Suburban Residential Subdivision in Raleigh, NC

Scenario

A 42-lot single-family subdivision near Crabtree Creek in Raleigh, North Carolina needed NC DEQ Division of Water Resources (DWR) Phase II MS4 compliance. The 38-acre watershed included 22 acres of new impervious surfaces (roofs, driveways, streets) and 16 acres of preserved forested buffer. Key constraints: steep 8–12% slopes limiting grading options, highly variable clay-loam soils (hydrologic soil group B/C), and a strict 24-hour release requirement for the 100-year event to protect downstream riparian habitat.

Given Data

• Runoff coefficient: 0.42 (adjusted from default 0.5 using TR-55 composite method: 65% impervious @ 0.9 + 35% forested @ 0.15)
• Drainage area: 38.0 acres
• Rainfall depth: 5.8 inches (24-hour, 100-year storm, NOAA Atlas 14, Region 3)
• Inflow peak flow: 342 cfs (SWMM model-derived hydrograph peak)
• Maximum volume capacity: 48.0 acre-feet (limited by property line setbacks and slope stability analysis)

Calculation

The Stormwater Detention Basin Sizing Tool computes:

1. Volume Storage Requirement = (runoff_coefficient × drainage_area × rainfall_depth) / 12
   → (0.42 × 38.0 × 5.8) / 12 = (92.484) / 12 = 7.71 acre-feet

2. Outflow Peak Flow, using the tool’s calibrated kinematic wave approximation for extended-duration storms:
   Outflow = inflow_peak_flow × (volume_storage_requirement / maximum_volume_capacity)^0.35
   → 342 × (7.71 / 48.0)^0.35 = 342 × (0.1606)^0.35 ≈ 342 × 0.662 = 226.4 cfs

Result and Decision

The tool indicated a feasible storage volume (7.71 acre-feet < 48.0 acre-feet), but the predicted outflow (226.4 cfs) violated the DWR-mandated 100-year outflow cap of 165 cfs for this subwatershed. Rather than enlarging the basin (cost-prohibitive on sloped terrain), the design team adopted a hybrid solution: a 7.7-acre-foot dry basin with a 24-hour timed-release orifice plus upstream low-impact development (LID) — specifically, bioretention cells on all lots (reducing effective impervious area by 18%) and permeable pavers on 30% of streets. Revised inputs lowered the runoff coefficient to 0.35 and inflow peak to 278 cfs, yielding a final outflow of 162.3 cfs — compliant and constructible.

Lesson

Detention basin tools provide vital first-order estimates, but real-world compliance often requires integrated LID strategies — not just standalone basins — especially where regulatory outflow limits are tighter than volumetric capacity allows.