SPATIAL PATTERNS OF SEDIMENT MOBILITY IN RIVERS DISTURBED BY DAM REMOVAL

Bedload dominated channels are often observed to be configured such that the onset of significant bedload transport occurs at bankfull discharge. It follows that equilibrium channel form depends on the critical discharge required to mobilize sediment. Using the onset of bedload transport as a predictive metric of channel equilibrium is complicated by observations that, in addition to discharge, the onset of sediment mobility depends on grain size, morphology and hydrograph shape. To date, ideas surrounding the relationship between sediment mobility and grain size, morphology and hydrograph shape have only been tested in flumes. The purpose of this research is to use the “natural experiments” of disturbed channels due to dam removal to quantify the conditions (grain size, morphology and hydrography shape) required for sediment mobility. The two dam removal sites are representative end members with respect to sand content. Spatially explicit bed shear stress determined using 2D flow models and direct measurements of sand content at locations of 400 passive and 70 active tracers will be used to quantify sediment mobility across multiple mobilizing flows throughout the dam removal sites. Movements of 70 active tracers will be used to constrain the critical discharge for the onset of tracer mobility. As observed in flume studies, I hypothesize that high amounts of sand on the bed will decrease the critical Shields value. I also hypothesize that erosional reaches, as determined by longitudinally increasing gradients in stream power, will have higher tracer virtual velocities. Conversely, depositional reaches, as determined by longitudinally decreasing gradients in stream power, will have relatively lower tracer virtual velocities. The results of this work will quantify the conditions required for sediment mobility and define reaches that are either equilibrating from disturbance (e.g. dam removal) or that remain disturbed over mobilizing flow events.