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Water Quality Infiltration Practices Introduction
> Infiltration Basins > Infiltration
Trenches > Porous Pavement > Infiltration practices generally involve allowing collected runoff water to percolate to the ground water table through a porous soil media. Infiltration facilities are thought to be potentially the most effective type of BMP for treating polluted stormwater since the soil can be an efficient treatment medium to filter out fecal bacteria, and since soluble heavy metals, phosphorus, and some organic compounds tend to be retained by soil minerals. In addition, pollutant uptake or transformation may be accomplished by soil bacteria. Particulate pollutants are also filtered out in infiltration facilities. Infiltration practices can reduce the need for downstream conveyance and storage facilities by reducing runoff flow rates and volumes. The failure rate of infiltration facilities due to clogging is high. In such systems, particulates should be removed to the maximum extent possible by a pre-treatment system, such as a detention basin, to prevent clogging of the media. Two primary problems in the use of infiltration facilities have been poor construction techniques and a lack of sediment control before the contributing catchment ground surface has been stabilized (i.e., vegetated). Infiltration facilities should be conservatively designed using low loading rates, or the pores in the media and surrounding soil tend to become clogged. Rejuvenation of the pore space requires "rest" periods between runoff events. The stormwater infiltration practices commonly described in the literature are generally modifications of four major types: infiltration basins, infiltration trenches, porous pavements and modular pavements (MELP [now WLAP], 1992).
Infiltration basins are water impoundments lined with relatively permeable soils and made by excavation or by construction of dams or embankments. With long detention times and grass bottoms, infiltration basins enhance pollutant removal by allowing more time for settling and adsorption of sediment and adsorbed pollutants. Although infiltration is a simple concept, infiltration devices must be carefully designed and maintained if they are to work properly. Poorly installed or improperly located devices fail easily. It is critical that infiltration devices only be used where the soil is porous and can absorb the required quality of stormwater. Maintenance needs for infiltration devices are higher than other devices partly because of the need for frequent inspection. Nuisance problems can occur, especially with insect breeding, odours and soggy ground.
Infiltration basins are designed to temporarily store surface runoff water for a selected design storm or runoff volume, and to increase ground water recharge through infiltration of stored water into the surrounding soil. Detailed design procedures and recommendations for infiltration basins are available in WEF (1998), KC (1998), FHWA (1996), Horner et al. (1994), WSDOE (1992), Stahre and Urbonas (1989) and Schueler (1987).
Infiltration basins, and other infiltration devices, attenuate both peak flows and runoff volumes. As a result it is possible to reduce the size of downstream conveyance and detention facilities. Infiltration basins can provide more effective contaminant removal than other structural BMPs as well as promote ground water recharge. The retention of stormwater using these devices can also help to preserve and enhance local vegetation.
Infiltration basins have a high failure rate because of clogging and poor drainage. Sediment must be removed before the stormwater enters the basin to prevent clogging of the soil. The water table must be at least 0.6 m under the bottom of the device and unless contaminated runoff is treated prior to infiltration, there is a potential for ground water contamination.
Maintenance requirements include regular inspections, cleaning of inlets, mowing and possible use of observation wells to maintain proper operation. Infiltration basins and associated pre-treatment devices must have sediment removed regularly. If an infiltration device becomes clogged, it may need to be completely rebuilt.
An infiltration trench consists of an excavated trench, generally 0.5 m - 3 m in depth and backfilled with porous material (usually a coarse aggregate), to form a temporary underground storage reservoir. Stored runoff then infiltrates into the surrounding soil. Alternatively, some or all of the stored water may be collected by perforated underdrains and routed to an outflow facility. A specialized type of infiltration trench is the roof downspout system, which receives only the runoff from roof downspout drains (MELP [now WLAP], 1992). Rooftop runoff is generally considered "relatively clean" in design manuals and is consequently considered safe for infiltration without prior treatment (WSDOE, 1992). A more complete description of roof downspout systems is provided in GVSDD (1999). Variations on the idea of infiltration trenches include dry wells, infiltration tanks and surface dispersion trenches. Dry wells are cylindrical, open bottom containers with no fill material and placed on a bed of drain rock. They can also have a catch basin and piping at the bottom to take excess water to water courses or storm sewers as required. These are deep holes, optionally with a grate or inlet structure at the bottom to allow water into the piping system. The piping system is filled with clean, round even sized stones and can be used where the subsoil has good permeability. Infiltration tanks are rectangular, perforated tanks surrounded by drain rock and soil. They are usually about 1.5 m wide by 1.5 m deep and contain no fill material. The top of the tank is buried one to two metres below the final grade of the finished surface. More details are provided in Konrad et al. (1995). Surface dispersion trenches are shallow trenches excavated and backfilled with drain rock. They are generally one metre wide by 0.3 m deep and may have perforated PVC pipe running the length of the trench. Fill material should be clean aggregate and surrounded by filter fabric. For more information on surface dispersion trenches refer to GVSDD (1999), Konrad et al. (1995) and WSDOE (1992).
Design and Uses Porous pavement is an alternative to conventional pavement that is intended to reduce imperviousness and consequently minimize surface runoff. Porous pavement follows one of two basic designs. First, it may be comprised of asphalt or concrete that lacks the finer sediment found in conventional cement. This formulation is usually laid over a thick base of granular material. Second, porous pavement may be formed with modular, interlocking open-cell cement blocks laid over a base of coarse gravel. A geo-textile fabric underlying the gravel prevents the migration of soil upward into the gravel bed. Both designs typically include a reservoir of coarse aggregate stone beneath the pavement for stormwater storage prior to exfiltration into surrounding soils (Schueler et al., 1992). The use of porous pavement requires permeable soils with a deep water table. Traffic must be restricted to exclude heavy vehicles. Its use is not advisable in areas expecting high levels of off-site sediment input, including chemicals and sand used in snow removal operations.
The porous pavement itself functions less as a treatment BMP and more as a conveyance BMP to the other necessary component of the design, the underlying aggregate chamber, which functions as an infiltration device. As with other infiltration devices, treatment is provided by adsorption, filtration, and microbial decomposition in the sub-soil surrounding the aggregate chamber, as well as particulate filtration within the chamber. Porous pavement is not intended to remove sediments and accumulations will decrease its effectiveness. The following long term contaminant removal efficiencies were reported in Schueler (1987):
Positive attributes include the diversion of potentially large volumes of surface runoff to ground water recharge, providing both water quality and quantity benefits. While more expensive than conventional pavement, it can also eliminate the need for more involved stormwater drainage, conveyance, and treatment systems, offering a valuable option for spatially constrained urban sites. Porous pavement may be most beneficial in watersheds with high percentages of impervious surface and high volumes of runoff. Its use is typically recommended for lightly trafficked satellite parking areas and access roads with low traffic volumes. Increased infiltration at the source (parking lots, etc.) will reduce the both volume of runoff and the delivery of associated pollutants to water bodies.
The big disadvantage of porous pavement is that sites have a high failure rate, due to clogging either from improper construction, accumulated sediment and oil or resurfacing (Schueler et al., 1992). Excessive sediment will cause the pavement to rapidly seal and become ineffective (Urbonas and Stahre, 1993). The modular, interlocking, open-cell concrete block type tends to remain effective for considerably longer than asphalt or concrete porous pavement. Porous pavement must be maintained frequently to continue functioning. Quarterly vacuum sweeping and/or jet hosing is needed to maintain porosity, and this may constitute one to two percent of the initial construction costs (Schueler et al., 1992). More information on porous pavement systems is provided in the GVRD's Best Management Practices Guide for Stormwater (1999).
Concrete Grid and Modular Pavement Concrete grid and modular pavements, sometimes referred to as "grasscrete", consist of strong, structural materials with regularly interspersed void areas filled with pervious materials such as sod, gravel or sand. The purpose of the modular pavement is to provide a load-bearing surface having adequate strength to support vehicles while allowing infiltration of surface water to the underlying soil. The potential applications of modular pavements are similar to those described for porous pavements. Other suggested uses include on-street parking aprons in residential neighborhoods, recreational vehicle parking pads, private roads, easement service roads and fire lanes, industrial storage yards and loading zones, residential and light commercial driveways, bike paths, walkways and patios. General design information is provided in GVSDD (1999). Specific design procedures and recommendations for modular pavements are available in WSDOE (1992), WEF (1998), FHWA (1996), WCC et al. (1995) and Konrad et al. (1995). The bearing surface may consist of poured-in-place concrete slabs, pre-cast concrete grids or modular unit pavers such as bricks or concrete blocks. Facilities using a vegetative cover must provide for percolation of stored water within the time limits necessary to avoid damage to ground cover vegetation. Efficiencies similar to those of infiltration basins and trenches can be expected for modular pavements (MELP [now WLAP], 1992).
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