Trash Rack Blockage in Supercritical Flow

A physical model of an urbanized storm-drainage conveyance system was conducted to identify the location and nature of channel overtopping. The drainage system is characterized by supercritical flow (F = 1.30 to 1.76) and a heavy debris load. A trash rack became the study focus. Trash rack orientations of 1:2 and 3:1 slopes were tested and evaluated for self-cleaning operations. The 1:2 slope rack tended to clog, while the 3:1 slope rack exhibited self-cleaning tendencies in supercritical-flow conditions. A blockage analysis was conducted, indicating that the 3:1 slope rack would cause localized flooding with only 41% blockage. Further, blockage near the bottom of the channel (nonfloating debris) resulted in more severe localized flooding than blockage placed near the water surface (floating debris). The present study suggests that the traditional procedure for evaluating system-design adequacy by assuming 50% blockage of the trash rack is not necessarily conservative for supercritical applications. It is recommended that a 40% blockage be assumed for trash-rack analysis in supercritical flow.