Sandy Borrow Area Sedimentation—Characteristics and Processes Within South Pelto Dredge Pit on Ship Shoal, Louisiana Shelf, USA

Ship Shoal is an inner-shelf submarine shoal with large amounts of restoration quality sand that was dredged in 2013–2016 for the Caminada Headland Restoration Project in central Louisiana, USA. Here we provide the first assessment of the characteristics and rate of seasonal sediment infilling the South Pelto dredge pit on eastern Ship Shoal, which will inform future restoration initiatives. Vibracore and multicore samples collected in 2017 and 2018 from within and outside of South Pelto dredge pit revealed new interlaminated silts and clays (median grain size 4.3–6 ϕ, 16–48 µm) deposited at the bottom of the dredge pit, underlain by original Ship Shoal sand and older pro-delta deposits. Through analyzing beryllium-7 ( 7 Be) inventories in multicores, we found that sedimentation rate varied seasonally from 4 to 12 cm (sedimentation rate 0.02–0.06 cm day −1 ) in fall 2017 to 8 to16 cm (0.05–0.15 cm day −1 ) in spring 2018. Mapping plume distribution along the continental shelf with satellite imagery corroborated with local wind/wave data revealed that the sediments deposited in South Pelto dredge pit are likely sourced from a combination of the Atchafalaya and Mississippi River plumes during the study period, in addition to possible resuspension from the ambient inner continental shelf and fine-grained material exported from proximal bays. Sedimentation in the dredge pit is not greatly affected by wall slope failure or bedload influx of ambient Ship Shoal sand. Infilling rate and material are slower and finer than past model prediction. Restoration quality sand on this sandy shoal in Louisiana is thus not renewable.