Fully coupled free-surface flow and sediment transport modelling of flash floods in a desert stream in the Mojave Desert, California

Abstract

For most of the year, a dry-bed desert wash is void of water flow. Intensive rain events, however, could trigger significant flash floods that bring about highly complicated hydrodynamics and morphodynamics processes within a desert stream. We present a fully coupled three-phase flow model of air, water, and sediment to simulate numerically the propagation of a flash flood in a field-scale fluvial desert stream, the so-called Tex Wash located in the Mojave Desert, California, United States. The turbulent flow of the flash flood is computed using the three-dimensional unsteady Reynolds-averaged Navier-Stokes equations closed with the shear stress transport k - omega model. The free surface of the flash flood at the interface of air and water phases is computed with the level-set method, which enables instantaneous tracking of the water surface as the flash flood propagates over the dry bed of the desert stream. The evolution of the desert fluvial stream's morphology, due to the action of the propagating flash flood on the mobile bed, is calculated using a Eulerian morphodynamics model based on the curvilinear immersed boundary method. The capabilities of the proposed numerical framework are demonstrated by applying it to simulate a flash flood event in a 0.65-km-long reach of the Tex Wash, the intricate channel morphology of which is obtained using light imaging detection and ranging technology. The simulated region of the stream includes a number of bridge foundations. The simulation results of the model for the flash flood event revealed the formation of a highly complex flow field and scour patterns within the stream. Moreover, our simulation results showed that most scour processes take place during the steady phase of the flash flood, that is, after the flash flood fills the stream. The transient phase of the flash flood is rather short and contributes to a very limited amount of erosion within the desert stream.