Small-scale Sediment Transport Processes and Bedform Dynamics

Book excerpt: ABSTRACT: The generation of small-scale sedimentary structures in the coastal environment is a complex process that occurs over a wide separation of scales in both time and space. These bedforms are ubiquitous features of the nearshore region, and yet specific information regarding their behavior and characteristics is still lacking. Specifically, it is unclear whether the bedload-dominated processes of the linear regime are as equally responsible for the generation of bedforms in the nonlinear regime, where flow separation, and subsequent vortex formation, tend to govern the dynamics of the bottom boundary layer. While a simple one-dimensional model is derived and used to explain incipient bedform growth in the linear regime, such an approach is not well-suited at addressing the complexities of the wave bottom boundary layer. Utilizing a new three-dimensional phase-resolving live-bed model, we simulate the dynamics of bedforms, such as sand ripples, in the nonlinear regime. Through forty-three independent simulations, the model has been found to reproduce oscillatory boundary layer flow, as well as provide accurate predictions of ripple geometry in both lab- and field-scale flows. Model results confirm that in the linear regime, bedform growth is promoted purely through bedload sediment transport, but inertial properties of the sediment are equally as important. In the nonlinear regime, bedform growth is also dominated by bedload transport; however, the entrainment and deposition of bed material plays an important role in maintaining ripple equilibrium, whereas it is mostly responsible for ripple decay.