A Three-Dimensional Experimental and Numerical Study of Nearshore Hydrodynamics and Morphology Change on a Steep Sand Beach

Abstract

Storm waves breaking on steeply-sloping sand beaches can generate strong currents across a narrow surf zone, and experience different rates of sediment transport and morphodynamic change compared to beaches with milder slopes. To investigate the dynamics on this type of beach, a three-dimensional physical model is implemented in a wave basin and a long steeply-sloping sandy beach is exposed to a series of different conditions generated by a paddle. Three different experiments are performed to observe the nearshore responses to surface waves. A momentum balance approach is applied to the nearshore hydrodynamics inside and outside the surf zone to investigate the transfer of wave momentum to the alongshore flow. This approach reveals that radiation stress gradients and cross-shore advection terms are dominant in the shoaling region. In the breaking region, the momentum exchange is mainly governed by the radiation stress gradients with a smaller contribution from cross-shore advection that relatively balances with the bottom friction and horizontal mixing terms. To investigate the beach morphology change, topographic surveys of a beach-dune system are performed and related to changes in wave energy during a storm. The results indicate that a near-vertical beach scarp forms, retreats landward, and then becomes controlled by the nearshore beach slope. Sediment eroded to form the scarp is initially deposited at the dune toe, however, as wave energy increases sand is transported by the alongshore current and is deposited in the nearshore zone increasing the surf zone width. A non-hydrostatic phase-resolving numerical model is used to investigate the wave-driven hydrodynamics that correspond to observed beach morphology changes during a storm. The numerical results indicate that the alongshore current is generated by wave breaking in the nearshore zone and it increases in magnitude across a widening surf zone for increasing wave energy. This is associated with a bottom shear stress that exceeds the critical bed shear stress, with the potential to initiate motion and transport sediment in agreement with the experimental observations. Overall, the results of this thesis provide insight into the coupled interaction between the three-dimensional wave-driven hydrodynamics and morphologic evolution of a steep sandy beach.