Author(s): L. Chang; S. N. Chan; J. H. W. Lee
Keywords: Intake structure; Supercritical flow; Transverse barrier; Storm water interception; Volume-of-fluid; CFD
Abstract: The Hong Kong West Drainage Tunnel (HKWDT) system consists of 34 storm water intake structures designed to intercept storm water runoff in upland catchments. Each intake intercepts and diverts the upstream supercritical flow into a bottom rack chamber connected to a supercritical vortex drop. To satisfy the minimum environmental flow requirement, a small flow is intercepted by an inclined barrier across the main channel and conveyed to the downstream drainage system through a 300mm wide low flow channel (LFC) along one side of the intake. Observations during rainstorms suggest that the storm water runoff being conveyed via the low flow channel might be more than originally designed. The hydraulic performance of the intake structure is assessed via three-dimensional Computational Fluid Dynamic (CFD) modelling using the Volume-of-Fluid (VOF) method. The predicted un-intercepted flow compares well with experimental measurements on a 1:12 undistorted model. The CFD computations show that in a typical rainstorm event, the supercritical inflow impinges on the barrier and is deflected upwards, resulting in complex cross-currents between the main stream and the LFC. It is found that the ratio of un-intercepted flow to the total flow decreases with increasing inflow. Supported by field observations the CFD model predicts that the low flow channel flow is significantly higher than designed; the effect of the barrier is less significant for higher flows.