The Role of Sediment Budgets in the Implementation and Evaluation of Controlled Floods to Restore Sandbars along the Colorado River in Grand Canyon, Arizona

Abstract:

The measurement and prediction of the fine sediment budget for the Colorado River in Grand Canyon has been of strong scientific and management interest since erosion of sandbars was first reported in the early 1970s, about 10 years after Glen Canyon Dam began regulating streamflow and eliminated the upstream sediment supply. Efforts to rebuild eroded sandbars have consisted largely of the experimental release of controlled floods, during which sand is redistributed from the bed to eddy sandbars along the channel margin. Flood-aggraded sandbars are, however, inherently unstable and inevitably erode between floods. Thus, sandbars cannot be “preserved,” but are dynamic landforms that require periodic rebuilding by recurring floods. Such a strategy, with the goal of achieving a long-term increase in the size and number of sandbars, was recently implemented as a policy initiative of the U.S. Department of the Interior. This High Flow Experiment Protocol is being implemented by a unique collaboration of scientists, engineers, and policy makers and provides a rare example of a case in which management decisions are fully integrated with scientific monitoring. Controlled floods are scheduled based on real-time monitoring of sediment flux, computations of sediment budgets, and use of flow and sediment models. Floods are scheduled to occur within a few weeks of measurement results becoming available, assuming that threshold triggers for sediment accumulation are met. Sandbar building results are evaluated within weeks to months using remotely deployed time-lapse cameras. The protocol has been implemented in fall 2013 and fall 2014. Preliminary results suggest that the program may be resulting in the desired effect of cumulative increases in sandbar size. These results are tentative, because recent years have been relatively favorable, with large fine-sediment inputs and low annual dam-release volumes. Successive years with low fine-sediment supply or above-average dam releases could cause the flood protocol to result in progressive fine-sediment depletion. Ultimately, success depends on factors that cannot be controlled and are difficult to predict. For these reasons, the protocol is designed as a long-term experiment whose success will be evaluated based on implementation and evaluation over 10 to 20 years.