Pennsylvania Handbook of Best Management Practices for Developing Areas

§6. BMP Retrofitting -- Remediating Existing Problems

§6.1 Introduction
§6.2 The Benefits and Appropriate Circumstances for Retrofitting BMPs
§6.3 Effectiveness of Retrofitted BMPs
§6.4 Incorporating BMPs into Existing Developments
§6.4.1 Retrofitting Filter Strips
§6.4.2 Retrofitting Infiltration Devices
§6.4 3 Retrofitting Detention Devices

§6.1 Introduction

Previous sections described incorporating BMPs into new development and redevelopment to minimize the adverse effects of the developments. Stormwater BMPs also may be incorporated in existing developments to remedy problems. Retrofitting may require minor changes to an existing facility, such as a detention basin, to improve water-quality mitigation functions or require constructing a new facility or device.

Retrofitting may be most feasible in upgrading a stormwater facility that was designed without fully considering multipurpose benefits. For example, many detention facilities constructed in the 1970s and 1980s were designed to address only flooding. As a result, many of the facilities do very little to mitigate water quality, do not effectively control small flood events smaller than a 2-year storm, and offer little in the way of aesthetic or recreational benefits.

Recently, local governments have been increasingly interested in retrofitting detention basins that are difficult to maintain, have internal drainage problems, or contain excess sediment or noxious vegetation. Retrofitting may be implemented to address local concerns as well as downstream water-quality concerns.

This section addresses three questions about achieving stormwater BMP objectives in developed areas:

1. What are the benefits and appropriate circumstances for retrofitting BMPs?
2. How can BMPs be retrofitted into existing developments?
3. What other measures can be used to achieve water-quality objectives in existing developed areas?

§6.2 The Benefits and Appropriate Circumstances for Retrofitting BMPs

Incorporating BMPs in existing developments can reduce the adverse hydrologic, hydraulic, and water-quality effects that the developments generate. Before 1970, drainage requirements for new development focused almost exclusively on conveying stormwater safely off the site. Drainage was achieved by using storm sewers, swales, and ditches. In fact, in many suburban areas, swale drainage was much more common than it is currently. As a result, pollutants are filtered and runoff volume reduced to a limited degree in many older developments. Without detention, however, the older areas commonly have serious local drainage problems and contribute to downstream flooding and erosion problems. Retrofitting detention basins or other storage devices into vacant lots or park sites could effectively address both stormwater quality and drainage problems. Because of the expense of excavating, this type of retrofitting has occurred only to a limited degree in the region. Some older communities where the houses are subject to basement flooding have constructed expensive underground storage devices; the facilities are not designed to address stormwater pollution, however.

Since the early 1970s, most new developments have been constructed with curb and gutter drainage and stormwater detention. However, most older detention facilities were designed to control the discharge rate for the 10-year, 25-year, and 100-year flood events. Smaller storms, which are responsible for stormwater pollution and channel erosion, were not addressed in the designs. Most existing wet-bottom basins, however, remove significant amounts of pollutants although the basins were not explicitly designed to remove pollutants. Important opportunities exist for retrofitting dry-bottom facilities, following the design standards of Section 3, and retrofitting the outlet controls of wet-bottom facilities.

Another important benefit of BMP retrofitting is remedying local nuisance conditions, maintenance problems, and aesthetic concerns. Many local governments, park districts, corporations, or homeowners associations that are responsible for maintaining detention and drainage systems have been challenged to remedy the problems. Often, poorly designed or maintained facilities could be dramatically improved and protected through effective retrofitting.

Numerous problem dry-bottom facilities have been identified throughout the state. Problems typically encountered are excessive sedimentation from construction-site erosion; invasion by nuisance vegetation; erosion along concrete low-flow channels; undesired ponding caused by unplanned backwater conditions or poor grading; litter, debris, and other aesthetic concerns; and obstructed outlet structures. In addition, wet-bottom facilities commonly have other problems, including shoreline erosion, excess algae and aquatic vegetation, and excessive turbidity. Many of the problems could be fixed by effectively retrofitting the facility and installing watershed-control measures.

Retrofitting BMPs can:

• Improve the multi-use functions and appearance of existing facilities and reduce maintenance needs
• Reduce the pollutant loadings to downstream waterbodies and wetlands
• Reduce downstream storm peaks and flow velocities which may be causing streambank erosion

§6.3 Effectiveness of Retrofitted BMPs

Ultimately, BMPs designed on the basis of the recommendations in Section 8 should be as effective when retrofitted into existing developments as they are in new developments. However, because of site constraints, BMPs may not be able to be incorporated to the same level or to the same standards as recommended. For example, to maximize the pollutant-removal effectiveness of an existing detention basin, the amount of storage available during large flood events may need to be reduced. This is an unacceptable alternative in most communities. Site constraints may necessitate using a series of BMPs that are smaller but still effective.

When designing a system of retrofit BMPs to achieve a target removal rate, using a water-quality model to estimate loads to and removal rates from various devices may be beneficial. Modeling is particularly beneficial when methods that rely on settling are being used in series because removal efficiency decreases with each successive device. The heaviest particles are removed in the first device and the smaller particles, which are more difficult to remove, are passed to the next device.

§6.4 Incorporating BMPs into Existing Developments

BMPs can be retrofitted into existing developments by using the same design concepts described for new developments. The major accommodation for existing developments is the need for greater ingenuity in identifying locations and opportunities for the individual BMPs. The following paragraphs briefly describe some common opportunities for retrofitting some of the BMPs discussed in Section 8.

§6.4.1 Retrofitting Filter Strips

Filter strips can be readily incorporated into some existing developments if relatively large vegetated surfaces can be used. On individual lots, the benefits of filter strips can be realized by re-routing rooftop and sump-pump discharges across the lawns. Care must be taken to extend discharge points away from foundations. Discharge points can be extended by using a variety of commonly available devices, from elbows and downspout extensions to splash pads.

For large development sites, runoff from paved areas can receive the benefits of filter strips if the paved and grassed surfaces are graded to route drainage to and across vegetated areas. Sometimes, the rerouting requires only removing or slotting the curbs along the edge of roads or parking lots. The capacity of existing swales for conveying runoff also may have to be assessed, and expanding them where necessary. Parking lots with vegetated aisle dividers may be particularly amenable to this type of filter strip development.

§6.4.2 Retrofitting Infiltration Devices

Infiltration measures, including infiltration trenches, permeable pavement, and bioretention, can be introduced at most sites where the soil permeability and depth to groundwater are sufficient. Prime areas for developing bioretention facilities include natural depressions and roadside swales. Parking lots, fire lanes, and other paved surfaces with low traffic can become infiltration areas by using permeable pavements. Infiltration trenches can be employed effectively along the downgradient edge of parking lots, where they can also be interconnected to permeable pavement. Alternatively, infiltration trenches can be developed along parts of an existing drainage system by reconstructing the drainageway with appropriate porous materials. For example, drain pipes can be replaced with infiltration trenches.

§6.4.3 Retrofitting Detention Devices

Roofs are one of the largest sources of concentrated runoff from developed sites. If runoff is retarded at the source, pressure can be taken off of existing undersized detention basins. Therefore, rooftop runoff management should be considered as part of any effort to retrofit runoff-peak detention in highly urbanized areas. In general, effective rooftop runoff management will require that buildings be flat-roofed. Recent advances in geosynthetic technology, now make retrofitting traditionally constructed tar roofs with vegetated covers cost-effective. Vegetated roof covers are a proven technology in central Europe where urban population density is higher than in most American cities. Several European cities, in an effort to reduce the overloading of existing urban drainage systems, provide incentives for homeowners to install vegetated roof covers. Another approach for managing rooftop runoff is to direct water to a dry well with detention capacity or to a below-grade detention basin.

Either existing detention basins can be retrofitted to improve their pollutant-removal characteristics or new detention basins can be retrofitted into existing developments. Basins designed primarily for preventing floods often can be retrofitted to provide greater hydrologic and water-quality benefits. Recommendations for significantly enhancing the achievement of BMP objectives include:

• Modifying the outfall to create a two-stage release to better control small while not significantly compromising the major storm detention required for flood control
• Eliminating paved low-flow channels and replacing them with meandering vegetated swales
• Eliminating low-flow bypasses
• Incorporating low berms to lengthen the flow path and eliminate short-circuiting
• Incorporating stilling and settling basin at inlets
• Regrading the basin bottom to create a wetland area near the outlet or revegetating parts of the basin bottom with wetland vegetation to enhance pollutant removal, reduce mowing, and improve aesthetics
• Creating a wetland shelf along the periphery of a wet basin to improve shoreline stabilization, enhance pollutant filtering, and enhance aesthetic and habitat functions

Although retrofitting can effectively reduce existing water-quality problems and address certain maintenance or operation problems of detention basins, additional measures sometimes are needed. In particular, retrofitting will not eliminate the need for effectively maintaining detention basins. Vegetation management (e.g., mowing, burning, replanting), occasional sediment removal, and inspection and cleaning of outlet structures should be part of a long-term maintenance plan for all detention basins.

Watershed controls also should be considered with retrofitting activities. In particular, source controls for household wastes, fertilizers, and pesticides can dramatically reduce contributions of problematic pollutants that adversely affect both multipurpose detention facilities and downstream water quality. For example, reducing fertilizer application in the watershed draining to a wet-detention facility will reduce the potential for eutrophication of that facility. Similarly, effectively enforcing erosion- and sediment-control standards will eliminate excessive sediment loadings to online detention basins, which store runoff from developing lands.