The discharge for gated crests at partial gate opening is similar to flow through a low- head orifice and may be computed by the following equation recommended by the Bureau of Indian Standards code IS:6934-1998 “Hydraulic design of high ogee overflow spillwaysrecommendations”.
Q Cg G0 L2ge (4) He
Where Q is the discharge (in m3/s), Cg is the gated coefficient of discharge, G0 is the gate opening (in m), Le is the effective length of crest, g is the acceleration due to gravity, and He is the hydraulic head measured from the centre of the orifice (in m).
Usually for high head spillways, radial gates are common and Figure 34 shows the position of a partially opened radial gate over an ogee-crested spillway. The gate opening G0 may be seen to be measured as the shortest distance from the gate lip to the ogee crest profile meeting at G. The angle is seen to be measured between the tangent at G and the tangent of the radial gate at gate lip. Figure 35 presents a curve relating the coefficient of gated discharge Cg with the angle
The curve presents an average value of Cg determined for various approach and downstream conditions and may be used for preliminary design purposes. In fact, it may be noticed that the discharge equation mentioned above for calculating flow through a gated spillway as recommended in IS: 6934-1998 may not be strictly correct as the gate opening becomes larger, comparable to the hydraulic head Hc.