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A. Existing Development Management

Develop and implement watershed management programs to reduce runoff pollutant concentrations and volumes from existing development:

  1. Identify priority local and/or regional watershed pollutant reduction opportunities, e.g., improvements to existing urban runoff control structures;
  2. Contain a schedule for implementing appropriate controls;
  3. Limit destruction of natural conveyance systems; and
  4. Where appropriate, preserve, enhance, or establish buffers along surface waterbodies and their tributaries.

1. Applicability

This management measure is intended to be applied by States to all urban areas and existing development in order to reduce surface water runoff pollutant loadings from such areas. Under the Coastal Zone Act Reauthorization Amendments of 1990, States are subject to a number of requirements as they develop coastal NPS programs in conformity with this management measure and will have flexibility in doing so. The application of management measures by States is described more fully in Coastal Nonpoint Pollution Control Program: Program Development and Approval Guidance, published jointly by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA).

2. Description

The purpose of this management measure is to protect or improve surface water quality by the development and implementation of watershed management programs that pursue the following objectives:

  1. Reduce surface water runoff pollution loadings from areas where development has already occurred;
  2. Limit surface water runoff volumes in order to minimize sediment loadings resulting from the erosion of streambanks and other natural conveyance systems; and
  3. Preserve, enhance, or establish buffers that provide water quality benefits along waterbodies and their tributaries.
Maintenance of water quality becomes increasingly difficult as areas of impervious surface increase and urbanization occurs. For the purpose of this guidance, urbanized areas are those areas where the presence of "man-made" impervious surfaces results in increased peak runoff volumes and pollutant loadings that permanently alter one or more of the following: stream channels, natural drainageways, and in-stream and adjacent riparian habitat so that predevelopment aquatic flora and fauna are eliminated or reduced to unsustainable levels and predevelopment water quality has been degraded. Increased bank cutting, streambed scouring, siltation damaging to aquatic flora and fauna, increases in water temperature, decreases in dissolved oxygen, changes to the natural structure and flow of the stream or river, and the presence of anthropogenic pollutants that are not generated from agricultural activities, in general, are indications of urbanization.

The effects of urbanization have been well described in the introduction to this chapter. Protection of water quality in urbanized areas is difficult because of a range of factors. These factors include diverse pollutant loadings, large runoff volumes, limited areas suitable for surface water runoff treatment systems, high implementation costs associated with structural controls, and the destruction or absence of buffer zones that can filter pollutants and prevent the destabilization of streambanks and shorelines.

As discussed in Section II.B of this chapter, comprehensive watershed planning facilitates integration of source reduction activities and treatment strategies to mitigate the effects of urban runoff. Through the use of watershed management, States and local governments can identify local water quality objectives and focus resources on control of specific pollutants and sources. Watershed plans typically incorporate a combination of nonstructural and structural practices.

An important nonstructural component of many watershed management plans is the identification and preservation of buffers and natural systems. These areas help to maintain and improve surface water quality by filtering and infiltrating urban runoff. In areas of existing development, natural buffers and conveyance systems may have been altered as urbanization occurred. Where possible and appropriate, additional impacts to these areas should be minimized and if degraded, the functions of these areas restored. The preservation, enhancement, or establishment of buffers along waterbodies is generally recommended throughout the section 6217 management area as an important tool for reducing NPS impacts. The establishment and protection of buffers, however, is most appropriate along surface waterbodies and their tributaries where water quality and the biological integrity of the waterbody is dependent on the presence of an adequate buffer/riparian area. Buffers may be necessary where the buffer/riparian area (1) reduces significant NPS pollutant loadings, (2) provides habitat necessary to maintain the biological integrity of the receiving water, and (3) reduces undesirable thermal impacts to the waterbody. For a discussion of protection and restoration of wetlands and riparian areas, refer to Chapter 7.

Institutional controls, such as permits, inspection, and operation and maintenance requirements, are also essential components of a watershed management program. The effectiveness of many of the practices described in this chapter is dependent on administrative controls such as inspections. Without effective compliance mechanisms and operation and maintenance requirements, many of these practices will not perform satisfactorily.

Where existing development precludes the use of effective nonstructural controls, structural practices may be the only suitable option to decrease the NPS pollution loads generated from developed areas. In such situations, a watershed plan can be used to integrate the construction of new surface water runoff treatment structures and the retrofit of existing surface water runoff management systems.

Retrofitting is a process that involves the modification of existing surface water runoff control structures or surface water runoff conveyance systems, which were initially designed to control flooding, not to serve a water quality improvement function. By enlarging existing surface water runoff structures, changing the inflow and outflow characteristics of the device, and increasing detention times of the runoff, sediment and associated pollutants can be removed from the runoff. Retrofit of structural controls, however, is often the only feasible alternative for improving water quality in developed areas. Where the presence of existing development or financial constraints limits treatment options, targeting may be necessary to identify priority pollutants and select the most appropriate retrofits.

Once key pollutants have been identified, an achievable water quality target for the receiving water should be set to improve current levels based on an identified objective or to prevent degradation of current water quality. Extensive site evaluations should then be performed to assess the performance of existing surface water runoff management systems and to pinpoint low-cost structural changes or maintenance programs for improving pollutant-removal efficiency. Where flooding problems exist, water quality controls should be incorporated into the design of surface water runoff controls. Available land area is often limited in urban areas, and the lack of suitable areas will frequently restrict the use of conventional pond systems. In heavily urbanized areas, sand filters or water quality inlets with oil/grit separators may be appropriate for retrofits because they do not limit land usage.

3. Management Measure Selection

Components (1) and (2) of this management measure were selected so that local communities develop and implement watershed management programs. Watershed management programs are used throughout the 6217 management area although coverage is inconsistent among States and local governments (Puget Sound Water Quality Authority, 1986).

Local conditions, availability of funding, and problem pollutants vary widely in developed communities. Watershed management programs allow these communities to select and implement practices that best address local needs. The identification of priority and/or local regional pollutant reduction opportunities and schedules for implementing appropriate controls were selected as logical starting points in the process of instituting an institutional framework to address nonpoint source pollutant reductions.

Cost was also a major factor in the selection of this management measure. EPA acknowledges the high costs and other limitations inherent in treating existing sources to levels consistent with the standards set for developing areas. Suitable areas are often unavailable for structural treatment systems that can adequately protect receiving waters. The lack of universal cost-effective treatment options was a major factor in the selection of this management measure. EPA was also influenced by the frequent lack of funding for mandatory retrofitting and the extraordinarily high costs associated with the implementation of retention ponds and exfiltration systems in developed areas.

The use of retrofits has been encouraged because of proven water quality benefits. (Table 4-17 (22k) illustrates the effectiveness of structural runoff controls for developed areas and retrofitted structures.) Retrofits are currently being used by a number of States and local governments in the 6217 management area, including Maryland, Delaware, and South Carolina.

Management measure components (3) and (4) were selected to preserve, enhance, and establish areas within existing development that provide positive water quality benefits. Refer to the New Development and Site Planning Management Measures for the rationale used in selecting components (3) and (4) of this management measure.

4. Practices

As discussed more fully at the beginning of this chapter and in Chapter 1, the following practices are described for illustrative purposes only. State programs need not require implementation of these practices. However, as a practical matter, EPA anticipates that the management measure set forth above generally will be implemented by applying one or more management practices appropriate to the source, location, and climate. The practices set forth below have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the management measure described above.

  • a. Priority NPS pollutants should be targeted, and implementation strategies for mitigating the effects of NPS pollutants should be developed.

  • b. Policies, plans, and organizational structures that ensure that all surface water runoff management facilities are properly operated and maintained should be developed. Periodic monitoring and maintenance may be necessary to ensure proper operation and maintenance.

  • c. Remnant pervious areas in already-built areas should be subject to enforceable preservation requirements. For example, set green space goals to promote tree plantings and pavement reclamation projects.

  • d. Developed areas in need of local or regional structural solutions should be identified and put in priority order.

  • e. Regional structural solutions, retrofit opportunities, and nonstructural alternatives should be identified, inventoried, and put in priority order.

  • f. Where possible, modify existing surface water runoff management structures to address water quality.

  • g. As capital resources allow, implement practices such as those in Table 4-17 (22k).

    5. Effectiveness Information and Cost Information

    The following is a general description of various retrofit options and their effectiveness. Since each retrofit situation is different, the costs will depend on site-specific factors such as climate, drainage area, or pollutants. Table 4-17 (22k) discusses the effectiveness of several practices often implemented when correcting existing NPS pollution problems in urban areas.

    a. Construction or Modification of Pollutant Removal Facilities

    Many of the management practices described in Section II of this chapter cannot be used in already urbanized areas because they require space that is typically not available in urbanized areas. However, two types of pollutant removal retrofits can be used to treat runoff: new treatment facilities can be built in limited land space, and existing facilities can be modified to obtain increased water quality benefits.

    New Facilities. If there is space available, the management practices described in Section II can be applied to provide water quality benefits. Typically, however, there are space constraints in urbanized areas that will not allow construction of these facilities. Water quality inlets may be appropriate in areas where space is limited and runoff from highly impervious areas such as parking lots must be treated. The effectiveness and costs of these facilities would be similar to those previously discussed. There are several types of water quality inlets catch basins, catch basins with sand filters, and oil/grit separators. These are described in detail in Section II.

    Retrofit of Existing Facilities. In the past, many surface water runoff management facilities were constructed to provide peak volume control; however, no provisions for pollutant removal were provided. These existing facilities can be modified to provide water quality benefits. Two common modifications are dry pond conversion and fringe marsh creation.

    • Dry Pond Conversion. Many dry ponds for surface water runoff management that provide peak volume control, but no water quality benefits, have been constructed. Many of these ponds can be modified to provide water quality control. These modifications can include decreasing the size of the outlet to increase the detention of the dry pond. A dry pond's outlet may also be modified to detain a permanent pool of water and thus create a wet pond or extended detention wet pond. Prince George's County, Maryland, has a successful program for urban retrofits. They are usually off-line facilities with forebays, vegetative benches, and deeper portions for storage.

    • Fringe Marsh Creation. Aquatic vegetation can be planted along the perimeter of constructed wet ponds or other open water systems to enhance sediment control and provide some biological pollutant uptake.

    b. Stabilization of Shorelines, Stream Banks, and Channels

    Urbanization can significantly increase the volume and velocity of surface water runoff that has the potential to erode streambanks and channels. This erosion can create high sediment loads in surface water. Streambanks can be stabilized by providing plantings along the streambank or by placing boulders, riprap, retaining walls, or other structural controls in eroding areas. Where feasible, vegetation and other soft practices should be used instead of hard, structural practices. See the Shoreline and Streambank Protection section of Chapter 6 for additional information.

    c. Protection and Restoration of Riparian Forest and Wetland Areas

    Riparian forests and wetlands are very effective water quality controls. They should be protected and restored wherever possible. Riparian forests can be restored by replanting the banks and floodplains of a stream with native species to stabilize erodible soils and improve surface water and ground water quality. Refer to Chapter 7 for additional information.

    Some examples of urban watershed retrofit programs are presented below. The first case study, the Anacostia watershed, involves a developed urban area suffering from multiple NPS pollution impacts. As with many of the examples given, the project has advanced only through the planning and early implementation stages. Therefore, performance data are not currently available.


    Opportunities for urban retrofitting are limited in developed watersheds, but they can be implemented through extensive onsite evaluations. For example, between 1989 and 1991 over 125 sites in the 179-square-mile Anacostia watershed in Montgomery County, Maryland, were identified as candidates for retrofitting after extensive on-site evaluation (Schueler et al., 1991). Retrofit options developed in the watershed included source reduction, extended detention (ED) marsh ponds or ED ponds to handle the first flush, additional storage capacity in the open channel, routing of surface water runoff away from sensitive channels, diversion of the first flush to sand-peat filters, and installation of oil/grit separators in the drain network itself. The most commonly used retrofit technique in the Anacostia watershed is the retrofit of existing dry surface water runoff detention or flood control structures to improve their runoff storage and treatment capacity. Existing detention ponds are maintained by excavation, adding to the elevation of the embankment, or by construction of low-flow orifices. The newly created storage is used to provide a permanent pool, extended detention storage, or a shallow wetland. Nearly 20 such retrofits are in some stage of design or construction in the Anacostia watershed.


    (Stack and Belt, 1989)

    Loch Raven Reservoir, a water supply reservoir serving Baltimore, Maryland, had a eutrophication problem due to excessive phosphorus loads. To address this problem, the city examined the effectiveness of its existing phosphorus controls. They found that the more than 24 extended detention dry ponds that had been originally constructed for surface water runoff management had been designed to treat the once-in-10-year or once-in-100-year flood. The extended detention ponds were thus inefficient at treating runoff from frequent storm events, and the city was receiving few water quality benefits from these structures. Modifications, or retrofits, allowed the basins to collect runoff from smaller events and reduce pollutant loadings without affecting their capacity to contain runoff from larger storms.

    Difficulties in obtaining permission from private pond owners restricted the number of ponds with planned retrofits to six ponds owned by the county and one privately owned pond. Private owners were concerned about the maintenance costs associated with the retrofits. Changes to the ponds usually involved alteration of the size of the orifice of the low-flow release structure. Computer modeling was used to determine the minimum size that would not interfere with the pond's design criteria (i.e., containing the 2-, 10- and 100-year storms) while providing sufficient detention time to settle the majority of the solids in urban runoff from the more frequent storms. Each retrofit was tailored to the basin's unique outlet and site characteristics, and costs reflect the differences in approach. For example, one of the ponds was modified as a urban runoff wetland for an estimated cost of $27,800. Retrofits of dry ponds were the least expensive, with costs of less than about $2,000. Draining and dredging boosted the cost of retrofitting a wet pond with a clogged low-flow release structure to approximately $13,000.

    Monitoring of the performance of the retrofits during 12 storm events measured removal efficiencies for particulate matter of over 90 percent and removal efficiencies for total phosphorus of between 30 and 40 percent. All of the storms monitored were less than the 1-year storm, and detention times ranged from 1 to 5 hours. Trash debris collectors were effective at reducing clogging; thus no maintenance was necessary in the first year of operation.


    (Bennett and Heaney, 1991)

    Improper surface water runoff drainage practices have degraded the quality of Florida's Indian River Lagoon by increasing the volume of freshwater runoff to the estuarine receiving water, as well as increasing the loading of suspended solids. Draining of wetlands for urban and agricultural development has led to nutrient loading in the lagoon.

    The study area, typical of most Florida flatwood watersheds, was selected as a representative drainage catchment. EPA's Storm Water Management Model (SWMM) was used to summarize the relationship between catchment hydrology, channel hydraulics, and pollutant loads. The model, calibrated for the study region, was used to evaluate the effectiveness of the proposed watershed control program and to project performance levels expected after the study region becomes fully developed. The retrofit of multiple structural measures was undertaken as a demonstration-scale project. An existing trunk channel was modified to act as a wet detention basin. Flow from the trunk channel enters a partially disturbed, interdunal, freshwater wetland. The wetland system provides nutrient assimilation, additional water storage capacity, sediment attenuation, and enhanced evapotranspiration. SWMM predicted that the project will remove between 80 percent and 85 percent of the total suspended solids, depending on the level of future development. The cost of the project in 1989 dollars, including operation and monitoring costs over a 10-year period, was $198,960.

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    This page last updated October 4, 1999