The stability of rock on mild slops under wave attack

Abstract

Momentarily the determination of the static stability of rock on mild slopes can be split in roughly two paths, the empirical and the physical approach. However, both the approaches have their limitations from which one could conclude that neither approach is currently an accurate description of the stability of rock on mild slopes under wave attack. The empirical approach by van der Meer [1988] has to be extrapolated outside its validity region and is deemed to be too conservative by Schiereck and Fontijn [1996]. The physical approaches on which the software XBeach-G[1], runs are capable of globally determining dynamic profile response reasonably well however they cannot accurately determine static stability. From this lack of an accurate approach to determine the stability of rock on mild slopes under wave attack, follows the research question of this thesis: “How should the stability or rock on mild slopes be described?” To determine how to describe the stability of rock on mild slope under wave attack, a test plan was set up. The test plan was split into two parts, firstly the empirical approach and secondly the physical approach. The empirical approach is based around the formula of Van der Meer [1988] for plunging waves. Whereas the physical approach follows the methodology of Xbeach-G. For which the input parameters have been derived by the means of a combination Bubble Image Velocimetry (BIV) and Particle Image Velocimetry (PIV). Concluding on the test results, the empirical Van der Meer [1988] gives good results for slopes up to 1:10. For slopes milder than 1:10, the damage should no longer be described by the damage level ‘S’. A good alternative seems to be the damage depth ‘E3’. The fact that ‘S’ no longer is a proper way to determine the damage, suggests that the Van der Meer [1988] formula should not be used for mild slopes any more. However, for all cases the extrapolation of Van der Meer [1988] results in (very) conservative approach and can therefore still be used in this form, but should serve as a first indication. The Van Rijn [2007] approach seems to be preferable over the Nielsen [2006] approach, because the Van Rijn [2007] approach aligns with the visual observations and profile change measurements. Apart from better results, the Van Rijn [2007] is more pragmatic than the Nielsen [2006] approach. This is because the parameters used in Van Rijn [2007] can easily be determined with basic details of the slope, whereas the Nielsen [2006] approach significantly depends on the accuaracy of the parameters (phase lag angle and the sediment friction coefficient), which both have been proven hard to determine.