(Similarity) PHYSICAL MODEL AND VALIDATION OF LOCAL SCOUR PATTERN DOWNSTREAM OF TRAPEZOIDAL THRESHOLD

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).
Keywords: Local scour, Trapezoid threshold, Scour length, Scour depth, Grain diameter, Froude number