Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Infiltration Trench
An infiltration trench is an excavated trench that has been lined and backfilled with stone to form a subsurface basin. Stormwater runoff is diverted into the trench and is stored until it can infiltrate into the soil, usually over a period of several days. Infiltration trenches are very adaptable BMPs, and the availability of many practical configurations make it ideal for small (less than 4 ha [10 ac]) urban drainage areas, such as ultra-urban sites. Infiltration trenches can be either on-line or off-line systems. They are most effective and have a longer life cycle when some type of pretreatment is included in their design. Pretreatment may include techniques such as vegetated filter strips or grassed swales.
Infiltration trenches provide the majority of treatment by processes related to soil infiltration, which include sorption, precipitation, trapping, filtering, and bacterial degradation. That the soils surrounding infiltration trenches are effective filters is best indicated by the tendency for these soils to clog if heavily loaded with oil, grease, and sediment. The extent of sorption and filtration is a function of the soil type; for example, highly permeable soils (i.e., sandy soils) usually have low cation exchange capacities (CECs, or the affinity for capturing positively charged pollutants). However, as an infiltration trench is used, fine material suspended in stormwater is captured within the natural soil, creating a more effective filtering matrix and increasing the pollutant removal. Based on the limited information available on chemical/biological changes in the soils surrounding infiltration trenches, the soil/stormwater interaction is complicated and site specific. It is difficult to generalize regarding the extent to which the soils operate aerobically or anaerobically.
Infiltration trenches are appropriate for ultra-urban applications, particularly subsurface designs that are covered with grating or pavement (Figure 3). Essentially all of the surface above a subsurface infiltration trench can be used as parking or public areas. Unfortunately, subsurface infiltration trenches are relatively expensive BMPs; the expense is due to construction of an underground vault, which must be placed among other subsurface utilities. Surface trench designs can be moderately expensive BMPs and can be easier to construct and operate, but they require greater space commitments because they are usually combined with area-intensive pretreatment such as grass filter strips (Figure 4). Surface infiltration trench designs are better suited to roadside application where space is at less of a premium.
Figures 3 and 4 indicate only two of many possible configurations. Both of these configurations illustrate the essential design features, which include pretreatment of runoff to minimize sediment loading, stormwater storage in a subsurface trench filled with stone, and discharge of all captured stormwater into underlying ground layers.
Both configurations shown in Figures 3 and 4 are complete trench designs or designs that discharge all treated stormwater into a highly permeable underlying soil trench. Where a complete trench design is undesirable or not feasible, a partial trench design can be employed to infiltrate only a portion of the stormwater runoff. Partial trench designs may incorporate an underdrain system placed several feet below the invert to intercept exfiltrating stormwater. This approach enables trench placement where there are relatively impermeable soils or there is a confining soil layer. As an alternative, a partial trench design can integrate a discharge pipe that limits the storage depth in the trench and routes all surplus stormwater to an outlet. The principal advantage of this design is it permits diversion of high flows and if the soils become clogged stormwater can still be discharged. Partial trenches can also be used as off-line facilities and can easily be retrofitted onto existing subsurface storm drains.
For infiltration trenches, effectiveness is solely a function of the amount of stormwater infiltrated; that is, the only pollutants not treated are those associated with the stormwater that bypasses the trench and are not infiltrated. The pollutants discharged to surficial groundwater aquifers are not generally accounted for in reported removal rates. Projected removal rates reported for two different designs are shown in Table 7.
In variable climates, harsh winter temperatures can freeze the water in infiltration trenches and eliminate the ability of the trench to store and infiltrate water. It is recommended that information on the soil freeze depth be obtained and the trench invert be located below this depth.
Siting and Design Considerations
For most ultra-urban applications designers should look for soils with high percolation rates below the proposed trench invert, surficial groundwater aquifers that are not used for drinking purposes, and ample clearance over bedrock. A range of recommendations have been made regarding the minimum permeability of the soil surrounding the infiltration trench; some suggest a minimum infiltration rate of 12.7 mm/h (0.5 in/h) (Yu and Kaighn, 1992; Schueler et al., 1992), but some states accept minimum values of 6.9 mm/h (0.27 in/h) (MDE, 1986). Minimum infiltration rates between 6.9 and 12.7 mm/h (0.27 and 0.50 in/h) are usually associated with loamy sand, sandy loam, loam, and silt loam texture soils; however, site-specific infiltration rates are a function of more than the soil texture. It is recommended that site-specific infiltration be measured in soils located below the proposed invert of the infiltration trench. In addition, soils should be examined to a depth at least 1.52 m (5 ft) below the proposed invert to identify if there are any underlying impermeable soil layers (clay lenses, fragipans, or hardpans). It should be noted that ultra-urban developments are frequently placed on disturbed cut/fill soils. This greatly increases the importance of site-specific infiltration testing.
Designs can be sized to manage a range of runoff volumes to meet specific water quality and quantity objectives. Small-scale units can be designed just to manage the first flush runoff volume; these designs are sometimes referred to as water quality exfiltration systems. Conversely, the size of the trench can be increased to significantly decrease the postdevelopment runoff rates and limit flooding.
While placing infiltration trenches in low permeability soils is questionable, trench designs can be made to work in lower infiltrating soils, but the surface area or size of the trench may become prohibitively large. Designers should note that the invert of the infiltration trench should be at least 1.22 m (4 ft) above underlying bedrock and at least 1.22 to 2.44 m (4 to 8 ft) over the seasonal high groundwater elevation (Yu and Kaighn, 1992). The trench bottom should be rototilled after excavation. The addition of a sand filter layer at the trench bottom should be considered to facilitate movement of water between the stone storage area and the subgrade. Designers considering application of infiltration trenches can roughly estimate 121 m2 (1300 ft2) of trench bottom area (a 1.22 m [4 ft] deep trench) is needed to store 12.7 mm (0.5 in) of runoff from a 0.4 ha (1 ac) impervious service area. In addition, the minimum recommended drain time is 24 hours and the maximum recommended drainage time is 72 hours. Finally, it is recommended that trenches should be located a minimum of 3.05 m (10 ft) downgradient and 30.5 m (100 ft) upgradient of any buildings and the ground slope should be less than 20 percent. There are several good sources available for detailed design and construction procedures and information, including Urban Drainage Design Manual Hydraulic Engineering Circular 22 (Brown et al., 1996), Evaluation and Management of Highway Runoff Water Quality (Young et al., 1996), and Maintenance of Stormwater Management Structures (MDE, 1986).
If appropriate sediment removal pretreatment is not provided, the life expectancy of an infiltration trench may be only five years (Schueler et al., 1992) due to the pore space and trench bottom becoming clogged. With proper regular maintenance, however, a trench may last as long as 10 or 15 years before major rehabilitation of the trench is required (Schueler, 1987). Following installation, frequent inspections are recommended at first, but these can be decreased to twice per year. These inspections should look into water levels in the infiltration trench, clogging of inlets or outlets, and accumulation of sediment in upstream pretreatment units. Immediate failure of the trench might occur if sediment is not directed away from the trench area during construction. Consequently, it is recommended that all upstream areas be stabilized before the trench is constructed.
Failure of an infiltration trench is determined by the continued presence of pooled water three days after rainfall has ended. A failure of this type leads to removal or replacement of part or all of the rock backfill. Surface infiltration trench rehabilitation can be estimated to cost approximately 20 percent of the initial construction costs, whereas rehabilitation of an underground trench can exceed the initial construction cost (Young et al., 1996). Clearly, proper, regular maintenance is essential to avoid costly trench rehabilitation.
Numerous design features can simplify maintenance. An example includes placing a filter fabric on top of the rock media, which can easily be stripped off when it is full of debris.
Infiltration trenches are most cost-effective for small drainage areas where space is at a premium and the water quality storage volume is less than 280 m3 (10,000 ft3 or approximately 12.7 mm [0.5 in] of runoff from 2 ha [5 ac]). Trench construction costs (1995 dollars) can be estimated using the following equation where V is the storage volume in cubic meters (Young et al., 1996):
This cost estimation is valid only for trenches that have storage volume on the order of 280 m3 (10,000 ft3). This formula does not include the cost of special inlets or grass filters for pretreatment of runoff but does include costs for excavation, backfill, filter cloth, inlet and outlet pipes, and fixtures.
Brown, S.A., S.M. Stein, and J.C. Warner. 1996. Urban Drainage Design Manual Hydraulic Engineering Circular 22. FHWA-SA-96-078. Federal Highway Administration, Office of Technology Applications.
Maryland Department of the Environment (MDE). 1986. Maintenance of Stormwater Management Structures: A Departmental Summary. Maryland Department of the Environment, Sediment and Stormwater Division, Annapolis, MD, and Maryland Department of Natural Resources, Water Resources Administration, Annapolis, MD.
Schueler, T.R., P.A. Kumble, and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices-Techniques for Reducing Non-Point Source Pollution in the Coastal Zone. Metropolitan Washington Council of Governments, Department of Environmental Programs, Anacostia Restoration Team, Washington, DC.
Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.
Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. FHWA-PD-96-032. Federal Highway Administration, Office of Environment and Planning.
Yu, S.L., and R.J. Kaighn. 1992. VDOT Manual of Practice for Planning Stormwater Management. Federal Highway Administration, FHWA/VA-92-R13, Virginia Department of Transportation, Virginia Transportation Research Council, Charlottesville, VA.