Laboratory Investigation on Effects of Flood Intermittency on Fan Delta Dynamics

To simplify the complex hydrological variability of flow conditions, experiments on delta evolution are often conducted using a representative channel-forming flood flow and results are related to field settings using an intermittency factor, defined as the fraction of time in flood. Although this factor provides an approximation of dominant flow conditions and makes modeling deltas easier by turning their complex hydraulics into a single representative value, little is known about how this generalization affects delta processes. We conducted experiments with periodic flow conditions to determine the effects of intermittent discharges on fan deltas. For each run, the magnitude of floods was held constant, but the duration changed, thus varying the intermittency factor, between 1 and 0.2. Floods consisted of higher water and sediment discharge, while base flow periods had lower water discharge and sediment input ceased, causing the system to become erosional during these periods. We find that as the duration of floods decreases, the delta topset is larger in area with a shallower slope due to reworking on the topset during base flow conditions. During base flows, the experimental system adjusts toward a new equilibrium state that in turn acts as the initial condition for subsequent flood periods. These results suggest that the adjustment timescale is a factor in determining the behavior of deltas and their channels. We conclude that both periods of flood when most of the sediment is supplied to the system and periods of base flow when topset sediment is reworked contribute to delta dynamics.