Safety Assessment of Irrigation Weirs by Modeling Local Scour Potential Downstream of Energy Dissipator

Abstract

The phenomenon of local scour downstream of irrigation weir energy dissipators is one of the basic degradation problems in hydraulic structures on rivers. The hydraulic jump in the stilling basin, which is triggered by flow turbulence due to energy dampening, forms current vortices in horizontal and vertical directions. Eddy currents that are not fully controlled and damped in the stilling basin can cause scouring, especially in the vertical direction just downstream of the energy dissipators construction. Simultaneous accumulation of scour in the long term can endanger buildings around the weir, especially the stilling basin, both downstream wings and the main weir. This paper aims to assess weir safety by predicting scour characteristics under end sill energy dissipators using the hybrid model: HEC-RAS and Scour Empirical Equation. The simulation is carried out on three conditions, namely minimum, average and maximum discharges with input of sediment grain gradations obtained from field measurements. The three types of discharge are obtained from the transformation of daily rainfall into daily discharge using the HEC-HMS Model. The results of scour predictions in the 3 scenarios were compared with the results of scour measurements at the study location. The results of the study indicate that the greatest scour depth at the maximum discharge of almost 1.5 meters has reached the depth of the end sill of the energy dissipator. This prediction result was also verified by the observation scour depth of more than 1.5 meters. Observation results also show that the Energy Dissipator II has collapsed massively both in the center and on the left and right sides of the energy dissipator. Based on the results of the study and investigation, it can be concluded that the weir safety at the study location can be categorized as low with the potential for very large scour.