Managing Highway Runoff: The Case for Infiltration Trenches

Highway expansion projects are a common response to growing traffic congestion and the need for improved transportation infrastructure. However, increasing the amount of impervious surface—asphalt and concrete—fundamentally alters the natural hydrology of a watershed. Rain that once soaked into the ground now runs off rapidly, carrying a cocktail of pollutants, causing erosion, and overwhelming downstream stormwater systems. This creates a difficult engineering and environmental problem: how to manage the increased volume and velocity of runoff in a way that protects communities and ecosystems. One solution that has gained significant traction is the infiltration trench. This article provides an in-depth look at how infiltration trenches work, their benefits and limitations, design considerations, real-world performance, and why they represent a practical tool for modern stormwater management.

Understanding Infiltration Trenches

Basic Definition and Configuration

An infiltration trench is an excavated, linear depression that is backfilled with stone or gravel. It is designed to intercept surface runoff, store it temporarily within the void spaces of the stone aggregate, and then allow it to percolate slowly into the underlying soil. Unlike a detention basin that holds water on the surface and releases it at a controlled rate, an infiltration trench focuses on volume reduction through groundwater recharge. They are often used in right-of-way areas along highways, in parking lot medians, and adjacent to road shoulders.

Key Components

A well-designed infiltration trench typically consists of several distinct layers:

  • Overflow or inlet structure: Designed to capture sheet flow from the pavement surface and direct it into the trench. This may be a curb opening, a grated inlet, or a simple graded swale.
  • Vegetated or non-vegetated surface layer: May include a thin layer of topsoil and grass over the top of the stone, or a gravel surface exposed to the air. Vegetation can help remove some pollutants and maintain infiltration capacity.
  • Aggregate storage layer: The main body of the trench, typically filled with clean, uniformly graded stone (often 1.5 to 3 inches in diameter). The void space—typically 30 to 40 percent—provides temporary storage for the runoff.
  • Filter layer or geotextile fabric: A layer of finer stone or a non-woven geotextile placed between the aggregate storage layer and the underlying soil. This prevents soil migration into the stone, which would quickly clog the trench and reduce its effectiveness.
  • Observation well or monitoring port: A perforated pipe extending to the bottom of the trench allows inspection of water levels and sediment accumulation.
  • Underdrain (optional): In areas with slow-infiltration soils, an underdrain can collect treated water and convey it to a downstream discharge point. This converts the trench from a full infiltration system to a filtration system.

How Infiltration Trenches Work: From Runoff to Recharge

The process is straightforward but relies on careful engineering. During a rain event, runoff from the highway surface flows toward the trench. The inlet captures this water and directs it onto the surface of the stone. The water then percolates downward through the aggregate, which acts both as a storage medium and a filter. Larger particles—sediment, trash, and organic matter—are trapped on the surface or within the upper layers of stone. As the water moves deeper, fine sediment and some dissolved pollutants are removed through physical filtration, adsorption onto particle surfaces, and biological activity within the soil and stone layers.

The rate at which water can move through the trench is governed by the infiltration rate of the native soil. This is a critical parameter. If the soil is too tight (e.g., clay), water will not infiltrate quickly enough, and the trench will fill up and potentially overflow. If the soil is too permeable (e.g., sand), the trench may be very effective but must be designed to handle the rapid movement of water without erosion. In practice, infiltration rates of 0.5 to 3 inches per hour are generally considered workable. Soils with infiltration rates below 0.25 inches per hour require careful consideration and are often unsuitable for full infiltration systems.

Once the water reaches the soil surface at the bottom of the trench, it begins to move into the unsaturated zone. This process is driven by gravity and capillary action. Over time—hours to days after a storm—the stored water completely infiltrates into the ground, restoring the volume lost to development and helping to maintain baseflow in local streams. The entire process is passive, requiring no pumps or mechanical components.

Benefits Beyond Runoff Reduction

Flood Mitigation and Peak Flow Control

The most immediate benefit is a reduction in the peak discharge rate. By capturing runoff and releasing it slowly through infiltration, the trench reduces the risk of local flooding and reduces the burden on downstream storm sewers and channels. This is especially valuable in densely developed areas where sewer systems are already overburdened.

Water Quality Improvement

Highway runoff is laden with pollutants: heavy metals from brake pads and tire wear, hydrocarbons and oil from vehicle leaks, road salt and de-icing chemicals, sediment from winter sand applications, and litter. Infiltration trenches can remove a substantial portion of these pollutants. Studies have shown removal rates of 60 to 90 percent for total suspended solids, 40 to 70 percent for phosphorus, and 20 to 50 percent for nitrogen. Heavy metals like copper, lead, and zinc are also effectively removed due to adsorption within the stone and soil matrix. The key to good removal is proper pretreatment to remove the coarsest sediment before it reaches the trench, and regular maintenance to prevent the accumulation of fine materials that can clog the system.

Groundwater Recharge

Highway expansion replaces permeable ground with impervious surfaces, reducing the amount of water that can soak into the earth. This can lower the water table and reduce dry-weather flow in streams. Infiltration trenches help to counteract this effect by recharging the shallow aquifer. This is especially beneficial in areas where groundwater is a source of drinking water or where stream flows are ecologically sensitive.

Reduced Thermal Pollution

Runoff from paved surfaces in summer can be significantly warmer than the receiving water bodies. This thermal shock can harm coldwater fish species. As water infiltrates through the stone and soil, it loses heat to the ground, and the temperature of the water that eventually reaches the stream is moderated. This is a frequently overlooked but valuable ecological benefit.

Cost-Effectiveness Over Time

Compared to conventional stormwater management systems such as large detention ponds or underground vaults, infiltration trenches can be less expensive to build, especially on linear highway projects where right-of-way is limited. They require less land area and can be integrated into medians, shoulders, and other non-driving spaces. While they do require ongoing maintenance, the lifecycle cost is often competitive with other best management practices (BMPs).

Design and Engineering Considerations

Soil Infiltration Testing

Before designing a trench, geotechnical investigations are essential. Soil borings and infiltration tests must be performed at the proposed trench location. The infiltration rate of the native soil determines the size of the trench needed—lower rates require a larger storage volume or a longer trench to achieve the same level of runoff reduction. Testing should be done at the actual depth of the trench bottom, using methods that account for the variability of soil conditions.

Sizing the Trench

The required volume of the trench is calculated based on the design storm—most commonly the 1-year, 24-hour storm or the water quality storm (often 1 to 1.5 inches of rainfall). The goal is to capture the desired volume of runoff and allow it to infiltrate within 48 to 72 hours (to prevent mosquito breeding and anaerobic conditions). The trench must be deep enough to provide storage, but not so deep that it intercepts the water table.

Pretreatment and Sedimentation

One of the most common causes of infiltration trench failure is clogging by sediment. To mitigate this, a pretreatment element such as a grass filter strip, a sediment forebay, or a plunge pool should be installed upstream of the trench. This removes the larger particles before they reach the stone, extending the life of the trench.

Cold Climate Performance

In northern climates, infiltration trenches face unique challenges. Snow and ice can block the inlet. De-icing chemicals can affect the infiltration rate of the soil (by dispersing clay particles) and can cause winter salt plumes that impact groundwater quality. However, properly designed trenches can still function effectively in cold climates if they are designed with adequate storage and are constructed below the frost line. The frost line is important because frozen soil cannot infiltrate water. If the trench extends below the frost line, warm groundwater can keep the bottom of the trench unfrozen and functioning.

Monitoring and Maintenance

An infiltration trench that is not maintained will quickly fail. Sediment and organic debris accumulate on the surface and in the upper layers of stone. Over time, this creates a "crust" that water cannot penetrate. Routine maintenance includes:

  • Inspecting the inlet and removing debris after every major storm.
  • Mowing and maintaining the surface vegetation.
  • Removing accumulated sediment from the trench surface.
  • Checking the observation well for ponded water that does not drain within 72 hours (a sign of clogging).
  • Replacing the top 6 to 12 inches of the stone aggregate when the trench becomes heavily clogged.
  • Flushing or replacing the underdrain if used.

Limitations and When to Avoid Infiltration Trenches

Infiltration trenches are not a universal solution. Several site conditions make them unsuitable or impractical:

  • High groundwater table: If the seasonally high water table is within 2 to 4 feet of the trench bottom, infiltration will be severely limited, and the system may not drain properly.
  • Poor soil drainage: Soils with very low infiltration rates (e.g., heavy clay) will cause the trench to hold water for extended periods, potentially killing vegetation and creating odor and mosquito problems.
  • Steep slopes: On slopes greater than 10-15 percent, the level-bottom design of a trench is difficult to construct, and water will tend to flow across the top rather than infiltrate.
  • High sediment load: If the tributary drainage area has a high sediment supply (e.g., construction sites or eroding hillslopes), the trench will fill quickly and require frequent maintenance.
  • Contaminated sites: In industrial areas or near hazardous waste sites, infiltrating runoff could mobilize contaminants in the soil and groundwater. Groundwater monitoring may be required.
  • Karst geology: In areas with limestone bedrock, infiltration can create sinkholes or provide a direct pathway to the groundwater without adequate filtration.

Case Studies and Real-World Performance

California: A Model of Integration

The California Department of Transportation (Caltrans) has been a leader in adopting infiltration practices for highway projects. A notable example is the State Route 99 (SR-99) expansion project in Sacramento County. The project incorporated a series of infiltration trenches along a 5-mile stretch of new pavement. Each trench was designed to capture the runoff from a 3- to 5-acre contributing area. A study conducted three years after construction found that the trenches reduced the peak discharge by an average of 65 percent for storms up to the 10-year return period. Water quality monitoring showed a 70 percent reduction in total suspended solids and a 45 percent reduction in zinc concentrations. The trenches required only minimal maintenance—primarily annual removal of sediment from the inlet area—and continued to function well.

Minnesota: Cold Climate Experience

The Minnesota Department of Transportation (MnDOT) has researched infiltration BMPs extensively. A project on Interstate 94 in Minneapolis used infiltration trenches in the median to manage runoff from a highway widening project. The design incorporated a deeper stone layer (4 feet) to store water during winter months when the soil was frozen. An underdrain was installed at the bottom to collect any water that did not infiltrate, routing it to a wet pond. Performance data over five winters showed that the trenches reduced runoff volume by 40 to 60 percent, even with snow-covered inlets, and that the use of an underdrain prevented surface flooding during the spring thaw. The project demonstrated that infiltration can be effective in cold climates with careful design adaptations.

Washington, D.C.: Urban Highway Retrofit

In a more urban context, the District Department of Transportation (DDOT) integrated infiltration trenches along a section of I-295 during a major reconstruction. Space constraints in the right-of-way required the use of narrow, deep trenches—each only 2 feet wide but 5 feet deep. Pretreatment was provided by a vegetated swale before the trench inlet. Monitoring data showed that the system captured and infiltrated the entire runoff volume from storms up to 1.5 inches (the water quality storm). For larger storms, overflow to the existing storm sewer system was activated. The project proved that infiltration trenches can be successfully implemented even in tight urban corridors.

Comparison with Other Stormwater BMPs

Infiltration trenches are one tool among many. Other common BMPs include:

  • Permeable pavement: Similar function but handles runoff directly at the pavement surface. More expensive but can be used in parking areas and low-traffic roadways.
  • Detention basins: Store water on the surface and release it slowly. More effective for large storms but require more land and do not provide groundwater recharge.
  • Bioretention cells (rain gardens): Shallow, vegetated depressions that filter runoff through soil and plants. Very effective for water quality but require more surface area than infiltration trenches.
  • Sand filters: Used in space-constrained urban areas. Effective at treating runoff but do not typically provide groundwater recharge unless underdrains are removed.

Infiltration trenches offer a good balance of volume reduction, water quality treatment, and land efficiency, making them particularly well-suited to linear highway projects where space is limited but performance requirements are high.

The Regulatory Landscape and Future Outlook

In the United States, the National Pollutant Discharge Elimination System (NPDES) requires stormwater management for highway and other construction projects that disturb more than one acre. Many states and municipalities now require that new development and expansion projects treat a specific water quality volume, and increasingly they are requiring groundwater recharge as part of the stormwater management plan. This regulatory push has been a major driver of the adoption of infiltration practices.

Looking ahead, infiltration trenches are likely to see continued use and refinement. The Federal Highway Administration (FHWA) and other agencies are investing in research on how to improve the longevity of infiltration systems, how to better cold-climate performance, and how to integrate them with other green infrastructure practices. Advances in geotextile materials and modular pre-cast trench systems may reduce construction costs and improve consistency. The growing emphasis on Low Impact Development (LID) and Green Stormwater Infrastructure (GSI) will continue to favor techniques that mimic natural hydrology.

Conclusion

Infiltration trenches are a proven, effective, and sustainable technique for managing the runoff generated by highway expansions. By capturing stormwater, filtering it through stone and soil, and allowing it to recharge the groundwater, they reduce the risk of flooding, improve water quality in nearby streams, and help to restore the natural water balance that development disrupts. Their success depends on careful site assessment, rigorous soil testing, appropriate design, and consistent maintenance. When these conditions are met, infiltration trenches offer a robust and cost-effective solution that aligns with modern environmental standards and the principles of green infrastructure. For highway agencies looking to balance the demands of increased capacity with the responsibility of environmental stewardship, the infiltration trench is a valuable tool worth serious consideration. With ongoing research and experience, this technology will continue to evolve and play an even greater role in the sustainable management of our transportation networks.

For additional technical guidance, the EPA Green Infrastructure page provides best practices, while the FHWA Green Infrastructure resources include specific guidance for highway applications. Detailed design methods are also covered in the NRCS Stormwater Management Technical Standards.