Stormwater overflow in stepped channel

Sharp bends in drainage channels are sometimes engineered to suit local topography, especially in densely populated cities where land is limited. A hydraulic model study of a steep urban drainage channel is carried out to investigate the causes of an overflow incident in Hong Kong urban area, and to develop the drainage improvement measures to prevent flooding. The stepped drainage channel has a sharp bend in the middle and several trash racks are located along the channel. Experiments are performed on a 1:16 Froude scale model. In the first phase, experiments on a purposely built straight channel model are performed to study the bottom roughness of the stepped channel flow isolating the effect of the bend. After the straight channel experiments, four trash rack scenarios are tested in the exact model with detailed channel features. The flow in the drainage channel is supercritical and highly aerated. Based on the theory of skimming flow in stepped channels, a hydraulic assessment suggests that the bend and trash racks may be dominant causes of the overflow. The experiments show that sharp bend causes a spiral flow leading to significant lateral overflow. The trash racks are observed to generate a serious ski-jump type overshooting flow. It is found that the stormwater overshoot and flooding can be prevented by the repositioning of appropriately sized trash racks and the containment of the spiral flow. The proposed channel improvement design has been implemented on site and found to work successfully in heavy rainstorms after installation. The present study offers novel insights into the design of urban drainage works.