Minimum critical reservoir top elevation

Given: $Q_{max}=5127$ m³/s; 5 openings, each 12.2 m wide; crest level 1250 m; $C_d=0.6$ for a broad-crested weir.

The minimum critical (maximum flood) level of the reservoir is the crest level plus the head $H$ that is needed to pass the maximum discharge.

Effective length

$$L=5\times12.2=61.0\ \text{m}$$

(Pier contraction is neglected, since no pier or abutment coefficient is given.)

Discharge equation (broad-crested weir)

$$Q=\frac{2}{3}C_d\sqrt{2g}\,L\,H^{3/2}$$ $$\begin{aligned} 5127&=\frac{2}{3}(0.6)\sqrt{2\times9.81}\,(61.0)\,H^{3/2}\\ 5127&=108.08\,H^{3/2}\\ H^{3/2}&=47.44\ \Rightarrow\ H=13.10\ \text{m} \end{aligned}$$

Reservoir level

$$\text{RL}=1250+13.10=1263.10\ \text{m}$$

Answer: the top (flood) level of the reservoir must not be lower than 1263.10 m (head over the crest $=13.10$ m).