Urbanization and Detention Sizing in a 240 ha Watershed
A 240 ha watershed is being converted from 50% forest (CN 55), 40% pasture (CN 70), and 10% impervious (CN 98) to 20% forest, 35% lawn (CN 70), and 45% impervious. The design storm is 100 mm in 24 h. A proposed detention basin has 45,000 m3 of storage below the emergency spillway and an outlet designed to pass 0.8 m3/s at the spillway elevation. The applicant claims the basin offsets the post-development increase in runoff volume. The local streamgage has only 3 years of record and no flow data for the 100 mm event. The review engineer must evaluate the claim with NRCS Curve Number hydrology and identify the dominant uncertainty.
1. Compute the area-weighted curve number and runoff depth for pre- and post-development conditions, and estimate the increase in runoff volume.
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Area-weighted CN: pre = 0.50(55)+0.40(70)+0.10(98)=65.3; post = 0.20(55)+0.35(70)+0.45(98)=79.6. For P=100 mm, S=25400/CN-254 mm. Pre: S=135 mm, Ia=0.2S=27 mm, Q=(100-27)^2/(100+0.8135)=25.6 mm. Post: S=65.1 mm, Ia=13.0 mm, Q=(100-13.0)^2/(100+0.865.1)=49.8 mm. Increase=24.2 mm over 240 ha=2.4e6 m2, so 0.0242*2.4e6=58,100 m3. Pre volume is about 61,400 m3; post volume is about 119,500 m3. This is a curve-number estimate, not a routed hydrograph.
2. If the basin were required to store the entire increase in runoff volume, would 45,000 m3 suffice? Explain why this check is not a routing analysis and what it does and does not prove.
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45,000 m3 is less than 58,100 m3, so the basin cannot store the entire computed volume increase; the shortfall is about 13,100 m3. But a volumetric check is not a routing analysis. The outlet can release 0.8 m3/s; over 24 h that is 69,100 m3, so the basin may still attenuate the peak while passing volume. Conversely, passing volume does not prove peak control or downstream protection. The engineer needs to route the design hyetograph through storage, compare peak outflow with the pre-development peak, and check freeboard, emergency spillway capacity, and outlet blockage. The CN method also depends on antecedent moisture and the initial-abstraction assumption, so the volume estimate itself carries uncertainty.
3. What analyses and field data would you require to address the 3-year streamgage record and the reliance on a single design storm?
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Three years of record cannot define the frequency of a 100 mm event or calibrate a rainfall-runoff model. Require regional regression or nearby long-record gages, continuous simulation with local rainfall, field verification of soils and imperviousness, and sensitivity to CN plus or minus 5 and to antecedent moisture. Because detention performance depends on both peak and volume, test multiple storm durations, including shorter high-intensity bursts and the 100-year event. Inspect as-built grading, inlet and outlet condition, and maintenance access. A basin that passes one storm may fail after sediment accumulation or debris blockage, so the review should include a maintenance and inspection plan, not only a single-event calculation.