Control of flow hydrograph sequencing on modeling debris-flow routing in river networks

Debris flows can have long-term effects on a watershed as deposited sediment at the upstream end of a river network can act as a sediment supply source for decades to centuries. Therefore, long-term simulation is critical to predict the combined effects of flow magnitude, duration, sequence, and intermittency for debris-flow sediment routing. While such modeling scope in large spatial and temporal scale is often restricted by computational capacity, simplifying the flow hydrograph can help make the modeling tractable. Murphy et al. (2019) developed a novel framework to predict post-wildfire sediment generation and sediment impacts downstream from burned areas by linking debris-flow generation and network-scale sediment routing models. We advance and apply the coupled framework focusing on the sediment routing component (Czuba, 2018) to explore the control of flow hydrograph sequencing on the transport of debris-flow sediment deposits with different sediment grain size distributions through the Provo River Network (upstream of the Jordanelle Reservoir in Northern Utah) at different time horizons. Hydrographs simulated included a full 30-year daily hydrograph, a reduced 30-year daily hydrograph excluding all the flows lower than half of the 2-year flow, different arrangements of one-year sections of this reduced hydrograph, and constant 2-year flow hydrograph. Simulation results from the reduced hydrographs (of constant flow and different sequences) show that these can produce long-term transport comparable to the original flow record. Although the initial (1-5 years) discrepancy is high, these differences decrease over time (after 10 years). The effect of flow sequences was less important for sand than gravel, as both high and low flow would move sand initially. This resulted in higher total transport when the low flow years were followed by high flow years, because later high flows can move the coarse gravel after the early sand removal by low flows. With the expected future increase in magnitude and frequency of floods due to climate change, the long-term simulation of flow sequences can inform river managers about how to better prepare to reduce loss from debris flows, and to improve overall river and watershed management.