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 2
For work package 775 on the regional trail bridge, the project controls engineer performs a contractor change evaluation and uses the Rational Method for drainage area 27 acres, runoff coefficient 0.53, and rainfall intensity 2.05 in/hr. What peak flow is estimated?
- A29.3 cfs
- B18.5 cfs
- C22.2 cfs
- D31.7 cfs
Show the answer and worked solution
Answer: A — 29.3 cfs
Worked solution
- Step 1. Use Rational Method
Q = C i A
Q = 0.53(2.05)(27) = 29.3 cfs
Why the other choices are wrong
- Choice B
- This peak flow scales the direct C * i * A product upward by roughly 18 percent rather than reflecting any substitution among the runoff coefficient, rainfall intensity, or drainage area, so it overstates what the rational method equation actually returns.
- Choice C
- This peak flow scales the direct C * i * A product downward by roughly 22 percent while leaving the runoff coefficient, rainfall intensity, and drainage area unchanged, so it understates what multiplying those three terms together actually returns.
- Choice D
- This peak flow pushes the direct C * i * A product roughly 45 percent above its true value, a jump too large to come from any single misread runoff coefficient, intensity, or area, and overstates the rational method result.