By Alisha Renfro, Coastal Scientist, National Wildlife Federation
In 2011, high water levels in the Mississippi River prompted the U.S. Army Corps of Engineers to open the Bonnet Carré Spillway in order to reduce pressure against the river levees and safely shunt the flood waters past nearby communities. In a recent study published in the Journal of Hydrology (Vol. 477, pg. 104-118), a group led by Mead Allison, Ph.D., examined the effects of the 2011 operation of the Bonnet Carré Spillway on local sediment transport and deposition in the river. By examining the effects of a large flood control structure, this study can help inform the design and operation of large sediment diversions that could increase sediment delivery into nearby basins and decrease the potential negative impacts on the navigability of the river.
The Bonnet Carré Spillway was constructed in response to the Great Mississippi Flood of 1927 which caused devastation from Illinois to Louisiana. The spillway, located upriver from New Orleans, is 7,000 feet long, consists of 350 bays and has the capacity to divert 250,000 cubic feet per second of water from the uppermost part of the river’s water column into Lake Pontchartrain. The spillway is only opened when river discharge going past New Orleans exceeds 1.25 million feet per second. This strategic opening of the spillway reduces water levels in the river, providing flood relief during high water events. The Mead Allison led study focused on the most recent opening of the spillway – the 2011 flood – when the Bonnet Carré Spillway was open from May 9 to June 19 and reached a maximum discharge of 315,000 cubic feet per second, 26 percent above flood control design.
To examine the immediate effects of the opened spillway, the researchers conducted three boat-based surveys – before, during and after the spillway was open – to map changes to the river’s bottom and to estimate the amount of sediment transported in the water column and along the river bed. A year later, a fourth survey was carried out to examine continuing effects of the 2011 spillway opening.
During the 42 days the spillway was open in 2011, 9.1 million tons of sediment, mostly sand, was deposited downriver from the spillway structure. This deposition hot-spot was determined to be the result of surface water exiting the river through the spillway. The loss of water decreased the energy available to transport the sediment, particularly the larger, heavier sands typically transported in the lower part of the water column. However, by June 2012, almost 70 percent of the sediment deposited downstream of the spillway was transported out of the study area.
The Bonnet Carré spillway was not built to capture sediment; rather its purpose is to siphon the Mississippi River’s surface water during high flow events, decreasing pressure against levees in order to protect communities and infrastructure. The authors of the study suggest that its results can influence the future design and operation of the planned sediment diversions outlined in Louisiana’s 2012 Coastal Master Plan. In order to minimize the shoaling observed near the spillway in 2011, there is a need for sediment diversion structures to capture more sediment relative to the water withdrawal from the river. This can be partially addressed by designing diversion structures that draw sediment and water from deeper depths of the river, where more sand – essential for land-building – is located and available for capture. Additionally, the management of sediment diversions that approach a scale similar to the Bonnet Carré could take advantage of local shoaling through regular dredging. The dredged material could then be pumped through the diversion structure.
During 2011, the flow of water in the Mississippi River below the Old River Control Structure peaked at 1.6 million cubic feet. This was the highest river flow ever recorded at this station since it began recording more than 75 years ago. In Louisiana, this high flow triggered the operation of the Bonnet Carré Spillway for the 10th time in its 80-year history. The structure succeeded in reducing water levels along the levees that protect New Orleans and other nearby communities. It also provided a unique opportunity to examine the river’s dynamics. This type of study will help inform and influence the design of future sediment diversions that will focus on maximizing sediment capture for efficient and effective land building in the sediment-starved Mississippi River Delta.