Numerical Assessment of Shear Boundary Layer Formation in Sewer Systems with Fluid-Sediment Phases

Numerical and empirical studies of soil slurry transport and deposition in urban stormwater sewers are few, presumably due to the difficulty of direct observation of soil slurry flow in stormwater pipes. Slurry in a sewer system includes both suspended load and bedload, but few studies have attempted to demarcate these two components. A boundary layer is a crucial determinant of sediment transport capacity. Stormwater runoff enters the sewer in turbulent flow, mostly mixed with soil slurry generated by rainfall. In this paper, we attempt analysis using ANSYS Fluent commercial CFD software. We describe the development of a numerical analytical methodology capable of predicting the flow of soil slurry in stormwater pipes, and propose a method for estimating the sediment–flow boundary layer. Using this model, we simulated stormwater runoff with a large content of soil slurry during a rainfall event. We investigated soil slurry transport and predict the formation of shear boundary layer by varying the inlet conditions (volume of soil slurry entering the stormwater sewer system) and by analyzing the flow velocity field and soil slurry volume fraction in the pipes under various experimental flow conditions. Based on the shear and settling velocity of sediment particles, we propose criteria for the formation of a shear boundary layer in stormwater pipes.