Help with this fluid mechanics problem step by step $.$

The Tupperware dam on the Blackstone River is $12$ ft high, $100$ ft wide, and sharp$-$

edged. It creates a backwater similar to Fig. below. Assume that the river is a weedy$-$

earth rectangular channel $100$ ft wide with a flow rate of $800f\frac{t^3}{s}.$ Estimate the value of

H and also the initial water depth as shown in the picture below.