Large-eddy simulation of flash flood propagation and sediment transport in a dry-bed desert stream

Abstract

A Large-Eddy Simulation (LES) model is used to study flow dynamics of a flash flood event in a dry-bed, desert wash, the so-called Tex Wash, near the Tex Wash Bridge on Interstate 10 in the Mojave Desert of California. The evolving free surface of the flash flood is tracked using the level-set method. A bed morphodynamics module is coupled to the hydrodynamics model to calculate the erosion and bed evolution of the mobile bed of the wash under flash flood conditions. Flash floods in a desert wash can be characterized with a number of salient features such as the (1) existence of both the dry- and wet-cells on the bed surface of the wash that correspond to the air and water phases, respectively; (2) presence of various flow regimes, critical, sub-critical, and super-critical in the flow domain; and (3) occurrence of a highly transient and complicated flow field and, subsequently, sediment dynamics throughout the wash. A numerical modeling effort is presented to study a recorded flash flood and the corresponding scour processes in the Tex Wash. The flood event occurred in 2015 and lead to the collapse of the Tex Wash Bridge. The of the current study is to gain insight into the flood flow and sediment transport mechanisms, which resulted in the collapse of the bridge. To that end, a study area, which includes a 0.65 km-long reach of Tex Wash at its intersection with the Tex-Wash Bridge, was selected. The bathymetry of the wash was obtained using light-detection-and-ranging (LiDAR) technology and used to construct the computational domain of the wash and bridge foundations. The transient flow of the flash flood, in both air and water phases, and the evolving morphology of the wash are numerically simulated. The site-specific numerical simulation revealed the formation of deep scour regions adjacent to the right abutment of the upstream bridge, where significant erosion caused the collapse of the bridge. Moreover, the results show that most of the scour processes take place during the steady phase of the flash flood when the desert stream is filled with water. However, the transient phase of the flash flood is rather short and contributes to a very limited amount of erosion within the stream.