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85 Tables D1 and D2 provide general descriptions of several common structural stormwater BMPs, along with brief examples of benefits, challenges, and costs. For more detailed informa- tion, please see the references cited. Table D1 includes the most common GSI BMPs. Table D2 summarizes information for the two conventional, conveyance-focused BMPs that were used in the hypothetical Bayside Airport scenario. These examples, in conjunction with information in Chapters 4 and 5 and local and state BMP guidance, will help in assessing costs and benefits for other conventional and GSI BMPs. A P P E N D I X D Stormwater BMPs (continued on next page) GSI Types General Description Benefits Challenges Costs References/ Sources of Information Green Roofs Green roofs A layer of vegetation grown on the roof of a building or structure, ranging from a thin layer of soil and a few small plants to a deep layer with trees Reduces total runoff volume by 25%â90%, reduces energy costs due to insulation, lasts 40â50 years vs. 15â20 for a conventional roof, creates a usable space for building occupants Potential hazardous wildlife attractant and source of foreign object debris (FOD), requires vegetation maintenance, requires additional inputs in hot, dry climates Construction: $15 to $25 per square foot, although the price of vegetation and cost of design are highly variable Additional: O&M, especially at beginning to establish new vegetation Jolley et al., 2017 Table D1. GSI BMPs.
86 BenefitâCost Analyses Guidebook for Airport Stormwater GSI Types General Description Benefits Challenges Costs References/ Sources of Information Harvesting and Reuse Rain barrels and cisterns A holding device that collects rainwater from roofs; water can then be used for irrigation, washing hangars and airplanes, or other purposes Reduces total runoff volume up to 100%, peak-flow attenuation or elimination, stores anywhere from 200â 25,000 gallons of water for use, can be located underground, lasts 20 years or more, savings in potable water use Potential for gutters or downspouts to clog, potential for roofing materials to leach into water, local laws may prohibit rainwater capture Construction: Cistern costs range from $0.50 to $4 per gallon of storage capacity. Additional: O&M for checking gutters and gutter guards, annual cistern cleaning, initial costs and O&M associated with treatment systems Jolley et al., 2017; North Carolina Department of Environment and Natural Resources (NCDENR), 2012 Bioretention Strategies Rain gardens, bioretention cells, and extended detention dry ponds Depressed plots that collect water and naturally filter it through soil and plant roots to achieve pollutant reduction and percolation into underlying soils Effectively removes pollutants (especially sediments, nutrients, and heavy metals), reduces total runoff volume, lasts up to 20 years, aesthetically pleasing Potential hazardous wildlife attractant due to standing water and vegetation, potential source of FOD, potential for filter media to clog; potential export of nutrients from soil media if organic-rich; potential for infiltration of pollutants and percolation to groundwater (e.g., from spills) if BMP is poorly sited Construction: $10 to $40 per square foot, although the price of vegetation is highly variable Jolley et al., 2017; NCDENR, 2012; FAA, 2013; WSDOT, 2008 Infiltration Devices Infiltration trenches, vaults, and basins A stone-filled basin that collects and holds water, allowing it to percolate into the underlying soil through the amount of porous space between the rocks Removes pollutants up to 100%, reduces total runoff volume up to 100%, mimics natural hydrology, not a potential hazardous wildlife attractant due to lack of vegetation Cannot be in areas with high water tables or impermeable soils, water may need pretreatment before entering, excess sediment can clog, life span 5â15 years, mobilization of existing groundwater pollutants possible if BMP is poorly sited New construction: $27,300 per acre for areas <3 acres $9,700 per acre for areas >3 acres Retrofit: $92,700 per acre for areas <3 acres $32,800 per acre for areas >3 acres Jolley et al., 2017; NCDENR, 2012; FAA, 2013; WSDOT, 2008 Table D1. (Continued).
Stormwater BMPs 87 GSI Types General Description Benefits Challenges Costs References/ Sources of Information Bioswales Vegetated and grassy swales A concave, gently sloping, vegetated channel that transports water and allows it to infiltrate soils below Moderately removes pollutants, potentially effective total runoff volume control, lasts indefinitely, functions as a stormwater transport device Sediment buildup can cause pooling and can potentially attract hazardous wildlife, requires some maintenance depending on the vegetation New construction: $19,500 per acre for <3 acres $2,300 per acre for >3 acres Retrofit: $37,500 per acre for <3 acres $4,400 per acre for >3 acres Jolley et al., 2017; NCDENR, 2012; WSDOT, 2008 Sand Filters Sand and media filters A sand-filled basin that collects water from the first flush of stormwater runoff and filters it into underlying soil or an underdrain piping system Removes sediments and pollutants effectively, potentially effective total runoff volume control, highly adaptable, lasts approximately 50 years Potential for standing water to attract hazardous wildlife, potential to clog from high sediment loads, requires frequent maintenance, possible export of nitrogen, mobilization of existing groundwater pollutants possible if BMP is poorly sited New construction: $88,000 per acre for <3 acres $48,100 per acre for >3 acres Retrofit: $113,800 per acre for <3 acres $62,300 per acre for >3 acres Jolley et al., 2017; NCDENR, 2012; FAA, 2013; WSDOT, 2008 Filter Strips Vegetated filter strips A shallowly sloped area containing vegetation that allows sheet flow of water across its surface; typically located adjacent to runways and taxiways Removes sediments and pollutants effectively, controls 50%â100% of total runoff volume, can be used airside along runways, lasts indefinitely Depressions in the strip that collect water or vegetation could potentially attract hazardous wildlife, requires frequent mowing Cost neutral since runway safety requirements typically create one naturally (only maintenance requirements) Jolley et al., 2017; NCDENR, 2012; WSDOT, 2008 Porous Pavement Permeable pavement, porous asphalt, pervious concrete, and pavers A type of pavement that allows water to pass through; typically lies on top of an underlying drain system or gravel to facilitate percolation into the ground Allows permeability in surfaces that typically prevent infiltration, reduces total runoff volume up to 100%, removes pollutants, life span of 20 to 40 years Limited load-bearing capacity, potential source of FOD, should not be used in areas prone to spills (loading docks, fueling areas, etc.), mobilization of existing groundwater pollutants possible if BMP is poorly sited Construction: On average, $2 to $6.50 per square foot Additional: O&M â periodic vacuuming, other maintenance Jolley et al., 2017; NCDENR, 2012; WSDOT, 2008 Table D1. (Continued). (continued on next page)
88 BenefitâCost Analyses Guidebook for Airport Stormwater GSI Types General Description Benefits Challenges Costs References/ Sources of Information Wetland Treatment Systems Constructed wetlands and conventional stormwater wetlands A manmade water collection system that facilitates the growth of marsh and wetland plant species either above or below ground Removes sediments and pollutants highly effectively, controls total runoff volume highly effectively, uses natural processes to filter water, lasts 20â 30 years Potential to attract hazardous wildlife given water and vegetation, high functionality requires careful design and construction New construction: $32,800 per acre for <3 acres $13,700 per acre for >3 acres Retrofit: $52,300 per acre for <3 acres $21,900 per acre for >3 acres Jolley et al., 2017; Shi et al., 2015 Table D1. (Continued). Stormwater BMP Type Potential Benefits Additional Potential Costs (all projects have construction costs) Special Considerations Dry detention basins â¢ Peak-flow attenuation â¢ No permanent pool â minimizes wildlife concerns â¢ Reduction of sediment and sediment-associated pollutants â¢ Relatively low cost â¢ Basins can be large â loss of land area â¢ Does not reduce volume Underground detention (vaults and piping) â¢ Some runoff volume reduction if designed to infiltrate â¢ Peak-flow attenuation â¢ Below ground â avoids wildlife concerns â¢ Leaves surface area free â¢ High storage volume â¢ Rapid installation â¢ Maintenance more difficult than above- ground systems â¢ No water quality treatment unless equipped with forebay for pretreatment prior to infiltration â¢ Can be designed to infiltrate (a benefit) Table D2. Examples of conventional stormwater BMPs.