Urban Commercial Redevelopment in Austin, TX

Scenario

A 12-acre mixed-use commercial redevelopment (retail, office, and parking) in Austin, Texas required compliance with City of Austin Watershed Protection Department (WPD) Ordinance §3-5-12, mandating post-development peak flow control to pre-development levels. Site constraints included limited available land (only 0.8 acres allocated for stormwater infrastructure), underlying shallow limestone bedrock limiting infiltration, and adjacency to a flood-prone tributary of Waller Creek. Pre-development runoff coefficient was estimated at 0.25 (pasture/wooded); post-development imperviousness pushed it to 0.75.

Given Data

• Runoff coefficient: 0.75 (validated via site survey and TR-55 land use tables)
• Drainage area: 12.0 acres
• Rainfall depth: 3.2 inches (6-hour, 10-year design storm per NOAA Atlas 14, Region 8)
• Inflow peak flow: 215 cfs (computed using Rational Method with time of concentration = 12 min)
• Maximum volume capacity: 15.0 acre-feet (geotechnically constrained by bedrock depth and adjacent utilities)

Calculation

The Stormwater Detention Basin Sizing Tool computes:

1. Volume Storage Requirement = (runoff_coefficient × drainage_area × rainfall_depth) / 12
   → (0.75 × 12.0 × 3.2) / 12 = (28.8) / 12 = 2.40 acre-feet
   (Note: Division by 12 converts inch-acres to acre-feet: 1 acre-foot = 12 inches over 1 acre)

2. Outflow Peak Flow is derived from routing analysis embedded in the tool’s empirical outlet sizing logic:
   Outflow ≈ Inflow × exp(−0.4 × √(volume_storage_requirement / maximum_volume_capacity))
   → 215 × exp(−0.4 × √(2.40 / 15.0)) = 215 × exp(−0.4 × √0.16) = 215 × exp(−0.4 × 0.4) = 215 × exp(−0.16) ≈ 215 × 0.852 = 183.2 cfs

Result and Decision

The calculated storage requirement (2.40 acre-feet) was well within the geotechnically feasible 15.0 acre-feet limit. However, the resulting outflow (183.2 cfs) still exceeded the pre-development peak (92 cfs). To meet regulatory compliance, the team increased the detention time by adding a multi-stage orifice plate outlet and raised the basin’s design volume to 8.5 acre-feet — yielding an outflow of 89.7 cfs (within ±5% of pre-development). A 0.7-acre, 12-ft-deep wet-detention basin with forebay and vegetated filter strip was constructed.

Lesson

Even when storage volume requirements appear modest, regulatory peak flow reduction targets often demand intentional outlet control design—not just basin size—making iterative routing analysis essential beyond initial tool output.