QUANTIFYING SEDIMENT TRAPPING EFFICIENCY AND ACCRETION RATES OF A RECENT CREVASSE SPLAY IN THE MISSISSIPPI DELTA: IMPLICATIONS FOR COASTAL RESTORATION

Crevasse splays form as a result of the breaching of natural levees and concurrent sediment deposition in adjacent wetlands, a process similar to artificial river diversion for coastal restoration purposes. We studied the lithology and chronology of a crevasse splay near Napoleonville, Louisiana, that was formed by Bayou Lafourche when it breached its natural levee and prograded into a cypress swamp. This crevasse splay covers an area of ~70 km² and contains up to 9 m thick deposits in its proximal portions that taper to 3 m distally. The splay deposits consist of >70% silt and clay. Assuming that the sediment load of Bayou Lafouche had a similar grain-size distribution as the present-day Mississippi River, we calculate a sediment trapping efficiency of >67%. Optically-stimulated luminescence (OSL) dating demonstrates that the splay was formed between 600 and 1200 years ago with predominant deposition occurred at ~AD 800 to 1000 in the proximal portions and between AD 1100 and 1400 toward the distal portions. Sediment accretion rates derived from the OSL chronology are typically >2 cm/yr.

Sediment trapping efficiency estimated for the crevasse splay is significantly larger than the ~30% sediment trapping efficiency of the Wax Lake Delta which is commonly used as an analog for river diversions. In the Wax Lake Delta, large proportions of the sediment load (notably the mud fraction) are lost to currents and waves given its location on the open coast. In contrast, sedimentation in a densely vegetated setting farther inland enables a much higher sediment trapping efficiency and thus higher sediment accretion rates which are needed to sustain wetlands under conditions of future accelerating sea-level rise. We conclude that it is critical that sediment trapping efficiency be considered in river diversion planning.