Abstract
Sandy sediments preserved as paleo-channel fill on the inner shelf, some of which are overlain by modern muds, have been mined for barrier island restoration along the northern Gulf of Mexico. These mined areas have been termed “mud-capped” dredge pits. The processes governing the morphological evolution of the pits are poorly constrained due to limited observational data. Physical oceanographic (e.g., currents and waves) and sedimentary data were collected at Sandy Point dredge pit offshore Plaquemines Parish, Louisiana in summer 2015. Currents outside the pit flowed southward and/or southeastward at speeds of 8–20 cm/s, while currents inside the pit had speeds less than 2 cm/s with no clear dominant direction. Wave heights detected inside the pit were less than 0.4 m. A high turbidity layer with suspended sediment concentration around 4 g/L was observed above the pit floor, and its thickness was ~0.5 m. With observational data as input, three 2–D numerical models were employed to predict pit morphological responses, including pit infilling, margin erosion and slope change. The model results suggest that resuspension events were rare on the seafloor adjacent to the pit under summer fair weather conditions. Modeled pit margin erosion was very limited. With little resuspension of seafloor sediment locally, weak margin erosion and stable pit walls, the dominant process governing pit evolution was infilling sourced by the deposition of suspended sediments from the Mississippi River plume.
Highlights
Due to eustatic sea level rise, local subsidence, and dam construction, coastal lands are diminishing worldwide [1]
suspended sediment concentration (SSC) for sea surface water was higher at T1, and a slight decrease was captured when river discharge dropped from July to August 2015 (Table 2 and Figure 2A)
Pit migration caused by net sand transport in one direction and (SDDPS) is closely related to: (1) pit migration caused by net sand transport in one direction and in water column caused by the reduction of current speeds over pit [13,24]
Summary
Due to eustatic sea level rise, local subsidence, and dam construction, coastal lands are diminishing worldwide [1]. In order to restore coastal lands, beaches and barriers, dedicated dredging and river diversions are employed [2,3]. Offshore sand mining provides sand resources to implement coastal restoration projects. Sand has been deposited in paleo-river channels along the Gulf of Mexico inner shelf and is sometimes overlain by a several-meter-thick shelf mud layer. The use of offshore sand from paleo-channels to nourish beaches and barriers has greater benefits to the coastal system by supplementing a deficit in the coastal sand budget with ‘out-of-system’. It is important to understand its potential effects to local marine ecology and fisheries, borrow area indirect impacts, and risks associated with offshore infrastructure [4,5,6,7,8]
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