Question

Suppose a developer purchased a 3-acre farm in the middle of the state of Tennessee. As part of the project, a 30,000 sqft. asphalt parking lot area will be placed on one side of the 3-acre farm. Looking at the 10-year 1-hour rainfall chart See map), the farmer determines that the rainfall amount is 2.2 inches/hour. Solve the Rational Formula to determine the peak flow of the property prior to development ( pre-development) and after the addition of the asphalt parking lot (post- development).

Answer 1

Answer: the peak flow rate prior to development (pre-development) is 15.51 cfs, and after the addition of the asphalt parking lot (post-development) is 76.21 cfs.


Explanation:

Q = CIA

Where:
Q = peak flow rate
C = runoff coefficient
I = rainfall intensity
A = drainage area

For the pre-development condition, we can assume that the entire 3-acre farm is a grassy area with no impervious surfaces. Therefore, the runoff coefficient will be 0.20.

Q(pre-development) = 0.20 x 2.2 x (3 x 43,560) / 3600
Q(pre-development) = 15.51 cubic feet per second (cfs)

For the post-development condition, we need to take into account the 30,000 sqft. asphalt parking lot. The runoff coefficient for asphalt is 0.90.

Q(post-development) = 0.90 x 2.2 x ((3 x 43,560) + 30,000) / 3600
Q(post-development) = 76.21 cubic feet per second (cfs)