GuideSeaandLakeDikes.pdfGuide on Sea and Lake Dikes
(Translation of ‘Leidraad Zee- en Meerdijken’)
Technical Advisory Committee for Flood Defences
December 1999
The Netherlands
2
Our DWW (Road and Hydraulic Engineering Institute) colleague Jan
Muijs passed away on 14 May 1999 during the completion of this
guide. As researcher and advisor Jan made a valuable contribution
to the knowledge collected in this guide in the field of the
erosion sensitivity of grass as dike revetment. Jan was a valued
and versatile colleague. His versatility is no better illustrated
than the painting that he made of the dike at Uitdam along
Markermeer lake.
The inclusion of this painting on the cover of this guide is a
fitting tribute to Jan.
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Indemnity
Preface
1 Introduction 7 Subject, objective, status, advice to readers,
overview of guides
2 Social Framework 9 2.1 General 2.2 Safety 2.3 Landscape, nature
and cultural heritage (LNC) 2.4 Other functions
3. The System of Flood Defences 13 3.1 General 3.2 The types 3.3
The categories
4. Care of Flood Defences 23 4.1 Introduction 4.2 Management cycle
4.3 Area-specific knowledge 4.4 Interrelationship with spatial
planning 4.5 The development of a vision and environmental impact
reports
5. Dimensioning 29 5.1 Introduction 5.2 Design of the cross section
on the basis of the ‘water retaining’ function 5.3 Design of the
cross section based on the other functions 5.4 Revetments 5.4.1
Introduction 5.4.2 Methodology for selecting a revetment 5.4.3
Transitional structures 5.5 Connecting structures 5.6 Manmade and
artificial structures and objects 5.6.1 General 5.6.2 Furniture and
fencing 5.6.3 Buildings 5.6.4 Vegetation 5.6.5 Windmills 5.6.6
Roads 5.7 Particular structures 5.7.1 General 5.7.2 Boulevards
5.7.3 Moles 5.7.4 Sand dikes 5.8 Layout with respect to day-to-day
management
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6. Implementation 56 6.1 General 6.2 Land acquisition 6.3
Accessibility of the structure and the surrounding area 6.4
Extraction and processing of materials 6.5 Summer preconditions 6.6
Specific selection and permit criteria 6.7 Concord between design
and implementation 6.8 Revision and maintenance
7. Day-to-day Management 57 7.1 Introduction 7.2 Monitoring
functions 7.3 Evaluation 7.4 Fixed and variable maintenance 7.4.1
Introduction 7.4.2 Strategies 7.4.3 Planning
Annexes I definitions II Symbols III Documents preparation IV
Compensation Principle V Realisation of the Guide on Sea and Lake
Dikes
References
Software
5
Indemnity
The Technical Advisory Committee on Water Defences has compiled and
arranged the data included in this publication with the greatest of
care. These data represent the state of the art at the moment of
publication. Nevertheless, no guarantee can be given that incorrect
information is not included in this guide. Users of this
publication accept the risks related to this. The TAW is unable to
accept any liability for damage that may occur as a result of the
use of these data on behalf of itself and the individuals who have
worked on this publication.
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Preface
No national guideline for the design of sea dikes has been issued
since the publication of the Delta Commission’s report in 1960. A
specific design guideline has never existed for lake dikes. The
Technical Advisory Committee for Flood Defence (TAW) has filled
this void by collating current knowledge and policy objectives in a
Guide on Sea and Lake Dikes. The decision to handle sea and lake
dikes in one guide is based on the similarities between the two
types of flood defence.
The Guide on Sea and Lake Dikes addresses the dimensioning of flood
defences in accordance with the present safety approach based on
the thoughts of the Delta Commission, and takes account of the
newest insights in the field of geo-technology and constructional
hydraulics. This applies to the calculation of wave run-up and
overtopping and the determination of overtopping discharge. The old
2% wave run-up requirement has been replaced by critical
overtopping discharges. In addition, new insights have been
processed with respect to the geo-technical dimensioning of flood
defences and handling objects in the flood defence. The earlier
guides for the design of river dikes and other flood defences have
been adapted for dimensioning wherever possible. The Guide on Sea
and Lake Dikes is explicitly geared to Grondslagen voor Waterkeren
(Guide on Fundamentals on Water Defence, 1998).
The increased appreciation for the dikes as recognisable elements
in the landscape and for nature and cultural historical values of
and on dikes has also been given a place in this guide. New
developments, reinforcement and maintenance of the flood defence
can effect the other functions of the dike. Inventories and
evaluation of functions, development and consideration of other
solutions are gradually becoming the common property of modern
flood defence managers. The Guide on Sea and Lake Dikes supplies
instruments to achieve integrated flood defence management.
The Guide on Sea and Lake Dikes also has a number of white and grey
patches, including the fall in the ground level due to salt, gas
and oil extraction, the time-dependent description of piping,
connecting structure, trembling and settlement slides. The study
programme of the Road and Hydraulics Engineering Division (DWW) is
dedicated to filling this knowledge void in order to improve this
guide further.
The Guide on Sea and Lake Dikes is the first new integrated guide
as referred to in the Grondslagen voor Waterkeren report
(Fundamentals on water Defences). In the future, it will be
possible to find all the technical know-how needed for the design
of (parts of) flood defences in the various Technical Reports. For
example, the information contained in the Guide on Sea and Lake
Dikes Design Basis Memorandum will be included in the Technische
Rapport Waterkerende Grondconstructies (Water Retaining Soil
Structures Technical Report) to be published in 2000. The Design
Basis Memorandum will then no longer be valid.
The publication of this guide completes the series for all types of
flood defence. A significant step has been taken in the unification
of the foundations for design and management of the flood defences
in the Netherlands. Testing of these foundations and the
accompanying instruments in practical situations and improvement by
using the results of studies will be given much attention in the
next few years.
Ir. W. van der Kleij TAW Chairman The Hague, December 1999
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1. Introduction
Subject This guide deals with the realisation and maintenance
process for sea and lake dikes, dams, separation embankments and
compartment dikes, in so far as they are primary flood defences.
This guide is part of a coherent series of guides on flood defence
(see table 1.1) and builds on the general TAW memorandum Gronslagen
voor Waterkeren (Fundamentals on water defences) [2], shortened to
Fundamentals in this publication.
Objective This guide provides recommendations for the management of
the dike or dam and the direct vicinity that enables the optimum
use of the various values and functions that the dike or dam can
have, with unconditional observance of the preconditions that apply
to the water retaining function.
Status The use of guides is recommended in Article 5 of the Flood
Defences Act [1] to everyone who is charged with managing or
supervising a primary flood defence.
Advice to readers The fundamentals and this guide must be used
together. - The fundamentals and chapter 2 of this guide provide
general information . They describe the social framework, handle
the elaboration of the concept of ‘safety from flooding’ and give
more details on multi-functionality and managing LNC (landscape,
nature, cultural) values.
Chapter 3 handles the system of flood defences. The following
chapters handle all aspects of the lifecycle of a sea and lake
dike.
- Chapter 4 handles the water retaining care, with the cyclical
process of management, as shown in the flowchart in figure 4.2.1.
The activities in this flowchart are looped. Often one part must be
dealt with in anticipation of a following phase. An example is the
(general) dimensioning of options. It is also indicated how any
link can be made to an environmental impact procedure. - Chapter 5
handles dimensioning. - Chapter 6 contains a number of points for
attention with respect to implementation. - Chapter 7 handles the
day-to-day care of the flood defence: control of the functions
after construction and evaluation of the actions this
necessitates.
The background information needed is so extensive, especially for
chapter 5, that it has been collected in a separate Design Basis
Memorandum. There are repeated references to this Design Basis
Memorandum on Sea and Lake Dikes in the guide. Its contents are
brought up-to-date in Technische Rapport Waterkerende
Grondconstructies (Water Retaining Soil Structures Technical
Report) [12] which will replace the Design Basis Memorandum once it
is published in 2000.
Overview of Guides The Technical Advisory Committee on Water
Defences (TAW) publishes a coherent series of guides. The
connection between the Guide on Sea and Lake Dikes and the other
TAW guides and publications is illustrated in Table 1.1.
There are two types of guide.
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The integral guides provide general rules important for all types
of flood defence. For example, Fundamentals covers the social and
administrative framework, the safety and multi-functional approach,
a general description of the failure mechanisms, dimensioning
techniques and other generally applicable matters. The fundamentals
are the hinge connecting the Flood Defences Act and the other TAW
guides. Leidraad Toetsen op Veiligheid (Guide on Safety Monitoring
for Water Defences) provides the calculation rules for the five
years safety test of the primary flood defence, which occurs within
the framework of the Flood Defences Act.
For every type of flood defence the guides provide flood defence
managers with instruments with which they can carry out their
management task. The design, management and maintenance of each
type of flood defence are given. In principle, these guides can be
used independent of each other. Any general aspects, such as
materials and background information is handled in separate
publications, Design Basis Memorandums or technical reports.
Table 1.1 Interrelationship TAW guides and publications/technical
reports Integral guides
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2. Social Framework
2.1 General Law, the administrative organisation and the
implementation of policy are the solid foundations of flood defence
in the social order. Within this framework the objectives are set
and the means specified, which are described in the fundamentals
[2].
Flood defences border or are part of water systems. Attention must
be given to the quality of the ground and surface water and water
floors in the construction and maintenance of a flood defence.
Functions are determined water system by water system with target
situations (and requirements based thereon) which must be met (in
time). These functions stand for the area in national plans
(including the Management Plan for National Waters, [14]) examined
in this guide, regional plans and zoning plans. This chapter
handles the safety, landscape, nature and cultural heritage
functions and a number of additional ones.
2.7 Safety Dealing with the concept ‘safety from flooding’ is a
theme of the Fundamentals. For the manager the concept of ‘safety’
must be measured and unitised, otherwise it cannot be used. Rules
are need to measure and use this safety and three aspects are
important in this: (A) the social norm; (B) the preconditions
(loads on the structure); (C) the strength of the structure.
These three aspects have all undergone development, and this
development naturally continues, making the interpretation of the
concept of ‘safety’ a continuing process. The three aspects undergo
permanent study and evaluation. This evaluation shows whether and
when the administration should be advised to adapt the safety norm.
The Flood Defences Act [1]divides the Netherlands into dike
enclosure areas with the accompanying legal safety norm (aspect
(A)); this is handled in fundamentals. This is the basis of
determining which flood defences should be classified as primary
flood defences and which norm should be used for which category of
flood defence.
The loads derived from the legal norm (aspect (B) for outside
waters are made available every five years by ministerial decree.
See the memorandum Hydraulische randvoorwaarden voor primaire
waterkeringen (Hydraulic Boundary Conditions for Primary Flood
Defences) [13].
For primary defences that do not directly retain outside water the
norm, until the moment that the inundation norm is fixed, is that
they must offer the same degree of safety as the day the Flood
Defences Act came into effect (15 January 1996).
Managers who want to anticipate the future situation when carrying
out necessary modifications must underpin, determine and register
the hydraulic conditions themselves. It is recommended that this be
done in consultation with the Helpdesk (at the Directorate-General
for Public Works and Water Management, Road and Hydraulics
Engineering Division, Delft).
The soil mechanical conditions and the current state of affairs
with respect to aspect (C) are addressed in chapter 5.
No safety norm has yet been given for the non-primary flood
defences (regional flood defences), which could now become
important in limiting the risks of flooding by outside water. These
are the flood defences that used to be referred to as ‘secondary’
flood defences or compartment dikes
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and as such also enjoyed the protection of public law. Drainage
canal embankments are also regional flood defences. A study of the
role of a non-primary flood defence in limiting the consequences of
the collapse of a primary defence must show whether its existence
is desired and if so which functions can be allocated to the
defence. These include limiting the inundated surface area,
retaining drainage canal water, limiting inundation in height and
scale, supply road and escape route or limiting tide storage in the
case of dike repair. In determining the loads to be calculated a
distinction must be made between the loads occurring directly after
the collapse of the primary defence and the load occurring in the
period following in which the primary defence is collapsed. In the
first case preconditions must be used that are derived from the
design preconditions for the primary flood defences with the help
of a plausible scenario for the collapse and the consequent
development of the inundation. In the second case, selection of an
exceedence frequency between 0.1 and 0.5 per year is recommended
for the outside water at the site of the collapse of the primary
defence. In both cases account must be taken of such local effects
as wind gusts and wave run-up. The large scale overflowing of the
defence need not be ruled out. This is in anticipation of a
fully-fledged norm setting in accordance with Article 3 of the
Flood Defences Act.
If the presence of the defence is no longer desirable it must be
taken out of service. The defence may have to be (partially)
demolished.
If the compartment dike fulfils a function in the security system
of a dike enclosure area to combat flooding (an escape route for
example), the dike must be included in the management plan, the
register and the management register.
The determination of the safety level of the regional flood
defences is the responsibility of the provincial authorities.
Stipulations in relation to these defences (to be determined later)
are found in the provincial flood defence regulations.
It is very important that the manager of the flood defence ensures
that necessary flood defence raising and reinforcement in the
future remains possible and that he can enforce those things
essential for the water retaining capacity in the area between the
structure’s stability borders.
2.3 Landscape, nature and cultural heritage (LNC)
There are no generally accepted integral assessment criteria for
the LNC values (including archaeological and geographical values).
Weighing up is not a process that is based on fixed objective
formulas, but rather the setting of aims and the making of choices.
By formulating aims and arguing choices the relationship between
the care of flood defences (construction, improvement and
day-to-day management) and the specific characteristics of the area
becomes clear. Bearing in mind that the setting of priorities and
aims is area-specific, it must be carried out for every dike route.
The way in which this is realised is set out in the
fundamentals.
2.4 Other functions
Agriculture and horticulture The assessment of a design can be
twofold on this aspect. On the one hand, agricultural and
horticultural land may be lost as a result of intervention, on the
other, the grass slopes and berms are usually used for agrarian
purposes. The construction of a flood defence may lead to a change
in the parcelling or accessibility of land. There may also be an
indirect link between intervention and consequences for agriculture
via water management.
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The degree to which the desired hydraulic quality of the grass can
be reconciled with the agrarian industry is found in chapter
5.
Recreation The waterside is a much used recreational area. Besides
such things as accessibility for cars, bicycles, anglers and horse
riders, facilities for fishing boats and vessels are also part of
the joint use of the flood defence. The aspects for assessment are:
- the degree of damage to (current) recreational facilities (size,
accessibility); - potential for recreational developments; -
sensitivity of the defence to damage due to recreational use.
Industry Industrial sites often have a water-bound location,
because they need water for production processes or for transport.
This leads to extra dike road and pipe crossings. In a number of
cases the flood defence is constructed behind such complexes or the
site is constructed on the seaward side of the flood defence
structure. The site itself, and the installations established in it
typically offer more limited safety from flooding. That is often
the result of a conscious decision by the business and the flood
defence manager can have some input here. This position on the
outside of the dike must be examined thoroughly by both parties.
The aspects for assessment are: - changes to the drainage system; -
changes in water levels and in discharges; - flooding by (salty)
seepage and overtopping water.
Traffic/transport
- road traffic A flood defence’s traffic function usually develops
‘naturally’ in relation to the high, and so dry, location of the
inside berm or crown of the defence structure. In the past the dike
proved to be the shortest route between two residential areas
situated on the flood defence. The traffic function sets a number
of specific demands. In the first place, a certain degree of space
is taken up by this function. The breadth of the road, perhaps with
parking strips and verges, will claim extra metres of the cross
section. And if traffic approaches from the side then connections,
crossings and suchlike will be needed, all of which must not affect
the water retaining capacity. It is often necessary to allocate a
relatively large amount of space for access and exit roads, which
is one of the decisive factors in the selection of the route. This
results in the following aspects for assessment: - space for
supplementary facilities (parking strips, verges); - connections,
crossings, access and exit roads; - traffic safety; - separation of
traffic functions; - traffic load.
Both the construction phase, temporary closures for instance, and
the final situation will have to be considered.
- shipping Wherever channels hug it, a dike will be subject to wave
attack caused by shipping. Flood defence and channel crossings also
necessitate the construction of navigation locks.
The following points for attention must be considered in relation
to shipping: - the effect on loading and mooring facilities;
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- the risk of collision with dikes.
Housing Building on, along or in the vicinity of flood defences has
always been a very specific problem, which can have great
consequences for security. An establishment location where land and
water meet has often been determined by historical development. As
a result the function of water retaining on the one hand and
housing and work on the other are often tightly interwoven. As
such, existing buildings have typically stood where they stand
since time immemorial. In some cases a continuous water retaining
profile is ingeniously constructed around the buildings, and a
number of construction details are accentuated that place very
special demands on the buildings. An ideas about retaining these
buildings must be addressed very carefully (see chapter 5). The
points for attention are: - changes to the quality of habitation
(enjoyment, views, transecting a residential area, for example); -
changes to building patterns (number of houses to be pulled down);
- changes in the accessibility of houses.
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3. The System of Flood Defences
3.1 General A flood defence has a characteristic place within the
water system. Those characteristics (norms, preconditions,
functions, defence type and form) are determined by the development
history of the dike enclosure area.
The development of the flood defence has had a steering influence
on the forming of the Dutch landscape and the ecosystem. The
interweaving of the flood defence function and other functions of
the defence are of all time: besides safeguarding against flooding
the flood defences also serve as road connections, as residential
area, as a mechanism to prevent the water becoming brackish and to
retain freshwater for cattle and agriculture. Furthermore, the
flood defence structures should also offer the possibility to
discharge and let in water and allow shipping to pass through. The
construction of dikes has played a role in the creation of salt
marshes and mud flats. New nature areas were created as a result of
land reclamation works, a good example of which is the
Oostvaardersplassen. Consequently, the flood defences also fulfil
an important cultural-historical information function.
Flood defences were sometimes temporary phenomena in the process of
bringing the land under cultivation and land reclamation. Now the
defences that are still functional, are designed and managed ‘for
eternity’. That does not mean that the structure must have an
eternal lifespan, but that a flood defence will be necessary in
those places where there is one at the moment. It is to be expected
that in the future we will need the flood defences even more than
we do now, bearing in mind the rise in the sea level and the
continuing fall in the ground level.
In the past century the safety level offered has risen
substantially. The rise in the safety level means a transition from
tangible to abstract safety and generally leads to habituation and
carelessness. As the tangible safety recedes further into the past
due to the lack of (near) disasters, the ‘third generation’ problem
comes into play: the generation that only knows of the disasters on
the basis of hearsay, swings into action against interventions it
considers absurd, especially when the works encroach upon rural
life in an environment where inhabitants feel safe and
secure.
The rising safety level has unfortunately disrupted the equilibrium
in the other functions. The necessary interventions in the existing
situation became ever bigger. As disasters were usually the
stimulus, the reinforcements were welcomed without too much
opposition well into the seventies. The closing off the Zuider Sea
was an exception, when the Zuider Sea fishermen saw their means of
making a living taken from them while not finding enough support in
building up their new lives.
By the end of the sixties the feeling was growing that, when
planning necessary interventions in the environment, it was not
only important to consider (economic) functions (such as ‘dry feet’
and ‘less brackish water’) that could be directly improved, but
also the consequences to nature and landscape. In the Delta Plan
attention centred on the closing off of the Oosterschelde. Among
other things, it was pointed out that the sea arm had a significant
nature function and that the Oosterschelde should be kept open. The
Klaassesz commission was appointed. After six months of intensive
investigation a report was published, showing that a compromise was
possible in the form of a movable storm tide defence structure. The
decision to build was taken by the second chamber of the Dutch
parliament in 1974. This structure proved invaluable to the
introduction of integral water management at the end of the
seventies.
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A discussion was started around the dike reinforcement works along
the Wadden Sea about the continued implementation of the age-old
three step land reclamation-salt marsh forming- enclosure process.
Against the backdrop of the expectation that there would be
scarcely any new forming of these areas, so important to nature,
after the enclosure of the salt marshes and mud flats, nature
protection agencies saw that an important loss of nature values was
ongoing. On the other hand, the earlier need for new agricultural
land was being opposed with increasing force. In Friesland after
long discussions it was decided to carry out the dike improvement
in 1993 in accordance with the existing route and to reform the
land reclamation into ‘salt marsh works’. Allowing salt marshes to
form and their preservation are central factors. Summer embankments
were even cut in front.
Within the scope of nature development, mentioned in part in the
Nature Policy Plan of the Fourth Memorandum on Water Management
(NW4), it may be desirable to permit tides in the polder. This is
called polder removal. The purpose of removing polders is to regain
important international nature values which are located on the salt
marshes and mud flats and are linked to the tides. Many of these
valuable salt marshes mud flats were lost due to the implementation
of the Delta Works.
In practice, removing polders means that the dike enclosure is
relocated to another site. Due to the presence of former dikes, in
Zeeland there are often possibilities in the vicinity of current
site of the flood defence. In the area in which the polders have
been removed lay out measures will have to be introduced in
combination with leaving the outside enclosure to develop the
nature values aimed for.
Removing polders may also be important in relation to the widening
of the flow area of a sea arm for safety reasons, for example the
Westerschelde.
The point of departure for the policy is the integral development
of the water system oriented to sustainability. That means that
environmental and liveability motives (should) play just as great a
role as economic motives. Flood defences are part of the water
systems. The following points for attention, some derived from the
Fourth Memorandum on Water Management are important: - for
centuries, protection against high water has almost synonymous with
flood defence construction and maintenance; however, sustainable
high water protection can best be realised by working with natural
processes as much as possible; humankind takes a step back to give
rivers, estuaries and coasts more space; that means responsible
building policy and timely reservation of land that will probably
be needed to maintain safety levels in the future; - for the Natte
Hart (the Wet Heart of the Netherlands: IJsselmeer, Markemeer,
Randmeren) nature development can be stimulated by constructing a
brackish water zone along the IJsselmeer closure dam and
constructing natural banks along the other parts of the Natte Hart
in combination with dike reinforcements; attention must also be
given to a more flexible water level management and the
consequences thereof.
In chapter 2 this is addressed in more detail for the various
functions that play a role in the management of the flood defence
structure.
3.2 The types Under the generic term ‘sea and lake dikes’ this
guide handles the dikes and dams on the seaward side of the sphere
of action of Leidraad Benedenrivieren (Guide on River Dikes – part
II: Lower River Area) [5] (figure 3.2.2).
The following types exist:
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- sea dikes; - lake dikes; - connecting flood defences: closure
dams - connecting flood defences: compartment dams - separation
dikes; - compartments dikes.
Closable defences fall under the sphere of action of Leidraad
Waterkerende kunstwerken en bijzondere constructies (Guide on
Water-retaining Structures and Special Objects)[4].
The most characteristic qualities of the flood defences are
summarised in figure 3.2.1.
Figure 3.2.1 Functions and characteristic location of
dikes/dams
In practice, it is not so that the characteristics mentioned are
found over the whole dike profile. They are often expressed in
different ways on the various construction elements (underwater
bank, foreland, dike body, inside berm).
The characteristics of the flood defence also occur in different
ways in relation to the materials used in the dike construction.
Both aspects emphasise the versatility of the structure and so the
varied relationship with the immediate vicinity of the flood
defence.
+ Sea dikes (figure 3.2.3) are found in the northern provinces on
the Wadden Sea, Eems and Dollard, and in the head of Noord-Holland.
Along the North Sea coast the flood defences chiefly consist of a
combined system of first bank, beach and dunes, in which a number
of artificial protection forms are found on a small scale, such as
dune foot protections, beachheads, bank structures and quay walls
(boulevards). This dune coast is only interrupted by the sea dikes
at the Hondsbossche seawall, the Pettemeer seawall, IJmuiden,
Katwijk, Scheveningen, Stellendam, Flaauwe Werk, the Westkapelse
sea dike and the Westzeeuwsvlaamse dikes. The dikes along Nieuwe
Waterweg on the seaward side of the storm tide defence structure
and the adjacent Europoort defence structure up to the Brielse
Gatdam also belong to the sea dikes.
+ Lake dikes (figure 3.2.4) are found along the inside lakes formed
by the closure dikes. These include the dikes along the former
Zuider Sea and around the IJsselmeer polders, Grevelingen,
16
Veerse Meer, Markiezaat, Volkerak/Zoommeer and
Eendracht/Spuikannaal Bath/Antwerp canal section.
The dikes on the riverwards side of the storm tide defence
structure in Nieuwe Waterweg, on the riverwards side of the
Europoort defence structure, and around Haringvliet and Hollansch
Diep fall under the sphere of action of Leidraad
Benedenrivieren.
+ Connecting flood defences (closure dams and compartment dams,
figure 3.2.5) are found between the Wadden Sea and IJsselmeer
closure dam, between IJsselmeer and Zwarte Meer (Ramspol), between
IJsselmeer and Markemeer (Houtribdijk) and in a number of former
tidal channels in the Delta area (Europoort, Haringvliet,
Brouwershavense Gat, Oosterschelde and Veerse Gat). Further
compartmentalisation was realised through the construction of the
Philips dam, the Oester dam, the Zandkreek dam, the Grevelingen
dam, Markiezaatskade, the Hellegats dam, the Roggebot lock, the
Nijkerker lock and the Kadeolen defence structure.
+ A few separation dikes and compartment dikes (figure 3.2.6)
without a direct water retaining function have been designated as
primary flood defences in order to realise a system of closed dike
ring areas or to compartmentalise sizable dike ring areas.
Separation dikes separate two dike ring areas with a different
safety norm (the dike between the Wieringermeer polder and the head
of Noord-Holland, the Linde dike between Friesland and Overijssel;
the defence structure along the national border south of Nieuwe
Statenzijl).
Compartment dikes are found between dike ring areas with the same
safety norm (between the Noordoost polder and Friesland). In the
future, these will also include any regional flood defences to be
upgraded to primary flood defences (including former compartment
dikes and secondary flood defences) as mentioned in chapter
2.
For a number of areas in the Netherlands, the primary defence
structures are complemented with a system of ‘secondary flood
defences’ or secondary dikes. Particularly in Zeeland the driving
idea is the probability of damage as a consequence of disasters
(dike and bank collapse). The provincial flood defence regulations
include stipulations on how these regional flood defences, which
still have to be designated, must be used. This is in anticipation
of the continued development described in chapter 2.
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18
21
22
3.3 The categories The Safety Monitoring Guide [3] gives the
flowing classification of categories for the primary flood
defences, on the basis of the type of water body to be retained and
the location in relation to the dike ring area to be protected. (1)
belongs to the system that directly surrounds the dike ring area
and retains outside water; (2) is as category (1), but is not meant
to directly retain outside water; (3) lies in front of the dike
ring area and retains outside water; (4) is as category (3), but is
not meant to directly retain outside water; (5) is one of the
categories (1) through (4), but is beyond the national
borders.
This classification is based on the fact that safety testing is
directly related to the Flood Defences Act. For management
purposes, the choice of classification is freer and so this guide
is in line with the hydraulic situation and the design of the
dike.
The inter-relationship between the type of water defence and the
categories is shown in figure 3.3.1.
Figure 3.3.1 Relationship between dike/dam type and category
23
4. Care of Flood Defences
4.1 Introduction Management comprises all activities to guarantee
that the functions that the flood defence has can be fulfilled.
Interpretation is based on a vision on the various functions and
how to approach them. The provincial flood defences regulations
include the obligation to draw up a management plan, in which the
vision and its implementation are recorded (see
Fundamentals).
This chapter shows the management cycle of a flood defence. Section
4.3 details the area-specific knowledge needed, followed by an
examination of the relationship with spatial planning in § 4.4. The
synthesis between addressing a structure’s inadequacies and
retaining or developing values is shaped within the development of
the vision and the environmental impact report. The steps to be
followed are found in § 4.5.
4.2 Management cycle The distinct parts of the realisation and
maintenance process are: - day-to-day management based on the
safety function; - day-to-day management based on the other
functions; - large-scale adaptations or new developments.
(Day-to-day) management based on the safety function The Flood
Defences Act lays responsibility for the maintenance of the
quantified safety level with the manager. The Leidraad Toetsen op
Veiligheid (Monitoring of Safety)[3] indicates how the safety level
is tested every five years. If testing shows that the dike does not
meet the legal norm, reinforcement or new development, not always
on the same site, is needed (figure 4.2.1). Care consists of all
responsibilities and instruments for policy selection, design,
construction and maintenance of the flood defence.
First responsibility rests with the flood defence manager (water
board or regional director of the Directorate-General for Public
Works and Water Management). The province supervises the care and
the integration of safety with other institutions. The state
creates the legal and policy frameworks and has supreme
control.
(Day-to-day) management based on the other functions Besides the
main function, safety, which a flood defence always has, a dike
section is usually also given one or more of the other social
functions landscape, nature, cultural heritage, traffic and living.
Knowledge of these functions and the setting of priorities is
needed to gain a picture of where the care should be directed. The
other functions are designated in plans and policy memorandums
issued by state, province and municipality. Many dikes are included
in the Ecological Main Structure of the Environmental Policy plan.
That means that it has acquired great significance in the
preservation of the nature it accommodates or in the development of
new nature by technical management of the environment. The
functions of dikes and dams that lie along managed waters around
the whole country are included in the Management Plan for National
Waters [14]. At provincial level the functions are often included
in the Regional Plan. Some functions are included in more detail in
a provincial environment and landscape policy plan for example, or
in a LNC guideline for dikes and ultimately in the municipal zoning
plans. Failure to fulfil one of the other functions will be
addressed in first instance by adapting day-to-day management. If
this fails to have the desired effect, repair measures may prove
necessary, after a multifunctional consideration on the basis of
the management plan. With respect to the other functions, the
following are valid reasons to structurally adapt the flood
defence.
24
+ small-scale adaptations: - expansion of joint use, in the form of
accessibility and passability of the outside berm/crown by
improving roads, construction of extra steps/dike crossings and
revetments/metalling for instance; - changing of the selected
management form for the grass slopes, for example production grass,
a recreational sunbathing area, an environment-oriented management
form; - maintenance of the grass slopes (grass length versus
possible uses). + large-scale adaptations: - relocation of the
flood defence, as a consequence of the construction of a harbour
for example; - reintroduction of joint use by the construction of
beach nourishment for dikes, as a consequence of a reduction in the
height of the foreland (a beach for example).
Large-scale adaptations or new developments In the future,
large-scale adaptations or new developments, such as those
introduced in the past few decades, are expected to be needed only
occasionally in the Netherlands. Examples of situations which
necessitate new development are: - when diking in and moving the
coast; - when constructing pump accumulation installations; - at
artificial islands; - when adapting compartmentalisation within
dike enclosures; - closure and connecting dikes; - large-scale
dredging depots; - removing polders to stimulate nature
development.
Cycle The adaptation of the flood defence can accordingly vary from
the modification or renovation of part of a dike or bank revetment
to the full construction of a new dike section. In all cases there
are several solutions from which a choice must be made. This is
part of a flood defence’s realisation and maintenance process
cycle, as shown in figure 4.2.1.
The activities in the flowchart are looped. Often the activities of
a following phase must be carried out in advance for the sake of
one component. An example is the (rough) dimensioning of the
options. The directional, form dimensional and constructional
requirements an existing primary flood defence must fulfil are
recorded in the register. The legal (by-law) restrictions are also
included. On the basis of the content of the register, it can be
determined which loop should be used: - maintenance works that
conform to agreements recorded in the register follow loop (A); -
works that necessitate a change to the register follow loop
(B).
When constructing a new flood defence loop (B) must be followed.
The first activity is typically the evaluation at the bottom of the
flowchart.
25
Figure 4.2.1 Realisation and maintenance process (management
process) of a flood defence
4.3. Area-specific knowledge In addition to knowing the objectives
anchored in the policy of the various administrative layers,
area-specific knowledge is also needed to draw up a flood defence
improvement plan. That knowledge is held by the flood defence
manager. He knows in which places and in which respect the flood
defence fails to fulfil the safety requirements. He knows which
functions have been allocated to the flood defence in terms of
planning. He also knows how each part of the flood defences will be
used in social terms (housing, traffic, industry, agriculture,
recreation or nature). The province and the state also hold a lot
of this knowledge, in the provincial environmental inventories for
example, the national council for the preservation of monuments and
historical buildings or archaeological research records. Private
nature and environmental organisations are also an important source
of information. Some of them are national organisations, such as
the Society for the Preservation of Nature Reserves in the
Netherlands, Bird Protection; others are
26
provincially oriented, including the Provincial Nature Conservation
Society, or local, such as history circles and nature groups.
4.4 Interrelationship with spatial planning Just like any other
infrastructure work, space must be found for flood defences in our
living environment, including built up areas. We live with, near to
or on flood defences. It is this interweaving that makes
modifications to flood defences so difficult. The freedom to modify
flood defences to fit the contemporary demands (and so often
relatively high water levels) is rigorously constrained by other
spatial demands, and sometimes even excluded. As well as a
hydraulic problem there is a planning problem, with the
accompanying social-cultural problems.
Formally, dike users (so also residents) have a permit for use
specifying that they must leave if the dike has to be reinforced.
In practice, this stipulation is not easily enforced. This issue is
on a totally different scale than the hydraulic aspect would have
us believe.
A solitary structure has a (technical) lifespan of approximately
50-100 years. At the end of this period the new design requirements
can be used to design the replacement structure. That implies that
the relative rise in the water level expected in the coming 50-100
years must be taken into account in the design of a solitary
structure.
In a complex planning situation however, the lifespan of the
individual structure is no longer important. The conglomeration of
buildings actually fixes the whole situation at today’s standards
until an unspecified time well into the future and the demolition
of individual premises in favour of individual rebuilding or
renovation precludes the possibility of revising the situation
painlessly. Usually disasters are needed to stimulate an integral
adaptation of the situation. The fact that Rotterdam was the target
of bombardment in 1940 and was hit by flooding in 1953 made the
integral improvement of its flood defence possible in practice. A
second round of improvement (necessitated by higher design
standards) now proves impractical, also because the choice has
fallen on a storm barrier defence structure in Nieuwe Waterweg
(Rotterdam New Waterway).
It is also possible to make it clear in other ways that large
timescales must be taken into account. In a town a distinction is
made between the elements of an individual building, a street and a
district. Each of these elements has its own timescale of
existence. Roughly speaking, the timescale for the succeeding
elements is always 2 to 3 times greater, if not markedly greater.
Looking ahead in planning terms, it is possible to design a
building for thirty years for example, but for the succeeding
elements the accompanying lifespan must be respected, which is many
times greater. This illustrates a fundamental principle that is
little used in the projection of flood defences. With respect to
the time factor, due to its location a flood defence that runs
through a town does not have the character of a solitary structure,
but of a district at the least.
If a flood defence runs through an industrial area for example,
Europoort (Rotterdam Port Area) is one such area, then it
determines the height of the rest of the area too. Conversely, the
height of the dike is actually set by the height of the area. This
applies for more than a century. In a district, the hydraulic
engineer must therefore not only have an eye for the lifespan of a
single hydraulic structure such as a dike or a lock; he must also
consider planning aspects. Conversely, the planner must not only
ask the hydraulics engineer how high ‘for the moment’ the flood
defence must be, but he must also ensure his colleague realises
that, once constructed, the height of the structure will be fixed
for the lifespan of the whole conglomeration. The estimation of the
lifespan of the whole conglomeration is not the responsibility of
the hydraulic engineer, but of the planner. Together they must
ensure that the flood defences and reserve strips is in line with
the zoning
27
plan, looking forward some hundred years or more (in the Coastal
Memorandum a period of 200 years is mentioned). The reserve strips
could be used for temporary matters.
4.5 The development of a vision and environmental impact reports
Area-specific knowledge is needed to develop a vision on the flood
defence marked for construction or reinforcement. The knowledge of
the policy objectives show what the aims are. That may mean that a
certain change must be realised, that is the desired safety. Other
policy objectives are oriented to minimising change, preserving LNC
values. The objectives aimed at preservation are not inflexible,
but imply a dynamic situation (reinforcement, development), such as
the switch to environmental management of grass vegetation, the
creation of new habitats such a frog pools, or the development of
an ecological connecting system zone or a bicycle path. The policy
objectives are oriented to preservation and development of the
values authorised by politicians. If there are LNC values at local
level that have not been allocated an official value, this will
have to be rectified. That occurs during the development of a
vision, starting with the inventory. In preparation, the LNC
aspects for the part to be improved are described thoroughly.
Sources that can be used are the provincial nature and
environmental inventories, the private nature organisations and the
public and private cultural history documentation. At this stage
the wishes of the other social functions, housing, work and traffic
are also collected. The municipality is the more important source
in this.
This data must be the basis for the selection upon which a value is
allocated. The fact that it is a choice means that it is
subjective. That means that political responsibility must be taken
for the new values. Social support is therefore essential. Advisory
committees must be set up to realise this support, in which all
interest groups are represented. Experts and administrators also
take part in the consultations. The advisory committees are
oriented to achieving consensus. The outcome is a list of LNC
values and other social functions to be preserved or developed.
This list is added to the authorised values and the functions fixed
and desired in the planning. That completes the overview of values
and functional requirements.
The vision now consists of two essential elements: the requirements
which the new flood defences must fulfil and the faults in the
existing flood defence and the values.
The following step is the search for measures to improve the safety
that spare the values as much as possible or even enhance them.
Solutions that fulfil the technical requirements, but score low on
the preservation or generation of the values are not considered
further. The most likely solutions are left. Working them out
further in the direction of the project memorandum makes the losses
and gains of the various solution values tangible. That information
is the basis of the ultimate choice of the technical approach to
the construction or improvement.
The above comprises five steps. 1. Inventory of the demands on the
new flood defence or on the faults of the existing flood defence,
of the LNC values and of the other functions. 2. Definition of the
improvements demanded for safety reasons, of the authorised LNC
values, of the additional area-specific LNC values and of the other
user wishes. 3. Finding the bottlenecks. Those are situations in
which the value mentioned is threatened by the technical
intervention demanded. 4. Selection of technical solutions (cross
section and choice of route) that fulfil the safety requirement,
with maximum orientation to preserving and developing values and
the best possible introduction of other functions. 5. Working out
one or more likely designs and choosing the preferred design.
28
The first four steps relate to the development of a vision, the
last one to the choice of the design.
The process of developing the vision corresponds to policy
analysis, that is the generation and selection of options. In terms
of the environmental impact report the vision is the essence of the
introductory memorandum and the working out of likely options plus
the selection of the preferred alternative is the essence of the
environmental impact report. In this way, vision development
(policy analysis) and project memorandum become a seamless part of
the environmental impact report process.
Accordingly only one procedure is needed for the whole dike
improvement process and it is characterised by the funnel
structure, efficiently leading to a solution that is optimal, based
on all considerations.
The use of a multi-criteria analysis continues to be worthwhile if
many different solutions are possible that have many
characteristics still lacking a value. The policy at the flood
defences has typically already passed that station and been
recorded in a higher framework. This is explained in annex
II.
29
5. Dimensioning
5.1 Introduction In the total realisation and maintenance process
of a dike (see figure 4.2.1) dimensioning plays a role in three
ways. - In general terms in considering options - In more detail at
plan level in working out the solution selected - For any flexible
maintenance (renovation and replacement)
This chapter gives the rules for the dimensioning of (parts of) a
dike body. It is set up on the basis of the following train of
thought. (a) Following from the safety function are the
requirements on water retaining capacity; these requirements are of
primary concern to dimensioning; a robust profile is preferred (see
section 5.2 for these considerations); the current approach to
hydraulic load is the overload approach by dike section, in which
the normative load is characterised by a tolerated value for the
wave overtopping discharge, depending on the construction of the
dike, its dike and the characteristic of the ground behind it; (b)
All functions together are decisive for the ultimate design (a
robust profile or a profile with special structures) and the place
of the defence (the route); (c) The structural development follows
the selection of the form and the route, including the organisation
of daily management.
These three steps are shown in figure 5.1.1, which must be seen in
connection with the figure 4.1.1.
(a) Requirements for the ‘water retaining’ function Reasoning from
the safety function point of view, a dike must have a sufficient
water retaining capacity to protect the hinterland from flooding.
In the Flood Defences Act (FDA) [1] the required standard for
protection is in the form of a load: the exceedence probability of
an extreme high water level (design height water level, see figure
5.2.2), which must be retained (the area frequency).
The safety requirement set for sea and lake dikes follows on from
the approach proposed by the Delta Commission. Each dike section
must be able to safely resist the hydraulic loads corresponding to
the area frequency in the FDA. For components such as the
collective statistics of the hydraulic loads (wind, water level,
waves) and the permissible overtopping discharge, the Delta
Commission’s approach is replaced by a more modern
formulation.
The effect of the safety requirement is twofold: a requirement that
the overtopping discharge must comply with for the determination of
the crown height and a requirement that the other components must
comply with. In words and formula form:
- the probability that the permissible overtopping discharge fixed
for the relevant dike section is exceeded (‘overload’ is the
consequence) must be less that the standard given in the FDA. The
permissible overtopping discharge follows on from the
characteristics of the dike section and the area behind it. This
gives the value (1): P{q > qt l h < NHW norm.
- the probability of failure of the defence as a consequence of the
occurrence of all other failure mechanisms (such as piping,
insufficient strength and stability of the dike body and the
revetment) if overload does not occur must be very small.
30
This gives the condition (2) P{failure as a result of other
mechanisms ¦ q < qt} < 0.1* standard where P{...} =
probability that the event between {} occurs in one year
q = overtopping discharge, following on from the geometry and the
hydraulic conditions
qt = permissible overtopping discharge, following on from general
construction characteristics and characteristic of the area behind
it
h = water level standard = the area frequency as recorded in the
FDA
Failure is said to occur if the functional criteria are no longer
fulfilled. For the safety criteria this means that the flood
defence no longer possesses the water retaining capacity referred
to in the FDA.
The failure criteria are defined by failure mechanism (see section
5.2 and figure 5.2.5).
The water retaining capacity (the strength) of a dike is determined
by the height of the crown, and the stability and the watertight
capacity of the (covered) dike body and the foundation
(substrate).
The design of a cross section from the point of view of the safety
function, bearing in mind the general requirements for the design
as a result of the other functions, is given in section 5.2. The
information needed on the loads to be taken into account and the
(soil-mechanical) strength parameters is handled in the Design
Basis Memorandum.
Dimensioning starts with the exploration of the possibility of a
robust profile. In step (a) a large- scale robust profile is
designed on the basis the safety function. Then in step (b) a check
is made of whether the requirements based on the other functions
can be fitted in to a robust profile in an acceptable way. If this
is possible a check will have to be made of whether the profile
really complies with all safety requirements. In other words, the
failure mechanism must be checked.
(b) Requirements for the other functions From the other functions
additional requirements and wishes can be formulated with regard to
the architecture of the dike; the crown width, the design of the
slopes, the revetment and the berms and the route. The gist has
been included in the preliminary design in step (a). It is handled
in more detail in section 5.3. Once the five steps in section 4.5
have been implemented the result will normally be the adaptation of
the design and location of the robust construction design, and/or
to a cross section of special structures, and in some cases even to
a completely new dike location.
(c) The structural development A few of the building blocks for the
structural development of the cross sections are discussed in
sections 5.4 through 5.8. - The cross section must be protected by
a revetment (section 5.4); - The connection of a dike body to
structures, dunes and high grounds demands tailored structures
(section 5.5); - In some cases a place must be found on, in or
around the dike body for, or account taken of existing objects
which do not contribute to the water retaining function (section
5.6); - Section 5.7 provides an overview of cross sections with a
special design, which can be used alone or in combination with a
robust profile when there are specific functional demands; - The
organisation in terms of daily management is handled in section
5.8.
31
32
5.2 Design of the cross section on the basis of the ‘water
retaining’ function
A number of steps in the design and dimensioning of the cross
section of the defence on the basis of the safety function is
handled in this section (see figure 5.1.1). (a1) Starting with a
robust profile; (a2) Making an initial sketch of the cross section:
outlining the contours and the location of the cross section on the
basis of a number of starting values, taking into account the
requirements for other functions; (a3) Checking of failure
mechanisms: examining whether all safety aspects are being complied
with. Implementation should also be included in this phase, even if
it has not yet been fully worked out. For instance, implementation
may necessitate a cunet/ground improvement and that can in turn
have consequences for stability under normative conditions. (a4)
Optimisation of the cross section: achieving the most economical
profile by varying a number of parameters. Most economical profile
here often means optimally cost-effective. However, this will not
always be possible taking into consideration all (weighed)
environmental effects.
Steps (a2), (a3) and (a4) form an iterative process.
(a1) Type of cross section In the selection of the type of cross
section there are two extremes. - On one side the tried and tested
concept of a solid dike body, constructed of sand, clay and stony
materials. - On the other side a concept on the basis of the
application of special (more sophisticated) structures, such as
water retaining screens in the dike, cofferdams, and movable flood
defences.
The following considerations are important in the selection. - The
extreme conditions that accompany the present safety standard
deviate considerably (especially on sea) from what has been
incorporated into designs in the past (hundred) years on the basis
of experience; this means that unknown failure mechanisms may
originate or known failure mechanisms develop in a different way
under these conditions. - With reference to the water retaining
function, the reliability of a dike is determined by quality of the
design, construction and daily management respectively. To a great
degree the quality that can be achieved in all three phases is
determined by the complexity (the proportion of special elements)
of the cross section. The increase in complexity heightens the
probability of certain factors not being evaluated correctly, while
the resistance to disintegration (a kind of residual safety in case
anything goes wrong) decreases as deviation from the robust profile
increases. - Sand, clay and stony materials are everlasting. The
dike body is integrated into the substrate. Gentle slopes ensure
favourable pressure distribution and stability, and optimal
absorption of wave energy; future heightening and strengthening of
the dike is usually feasible. - The use of special structures
intensifies the efforts of daily management in monitoring
(continuous checking of the fulfilment of the function, especially
hidden elements), maintenance, replacement and improvement, and any
gate operation.
Although deviation from a good-sized solid dike body increases the
number of uncertainties, generally the aim is an optimal robust
profile (naturally only using generally accepted technical insights
and models). Application of special structures is only considered
if there are functions and values that justify such a choice. The
capitalised costs of construction, maintenance and improvement
generally increase too.
33
A list of points for attention is derived from the above and can be
used in the selection.
* The design (with an eye to total management) should be
implemented in such a way that - heightening and strengthening
continue to be possible well into the future; - the probability of
unknown failure mechanisms is minimised; - the dike does not
disintegrate immediately due to neglectful management; - the
development of subsequent failure mechanisms is slowed down as far
as possible when failure does occur; - the fulfilment of the
function continues to be controllable (this becomes more difficult
as the cross section becomes more complex and more hidden elements
are used).
* When considering special (more sophisticated) structures the
following points demand extra attention. - The way in which the
failure mechanisms are addressed in the design; - The flexibility
of the design: that is a measure of the ease with which the defence
can be modified if standards, preconditions, know-how and social
views change; - The degree of complexity (experience) in the
construction phase; - The scale of the organisation and efforts of
management (including the necessity of accumulating experience) for
normal management and extreme (crisis) situations; the dependence
on third parties (both contractors and volunteers); - The
reliability of a reliability analysis, linked to residual safety in
case of failure; - The operational safety of closure operations; -
The controllability over a great distance of a temporary dike in
operation (with an eye to vandalism); - The costs of investment,
maintenance and future modification/replacement; also the question
of the impact of today’s choice on future generations (social
sustainability); - The technical sustainability of the materials;
sand and clay do not age (note the structure forming of clay) and
so do not demand maintenance, whereas steel, synthetic materials
and concrete do and can be the root of serious problems, not in
elements which are easy to replace but in foundations and hidden
structures; - The selection of a structure that does not collapse
at once, but gives a warning as the point of overload approaches; -
The laying down of good management registers.
(a2) Initial sketch of a cross section. In this step the first
rough contours of the dike body are outlined on the basis of the
above- mentioned considerations. A number of rules of
thumb/assumptions are given, which should be considered purely as
starting values.
The first choice is for a robust profile. The diagram in figure
5.1.1 shows however that a special structure may ultimately be
selected. The design value of the robust profile is generally 50
years. A longer design value is recommended for determining the
strength and the height of structures that are expensive and
difficult to modify.
Figure 5.2.2 shows a cross section of a dike with accompanying
designations. Important factors when making a first (preliminary)
sketch are - the crown height and width; - the incline of the slope
in relation to general ideas on the method of covering; - the
positioning and dimensions of any berms, and the access
provisions.
34
(-) The construction level of the crown The construction level of
the crown (see figure 5.2.1) is determined by the sum of the
contributors referred to below (a) + (b) + (c) + (d) + (e) + (f):
a. the water level with a probability of exceedence equal to the
statutory standard: the normative high water level NHW xxxx, where
xxxx stands for the year of fixing; b. the rise in high water
(including the fall in NAP) over the plan period ; c. the excess
value for storm oscillations, gust bumps and seiches; (local) gusts
are only taken into account if they have not been processed in the
water level statistics; d. the wave run-up height which corresponds
to overtopping of 1l/m/s (as starting value from the logarithmic
series 0.1 -> 1 -> 10l/m/s); e. the sinking or settlement of
the bottom expected locally over the plan period; f. the expected
sinking of the crown due to settling of the dike body and of the
subsoil over the plan period, after delivery.
The contributor (a) and the contributor (b) represent the outside
water in Hydraulische randvoorwaarden voor primaire waterkeringen
(Hydraulic Boundary Conditions)[13]. For inside waters, the design
height water level to be maintained is found in the register and
the manager’s management plan (see chapter 2).
The contributors (a), (b), (c) and (e) cannot be influenced; a
description of the corresponding loads is given in chapter B2 of
the Design Basis Memorandum. Contributor (f) can be influenced.
Contributor (d) is dependent on the slope and the shape of the
outer slope, the foreshore and wave damping measures; calculation
occurs when the water level is equal to (a) + (b) + (c) and the bed
attitude takes account of the changes in the lie of the foreland,
including the subsidence in the plan period.
More information on how (a) through (f) are determined is provided
in chapter 5 of the Design Basis Memorandum.
(-) The crown width To begin with the practical measure of the
crown width is taken to be 2 metres; a crown of this width is just
passable and adequate if a maintenance/inspection road is situated
on an inside berm. A passable maintenance/inspection road is at
least 3 metres wide. Roads for public traffic naturally demand more
space.
(-) The slopes The outer slope between berm and crown is given an
average (camber) slope of 1:5 on sea or 1:4 along the lakes. If
there is no clay hard revetments are used. When using an asphalt
concrete revetment, the slope must be no greater than 1:5 in
connection with the maintenance recommendation. If there are good
clay slopes of 1:5 or less can be maintained, depending on the
strength of the wave attack. A slope is usually more attractive the
less steep it is. The inner slope is given a slope of 1:3.
(-) The position of the cross section For dike improvement the
choice is between - dike reinforcement towards the inside; - dike
reinforcement towards the outside; - dike reinforcement on both
sides and across the existing dike; - a new route inside or outside
the existing dike. In this choice the technical principles in terms
of flood defence, the other functions and the costs are combined to
achieve a socially acceptable solution.
35
36
The strip of land on which the underside of the dike, the sole,
rests, is called the foundation layer or footing. The slope on the
side where the highest water levels occur is the outside slope or
the outside of the dike. Its lower edge is called the foot, or the
toe or outside toe. If the outside of the dike consists of two
slopes, separated by an approximately horizontal part, then the top
part is called the upper slope, the lower part the lower slope and
the horizontal part, depending on its purpose and height, the high
outside berm, maintenance berm, high water berm, storm tide berm or
flat berm. The highest, approximately horizontal part of the dike
is the crown. The slope on the land or polder side is the inside
slope or the inside of the dike, also subdivided in many cases into
an inside berm (maintenance/access berm). The lower edge is the
heel, but it is also called the inside toe or the foot. The strip
of land along the heel is the low inside berm. If it is bordered on
the landside by a ditch, then it is called the berm ditch or inside
berm ditch; if the dimensions are large the name dike canal or reed
is used. If the strip of land is bordered along the outside toe by
a partition (ditch, fencing…) that runs approximately parallel to
the dike then that strip is called an outside berm. If the toe is
under high water and there is a berm covered with stones then it is
called a low water berm, low outside berm or water berm. If there
is land above water on the sea side of the dike, then it is called
the foreland or first bank. The part of the land beside the outside
toe and under low water is the underwater slope.
Figure 5.2.2 Designations (definitions) dike profiles
(-) Choice of berm at storm surge level At most dikes, a berm has
been introduced with an eye to the reduction of wave overtopping at
level (a) + (b) + (c) + (e) + (f, only the part to the berm
height). The width is approximately 4*HS
with a minimum of 5 metres. The berm slopes under 1:20 to the
outside. The following points are important in considering the need
for a storm flood barrier. - The effect of such a berm is small for
a slope less than 1:5; - The design of the revetment must take
account of concentrated wave attack at the crack
points and the fact that these crack points are often weak points
in the revetment; - A future modification of the contributors (a),
(b) and/or (c) can necessitate a radical change
(heightening of the berm) to the outside slope.
(-) Water berm A water berm or berm to transition is introduced as
a transition between the rubble layer on a crib via a toe partition
or dike wall to a stone revetment on the outside just above low
water (at lake dikes at the level of the lowest lake level), when
the foreland is low. This berm is not needed when the rubble layer
ends at a higher level.
(-) Accessibility Although the height and width of the inside berm
must ultimately at least comply with stability requirements for the
whole dike body, in the preliminary design an inside berm is
introduced at the recommended height of AHW +1, topped with a
maintenance and inspection road with a minimum width of 3m. This
road can also serve as a transport route for repair after a breach.
The dike must also be accessible length-wise to car traffic on the
outside for maintenance and
37
inspection. Such a facility can often be included in the hard
revetment, but when the slope is very wide the demands in relation
to the material to be deployed for maintenance is increased.
Figure 5.2.3 shows examples of a new dike with a robust
profile.
Westerschelde design height water level NAP + 5.90m storm tide
level 1953 NAP + 5.08 concrete uprights thickness 0.40 /
2300kgm3
on stone layer 0.10m on geotextile on mined stone 1.00m thick
basalt columns min. 0.30 thick washed in with broken stone on
geotextile; soil fill-in maintenance strip 3.00 wide clay 0.80 and
0.60m thick country road 4m, drainage trench centre to centre 25m
ditch drainagecentre to centre 100m
Grass dike Wadden Sea design height water level NAP + 5.00m
concrete blocks 0.30 x 0.25 x 0.20 cell blocks 0.12 thick drain
pipe blast-furnace slag 0.20 thick asphalt concrete 0.13 thick cell
blocks 0.12 and 0.09m thick clay 1.50, 0.80, 1.60mm thick stony
materials 0.20 thick clinkers (bricks) 0.07 thick street sand 0.07
thick stony materials 0.15 thick sand foundation 0.40 thick
Den Helder design height water level NAP + 5.00m fascine
mattress/crib with covering basalt uprights 0.40 thick 0.30 thick
clay 1.00 thick clay 0.60 thick
Figure 5.2.3 Examples of dikes with soil profiles
(a3) Control on failure mechanisms The height of a dike is of
primary importance in determining the quantity of overtopping
water. The height is guaranteed by the quality of the dike. The
quality is determined by the relationship between strength and
normative loads. Insufficient strength can lead to the occurrence
of the following failure mechanisms (also see figure 5.2.4):
38
Figure 5.2.4 Failure mechanisms
• Vertical and horizontal deformation and tectonic subsidence -
vertical deformations occur as a result of settlement of the
subsoil and setting of the raised
material; - horizontal deformations occur at thick and weak clay
and peat packages in, under and alongside the dike, and can lead to
loads on structures in and in the vicinity of the defence, such as
conduits and building foundations; - tectonic subsidence occurs in
water extraction or mineral mining.
• Inadequate macro-stability, including horizontal shearing of the
total dike body Macro-stability is the stability in relation to
shearing by an earth body or large parts of it along straight or
curved sliding planes.
• Loss of stability as a result of erosion of the outside slope. •
Loss of stability as a result of erosion of the foreland. •
Inadequate micro-stability
Micro-stability is the stability of earth layers of limited
thickness at the surface of a slope under the influence of a
groundwater flow. Micro-stability is caused by a high water table
in the dike.
• Stability in case of overtopping Overtopping can cause water to
infiltrate the inside slope. As a result the top layer of the
inside slope is saturated and can shear.
• Erosion of crown and inner slope in case of overtopping If there
is a large quantity of overtopping water, erosion of the inner
slope can occur as a result of water flowing along or off the inner
slope. Infiltration due to overtopping can lead to shearing of the
inside slope.
• Loss of stability due to sand boils (piping) Piping can be
described as a concentrated outflow of groundwater on the inside at
high outside water levels, where the velocity of the outflowing
water is such that soil particles are carried along and cavities
and tunnels originate due to receding erosion which threatens
stability.
• Loss of stability as a result of loss of consistency due to
settlement flow at the foreland or due to softening of the dike
body Loss of consistency due to settlement flow is a mechanism in
which a water-saturated mass of sand is subjected to a great
displacement (‘flows’) as a result of softening. Softening of
sand
39
with a loose packing is the result of an increase in shear
strength, where, owing to a rearrangement of the grain structure
(decrease in volume) an increase of the water and air pressure
occurs in the pores to such a degree that the contact pressure
between the individual grains decreases to a significant degree and
the sand mass behaves like a heavy fluid. This plays an important
role along parts of the Westerschelde (see Figure 5.2.5).
• Loss of stability of hard revetments, toe structures and bank
protection inside the dike by wave forces, internal water pressure
etc
• Damage to the dike revetment and the crown as a result of a
collision, floating objects and ice.
Figure 5.2.5 Routes (locations) in the Delta sensitive to
softening
The methods for controlling the above-mentioned failure mechanisms
in relation to dimensioning are handled in the Design Basis
Memorandum, chapters B5 and B6.
(a4) Optimisation of the cross section For optimisation of the
cross section of a soil structure (in terms of both loads and
strength) the following variables are available to the designer: -
Wave overtopping (d). Reduction in wave overtopping means a lower
crown height; this can be achieved by a more gentle outside slope,
a higher foreland, a wave damping structure, a coarse outside slope
and/or a outside berm at the correct height. Figure 5.2.6 shows an
example of the influence of the incline of the outside slope and a
berm situated approximately at the level of design height water
level. - The dimensioning of the crown and inner slope. By
permitting greater wave overtopping, the crown height can be
lowered. The strength of the crown and inside slope must then be
increased by introducing a gentler slope incline and/or a heavier
revetment for the slope. Measures must also be taken for drainage
on the inside slope. The crown width can only be linked to the
safety aspect to a limited degree. This is the most likely
explanation for why different crown widths are used in different
regions, also with an eye to other functions and maintenance
aspects.
40
- The choice of revetment on the outside slope, the crown and the
inside slope (see section 5.4). Architectonic values and LNC (form,
regional material, colour…) wishes form the basis to which more
details are added. - The choice of the incline of the inside slope,
the height and the width of the berm These variables can be
modified in relation to each other in connection with the variation
in the crown height. For example, optimisation of the wave
run-up/overtopping due to a less steep slope provides a lower crown
height, taking up more space, but means that the inside berm need
not be as wide and redresses part of the extra space utilisation,
etc. - The location of the cross section (place in traverse
direction). Also in this step local adaptations to the location can
offer a solution. - Soil improvement if the subsoil is weak. Here,
the crown height can be reduced during construction (in connection
with settlement allowance) and/or the dimensions of an inside berm
reduced. - Soil improvement in front of or behind the dike if the
subsoil is (too) permeable. Here the space taken up inside the dike
can be limited in the case of piping. - Water retaining in the dike
body and drainage. Depending on a variety of circumstances in the
given situation and construction considerations the choice will
first have to be made between a) a dike with the (salt)water
deflection structure on the outside and in the sole, combined with
a drainage to the inside and b) a dike with an open outside toe and
possibly also the sole and a (salt)water deflection structure on
the inside of the dike body. Bearing in mind that there is mostly
no risk of piping a drainage ditch is usually built along the
inside berm, in order to keep the dike body as dry as possible in
day-to-day conditions. This means that the dike body is as dry as
possible at the moment that a high water level has to be deflected.
At dikes along IJsselmeer the drainage system, supplementary to a
ditch, is an important precondition for a good design.
Figure 5.2.6 Reduction crown height due to gentler outside slope
and outside berm
Optimisation is detailed further in chapter B4 of the Design Basis
Memorandum. The cross section generated in this way is now
acceptable in terms of height, but must still be controlled in
terms of failure mechanisms.
41
5.3 Design of the cross section based on the other functions From
the point of view of the other functions, requirements and wishes
can be formulated with respect to the retention of certain elements
on or nearby the dike and to the architecture of the dike: the
crown width, the design of the slopes, revetments and berms. They
are already included in the outline design in step (a) section 5.2.
This section contains a further detailing. After completion of the
five steps in section 4.5, this will often result in local
adaptation of the form of the soil profile, or its location and/or
to a special cross section, as handled in section 5.7. This can
even result in local changes to the position of the water retaining
structure.
The selection process is outlined in figure 5.1.1. When processing
the function requirements the same approach is followed as in
paragraph 5.2: the initial sketch is optimised and controlled on
failure mechanisms to arrive at the special cross section. The
points of attention to realise this are: - landscape, nature and
cultural history; - agriculture; - recreation; - industrial and
residential aspects; - water management; - traffic and
transport.
In some cases use is made of sections 5.6 (Objects) and 5.7
(Special Structures).
A number of relevant aspects and assessment criteria have already
been initially introduced for these functions in sections 2.3 and
2.4. The text below serves as supplementation and
realization.
Landscape, nature and cultural history The dike forms a hard
separation between, on one side, the agricultural, cultivated
polder landscape and on the other side the extensive, natural
vitality of a lake, the sea, an inlet or mud flats. For a treatment
of the design of a dike from the viewpoint of landscape, nature and
cultural history, reference is made to the fundamentals.
Supplementary information can be found in the memorandum
Natuurvriendelijke waterkeringen langs de Westerschelde en de
Oosterschelde (Nature-friendly Flood Defences along the
Westeschelde and the Oosterschelde) [18].
Some points of attention are: * Try to compensate the loss of a
natural area due to the construction or improvement of a dike
through the simultaneous construction of new small-scale natural
areas. The compensation principle is in force at dikes in or along
the ecological main structure (appendix III).
* Introduce a grass revetment where possible (Design Basis
Memorandum, chapter B5.6.4). Any other planting on crown and inner
slope requires the application of a settlement allowance (see
section 5.6.4).
* Vegetation zones occur on the hard revetments and toe structures
on the outer slope of sea dikes along the sea, depending on type of
revetment, slope, type of substrate, current strength, tidal
amplitude and elevation in the tidal zone. The test dike on Neeltje
Jans offers some examples. Rough surfaces offer better
possibilities for attachment than smooth surfaces. Smooth surfaces
are difficult to negotiate, also for animals, especially at the
waterline. Loosely packed stone slopes have all sorts of hollows
and niches in which animals can hide. The use of dike protecting
materials containing substances that are toxic if taken up by
organisms growing on it should be avoided (for example the formerly
much-used lead and copper slag bricks, ore slag and a number of
asphalt products. See Handboek voor natuurvriendelijk oevers (Guide
on Nature-friendly
42
Banks) [19]). If this is taken into consideration in the design of
a dike, then the nature and landscape aspects will benefit. The
different communities on hard substrates are not only worth
studying from a scientific and aesthetic point of view however.
Various species can be used as indicators for the water quality,
and as such are important factors in integral water
management.
Damage can occur to the natural values when replacing a revetment.
This can be compensated in part by applying a berm to transition
consisting of blocks with a diameter greater than approximately 0.2
metres. It is possible to retain water by means of a special design
of the berm to transition .
The following can be stated with respect to the opportunities for
growth and development of the different communities living on
revetments. - Concrete blocks achieve high scores; - Basalt scores
reasonably well, depending on the exposition; cast in with asphalt
scores bad; - A special structure of basalton scores well; - Open
stone asphalt achieves good scores in a number of cases; - 100%
revetment with asphalt scarcely offers hardly any possibilities for
development of species-diversity.
* The various revetments of old dike sections reflect the insights
and possibilities of hydraulic engineering with respect to the
building of dikes and the maintenance in a certain period. Certain
materials are no longer used. For this reason, old revetments have
a historical value in hydraulic engineering terms. Preservation of
such dike sections or the re-use of local materials or colours
(Vilvoord and Lessine stone in places with a small wave attack for
instance) has three advantages. - Acknowledgement of culture
historical elements; - Dike sections with a certain historical
value are preserved; - Certain types of substrate continue to
exist, which means that existence of certain communities is made
possible.
* The dike as a whole is also a reflection of the past. At various
locations, such elements as diking in, harbours, remains of dike
slides and breakwaters are historical manifestations. The aim is to
preserve these elements as much as possible.
* It is possible and sometimes desirable to pack the dike as a dune
at certain locations. In this case the dike is constructed
‘normally’ and, in connection with the dynamic character of dunes,
the packing in sand is mainly left to nature. Nature may be given
some assistance by securing parts covered with sand with marram
grass.
* The diversity in dike design can be enhanced by ‘playing’ with
slopes and crown heights, always taking account of the dike as a
whole and the surrounding area. Also in the design of the
lengthwise location of the dike, there must be attention for the
aesthetic rules, as they are applied in the field of road building,
span radii, transitions to uprights, no short uprights between
common spans, and following the building alignment of buildings in
the vicinity. Also the well-considered choice of camber and the
careful choice of the dike furniture such as fencing (material,
number and distance of laths) slope steps, and colours of the
materials used can improve harmony with the surroundings.
Agriculture Agriculture is a factor in the dike design that is
almost only present in the form of ‘loss of agricultural ground’.
The relocation of farms may be necessary. The accessibility of
agricultural plots falls under the traffic function, the ground
water problems under the water management
43
function. Pasturing of slopes covered by grass can be included
under agriculture if the decision has to be made as to whether
management of the grass is to be switched to a form that has no
value to the agricultural economy. Aspects with respect to the
agrarian management of the grass as a hydraulic slope protection
are handled in the Design Basis Memorandum, chapter B6.
Recreation Wishes founded on recreation function considerations may
lead to the desire to introduce recreational facilities (parking
places, art structures, viewing points, picnic areas, special
plants, alternative bicycle-path function for inspection paths
etc). This introduces an extra element to the design over and above
those in mentioned section 5.2. See section 5.6.2 for furnishings
and fencing.
Dike sections with a recreational function place special demands of
the revetment. Improper use of the dike, such as campfires and
vandalism, should be taken into account.
In the case of a raised or natural sandy beach in front of a dike
with a grass revetment the grass may be choked due to covering with
sand. Do not use grass in those places.
All of this results in extra connections and transitions, and so to
potential weak points in dike design. That demands special
attention, also in the day-to-day management.
Industrial and residential aspects The presence of buildings
outside the dike is rooted in a desire to cross cables and conduits
with the dike. This aspect is handled in section 5.6.3.
Windmills are often projected in the vicinity or even on top of
dikes. The technical consequences are addressed in section
5.6.5.
Buildings that remain intact place special demands on the design
and the method of construction of works. Measures should be taken
in relation to the accessibility of buildings and public utilities.
The Technische Rapport Waterkerende Constructies (Guide on Water
defence Soil Structures)[12] indicates in which way the probability
of damage due to setting can be determined and how the damage can
be restricted by means of construction measures
Regulations on the building of new houses are found in section
5.6.3.
Water management Water caused by overtopping and/or seepage must be
drained away. When dimensioning the drainage system, the danger of
bursting should be taken into account (see Design Basis Memorandum,
section B5).
At sea dikes, depending on the land use behind the dike, salty
seepage may have a positive or a negative effect on plants and/or
nature. Screens may be used or flushing of waterways introduced if
the effect is negative. If there is a positive effect, it may be
beneficial to the cultivation of vegetables which grow well in a
salty environment or the preservation of a salty nature area,
measures to activate the salty seepage are possible by means of
drainage systems driven by the tidal motion.
Traffic and transport A ramp or access road is required wherever
the crown of the dike or the inner berm has a traffic function. Its
dimensioning is a road construction matter demanding input from
experts.
44
Road crossings are often troublesome, because of their great
height. If buildings are located close to the dike, there is often
simply no space for them. In those extreme cases a cut-off in the
dike may be considered.
The influence of a road on the water retaining capacity of a dike,
relates in part to the traffic load and in part to possible erosion
at the connection