Mobile Bay Sediment Budget 2012 Released

We've just posted the Mobile Bay Sediment Budget 2012 to our Library of documents.


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. The Mobile-Tensaw River Delta at the head of Mobile Bay is the largest inland delta complex in the U.S. and encompasses a variety freshwater and estuarine habitat critical to the environmental sustainability of Mobile Bay. Clastic sediment transport within the drainage basin is the primary source of sediment for delta growth at its boundary in northern Mobile Bay. 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 general configuration of the Bay, relatively low velocity discharge from the river, and quiescent wave and current conditions within much of the Bay result in significant quantities of fine-grained sediment deposited in the Bay.

Historical shoreline and bathymetry surveys, as well as detailed channel dredging and placement records, were the primary sources of data compiled for evaluating sediment transport quantities and patterns within Mobile Bay toward development of an operational sediment budget. Channel dredging records indicate that maintenance dredging in the 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. The amount of sediment entering the Bay from adjacent watersheds is about 2.87 mcy/yr or 64% of annual maintenance dredging.

Sedimentation in Mobile Bay for the period 1917/18 to 1984/2011 revealed a pattern of net deposition, except for channel areas. Approximately 2.08 mcy of sediment was deposited in the Bay on an annual basis for the period of record. Sediment input from watersheds surrounding the Bay contributed about 2.87 mcy of sediment to the Bay, indicating that approximately 0.79 mcy of sediment were exported annually from the Bay during this period. Of this quantity, about 0.63 mcy/yr were transported to the Gulf of Mexico and Mississippi Sound by tidal currents. These quantities indicate that net deposition in the Bay accounts for approximately 72% of sediment input from watersheds and 28% is transported from the Bay through Pass aux Herons and Mobile Pass through natural transport processes and offshore disposal of dredged sediment. Channel maintenance dredging quantities between 1917/18 and 1984/2011 exceeded sediment input from Bay watersheds by about 1.6 mcy/yr, suggesting that about 36% of maintenance dredging material placed in the Bay was transported back to the channel.

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 would be beneficial to channel dredging operations and benthic ecology. Dredged material placement farther from the channel may prevent 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.