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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 flow. 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.

Little is known about the strength of the land-side slopes of the Vietnamese sea dikes under overtopping attack. Some grass-covered slopes were tested by a wave overtopping simulator. The paper describes the simulator and presents the test results. The tested grass covers could withstand overtopping rates varying from 20 l/s to 100 l/s per meter of dike length in some hours. Damage usually started at bare spots, at the transition between the slope and the toe, at the transition between different materials, and around objects (e.g. big trees). These features reduce the strength of a grass cover and therefore should be avoided.

Physical model tests were done on an armour of Tetrapods, placed in a single layer. The objective of the investigations was to study the stability of the secondary layer, and to see if the material of this secondary layer could be washed out through the single layer of Tetrapods. It was concluded that secondary armour is not washed out through an undamaged layer of Tetrapods, and that all damage to the secondary layer is related to damage in the primary layer. To prevent damage to the Tetrapod layer, a high placing density is needed. In case of low placing density, the area around the waterline will slide down, creating gaps in the main armour and exposing the secondary layer. Because of this process the placing density in lower sections of the breakwater increases, and consequently also the strength increases at those places.

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.

For management of coastal breaching hazards it is critical to be able to assess the potential of coastal barrier breaching as a result of wave actions during storm surges (wave overwash). This phenomenon is known as the breach initiation phase; the mechanisms behind this are not very well understood. In the present study, laboratory experiments of mobile-bed (sand) barrier were carried out to increase the understanding of the processes of the barrier response during storm surges and also to generate data for calibrating a new numerical overwash model. A numerical model of the barrier response, which integrates the processes of beach and dune erosion and of wave overwash, has been developed. The approach of the UNIBEST-TC model (Bosboom et al., 2000) was adopted for modelling the processes of beach and dune erosion. For modeling wave overwash new overtopping parameters are introduced, based on the approach of Tuan et al. (2006). The model is capable of simulating the time-dependent barrier response during storm surges with occurrence of moderate to severe overwash. Overwash-induced effects on the cross-shore transport processes are also effectively incorporated. The model has been calibrated with the laboratory data on the barrier response (the barrier profile response and the overwash channel development). The process of the barrier response together with its major morphological features was fairly well predicted.

Experience has shown that stability parameters for breakwater armour are different for steep and shallow foreshores, as well as for deep and shallow water. A change from a deep water spectral information (T_m0) to shallow water spectral information (T_m−1,0) does not completely explains this difference. Tests have shown that stability also depends on parameters not described by the spectrum at the toe of the breakwater. It is suggested that these parameters include for example the skewness of the waves.

Theoretical and experimental study on placement of single layer armour units, the study is performed with Xbloc. Parameters to assess quality of placement grid, like packing density and potential interlocking coefficient are presented. Recommended placement pattern for roundhead and curved sections is determined.

Modern design formula for coastal structures (like rock stability formula and overtopping formula use wave parameters (H_2% and T_m-1,0) which are not readily available from standard boundary condition wave data. For transforming values like H_s and T_p to the parameters used in the new formulas, often-fixed conversion factors are used. However, this may lead to significant errors. Therefore, it is better to calculate these new parameters with an appropriate wave transformation model. The one-dimensional Graphical User Interface for SWAN (SwanOne) is presented as a simple tool to perform the required transformation.

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 effect of single layer interlocking armour unit breakage on the hydraulic armour layer stability and potential damage progression is addressed in this paper. A 2-dimensional scale model of a rubble mound breakwater with an armour layer consisting of Xbloc armour units was tested. The residual armour layer stability with broken units was determined. The armour unit displacement and damage progression was assessed. According to the test series breakage of the single layer armour units has a significant negative effect on start of damage of the armour layer. Breakage of units has however no significant effect on failure of the armour layer. This leads to a long and gradual damage progression compared to an armour layer without broken units.

To assess the hydraulic performance of coastal structures – viz. wave run-up, overtopping and reflection – and to evaluate the stability of the armour layers, use is made of the dimensionless surf similarity parameter, as introduced by Battjes (1974). The front side slope of the structure and the wave steepness are combined in this parameter, also called the Iribarren number. The introduction of the wave steepness was based on the wish to include the effect of the wave period, T, in the surf similarity parameter and hence in the various methods that describe the hydraulic and structural response to waves. The wave steepness to be used in the various methods is the fictitious wave steepness: the ratio of the wave height at the toe of the structure (H) and the fictitious deep-water wavelength (L_o), or rather, the squared value of the local wave period, multiplied by g/2π. In deep water the fictitious wave steepness equals the real wave steepness (H_o/L_o), but this is not the case in shallow water, H/L_o ≠ H/L. The characteristic wave period of a wave field travelling into shallow water is subject to change, due to bathymetry, initial wave breaking, etc. Using the real deep-water wavelength in the expression for the fictitious wave steepness may, therefore, lead to incorrect conclusions when evaluating the key response characteristics in (very) shallow water. To avoid ambiguities and mistakes, it is therefore suggested to refrain from using the wavelength in the expression of the fictitious wave steepness, but to rather only use the local wave period: s_o = 2πH_s/(gT²). A logical next step would be to use "s_f" as the notation for the fictitious wave steepness.

The current research is aimed at finding a proper relation between flow forces acting on the bed and the bed response. To this end, experiments were carried out in which both the bed response (quantified by a dimensionless entrainment rate) and the flow field (velocity and turbulence intensity distributions) are measured. The three available stability parameters, which are used to quantify for the flow forces, were evaluated using the measured data. The focus of the evaluation is on the correlation of these stability parameters with the measured bed damage expressed in terms of the dimensionless entrainment rate. The experimental results confirm that the Shields stability parameter fails to predict bed damage for non-uniform flow conditions (R² = 0.18). In contrast, the stability parameters of Jongeling et al. (2003) and Hofland (2005) give better damage predictions (R² = 0.77). The results confirm the strong influence of the velocity and turbulence intensity distributions on the stability of bed material.

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 1/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.

In de Van der Meer formulas for armour stability the Notional Permeability is used as a parameter. Unfortunately the physical basis of this parameter is weak. It is therefore suggested to use a relation between the Notional Permeability P and the reduction of wave run-up due to infiltration into the breakwater. The advantage is that the latter can be computed with VOF models. This makes it possible to estimate the value of P from mathematical models. Also the run-up reduction can be measured in a physical model, which has the advantage that physical tests for run-up are much faster to execute than models for armour stability.