Stormwater Discharge from an 8 ha Development into an Existing Culvert
A developer proposes 8.0 ha of medium-density housing on greenfield land that currently drains to a 900 mm diameter brick culvert under a public road. The culvert was built in 1968 and serves a 40 ha catchment. The local authority requires no increase in peak discharge for the 100-year storm. Pre-development runoff coefficient is 0.35 and post-development is 0.75. The 30-minute design intensity for the 100-year event is 80 mm/h; the existing culvert capacity is estimated at 1.10 m³/s. The site has 1.5 m of clay over weathered mudstone and only 0.6 m of fall to the outfall. The developer's consultant proposes a 300 mm orifice flow control and a dry detention basin of 1,200 m³. Downstream residents report that the culvert already surcharges in heavy rain, and there is a 0.8 m diameter water main crossing the only basin location.
1. Calculate pre- and post-development peak flows using the rational method and assess whether the proposed basin volume is credible.
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Rational method: Q = 0.278 C I A, with A = 0.08 km². Pre-development: Q = 0.278 × 0.35 × 80 × 0.08 = 0.62 m³/s. Post-development: Q = 0.278 × 0.75 × 80 × 0.08 = 1.33 m³/s. The post-development flow exceeds the estimated culvert capacity of 1.10 m³/s by about 21%, and the authority's requirement is no increase, so the target is 0.62 m³/s. The required attenuation is therefore about 0.71 m³/s. For a 30-minute storm the storage would be roughly 0.71 × 1,800 = 1,278 m³, before allowing for climate change or a longer critical duration. A 1,200 m³ basin is therefore at best marginal and probably insufficient. This is only a screening check; a full hydrograph routing is needed.
2. What additional checks would you require before accepting the detention basin design, and how would the water main affect it?
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Requirements: (1) full rainfall-runoff routing for durations from 15 minutes to 24 hours to find the critical duration, not just the 30-minute intensity; (2) apply a climate-change allowance of at least 20% on rainfall intensity, and check the 100-year plus allowance; (3) CCTV and condition survey of the 1968 culvert, because its actual capacity may be below 1.10 m³/s due to silt and deterioration; (4) confirm tailwater and outfall levels; (5) check blockage and safety during extreme events. The 0.8 m water main is a fatal constraint: an easement will prevent excavation under or close to it, and the basin may not be buildable at that location. The main also needs protection from loading and settlement. The basin location must be moved or the design changed to tanks or a smaller footprint.
3. If the culvert cannot be upgraded and the basin cannot be enlarged, what options remain and what would you recommend?
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Options: reduce the connected impermeable area through permeable paving, green roofs, rain gardens and swales; reduce density or phasing; provide off-site attenuation; upgrade the culvert in agreement with the authority; or pump the discharge. In practice, reducing the post-development coefficient from 0.75 to about 0.55 would bring the peak close to 0.98 m³/s, still above 0.62 but below culvert capacity. My recommendation would be not to approve the drainage scheme as submitted. The consent should be conditional on a verified storage volume, a flow-control design with a fail-safe overflow, a culvert condition survey and a maintenance regime. If the basin cannot be relocated away from the water main, the development should be reduced in scale.