Impact of hydrograph form on bedload transport processes in armored channels

Abstract:

Nearly all channels experience unsteady flows, from gradually varying snowmelt driven hydrographs to rapidly changing rain driven or regulated hydrographs. Despite this, the impacts of hydrograph form on bedload transport processes remain poorly understood. A set of flume experiments was conducted at the University of Idaho’s Stream Lab to investigate whether hydrograph form (rapid vs gradual changes in flow) alters transport rates and mobile grain sizes for a given discharge as well as armor persistence and total transport volume during a high flow event. Prior to experiments the flume was run to equilibrium with limited sediment feed to armor the bed. Experiments included four steady state runs and nine hydrograph runs that used five different hydrograph forms. The rate of change in flow between time-steps varied in the hydrographs but minimum and peak flows and total estimated transport capacity were held constant.

Initial data analysis suggests that hydrograph form does indeed impact total transport volumes, transport rates, and transported grain sizes. More rapidly changing hydrographs transported a larger total volume of sediment and had greater transport rates at peak flow than hydrographs with slower rates of change. All hydrographs displayed counter-clockwise hysteresis in transport rates with the exception of the most gradually varying run. More rapidly changing hydrographs had coarser bedload material on the rising limb but finer bedload material on the falling limb compared the same flow in more gradually changing hydrographs. Many of these observations may be linked to more significant armor loss promoted by more rapid changes in flow. On the rising limb, rapid changes in flow may increase the unsteady component of shear stress to mobilize larger grains and greater volumes of sediment over short durations compared to the effects of gradual flow increases. This has the potential to destabilize the armor surface and further mobilize surface and subsurface material. Repeat photos and high speed video of the bed surface during experiments will be used to investigate this hypothesis and further work will identify and quantify significant relationships between hydrograph form and bedload transport processes.