Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result, and the arithmetic was re-run as an executable calculation. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.
Rational Method Peak Runoff: FE Civil practice problem 3
For work package 790 on the regional trail bridge, the agency plan reviewer performs an independent check calculation and uses the Rational Method for drainage area 22 acres, runoff coefficient 0.38, and rainfall intensity 2.3 in/hr. What peak flow is estimated?
- A11.7 cfs
- B19.2 cfs
- C20.6 cfs
- D22.9 cfs
Show the answer and worked solution
Answer: B — 19.2 cfs
Worked solution
- Step 1. Use Rational Method
Q = C i A
Q = 0.38(2.3)(22) = 19.2 cfs
Why the other choices are wrong
- Choice A
- This figure runs about 45 percent above the checked peak flow, and no single swapped value among the runoff coefficient, intensity, or area reproduces it cleanly; multiplying 0.38 by 2.3 by 22 gives the verified 19.2 cfs.
- Choice C
- No single formula substitution or unit slip in Q = C x i x A lands on this higher total; the Rational Method calculation with the stated 0.38 coefficient, 2.3 in/hr intensity, and 22 acre area yields 19.2 cfs.
- Choice D
- This lower total cannot be traced to one specific arithmetic or formula error in the Rational Method; multiplying the given runoff coefficient, rainfall intensity, and drainage area together in Q = C i A produces the verified 19.2 cfs peak flow.