Sediment flux and fate in the Mississippi River Diversion at West Bay: observation study

Land-building sediment diversions, such as the Mississippi River Diversion at West Bay, can be used as important building blocks in the conservation and restoration of Mississippi Delta wetlands. Sediment deposition and retention patterns were studied in the West Bay diversion outfall area through analyses of sediment cores, hydrodynamics, and bathymetry. Cores and x-ray samples were collected at twenty stations in March 2004, August 2004, November 2005, and April 2006. Cores were analyzed for grain-size distributions and 7Be activities, in order to examine patterns of recent sediment deposition. In comparing ratios of total 7Be inventory to atmospheric 7Be deposition rates, it was estimated that 10%, 60%, and 30% of the cores experienced net accretion, net erosion, and no deposition, respectively. Turbidity and velocity data collected during instrument deployments along with historical Mississippi River flows and sediment loads were used to estimate an average sediment influx of 2.9 x 106 tons/year (2.6 x 109 kg/year). A sediment trapping efficiency of 25 - 50% was estimated by comparing sediment influx with 7Be penetration depths observed. This estimation could not be verified by bathymetric surveys conducted in April 2006 and compared to pre-construction surveys from 2003. Volumetric change calculations estimated a loss of over 4.16 x 106 yd3 (3.18 x 106 m3) of sediment from the bay bottom. It is speculated that these losses were caused by Hurricane Katrina which made landfall on August 29, 2005 approximately 15 nautical miles (28 km) from the study area. Potential deltaic growth rates of between 51 ac/year (21 ha/year) and 143 ac/year (59 ha/year) were estimated for the diversion by comparing delta growth parameters estimated in this study with the Wax Lake Delta. These estimates suggest that delta-building processes at West Bay may continue following a typical subdelta growth curve of 100 - 150 years, meaning peak development of deltaic wetlands could be decades away. Therefore, future planning for diversions should consider coastal process which could potentially counter environmental benefits and engineering strategies should place as much focus on receiving area configuration and trapping efficiency as sediment delivery in order to maximize sediment retention