Discharge over overflow spillway

Given: crest length $L = 60$ m, crest elevation $= 133$ m, reservoir water surface $= 135$ m, coefficient of discharge $C_d = 0.74$.

Head over crest:

$$H = 135 - 133 = 2\ \text{m}$$

Formula (flow over a weir/overflow spillway, approach velocity neglected):

$$Q = \frac{2}{3}\,C_d\,\sqrt{2g}\;L\,H^{3/2}$$

Substitution:

$$\begin{aligned} Q &= \frac{2}{3} \times 0.74 \times \sqrt{2 \times 9.81} \times 60 \times 2^{3/2} \\ &= 2.185 \times 60 \times 2.828 \\ &= 370.8\ \text{m}^3/\text{s} \end{aligned}$$

(The equivalent spillway coefficient $C = \tfrac{2}{3}C_d\sqrt{2g} = 2.185$, which is close to the usual value of about 2.2 for ogee crests in $Q = C L H^{3/2}$.)

Answer: discharge over the spillway $Q \approx 370.8$ m³/s.