Stormwater Detention

Calculate detention basin volumes and outlet sizing for stormwater management.

Design a detention basin for a commercial development:
- Total Site Area: 5.2 acres
- Pre-development runoff coefficient (C): 0.25
- Post-development C: 0.75 (80% impervious)
- Design Storm: 10-year, 24-hour event
- Rainfall depth: 5.5 inches
- Time of concentration (Tc): 15 minutes
- Allowable release rate: Match pre-development peak
- Local hydrology: Rational Method / SCS

Determine required storage volume and outlet orifice size.

DETENTION BASIN DESIGN (RATIONAL METHOD)

1. PRE-DEVELOPMENT PEAK FLOW
   Rational Method: Q = CiA

   10-yr intensity (15 min Tc):
   i = 4.5 in/hr (from IDF curve)

   Q_pre = 0.25 × 4.5 × 5.2
   Q_pre = 5.85 cfs (allowable release rate)

2. POST-DEVELOPMENT PEAK FLOW (Undetained)
   Q_post = 0.75 × 4.5 × 5.2
   Q_post = 17.55 cfs

3. REQUIRED STORAGE VOLUME
   Using Modified Rational Method:
   V_s = (Q_post - Q_allowable) × T_d

   Assume detention time T_d = 60 min
   V_s = (17.55 - 5.85) cfs × 60 min × 60 sec
   V_s = 42,120 CF

   With safety factor of 1.2:
   V_required = 50,544 CF

   Basin dimensions (rectangular):
   Assume depth = 6 ft average
   Surface area = 50,544 / 6 = 8,424 SF
   Use 100' × 85' × 6' deep = 51,000 CF ✓

4. OUTLET ORIFICE SIZING
   Orifice equation: Q = CdA√(2gh)
   Where:
   - Cd = 0.6 (sharp-edged orifice)
   - h = 6 ft (maximum head)
   - Q = 5.85 cfs (allowable release)

   A = Q / (Cd√(2gh))
   A = 5.85 / (0.6 × √(2 × 32.2 × 6))
   A = 0.625 SF = 90 in²

   Circular orifice diameter:
   d = √(4A/π) = √(4 × 90/π)
   d = 10.7 inches

   Use 11" diameter orifice

5. OVERFLOW SPILLWAY (100-year storm)
   Design for 100-yr event (emergency overflow)
   Q_100 = 25 cfs (estimated)

   Weir equation: Q = CLH^(3/2)
   Use 20-ft wide rectangular weir
   H = 1.3 ft (allowable freeboard)

FINAL DESIGN:
✓ Basin: 100' × 85' × 6' deep (51,000 CF)
✓ Primary outlet: 11" diameter orifice
✓ Emergency spillway: 20' wide @ elev. 6.0'
✓ Release rate: 5.85 cfs (matches pre-dev.)

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