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.
Size an infiltration trench — a linear, stone-filled excavation that captures stormwater runoff and lets it soak into the surrounding soil. Enter your drainage area, design storm, trench dimensions, aggregate and native soil to get the required trench length, available storage, infiltration capacity and drawdown time, using EPA and ASCE methodology.
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
How infiltration trench sizing works
The design balances the runoff volume that must be captured against the storage available in the trench's aggregate voids, then checks that the stored water infiltrates into the native soil quickly enough. The calculation follows these governing relationships:
1. Runoff volume
Vrunoff = C × P × A — the composite runoff coefficient
C times design rainfall depth P times drainage area A.
C is area-weighted from impervious (0.95) and pervious (0.20) surfaces by your
impervious percentage.
2. Required trench length
L = Vrunoff ÷ (W × D × n) — because only the
aggregate void space stores water, the storage per unit length is width W
times depth D times porosity n. Storage volume is then
Vstorage = W × D × L × n.
3. Design infiltration rate
fdesign = fmeasured ÷ SF — the measured (or
tabulated) soil infiltration rate divided by a safety factor SF (typically
2–3) to account for variability and clogging.
4. Infiltration capacity & drawdown
Qinf = f × Abottom + 0.5 × f × Aside
— bottom infiltration plus sidewall infiltration, with sidewalls taken at 50% of the bottom
rate to reflect horizontal flow. Drawdown time is then
tdrain = Vstorage ÷ (Qinf × 3600)
hours.
Variable definitions: Vrunoff = runoff volume (cf or m³);
C = composite runoff coefficient; P = design rainfall depth (in or mm);
A = contributing drainage area (sf or m²); W, D,
L = trench width, depth and length; n = aggregate porosity (void ratio);
f = infiltration rate; SF = safety factor;
Qinf = total infiltration flow; Abottom,
Aside = bottom and sidewall areas.
The design is judged adequate when the storage ratio
(Vstorage ÷ Vrunoff) is at least 1.0 and the drawdown
time is within your target. As a planning check, the trench footprint is typically 2–10%
of the contributing impervious area.
Soil infiltration rates by texture
Typical saturated infiltration rates used when a field test is not available. Apply a safety factor of 2–3 to obtain the design rate, and always prefer a site-specific infiltration test on the actual trench bottom.
| Soil class | Min (in/hr) | Typical (in/hr) | Max (in/hr) | Suitability |
|---|---|---|---|---|
| Sand | 2.0 | 8.0 | 12.0 | Excellent |
| Loamy sand | 1.0 | 4.0 | 8.0 | Very good |
| Sandy loam | 0.5 | 2.0 | 4.0 | Good |
| Loam | 0.25 | 1.0 | 2.0 | Moderate |
| Silt loam | 0.15 | 0.5 | 1.0 | Moderate |
| Sandy clay loam | 0.10 | 0.3 | 0.5 | Slow |
| Clay loam | 0.05 | 0.2 | 0.4 | Slow — needs underdrain |
Source: EPA SWMM Manual; USDA NRCS Soil Survey. Soils below roughly 0.5 in/hr often cannot meet a 24–72 hour drawdown and may require an underdrain.
Design assumptions & when not to use a trench
Assumptions
- Uniform soil infiltration rate around the trench
- Clean aggregate at the specified porosity
- Groundwater table at least 2–4 ft below the trench bottom
- Adequate pretreatment to prevent clogging
- No standing water on the trench surface
- Safety factor covers soil variability and clogging
Contraindications
- Very tight clay soils (< 0.1 in/hr)
- High groundwater table (< 4 ft separation)
- High sediment loads without pretreatment
- Contaminated runoff from industrial/commercial hotspots
- Karst terrain without a liner
- Wellhead protection areas
Infiltration trenches provide limited removal of dissolved pollutants and require ongoing maintenance; sidewall infiltration can also decline over time. Always confirm against your local stormwater design manual.
Frequently asked questions
How long should an infiltration trench take to drain?
Most stormwater BMP guidance targets a drawdown (dewatering) time of 24 to 72 hours, with 48 hours a common design value. Draining within this window restores storage before the next storm, limits standing water and mosquito habitat, and avoids prolonged saturation of the surrounding soil. This calculator computes drawdown as the available void storage divided by the total infiltration rate (bottom plus sidewall) and flags the design if it exceeds your target.
Why do I apply a safety factor to the soil infiltration rate?
A safety (correction) factor of about 2 to 3 is applied to the measured or tabulated infiltration rate to account for soil variability, gradual clogging by fine sediment, and the difference between short-term test conditions and long-term field performance. The design infiltration rate used in sizing is f_design = f_measured ÷ SF, so a soil tested at 2.0 in/hr with SF = 2 is designed at 1.0 in/hr. A field infiltration test on the actual trench bottom is strongly recommended over relying on textbook soil-class values.
What soils are suitable for an infiltration trench?
Infiltration trenches work best in well-drained soils with an infiltration rate above roughly 0.5 in/hr — sand, loamy sand, sandy loam and loam. Tight soils such as clay loam (typical 0.2 in/hr) or anything below about 0.1–0.5 in/hr generally cannot drain within 24–72 hours and are considered contraindicated without an underdrain or other modification. You also need at least 2–4 feet of separation between the trench bottom and the seasonal high groundwater table or bedrock.
How does aggregate porosity affect trench size?
Only the void spaces between stones store water, so storage volume is the trench volume multiplied by the aggregate porosity (void ratio). Clean 1.5–2.5 inch washed stone has a typical porosity of about 0.40, meaning 40% of the trench volume holds water; rounded gravel is lower at about 0.32. Because required length L = V_runoff ÷ (W × D × n), a lower porosity directly increases the trench length needed to store the same runoff volume.
Why does pretreatment matter for infiltration trenches?
Infiltration trenches are highly susceptible to clogging from sediment carried in by runoff, which is the leading cause of premature failure. A pretreatment device — a sediment forebay, grass filter strip, or inlet sump — captures coarse sediment before it reaches the stone, extending service life. In this calculator a sediment forebay is sized at about 25% of the water quality volume and other pretreatment at about 10%.
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Last verified: February 2026