Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 This primer on green stormwater infrastructure (GSI) describes the essentials of stormwater management and GSI for airport managers, planners, engineers, and aviation consultants. The Problem: Stormwater Management at Airports Rainfall or melting snow creates stormwater runoff, which, in an undeveloped, pervious area, will infiltrate into the ground (and provide groundwater recharge), be taken up by vegetation, and/or evaporate. Excess runoff accumulates and flows across the landscape into local water bodies, including streams and wetlands. In a developed area, like an airport, stormwater runoff accumulates on the paved or impervi ous areas and is collected in a series of drainage ditches, stormwater catch basins, and sewers and discharged to the nearest receiving waters. As noted in Figure 1, the volume and speed of stormwater runoff and pollutant load are increased in paved, impervious areas, in contrast to unpaved, pervious areas. These increases in stormwater runoff discharge speed and pollutant load are the challenge in stormwater manage ment. If not managed, they can cause (1) flooding, (2) erosion, and (3) adverse effects on down stream receiving waters (Figure 2). Pervious and Impervious Areas at Airports As illustrated in Figure 1, the amount of impervious area (i.e., paved areas) in relation to per vious area (i.e., vegetated, nonÂcompacted, unpaved areas) determines the volume and speed of runoff from an airport. The proportion of impervious area can vary depending on the airport location and size. At airports, there is often a significant amount of pervious area, especially on the airfield (see Figure 3). Airports can also have large amounts of impervious area (e.g., taxiways, aprons, and parking lots). To some extent, runoff from impervious areas may be directed to drain to veg etated areas before discharging to receiving waters; however, runoff from such areas is typically directed to conventional stormwater collection systems. Certain portions of an airport, such as landside parking lots and commercial areas, may resemble commercial and business districts, which can have impervious cover of up to 85 per cent (as noted in Table 1). However, the percentage of impervious cover over the entire airport (including the airfield and other airside land uses) may be substantially lower. For example, at the Georgetown (Texas) Introduction
2 Green Stormwater Infrastructure Municipal Airport, the percentage of impervious cover of the entire area is approximately 22 percent (City of Georgetown 2015; Figure 4). Similarly, in 2000, Dulles International Airport had an overall percentage of impervious cover of 10 to 20 percent, even though it is a larger airport (Dougherty et al. 2004). The percentage of impervious cover at some airports, such as McCarran International Airport, Los Angeles International Airport, Phoenix Sky Harbor Inter national Airport, and San Diego International Airport, can be greater than 50 percent. Source: Jane Hawkey, Integration and Application Network, University of Maryland Center for Environmental Science (http://ian.umces.edu/imagelibrary). Figure 1. Runoff from impervious and pervious surfaces. Source: Michael Baker International. Figure 2. Damage from uncontrolled stormwater runoff.
Introduction 3 Source: © 2016 Google Earth. Figure 3. Airfield pervious areas at Austin Bergstrom International Airport. Information source: University of Delaware, Water Resources Agency, 1998; USDA, Soil Conservation Service, TR-55, 1983. Source: Delaware Sea Grant College Program (2005). Table 1. Percentage of impervious cover by land use. Source: © 2016 Google Earth. Figure 4. Aerial view of Georgetown Municipal Airport.
4 Green Stormwater Infrastructure Water Quality Characteristics of Stormwater Runoff from Airports Stormwater runoff from impervious surfaces at airports is often a source of water pollu tion. The runoff from runways, ramp areas, and buildings potentially contains pollutants from aircraft fuels, equipment cleaning agents, oil, and other fluids used in maintenance activities (see Table 2). These pollutants can enter surrounding receiving waters (e.g., streams, lakes, and marine systems), where degradation to water quality and aquatic life may occur. History of Stormwater Regulations at Airports Since its inception in 1990, airport and tenant activities have been subject to the National Pollutant Discharge Elimination System (NPDES) industrial stormwater permitting program, either under individual permits or under MultiÂsector General Permits (MSGPs) as summarized in Table 3. A majority of states have been delegated by the United States Environmental Protec tion Agency (U.S. EPA) to manage the NPDES stormwater program. The stateÂmanaged pro grams are often called State Pollutant Discharge Elimination System programs. Under these programs, airports must implement a stormwater pollution prevention plan that includes standard stormwater treatment best management practices (BMPs) and water quality moni toring and reporting. Activities subject to the MSGP at airports include but are not limited to (1) servicing, repairing, or maintaining aircraft and ground vehicles; (2) equipment cleaning Land Use Sediment Nutrients Heavy Metals Bacteria Spills Notes Runways and Taxiways Low Medium High Unlikely Highly unlikely Metals from brake pad wear and rubber accumulation Ramp Area Medium Medium Medium Low Likely Isolate to manage spills Service Roads Low Medium Medium Unlikely Highly unlikely Terminal and Other Buildings Low Medium Low Unlikely Highly unlikely Avoid metal structural materials and cladding (e.g., metal roofs) to keep metal concentrations low Parking Lots Medium Medium Medium Unlikely Highly unlikely Rental Car Centers (quick turnaround areas) Low Low Low Unlikely Unlikely, except in areas where fueling occurs Fixed Base Operators Medium Medium Medium Low Likely Isolate to manage spills Maintenance Hangars Medium Medium Medium Low Likely Isolate to manage spills Outdoor Parking of GSE Medium Medium Medium Low Likely Isolate to manage spills *Based on the professional judgment of the authors. Table 2. Typical pollutant loads for airport land uses.* Resources for Stormwater Permitting Programs Applicability to Airports NPDES MSGP Associated with Industrial Activity https://www.epa.gov/npdes/final-2015-msgp-documents All airports [operating under standard industrial classification (SIC) codes 4512â4581]. NPDES Permits for Municipal Separate Storm Sewer Systems (MS4s) https://www.epa.gov/npdes/stormwater-discharges-municipal- sources#overview Airports discharging runoff to MS4s, or airports designated as MS4s. NPDES Construction General Permit https://www.epa.gov/npdes/epas-2012-construction-general-permit- cgp-and-related-documents Airports with areas of construction of one acre or more. Table 3. Summary of stormwater permitting programs.
Introduction 5 and maintenance (including vehicle and equipment rehabilitation, mechanical repairs, painting, fueling, and lubrication); and (3) deicing/antiÂicing operations. Specific airports categorized as municipal separate storm sewer systems (MS4s) can also be permitted directly under the NPDES municipal stormwater permit program. Other airports are subject to the MS4 requirements indirectly if their stormwater discharges to the municipal storm sewer system. In addition, airports are subject to the NPDES stormwater construction program. The construction program requires a stormwater pollution prevention plan that implements construction BMPs and an erosion and sediment control plan. On a local level, ordinances promulgated by municipalities and other local governments typi cally regulate stormwater management to implement flood control measures. These ordinances often incorporate state or federal requirements from the NPDES program. For example, in Los Angeles, state and local regulations and ordinances, some of which were developed to comply with the NPDES MS4 program, have forced Los Angeles International Airport (LAX) to imple ment a combination of conventional stormwater infrastructure and GSI programs to manage its stormwater. Similarly, compliance with the state and federal regulations, as well as the influence of local nonÂgovernment organizations, prompted SeattleâTacoma International Airport (SEA) to implement GSI and to monitor effectiveness at NPDES outfalls. Additional Resources ⢠ACRP Report 14: Deicing Planning Guidelines and Practices for Stormwater Management Sys- tems, Chapter 2: Guidelines for Developing Integrated DeicingÂRunoff Management Systems, pages 5â10. CH2M HILL, Gresham, Smith and Partners, and Barnes & Thornburg LLP. 2009. Transportation Research Board of the National Academies. http://www.trb.org/main/blurbs/ 161758.aspx. This chapter describes Clean Water Act requirements, including the various types of NPDES permits required by airports. ⢠Environmental Desk Reference for Airport Actions, Chapter 6: Construction Impacts and Chap ter 20: Water Quality. FAA. 2007. http://www.faa.gov/airports/environmental/environmental_ desk_ref/. ⢠FedCenter.gov Stormwater web page for federal facility managers and their agencies: https:// www.fedcenter.gov/assistance/facilitytour/construction/stormwater/. Two Approaches: Conventional Versus Green Stormwater Infrastructure There are two approaches to stormwater management: conventional (or gray) and green. Conventional stormwater infrastructure relies on stormwater pipes and structures, mechani cal equipment, and stormwater detention/retention ponds. Green stormwater infrastructure maintains, mimics, or restores natural processes to manage stormwater as close as possible to its point of origin. The key difference between conventional stormwater infrastructure and GSI is the method by which each moderates the increase in peak runoff created from paving or other development. During the typical rain storm, the runoff increases during the storm to a peak and gradually decreases until the storm ends. To prevent downstream flooding and erosion, both conventional and GSI approaches ensure that the peak runoff does not increase (i.e., in comparison to the preÂdevelopment runoff). The conventional approach, using a typical stormwater detention pond, does not manage the volume and duration of the stormwater runoff (which have both increased in comparison to preÂdevelopment conditions). The GSI BMP approach, on the other hand, more closely replicates preÂdevelopment storm runoff by minimizing both the increase in volume and the duration of the stormwater runoff.
6 Green Stormwater Infrastructure At many facilities, including airports, the best stormwater management system may be a com bination of conventional stormwater infrastructure and GSI approaches. Figure 5 illustrates, at the large scale of a watershed, the spectrum of management practices and their characteristics across this spectrum. Conventional Stormwater Management at Airports The goals of conventional stormwater management and infrastructure at airports are (1) flood management and erosion control and (2) water quality enhancement/pollution control. Flood Management and Erosion Control The proposed solution in most jurisdictions is to manage the stormwater runoff from the site so that the postÂdevelopment peak discharge is equivalent to the preÂdevelopment peak. This solution is designed to prevent flooding and erosion but does not necessarily address the other potential adverse effects on downstream receiving waters (i.e., negative impacts to aquatic life from stormwater pollutants). At airports, the infrastructure designed to attenuate the peak discharge is often one, or a series of, detention/retention basin(s) located at the downstream end of a stormwater collection sys tem before discharge to the receiving water, or at the property line (see Figure 6). Water Quality Enhancement/Pollution Control Preventing downstream adverse impacts by managing the pollutant load from a developed site (in comparison to preÂdevelopment) can be addressed at the lower end of the drainage area before discharge. This can be accomplished via stormwater wet ponds (which are designed to remove sediments and associated nutrients), constructed wetlands, and proprietary manufac tured treatment systems. At airports, conventional deep wet ponds and proprietary systems can be used if their poten tial as wildlife attractants is considered and mitigated as necessary. For example, a treatment system to remove metals was installed downstream of conventional stormwater detention basins at SeattleâTacoma International Airport (Figure 7). Source: Francis (2010). Figure 5. The spectrum of stormwater management infrastructure.
Introduction 7 Green Stormwater Infrastructure Green stormwater infrastructure maintains, mimics, or restores natural hydrology to manage and treat stormwater at or near its source. GSI can involve the use of various structural BMPs such as bioswales, green roofs, constructed wetlands, or bioretention. These BMPs promote infiltration and evapotranspiration, reducing runoff and reducing peak flows. Use of these BMPs leads to improved water quality and reduces erosion and sedimentation. Table 4 summarizes common GSI BMPs and their potential airport application. Source: Port of Seattle (n.d.). Figure 6. Concrete-lined detention basin at SeattleâTacoma International Airport. Source: Port of Seattle (n.d.). Figure 7. Proprietary stormwater treatment system at SeattleâTacoma International Airport.
8 Green Stormwater Infrastructure GSI Definition Bioretention Process consisting of shallow, vegetated basins that collect and absorb runoff from rooftops, sidewalks, and streets. Mimics natural hydrology by infiltrating and evapotranspiring runoff. Other common names: bioretention cells, rain gardens, biofiltration cells. Green Roofs Rooftops covered with growing media and vegetation that enable rainfall infiltration and evapotranspiration of stored water. Particularly cost effective in dense urban areas where land values are high. Harvesting and Reuse Process consisting of receptacles or tanks to collect and store rainfall for later use. Systems include rain barrels, tanks, or cisterns. Infiltration Galleries Gravel/rock-filled trench that receives stormwater runoff to encourage infiltration to groundwater. Before entering the trench, stormwater runoff passes through some combination of pretreatment measures, such as a bioswale and/or detention basin. Porous Pavement Paved surfaces that infiltrate, treat, and/or store rainwater where it falls. May be constructed from pervious concrete, porous asphalt, or permeable interlocking pavers, among several other materials. Particularly cost effective where land values are high and where flooding or icing is a problem. Sand Filters Basins that capture stormwater runoff and then filter the runoff through a bed of sand in the floor of the facility. Filter Strips Vegetated areas with shallow slopes that are designed to treat runoff as overland sheet flow. Bioswales Vegetated, mulched, or xeriscaped channels that provide treatment and retention as they move stormwater from one place to another. Bioswales slow, infiltrate, and filter stormwater flows. As linear features, bioswales are particularly suitable along streets and parking lots. Wetland Treatment Systems Treatment systems designed to improve water quality through natural processes involving wetland vegetation, soils, and their associated microbial assemblages. Often less expensive to build than traditional wastewater treatment options, wetland treatment systems have low operation and maintenance expenses. Source: U.S. EPA (2015, 2013). Table 4. GSI BMP definitions.
