Effect of Flood Hydrograph Duration, Magnitude, and Shape on Bed Load Transport Dynamics

Bed load sediment transport is an inherently challenging process to measure within a river, which is further complicated by the typically transient nature of the hydrograph. Here we use laboratory experiments to explore how sediment flux under transient—unsteady and intermittent—flow differ from those under steady flow. For a narrow unimodal sediment distribution, we calculated fluid stress and measured sediment flux for a range of hydrograph durations, magnitudes, shapes, and sequences. Within a hydrograph, we find considerable variability in sediment flux for a given stress above the threshold for motion. However, cumulative bed load flux resulting from a flood scales linearly with the integrated excess transport capacity (flow impulse). This scaling indicates that, to first order, flow magnitude, duration, shape, and sequence are only relevant to bed load flux in terms of their contribution to the total flow impulse, in agreement with prior field results. The flood impulse represents a quantitative parameter through which the effects of transient flow on coarse sediment transport may be parsed.