Delta Islands and Flood Intermittency

These experiments were conducted in the Sediment Transport and Earth-surface Processes (STEP) Basin, located at the Morphodynamics Laboratory at the University of Texas at Austin. In each experiment, the domain is 1.85 m x 2 m with water depth of 50 mm over a 50-mm thick sediment bed. The sediment bed acts as a rough base and ensures the flows are transport-limited rather than supply-limited. At every moment the flow transports as much sediment as it is capable. The bed is composed of the same sediment as the input.

A computer controlled pump and auger feed the sediment and water (0.355 l/s at a concentration of 1:100) mixture into a bucket with an Arduino-controlled knife valve that connects to the domain inlet by a 2 inch PVC pipe. The supplied sediment is composed of fine quartz sand, with a D50 of 171 µm and a density of 2650 kg/m3. The water level in the STEP basin is maintained by a computer controlled weir, constraining water level within 1 mm of the 50 mm above the initial sediment bed height. The tank does not recirculate, and all the sediment from the flows remains in the domain during the experiments.

During floods, the knife valve closes for a programmed duration of 15 seconds, filling the bucket to 5.325 liters, and then opens for a flood and subsequent interflood flow. The inlet hose is placed at an angle to the bucket wall, forming a strong circulation cell in the bucket that keeps sediment in suspension. Because the system is pre-programmed through the Arduino, the floods are all the same magnitude and duration – flood discharge is 6 l/s, while interflood discharge is 0.355 l/s. This setup also allows for broad comparability across different runs with different intermittencies. Each run has the same sediment and water volume input. The frequency of floods is the only changing variable.