Morphologic Change and Sediment Transport in a Dredge Pit on an Energetic Submarine Shoal of the Louisiana Shelf: Evaluating the Efficacy of Restoration

Abstract:

Coastal barrier island restoration projects have become increasingly popular in recent years when dealing with rapid land loss and relative sea level rise. The demand to find high quality, large volume and locally available sediment resources has forced decision makers and managers to explore multiple types of sediments and dredging methods. Block 88 on Ship Shoal, centered on the Louisiana continental shelf, is the largest sand borrow area in Louisiana’s history in terms of excavation volume (1.5 million cubic yards), and has been identified as the primary borrow area for several nearby barrier island restoration projects. Findings from preliminary research and modeling of the region gave decision makers the confidence to establish setback areas and designate dredge pit locations deemed appropriate for restoration projects. In order to further investigate these early findings and test their efficacy for future restoration efforts, this study uses multiple geophysical methods including bathymetry, sidescan, and subbottom profiling to study sediment transport and morphological change at the Block 88 dredge pit immediately following, one-, and two- years post-dredging. Our objectives are to quantify the sediment infilling processes, characterize morphological evolution (e.g., dredge cuts, slopes, pit wall migration and stability) and investigate the relative contributions from proximal rivers (Atchafalaya/Mississippi), energetic waves, tides, longshore currents, and shoal reworking. Our results indicate that the deeper parts of the Block 88 dredge pit show lower-reflectivity backscatter that correlate to a relatively finer sediment texture (i.e., mud) compared to the adjacent sandy shoal. These results increase our decision-making ability regarding sea floor stability and protection of both environmental and cultural resources, as well as provide for better management of valuable sand resources.