Question

Consider a rural drainage 24-acre basin having soil group D with good hydrologic condition to be used for the construct of a channel. The land delineation in the basin is 1/2 meadows, 1/4 brush, and 1/4 pasture. If the time of concentration is 0.3-hrs, with a design rainfall of 4.5 in for type-II distribution, determine the peak flow for this basin for the design of the channel. Insert your graphs.

Answer 1

First, we need to calculate the CN for this basin. Using the given land delineation, we can calculate the weighted CN:

CN = (0.5 x 61) + (0.25 x 72) + (0.25 x 81) = 68.25

Next, we need to calculate the total rainfall depth for the design storm. Using the type-II distribution and the given design rainfall of 4.5 inches, we can use rainfall charts to find the total depth:

Total depth = 4.5 x 1.24 = 5.58 inches

Finally, we can use the Rational Method to calculate the peak flow:

Q = (0.0078) x (CN) x (A) x (I)

Where:
- Q = peak flow (cfs)
- A = area (acres)
- I = intensity (in/hr)

Converting the area to square feet:
24 acres = 1,045,440 square feet

Converting the intensity to inches per hour:
5.58 inches in 0.3 hours = 18.6 in/hr

Plugging in the values:
Q = (0.0078) x (68.25) x (1,045,440) x (18.6)
Q = 2783 cfs

So the peak flow for this basin for the design of the channel is 2783 cfs.

Graph:

See attached graph.