Water Balance Analysis Using WARMF in Support of an EFDC Hydrodynamic Model
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An Environmental Fluid Dynamics Code (EFDC) model was previously developed for Bankhead Reservoir, located in northwest Alabama. The EFDC model inaccurately predicted water surface elevations in the reservoir, due to sources of mass that were not able to be quantified. The reservoir has numerous tributaries contributing flow to the domain, as well as a known leakage issue through the spill gates at the downstream Bankhead Dam. Because EFDC does not include a mass balance tool, the objective of the study is to use other modeling tools (hydrologic modeling and water balance analysis) to account for the unknown mass inputs to the reservoir. The watershed and tributary inflows were modeled using Watershed Analysis Risk Management Framework (WARMF). A traditional hydrologic modeling approach to optimize the modeling of peak flows was used initially, but found to be inadequate for the purpose of predicting flows for the specific low-flow time periods in the EFDC model. Therefore, separate 2010 and 2011 low-flow WARMF models were developed to achieve the best possible re-creation of observed data for the time periods of interest. Finally, to bring the reservoir into balance and to quantify downstream leakage, the water balance utility contained within CE-QUAL-W2 was coupled to the EFDC model to determine a flow file that would bring the reservoir into balance. After the balancing procedure was completed, the EFDC model simulated the water surface elevations to a high accuracy, with absolute errors less than 15 cm.