HYDRODYNAMICS, SEDIMENT TRANSPORT, AND WATER QUALITY OF TWO CONTRASTING DREDGE PITS ON THE LOUISIANA SHELF

Sediment is needed for coastal restoration in Louisiana and is often excavated offshore from both mud-capped and sandy dredge pits. The mined sand from mud-capped pits, like Sandy Point which is west of modern Mississippi Delta of Louisiana, was originally from a paleo river channel and later covered by mud from modern coastal processes. Sandy pits, like Caminada pit in eastern Ship Shoal on Louisiana shelf, generally experience higher energy conditions. To better understand the post-dredging effects on hydrodynamics, sediment transport, and water quality, two tripods were deployed at Caminada and Sandy Point pits in summer 2018 and 2019, respectively, and profiling data were collected during a total of 34 casts in 2018 and 2019. At both pits, one tripod was positioned inside the pit while the other was outside for comparison. An Acoustic Doppler Current Profiler, HydroCAT, Acoustic Doppler Velocimeter, and a wave gauge were mounted on each tripod and deployed for 1-2 months of 2018 and 2019, respectively, to collect high temporal resolution data. The results reveal similarities and differences between pit and adjacent waters and allow us to compare two unique locations. Tropical storm events were observed during each deployment which provided insight concerning the effects these storms have on dredge pits. Water masses inside Caminada pit were relatively stable and sluggish which resulted from the pit’s deeper depth. Sandy Point dredge pit was highly impacted by the Mississippi River plume due to its close proximity, which effectively stratified the water column in summer 2019, affecting ventilation near the seabed. Strong winds, taller waves, higher shear stresses, elevated turbidity and increased dissolved oxygen were found during the passages of tropical storms over both pits. When comparing with outside pit tripod stations, the inside station of Caminada pit experienced a longer duration of low dissolved oxygen in bottom water column. Water mass movements and oxygen consumption from resuspension of pit bottom are key mechanisms driving oxygen dynamics. The results provide key observational data for future studies on post-dredging morphologic change, marine communities, and oxygen dynamics of dredge pits on the Louisiana shelf and other coastal dredge areas worldwide