What This Solves
Sizes an infiltration trench for stormwater management by calculating required dimensions, aggregate storage volume, infiltration capacity, and drawdown time.
Best Used When
- You are designing a linear infiltration BMP along a road, parking lot, or building perimeter
- You need to calculate the trench dimensions to capture and infiltrate a design storm
- You want to verify that drawdown will occur within the required time (typically 48-72 hours)
Do NOT Use When
- You need a vertical point infiltration system rather than a linear trench — Use Dry Well Calculator
- You are designing a surface bioretention area with vegetation and amended soil — Use Rain Garden Calculator
- You are designing a subsurface drain to convey water away rather than infiltrate it — Use French Drain Calculator
Key Assumptions
- Aggregate void space (porosity) provides the effective storage volume
- Infiltration occurs through the bottom and sides of the trench into native soil
- Native soil infiltration rate is constant throughout the drawdown period
- No groundwater mounding reduces the effective infiltration rate
- The trench is wrapped in filter fabric to prevent soil migration into the aggregate
Input Quality Notes
Field infiltration testing at the trench bottom depth is essential. Apply a safety factor of 2-3 to measured rates to account for long-term clogging and siltation.
Design Infiltration Trench
For educational purposes only. Not a substitute for professional engineering judgment.
Infiltration Trench Design Overview
Infiltration trenches are linear rock-filled excavations that capture stormwater runoff and allow it to infiltrate into the surrounding soil. They provide both storage and treatment through infiltration.
- Storage Volume - Water stored in aggregate void spaces
- Infiltration - Water percolates through bottom and sidewalls
- Pretreatment - Sediment removal prevents clogging
- Drawdown Time - Typically 24-72 hours maximum
Aggregate Porosity Values
| Aggregate Type | Min | Typical | Max |
|---|---|---|---|
| 1.5-2.5" Clean Washed Stone | 0.35 | 0.4 | 0.45 |
| 0.75-1.5" Clean Washed Stone | 0.32 | 0.38 | 0.42 |
| Crushed Angular Stone | 0.3 | 0.35 | 0.4 |
| Rounded Gravel | 0.25 | 0.32 | 0.38 |
Source: EPA Stormwater Manual (2004), ASCE MOP 77 (2006)
Soil Infiltration Rates
| Soil Class | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Sand | 2 | 8 | 12 |
| Loamy Sand | 1 | 4 | 8 |
| Sandy Loam | 0.5 | 2 | 4 |
| Loam | 0.25 | 1 | 2 |
| Silt Loam | 0.15 | 0.5 | 1 |
| Sandy Clay Loam | 0.1 | 0.3 | 0.5 |
| Clay Loam | 0.05 | 0.2 | 0.4 |
Source: EPA SWMM Manual (2020), NRCS Soil Survey
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Last verified: February 2026