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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 (7Be) 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. [ABSTRACT FROM AUTHOR]

Many research efforts have been undertaken over many decades in the field of Louisiana coastal restoration, but long-term experiments for sediment suspension and consolidation in diversion-receiving basins are still limited, despite significance of this topic to ongoing restoration strategies. Sediment samples were collected from two active diversions on the Mississippi River: West Bay, a semi-enclosed bay located on the Mississippi River Delta and fed by the West Bay Diversion, and from Big Mar pond, a receiving basin of the Caernarvon freshwater diversion from the lower Mississippi River, Louisiana, USA. A dual-core Gust Erosion Microcosm System was used to measure time-series (0.5, 1, 2, 3, 4, 5, and 6-months after initial settling) erodibility at seven shear stress regimes (0.01–0.60 Pa) using experimental cores prepared with two initial sediment concentrations (60 and 120 kg m −3 ). A 230-cm tall settling column with nine sampling ports was used to measure the consolidation rates for initial sediment concentrations ranging from fluid mud (10 kg m −3 ) to dredge effluent (120 kg m −3 ), in combination with two levels of salinity (1 and 5). The erodibility of West Bay sediment decreased with increasing time of consolidation. The critical shear stress for resuspension increased from 0.2 Pa after 2 months to 0.45 Pa after 4 months of consolidation. The consolidation rates were inversely and exponentially related to initial sediment concentrations. Consolidation tests in salinity of 1 generally settled faster than that in salinity of 5, and consolidation tests with low sediment concentration tests generally settled faster than high-concentration tests. An exponential coefficient was added in the Sanford (2008) model to better predict the consolidation profile of both rapid early settling and slow self-weight consolidation processes. Our study suggests that enclosed basin, low salinity, relatively low sediment concentration and minimized disturbance for 4 months all favor of mud deposition and retention, and these factors should be considered in the design of coastal restoration projects such as dredging effluent placements and sediment diversions. [ABSTRACT FROM AUTHOR]

Erodibility is critical to the sediment resuspension process but has not been measured systematically in large river-dominated muddy continental shelves before. During early summer of 2011, the Mississippi River experienced a major flood event. This flood provided a unique opportunity to examine how shelf seabed erodibility responded to a large river flood, and the ultimate fate of flood deposition is important to geological and biogeochemical processes (e.g., stratal formation, carbon sequestration). A total of 106 sediment cores were collected on the Louisiana shelf during five cruises in 2010 and 2011, and a new dataset was used to evaluate the response of the seabed to the recent conditions. The localized flood deposit was mainly within tens of kilometers of river sources, and little sediment accumulated on the middle Louisiana shelf. Seabed erodibility was measured using a dual-core Gust Erosion Microcosm System. The erodibility of sediment collected in April 2011 exceeded that for August 2010 and August 2011. The springtime increase in erodibility seemed to be related to the recent presence of energetic waves that mobilized the seabed. Erodibility was highest on the inner shelf southwest of Atchafalaya Bay, intermediate on the middle shelf, lowest in the Mississippi Canyon, and highly variable on the Mississippi subaqueous delta. These spatial patterns were influenced by proximity to river sources, flood-deposit thicknesses, intensity of wave-driven bed stresses, and bioturbation. The flood-deposit thickness itself, however, was not sufficient to explain all the spatial variations of erodibility after the peak of the Mississippi flood. Comparing values to published data, the depth-varying erodibility on the Louisiana shelf was close to the “low erodibility” level for the York River of Virginia, and similar to the data collected from Baltimore Harbor in Maryland and the main stem of upper Chesapeake Bay. Our findings promote understanding of the resuspension of fluffy organic-rich layer at the water–sediment interface, which influences sediment oxygen demand on the Louisiana shelf. This dataset is also valuable to observational and modeling studies of large river sediment dispersal systems worldwide. [ABSTRACT FROM AUTHOR]