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Infiltration Trench

General

Infiltration trenches and beds are rock filled depressions that collect and store stormwater until it can infiltrate into the subsoil. Sediment settles out in the device, and stone and subsoil adsorb nutrients, metals, and organic material as the water infiltrates. Infiltration trenches and beds may also be designed to reduce peak flows from a site if the storage capacity of the device is increased and an outlet structure is included in the design.

These structures are applicable on sites with highly permeable soils and drainage areas of less than 15 acres. Infiltration trenches should not be used near foundations, basements, or roads or on sites with high water tables, steep slopes, or clay soils. In addition, these devices shall not be used on sites with large concentrations of soluble pollutants, as groundwater contamination may result.

While these structures effectively treat the runoff volume from small storms, larger storm events quickly overwhelm the capacity of the device and render it ineffective. Trenches are also susceptible to clogging from large sediments and, as a result, they should be used in conjunction with other management practices.

Advantages

  • Removes sediment, nutrients, and organics material from stormwater
  • May be designed to reduce peak flows

Disadvantages

  • Limited functionality with frozen ground
  • May cause groundwater pollution if not sited properly
  • Susceptible to clogging
  • Requires frequent maintenance
  • Not applicable on sites with high sediment loads or sites with large concentrations of hydrocarbons

Design

Infiltration trenches and beds are shallow excavations that are filled with coarse stone and collect and temporarily store runoff. The distinction between trenches and beds is typically based upon geometry, with trenches having a greater depth than width and beds having a greater width than depth. They are appropriate on sites with relatively small drainage areas and are often used to treat the runoff from impervious surfaces.

These structures are 3-10 feet deep and are filled with 3-4 inch diameter clear stone.

The trench width should be at least 4 feet wide, while the length will depend upon the individual site characteristics, but should be designed with a maximum retention time of 24 hours and must be large enough to safely handle the runoff from small storms.

Infiltration trenches should be constructed in soils with a design infiltration rate of at least 0.13 inches per hour (refer to Infiltration Modeling). In addition, the bottom of the trench must be at least 3 feet above the seasonally high water table to prevent groundwater contamination and to prevent groundwater from flooding the trench and rendering it ineffective.

Infiltration trenches are highly susceptible to clogging with sediment. As a result, geotextile filter fabric must be used to line the trench to prevent the surrounding soil from mixing with the stone. Runoff directed to a trench must be routed through a pretreatment device before entering the trench. Pretreatment of the runoff will help to prevent clogging of the trench from sediment as well as preventing groundwater contamination from pollutants contained in the runoff that is being infiltrated to the groundwater. Basins receiving runoff from rooftops only do not require pretreatment.

To divert large flows, a subsurface drain may be placed in the center of the trench, 2 feet from the surface, to prevent these flows from bypassing treatment (refer to Subsurface Drain).

Construction

  • Do not construct until the entire site has been stabilized to prevent clogging
  • Care must be taken during all phases of construction to prevent the compaction of soils in and around the practice

Maintenance

  • Remove accumulated sediment in pretreatment devices at least twice a year
  • To monitor drainage from infiltration trenches, an observation well may be installed and should be checked after all storm events to ensure that water is draining properly

Method to Determine Practice Efficiency

When properly designed and used in conjunction with proper pretreatment, stone trenches (not beds) meet the county standard for oil and grease removal.

In order to determine the infiltration performance of this practice SLAMM, RECARGA or some other approved model may be used. Additional information regarding acceptable modeling of infiltration practices is found on the infiltration modeling page.

References