XBeach-G as a Design Tool for Rock on mild slopes under wave loading.

Abstract

This study describes the stability of rock on a mild slope (e.g. milder than 1:6) under wave loading. This is done because an increasing number of situations occur where mild foreshores are protected from the wave and currents. The empirical stability formula, designed by VAN DER MEER [1988], is not valid for these kind of slopes. Nonetheless, an extrapolation of this VAN DER MEER [1988] formula is currently used to design for these mild slopes. Recent research shows that an optimisation is possible as the extrapolated VAN DER MEER [1988] formula seems to overestimate the erosion. This research tests XBeach-G on mild slopes to verify its applicability as a design method for mild slopes. XBeach-G is a process based 1D numerical model, designed to model the physics occurring on mild gravel beaches. To verify the potential XBeach-G as a design tool, some of the VAN DER MEER [1988] tests are reproduced and the occurred damage is compared to two sediment transport formulas, the VAN RIJN, [2007] and NIELSEN, [2006]. Because the VAN DER MEER [1988] tests are executed on relative steep slopes, the slope angle is changed to more mild slopes. The observed trend is analysed on several hydrodynamic and morphodynamic parameters, such as the velocity, the acceleration, the shear stress and the sediment transport rate. Beside the slope, several other parameters such as the stone size; the phase lag angle and layer thickness are changed as well. A comparison of the test results to the formulas show that neither of the two formulas are able to predict the trend of damage levels as is found in the tests. The NIELSEN, [2006] formula gives unexpected results for steeper slopes, and the VAN RIJN, [2007] formula for mild slopes. This report proposes to use the VAN RIJN, [2007] formula for steeper slopes and to use the NIELSEN, [2006] formula for mild slopes. (milder than 1:6). Considerable attention should be given to the calibration factors in the NIELSEN, [2006] formula as these have a significant effect on the formed erosion. The model functions well enough for less detailed erosion profile estimations. The overall erosion depths and profile do not deflect that much and can be used for more dynamic profile descriptions. Because there is not a lot of data for comparison, it is recommended to do additional tests to verify the observed results. The test programme should focus on the point of incipient motion for coarse sediment under an angle. Both formulas are designed for horizontal sandy beds. Adjustments have been made to use them for rocks on slopes. Further research should focus on these correction factors as these do not yet seem to be correct