Infiltration Trenches: A Low-Impact Stormwater Solution

As urban development expands, managing stormwater runoff becomes increasingly important to protect water quality, reduce flooding, and prevent erosion. One proven low-impact development (LID) solution is the infiltration trench – a simple yet effective stormwater management practice that supports sustainable site design.

Infiltration trenches help mimic the natural water cycle by allowing rainfall to soak back into the ground instead of being carried away by storm drains and pipes. By promoting groundwater recharge and filtering out pollutants, these trenches help restore a site’s natural hydrology, improve the quality of nearby streams and rivers, and reduce the burden on municipal stormwater systems. They’re an accessible, cost-effective option for property owners and communities looking to balance development with environmental stewardship.

What Is an Infiltration Trench?

An infiltration trench is a shallow, excavated trench filled with clean, uniformly graded gravel or stone. Its main purpose is to temporarily store stormwater runoff and allow it to infiltrate slowly into the underlying soil. Unlike traditional stormwater systems that direct runoff to pipes and outfalls, infiltration trenches recharge groundwater and filter pollutants naturally.

Infiltration trenches are commonly used in:

• Parking lots and roadways
• Residential subdivisions
• Commercial developments
• Small-scale retrofit projects

How Do Infiltration Trenches Work?

When it rains, runoff from impervious surfaces (like rooftops and pavement) flows into the trench via a swale, curb cut, or storm drain inlet. The runoff temporarily fills the voids between the gravel or stone, where it is stored until it can infiltrate through the bottom and sides of the trench into the native soil.

The layers typically include:

• Filter fabric: Prevents soil from clogging the stone reservoir.
• Stone bed: Provides storage for runoff.
• Perforated pipe (optional): May be installed for overflow conveyance or underdrainage if infiltration rates are low.

Benefits of Infiltration Trenches

✅ Improves Water Quality: By filtering out sediment, nutrients, and pollutants as water infiltrates through soil layers.

✅ Reduces Runoff Volume: Helps mimic natural hydrology, reducing peak flows and downstream flooding.

✅ Recharges Groundwater: Supports baseflow to nearby streams and aquifers.

✅ Cost-Effective: Simple design, low maintenance costs, and minimal footprint.

✅ Supports Regulatory Compliance: Helps sites meet stormwater management requirements, including Total Maximum Daily Loads (TMDLs) and local MS4 permit goals.

Key Design Considerations

Proper siting and design are critical for infiltration trenches to function effectively:

• Soil Infiltration Rate: A minimum infiltration rate of 0.5 inches per hour is generally recommended. Conduct a soil percolation test to confirm.
• Depth to Seasonal High Water Table: Maintain adequate separation (commonly 2–4 feet) between the trench bottom and the water table to prevent groundwater contamination.
• Pretreatment: Use vegetated filter strips, sediment forebays, or sumps to remove coarse sediments before runoff enters the trench.
• Overflow and Emergency Spillway: Include a safe overflow path for extreme storms.
• Setbacks: Follow local regulations regarding setbacks from buildings, foundations, wells, and septic systems.

Best Practices for Installation

✔️ Excavate during dry weather to avoid soil compaction.
✔️ Use geotextile fabric to separate the trench stone from surrounding soils.
✔️ Backfill carefully with clean, washed stone.
✔️ Protect the trench from sediment during construction.
✔️ Establish vegetation around the trench to enhance pollutant removal.

Maintenance Requirements

Like all stormwater BMPs, infiltration trenches require regular inspection and maintenance:

• Inspect inlets and pretreatment devices quarterly.
• Remove accumulated sediment and debris.
• Monitor for signs of clogging or ponding.
• Restore vegetation as needed to maintain treatment efficiency.

Are Infiltration Trenches Right for Your Site?

Infiltration trenches can be a practical solution for residential, commercial, or institutional sites where soils are suitable for infiltration and space is limited. They are especially valuable in highly developed areas where larger BMPs like infiltration basins or constructed wetlands may not fit.

Final Thoughts

Infiltration trenches are a simple, cost-effective, and environmentally friendly way to manage stormwater runoff while protecting local waterways and replenishing groundwater. When properly designed, installed, and maintained, they offer long-term benefits for both property owners and the community.

Need help designing, inspecting, or maintaining an infiltration trench?
📞 Call us at (410) 231-3455 or contact us online to evaluate your site and determine the best LID solutions for your property.