Simulated sediment-pulse dynamics in a gravel-bedded mountain river
Abstract
Mountain rivers often receive sediment in the form of pulses from events like landslides or debris flows that interact with channel morphology, habitat conditions, and channel-floodplain connectivity in downstream reaches of the river network. The movement of sediment depends on the flow, grain size distribution, interactions among grain sizes, bed morphology, as well as the geometry of the channel and the network. In this numerical modeling work, episodic, discrete pulses of sediment of different grain size, volume (19000 m3 and 57000 m3), and type (uniform and mixture) are supplied to a river under medium and high flow conditions to observe the downstream impacts. This study builds on a previous Lagrangian, bed-material sediment transport model and serves as an essential validation of the space-time evolution of sediment pulses. Key modeling advancements include the consideration of particle abrasion, stress partitioning, and allowance for different upstream sediment supply conditions, including transporting at capacity, from a sediment pile, or applying a sediment rating curve. The one-dimensional model is applied to a 27-kilometer reach of the mainstem Nisqually River (WA) draining Mt. Rainier and terminating in Alder Lake. First, using measured flow (66-year hydrograph) and grain sizes, we validate the model against measurements of bed-elevation change (at a USGS gage) and accumulation rates in Alder Lake. Then, we investigate the magnitude, timing, and persistence of downstream changes in bed elevation and grain size distributions due to introduction of sediment pulses. Comparing against the baseline condition (without pulse), we found that pulses with smaller grain sizes (finer than the bed) mobilize more bed sediment and thus, may lead to more downstream channel degradation. Different pulse volumes showed similar downstream impacts, but pulses with larger volumes moved more rapidly, because they were less likely to mix with existing bed sediment. Pulses with a mixture of grain sizes moved through the system more slowly than pulses with a uniform grain size (both with same mean gravel size), and have a more diffuse impact downstream. This ongoing work will allow us to better understand fluvial geomorphic responses to variations in sediment supply.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP016..05A
- Keywords:
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- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY