Design Storm Selection for a Small Urban Catchment
A 12 km² urbanizing catchment needs a design peak flow for a culvert. Available data: 9 years of peak-flow record (too short for at-site frequency analysis), and a regional regression equation for the 100-year peak. The catchment is 35% impervious and projected to reach 55% impervious at build-out. A 24-hour design storm depth of 180 mm is proposed, but the time of concentration is only 1.5 hours. A colleague proposes using the 24-hour depth directly with the rational method.
1. Critique the proposed design-storm and method choice.
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Two errors. First, a 24-hour storm depth applied to a catchment with a 1.5-hour time of concentration is inconsistent: the critical storm for peak flow is one whose duration approximates the time of concentration (or slightly exceeds it), not 24 hours. Using the 24-hour depth with the rational method overstates the intensity-duration mismatch and can misrepresent the peak. Second, the rational method with a single intensity assumes steady rainfall; for urban catchments, the design storm should be a short-duration, high-intensity event (or an intensity-duration-frequency value at the time of concentration). The 9-year record is too short for at-site 100-year estimation, so regional regression is appropriate but must be adjusted for imperviousness.
2. How should imperviousness change be handled between current and build-out conditions?
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Design for build-out (55% impervious), not current conditions, because the culvert must serve the developed catchment. The rational method runoff coefficient C should reflect 55% impervious (higher C, e.g., 0.6–0.75 depending on soils and routing) and the time of concentration will shorten as imperviousness and drainage efficiency increase, raising peak intensity. Apply the regional regression, then adjust peak for the imperviousness difference between the regression's calibration catchments and build-out. Report both current and build-out peaks; size the culvert for build-out with a freeboard/safety margin. Document the imperviousness assumption as a design condition.
3. What is the defensible design statement given the short record?
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State that the 100-year peak is estimated from regional regression adjusted to build-out imperviousness, with a design storm of duration near the time of concentration (not 24 hours). Provide an uncertainty range reflecting the short record and regression error (often ±30–50%). Recommend the culvert be sized to the upper bound or with freeboard if failure consequences are high. Note that the 9-year record cannot support at-site frequency analysis and should not be used alone. The defensible answer is a bounded, assumption-explicit estimate, not a single rational-method number derived from a mismatched storm duration.