PHYSICAL MODEL AND VALIDATION OF LOCAL SCOUR PATTERN DOWNSTREAM OF TRAPEZOIDAL THRESHOLD

Abstract

Trapezoidal thresholds are often used to measure discharge in irrigation canals. A trapezoidal sharp threshold can be made with a slope (n) varying from n₁ = 1, n₂ = 0.5 and n₃ = 0.1. Discharge measurement with this method is not only easy, but also has good accuracy, especially for irrigation water distribution. However, the weakness in the downstream threshold can occur in local scour. To observe the local scour pattern downstream of the trapezoidal threshold, a physical model test was conducted in the laboratory. Forty (40) experiments were carried out on 3 variations of the slope of the trapezoidal frame wall and using 2 channel base materials. From the results of this study, the equations for scouring length (Ls) and scour depth (ds) are obtained which are influenced by downstream water height (H), grain diameter (d50), froude number (Fr), and threshold wall slope. After validating the resulting model using a slope of n₄ = 0.6 and n₅ = 0.3, a relative error rate of 18.7% was obtained (NSE = 0.1, MAE = 0.2 and RMSE = 1.4)