8. Drop Spillway

Drop spillway, one of the most widely used soil conservation structures, is primarily used for controlling and stabilising grades in a gully. The chapter focuses on the hydrologic, hydraulic and structural designs of drop spillways. The hydrologic design approaches for estimating the peak flow rate, i.e., the rational method, empirical or frequency factor method of frequency analysis and the hydrological or hydraulic modelling, are discussed. The hydraulic design of straight and box-inlet drop spillways under free and submerged flow conditions is presented. This chapter also includes the critical depth concept and its application in determining the dimensions of various components of the straight and box-inlet drop spillways. The structural design contains the analysis of the horizontal forces acting against the structure due to the hydrostatic pressure of the water column upstream and the earth pressure caused by the backfill. It also comprises the uplift pressure caused due to water seepage through the saturated foundation material. A detailed procedure to analyse the stability of the structure against overturning, sliding, piping, tension, and compression or contact pressure is demonstrated through a solved example.