SIMULATED SPATIAL AND TEMPORAL EVOLUTION AND DOWNSTREAM EFFECT OF SEDIMENT PULSES IN RIVERS DRAINING MOUNT RAINIER, WASHINGTON (Invited Presentation)

A variety of natural and anthropogenic processes supply episodic, discrete pulses of sediment to rivers. On Mount Rainier, a glaciated stratovolcano in Washington State, debris flows commonly deliver sediment from steep slopes to the Nisqually, Carbon, White, and Puyallup Rivers. One-dimensional river-hydraulic and sediment-transport models were developed using The Unified Gravel-Sand (TUGS) model for each of these four rivers with downstream variations in slope, width, and bed-sediment grain size. The spatial and temporal evolution and downstream effect of sediment pulses in rivers were investigated by varying the streamflow hydrology, sediment-pulse volume, and sediment-pulse grain size. Sediment pulses supplied to each river model were used to characterize and better understand the timing, duration, location, and effect of sediment pulses in downstream reaches for informed river management. An equation for the arrival time of a sediment pulse was obtained that distills the complex formulations within the TUGS model to a succinct description of the most important factors. Sediment pulses were mobilized quickly through reaches (1) with high sediment-transport capacity (often narrow reaches), (2) where the sediment pulse was finer than the bed surface, and/or (3) with large magnitude or high frequency of sediment-transporting flows. In general, sediment pulses transported progressively slower with downstream distance. The magnitude of the change in sediment load, bed volume, and bed-surface grain size decreased downstream and was proportional to sediment-pulse volume. Wide alluvial reaches with low sediment-transport capacity acted as upstream geomorphic controls on downstream sediment transport.