Gravel-bed river morphodynamic processes throughout a very large flood event from numerical modeling

Abstract

Due to the lack of numerical modeling and continuous in-situ monitoring of flow and sediment transport data, the mechanism of local-scale morphodynamic processes during very large floods are not fully understood. Thus, this study employs Delft3D to develop a two-dimensional (2-D) morphodynamic model to simulate and analyze the morphodynamic process of a gravel-bed river during an 80-year flood event in Calgary, Canada. The model was calibrated using velocimetry data and validated against the measured post-flood bed elevation data. The coefficient of determination (R2) and ratio of the root-mean-square error to the standard deviation (RSR) between the modeled and measured bed elevation was 90% and 0.33, respectively, which demonstrates the reliability of the model. The modeled flow velocity, bedload transport rate, surface sediment sizes, and corresponding morphological changes at different flood stages are presented and analyzed. Results show that bed incision and the mid-channel bar continuously developed throughout the flood while bank erosion and the growth of bank-attached bar mainly happened during the rising and peak periods. We found that the timing and duration of major morphological changes during a flood event varies from site to site within a reach, but is similar for similar morphological units. We also found that the spatial variation of channel planform is the dominant determinant of major morphological changes during floods, while flood events trigger the sediment motion and result in actual deposition or erosion. Improvements are needed in terms of the modeling of bedload transport and bed stratigraphy.