Introduction
An infiltration trench is a narrow excavation filled with clean, open-graded aggregate (or engineered media) that temporarily stores stormwater in the voids between stones and then releases it into the underlying soil. Trench systems are commonly used along parking lots, driveways, building perimeters, and downslope of compacted lawns where natural infiltration has been reduced. Compared with surface basins, trenches can fit into tight sites because most of the storage is below grade.
This calculator provides a first-order estimate of (1) the runoff volume generated by a design storm, (2) the trench length required to store that volume given a width, depth, and aggregate void ratio, and (3) an estimated drain-down time based on the native soil infiltration rate. The results are useful for early sizing and for understanding tradeoffs (for example, deeper trenches store more per meter but may drain more slowly).
How to use the calculator
- Enter the drainage area that contributes runoff to the trench (for example, a roof area or paved area). Use square meters (m2).
- Enter the design rainfall depth in millimeters (mm). This is the storm depth you want the trench to capture.
- Enter trench width and depth in meters (m). These are the internal dimensions of the aggregate storage zone.
- Enter the aggregate void ratio (between 0 and 1). This represents the fraction of the trench volume that is empty space available for water storage. Typical clean stone is often around 0.35–0.50.
- Enter the soil infiltration rate in mm/hr. Use a measured value if available (field tests are best). If you only have a rough soil texture, use the reference table below as a starting point.
- Click Calculate to see runoff volume, required trench length, and estimated drain time.
Tip: If the drain time is very long, consider increasing bottom area (wider trench, multiple trenches, or a shallower design) or reassessing the infiltration rate with site testing. Many local standards target drain-down within about 24–48 hours to reduce prolonged saturation.
Formula and assumptions
The calculator uses consistent metric units internally. Rainfall depth and infiltration rate are entered in millimeters and converted to meters. The equations below are intentionally simple so you can sanity-check results and understand which inputs drive the design.
1) Runoff volume
Runoff volume is estimated as:
V = Ad × Dr
Where V is runoff volume (m3), Ad is drainage area (m2), and Dr is rainfall depth (m). This is a simplified approach that effectively assumes the contributing area behaves like an impervious surface (runoff coefficient ≈ 1). If your site includes pervious areas, interception, or significant depression storage, the true runoff volume may be lower.
2) Trench storage and required length
Storage per meter of trench length is:
S1m = B × Z × n
Where B is trench width (m), Z is trench depth (m), and n is aggregate void ratio (dimensionless). Required trench length is then:
L = V / (B × Z × n)
3) Estimated drain-down time
The drain-down time is estimated assuming infiltration occurs uniformly through the trench bottom area and that the infiltration rate remains constant:
td = Z / (f × n)
Where td is time (hours), f is soil infiltration rate (m/hr), Z is trench depth (m), and n is void ratio. This simplified relationship highlights why low-permeability soils or low void ratios can lead to long drain times.
Worked example
Suppose a 200 m2 roof drains to a trench and you want to capture a 25 mm design storm. The estimated runoff volume is:
V = 200 × 0.025 = 5.00 m3.
If the trench is 0.6 m wide, 1.0 m deep, and uses clean gravel with void ratio n = 0.40, then storage per meter is:
S1m = 0.6 × 1.0 × 0.40 = 0.24 m3 per meter.
Required length is:
L = 5.00 / 0.24 ≈ 20.8 m.
If the soil infiltration rate is 10 mm/hr (0.01 m/hr), the estimated drain-down time is:
td = 1.0 / (0.01 × 0.40) = 250 hours (about 10 days).
That drain time is typically too long for many design standards, suggesting the soil is too tight for an infiltration trench at this depth, the infiltration rate is overestimated/underestimated, or the design should be changed (for example, shallower and wider to increase bottom area, multiple trenches, or an alternative BMP).
Reference values (starting points)
Use measured site data whenever possible. The values below are only rough planning-level ranges and can vary widely with compaction, layering, groundwater conditions, and clogging potential.
| Aggregate type | Void ratio n | Soil texture | Infiltration rate (mm/hr) |
|---|---|---|---|
| Crushed stone | 0.35 | Sand | 25 |
| Clean gravel | 0.40 | Loam | 10 |
| Large river rock | 0.45 | Clay loam | 5 |
| Open-graded aggregate | 0.50 | Clay | 2 |
Interpretation guide (what the results mean)
The calculator returns three values: runoff volume, required trench length, and estimated drain time. Each value answers a different design question. Runoff volume is the amount of water you are trying to manage for the selected storm depth. If you increase the rainfall depth or the drainage area, the volume increases linearly. Required trench length is the length of trench needed to provide enough void storage to hold that volume at once. Length decreases if you increase width, depth, or void ratio, because each meter of trench stores more water. Estimated drain time is a simplified indicator of how quickly the stored water can infiltrate into the soil.
A common planning check is whether the drain time is within a target window such as 24 to 48 hours. Faster drain-down reduces the chance of prolonged saturation and helps the trench recover storage capacity before the next storm. However, extremely fast infiltration is not always better: in sensitive groundwater areas, designers may need additional treatment or separation distances. Treat the drain time as a screening metric and confirm with local requirements.
Practical sizing tips
If the required length is impractically long, you can often adjust the design without changing the captured storm depth. Increasing width is usually the most straightforward way to add storage per meter, but it requires more footprint. Increasing depth adds storage too, but deeper excavations can encounter groundwater, utilities, or unsuitable soils. Increasing void ratio is limited by the aggregate type; clean, uniformly graded stone tends to have higher void space than well-graded mixes.
If the drain time is too long, the controlling factor is typically the infiltration rate. In low-permeability soils, a trench may need to be shallower, wider, or split into multiple parallel trenches to increase the infiltrating area. In some projects, an underdrain or a lined system with controlled discharge is used instead of full infiltration. Also consider pretreatment: sediment is the most common cause of long-term performance loss, so a filter strip, sump catch basin, or forebay can protect the trench.
Common unit checks and input guidance
Unit mistakes are a frequent source of unrealistic results. Rainfall depth is entered in millimeters, so a 1-inch storm is about 25.4 mm. Infiltration rate is also entered in mm/hr. If you have an infiltration rate in inches per hour, multiply by 25.4 to convert to mm/hr. Trench width and depth are entered in meters; for example, 24 inches is about 0.61 m.
The void ratio input should be a decimal fraction between 0 and 1 (for example, 0.40), not a percentage (40). If you enter 40, the calculator will reject it because it is outside the valid range. If you are unsure, start with 0.40 for clean gravel and refine later based on material specifications.
Limitations and design notes
This tool is intentionally simplified. Use it for preliminary sizing and education, not as a final engineering design. Key limitations include:
- Runoff coefficient not included: The runoff volume calculation assumes the full rainfall depth becomes runoff from the drainage area. Real designs often apply a runoff coefficient (C) or use hydrologic modeling.
- Constant infiltration rate: Soil infiltration can decrease as the soil becomes saturated, and it can vary by depth due to layering. Field testing (e.g., double-ring infiltrometer) is recommended.
- Clogging and maintenance: Sediment and fines can reduce performance over time. Pretreatment (filter strip, swale, catch basin) and maintenance are critical. Consider a safety factor or additional length to account for long-term degradation.
- Groundwater and separation: Many jurisdictions require vertical separation to seasonal high groundwater or bedrock. Always check local codes and site constraints.
- Water quality considerations: Infiltration may be inappropriate where runoff contains high pollutant loads or where groundwater protection is required.
- Geometry simplification: The calculator treats the trench as a uniform rectangular prism and does not account for sidewall infiltration, pipe distribution, overflow structures, or underdrains.
If your calculated drain-down time is much greater than 48 hours, consider alternative stormwater practices (rain garden/bioretention with engineered soil, detention with controlled release, or a lined system) or consult a qualified professional.
Frequently asked questions
Does this calculator include a runoff coefficient?
No. The runoff volume is computed as drainage area multiplied by rainfall depth, which is equivalent to assuming a runoff coefficient of 1.0. If you want a quick adjustment for partially pervious drainage areas, you can approximate by reducing the drainage area to an “effective” area (for example, multiply the actual area by your chosen coefficient) before entering it.
Why is the drain time so sensitive to infiltration rate?
In the simplified drain-down equation, infiltration rate appears in the denominator. Cutting the infiltration rate in half doubles the estimated drain time. Because infiltration rates can vary by orders of magnitude between sands and clays, it is normal to see very different drain times for the same trench geometry.
What if my project requires overflow?
Many trenches are designed with an overflow to safely pass storms larger than the design event. This calculator sizes storage for the selected storm depth only; it does not design overflow structures. In practice, you would provide a stabilized overflow path or connection to a downstream conveyance system.