Narrow Results

The stability of the rear side of rubble mound structures under wave attack has been studied by means of physical model tests in a wave flume. The damage at the rear side slope has been measured for several structure types while the main hydraulic and structural parameters affecting the damage have been varied. The data-set was analysed to obtain estimates of the amount of damage for a wide range of conditions. By using estimates of velocities at the crest during wave overtopping events the amount of damage at the rear-side slope could be predicted. The estimates of velocities can be obtained by using an approach that estimates fictitious wave run-up levels for a situation as if the structure was non-overtopped. Depending on the acceptable damage level the required stone diameter at the rear side can be estimated based on the obtained prediction formula.

A method has been developed to estimate wave overtopping discharges for a wide range of coastal structures. The prediction method is based on the technique of Neural Network modelling. For this purpose use is made of a data set obtained from a large number of physical model tests. Moreover, a method was developed to obtain the confidence intervals around these predictions.

One of the main objectives within the EU-project CLASH (www.clash-eu.org) was to create a generic prediction method for wave overtopping at coastal structures by means of the Neural Network technique. An extensive and homogeneous database on wave overtopping was set up within CLASH, mainly with the aim to be used for the training process of the Neural Network (NN). A total number of 10,532 tests from 163 independent test series were screened and included in the database. The final database consists of far more information than needed for the training of the NN: 31 parameters are included to describe each overtopping test of which only 17 are used for the NN development. This explains the possible use of the overtopping database on its own. Plotting various parameters of the database together in graphs gives a clear view on the contents of the database. Also the ranges covered by the parameters can be detected in this way. The creation of the database, the analysis of the database, and the possible use of the database on its own are described in this paper.

The stability of rock slopes subject to breaking waves over a shallow foreshore is addressed in this paper. Physical model tests have been conducted on a 1:100 foreshore slope with a wide range of wave conditions including single and double peaked spectra and severely breaking wave conditions. The measured damage levels are compared to the Van der Meer design formulae, which were developed for conditions with no breaking on the foreshore. It has been found that the results of the design formulae are sensitive to the choice of input parameters (H_1/3 or H_m0 and T_m or T_m-1,0) and that the inclusion of the reduction factor for breaking waves (H_2%/H_s) in the design formulae should be applied only with caution, depending on the choice of input parameters. These results apply for the conditions tested and should be verified for other foreshore configurations before a definite recommendation pertaining to the use of the Van der Meer formulae for shallow foreshores can be given.

Wave overtopping over dikes may cause dangerous situations, one of which is breaching of dikes. Wave overtoppmg can be characterised by mean overtopping discharges but this does not provide information on individual wave overtopping events. This paper focuses on parameters related to individual wave overtopping events with a low probability of exceedance. Measurements of velocities and the thickness of water layers have been performed at the crest and inner slope of dikes. The height, width and roughness of the crest have been varied and also the angle and roughness of the inner slope. Based on these tests, prediction formulae have been derived for velocities, the thickness of water layers, and low-exceedance wave overtopping discharges. The estimates of these quantities can be used to characterise the wave loading on the crest and inner slope of dikes, especially as the characteristic wave loading for the initial phase of breaching of dikes initiated by erosion at the crest or inner slope. Besides above mentioned quantities the measurements were used also to obtain more information on for instance mean wave overtopping discharges and on volumes of water within individual overtopping waves.

All over the world breakwaters exist that are armoured with Antifer units. However, the influence of different placement methods on the stability of Antifer-block armour layers is still not clear. This paper presents the results of experiments with different placement methods performed in the wave-flume of the Delft University of Technology (Frens, 2007). Irregular waves were generated with a JONSWAP spectrum. It is concluded that, if the under layer and the toe are smooth and the blocks can be placed accurately, the best performing placement methods are the so-called 'closed pyramid placement method' for packing densities around 45% and 50% and the so-called 'double pyramid placement method' for packing densities around 55% and 60%.

The stability of the rear side of rubble mound structures under wave attack has been studied by, amongst others, Van Gent and Pozueta (2004). This was done for rubble mound structures without crest elements. This paper deals with the stability of the rear side of rubble mound structures with crest elements. Based on an extensive data-set, obtained from physical model tests in a wave flume, estimates of the amount of damage to the rear side have been obtained for a wide range of wave conditions and structure geometries. The required stone diameter at the rear side can be estimated based on the obtained prediction formula.

Wave flume experiments were carried out on wind affected overtopping of rough and smooth sloping coastal structures with crest elements in the low overtopping regime (q* = q/√gH_m0³ < 2 × 10^-4). The wind influence was modelled by a paddle wheel, a technique which has been shown to be reliable in assessing the maximum effect of wind on overtopping. Rough and smooth sloped coastal structures with slopes of 1:2 and 1:1.5 were tested. The crest of the coastal structure was modelled as a sharp crest using vertical crest elements of varying height. The results show that the mean overtopping discharge for rough and smooth sloping coastal structures under wind influence is in the range of 1.2-6.3 times the mean overtopping discharge without wind (q_w/q ≈ 1.2 and 6.3) and thus in the same range as previously published for vertical structures.

Computations of the Boussinesq-type wave model TRITON of wave propagation over a shallow foreshore with a varying low-tide terrace configuration were carried out. The results were compared with Scheldt flume physical experiments, using the same settings and post processing. The comparisons show that TRITON can model the wave propagation along shallow foreshores quite accurately as long as highly nonlinear effects do not play a significant role. Furthermore, the level of the low-tide terrace was found to influence the wave conditions at the toe of a fictive structure as follows: a) As the level of the low-tide terrace increases there is more energy dissipation and the significant wave height at the toe decreases, decreasing the hydraulic load. b) On the other hand, as the waves dissipate low-frequency energy is released and the spectral wave period at the toe increases, increasing the hydraulic load.

Physical model tests on the stability of gravel beaches under the combined loading of waves and currents are performed in the Delta Basin at Deltares | Delft Hydraulics. The physical model tests provide data on the profile development and longshore transport rate under these circumstances. The morphological response of gravel beaches seems to be governed predominately by either the cross-shore beach profile development or the longshore current velocity depending on the combination of longshore current velocity and angle of wave incidence. The dataset obtained is limited, but the obtained information is valuable for gravel beaches that serve as man-made sea defence.

Physical model tests were performed to obtain information on the stability of rock slopes with a horizontal berm. This paper is focussed on the rock slope stability of the slope above the berm. By applying a berm the rock size in the upper slope can be significantly smaller than for a straight slope without a berm. The influence of the width of the berm, the level of the berm, and the wave steepness have been investigated. Based on the test results a prediction formula has been derived for 1:2 slopes with various dimensions of the berm.