Modeling the impact of abrasion on the fluvial morphodynamics of a volcanic sediment pulse, Suiattle River, WA

Large sediment pulses deposited in river channels can alter channel morphology and amplify downstream flood hazards. In the Suiattle River of Washington State, abrasion controls the downstream impact of sediment supply from Glacier Peak, a stratovolcano that regularly supplies the channel with large sediment pulses. This phenomenon is evident by the persistence of strong volcanic grains on the streambed and the rapid downstream loss of weak volcanic grains to fine sediment. Although cobbles and boulders dominate pulses in the channel, the Suiattle has an unusually high supply of fine sediment and contributes to fine sediment impacts downstream. Despite the evidence that the abrasion of coarse sediment during downstream transport drastically impacts the balance of fine and coarse sediment in the channel, little to no work - in the Suiattle or other real landscapes - has been done to determine the importance of abrasion on the dynamics of sediment pulse evolution in a highly abrasion-prone setting. In this study, I use the Suiattle River's geometry and sediment characteristics as a real-world framework to explore how sediment pulses evolve under different modeled scenarios of abrasion. I apply the Network Sediment Transporter (NST), a 1D morphodynamic and Lagrangian sediment transport model within Landlab, to simulate channel response to a large sediment pulse. I evaluate three scenarios: (1) no abrasion, (2) abrasion rates based on Schmidt Hammer Rock Strength (SHRS) measurements as a proxy for lab-derived tumbler abrasion rates, and (3) doubled SHRS values to account for known underprediction. Each scenario is driven by a ~2% exceedance flow derived from a distributed hydrology model, with intermittent high flows (~0.1% exceedance) to enable the transport of large particles. The model results show that increased abrasion enhances the mobility of bed material and substantially increases the production and transport of fine sediment. In contrast, changes in bed elevation are relatively insensitive to abrasion alone. Instead, channel geometry (channel width) emerges as a key control on sediment deposition and export. These findings emphasize the utility of abrasion-inclusive sediment transport modeling and highlight how topographic and geomorphic factors shape sediment pulse evolution. Future work should continue to develop the numerical modeling tools associated with the NST, which will help facilitate a more detailed exploration of the effects of abrasion and heterogeneous sediment characteristics in the rivers of the western Cascade Mountains.