The effect of step on the hydraulic characteristics of the subcritical free surface flow in conveyance tunnel

Due to the water scarcity and uneven distribution of water resources, conveyance systems are designed to carry water through basins. For this purpose conveyance tunnels which carry supper/subcritical flows are commonly constructed. The occurrence of steps in the tunnels created by segment off-sets during the TBM operations would cause significant local head losses. As a result, the flow discharge may reduce. In this study subcritical free surface flow in conveyance tunnel is simulated using the one-dimensional HEC-RAS model. Impact of the invert segment off-set on the discharge rate is then estimated. Similarly a two-dimensional numerical model based on the Volume of Fluid (VOF) scheme is applied. The effects of the step height and angle are also quantified. The results show that the existence of steps in the tunnel reduces the flow discharge considerably. Decreasing the approaching angle of the step would improve the flow condition. HEC-RAS simulations showed reliable results in the study of local head losses for sub-critical flow conditions. (C) 2011 Elsevier Ltd. All rights reserved.