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The wave overtopping simulator is a device which is able to simulate overtopping waves at the crest of a dike and at the inner slope. The device has been used in the ComCoast project to test a traditional dike section with grass and the SGR, the Smart Grass Reinforcement, placed in May 2006. This report describes in the first part (Chapters 1-6) the design, construction and calibration of the 1 m wide prototype of the wave overtopping simulator, as it was performed from May July 2006, under the ComCoast programme. This first part has also been reported as a final version 2.4 in November 2006. The second part continues with the actual construction and testing of the real wave overtopping simulator and the use of it during testing at the dike at Delfzijl, all under the ComCoast programme. Further, the measurements itself and analysis of velocity and flow depth during this testing has been described. Also the work of Bosman, as performed in this project as a MSc-thesis, has been summarised. This second part describes the period from November 2006 to August 2007.

Report on laboratory tests on a crest drainage dike; investigation if a channel in the crest of the dike is able to decrease the amount of overtopping over the dike. Chapter 2 provides details about findings from previous studies and the relevance of those findings to this research project. Available literature on wave run-up and wave overtopping are also reviewed and summarised. Research performed on wave overtopping layer thickness and the overtopping flow velocities are studied. Formulae to calculate layer thickness and velocity are summarised in this section. Additionally, scaling laws and scale and model effects will be explained. Section 2.6 describes the fundamentals of the crest drainage dike and the expected behaviour during an overtopping event. After the literature review, the objectives and methodologies were considered. The final section of Chapter 2 is focused on revising objectives and methodology. In Chapter 3 the model setup, measurement techniques and test programme are described. Chapter 3 also includes the calibration of measuring devices and methods of data analysis. During the physical model study four different measurements (wave parameters, layer thickness, velocity, weight of overtopping water) were taken. Methods for measure each of these parameters are described with instructions about the instruments. Calibration methods for each measuring devise are then explained. Chapter 4 presents the most important results from the model testing: primarily the reductions of mean overtopping rates due to different configurations. The influence of the crest drainage dike in reducing the overtopping volume per wave is explained and results from tests with different drainage options, overtopping layer thickness, and overtopping flow velocities for the crest drainage dike are given. Lastly, the results from selected tests on re-curve wall configurations are presented.

In the present report field erosion tests of the inner slope of a sea dyke in the province of Groningen (near Delfzijl) are described for the situation of severe wave overtopping. Three types of tests have been performed: tests at the present grass cover, tests at a reinforced grass cover and tests at a section of bare clay. At the reinforced grass cover section a provisional Smart Grass Reinforcement (SGR) system was installed in May 2006 (Royal Haskoning & Infram, 2005). The test sections were 4 m wide and extended over about 16 m along the inner slope, down from the dyke crest. It should be remarked that the erosion tests focused on the inner slope from the crest to some distance above the level of the service road. Below the service road, the berm slope was artificially strengthened with riprap as to avoid any damage on beforehand. The tests have been carried out under the framework of ComCoast, Work Package 3 (WP3). Additional measurements have been commissioned by the SBW program (Sterkte & Belastingen Waterkeringen or in English: Strength & Loads Water Defences), a research program for improvement of knowledge of present defences. These measurements included: measurement of flow velocities and water depths, infiltration tests, determination of the soil shear strength and an additional erosion test at a bare clay section. The present report is an overall report, presenting a full overview of all test activities, as far as conducted under the responsibility of the consortium. At a later stage, the SBW measurements will be analysed further within the scope of the SBW program. Where applicable, however, some preliminary results are included in this report. For further information, the present report addresses background reports that were written within the framework of ComCoast or SBW. Special reference is made here to the report on the wave overtopping simulator (Infram & Royal Haskoning, 2007), a summary of which has been presented in the present report in Chapter 4.

A number of equations has been developed for calculation overtopping quantities over breakwaters, as well as equations to describe the stability of the inner slope. However, not all the water which overtops the outer boundary of the breakwater will reach the inner slope; part of the water is infiltrated in the crest. In this paper data are presented on the percentage of water infiltrating in the crest of the breakwater. The results are compared with the equation presented in the UK overtopping manual.

This thesis shows a set of equations that define the micro-stability and macro-stability of reinforced grass revetments with geosynthetics on inner dike slope during an overtopping event. At micro-stability level, it has been analyzed that the equilibrium in a soil particle using the criterion of incipient motion of soil particle. The forces in normal direction to water flow are considered the most important to keep in equilibrium the soil particles of the reinforced grass revetment. As unstable forces are considered: Lift force, this force can be defined in function of depth-averaged relative turbulence intensity and averaged-velocity (Hoffmans’ Model, 2006). Likewise, the averaged-velocity can be defined in function of concentration factor of stems, bent vegetation height and vegetation height (Carollo’s Model, 2005). As stable forces are taken account: Friction force due to cohesion stress, the cohesion forces produced by root and geosynthetic in the soil are assumed in the same direction of the water flow and they cause a frictional forces in the normal direction of water flow when the particle tend to detach. Also, gravity force, as it is referred to submerged weight of soil particle. The stability equation of a soil particle of reinforced grass revetment is deduced from the application of the equilibrium between these forces; it means unstable forces must be equal or less than stable forces. At macro-stability level, it has been analyzed the acting forces on reinforced grass revetment during an overtopping event has been applied to the equilibrium in the system. The diagram of free body of reinforced grass revetment is divided in two wedges and identified like active and passive wedges (Koerner’s model 1991). The wedge located at top of the slope exerts an active function in the system that means it tends to push away the soil of the revetment so that is in the toe of the slope. The other wedge located at toe exerts a passive function that consist to hold the active wedge giving equilibrium to the system. The forces taken account in the diagram of free body of both wedges are: the weight of the soil, the shear force of the water, the cohesion forces produced by cohesive soil, roots and geosynthetics, reactive forces located in the failure plane and pressure forces between the passive and active wedges. To define the stability of the system, the safety factor is introduced in the friction angles of different soils and soil-geosyntethic besides in the cohesion produced by root and geosynthetics. The polygon method is applied in the forces acting on each wedge and it is deduced in a quadratic equation in function of safety factor. A safety factor higher than one defines a stable system. The geosynthetic analyzed are the geogrid and geocell. Unlike geogrid for the geocell an additional force is considered and this is the shear force which is generated in the contact between geocell wall and soil and it is in normal direction of the surface. This additional force is independent of the geocell tension; it means the geocell does not need to be in tension to help the stability of the system. In the case of geogrid could not be affirm the same. With much analysis of micro-stability as macro-stability of the system, the cohesive forces produced by roots and geosynthetics are important. In this thesis is described in brief mathematical models, based on equation’s coulomb, which defined these cohesions. The cohesion produced by root in the soil is calculated using the simple root perpendicular model (Model of Wu et al, 1979). The additional cohesion produced by the geosynthetic uses the model of Jewell, 1980, to be calculated. Both cohesion models are based on the tension stress of the root or geosynthetic located inside of the soil layer produce shear stress in this layer. The shear stress could be in any plane but it is used the failure plane. This shear stress in the failure plane is composed of two components, one of the component is the projection of tension stress in the failure plane and the other is the friction stress produced by the normal component to the failure plane of tension. The tension of roots and geosynthetics are important in both cohesion models. The tension of root is easily to obtain from laboratory.

Nowadays the protection of our country for high sea levels and heavy storms is daily news. The ComCoast project (COMbined functions in the COASTal defence zones) is originated by ten organisations out of five European countries bordering the North Sea coasts in order to develop innovative solutions for flood protection in coastal areas. Instead of automatically raising the coastal defence zone on places where more protection is needed, ComCoast creates multifunctional flood management schemes with a more gradual transition from sea to land. Part of the new solutions is the wave overtopping resistant dike. This so called "overtopping durable dike", should withstand wave overtopping during a storm in much better than the current ones. More knowledge about the loads on the dike by wave overtopping is therefore needed. Recently formulae have been derived for maximum flow depths and velocities on the crest and inner slope. These formulae are based on the difference between fictive wave run-up and the crest freeboard. This is a good measure to determine the flow depths and velocities on the dike. These formulae have been calibrated by two independent physical model test programs in different wave flumes by Schpf in Germany and by Van Gent in the Netherlands. If these two studies are compared there appears to be a large difference in the empirical coefficient of the flow depth equation of a factor 2.2. They collectively wrote a paper and found the test set-up as primary cause for the discrepancy in the flow depth coefficient. The differences between the test set-up and analysis have been studied in the present thesis. The overtopping time and the variation of velocity and flow depth in time have been investigated as well. These quantities are also necessary to be able to give a full description of the loads on the dike during wave overtopping. The present study shows that the outer slope is of great importance in the flow depths and velocities on the crest. This is new knowledge. Schpf performed his tests on a dike model with an outer slope of 1:6 and Van Gent used a dike model with an outer slope of 1:4. The empirical coefficients appeared to be dependent on the outer slope steepness. Subsequently a formula for the overtopping time is created, based on the difference between fictive wave run-up and crest freeboard. The overtopping time appeared not to be a function of the outer slope. The variation of flow depth and velocity in time can be approached with a linear function. The new equations for wave overtopping are compared to the results obtained by tests with the wave overtopping simulator. The wave overtopping simulator is a machine which is able to simulate wave overtopping on a dike on full scale. Despite the difficulties in measuring velocities and flow depths during these tests, one can conclude that the wave overtopping simulator works very well. Storms with high overtopping discharges can be simulated accurately for testing strength and stability of a dike.

In the framework of the ComCoast project, the concept of the Crest Drainage Dike has been studied regarding the reduction of wave overtopping. This study only focuses on the average wave overtopping discharge. The basic concept of the Crest Drainage Dike is a basin, integrated in the crest of the dike, that collects overtopping water and thus reduces the load on the inner slope of the dike. The collected water in the crest basin is drained landward or seaward through pipes. The main goal of this report is to identify the physical background of the concept of the Crest Drainage Dike and to predict the wave overtopping discharge as a function of hydraulic and geometric boundary conditions. Therefore two different types of theoretical studies have been executed. The first study is process-based and serves as a basis for the numerical program that has been developed. Since this model is partly based on several assumptions, several physical model tests have been executed to verify or reject the stated hypotheses. In the physical model tests, several hydraulic and geometric boundary conditions, such as the wave height, the crest freeboard, the use of berms, the wave spectra, the wave steepness and the drain layouts, have been varied. Since the predictions of the numerical program are well in line with the measured wave overtopping discharges, the numerical program is used to investigate the use of a Crest Drainage Dike in two case studies. The case studies are the Hondsbossche Sea Defence and the Perkpolder Sea Defence. Both dikes are located in the Netherlands The use of this numerical program gives a better insight in the physical background of the Crest Drainage Dike. The description of these physics is the second part of the theoretical study. For dikes with severe wave attack, such as the Hondsbossche Sea Defence, only a small fraction of the waves is reaching the crest of the dike. However, the waves that do reach the crest of the dike have a relatively large volume and the buffer capacity of the Crest Drainage Dike limits the effectiveness of the Crest Drainage Dike. Besides this, there is a high statistical uncertainty since the average wave overtopping discharges are determined by only a couple of waves. For dikes with a lower wave attack, such as the Perkpolder Sea Defence, more waves with a lower volume per wave are overtopping and therefore the concept of the Crest Drainage Dike works well. However, the crest freeboard reduction with the use of the Crest Drainage Dike is in these specific cases is only minor. Based on the numerical studies and the current Dutch overtopping criteria, the reduction of the crest freeboard with the use of the Crest Drainage Dike is determined and is significantly lower then the assumed reductions in earlier studies.

The purpose of this report is to investigate the differences in overtopping characteristics over the crest of a rubble mound breakwater when the crest is made either impermeable or permeable. Among numerous characteristics that are effected by a modification in the permeability of the crest, this report looks specifically into three separate aspects: the design level changes that are caused by modifications in the permeability of the crest, a comparison of the total and sector-wise overtopping discharges and finally the differences in spatial overtopping intensities between the two. By looking into two well-known overtopping design guidelines for overtopping, namely, Owen and Eurotop, this report aims to look at the differences it would make in designing a breakwater with either an impermeable or a permeable crest. This is done by building a breakwater model in a wave flume and comparing it with the existing guidelines and assessing the changes that best represent the modified model. It is also important to observe how this physical modification of the crest affects the overtopping discharges and spatial overtopping intensities behind the crest of the breakwater. This will be relevant for designers or contractors tasked to make changes to an existing breakwater that results in its crest becoming impermeable. An insight into the overtopping discharges and intensities will be extremely useful to be able to predict the overall changes and cater for them.

Experimental research on wave run-up of regular waves on smooth slopes. Detailed run-up measurements and derivation of a run-up formula. A mathematical model to describe run-up. Some notes about wave overtopping

The main goal of this study is to determe the relation between very oblique wave attack and overtopping, and to accordingly adjust the formulae for oblique wave attack. The required knowledge to be able to read and understand this report is of a Bachelor in civil engineering level. All around the world different types of structures are built to protect adjacent areas from river or coastal flooding during high water levels. Only limited research is available on the influence of oblique wave attack (for angles over 45 degrees) on wave overtopping. Hydralab is an EU-project, which gives researchers in the European Union the possibility to carry out research in large hydraulic facilities. Cornerdike is a part of the Hydralab IV program. The Cornerdike research project was performed at the shallow-water basin at DHI in Hørsholm, Denmark. To achieve the goal of this research, tests, data processing and analysis were done.

The present report deals with the application of a provisional Smart Grass Reinforcement (SGR) system in 2006 for full scale testing of increased overtopping at the Groningen sea dyke test section near Delfzijl, as envisaged in 2007. The SGR has been placed at two strips of 4 m wide: one primary strip at the basic test site and one at a secondary test strip for additional testing. The placement of the SGR followed the awarding of this system (Report of 10 November 2005) to the consortium of Royal Haskoning and Infram), as the most feasible system for reinforcement of the dyke crest and inner slope of wave overtopped dykes. The activities for the application of the SGR, included pre-engineering, preparations, installation and follow-up. The activities that have been conducted under the responsibility of the consortium of Royal Haskoning and Infram, many other parties contributed as well: subconsultants FlevoGreenSupport and Queens Grass, Huesker (Germany), four students of the Technical College Leeuwarden. Moreover the ComCoast project organization (RWS/CUR and partners Water Board Hunze and Aas, Province of Groningen and Municipality of Delfzijl) were actively involved. The main part of the present report lays down in brief the preparatory activities (Chapter 2), the installation activities (Chapter 3) and the follow-up treatment (Chapter 4). In Chapter 5 conclusions are drawn and recommendations made. Details can be found in the appendices attached to this report: Appendix A: Photographs of situation of the SGR during the inspection visits; Appendix B: Reconnaissance of placement method and type of geosystem; Appendix C: Typical results of grass cover analysis for fertilization (follow-up treatment). The provisional application of the SGR for obtaining a representative test site in 2007 has probably been successful, in spite of many difficulties that had to be overcome. This can be seen from the photographs in Appendix A and Chapter 3. To date, a positive outcome is not sure yet, as there is still doubt on the degree of intertwinement of the grass roots with the geosystem. Hence, further checking of the monitoring strips, next to the test sections, shortly before testing in Mach 2007, will be required to ascertain the required functionality of the SGR. Looking back, the application of the Big Roll method has appeared as not really feasible: elaboration of a smart system for placement, such as indicated earlier by the consortium, should have aimed at from the beginning. A firm statement can be made here that the Big Roll method cannot be made feasible for large-scale application of SGR and further research is needed unconditionally for smart installation techniques of SGR, in combination with a suitable reinforcement system. As regards the unique character of the tests, we certainly hope for a positive outcome of the adequacy of the provisional SGR and for positive test results. Hence, awaiting the outcome of the check on the SGR adequacy shortly before the beginning of the tests next year, full-fledge preparations for the tests, are being continued.

Climate change will cause increasing physical loads on the flood defence of the Province of Groningen along the North Sea over the next decades. This is caused by a sea level rise between 15 and 35 centimetres whilst the ground level is lowering between 38 and 48 centimetres until the year 2050. This results in salt-water intrusion into the land surrounding our coasts. The opinion of (DWW (Dienst Weg- en Waterbouwkunde of Rijkswaterstaat) is that it is no longer feasible to continue traditional flood management methods where the line between land and sea is keeping out tidal waters. In the ComCoast project2, we recognise the need to develop new sustainable flood management strategies in order to influence planners to anticipate future developments. With the facts mentioned above the main question (for this research) is formulated as following: Which ComCoast-solutions are possible in the Province of Groningen and what are the possibilities to implement these in the Province of Groningen? This question is answered by studying literature and interviews. To obtain the correct information four cases are selected and for those cases as many as possible stakeholders are interviewed. For one case only one stake-holder is interviewed. To obtain more information about ComCoast in general two persons are interviewed. A result of the literature study, research in the coastal area and the interviews is that it appeared that there are five ComCoast-solutions. Two of them are foreshore solutions and three landward solutions. 'Foreland protection' and 'foreshore recharge to restore the coastline' are foreshore solutions. The landward solutions are 'overtopping defence', 'managed realignment' and 'regulated tidal exchange'. There are three different motives for applying a ComCoast-solution. The first motive is to increase the safety of the inhabitants of the coastal areas. Reduce the increasing salt intrusion is the second motive. The last motive is to realise spatial needs. Each solution relays on different motives. Table 0.2 shows the motives for each solution including traditional flood management. In this report components are formulated to define right opinions of the potential stakeholders. The components are a result of studying literature. In the selected cases Breebaart, Perkpolder, Ulsderpolder and GOG Kruibeke are farmers (including agricultural organizations) in most cases the only opponent. Losing land is the mean reason. Nature conservation organisations and the higher authorities are in all cases patron. The waterboards are in the most cases neutral and the communities are in all cases at least neutral. In the case Perkpolder is one commercial partner also patron. The result of this information provides criterions to decide the possibilities of the solutions. There are two measurable criterions which define the possibilities of a solution in an area. The first is 'missing buildings' and the second is 'missing deep channel/fairway'. Measurable criterions which define the attraction are 'attendance secondary dyke', 'attendance silence area', 'attendance agricultural area' and in some cases missing deep channel/fairway.

ComCoast (Combined Functions in Coastal Defence Zones) is an INTERREG IIIB project funded by the EU. ComCoast aims to develop and demonstrate innovative solutions for flood protection in coastal areas. In ComCoast, five countries from the North Sea Region are involved: Belgium, Denmark, The Netherlands, Germany, and the UK. In total, ten partners constitute the project consortium. Climate change cause already increasing physical load on coastal defence along the North Sea. Over the next decades, problems will arise through the need of adapting the existing coastal defence structures to modified conditions of coastal development, e.g. nature conservation areas, limited budgets, or changing social pretensions. New approaches emphasizing a gradual transition from sea to land are being explored in order to incorporate land use management with regard to the increasing pressure from population growth. These transitional areas will offer new opportunities both to the environment and to the people. In the ComCoast project, the need to develop new sustainable flood management strategies will be recognised in order to influence planners to anticipate future developments. Work Package 1 is one of the six Work Packages of the ComCoast project. Work Package 1 aims to identify feasible ComCoast areas along the southern North Sea. Work Package 2 explores new socio-economic evaluation methods for the ComCoast concept. The design and development of alternative embankments and strategies for coastal defence zones are worked out in Work Package 3. Work Package 4 deals with new and innovative participation strategies to involve stakeholders in the development process of ComCoast concepts. This technical report gives a short overview about the activities of Work Package 1 from Mai 2004 until Mai 2005.

The present report provides a state-of-the-art inventory of relevant information and technical concepts for the ComCoast project, being the first phase of the research stages of Work Package 3 (WP3). This project was assigned to Royal Haskoning by CUR. The information scan was set-up in a systematic way. About 160 relevant references have been traced for technical aspects and related projects. These references have been presented in a table database, including a preliminary judgement of the relevancy of these references for ComCoast. Further references can be introduced to the database in subsequent phases of ComCoast WP3. Based on the information, a preliminary evaluation was made for related projects. Emphasis was put on a preliminary evaluation of potential protection systems for the inner slope of primary sea defences (embankments). To this end, an illustrative evaluation methodology has been presented in this report. As an illustrative outcome, a reinforced grass revetments showed a high score and therefore this system seems to be adoptable as a feasible system for strengthening of the crest and inner slope of overtopped defences. Other types of protection systems have been presented as well and are briefly indicated in the Fact Sheets attached to this report. Prior to further elaboration of the most appropriate strengthening systems in subsequent phases of WP3, the development of an overall judgement framework is recommended as a follow-up of the present methodology. This framework should not be limited to the stability of the protection system only, but should also include other failure mechanisms for overtopped embankments, with special emphasis on critical soil mechanical failure mechanisms. As regards the new field of application (strengthening of crest and inner slope of embankments under increased wave overtopping), there may be room for new and innovative systems to reinforce the defences, together with methods to reduce tidal impacts and wave overtopping. It is recommended to promote further innovative research within subsequent phases of WP3.

The present proposal deals with a presentation of three potentially feasible concepts for strengthening of the sea defences as to allow for increased wave overtopping, within the framework of ComCoast. In addition, for each concept an indicative proposal is given for a further theoretical study (Phase 2) and an indicative approach is shown for further testing. This proposal is denoted Phase 1: generation of alternative concepts. For the present proposal and the activities involved, as well as possible future assignments within Phase 2, a consortium has been formed between Royal Haskoning and INFRAM (Royal Haskoning, being the leading party), the two partners that also participated in the Inventory Study that preceded this proposal. We think that this combination brings about the outstanding expertise from both firms, which we hope is reflected in the present proposal. The three alternative concepts focus on different types of innovative reinforcement measures, i.e.: reduction of the wave overtopping flow attack, strengthening of the present grass revetments and strengthening of the subsoil. After an introductory chapter, Chapter 1, the relevant failure phenomena are indicated in Chapter 2. The Concepts A (low hedges), B (application of innovative grass reinforcement) and C (application of Smartsoils techniques) developed to cope with these phenomena are presented in Appendices A, B and C and are summarized and discussed briefly in Chapter 3. Chapter 4 presents the indicative set-up for the further theoretical studies of Phase 2 for each concept, whereas in Chapter 5 the approach for testing of the concepts is indicated. The Concepts A and B are highly promising solutions, and we think that they only need limited elaboration and research. The costs are very competitive as compared to the costs for traditional raising of a dike crest (=Reference). The savings can be so big, that further in-depth research on optimization seems anyhow to be useful. Concept C is still highly experimental for application on sea defences and clayey soils. Nevertheless, it would be an interesting option to further explore The charm of these Concepts is that they can also be combined in an economical way (A and B together being still much more economical as the Reference) as to cope with increasing Sea Level Rise in future in a flexible way).

The objective of ComCoast (Combined functions in coastal defence zones) is to investigate how a multifunctional wide sea defence zone might be created. Instead of just increasing the height and weight of the dike, solutions are searched in the wide sea defence zone landward. These is an number of technical solutions, one of them is increasing the dike strength in orde to resist the load of water flow upon the grass revetment during extreme storms and handling the incomming water. WP3 aims at developing new concepts or further study of existing concepts together with the market parties to increase the dike strength in order to resist wave overtopping. The new concepts need to be cost-effective and social and environmental acceptable. In this report the sollution based on a stilling/retention basin idea is described. Besidses the concept a theoretical phase approach, global plan for test phase, costs and planning are given.

Recently, formulae have been derived for maximum flow depths and velocities on the crest and inner slope of dikes or levees at wave overtopping. Two independent physical model test programs in different wave flumes showed, however, a large difference in the results. The present paper clarifies this discrepancy and shows that the empirical coefficients are dependent on the gradient of the outer slope. Subsequently, a formula for the overtopping time was created, based on the difference between fictive wave run-up and crest freeboard. The overtopping time appeared not to be a function of the outer slope. The variation of flow depth and velocity in time can be approached with a linear function.

The ComCoast concept involves measures in seaward and landward direction. One of the options in landward direction is an overtopping dike. In this concept the crest and/or inner slope of the dike is strengthened so that more overtopping of the dike can be allowed. Advantage is that heightening of the dike is not necessary and furthermore this can be combined very well with a wet and brackish zone behind the dike. This zone provides opportunities for nature development, recreation and possible concepts like living in the water (houses on poles). Instead of increasing the revetment strength of the entire inner slope, it is also possible to decrease the overtopping loads at the inner slope without alteration of the cross section of the dike. An alternative where the wave-overtopping load on the inner slope is reduced to an acceptable level by measures taken at the crest of the dike. The concept "Crest drainage dike" consists of a concrete construction in the crest of the dike (in the form of a wide Uprofile). The proposal for this solution is discribed in this report.

The ComCoast concept involves measures in seaward and landward direction. One of the options in landward direction is an overtopping dike. In this concept the crest and/or inner slope of the dike is strengthened so that more overtopping of the dike can be allowed. Advantage is that heightening of the dike is not necessary and furthermore this can be combined very well with a wet and brackish zone behind the dike. This zone provides opportunities for nature development, recreation and possible concepts like living in the water (houses on poles). Instead of increasing the revetment strength of the entire inner slope, it is also possible to decrease the overtopping loads at the inner slope without alteration of the cross section of the dike. An alternative where the wave-overtopping load on the inner slope is reduced to an acceptable level by measures taken at the crest of the dike. The concept "Crest drainage dike" consists of a concrete construction in the crest of the dike (in the form of a wide Uprofile). The theoretical study for this solution is discribed in this report.