A simple morphodynamic model to evaluate decadal-scale sandbar dynamics downstream from a large dam
Abstract
Downstream from large dams, sandbars often erode because reduced peak discharge lowers the elevation to which a sandbar may build, reduced sand supply limits the sediment available for deposition, and dam operations such as hydropeaking erode bars through seepage erosion and bar shaving along shorelines. Along the Colorado River in Grand Canyon, Arizona, controlled floods are now routinely released from Glen Canyon Dam to rebuild sandbars during periods of sand accumulation following tributary floods. While annual surveys and remote cameras provide an assessment of changes in bar size, a model to predict long-term sandbar dynamics to flow and sediment regimes is a vital management tool to evaluate different water release scenarios. Here we present a morphodynamic model to compute decadal-scale changes in sandbar volume for a subset of sandbars of the Colorado River in Grand Canyon. We simulate sandbars as a triangular wedge, in which only part of the bar may be submerged, depending on the bar volume and stage elevation. Deposition occurs as a function of sand grain size and concentration, building on the model of Andrews and Vincent (2007), using sub-daily records of flow and suspended-sand concentration from 2002-2019. Erosion occurs along the bar-water interface and follows an approximate exponential decay with bar volume. Two free parameters in the model - an eddy exchange coefficient and an erosion rate parameter - were optimized using periodic measurements of sandbar volume at a suite of sandbars in similar geomorphic settings. The model is relatively insensitive to the number of calibration data points, suggesting that physical processes are reasonably well-represented in the model equations. Overall bar behavior is well predicted using the calibrated model (R2~0.8). Sandbars are largest following controlled floods and decay in size toward some minimum volume associated with normal dam operations, during which smaller fluctuations in bar volume occur due to seasonal changes in flow release volume and sand concentration. Post-hoc modeling demonstrates that significant increases in sandbar volume occur with increasing frequency of controlled floods and that high concentrations of tributary supplied sand entrained during floods are a key factor in their effectiveness.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP027..03M
- Keywords:
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- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY;
- 1865 Soils;
- HYDROLOGY