Analysis of Longshore Sediment Transport on Beaches

Abstract

The present study investigates longshore sediment transport for a variety of bathymetric and wave conditions using the National Oceanic Partnership Program (NOPP) NearCoM Model. The model is used to determine the effects of wave shape and bathymetry changes on the resulting longshore sediment transport. The wave drivers, REF/DIF 1 and REF/DIF S, are used to assess the effects of monochromatic and spectral waves on longshore sediment transport, respectively. SHORECIRC is used as the circulation module and four different sediment transport models are used. Longshore transport comparisons are made with and without skewed orbital velocities in the shear stress and current velocities. It is found that the addition of skewed orbital velocities in shear stress and transport formulations increases longshore sediment transport by increasing time-varying effective shear stress. The addition of skewed orbital velocities greatly increases the transport due to advection by waves.

The localized longshore sediment transport is calculated using a generic physics based method and formulas by Bagnold, Bailard, and Bowen, Watanabe, and Ribberink. The transport results for each scenario are compared to the total transport CERC, Kamphuis, and GENESIS formulas. The bathymetries tested include an equilibrium beach profile, cusped beach profiles, and barred beach profiles with different bar locations. The longshore transport on an equilibrium beach profile is modeled for a 0.2 mm and 0.4 mm grain size and transport is compared to the CERC formula. The longshore sediment transport for d=0.2 mm is larger than d=0.4 mm when wave power is small, but as wave power increases the transport for the larger grain size dominates. The transport is also affected by the addition of cusps and bars on an equilibrium beach profile. The barred beach is modified to compare transport between waves breaking at the bar, before the bar, and after the bar. The features affect the transport when the wave powers are small, but as wave heights increase the cusp and bar features induce little change on the longshore sediment transport.