Mississippi River channel response to the Bonnet Carré Spillway opening in the 2011 flood and its implications for the design and operation of river diversions

SummaryThe large Mississippi River flood in 2011 was notable in the lowermost Louisiana, USA reach for requiring operation of several flood control structures to reduce stress on artificial levees: the largest diversion went through the gated Bonnet Carré Spillway, which was opened for 42 days in May and June. The removal of approximately 20% of the total flood discharge from the river provided an opportunity to examine the impact of large water diversion on the sediment transport capacity of large rivers.Boat-based, acoustic and water and bed sampling surveys were conducted in the Mississippi River channel adjacent to the Spillway immediately prior to the opening of the structure, at full capacity, and immediately following (June 2011) and 1 year after (June 2012) closure. The surveys were designed to examine (1) elevation change of the channel bed due to scour or aggradation of sediment, and (2) suspended and bedload transport variability upriver and downriver of the Spillway. The results indicate that approximately 9.1 million tons of sand were deposited on the channel bed immediately downriver of the water exit pathway and extending at least 13 km downriver at a rapidly and progressively reducing magnitude per river kilometer. The surficial deposit was of finer grain size than the lateral sand bars in the channel upriver of the structure. We argue the deposit was largely delivered from suspension derived from the observed deflation of lateral bars upstream of the diversion point, rather than from sand arriving from the drainage basin. Approximately 69% of the 2011 flood deposit was removed from the 13 km downstream reach between June 2011 and June 2012. We conclude that the source of the channel deposit was the reduction in stream power, and, thus, in the sediment transport capacity of the Mississippi, associated with the water withdrawal. The re-entrainment of this material in the following flood year indicates the system rapidly re-establishes an equilibrium to pre-opening conditions. Future diversions in the river for coastal restoration will have to address this issue to maintain a deep draft navigation channel in the Mississippi River.

► We examine impacts of the record Mississippi River flood of 2011.
► We examine the effects of removing 20% of the river flow through a river diversion on channel hydrodynamics.
► We provide data and analysis that will improve design and operation of river diversions.