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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

Hoe evenwichtsverlies bij golfoverslag ontstaat en welke krachten daarbij een rol spelen is onduidelijk. In dit rapport is onderzocht hoe groot de wrijvingskrachten op het binnentalud van een dijk zijn ten gevolge van het overslaande water. Omdat dit probleem niet toegankelijk is voor berekening, zijn een aantal proeven gedaan. Hiertoe is een schuifspanningsmeter ontworpen. Het ontwerp is gemaakt aan de hand van schattingen van de golfoverslag. De wrijvingskrachten t.g.v. overslag zijn gemeten bij één geometrie, één ruwhied en één positie van het instrument.

Medium height breakwaters are subjected to a considerable amount of overtopping. This implies the armour on the inner slope has to stable against overtopping waves. Existing equations do lack some basic understanding of the process of starting up movement of rock on the inner slopes. In order to get more insight into this phenomenon a special device has been constructed in the laboratory. In this device it is possible to generate one single, overtopping wave. So with this piece of equipment overtopping of single waves can be repeated several times and a relation can be looked for between the individual plunge and the stability of the individual blocks. In an other research program the relation between the random waves and the individual plunges is investigated. As a follow up the results of damage by individual waves will be combined with the distribution of overtopping water over a breakwater. This combination will result in a damage distribution due to random overtopping. It has been found from the first test series that stability of the blocks can be described as a function of the maximum water velocity on the crest, combined with some geometrical parameters of the inner slope. From this research followed that the crest height and inner slope also had an effect on stability

Sea dikes in Viet Nam have been built up during a long history. Every year, about 4 to 6 storms attack the coast and cause severe damages of the sea dikes. Till now, little is known about strength and stability of the inner slope covered with grass under impacts of wave overtopping during storm surges. Destructive tests have been performed with the Wave Overtopping Simulator in Viet Nam revealing that grass slopes are able to withstand a certain amount of wave overtopping. In Hai Phong, a slope section covered by Vetiver grass could suffer a maximum wave overtopping discharge of up to 120 l/s per m. In Nam Dinh, three tested locations within a short dike section of 50 m show a large variation in erosion resistance of the Bermuda grass slope with maximum discharge of 20, 40 and 70 l/s per m.

Report on destructive overtopping tests executed with the wave overtopping simulator along the coast of Vietnam to investigate the strength of the inner slopes of grass dikes. This report presents measurement and observation results obtained during destructive tests with the simulator in Do Son, Thinh Long and Thai Thuy. After the first chapter, Chapeter 2 discribes all test sections in detail and test scenario. The method and results of hydraulic measurements are presented in Chapter 3. Chapter 4 and Chapter 5 introduce the slope profiles and photograph of grass slopes. The formation and development of the damages induced by wave overtopping are discussed in Chapter 6. Chapter 7 deals with the resistance against wave overtopping and critical velocity of different grass covered slopes which were tested with the simulator. Finally, Chapter 8 are conclusions and future research, respectively. Detail of hydraulic measurement performed at three sea dikes are given in Appendix A. Data of the slope profiles measured at Thinh Long and Thai Thuy are presented in Appendix B. Appendix C contents of the photograph of the damages on the slope sections at Thinh Long and Thai Thuy sea dike. Characteristics of soil and grass taken at test sections are given in Appendix D. Appendix E summarises brief specifications of all test sections.

The resistance of various grassed slopes against wave overtopping has been appraised by means of the Wave Overtopping Simulator in situ for a couple of years in Viet Nam. Destructive test results show that a dike slope covered with grass could suffer a certain overtopping discharge not smaller than 20 l/s per m. During testing it was difficult to predict when a slope starts to be damaged, however, damages tend to follow a more or less similar process of development. The concept of “erosional indices” and the method of “cumulative overload” (van der Meer et al., 2010) are applied to the simulator tests in Viet Nam. Critical velocities were then estimated for different kind of grassed slopes.

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.

The safety of a large part of the Netherlands depends on sea dikes. Due to climate change, a sea level rise is predicted. Together with stronger storms and more wave attack, wave overtopping on the current dikes will increase. As a response, crest levels of the dikes need to be raised in order to meet the present regulations for wave overtopping. Alternatively, these regulations could be lowered if the dikes can be proven strong enough to cope with the increased loads. The SBW ('Sterkte & Belastingen Waterkeringen' or 'Strength and Loads on Water Defences') project with respect to grassed inner slopes focuses on the improvement of reliable overtopping criteria for the present dike structures. Experiments are carried out using a wave overtopping simulator. This device is developed to create full scale overtopping conditions on a inner slope. Experiments show initiation of erosion of the grass layer downstream of the transition of the slope and a horizontal part. The focus of the present research is modelling of the erosion process at a transition. The erosion process is described by two different models. The Transition Model (TM) is derived to describe the development of the scour hole due to the impinging forces of the overtopping wave tongue, known as surface erosion; The SSEA (Sites Spillway Erosion Analysis) model is taken to describe the headcut erosion process which occurs after the scour hole has reached a certain depth. The TM is based on a oblique impinging jet and a depth variable strength profile, in where near the surface the root system dominates, whereas with increasing depths cohesion and internal friction of the clay dominates. Since irregularities, found at the real dikes, are not taken over in the model discrepancies between model and experimental results occurred. According to the defined erosion model, the present guidelines are stern and the actual strength of a grassed inner slope is underrated. The work is done in cooperation with Deltares.

A new process-based approach is introduced for a more efficient computation of the overflow-induced growth of an erosional channel in a noncohesive homogeneous narrow landmass such as the breach growth in a sand-dike. The approach is easy to incorporate in a 1D/2DV morphodynamic model to compute the channel growth both vertically and laterally. The flow modeling is based on the shallow water equations. For modeling the channel growth, a set of closed equations describing the channel growth in both vertical and lateral direction has been derived in connection with several new morphologic parameters such as the representative channel width and the channel cross-sectional growth index. The approach has been applied to simulate the breach growth in sand-dikes and the morphological development of wave overwash across sand barriers. The computational results bear fairly good resemblance with existing experimental data.

The Simulator was applied to test the resistance against wave overtopping of grass covered dike slopes in Viet Nam. Observation and measurement during destructive tests were performed to investigate the development process of damage induced by overtopping flow. Damages were likely to be initiated at transition of either geometry or materials, at eroded holes and around objects existing on the slope. Grass cover could withstand a certain overtopping rate that varies widely and in order of 10 l/s per m.

Extensive laboratory experiments were carried out to investigate wave overtopping on seadikes with (vertical) crown-walls in Vietnam. It is shown that the wave overtopping reduction by walls is inadequately, although rather complexly, described by TAW-2002. A new approach has been developed, which can straightforwardly be incorporated in the original formulations of TAW-2002 to improve the prediction of wave overtopping on sea-dikes crowned with this type of walls.

This thesis focuses on wave overtopping at rubble mound breakwaters with a non-reshaping berm. The research was aimed at gaining insight into the influence of a permeable berm on the overtopping behaviour. Moreover it was desired to validate existing prediction methods for the spatial distribution of overtopping for breakwaters with a non-reshaping berm. Wave overtopping was investigated by means of a physical model. The breakwater scale model was divided into 8 collection bins. Overtopped volumes were collected and pumped into floating tanks further down the flume. After the experiment the mass of the floating tanks was measured and the mean overtopping discharge could be determined for 8 horizontal positions on the breakwater. The measured total overtopping discharges cannot be predicted accurately by existing prediction methods. On the basis of experimental data a new prediction method was proposed that achieves an excellent fit for total overtopping. The crest freeboard definition was adjusted to account for the permeability of the crest. The reduction factor accounting for slope roughness was made dependent on the Iribarren number. For Iribarren numbers higher than 6, this method calculates no reduction of overtopping due to slope roughness. The effect of a permeable berm on total overtopping was found to be remarkably different from the effect of an impermeable berm. Permeable berms below Still Water Level (SWL) lead to less reduction of overtopping than impermeable berms below SWL. Berms above SWL lead to wave breaking on the slope in front of the berm. Contrarily to impermeable berms above SWL, a permeable berm above SWL leads to significant reduction of overtopping. The measured spatial distribution of overtopping is associated with a lot of seemingly random behaviour. Large differences were found with the experimental data of Lioutas (2010). It is suspected that the used experiment setup gives rise to significant model effects for the spatial distribution of overtopping. An experiment setup was recommended that is expected to more accurately model the behaviour of the prototype situation. Data on the spatial distribution of overtopping could not accurately be predicted by existing prediction methods. In some cases existing prediction methods provided an upper limit for overtopping (Juul Jensen, 1984) but none led to a good fit with the experimental data. A new reduction factor was found that reduces the amount of scatter and provides a conservative prediction of the experimental data.

Natural or unintended breaching can be catastrophic, causing loss of human lives and damage to infrastructures, buildings and natural habitats. Quantitative understand-ing of coastal barrier breaching is therefore of great importance to vulnerability as-sessment of protection works as well as to spatial planning against flooding hazards. The main objective of the present research is to develop a reliable process-based nu-merical model of coastal barrier breaching, which is capable of simulating both the breach initiation during storm surges and the barrier breaching due to overflow. The model is limited to homogeneous coastal sand barriers or similar types only, such as sand-dikes and sand dunes. The newly-developed model is referred to as a site model, i.e. the simulation domain is restricted to a representative cross-section of a barrier and the question where in plan a breach will occur is not answered by the model.

To increase physical insight into wave overwash processes at low-crested beach barriers, wave overtopping discharge events rather than the conventional average overtopping discharge need to be quantified. Also, in order to make intelligent use of the many empirical formulations on wave overtopping discharge at breakwaters from literature, a single-valued appropriate slope for a natural beach needs to be derived. To resolve these issues, laboratory experiments of composite-slope low-crested barriers were carried out. The tests deal with overwash on a smooth non-uniform slope on shallow foreshores. The conventional average overtopping discharge concept does not represent the discontinuous character and associated strength of overtopping ow. Instead, e.g. for purposes of morphological modeling, wave overtopping should be treated as an event-based process. In this study, several new parameters such as the wave-averaged overtopping time, the relative total overtopping time, the overtopping asymmetry, the average maximum discharge and the average instantaneous discharge are defined and formulated. A new approach for defining an equivalent slope is proposed in the parameterization of the overtopping discharge that also takes into account effects of the wave period. It is experimentally shown to be an improvement over the conventional approach by Van der Meer (1998), especially eligible for low-crested sandy slopes such as barriers, dikes, dunes, etc on shallow foreshores.

Wave overtopping discharges at coastal structures are well described in the EurOtop Manual (2007), including the distribution of overtopping wave volumes. Each volume that overtops a dike or levee will have a certain flow velocity and depth record in time, often given by the maximum velocity and flow depth. This paper describes some further development of the theory on flow depth and velocities on the crest, but will also show an inconsistency with respect to the mass balance. The second part of the paper gives an analysis of measured values on real dikes, simulated by the Wave Overtopping Simulator. It gives also the method of "cumulative hydraulic load" to compare overtopping discharges for different wave conditions. A large wave height with less overtopping waves, but larger overtopping wave volumes, is more damaging than a small wave height with more, but smaller overtopping volumes, even if the overtopping discharge is similar. The reasons to develop the cumulative hydraulic load have been compared with the recently in the US developed method of erosional equivalence.

In this paper, the effect of beach nourishment on wave overtopping in shallow foreshores is investigated with the nonhydrostatic wave-flow model SWASH. Firstly, the applicability of SWASH to model wave overtopping is tested by comparing results with a physical model setup with different storm wall heights on top of an impermeable sea dike. The numerical results show good agreement with the physical model. After validation, sensitivity analysis of the effect of beach nourishment on wave overtopping is conducted by changing bottom configurations with the SWASH model. From the sensitivity analysis, it becomes clear that wave overtopping discharge in shallow foreshores is characterized by the bores generated in surf zone due to wave breaking. To reduce wave overtopping discharge in shallow foreshore, it is important to reduce the horizontal momentum of the bores.