(12) Effects of vegetation on delta morphodynamics and sediment transport
A. Piliouras1, W. Kim1, and B. Carlson1
1Department of Geological Sciences, University of Texas at Austin, Austin, Texas.
Coastal areas are rapidly losing land due to subsidence and increasing rates of sea level rise. The Mississippi Delta has lost nearly 5000 km2 of land in the past century. As a result, Louisiana has planned to implement diversions of water and sediment from the main Mississippi River into the surrounding bay to attempt to build new land. In order to ensure the success of these diversion projects, we need to understand the factors controlling delta growth. We conducted a set of experiments in the Sediment Transport and Earth-surface Processes (STEP) Basin at the University of Texas at Austin to examine the effects of vegetation on delta growth and dynamics. One experiment was conducted without vegetation, and three were conducted using alfalfa ( Medicago sativa) as a proxy for riparian vegetation. Results indicate that vegetation increased sediment trapping on the delta topset, thus increasing delta slope and decreasing progradation as compared to the unvegetated experiment. Vegetated experiments also resulted in a lack of channelization when the topset reached ~20% plant cover, after which progradational delta lobes were no longer evident. A fourth vegetated experiment was conducted to account for flood intermittency and fluctuations in water and sediment fluxes over time, as they occur in nature. We found that floods carrying high sediment loads were net-depositional and resulted in almost complete erasure of channels. Lower discharge periods with near-zero sediment loads were highly erosive and worked to maintain channels while actively reworking the delta surface. Thus, low-flow discharge may be the formative discharge for river delta channels and the consideration of water and sediment discharge fluctuations is necessary to maintain flow paths on a vegetated delta.