the background value and thus
is found to be zero in non storm conditions.
During the storm the calculated SSC at station V1 is 0 to 4 times higher than the
measured SSC. It is found to predict the increase and decrease in SSC at the same
Chapter 4: Results
Alice Lefebvre University of Southampton School of Ocean and Earth Science
75
time as the observed increase (20th of October at 21:00) and decrease (21st of
October at 8:30am, Figure 4.20).
The calculated suspended sediment concentration at station V2 at a height of 0.55m
is 0-0.5 times lower than the measured concentration during the storm. The increase
in SSC is calculated to occur half an hour after what is observed and the decrease
half an hour before.
Figure 4.19: Suspended sediment concentration (SSC) calculated at the height (z) of eachOBS for the three stations.
20/10 21/10 22/10 23/100
5
10
15
20
SS
C (
mgl
-1)
z = 0.45 m
SSC calculated
SSC measured
20/10 21/10 22/10 23/100
2
4
6
8
SS
C (
mgl
-1)
z = 0.55 m
SSC calculated
SSC measured
20/10 21/10 22/10 23/100
5
10
15S
SC
(m
gl-1
)
z = 0.45 m
SSC calculated
SSC measured
V3
V2
V1
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At station V3 it was found that the calculated SSC underestimates greatly the
measured SSC (4 times lower). It predicts an increase in SSC at 21:30 the 20th of
October, 2 hours later than the observed increase (19:30) and a decrease at 8:00am
on the 21st of October half an hour earlier than the observed decrease (8:30am).
The differences found between the calculated and measured SSC at stations V1 and
V2 may be due, amongst other reasons, to inaccurate estimation of parameters, non-
constant eddy diffusivity or error in the calibration of instruments.
Almost all of the formulas that we used are empirical formulas deduced from
measurements in a particular environment and can lead to inaccurate estimations
when used for other set of conditions.
We observe that each peak in the calculated SSC occurs slightly earlier than what is
measured. This could indicate the influence of currents, which even if there are very
small, can cause slight variations in the eddy viscosity.
On the other hand, the calibration curve of the OBS could be inaccurate and lead to
errors in the estimation of the SSC.
However, it is observed that the calculated SSC at the reference stations V1 and V2
predicts the increase and decrease of the measured SSC at almost the right moment
and both have the same order of magnitude. On the contrary, at station V3 the
calculated SSC is much lower, the increase is predicted to happen 2 hours after and
the decrease half an hour earlier than what is observed.
Chapter 5: Interpretation
Alice Lefebvre University of Southampton School of Ocean and Earth Science
77
CHAPTER 5: INTERPRETATION
5.1. Hydrodynamic effects
The results show that throughout the entire experiment the currents are slower at
station V3, inside the pit, than at the reference stations V1 and V2 and have
different directions. This difference is induced by the greater water depth in the
gravel-pit, which leads, as described by Klein (2003) in the Tromper Wiek Bay, to a
decoupling of the flow inside the crater from the flow above. The current direction
inside the pit, always towards the NW, may be caused by a morphological effect.
Furthermore a significant reduction of the orbital velocity is observed inside the pit
compared with outside the pit. This agrees very well with the wave linear theory
which predicts a diminution of orbital velocity when increasing the water depth.
Therefore it has been found, as expected by theories and previous research (Graca
et al., 2004, Kleinhans et al., 2004), that both wave and current components of the
flow are reduced in the pit and above compare with in the area surrounding the pit,
due to the increase in water depth.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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78
5.2. Effects on sedimentation
The pit seabed has been found to be entirely composed of sand whereas seabed at
the reference stations is composed of both sand and gravel. This indicates that the
gravel extraction induces a change in the seabed nature since the gravel is extracted
whereas the sand is spilt back into the sea and partially refills the pit in the same
area, as described by Diesing et al. (2004). Thereafter the sediments inside the
crater are finer and better sorted than outside.
Results found from the SSC and sediment mobility are summarised in Figure 5.1.
It has been calculated that the threshold of movement is reached half an hour later
in the pit than outside and sediment movement stops an hour and a half earlier
although the sediments are finer, which implies a smaller threshold of movement.
Thereafter the bed shear-stress is smaller inside the crater.
Contrary at what was expected from these results it has been observed that the SSC
at the height of the sensor increases significantly an hour and a half earlier inside
the pit than at the reference stations. Furthermore, the SSC inside the crater is found
to be significantly larger than the SSC outside the pit.
The calculated SSC at the height of each sensor also indicates differences between
the reference stations, V1 and V2, and the station inside the gravel-dredged pit, V3.
It predicts well the increase of SSC at stations V1 and V2 but at station V3 the
increase is observed 2 hours before that which is predicted. Additionally the
calculated SSC underestimates the measured SSC.
Chapter 5: Interpretation
Alice Lefebvre University of Southampton School of Ocean and Earth Science
79
The results indicate that a large part of the sediment is advected from outside the
crater rather that being of local origin. We can put forward the succession of events.
At 17:30 on the 20th the threshold of motion is reached outside the crater and the
bed shear-stress increases rapidly. The sediment begins to move principally as
bedload and the finer fraction is carried in suspension. The SSC at stations V1 and
V2 increases very slightly. Inside the pit the bed shear-stress is exceeded half an
hour later and is lower, here the sediments are transported essentially as bedload. At
19:30, the SSC at the height of the sensor inside the pit began to increase whereas
Figure 5.1: Measured SSC at the three stations during the experiment, time ofsediment movement calculated from the bed shear-stress and time of sediment suspension at the height of the sensor from the calculated SSC.
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5
10
15
20
SS
C (
mgl
-1)
20/10 21/10 22/10 23/100
5
10
15
20
SS
C (
mgl
-1)
20/10 21/10 22/10 23/100
5
10
15
20
SS
C (
mgl
-1)
Predicted sediment movement
V3
V2
V1
Predicted sediment suspension at the
sensor height
Observed increase in SSC
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
Alice Lefebvre University of Southampton School of Ocean and Earth Science
80
this is not predicted and does not occur outside the pit. This can be explained by the
lower waves and current speeds above the pit compared with around it which
induces the sediment put into motion and transported outside the pit to fall into the
pit. A vertical component of the current, enhanced by the depth of the pit, may also
contribute to the increase in SSC inside the pit by pushing the sediment into it. At
21:00 the wave action is strong enough to put the sediment into suspension at the
height of the sensor outside and inside the pit, the SSC therefore increases
significantly. For the entire period during which the sediment is in suspension at the
height of the sensor, the SSC inside the pit is significantly higher that outside and
made up of local and advective sediment. At 8:30 on the 21st, the bed shear-stress is
no longer high enough to maintain the sediment in suspension and therefore, the
SSC decreases significantly inside and outside the pit. It will take 12 and a half
more hours for all of the sediments to fall to the seabed and the SSC to equal the
background value.
Figure 5.2: Schematic representation of the sequence of events during the storm.
The blue arrows indicate the strength of the current, the ellipse represent the action of waves. In the non-dredged zone gravels are present in patchy deposit and the sediment size is bigger than inside the pit.
Chapter 5: Interpretation
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81
Therefore, this proves that due to the less significant wave and current action inside
and above the pit than outside and also due to a vertical component of the current,
the pit acts as a trap to the sediment which was put into motion outside the pit
(Figure 5.2).
Although this experiment demonstrates that the pit acts as a trap for fine sediment,
it has also proven, as described by Diesing et al. (2004), that at this water depth the
sediment is put into motion only when the wave height is relatively high (at least
0.45m), which is true on this coastline only during storm events when the wind
blows from the East. Therefore, the time scale of regeneration may be quite long.
However we also demonstrated that the pit acts as a trap for sediment. In the
Tromper Wiek Bay an important quantity of aggregates has been dredged and in the
northern part, a site of gravel extraction, the seabed is covered by a significant
number of pits. The pits, by trapping the sediment, remove it from its natural
pathway, and may alter the sedimentary budget of the beach. This can result in a
lack of sediment arriving at the coast and hence cause coastal erosion.
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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CHAPTER 6: CONCLUSIONS
This study allowed the description of waves, currents and suspended sediment
concentration in the Tromper Wiek Bay under storm and non-storm conditions. It
was observed that:
- the currents were permanently very low (~0.003ms-1),
- the wave height under non-storm condition was very small (~10cm) as was the
wave period (~4sec) and its increase was strongly dependent on wind direction;
only wind from the east produced an increase in wave height (up to 1.3m), which
was observed at the same time as an increase in wave period (up to 5s),
- the waves recorded in the Tromper Wiek Bay during the storm were locally
generated (generation in the Pomeranian Bay),
- the waves were very symmetrical, i.e. orbital velocities under the crest almost
equalled orbital velocities under the trough, and are well described by the wave
linear theory,
- the sediments were put in motion by waves since the wave height was high
enough (~0.5m), the currents were not found to be sufficiently strong to induce
sediment movement.
Furthermore, the results of this study allowed us to characterise some effects of the
isolated 3m-deep gravel-dredged pit studied:
- the currents were found to be decoupled inside the pit, i.e. current speed was
reduced inside and above the dredged zone and current direction was changed
compared with that of currents around the pit,
Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea).
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- the wave orbital velocities inside the pit were observed to be smaller than around,
as predicted by the wave linear theory, since the water depth increased,
- inside the pit, the seabed sediment size (sand) was found to be much smaller than
outside (mixture of sand and gravel), this was first induced by pit re-filling with
spilled back sediment during the dredging and then amplified by the fact that the pit
acted as a trap for sediment when put in movement by wave action. The pit refilled
with transported fine sediment due to the decrease in waves and currents speed
above and inside the pit.
Although we have proved that the pit is a trap for fine sediments, and will certainly
change the sediment budget towards the coast, we cannot quantify the influence that
it will have on coastal erosion. Therefore further studies could concentrate on the
quantification of sediment trapping and the influence on coastal erosion. This can
be done, for example, by modelling the waves, currents and sediment transport as
done on US coasts by Maa et al. (2004).
Furthermore, it would also be interesting to better evaluate the time scale of
regeneration of the dredged-area. This could be done by a survey of the gravel-pit
over a long time scale (several years at least) using, for example, a repeated Side
Scan Sonar.
Furthermore this research was done on a site of low energy where the threshold of
movement is reached only during storms. Other studies on the effect of pits in
smaller water depth or in high currents environment would increase the knowledge
of marine aggregate extraction impacts.
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Chapter 5: Annexes
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ANNEXES
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Annexe 1: Model to illustrate the formation of relict sand and gravel, example offshore south-eastern Britain (from BMAPA, 1995).
Chapter 5: Annexes
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Annexe 2: Statistical measures of grain size parameters and descriptive terms applied to parameter values (McManus, 1988).
Annexe 3: Sketch showing the Faraday effect, which forms the basis of the electromagnetic current meter. The effect results in a potential difference E= B V L induced between two electrodes (X and XX) with a separation L when a conductor (seawater) moves at a resolved velocity V perpendicular to the line X - XX and perpendicular to a magnetic field with a flux density of B induced by coil C (from Collar and Griffiths, 2001).
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Annexe 4: Sketch showing the principle of capacitive pressure sensors which use a thin diaphragm, quartz or silicon, as one plate of a capacitor. The diaphragm is exposed to the process pressure on one side and to a reference pressure on the other. Changes in pressure cause it to deflect and change the capacitance. The change may or may not be linear with pressure and is typically a few percent of the total capacitance. The capacitance can be monitored by using it to control the frequency of an oscillator or to vary the coupling of an AC signal. It is good practice to keep the signal-conditioning electronics close to the sensor in order to mitigate the adverse effects of stray capacitance. Circuit 6 is a schematic example (from Annexe 5: Constant pressure contours beneath a 100m wave. Water wave is 100m (from Pajala, 2002).
Chapter 5: Annexes
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Annexe 6: A selection of information from the Beaufort Wind scale (from Open University, 2000).
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Annexe 7: Nomogram of deepwater significant wave prediction curves of wind speed, fetch length and wind duration (from CERC, 1984).
Fetch length (km)
Win
d-st
ress
Fac
tor ,
UA, (
ms-1
)
Chapter 5: Annexes
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Annexe 8: Density and kinematic viscosity of water in function of temperature and salinity (from Soulsby, 1997).