GuideSeaandLakeDikes.pdfGuide on Sea and Lake Dikes
(Translation of ‘Leidraad Zee- en Meerdijken’)
Technical Advisory Committee for Flood Defences
December 1999
The Netherlands
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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
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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,
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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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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
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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.
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+ 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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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(-) 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.
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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
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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):
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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
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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.
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- 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.
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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
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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
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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.
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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