An Experimental Investigation of Alongshore Wave Momentum Transfer to Nearshore Flows in the Outer Surf Zone of a Steep Sand Beach

The nearshore hydrodynamics on a steep beach involve rapid wave transformation and generation of currents in a narrow surf zone close to the shoreline. For better understanding of the hydrodynamic processes in this type of coastal environment, a laboratory experiment was constructed with a 1:5 beach slope in a large wave basin. Obliquely incident irregular waves with a range of different statistical properties were generated at the offshore boundary to force the nearshore hydrodynamics, and spatially intensive wave and current measurements were collected across the beach. The beach is made of natural sand to achieve realistic bottom friction while the slope was periodically reset to original bathymetry to maintain a near-constant beach profile in all tests. Momentum was evaluated at cross-shore measurement sites inside and outside the surf zone to investigate the cross-shore transfer of alongshore wave momentum to the nearshore flows. This approach reveals that radiation stress gradients and cross-shore advection terms are dominant in the shoaling region. In the breaking region, the alongshore 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. The alongshore momentum comes close to balancing across a relatively short distance on the steep beach, and the momentum across both the higher flux breaking region must be combined with the lower flux shoaling region for closure of the momentum balance. Overall the experimental results indicate the importance of the alongshore momentum flux from wave breaking to advection and mixing that extend outside the narrow breaking region on a steep beach.