Investigation of riprap stability at downstream of spillway flip bucket without and with energy dissipators

Abstract

Spillway is one of the most cost-effective energy dissipating structures used in dams which leads to dissipation of flow energy at the end of the structure. Scouring is a phenomenon occurring next to the hydraulic structures, and which can lead to deterioration and even collapse of such structures. One of the methods to control the scouring downstream of a dam is using riprap. In this study it was investigated the stability of riprap based on relative particle diameter downstream of spillways of four different categories of energy dissipators, namely simple flip bucket, dentated flip bucket, simple triangular sill, and dentated triangular sill. The experiments were performed in a rectangular test flume. Ripraps of the same density and of four different diameters were investigated at five discharges at the movement threshold. Results of the study suggested that the relative particle diameter was an important factor in stability of the riprap. Besides, in all of the four aforementioned spillways, the stability number at the movement threshold was found to decrease with increase in the relative particle diameter. For all of the relative particle diameters, use of dentates led to an increase in the stability number 11% and 7% in spillways with simple flip bucket and triangular sill respectively. Also, a formula for estimating relative particle diameter was proposed. The correlation coefficient of the results yielded by this formula and the laboratory results was found to be 0.9 and 0.6 for the spillway with simple flip bucket and the other three types of the spillways respectively.

Keywords

• riprap
• stability number
• simple flip bucket
• dentated flip bucket
• simple triangular