Sediment Dynamics in Mobile Bay, Alabama: Dredging and Placement Practice for Cost Efficiency and Ecological Benefit

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Mobile Bay is the second largest estuary in the U.S. and the primary depositional basin for the sixth largest river system in the U.S.  Although a significant quantity of sediment enters the Bay from the delta, less than 30% of sediment eroded from the watershed reaches the Gulf of Mexico. The purpose of this study was to document long-term regional sediment transport patterns within the Bay to develop a sediment budget for assessing net changes in navigation channel and disposal site configuration relative to wave and current processes. Specifically, determine whether dredged sediment placement practices can be modified to limit environmental impacts to benthic communities and maximize cost efficiency of channel dredging and sediment placement.

Navigation channel dredging and placement records, as well as historical shoreline and bathymetric surveys, were the primary sources of data compiled for evaluating sediment transport quantities and patterns within Mobile Bay. Channel dredging records indicate that maintenance dredging in Mobile Bay ship channel has been very consistent since about 1913, regardless of channel depth and width changes, at annualized dredging volumes of about 4.15 mcy. Channel maintenance dredging exceeded sediment input from Bay watersheds by about 1.6 mcy/yr, suggesting that about 36% of maintenance dredging material placed along channel margins in the Bay was transported back to the channel before the next dredging cycle.

Net sediment movement within the Bay indicates that in-bay disposal of sediment is most similar to natural long-term depositional processes. Design of dredged material placement techniques that focus on thin-layer disposal farther from the margins of the channels was recommended as beneficial to channel dredging operations and benthic ecology. Dredged material placement farther from the channel is expected to limit excess maintenance dredging resulting from transport of sediment from channel margin disposal mounds into the channel. Furthermore, thin-layer disposal provides for faster recovery for bay-bottom benthic communities and has a less permanent impact on benthic ecology.