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

• Published in: Geophysical research letters 2018-08, Vol.45 (16), p.8264-8271
• Main Authors: Phillips, C. B., Hill, K. M., Paola, C., Singer, M. B., Jerolmack, D. J.
• Format: Article
• Language: English
• Subjects: Bed load; Capacity; Duration; Dynamics; experiment; Experiments; Flood hydrographs; Flooding; Floods; Flow; Fluctuations; Flux; geomorphology; hydrograph; Laboratories; Laboratory experiments; Landscape; Modelling; Rivers; Scaling; Sediment; Sediment distribution; Sediment load; Sediment transport; Shape; Snowmelt; Steady flow; Storms; Stress concentration; Transport; Unsteady flow
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078976

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. Plain Language Summary Mountain river floods produced from snowmelt can last months but remain relatively shallow, while floods resulting from storms are often shorter in duration and deeper. These floods have, in a sense, different shapes and sizes determined by their environment and climate. We performed laboratory experiments to understand how these flood shapes and sizes affect the amount of sediment they can move, a key precursor to understanding how rivers and flooding impact the landscapes in which they reside. Our experiments show that if one accounts for the forcing of the flood in a physically based manner, there is no difference between floods of different shapes and sizes in terms of how much sediment they move. We suggest that these results may make floods easier to characterize when modeling landscapes. Key Points Laboratory experiments reveal that total bed load flux for a flood is linearly related to the flow impulse (integrated transport capacity) For an equivalent flow impulse, transient and steady floods transport the same total bed load sediment flux Components of unsteady floods affect bed load flux in their contribution to the impulse but are otherwise identical for well sorted gravel