Wave Energy and Direction Observed near a Pier

Abstract

Alongshore gradients in wave energy and propagation direction were observed near a pier that extends 500 m from the Duck, N.C., shoreline to about 6-m water depth. When incident waves approached the beach obliquely, wave energy observed near the shoreline 200 m downwave of the pier was as much as 50% lower than observed 400 m downwave, and waves close to the pier were more normally incident than those farther downwave. Alongshore gradients were much smaller 400 m offshore of the shoreline, upwave of the pier, and with nearly normally incident waves, confirming that the gradients are associated with wave propagation under the pier. A spectral refraction model for waves propagating over the measured bathymetry, which includes a depression under the pier, accurately predicts the observations 400 m downwave of the pier, but overpredicts energy near the pier. Refraction model predictions that include partial absorption of wave energy by the pier pilings reproduce the observed alongshore gradients, suggesting that piling-induced dissipation may be important.