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Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-i CHAPTER 4: STORMWATER RUNOFF CONTROL PRACTICES TABLE OF CONTENTS 4.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2 FILTRATION PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 Buffer Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2 Grassed Swales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4 Sand Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5 Water Quality Inlets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 4.3 DETENTION PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Extended Detention Ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Constructed Wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 4.4 INFILTRATION PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11 Porous Pavers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11 Infiltration Trenches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-12 Bio-Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 4.5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17 Chapter 4 Stormwater Runoff Control Practices 4-ii Environmental Protection Handbook Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-1 CHAPTER 4: STORMWATER RUNOFF PRACTICES 4.1 INTRODUCTION During development, both the landscape and the hydrology of a parcel of land can be significantly altered. The following may occur both during and after development: • Soil porosity changes; • Impermeable surfaces increase; • Water retention on-site decreases; • Channels and other water conveyances are built; • Slopes change; • Vegetative cover decreases; and • Soil surface roughness decreases. These changes result in increased runoff volumes and velocities (or speeds) which can scour and erode guts, steep slopes, and unvegetated areas, and cause downstream siltation of roads, parking lots, yards, ponds, beaches, seagrass beds, coral reefs, and other coastal areas. In addition, impervious surfaces can also increase water temperatures. Stormwater management practices are designed to perform one or more of the following functions: decrease the erosive potential of increased runoff volumes and velocities caused by land development; remove sediment and other pollutants in stormwater runoff that result from activities that occur during and after development; preserve or improve drainage patterns and other hydrologic conditions so that they closely resemble conditions previous to development; and preserve natural systems. Stormwater control practices rely on three different processes to treat runoff: filtration, detention, and infiltration. 4.2 FILTRATION PRACTICES Examples of filtration practices that are suitable for conditions in the Virgin Islands include: • Buffer zones; • Grassed swales; • Sand filters; and • Water quality Inlets Filtration practices treat sheet flow runoff by using vegetation or sand to filter and settle pollutants. In some cases, infiltration and treatment of runoff in the subsoil may also occur. Some practices can also be installed in storm drains to remove pollutants from water, particularly sediment, oil and grease. After being filtered, the stormwater runoff can be routed into guts, drainage channels, or other waterbodies; evaporated; or infiltrated into the surrounding soil. The microclimate of the area must be considered in selecting vegetative systems. Chapter 4 Stormwater Runoff Control Practices 4-2 Environmental Protection Handbook Buffer Zones When and Where to Use Buffer Zones What to Consider Figure 4.1. Three-zone aquatic buffer system (CWP, 2001). Buffer zones are areas of vegetated land along a shoreline, wetland or gut where development is prohibited. Buffer zones are intended to physically protect and separate guts, wetlands and shorelines from development activities. They also provide stormwater management, filter pollutants from stormwater runoff, provide food and cover for wildlife and aquatic organisms, and provide cooling shade (CH2M Hill, 1998). There are three types of buffer zones: setbacks, vegetated buffers and engineered buffers (CWP, 2001). Setbacks are areas that separate waterways from potential pollution hazards (typically development sites). Vegetated buffers are natural areas that exist to divide land uses or provide landscaping. Engineered buffers are areas specifically designed to treat stormwater before it enters a gut, wetland or coastal area (CWP, 2001). They may closely resemble natural ecosystems, such as grassy pastures or forests. Buffer zones are designed specifically to protect waterbodies, slow stormwater runoff and remove pollutants from stormwater, and in this way differ from Filter Strips (see Chapter 3), which are designed specifically to filter sediment from runoff. Forested buffers are well-suited to protect water quality and improve aquatic habitat in urban and suburban landscapes. Buffer zones are generally used to protect guts, wetlands and/or shorelines from development activity, to treat stormwater runoff from low density residential or resort developments, or to treat agricultural runoff. One buffer zone design utilizes a three zone system with inner, middle and outer zones (Figure 4.1). Buffer zones can increase infiltration and recharge to groundwater, slow stormwater runoff, reduce erosion of guts and shorelines, improve and increase aquatic habitat, filter sediment, and remove soluble nitrogen and phosphorus from stormwater (CH2M Hill, 1998). Setbacks should have a minimum width of 100 feet to provide adequate protection of waterbodies from development activities. However, in urban and suburban areas where open space is limited, narrower buffers adjacent to guts and wetlands can still be beneficial. Effective buffer zone design (Figure 4.1) is based on criteria that determine how a buffer will be sized, delineated, managed and crossed (see Appendix C). For optimal stormwater treatment, buffers can be designed with three lateral zones: a stormwater depression leading to a grass filter strip in turn leading to a forested buffer. The stormwater depression (or basin) captures and stores stormwater during smaller storm events. Larger stormflows Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-3 How Effective Are Buffer Zones? are bypassed directly into a channel. The runoff captured in the depression can the be spread across a grass filter strip (see Chapter 3). The grass filter then discharges into a wider forest buffer designed so that all the stormwater is absorbed into the soil and not discharged to the water body (CWP, 2001). Maintenance will be needed for any type of buffer zone. Buffer boundaries should be well-defined and visible before, during and after construction. Buffers designed to capture and treat stormwater runoff from urban or suburban areas will also need regular maintenance to prevent gullies from forming and bypassing the treatment. Also, because buffer zones are maintained differently than park lawns, they may sometimes be seen as dangerous or unkempt public places. Concerns may arise that the development of shrubby vegetation will interfere with unimpeded views or be abused as dumping places for trash and litter. Therefore, an educational program highlighting the environmental and recreational benefits of buffers should be a part of restoration programs (CH2M Hill, 1998). Buffers lower runoff velocity, preserve natural vegetation around guts, wetlands and shorelines, stabilize banks of drainage channels and guts, and slightly reduce both runoff volume and watershed imperviousness. In some cases, buffers can also help to reduce the size and cost of downstream stormwater control facilities. The pollutant removal effectiveness of buffer zones ranges widely and depends on the buffer type and design. Setbacks are designed to prevent pollution from neighboring land uses, they are not designed for pollutant removal during a storm (CWP, 2001). Buffers, when designed correctly, can remove sediment, organic material, and many trace metals from stormwater runoff. The rate of pollutant removal is thought to be a function of the length, slope and soil permeability of the buffer, the size of the contributing runoff area, and the runoff velocity. Forested, or wooded, buffers tend to remove a greater variety of pollutants than grassed filter strips, but since there is less vegetative ground cover with forested buffers, they also need to be twice as wide as grassed buffers. Grass buffers are more effective for trapping sediment, but forested buffers are more effective in reducing nutrient loads. ADVANTAGES OF BUFFER ZONES • Relatively inexpensive to establish (cost almost nothing if preserved before the site is developed). • Can remove a high percentage of sediment and attached pollutants if designed properly. • Can provide privacy barrier, cooling shade, wind break, noise reduction, wildlife habitat, and protection of guts, wetlands, shorelines and their vegetation. • Can protect increase infiltration, reducing flooding potential. • Maintenance tasks and costs are minimal for “natural” buffer strips. • Can provide recreational benefits. DISADVANTAGES OF BUFFER ZONES • Potential loss of developable land. • Potential misuse of buffer zones as trash or litter dumps. • Potential increase in nuisance species (mosquitoes, mongoose, etc.) Chapter 4 Stormwater Runoff Control Practices 4-4 Environmental Protection Handbook Grassed Swales When and Where to Use Grassed Swales Figure 4.2. Grassed swale design (Schueler, 1987). What to Consider Grassed swales are shallow, vegetated, man-made ditches and channels designed to treat and infiltrate a specified volume of stormwater runoff. The grass in the swale prevents erosion, filters sediment, and provides some nutrient uptake. The underlying soil can also provide some filtering and treatment, absorb water, and decrease stormwater volumes. Grassed swales can also be used to convey stormwater runoff. Enhanced grassed swales, or biofilters, use check dams and wide depressions to increase runoff storage and promote settling of pollutants. A grassed swale is an infiltration/filtration method that is usually used to provide pretreatment before stormwater runoff is discharged to treatment systems or a water body. Swales are typically used in small residential or commercial developments and are well suited to treat road or highway runoff, or as an alternative to curb and gutter drainage systems. Swales are almost always used in combination with other stormwater control or treatment practices. Grassed swales are not appropriate in arid areas where it is difficult to establish good vegetative cover. Individual grassed swales should be used to treat small drainage areas (less than 5 acres). Grass swales are not effective if they are built on slopes greater than 5% because stormwater velocity becomes too great and causes erosion and prevents adequate infiltration or filtering. However, for steeper slopes (up to 15%), check dams (see Chapter 3) can be installed to slow runoff and provide greater filtration and infiltration. Grassed swales should also not be used in areas where groundwater is within two feet of the bottom of the swale. Swale slopes need to be graded as close to zero as possible, and side-slopes should be no greater than 3:1 (Figure 4.2). The flat channel should be between two (2) and eight (8) feet wide. A dense cover of hardy, low-growing, erosion-resistant grass or groundcover must be established (such as bermuda, bahia, hurricane or zoysia grass). Swales should be mowed frequently during the wet season to stimulate grass growth, control weeds, and maintain the system’s capacity. Swales should never be mowed shorter than 4 inches. The soils underneath grassed swales need to be very permeable (infiltration rate greater than 0.5 in/hr). Wood, rock or stone check dams can also be installed in swales to promote infiltration, but earth or soil should not be used because moderate to severe storms will blow out the dam. Swale maintenance primarily involves keeping the grass cover dense. This requires periodic mowing, occasional spot seeding, and weed control. Home owners are usually responsible for this maintenance. However, close mowing and excessive fertilizer and pesticide applications can adversely affect swale performance and negate any pollution reduction benefits the swale may have. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-5 How Effective Are Grassed Swales? Sand Filters When and Where to Use Sand Filters Grassed swales control peak stormwater runoff discharges by reducing runoff velocity and infiltrating a portion of the stormwater runoff volume that passes through the swale. However, runoff infiltration is limited, and is never more than 10% of total volume. Pollutants are removed by the filtering action of the grass, deposition into low velocity areas, or infiltration into the subsoil. However, if a swale is constructed in soils having low permeabilities, there will be no soluble pollutant removal and low to moderate sediment and other particulate pollutant removal. Also, swales require relatively level slopes to function properly. The over-application of fertilizers and pesticides by homeowners can make swales a source of pollutants, rather than a treatment practice. ADVANTAGES OF GRASSED SWALES • Are more economical to establish than curb and gutter drainage systems. • Have relatively low maintenance requirements. • Can protect surface infiltration trenches and storm drains from sediment clogging. DISADVANTAGES OF GRASSED SWALES • Will not work on slopes greater than 5%. • Provide low to moderate particulate (sediment, etc.) removal and minimal soluble pollutant removal. A sand filter is usually a two-staged practice; the first stage is a chamber for settling sediment and other particles in stormwater and the second is a sand-filled filter bed with a subsurface drain. Stormwater runoff is diverted into the first chamber where large particles (sand, debris) settle out, and then flows to the second chamber where small particles (clays) and some other pollutants (oil and grease, heavy metals) are filtered out. The stormwater that seeps through the sand is collected in underground pipes and can then be reused for irrigation or returned back to the drainage channel or gut. Enhanced sand filters use layers of peat, limestone, and/or topsoil, and may also have a grass cover crop, to improve pollutant removal efficiency. Sand-trench systems have also been developed to treat parking lot runoff. Sand filters can be used on most development sites and have few constraining factors, but are best to use on small sites (a maximum of 5 acres for surface sand filters or 2 acres for perimeter or underground filters). Sand filters used for larger drainage areas often clog, necessitating sand replacement. Most sand filters have been used on small parking lots. Sand filters are especially suitable for areas with thin clay soils, hilly terrain, high evapotranspiration rates, low soil infiltration rates, limited space, and frequent droughts (Schueler, 1994). Most sand filters have a contributing watershed area between a half an acre and ten acres, although the upper limit for usage is 50 acres. Sand filters can also be used for retrofit purposes and in small developments in urban or suburban areas. Figure 4.3 presents an example sand filter system. Chapter 4 Stormwater Runoff Control Practices 4-6 Environmental Protection Handbook Figure 4.3. Sand filter system (Austin, Texas, 1991). What to Consider Sand filters require relatively simple, but frequent (quarterly), maintenance such as raking, surface sediment removal, and removal of trash, debris and leaf litter. Replacement of the surface sand layer (top 2-3 inches) may also needed on a relatively frequent basis (every 2 to 3 years). Sand filters are costly (up to $10,000 to $20,000 per impervious acre treated), but are long-lived and have lower maintenance and rehabilitation costs than infiltration trenches, and are more widely applicable to environmental conditions in the Virgin Islands. Sand filters need flow splitter designs that will not clog. Flow splitters are used to bypass larger flows to the storm drain system or a stabilized channel. Sand filters also need adequate access and regular removal of surface sediment to ensure longevity. They also require two to four feet of available head (the vertical drop between the entrance and exit of a filter for gravity flow through the filter) for most off-line applications. The pretreatment chamber should be able to hold at least 25% of the runoff volume to be treated in the practice. The sand filter chamber should be able to hold at least 75% of the runoff volume to be treated in the practice. Typical runoff volumes used are those from a 1" storm or ½" of runoff over the entire area draining to the practice. Sand filters can be used in areas where groundwater quality is not a critical concern – they should NOT be used in wellhead protection areas unless treated water is routed to another practice for further treatment or removal. Cheaper geotextiles or soil liners can also be used instead of concrete liners. ADVANTAGES OF SAND FILTERS • Excellent longevity and very few environmental limitations. • Require little or no developable land (most are placed underground or on margins of parking lots). • Can be applied to most development sites and can be used in areas with thin soils and steeper slopes. • Have low maintenance/rehabilitation costs. • Are useful in watersheds where concerns over groundwater quality prevent the use of infiltration. • Have moderate to high pollutant removal capability. DISADVANTAGES OF SAND FILTERS • Have frequent maintenance requirements. • Large, surface sand filters without grass covers may not be attractive in residential areas. • Some stormwater runoff will bypass filter during a large storm event. • Do not provide stormwater QUANTITY control. • High installation cost. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-7 Water Quality Inlets Figure 4.4. Stormceptor® operation during normal flow conditions (Stormceptor® Technical Manual, 1997). When and Where to Use Water Quality Inlets What to Consider Figure 4.5. Schematic design of an oil-grit separator (Schueler, et al., 1992). Water quality inlets (also called oil-grit separators) are three-stage underground structures designed to remove oil, grease, other absorbed hydrocarbons, heavy sediment and other floating substances from stormwater runoff before it is discharged to the storm drain system and/or to guts, ponds or coastal waters (CH2M Hill, 1998). Other pre-fabricated products that perform a similar function are called storm drain or catch basin inserts. The many different kinds and models available range from filter devices to remove sediment to pre-fabricated oil-grit separators to multi-stage treatment units that incorporate vegetation to help remove nutrients and other pollutants (Figure 4.4). Water quality inlets are used on sites that are expected to receive heavy vehicular traffic or large amounts of petroleum. The inlets are usually placed to catch the oil and fuel that leak from cars and trucks in parking lots, service stations, or loading areas (Figure 4.5). The inlets can reduce maintenance of infiltration systems, detention basins, and other stormwater devices and be used as a first stage of treatment by removing oil and sediment from stormwater before it enters another larger stormwater pollution control practice, like a wet pond. Water quality inlets can be installed in most areas for drainage areas no larger than 1 acre. They can be installed in most any soil or terrain, and can be used near or at the impervious surface contributing the stormwater runoff. Inlets need enough land area for the structure and to allow access for proper maintenance. The pollutant removal efficiency of storm drain inserts or water quality inlets varies depending on the volume of the practice, flow velocity, and the depth of the baffles and elbows in the chamber. These practices should be inspected regularly and cleaned at least twice a year to remove sediment, accumulated oil and grease, floatables and other pollutants. The wastes removed may be hazardous (such as petroleum products) and may need to be disposed of with a licensed hazardous waste hauler. ADVANTAGES OF WATER QUALITY INLETS/OIL-GRIT SEPARATORS • Easily installed. • Can effectively remove oily pollutants. • Small inlets can be distributed over a large drainage area, instead of building a single large structure. • Are not unsightly, because are hidden underground. Chapter 4 Stormwater Runoff Control Practices 4-8 Environmental Protection Handbook Extended Detention Ponds When and Where to Use Extended Detention Ponds DISADVANTAGES OF WATER QUALITY INLETS/OIL-GRIT SEPARATORS C Have frequent maintenance requirements. C Low removal of contaminants other than oil, grease and coarse sediment. C Difficult to maintain because of enclosed, underground design. C Sometimes odor is a problem. C Do not provide stormwater QUANTITY control. 4.3 DETENTION PRACTICES Examples of detention practices that are suitable for conditions in the Virgin Islands include: • Extended detention ponds; and • Constructed wetlands Detention practices temporarily hold runoff to control runoff rates and volumes, and to settle and retain sediment and other pollutants. All detention practices use settling to remove particulate pollutants (sediment, organic matter, etc.). Properly designed extended detention ponds can minimize erosion of downstream channels by controlling water discharge speed. They can also remove nutrients and provide wildlife habitat if they are landscaped and designed properly. Constructed wetlands and multiple-pond systems can further reduce pollutants in runoff. Many of these systems are currently being designed to include vegetated buffer strips to provide enhanced wildlife habitat and scenic areas. Extended Detention (ED) ponds temporarily hold a portion of stormwater runoff for up to 24 hours after a storm, using a fixed outlet to regulate outflow at a specified rate. This allows sediment and other pollutants to settle out of stormwater. ED ponds are usually “dry” between storms and do not have any permanent standing water. They are typically made up of two stages: an upper stage that stays dry except for larger storms and a lower stage that is designed to treat average storms. Temporary and most permanent ED ponds use a riser with an anti- vortex trash rack (a type of trash screen that does not cause whirlpools to form in the pond or riser) on top to control trash. Enhanced ED ponds also have a plunge pool near the inlet, a micropool at the outlet, and an adjustable, reverse-sloped pipe as the ED control orifice (Figure 4.6). Extended detention ponds can be implemented in most new developments, and can also be used to retrofit existing dry ponds in older urbanized areas. They can be used on development sites 10 acres and greater. Soils should neither be extremely impermeable (D soils) or extremely permeable (A soils). ED ponds can also be lined with impermeable materials so that no infiltration occurs. The water collected can then be used to irrigate gardens or lawns. The space required for ED ponds is usually less than 5% of the total site area. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-9 Figure 4.6. Dry extended detention (ED) pond design (Schueler et al., 1992). What to Consider How Effective Are ED Ponds? Runoff should be detained for at least 24 hours to ensure sufficient pollutant removal. ED ponds should be designed, at a minimum, to store the stormwater runoff volume of a one-inch storm. Different areas of the U.S. have developed rules for sizing extended detention ponds – these rules specify both a volume of stormwater runoff to be detained and a length of time over which the runoff is released. Some sizing recommendations include: • A volume equivalent to ½-inch of runoff distributed over the contributing watershed and released over a 40-hour period; • Runoff volume generated from the one-year, 24-hour design storm and released over a minimum of 24 hours; • Runoff volume generated from the two-year, 24-hour design storm, released over 24 hours; • Runoff volume generated from a one-inch storm released over 24 hours; and • “First-flush” runoff volume (½-inch per impervious acre) released over 24 hours. Two-stage ED ponds are most effective in removing pollutants. Slopes leading to the pond should be gentle enough to prevent gully erosion of pond banks (i.e., side slopes no greater than 3:1 and banks no steeper than 2:1). If banks are steeper than 2:1 they should be stabilized with rip-rap to prevent erosion. The slope of the upper stage of the ED pond should be between 2 and 5% to promote rapid drainage. The drainage channel immediately below the pond outlet should be lined with large stone riprap and graded to a slope ~ 0.5%. A layer of filter cloth that conforms to the natural dimensions of the channel should be laid down and anchored with 18"-30" stone riprap. ED ponds require routine maintenance including mowing, inspections, debris and litter removal, erosion control, and nuisance control. Other maintenance that may be necessary includes structural repairs and equipment replacement, and sediment removal. Extending the detention time of dry ponds is an effective, low cost way to remove sediment from stormwater runoff and to minimize downstream channel erosion. If stormwater can be detained for 24 hours or more, as much as 90% of the particulate pollutants in stormwater runoff can be removed. However, ED ponds only slightly reduce the levels of soluble nutrients in stormwater runoff, unless a shallow marsh is created in the wetter portion of the pond. ED ponds can also significantly reduce the frequency of erosive downstream floods. Chapter 4 Stormwater Runoff Control Practices 4-10 Environmental Protection Handbook Constructed Wetlands When and Where to Use Constructed Wetlands Figure 4.7. Example of a shallow marsh planting strategy (Schueler et al., 1992). What to Consider How Effective Are Constructed Wetlands? ADVANTAGES OF ED PONDS • Significant pollutant removal, if designed properly. • Creation of local wetland and enhanced wildlife habitat. • Limited protection of downstream aquatic habitat. • Only 10% more costly than conventional dry ponds. DISADVANTAGES OF ED PONDS • Occasional nuisance problems (odor, debris, mosquitos, and weeds). • Moderate to high routine maintenance needs. • Eventual need for costly sediment removal and disposal. Constructed wetlands are engineered systems designed to imitate natural wetlands ability to improve water quality by treating and containing stormwater runoff and pollutants and decreasing pollutant loadings to coastal waters. Constructed stormwater runoff wetlands simulate natural wetlands and attempt to replicate all of the functions of natural wetlands. Wetlands can be constructed around the outside of a pond or in a sediment forebay (Figure 4.7). The most reliable method to create a constructed wetland is to transplant live plants or dormant rhizomes from nursery stock. Transplantation from existing wetlands is not usually as effective and can be detrimental to the source wetland. Most wetland species thrive in water less than one foot deep. To create these depths over a wide area, sites usually need to be graded. Also potential sites must have enough baseflow (subsurface water flow) or surface flow re-routing to ensure that severe seasonal evaporation does not damage wetland vegetation. To achieve optimal pollutant removal, the surface area of the wetland should be the size of about 2 to 3% of the total contributing watershed area. Establishing wetland species around new or existing wet ponds, dry ponds, or sediment basins is an effective, low cost way to remove sediment and soluble pollutants from stormwater runoff, to minimize downstream channel erosion, and to minimize stormwater runoff peaks. Constructed wetlands can also significantly reduce flooding frequency downstream. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-11 Porous Pavers Figure 4.8. Porous driveway option (Estate Lerkenlund, St. Thomas). ADVANTAGES OF CONSTRUCTED WETLANDS • Significant pollutant removal, if designed properly. • Creation of wildlife habitat and propagation of threatened or endangered wetland species. • Limited protection of downstream aquatic habitat. • Stabilizes pond or basin floors to prevent erosion. • Vegetation costs are small portion of pond construction costs. DISADVANTAGES OF CONSTRUCTED WETLANDS • Difficulty in establishing native wetland vegetation. • Engineering design assistance is limited. • Need for steady water source. • Increase of mosquitoes in stagnant areas. 4.4 INFILTRATION PRACTICES Some infiltration practices that are appropriate for use in the Virgin Islands include: • Porous pavers; • Infiltration trenches; and • Bioretention areas. Infiltration practices treat stormwater runoff by filtering it through the soil. Under natural conditions, water percolates through the soil, where filtration and biological action remove pollutants. However, stormwater treatment systems that use soil absorption require deep, permeable soils at separation distances of at least 4 feet between the bottom of the structure and the groundwater. Long-term effectiveness of these practices depends on proper operation and maintenance. Infiltration systems, some filtration devices, and sand filters should be installed AFTER construction has been completed and the site has been permanently stabilized. Infiltration and filtration systems that are clogged by sediment generated during construction activities or because of premature use of these systems will fail. There are two categories of porous paving or alternate pavers: paving blocks and other surfaces including gravel, cobbles, brick or natural stone. Porous paving materials are used in low-traffic areas (such as low-use parking lots, emergency areas, driveways, walkways) in place of asphalt or concrete. Concrete tire-tracks with grassed interiors can also be used for steeper driveways (Figure 4.8). Paving blocks can be concrete, cement or high-strength plastic grids placed on a pervious base such as gravel or sand (Figure 4.9) The grids or pavers are then filled with pervious materials such as sand, gravel or soil. Grids filled with soil can be seeded to attain a grassed or lawn surface. The resulting system provides a load-bearing surface that can to support vehicles while allowing infiltration of surface water into the underlying soil or bedrock (Figure 4.10). This reduces stormwater runoff volume and discharge rate and improves water quality. Chapter 4 Stormwater Runoff Control Practices 4-12 Environmental Protection Handbook Figure 4.9. Installation of interlocking plastic grid pavers at UVI-CES Demonstration Garden, St. Thomas (photo by Dale Morton, UVI-CES). When and Where to Use Porous Paving How Effective is Porous Paving? Figure 4.10. Diagram of concrete grid pavers (Empire State Chapter, Soil and Water Conservation Society, 1997). Porous paving should only be used for lower volume parking areas where heavy compaction will not be a problem, and is most effective on sites with gentle slopes and moderate to highly permeable soils. Webbed cellular confinement systems work very well in reducing erosion and stormwater runoff from gradual driveways and low-use roads. For these uses, it is necessary for rock (gravel or crusher run) to be used as fill material instead of soil, in order to provide sufficient load support (especially if filter fabric is not placed below the webbing and fill material). Grassed block/grid cellular confinement systems can work well in controlling erosion and stormwater on fairly level, low-use traffic areas, such as emergency access areas, driveways or parking lots. For higher or heavier used parking areas, gravel fill would be more appropriate, as continual vehicular traffic damages the grass base. Some disadvantages for use in the V.I. have been observed on the grassed demonstration system. First, the subsurface must be completely level for the blocks to lay properly. Secondly, for a grassed parking area that requires soil fill, the blocks should be filled by hand in order to avoid over-filling with soil. Over-filling reduces the blocks ability to protect grasses planted within the system so that continual traffic will damage the sod. Porous paving can provide hydrologic conditions that are similar to pre-development conditions. If installed properly, this practice can control peak stormwater discharges and can also reduce stormwater runoff volumes through infiltration. Porous paving materials are NOT intended to remove coarse sediment particles, but can provide significant pollutant removal if the subsurface soil has adequate infiltration capacity. Because sediments can rapidly clog the base of the pavers, it is essential that practices (such as filter strips, sediment traps, etc.) are used to keep coarse sediments and other particles from entering the pavement surface. The gravel or sand base beneath the pavers may also be lined with filter fabric to prevent the rock material from becoming imbedded. Grassed porous pavers should only be used for emergency or overflow parking areas because compaction caused by daily traffic flow will kill grassed areas. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-13 Infiltration Trenches Figure 4.11. Parking lot perimeter trench design (Schueler, 1987). When and Where to Use Infiltration Trenches ADVANTAGES OF POROUS PAVERS • Has load-bearing strength, longevity, and maintenance requirements similar to conventional pavement, when properly installed. • Increases stormwater infiltration, decreases runoff volume, removes some pollutants, and preserves the natural water balance at the site. • Reduces or eliminates the need for curbs and gutters in residential areas and for downstream conveyance systems. • Provides a safer driving surface with better skid resistance and reduced hydroplaning. DISADVANTAGES OF POROUS PAVERS • If it becomes clogged it is difficult and costly to repair. (The risk of premature clogging is high, and can only be prevented if sediment is kept off the pavement before, during and after construction by filter strips, sediment traps, etc.) • Not appropriate for high traffic areas. Infiltration trenches are shallow, excavated ditches that have been backfilled with stone or gravel to form an underground reservoir. Stormwater runoff is diverted into the trench and gradually infiltrates into the subsoil and eventually into groundwater. Variations in infiltration trench design include enhanced infiltration trenches (trenches that have extensive pretreatment systems to remove sediment and oil). Trench size depends on the design storm volume of runoff to be controlled and the degree of infiltration (or infiltration rate) of the soil or subsoil (Figure 4.11). Individual trenches are used primarily for on- site runoff control, and are rarely practical or economical on sites larger than five acres. Trenches are NOT designed to trap coarse sediments. Runoff water should be pre- treated (with filter or buffer strips or some other kind of sediment trapping device) to remove sediment and other particles to lower trench failure rates. Trenches should be placed on flat ground (5% slope or less), but the slopes of the site draining to the practice can be up to 15%. Trenches are not practical for use in soils with infiltration rates less than ½ inch/hour or more than 3 inches/hour, or with more than 20% clay or 40% silt/clay content (CWP, 1998). Trenches should be located in areas with deep soils – 2' to 5' of clearance from the bottom of the trench to bedrock or water table is recommended (Schueler, 1992). Chapter 4 Stormwater Runoff Control Practices 4-14 Environmental Protection Handbook What to Consider Different Uses Figure 4.12. Graveled infiltration areas in a parking lot, St. Croix (photo by Dale Morton, UVI-CES). Figure 4.13. Swale/trench design for a development (Schueler, 1987). Trenches have limited maintenance requirements aside from routine inspections and strict erosion and sediment control. However, if a trench is allowed to clog, partial or complete replacement of the structure may be required. Trenches have limited applications on steep slopes (>15%). Infiltration trenches can be located on the surface or below ground. Surface trenches may capture runoff directly from adjacent land areas, after it has been filtered through a buffer (Figure 4.12). Underground trenches are used for more concentrated runoff (from pipes, channels or storm drains). However, special inlets need to be installed in underground trenches to prevent coarse sediment, oil and grease from clogging the trench. Surface trenches are recommended for residential areas, where the smaller amounts of sediment and oil that are present can be trapped by grass filters (Figure 4.13). Because the surface is exposed, these trenches have a slightly higher risk of clogging than underground trenches, but they are also easier to maintain and inspect. Underground trenches are best suited for larger developments (10 acres or greater) where concentrated runoff from pipes or channels is directed to the trench. However, the runoff needs to be pre- treated before entering the trench, and it also needs to be evenly distributed within the trench. Underground trenches are also more difficult and costly to maintain. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-15 How effective are infiltration trenches? Infiltration trenches can potentially reduce stormwater runoff so that hydrologic conditions are close to pre- development conditions. Trenches designed to infiltrate all incoming water into the surrounding soil can effectively reduce peak discharge rates associated with the design storm (the size of storm a given practice is designed to handle). However, for both physical and economic reasons, it is not always practical to design trenches for very large or infrequent storms. In this case, trenches that are designed to infiltrate part of the runoff into the surrounding soil and collect the rest in a perforated underdrain at the bottom of the trench and direct it to a central outlet, are more appropriate. Trenches can also reduce the increases in post-development runoff volume that are produced by small or medium-sized storms. The effectiveness of a trench in reducing stormwater runoff volumes is a function of the amount of water that can be infiltrated into the surrounding soil profile. Infiltration trenches are not intended to remove sediment. They are designed to remove fine particles and soluble pollutants by filtration through the trench and surrounding soil profile. Pollutant removal in a trench can be enhanced by increasing the surface area of the trench bottom (i.e., make the trench shallow and broad rather than narrow and deep). This will provide more area to increase infiltration into the surrounding soil profile and provides more soil below the trench to further filter water. The greatest reduction of nutrients, metals, and bacteria will occur in soils that have higher clay contents and/or organic matter, and the least reduction will occur in sandy soils. Unfortunately, soils that maximize pollutant absorption also tend to have very low infiltration rates. However, these soils are most prone to blockage, so it is vital that large particles are removed from stormwater runoff (by a filter strip or other practice) before it enters the trench area. The trench should be designed to completely drain within three days after the maximum design storm event. If a trench is constructed over soils with a lower infiltration capacity, it may be advisable to adjust the depth of the trench so that it drains in two days or less, as a safety margin. However, if a trench drains in less than 6 hours, it will not provide adequate pollutant removal. Test wells should be installed in every trench to monitor draining times after installation. The water level in the well should be measured daily after a large storm. If the trench does not completely drain after 3 days it usually means that the bottom of the trench is clogged and needs to be cleaned out. Otherwise, if a partial exfiltration trench empties completely within one day, it means that either the underdrain is too large or that the bottom of the trench has clogged, or both. ADVANTAGES OF INFILTRATION TRENCHES • Preserve natural groundwater recharge capabilities. • Are relatively easy to fit into margins, perimeters and other un-utilized areas of a parking lot or development site. • Are one of the few BMPs that provide pollutant removal on small sites or developments wedged in between existing developments. DISADVANTAGES OF INFILTRATION TRENCHES • Difficulties in keeping sediment out of the structure during site construction (especially if construction occurs in phases). • The need for careful construction of the trench and regular maintenance. • Possible risk of groundwater contamination if toxic materials are introduced. • Require relatively flat area for the trench, 2 to 5 feet of soil profile, and well-drained soils. Chapter 4 Stormwater Runoff Control Practices 4-16 Environmental Protection Handbook Bio-Retention When and Where to Use Bioretention What to Consider Figure 4.14. On-line bioretention cross section - incorporated into a grass swale with a mild to moderate slope (CH2M Hill, 1998). Figure 4.15. Off-line bioretention system design (CH2M Hill, 1998). Bioretention systems are landscaped areas made of soil and sand mixtures and planted with native plants that are used to filter stormwater runoff on residential and non-residential sites (CH2M Hill, 1998). They are usually located in parking lot islands or within small pockets in residential land uses. Systems can be inline, located in grasses swales modified to enhance pollutant removal (Figure 4.14), but are more commonly located offline. In offline systems, surface runoff is directed into shallow, landscaped depressions. These depressions are designed to mimic many of the pollutant removal mechanisms that operate in forested ecosystems. During storms, runoff ponds above the soil and mulch, filtering through to the subsoil or to a perforated underdrain returning to the storm drain system (Figure 4.15, next page). Runoff from larger storms bypasses the system directly into the storm drain system. Bioretention is used on small sites (less than 5 acres) with flat slopes (5% or less), typically parking lots or small residential areas, and for small storm events (1- to 5-year, 24-hour storm). It can be used in almost any soil because the soil is made and placed in the system. Bioretention systems should be separated from the water table by at least 3 feet. Bioretention systems are most effective if they are as close as possible to the source of runoff. They have five basic features: pretreatment, treatment, conveyance, maintenance reduction and landscaping. Pretreatment practices like grassed filter strips capture and remove coarse sediment from runoff water to reduce maintenance and clogging of the system. A pea gravel level spreader (spreads flow evenly with no channels) can also be used. Treatment designs should size the system between 5% and 10% of the impervious area draining to it, install a sand/soil filter bed with a mulch layer above the soil bed, and allow runoff to pond 6" to 9" deep above the filter bed. Stormwater should be conveyed to and from the practice to minimize erosion. Bioretention systems include an underdrain system (perforated pipe in a gravel bed) that collects filtered runoff at the bottom of the filter bed and channels it to the storm drain system. An overflow structure also needs to be designed to route flow from large storms to the storm drain system. All parts of the system should be easily accessible for maintenance. Chapter 4 Stormwater Runoff Control Practices Environmental Protection Handbook 4-17 Different Uses How Effective are Bioretention Systems? Landscaping bioretention systems is vital for the system to function properly. Native vegetation should be used for landscaping, where possible. Plants should be selected that can tolerate both wet and dry conditions (such as cattails, typha sp.). The edges of the bioretention area are usually dry and so can be landscaped with native plants appropriate to that area. It is best to select a combination of trees, shrubs and ground covers. A partial exfiltration bioretention area can be used on sites with appropriate soils (see Infiltration Trenches, page 4-12) that can adequately absorb the “exfiltrated” stormwater runoff. In this type of system, the filtered stormwater runoff is dispersed into the surrounding soil. The underdrain is only installed on a small part of the bottom of the system, and acts as an overflow for larger storm events. When properly designed and sized, bioretention systems can provide stormwater control and treat runoff from small storms on small drainage areas. However, the five features of the bioretention system must be designed properly to achieve optimal performance. ADVANTAGES OF BIORETENTION SYSTEMS • Preserve the natural water balance of the site. • Can serve larger developments. • Can be used as sediment basins during construction. • Are cost-effective when compared with other stormwater management practices. DISADVANTAGES OF BIORETENTION SYSTEMS • Can have a fairly high failure rate due to unsuitable soils. • Need frequent maintenance. • Possible nuisance from odor, mosquitos, soggy ground. 4.5 REFERENCES Austin, Texas. 1991. IN: Schueler, T.R. 1994. “Developments in Sand Filter Technology to Improve Stormwater Runoff Quality,” Watershed Protection, 1(2):47-54, Center for Watershed Protection, Silver Spring, Maryland. CH2M Hill. 1998. Pennsylvania Handbook of Best Management Practices for Developing Areas, Pennsylvania Association of Conservation Districts, Harrisburg, Pennsylvania. Center for Watershed Protection (CWP). 2001b. “Aquatic Buffers Fact Sheet: Buffer Zones,” Stormwater Managers Resource Center, www.stormcenter.net, Ellicott City, Maryland. Center for Watershed Protection (CWP), Environmental Qaulity Resources and Loiederman Associates. 1998. Maryland Stormwater Design Manual, Draft. Prepared for: Maryland Department of the Environment, Baltimore, Maryland, website: www.mde.state.md.us/environment/wma/stormwatermanual/mdswmanual.html. Empire State Chapter, Soil and Water Conservation Society. 1997. New York Guidelines for Urban Erosion and Sediment Control - Update Package, Syracuse, New York. Chapter 4 Stormwater Runoff Control Practices 4-18 Environmental Protection Handbook Schueler, T.R. 1994. “Developments in Sand Filter Technology to Improve Stormwater Runoff Quality,” Watershed Protection, 1(2):47-54, Center for Watershed Protection, Silver Spring, Maryland. Schueler, T.R., P.A. Kumble, and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices: Techniques for Reducing Nonpoint Source Pollution in the Coastal Zone, Anacostia Restoration Team, Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, DC. Publication Number 92705. Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs, Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, DC. Publication Number 87703. Stormceptor Corporation. 1997. Stormceptor® Technical Manual, Rockville, Maryland, www.stormceptor.com. U.S. EPA. 1993. Guidance for Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters, U.S. Environmental Protection Agency, Office of Oceans, Wetlands and Watersheds, Washington, DC. Document Number 840-B-92-002. U.S. EPA. 1992. Stormwater Management for Construction Activities: Developing Pollution Prevention Plans and Best Management Practices, U.S. Environmental Protection Agency, Office of Wetlands, Oceans and Watersheds, Washington, DC. Document Number 832-R-92-005.