Measuring delta volumetric change and composition using a streakline-based method on the Wax Lake Delta

Estimating river delta growth is essential for determining coastal sustainability. We use a new method to quantify the recent accretion and composition of the Wax Lake Delta, a rapidly prograding delta in Louisiana. The method identifies the area of significant (subaqueous) delta aggradation from delta flow patterns, which are visible in aerial imagery through biogenic streaklines that naturally occur on the water surface. Comparison of bathymetric maps and streaklines shows that channel tips and the associated seaward extent of significant delta deposition occur where the divergence of flow direction equals 0 (+/- 400 m). Delta volumes calculated using this method, the 1974 pre-delta shoreline, and a measured average delta thickness of 1.6 m provide an estimate of the delta volume which we found to increase by 2.0*106 m3/yr between 1992 and 2014. We then compared this rate to the total volume of silt and sand discharged to the WLD using USGS measurements. Over the same period, the WLD received 1.4*106 m3/yr of sand and 9.0*106 m3/yr of silt and clay. Assuming all sand supplied to the delta is sequestered, volume comparisons show that the sand fraction of the WLD is between 18-38%, lower than previous estimates. Further, between 44-69% of the total suspended silt and clay bypasses the delta. These composition estimates can be used to predict land growth from engineered diversions, and the method can be used to monitor the resulting delta growth.