Retrofit Challenges

Physical Conditions Affecting Retrofit Feasibility

The primary challenges of using stormwater BMPs for retrofits are that most urbanized areas are characterized by highly connected impervious surfaces, aged infrastructure, and limited pervious or open areas. These challenges are often compounded further when the open space that is available is not particularly amenable to stormwater BMPs. Knowing the physical attributes and constraints within an area of interest will assist in evaluating the practicability of using specific stormwater BMPs for that area. Therefore, geographic data should be collected and analyzed on the following physical attributes:

  • Impervious areas (including impervious areas that could potentially be removed): Impervious areas can generally be defined as any location where the natural infiltration of water into the soil is inhibited. Impervious areas contribute nearly all of the stormwater runoff that enters the sewer system and are therefore the areas requiring the most attention when devising a BMP retrofit plan. When assessing impervious areas it is also important to understand their interconnectivity, as well as their connectivity to pervious areas and the stormwater conveyance system. It is also important to identify areas where impervious areas can be removed.

    Functionally impervious soils are also a common challenge when retrofitting urban areas. Soils may be functionally impervious because of a high clay content, a high percentage of urban fill, or because of a high degree of compaction from past construction activities. By not allowing percolation into the subsoil, functionally impervious soils may prevent significant volume reductions from occurring in infiltration BMPs such as rain gardens.

  • Pervious areas: Although often less prevalent than impervious areas, pervious surfaces are also present in the urban environment. Pervious areas often provide the best opportunities for incorporating stormwater BMPs, but the use of and demand for these limited spaces is often high. Urban pervious areas include landscaped areas, vacant lots, parks and playfields/lawns, and urban open space corridors.
  • Utilities: Stormwater BMPs with underdrains or overflows should ideally be located close to the drainage system so as to minimize piping costs, reduce chances for utility conflicts, minimize disturbed areas, and cut construction times. Locating BMPs near stormwater utilities is generally not a problem in urban areas. As with proximity to the drainage system, providing adequate vertical relief through siting or site modifications is important for stormwater BMPs that depend on the transportation of the effluent and/or the influent input flows from a different location through conduits such as pipes or open channels.
  • Building locations: One of the primary drivers for selecting and sizing a stormwater BMP is the existing infrastructure. Concern over the structural integrity of building foundations, roadways, bridge abutments, and retaining walls may discourage the use of certain BMPs. Design variations such as lining infiltration practices and providing underdrains can be used to prevent any potential damage to foundations from infiltrating stormwater.
  • Known groundwater contamination or hazardous waste facilities: Unrestricted infiltration is not advisable in cases where existing soils are contaminated because of the potential to leach contaminants into groundwater supplies. In situations where existing soils are determined to be polluted, infiltration practices are often avoided in favor of other volume reduction controls that utilize evaporation/evapotranspiration and storage/reuse processes. While there may be opportunities for incorporating stormwater BMPs during Brownfield development projects, the use of infiltration in these areas may be limited unless polluted soils are first removed or remediated or lined systems are used.
  • Environmentally critical areas (steep slopes): Results in accelerated erosion from runoff, soil loss, and changes in natural topography and drainage patterns. The disturbance of steep slopes in retrofit situations should be restricted or prevented because of the water quality impacts that could be incurred.

Perceived Conditions Affecting Retrofit Feasibility

Public acceptance is an important consideration because the successful and efficient operation of stormwater BMPs is facilitated in an atmosphere of cooperation and partnership with the people who use the spaces where the controls are located. Also, mass opposition to the construction and implementation of stormwater BMPs can lead to projects being delayed or abandoned altogether. Therefore, public perceptions and concerns about hydrologic performance (soil characteristics, standing water, and public health issues) safety, construction related inconveniences, and maintenance needs are challenges to overcome for community acceptance and implementation of stormwater BMPs in the urban environment. Community outreach consisting of public meetings and educative signage providing the benefits associated with stormwater BMPs can be used to assuage public concerns.


Maintenance is required for the long term performance of stormwater BMPs. Therefore, maintenance considerations must not be left until after construction is completed, but rather be planned and incorporated into the design process. Maintenance requirements for stormwater BMPs typically consist of the following activities:

  • Landscaping and vegetation management
  • Sediment and accumulated pollutant removal;
  • Structural repairs to BMP components
  • Regular inspections
  • Restoration and/or rejuvenation of BMP components (i.e., scarifying infiltration beds)
  • Repair of inlet and outlet structures and other BMP amenities and flow control structures
  • Waterproofing and/or BMP liner replacement/repair

Maintenance factors to be considered for each stormwater BMP are provided below.

Maintenance Factors to be Considered for Stormwater BMPs
BMP Responsibility Level of Effort Frequency
Downspout Disconnection Owner Minimal: No vegetation management; no removal of captured pollutants. Low: Provides few water quality benefits.
Filter Strips Owner Moderate: Management of vegetation; occasional removal of captured pollutants. Moderate: Need vegetation management.
Infiltration Practices Owner Moderate to High: Rejuvenation may be needed (scarifying surface/raking); possible removal of vegetation; removal of captured materials; media may have to be completely changed. Low: Need removal of accumulated pollutants. Need occasional vegetation management. Pre-treatment is very important.
Pocket Wetlands Owner Moderate to High: Management of vegetation; removal of floating debris and trash; sediment and vegetation removal. Low: Need removal of accumulated pollutants. Need occasional vegetation management. Sediment forebay.
Porous Pavement Owner/Municipality Moderate to High: Rejuvenation may be needed (vacuum sweeper/power washing); vegetation management; pavement may have to be completely changed. Low: May need vegetation management.
Rain Barrels/Cisterns Owner Minimal: No vegetation management; no removal of captured pollutants. Requires periodic emptying. Treatment for mosquitoes may be necessary. Low: Provide little or no water quality benefits.
Rain Gardens Owner Minimal to Moderate: Vegetation management required. Moderate: Need vegetation management.
Rooftop Storage Owner Minimal: No vegetation removal; minimal removal of captured pollutants. Low: Provides little pollutant removal.
Tree Box Filters Owner Moderate: Minimal removal of debris. Low: May require vegetation management.
Planter Boxes Owner Low: Underdrains may need to be replaced if clogged. Low: Periodic removal of weeds for aesthetics.
Vegetated Roofs Owner/Municipality Moderate: Vegetation management Moderate: Require vegetation management.
Vegetated Swales Owner Low to Moderate: Minimal removal of captured pollutants; vegetation management. Moderate: Require vegetation management; may require removal of accumulated material.

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Many factors influence the feasibility of using individual stormwater practices. Site constraints, performance objectives, and design preferences affect BMP selection. A summary of design considerations for each BMP is provided below.

Feasibility Assessment of Using Stormwater BMPs for Urban Retrofits
BMP Reliability / Confidence in Fundamental Design Design Flexibility
Downspout Disconnection Potential for large reduction in directly connected impervious area, but must infiltrate or provide storage May be used with many other source controls.
Filter Strips Provide hydrologic control depending on design. Sediment can be resuspended during large storms. Many options for incorporating into landscaped environment. Higher WQ performance associated with check dams, shallow slopes, and low velocities. Soil amendments can be used to improve infiltration and retention capacity.
Infiltration Practices Prone to clogging and bypass if not maintained frequently or sedimentation pretreatment not provided. Trenches more amenable to urban environment than basins. Various soil amendments possible to improve pollutant adsorption capacity. Can be used in conjunction with detention.
Planter Boxes Low potential for clogging. Provides volume reduction and peak discharge control. Can be integrated into most building landscape designs and practically any shape or configuration is possible.
Pocket Wetlands Effluent quality generally good for most parameters. May release nutrients during dormant periods. Some detention provided, but negligible volume reduction. More difficult than some practices to incorporate into the ultra-urban environment.
Porous Pavement Volume losses possible from clogging; requires vacuum sweeping or power washing to restore hydraulic capacity. Should be designed to infiltrate incident rainfall only, however, porous shoulders may receive runoff from the primary travel lanes.
Rain Barrels / Cisterns Typically smaller storage volumes. Minimal hydrologic benefits. Installed above or below ground, as well as on rooftops. Below ground may require pumping for reuse.
Rain Gardens Substantial hydrologic benefits possible due to retention, infiltration, and evapotranspiration. Pollutant reductions may also be high due to plant uptake and microbial degradation. Very flexible in that practically any shape is possible. Many options for incorporating into landscaped environment. Underdrains may be installed for low permeable soils.
Rooftop Storage Provide good peak control for small storm events, but no water quality benefits. Not many options for design alternatives, but easy to incorporate into urban environment. Innovative outlet controls possible.
Tree Box Filters Similar to rain gardens, with underdrain. Similar to rain gardens with underdrain.
Vegetated Roofs Substantial hydrologic benefits possible due to retention and evapotranspiration, but benefits are realized primarily for small, frequently-occurring storm events. Many options for design and configuration of vegetation. Roof gardens where plants are placed in pots and planter boxes are one design variation.
Vegetated Swales Substantial hydrologic control within a conveyance system. Many options for incorporating into landscaped environment. Higher WQ performance associated with check dams, shallow slopes, and low velocities.

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