Downstream Culvert Surcharge on an Infill Lot
A developer proposes a 14-lot subdivision on a 3.2-acre parcel. The site currently drains as sheet flow to a 36-inch CMP culvert crossing the county road at the low corner; the culvert invert is 2.1 ft below the road shoulder and the pipe is estimated at 40% full during the 10-year storm. The grading plan raises the rear 0.8 acre by up to 1.5 ft and adds 0.6 acre of new impervious surface (roofs, driveways, a private lane). Peak 10-year flow off the site is calculated at 9 cfs pre-development and 17 cfs post-development. The county requires that post-development peak discharge not exceed pre-development for the 2-, 10-, and 100-year events, and that no downstream structure be adversely affected. The engineer proposes a single dry detention basin sized for the 10-year volume with a 12-inch orifice outlet.
1. Evaluate whether the proposed single-basin, single-orifice design satisfies the county criteria, and identify the controlling deficiency.
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The design is deficient on two grounds. First, the county criterion is multi-event (2-, 10-, 100-year), but the basin is sized only for the 10-year volume. A 12-inch orifice passes flow roughly with the square root of head, so the small 2-year event may be detained adequately while the 100-year event overwhelms the same storage and discharges well above pre-development peaks. Detention must be shown to control all three events, typically by routing each storm through the stage-storage-discharge relationship and sizing the outlet (or adding a secondary/emergency outlet) so the 100-year peak is also attenuated. Second, the criterion that no downstream structure be adversely affected is not addressed. The existing culvert is already near capacity at the 10-year event; even a matched 10-year peak delivered at a different time-to-peak can raise the culvert headwater because the post-development hydrograph is peakier. The engineer must route the post-development hydrograph through the culvert to confirm headwater stays below the shoulder and does not back up onto upstream properties, and should consider extended detention or a staged outlet to mimic pre-development timing.
2. The basin is sited on the raised rear fill. What geotechnical and grading concerns does that create, and how should they be resolved before final design?
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Placing a detention basin on 1.5 ft of new fill creates a seepage and stability problem. The basin bottom must be below the surrounding grade to store volume, so excavation will penetrate the new fill and possibly the native soil, creating a path for stored water to seep under the embankment or toward adjacent lots. Required actions: (1) a geotechnical investigation of the fill and native soils to establish permeability, consolidation, and the depth to a suitable foundation; (2) confirmation that the fill was placed and compacted to specification, since uncontrolled fill under a basin will settle and crack any liner; (3) a liner or low-permeability bottom where seepage would affect adjacent foundations or slope stability; (4) a stability analysis of the basin side slopes and any embankment, with side slopes no steeper than needed for maintenance access; and (5) a positive gravity outlet and emergency spillway that do not depend on the raised fill remaining intact. The basin should not be located where its excavation undermines the raised building pads.