Modeling sheet-flow sand transport under progressive surface waves

Abstract

In the near-shore zone, energetic sea waves generate sheet-flow sand transport. In present day coastal models, wave-induced sheet-flow sand transport rates are usually predicted with semi-empirical transport formulas, based on extensive research on this phenomenon in oscillatory flow tunnels. However, recent sheet-flow experiments in large scale wave flumes, with progressive waves instead of oscillatory flow, have shown rather different results compared to the earlier tunnel experiments, namely significantly increased sand transport in onshore direction. This study investigates in detail how progressive waves affect the wave-induced bottom boundary layer flow, the sand transport rates and the behavior of the sheet-flow layer. Hereto, two numerical modeling tools have been developed and step by step validated, firstly on small scale flume data of wave boundary layer flow over fixed beds, subsequently on large scale flume data of sand transport rates and flow velocities above mobile beds, and finally on measurements of bed erosion. The models have been applied in a numerical parameter study to quantify the importance of various progressive wave effects over a range of wave and bed conditions. Thus, it was found how two competing streaming mechanisms, respectively the onshore directed progressive wave streaming and the offshore directed non-linear wave shape steaming, determine the wave-averaged current profile. Furthermore, it was found that for larger sand grains, progressive wave streaming is the major contributor to the increased onshore transport. However, for finer grains, also an alternating convergence and divergence in horizontal sediment advection contributes increasingly with decreasing grain size. The main result of this study is a detailed insight in how progressive wave effects contribute to sand transport. Next, parameterizations have been developed from the numerical results. These parameterizations form useful building blocks to improve practical sand transport formulas, which will contribute to better predictions of the coastal morphology in engineering practice.