The rational method estimates the peak rate of stormwater runoff from a small drainage area as the product of a runoff coefficient, a rainfall intensity, and the area. It is the standard first-pass tool for sizing inlets, culverts, and storm sewers on small urban catchments, and the FE Civil exam uses it constantly because the arithmetic is short and the assumptions are instructive. The runoff coefficient is the fraction of rainfall that becomes surface runoff, running from roughly 0.10 for flat lawns to 0.95 for pavement and roofs. A catchment with more than one surface type takes an area-weighted composite coefficient, which is the second most common step in these problems. The units are the reason the method is worth practising. In customary units, intensity in inches per hour times area in acres gives acre-inches per hour, and that number is within about one percent of cubic feet per second, so the formula is used with no conversion factor at all. That coincidence does not carry over to SI, where a factor of 360 converts millimetres per hour and hectares into cubic metres per second. The rainfall intensity itself is read from an intensity-duration-frequency curve at a duration equal to the time of concentration, which is the assumption that limits the method to small areas.
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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. 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.
FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES. All practice questions are original and generated for study purposes based on publicly available exam specifications.