Prediction of local scour downstream of singlestep broad crested porous weirs

Broad crested weirs are hydraulic structures used for flow measurement and water level regulator. In hydraulic structures which are obstacles to flow, the differences in flow pattern in some areas result local scour in these areas. Scour is the bed erosion at the downstream of the hydraulic structures due to high water flow or turbulent flow. The importance of examining the scour depth is evident when the scour depth is significant on river structure's foundation and affecting the stability of the structures or destroying them. In this study, flow was simulated using three –dimensional hydrodynamic analysis by FLOW-3D software and maximum depth downstream of the weir was predicted. Three models of Single Step Broad Crested Weirs [C1, C2, C3] with same step angles (8.7) and different porosities (20,25,30%) and different discharges (15,20,25) l/s respectively are tested under the same duration of 6 hours. There was a good agreement between experimental and FLOW-3D results. The numerical and the experimental results showed that, model C3 reduce the maximum depth of scour and also dump the scour holes away from the structure as compared to other models. The numerical study showed that the value of the scour depth at the downstream of the weir increased by increasing the discharge. The porosity effect was also studied in the scour depth at the downstream of the weir. It has been found that model C3 and porosity (30%) give a minimum scour depth and minimum local scour hole volume. The present technique reduces costs and improves the hydraulic performance of the broad crested weir.