Equivalent Manning's Roughness in Combining Open Channel Junction Flows

The present numerical study attempts to estimate the equivalent Manning's roughness in combining open channel junction flows. The three-dimensional flow field in a combining open channel junction is simulated using the CFD software, Fluent 17.2. Simulated results for water surface elevation and velocity distribution are validated against the experimental results available in literature for junction flow over smooth bed. The numerical model is used to simulate nine different flow conditions using three different discharge ratios (ratio of main channel discharge to total discharge) and three bed roughness heights. The head loss is calculated considering the energies at three sections (two at upstream of the junction and one at outlet). An equivalent Manning's roughness is obtained from the head loss. For discharge ratio of 0.75, values of equivalent Manning's roughness for the channel reach, starting from the location where junction starts and up to the channel outlet, are found to be 0.012, 0.016, and 0.018 for bed roughness height = 0 mm, 2 mm, and 5 mm, respectively. For discharge ratio of 0.25, values of equivalent Manning's roughness are found to be 0.016, 0.020, and 0.023 for bed roughness height = 0 mm, 2 mm, and 5 mm, respectively. The results show that the equivalent Manning's roughness in a channel junction flow is a function of the discharge ratio and bed roughness. The effect of bed roughness on water surface profile and bed shear stress is also presented. The present study will be useful for analyzing the flow resistance in channel junction in a simplified way.