Stormwater Management Fact Sheet: Infiltration Trench


Description

An infiltration trench (a.k.a. infiltration galley) is a rock-filled trench with no outlet that receives stormwater runoff. Stormwater runoff passes through some combination of pretreatment measures, such as a swale or sediment basin, before entering the trench. Runoff is then stored in the voids of the stones, slowly infiltrated through the bottom and into the soil matrix over a few days. The primary pollutant removal mechanism of this practice is filtering through the soil.

Applicability

Infiltration trenches need to be applied very carefully. While trenches can be applied in most regions of the country, their use is sharply restricted by concerns such as site feasibility, potential groundwater contamination, soils, and clogging.

Regional Applicability
Infiltration trenches can be utilized in most regions of the country, with some design modifications in cold and arid climates. In regions of karst topography, infiltration trenches should not be used due to concerns of sink hole formation and groundwater contamination.

 

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. Infiltration trenches can seldom be applied in the ultra urban environment. The two main reasons are the potential of infiltrated water to interfere with existing infrastructure, and the relatively poor infiltration capability of most urban soils.

 

Stormwater Hotspots
Stormwater hotspots are land uses or activities that generate highly contaminated runoff (concentrations of pollutants in excess of those typically found in stormwater). Infiltration trenches should not receive runoff from stormwater hotspots, unless the stormwater has already been fully treated by another stormwater treatment practice to avoid potential groundwater contamination.

 

Stormwater Retrofit
A stormwater retrofit is a stormwater treatment practice installed after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. Infiltration trenches may be used as a stormwater retrofit, but their use is somewhat restricted by two factors. First, infiltration trenches can only treat small sites (less than five acres). Small site practices are generally a high cost retrofit option (in terms of construction cost and maintenance) since a large number of small site practices are needed to serve an active watershed. Second, it is often hard to find suitable areas for infiltration in already urban or suburban watersheds because of hotspots and poor soils.

 

Cold Water (Trout) Streams
Infiltration trenches are an excellent option for cold water streams because they encourage infiltration of stormwater. Stormwater does not warm as it travels underground to the stream, which reduces the temperature impacts commonly associated with urbanization and stormwater practices. Infiltration is also very helpful in sustaining dry weather flows.

 

Siting and Design Considerations

While trenches can be applied in a variety of situations, their use is frequently restricted by concerns over groundwater contamination, soils, and clogging (see Figure 1).

Siting Considerations
Infiltration practices need to be located extremely carefully. In particular, designers need to ensure that the soils at the site are appropriate for infiltration, and that designs minimize the potential for groundwater contamination, and long term maintenance.

Drainage Area

Infiltration trenches generally are applied to relatively small sites (less than five acres) that have relatively high impervious cover.

Slope

Infiltration trenches should be placed on flat ground, but the slopes of the site draining to the practice can be as high as 15%.

Soils /Topography

Soils and topography are a strongly limiting factor when locating infiltration trenches. Soils must be significantly permeable to ensure that trenches can infiltrate quickly enough to reduce the potential for clogging.In addition, soils that infiltrate too rapidly may not provide sufficient treatment, creating the potential for groundwater contamination. The infiltration rate should range between 0.5 and 3 inches per hour. In addition, the soils should have no greater than 20% clay content, and less than 40% silt/clay content (CWP, 1998). The infiltration rate and textural class of the soil needs to be confirmed in the field; designers should not rely on more generic information such as a soil survey. Finally, infiltration trenches may not be used in regions of karst topography, due to the potential for sink hole formation, or groundwater contamination.

Groundwater

Designers always need to provide significant separation (2' to 5') from the bottom of the infiltration trench and the seasonally high ground water table, to reduce the risk of contamination and trench failure. In addition, infiltration practices should be separated at least 150 feet from adjunct drinking water wells.

Design Considerations
Infiltration trench designs vary considerably, depending on site constraints and the preferences of the designer and community. There are some features, however, that should be incorporated into every infiltration trench design. These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information see the Manual Builder Category).

Pretreatment

Pretreatment refers to design features that remove sediment before it enters the trench thereby easing the long-term maintenance burden. Pretreatment is essential for infiltration trenches. In order to make pretreatment effective, designers should incorporate "multiple pretreatment" into every trench, using grass swales, vegetated filter strips, sedimentation basins, or a plunge pool in series.

Treatment

Treatment design features enhance the pollutant removal capability of a trench. During the construction process, the upland soils of infiltration trenches need to be stabilized to ensure that the trench does not become clogged with sediment. Furthermore, the trenches should be filled with large clean stones that can retain the required volume water to be treated in their void space. Like infiltration basins, trenches should be sized so that the treatment volume can completely infiltrate through the trench bottom in twenty-four hours.

Conveyance

Stormwater needs to be conveyed through stormwater treatment practices safely, and in a method that minimizes soil erosion. Designers need to be particularly careful in ensuring that channels leading to infiltration trenches are designed to minimize erosion. Infiltration trenches should be designed to treat only small storms, (i.e., only for water quality or recharge). Thus, trenches should be designed "off-line," using a structure to divert only small flows to the trench. Finally, the sides of an infiltration trench should be lined with a geotextile fabric to prevent adjacent soils from clogging the practice.

Maintenance Reduction

Designers need to incorporate maintenance reduction features into the trenches to reduce future maintenance needs and make regular maintenance activities easier to perform. As with all practices, infiltration trenches should have a direct access path for maintenance activities. An observation well (i.e, a perforated PVC pipe that leads to the bottom of the trench) is needed to enable inspectors can monitor the drawdown rate. Where possible, trenches should have a means to drain the practice if it becomes clogged (such as an underdrain). An underdrain is a perforated pipe system in a gravel bed, installed on the bottom of filtering practices to collect and remove filtered runoff. An underdrain pipe with a shutoff valve can be used as an overflow in case the trench becomes clogged.

Landscaping

In infiltration trenches, there is no landscaping on the surface practice itself, but it is important to ensure that the upland drainage is properly stabilized with dense vegetation, both during and after construction.

Regional Variations

Arid or Semi-Arid Climates

In arid regions, infiltration practices are frequently recommended because of the need to recharge the groundwater. One concern in arid regions is the potential of infiltration trenches to clog, due to relatively high sediment concentrations produced in these watersheds. Consequently, pretreatment needs to be strongly emphasized in arid and semi-arid climates.

Cold Climates

In extremely cold climates (i.e., regions that experience permafrost), infiltration trenches are not feasible. In moderately cold climates, infiltration trenches may be feasible, but some steps need to be taken to make it work. The volume may need to be increased in order to treat snowmelt runoff values. In addition, if the trench is used to treat roadside runoff, it is desirable to divert flow around the trench in the winter to prevent infiltration of chlorides from road salt. Finally, a minimum setback is needed from roads to ensure that the trench does not cause frost heaving.

Limitations

While infiltration trenches are a useful treatment practice, they have several limitations. Whilethey do not detract visually from a site, they provide no visual enhancements. Their application is limited due to concerns over groundwater contamination (for more information see Risk of Groundwater Contamination from Infiltration of Stormwater, Article 104 in The Practice of Watershed Protection) and other soils requirements. Finally, maintenance can be burdensome, and infiltration practices have a relatively high rate of failure.

Maintenance Considerations

Infiltration practices require regular maintenance and inspection practices to perform properly. The table below outlines these required maintenance practices.

Table 1. Typical Maintenance Activities for Infiltration Trenches
(Source: Modified from WMI, 1997)

Activity

Schedule
  • Check observation wells following 3 days of dry weather. Failure to percolate within this time period indicates clogging
  • Inspect pretreatment devices and diversion structures for sediment build-up and structural damage.

Semi-Annual Inspection
  • Remove sediment and oil/grease from pretreatment devices, as well as overflow structures.

Standard Maintenance
  • If bypass capability is available, it may be possible to regain the infiltration rate in the short term by using measures such as providing an extended dry period.

5-year Maintenance
  • Total rehabilitation of the trench to maintain storage capacity within 2/3 of the design treatment volume and 72-hour exfiltration rate limit
  • Excavate trench walls to expose clean soil.

Upon Failure


Infiltration practices in general have historically had a high rate of failure, compared with other stormwater treatment practices. One study conducted in Maryland (Galli, 1992), revealed that less than half of the infiltration trenches investigated (of about fifty) were still functioning properly, and less than one third still functioned properly after five years. Many of these practices, however, did not incorporate advanced pretreatment. Infiltration performance should improve by carefully selecting the location and improving the design features of infiltration practices (for more information see Failure Rates of Infiltration Practices Assessed in Maryland, Article 101 in The Practice of Watershed Protection).


Effectiveness

Stormwater treatment practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal (for more information, see the Manual Builder Category) . Infiltration trenches can generally provide groundwater recharge, pollutant control, and can help channel protection.

Groundwater Recharge
Infiltration trenches recharge the groundwater because runoff is treated for water quality by filtering through the soil and discharging to groundwater.

 

Pollutant Removal
Very little data are available regarding the pollutant removal associated with infiltration trenches. It is generally assumed that they have very high pollutant removal, because none of the stormwater entering the infiltration trench remains on the surface. Table 2 provides pollutant removal estimates derived from CWP's National Pollutant Removal Performance Database for Stormwater Treatment Practices.

 

Table 2. Pollutant Removal Efficiency of Infiltration Trenches (Winer, 2000)

Pollutant

Pollutant Removal(%)

TSS

NA

TP

100

TN

42.3

NOx

82

1: Data based on less than five data points


Cost Considerations

Infiltration trenches are slightly expensive, when compared to other stormwater practices, in terms of cost per area treated. Typical construction costs, including contingency and design costs, are about $5 per cubic foot of stormwater treated (SWRPC, 1991; Brown and Schueler, 1997).

Infiltration trenches typically consume a relatively small 2% to 3% of the drainage site. In addition, infiltration trenches can fit into thin, linear areas. Thus, they can generally fit into relatively unusable portions of a site.

One cost concern associated with infiltration practices is the maintenance burden and longevity. If improperly maintained, infiltration trenches have a high failure rate (See Maintenance Considerations). In general, maintenance costs for infiltration trenches are estimated between 5% and 20%of the construction cost. In order to ensure long term functionality of the practice, realistic values are closer to the 20% range.

References


Brown, W. and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic Region. Prepared for: Chesapeake Research Consortium. Edgewater, MD. Center for Watershed Protection. Ellicott City, MD.

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1998. Maryland Stormwater Design Manual. Draft. Prepared for: Maryland Department of the Environment. Baltimore, MD. http://www.mde.state.md.us/environment/wma/stormwatermanual/mdswmanual.html

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.


Ferguson, B.K., 1994. Stormwater Infiltration. CRC Press. Ann Arbor, MI.


Galli, J. 1992. Analysis of Urban BMP Performance and Longevity in Prince George's County, Maryland. Metropolitan Washington Council of Governments. Washington, DC.


Minnesota Pollution Control Agency. 1989. Protecting Water Quality in Urban Areas: Best Management Practices. Minneapolis, MN.


Schueler, T. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments. Washington, DC.


Southeastern Wisconsin Regional Planning Commission (SWRPC). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Waukesha, WI..


US EPA. 1993. Office of Water. Guidance to Specify Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA-840-B-92-002. Washington, DC.


Watershed Management Institute (WMI). 1997. Operation, Maintenance, and Management of Stormwater Management Systems. Prepared for: US EPA Office of Water. Washington, DC.


Winer, R.� 2000.� National Pollutant Removal Performance Database for Stormwater Treatment Practices: 2nd Edition.� Center for Watershed Protection.� Ellicott City, MD.