Modeling Sandbar Effects on Nearshore Waves and Morphological Change using SWAN

Abstract

Numerical model simulations (Delft3D SWAN) are used to examine the impact of small alongshore variations in the bathymetry of an outer sandbar (in about 5-m water depth) on the nearshore wave field as the shallow (< 3 m) bathymetry changes from near alongshore uniform to strongly spatially variable to understand wave driven morphologic evolution. Waves were observed at Duck, NC with an array of 14 pressure gages between 1- and 3-m water depth spread over 250 meters alongshore. Bathymetry was measured between the dune toe and about 8-m water depth on September 26 and October 2, 2013. The bathymetry evolved from roughly alongshore uniform on September 26 to strongly alongshore variable on October 2. Between these dates incident significant wave heights ranged from 0.5 meters to 2.3 meters, with incident angles from 20 degrees north to 5 degrees south of shore normal. Simulations were run with observed bathymetry for both the outer bar and inner shallow bathymetry, with smoothed outer bar and observed shallow bathymetry, and with digital elevation model bathymetry to determine the effects of outer bar and shallow bathymetry on wave evolution.