ANALYSIS OF PLUNGE POOL SCOUR HOLE FORMATION BELOW A CHUTE SPILLWAY WITH FLIP BUCKET USING A PHYSICAL MODEL

Abstract

Plunge pool scouring due to water jet impingement from flip bucket spillways is a major concern for stabilization of hydraulic structures and their downstream channel. The objectives of this study are to select an appropriate movable riverbed material for reproducing scour hole in the physical model and to analyze the characteristics of its formation using the selected material. The whole tasks were carried out using Nam Ngum 3 Spillway Physical Model constructed with an undistorted scale of 1:75. Different types of movable riverbed material were tested and the most appropriate one was then selected in order to conduct the scour study. Based on a set of criteria, cohesive movable riverbed material of Sand (40): Cement (1): Water (5) was found to reproduce more reliable scour hole than non-cohesive material of 25 mm graded gravel. Utilizing the selected material, the overall model test demonstrated that spillway operation, river morphology and natural topography of the plunge pool are the main factors governing irregular trend of scour development. Scour hole tends to develop toward the left bank and the maximum scour depth, about 74 m deep below the original riverbed, is located approximately 252 m downstream of the spillway’s bucket lip.