Sandy Dredge Pit Sedimentation � Characteristics and Processes in Caminada Borrow Area, Ship Shoal, Louisiana Shelf, USA

Abstract

Mississippi River Deltaic Plain’s barrier islands are undergoing rapid disintegration due to high rates of subsidence and a deficit in coastal sediment supply. To mitigate for barrier island land loss, Louisiana has implemented a restoration program that supplements coastal sediment deficits by introducing sand from outside of the active coastal system. 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. Vibracore samples (1.5 - 3.5 m deep) collected in 2017 and 2018 in Caminada Borrow Area revealed new silts and clays deposited at the surface, underlain by original Ship Shoal sand and older pro-delta deposits. Through analyzing Beryllium-7 (7Be) inventories in shallower multicores, we find that in 2017, 4-12 cm of sediments were deposited in Caminada Borrow Area with sedimentation rates at 0.02 – 0.06 cm/day. During repeat coring in 2018, 8-16 cm of sediments were deposited and sedimentation rates were calculated to be 0.05 – 0.15 cm/day. There is little difference in grain sizes between the two years, although interlaminated silty packages with a few isolated sand lenses can be seen in x-ray images. Clays and fine silts deposited in Caminada Borrow Area were likely sourced from the Atchafalaya River plume during the study period, with some contribution from the main-stem Mississippi River and resuspension from ambient bays and inner shelf. Analysis of local wind/wave data revealed that Atchafalaya plume sediments extend southeastward to reach Caminada Borrow Area immediately following winter storms or tropical storms. Resuspension and redeposition during higher energy storm or hurricane events likely produced the observed coarse silt laminations. Sedimentation in the borrow area is not greatly affected by wall slope failure, evidenced by the lack of Ship Shoal sand in Caminada Borrow fill deposits. These results are in contrast with recent studies of mud-capped dredge pits on the Louisiana continental shelf and numerical models, as infilling rate is slower than predicted and the material is mainly silts and clays. Restoration quality sand on this particular sandy shoal in Louisiana is thus not renewable.