Conclusions

A numerical wave model was developed to study the potential impact of proposed sediment borrow pits on the wave characteristics and longshore sediment transport rate (LSTR) near Breton Island, Louisiana. The model consisted of four nested domains that were used to simulate a set of 116 scenarios derived from a wave climatology for a baseline (no pit) condition and two proposed borrow pit configurations. Surf zone wave characteristics for each scenario provided the input for an empirical model for the LSTR. Model results for bathymetries including the proposed pits were compared to the baseline case to determine the impact the particular borrow pit designs considered would have on wave climatology and alongshore transport at Breton Island.

Comparisons between the wave height predicted for the baseline case and the two borrow pit configurations showed that changes in significant wave height were less than 10 centimeters and largest in the immediate vicinity of the borrow pit. The LSTR direction and magnitude depended on the individual wave scenario. The change in magnitude of the calculated LSTR with the addition of the two borrow pits was an order of magnitude less than the LSTR calculated for the baseline case. The gradient of LSTR changed somewhat with the addition of the borrow pits, particularly in a region of slightly less erosion toward the southern end of the island, with slightly more erosion to the north and slightly less accretion to the south of that location; however, changes were less than an order of magnitude of the baseline gradient with no pits, and no new convergences or divergences of sediment transport were identified.