CONSTRUCTING A SEDIMENT BUDGET FOR THE MISSISSIPPI-ATCHAFALAYA RIVER IN LOUISIANA IN SUPPORT OF COASTAL RESTORATION

A team of Federal, State of Louisiana, and academic scientists have been tasked by the LCA Science and Technology Program to provide a comprehensive suspended sediment budget for the lower Mississippi-Atchafalaya River in Louisiana. While catchment sediment budgets have been done for the Mississippi in the past, the present effort is designed to examine the sediment transport cycle in the tidal and estuarine section of the river, and the impact of more than a dozen man-made and natural water exits downriver of the Old River Control split with the Atchafalaya. An understanding of the magnitude, timing and character of suspended sediment pass through the lower river annually is critical to planning future sediment diversions in support of coastal restoration in the delta. The focus of the study is flood years 2008-2010, to avoid issues of a documented longer-term decline in sediment loads, and to take advantage of the installation and operation of several new monitoring stations. The budget is constructed using a backbone of daily USGS and USACE monitoring stations, supplemented with boat-based, project-specific studies. Preliminary conclusions are that 1) 30-50% of the sediment in the main river below Old River exits the river prior to Head of Passes, 2) the timing of suspended load is strongly impacted in the tidal reach below Baton Rouge by an annual bed storage-resuspension cycle and 3) traditional methods of using sediment ratings curves to measure annual suspended throughput are underestimating true load during the rising limb and overestimating during the falling limb of discharge pulses.