Infiltration Trench
An infiltration trench is an engineered practice consisting of a long, narrow, stone-filled excavation that receives and stores stormwater runoff. It functions by temporarily holding this runoff in the void spaces between the stones, allowing it to gradually percolate into the underlying soil. This process both recharges groundwater and treats stormwater quality, as pollutants are filtered and adsorbed by the soil matrix. Infiltration trenches are designed without a conventional outlet, ensuring that all captured runoff is managed through exfiltration into the ground.
These systems are typically implemented to manage runoff from small, impervious drainage areas such as rooftops, parking lots, and roadways. To prevent premature clogging from sediment, runoff must pass through a pretreatment system—such as a vegetated swale, filter strip, or small sedimentation chamber—before entering the trench. Proper siting, design, and construction are critical to their long-term performance, as they are highly susceptible to failure if sediment loads are not adequately controlled.
Applicability
The successful application of an infiltration trench is heavily dependent on specific site conditions. While adaptable to many regions, their use is constrained by factors including soil permeability, groundwater proximity, and land use. They are particularly effective at mitigating thermal impacts to cold-water streams by cooling runoff before it reaches receiving waters.
Drainage Area and Site Conditions
Infiltration trenches are best suited for relatively small drainage areas, typically less than five acres, with a high percentage of impervious cover. The practice itself should be located on relatively flat terrain, with slopes not exceeding 6%. However, the contributing watershed may have slopes as steep as 15%. Because they are linear, trenches can often be integrated into unused or marginal areas of a development site, consuming only 2% to 3% of the land area they serve.
Soils and Topography
Soil characteristics are the most significant limiting factor for infiltration practices. The underlying soils must have a field-verified infiltration rate of at least 0.52 inches per hour. Soils with rates exceeding 3.0 inches per hour may require additional pretreatment to prevent groundwater contamination. The soil composition should have less than 20% clay content and less than 40% combined silt and clay content. Infiltration trenches are not suitable for sites with fill soils or in areas with karst topography, where their use could lead to sinkhole formation and rapid contaminant transport to groundwater.
Geotechnical investigation is not optional for infiltration design. Do not rely solely on published soil surveys. On-site testing, with at least two borings per proposed facility, is essential to confirm the infiltration rate, soil texture, and depth to the water table and bedrock.
Water Table and Setbacks
To ensure proper function and protect groundwater, the bottom of the infiltration trench must be at least four feet above the seasonally high water table and any bedrock layer. A minimum horizontal setback of 100 feet is required from any water supply wells. Additionally, trenches should be located at least 25 feet down-gradient from building foundations or other structures to prevent water-related damage.
Land Use Suitability
Infiltration trenches should not receive runoff from designated stormwater hotspots—land uses that generate high concentrations of pollutants—unless the runoff is fully treated by another practice first. In ultra-urban environments, their use is often impractical due to compacted soils, limited space, and potential conflicts with existing underground utilities. While they can be used in stormwater retrofits, finding suitable locations in previously developed areas can be challenging.
Design Criteria
Effective infiltration trench design incorporates specific elements for feasibility, conveyance, pretreatment, and treatment. These criteria work together to ensure long-term performance and minimize the risk of failure.
Feasibility and Sizing
A trench must be sized to capture and infiltrate the full water quality volume (WQv). The storage volume is calculated based on the void space of the stone aggregate, which can be assumed to be 0.32 for design purposes. The entire stored volume must be able to drain completely into the subsoil within 48 hours of a storm event to ensure storage capacity is available for subsequent rainfall and to prevent anaerobic conditions.
Conveyance
Infiltration trenches must be designed as “off-line” systems. This means that only the smaller, more frequent storm flows associated with water quality are diverted into the trench. Larger storm flows are safely bypassed through a separate conveyance system to a stabilized outfall, preventing erosion and hydraulic overload of the practice. Exit velocities from pretreatment devices should not exceed 5 feet per second during a 2-year storm event.
Pretreatment
Pretreatment is the most critical design element for preventing clogging and extending the functional life of an infiltration trench. A dedicated pretreatment system, such as a grass channel, filter strip, or sedimentation chamber, must be included to capture coarse sediments before they reach the stone reservoir. The pretreatment component should be sized to manage at least 25% of the total WQv. For sites with highly permeable soils (infiltration rate > 2.0 in/hr), at least 50% of the WQv should be treated by another method prior to infiltration to provide additional pollutant removal and protect groundwater.
Treatment Components
The trench itself is an excavation filled with clean, washed stone aggregate. The sides of the trench must be lined with a non-woven geotextile fabric to prevent the migration of surrounding soil into the stone voids, which would reduce storage capacity and lead to clogging. The bottom of the trench is typically left unlined to maximize the infiltrative surface area. Every infiltration trench must include an observation well—a vertical, perforated 6-inch PVC pipe extending to the bottom of the trench—to allow for monitoring of the drawdown time after storms.
Pollutant Removal
Infiltration trenches are presumed to have high pollutant removal capabilities because captured runoff is prevented from discharging directly to surface waters. Pollutants are removed through a combination of filtration, adsorption, and biological processes in the soil matrix. However, monitored performance data for this practice is limited. The data available suggests high removal for some pollutants, but these figures should be interpreted with caution due to the small number of studies. The full pollutant removal database provides more comprehensive information on BMP performance.
| Pollutant | Percent Removal |
|---|---|
| Total Suspended Solids (TSS) | NA |
| Total Phosphorus (TP) | 100%* |
| Total Nitrogen (TN) | 42.3%* |
| Nitrate-Nitrogen (NOx) | 82%* |
| Source: Winer, 2000. *Based on fewer than five data points. | |
Construction and Cost Considerations
The construction sequence for an infiltration trench is critical to its success. The entire contributing drainage area must be fully stabilized with vegetation before the trench is excavated and brought online. The trench area should be protected by diversion berms during site construction and must never be used as a temporary sediment control device. Compaction of the subgrade soil at the bottom of the trench must be avoided; excavation should be performed with equipment positioned outside the footprint of the practice.
Construction costs for infiltration trenches are approximately $5 per cubic foot of stormwater treated (Brown and Schueler, 1997). While the land consumption is low, the overall cost can be higher than some other practices due to the expense of excavation and stone aggregate. The most significant cost consideration is the long-term maintenance burden. Annual maintenance costs can range from 5% to 20% of the initial construction cost, with the higher figure being more realistic for ensuring the practice remains functional over its design life.
Maintenance
Consistent and proactive maintenance is essential for the long-term performance of infiltration trenches. Neglect, particularly of pretreatment systems, is the leading cause of failure. The maintenance schedule below outlines key activities required to keep the practice functioning as designed.
| Activity | Schedule |
|---|---|
| Inspect observation well after a storm to confirm water has drained within 48-72 hours. | Semi-Annually |
| Inspect pretreatment devices and diversion structures for sediment buildup, structural damage, and debris. | |
| Remove accumulated sediment, trash, and oil/grease from all pretreatment devices and overflow structures. | Annually, or as needed |
| If clogging is observed, use bypass capabilities (if present) to allow the trench an extended dry period to potentially restore infiltration capacity. | As Needed |
| Perform total rehabilitation, including excavation and replacement of stone aggregate and filter fabric, and scarification of trench bottom and walls. | Upon Failure |
Source: Adapted from WMI, 1997.
Limitations
Despite their effectiveness, infiltration trenches have significant limitations. Historically, they have exhibited a high rate of failure, primarily due to clogging from inadequate pretreatment or improper construction. A 1992 study in Maryland found that less than one-third of existing trenches were functioning properly after five years (Galli, 1992), though modern designs with improved pretreatment have better performance records.
The strict requirements for soil type, depth to water table, and setbacks limit their applicability to only the most suitable sites. The risk of groundwater contamination is a primary concern, precluding their use for runoff from stormwater hotspots. Finally, infiltration trenches are subsurface practices that offer no aesthetic value, wildlife habitat, or recreational space.
Frequently Asked Questions
What is the primary purpose of an infiltration trench?
The primary purpose of an infiltration trench is to manage stormwater runoff from small impervious areas by capturing it and allowing it to soak into the ground. This achieves two main goals: recharging local groundwater aquifers and providing a high level of water quality treatment. By infiltrating runoff, the practice mimics the natural hydrologic cycle and removes pollutants through soil filtration, which is particularly beneficial for protecting the health of nearby streams and watersheds.
How does an infiltration trench differ from an infiltration basin?
An infiltration trench is a narrow, deep, subsurface structure filled with stone aggregate. It is typically linear and occupies a small surface footprint. In contrast, an Infiltration Basin is a large, open, vegetated depression on the surface that looks like a dry pond. Basins manage runoff from larger drainage areas and rely on both the surface vegetation and underlying soil for treatment, while trenches rely solely on the subsoil. Trenches are best for tight spaces, while basins require more land area.
What is the most common reason infiltration trenches fail?
The most common cause of failure is clogging of the stone aggregate or underlying soil with fine sediment. This occurs when runoff entering the trench is not adequately pretreated to remove suspended solids. Over time, these sediments accumulate in the void spaces and on the infiltrative surface, drastically reducing the rate at which water can soak into the ground. Improper construction techniques, such as compacting the subgrade soil, can also lead to immediate or premature failure of the system.
Can infiltration trenches be used on any type of soil?
No, their use is restricted to sites with suitable soil conditions. The underlying soils must be sufficiently permeable to allow water to drain within the required 48-hour drawdown period. A minimum field-verified infiltration rate of 0.52 inches per hour is required. Soils with high clay or silt content are not appropriate. Conversely, extremely rapid infiltration rates can also be a concern, as they may not provide adequate time for pollutant removal, potentially risking groundwater contamination.
What is an “off-line” design and why is it important?
An off-line design uses a flow-splitting structure to divert only a specific volume of runoff—typically the smaller, more frequent storms that carry the most pollution—into the infiltration trench. Larger storm flows bypass the trench and are conveyed through a separate pipe or channel. This is critical for preventing the trench from being overwhelmed by large volumes of water, which could cause erosion, scour the stone media, and wash out the system. It protects the trench and ensures it is dedicated to its primary function of water quality treatment.
How do I know if my infiltration trench is clogged?
The observation well is the primary tool for diagnosing clogging. This is a perforated pipe that allows an inspector to see the water level inside the stone reservoir. After a rainstorm, the water level should drop steadily and the trench should be completely empty within 48 to 72 hours. If water remains in the observation well for longer than three days, it is a clear indication that the system’s infiltration capacity has been compromised and it is clogged.
Can I build an infiltration trench in a cold climate?
Yes, infiltration trenches can be adapted for use in cold climates, but not in areas with permafrost. Designers must increase the storage volume to account for snowmelt runoff, which can release large amounts of water over a short period. If the trench receives runoff from roadways, it is advisable to divert winter flows to prevent chloride-laden road salt from infiltrating and contaminating groundwater. A sufficient setback from paved surfaces is also needed to prevent frost heave issues.
How is the size of an infiltration trench determined?
The size is based on the required storage volume, known as the Water Quality Volume (WQv), which is determined by local stormwater regulations. The calculation considers the drainage area size and imperviousness. The physical dimensions of the trench are then calculated based on the void ratio of the stone aggregate (typically 32-40%) and the infiltration rate of the native soil, ensuring the entire WQv can be stored and infiltrated within the maximum 48-hour drawdown time. The infiltration sizing calculator can assist with these design computations.
Are there alternatives if a site is not suitable for an infiltration trench?
Yes, many alternatives exist. If a site has suitable soils but linear space is unavailable, Porous Pavement may be an option. If soils are unsuitable for infiltration, practices that filter runoff without relying on infiltration can be used. A Sand Filter, for example, provides excellent pollutant removal by passing stormwater through a sand bed before discharging it. The online BMP selector tool can help identify the most appropriate stormwater practices based on specific site constraints and treatment goals.