CONCEPT:Overflow Spillway:As per IS 6394:1998, cl. 4.2.1.It is also known as the ogee spillway as shown below.The design discharge over the spillway when He = Hd is computed by -Q = C × Leff × Hd3/2 Here,C = \(\frac{2}{3}\) × Cd × \(\sqrt{2g}\) = [1.88, 2.20]Here,Le - effective spillway length (m)Hd - Design energy HEAD = hd + \(\frac{V_o^2}{2g}\) mExplanation:The discharge corresponding to any energy head over the ogee spillway is given by -Q = \(\frac{2}{3}\) × Co × Le × \(\sqrt{2g}\) × He3/2 ---(1)The design discharge over an ogee spillway at the design head is given by -Q = \(\frac{2}{3}\) × Cd × Le × \(\sqrt{2g}\) × Hd3/2 ---(2)Here,He - any energy head over the spillway (m)Co and Cd - coefficient of discharge at head He and Hd respectively.Cd = \(f\left(\frac{P}{H_d} \RIGHT)\) as shown below -From the figure above,(Cd)minimum = 0.64(Cd)maximum = 0.738If \(\frac{H_e}{H_d}\) = 1 ⇒ \(\frac{C_o}{C_d}\) = 1 ⇒ Co = Cd = [0.64, 0.738]Also, We know,Discharge through the ogee spillway can be given by -Q = C × Leff × Hd3/2 ---(3)On comparing (2) and (3)C = \(\frac{2}{3}\) × Cd × \(\sqrt{2g}\) = [1.88, 2.17]From design consideration, we always consider maximum discharge.Hence, for the design of the spillway maximum value of C will be considered i.e. 2.17 ≈ 2.20.