The Effect of Episodic Sediment Supply on Bedload Variability and Sediment Mobility

The effect of sediment supply on sediment mobility is analyzed for a poorly sorted (0.5-64 mm) experimental bed. Water discharge was held constant over a sequence of seven runs, and 300 kg of sediment was supplied during each run in different magnitudes and frequencies. In runs with constant feed bedload transport rate increased gradually. In contrast, runs that received large sediment pulses showed pronounced increases in bedload rate as the bed surface got finer, followed by monotonic declines as the bed surface coarsened. We studied the temporal scales of bedload fluctuations by means of sample autocorrelation coefficients and the rates of decrease in bedload fluctuation with sampling time scale. The significant trends caused in bedload rate by large occasional sediment pulses increased long-term autocorrelation in bedload rate time series relative to runs with constant feed. Bed evolution and local changes in sediment storage caused multiple scales of variability in bedload rate, which increased autocorrelation and caused long-term persistence in bedload series over periods with a nearly constant mean. The scaling statistics of bedload transport fluctuation depended on grain size, and those for total bedload were similar to those for fine gravel (2-8 mm), which was fully mobile and dominated bedload transport. Grain size dependence of bedload fluctuation was not affected by changes in sediment feed because water discharge and sediment texture were held constant.