· · · Repository Hydraulic Engineering Reports

ELASTOCOAST revetments are highly porous structures made of crushed stones which are durably and elastically bonded by Polyurethane (PU). To improve the understanding of the physical processes involved in the wave-structure-foundation interaction and to develop prediction formulae for both hydraulic performance and wave loading more than 75 large-scale model tests using both regular and irregular waves were performed. Three ELASTOCOAST revetment alternatives with the same slope (1:3) and the same revetment thickness (0.20 m) but with different thicknesses of the underlying filter layer (0.00 m, 0.10 m and 0.20 m for Model Alternatives A, B and C, respectively) were tested. More than 85 measuring devices synchronously connected to two video cameras were used. Prediction formulae are developed for wave reflection, wave run-up and run-down as a function of the surf similarity parameter which illustrate the advantage of ELASTOCOAST revetments as compared to conventional revetments. For instance, more than 25% less wave runup may result on comparison to smooth impermeable revetments. Using a surf similarity-based wave load classification as well as a systematic parametrization in both time and space, prediction formulae are also developed for both impact loads on and just beneath the revetment. These include the peak pressure pmax, its location in relation to still water level zpmax, the spatial pressure distribution and the time related parameters (rise time and total load duration). Prediction formulae for the wave-induced pore pressure in the sand core beneath the revetment are also provided, including the maximum pressure at the upper boundary of the sand layer and its development in deeper layers. Formulae are also proposed for the flexural displacement ä of the ELASTOCOAST revetment, showing that for impact load much smaller displacements would result than for non-impact load and that ä linearly increases with peak pressure pmax for a given revetment thickness. Finally, a stability analysis of Model Alternative A is performed on basis of the results of the measurements and the simultaneously recorded videos. The results illustrate why Model Alternative failed due to local transient soil liquefaction while Model Alternative B tested synchronously under the same wave conditions did not fail.

CLASH is concentrating on investigations of wave overtopping for different structures in prototype and in laboratory. The model investigations have focussed on wave overtopping and the comparison of overtopping results from small-scale model tests and prototype measurements. Possible differences in the results from small-scale tests and prototype were analysed with respect to measurement accuracy as well as model and scale effects. This report proposes a methodology to assess the aforementioned effects and to provide the uncertainties and correction factors for quantifying the various influences when performing model tests. First, the available literature on scale and model effects has been reviewed. It was found that scale effects especially for wave run-up and overtopping have been reported in the past. Manydikes up to 25% higher wave run-ups were observed. Wave overtopping for armour slopes in front of vertical walls in prototype was reported to be up to 10 times higher than in model tests but it is still not clear whether this is due solely to scale effects. Second, some theoretical considerations were performed to derive critical Weber and Reynolds numbers which should always be exceeded during model tests. It was found that for wave run-up and wave overtopping Weber numbers should not fall below Wecrit = 10 and that water depths should always be larger than 2 cm and wave periods longer than 0,35 s. This is usually the case in all models. Additionally, the overtopping related Reynolds numbers should be larger than 1·103 which is also the case for most of the model tests. Results for all field and model investigations have been plotted for the investigated sites using data from the field and two models of smaller scale. Results have shown that model tests performed for the vertical wall in Samphire Hoe and the steep Zeebrugge rubble mound breakwater do not deviate much from the prototype data points. However, for the flatter slope in Ostia differences between prototype and model have been observed in the order of up to one order of magnitude. A Monte-Carlo simulation was used to determine the variation which may occur when different measurement uncertainties and scale effects are considered. The results show a large dependency on the magnitude of the overtopping rate itself which was also evident from the observation of the model tests. Differences of a factor of about 5.0 for large overtopping rates and a factor of about 40.0 for low ones are observed. Finally, a new parameter map for scaling was proposed taking into consideration the aforementioned findings (Fig. 22). The map depends on whether or not the structure is ‘rough and sloping’ and eventually suggests a scaling predictor. The latter was then applied to the test cases of Zeebrugge and Ostia.

Within WP4 laboratory investigations for the Zeebrugge rubble mound breakwater have started in the wave flume of Leichtweiß-Institute for Hydraulic Engineering of the Technical University of Braunschweig (hereafter LWI) and in the wave flume of the Universidad Politécnica de Valencia (hereafter UPVLC). In order to check any influence that is typical for laboratory measurements and to identify possible causes for differences in results, identical tests will be carried out in the two laboratories (LWI and UPLVC). This report describes the laboratory facilities at LWI and UPVLC and the results of LWI and UPVLC parametric tests as well as the UPVLC and LWI reproduction of storms and tests with wind conditions. The results of these tests will already give an indication for (i) scale effects to be expected; (ii) differences of various types of measurement techniques; (iii) model effects; (iv) relative importance of analysis methods and (v) wind effects; and (vi) magnitude of forces on persons induced by overtopping waves (hazards).

Wave overtopping is one of the most important processes for the design of seadikes which was responsible for many severe dike failures in the past. Wave overtopping can not be avoided due to the random nature of the waves and the uncertainties associated with the determination of the design water level. The present design on wave overtopping is based on average overtopping rates. Nevertheless, average overtopping rates are not sufficient for the design of seadikes, because overtopping velocities and overtopping layer thicknesses are required to assess the infiltration and erosion on the landward side of seadikes by overtopping water. The objective of the present thesis is the determination of the velocities and layer thicknesses on the seaward slope, the dike crest and the landward slope as a function of the relevant wave and dike parameters by means of theoretical and experimental investigations. Small scale model tests have been carried out in a wave flume with typical dike profiles by using regular waves and wave spectra. Small scale model tests are influenced by scale effects and the transfer of the results to nature might be affected. Therefore, theoretical investigations on the influence of viscosity and surface tension on the model results are performed. It can be concluded that the results of the present study are not significantly influenced by scale effects and can be transfered to prototype scale. Wave overtopping is dependent on the processes associated to wave breaking and wave run-up on the seaward slope. Therefore, these processes are investigated first. Breaker type, breaking water depth, wave run-up height, wave run-down and the impact point of the breaking wave on the seaward slope are determined and described by means of empirical equations. The main part of this thesis is the determination of layer thicknesses and velocities on the seaward slope, the dike crest and the landward slope. Layer thickness and wave run-up velocities on the seaward slope are closely connected to the wave run-up height. Empirical formulae for layer thicknesses and a theoretical approach for overtopping velocities are derived for the dike crest. Overtopping velocities and layer thicknesses on the landward slope are derived on the basis of the two-dimensional momentum equation and the continuity equation. All derived formulae are calibrated by model tests with regular waves and verified by model tests with wave spectra.

Cost comparision of an Elastocoast revetment in comparison with a classical revetment in coastal protection.