Elevation Change in Coastal Louisiana – Looking for Stable States in a Rapidly Prograding Fluvial Delta

Abstract:

River deltas form where sediment-rich waters debouche into bodies of standing water. These sediments are sorted, deposited, and re-worked, through the combined effects of buoyancy, rivers, tides, winds, and waves, creating a self-organized system. This organization is further controlled by vegetation where landforms are high enough relative to the standing water level to support it. Past work on wetland systems use histograms of elevation to infer rates of change in elevation as a function of elevation itself. We test this inference on Wax Lake Delta, a rapidly prograding sub-delta in Atchafalaya Delta system, Louisiana, using two lidar surveys collected in 2009 and 2013. The island surfaces have comparably low elevation relative to mean water level, with subaerial relief of only 0-1.5 m being composed of chevron-shaped channel-bounding levees bounding large, shallow internal lagoons. Island vegetation succeeds from aquatic to intertidal to emergent species along the elevation gradient. Peaks in the histogram of 2009 elevations suggest multiple stable elevations within the delta. Statistical analysis of the differences between the lidar surveys shows elevation change as a function of both location (within the delta as a whole and within individual islands) and initial elevation. These functional relationships can be attributed to the interaction of sediments, vegetation, water currents due to both riverine and bay processes, and local subsidence.