Question

We would like to set a Cipolletti weir in a small forest stream to measure flows in the watershed, and take water quality samples as part of a watershed ecology research project. The upstream area = 84.5 acres, the storm rainfall intensity was found to be 1.09 in/hr, the runoff coefficient =0.76. If our weir length (L) is 3 meters, what will the height of the water be during our peak flow storm event?

Answer 1

Given, area of the watershed = 84.5 acres = 84.5 x 43560 ft² = 3680820 ft²
Now, rainfall intensity = 1.09 in/hr = 1.09/12 feet/hr
Runoff coefficient = 0.76
Also, weir length = 3 meters = 9.8425 ft.
Therefore, discharge at the watershed outlet = 0.76 x 3680820 x 1.09/12 ft³/hr
= 254099.274 ft³/hr.
= 254099.274/3600 ft³/s = 70.583 ft³/s
Now, we know the discharge through a cipolleti weir is given by

Q = 3.367 L h^3/2
Where, Q = Discharge in cfs, L = weir length in feet and h = height of water in feet.

⇒ 70.583 = 3.367 x 9.8425 x h^3/2
⇒ h^3/2 = 2.129
⇒ h = 1.655 ft.