Analysis of Sediment Deposited In Front of Bonnet Carre Spillway Structure As A Result of Mississippi River High Water In 2020

The Mississippi River in New Orleans had elevated high water stages for 6 months in 2020. Bonnet Carre Spillway was operated on April 3 and remained open for one month. The spillway saw a max flow of 81,070 cfs through the structure. While this was not one of the major openings of Bonnet Carre, the high water still deposited an incredible amount of sediment in front of the structure. From a daily river operations standpoint, there is an interest is keeping a reasonable amount of material in place in front of the structure. A lack of water depth over the forebay during an opening has been proven to cause barge tows to run aground before approaching the structure. By the time the picket boat realizes the threat, the tows are sometimes already aground over the 1.4 mile long structure. HEC-RAS was used to do a hydraulic analysis with the objective of determining a forebay elevation and provide an opinion on potential flow restriction, or marginal areas that may restrict the flow as sediment builds up during the next opening. Analysis was done using varying heights of sediment deposits in front of the structure that would ensure flow diversion through the structure up to the design of 250,000 cfs without concern of restriction. It was found that up to an additional 3 feet of sediment above the gate sill would not restrict flow. This would ensure that we can, if needed, divert up to 250,000cfs without concern of flow restrictions in the forebay and provide a small buffer to fend off wayward barge tows, by restricting available draft up to the structure.