USING A SIMPLE PHYSICAL MODEL TO EVALUATE SANDBAR DYNAMICS IN GRAND CANYON

Controlled floods are released from Glen Canyon Dam to increase the extent of sandbars along the Colorado River in Grand Canyon National Park, Arizona. Since 2012, a new management protocol has resulted in five controlled floods following large inputs of sand from the Paria River. The effectiveness of individual floods depends on their magnitude and duration, the sand supply, and the hydraulic setting of individual sandbars. While repeat surveys and remote camera measurements are used to assess individual flood events, planning for future floods would benefit from a simple modeling tool to evaluate the potential sandbar changes under different management scenarios. Here we present a morphodynamic model using a simplified bar geometry to calculate decadal-scale changes in sandbar volume – on sub-daily timescales – for a subset of sandbars in Grand Canyon. In this physically-based model, we simulate sand exchange between the main channel and lateral recirculation zones (eddies) at a rate dependent on an eddy exchange coefficient and the concentration of sand in the main channel. Particle settling velocity and the average depth of flow over the bar surface control the rate of sand deposition, and erosion is modeled at each time step along the bar-water interface. The model inputs are continuous (15-minute) measurements of discharge, sand concentration, and median sand grain size in suspension from 2002 to 2018. Measurements of sandbar volume are used to constrain the eddy exchange coefficient and an erosion rate parameter. The model is relatively insensitive to the calibration data, suggesting that physical processes are reasonably well-represented in the model equations. Overall bar behavior is well predicted using the calibrated model (R²~0.80), with bar response during sustained intermediate flows resulting in the largest errors. Post-hoc modeling demonstrates that, while some floods are more effective than others, significant increases in sandbar volume occur with increasing flood frequency (e.g. annual). In the absence of controlled floods during this period, sandbars would have likely remained near their minimum observed size.