3.2 Stormwater Management Design Strategies
Section 3.2 provides guidance on the Philadelphia Water Department’s (PWD's) integrated design approach (Section 3.2.1), which specifies the use of non-structural design (Section 3.2.2), disconnected impervious cover (DIC) (Section 3.2.3), and stormwater management practice (SMP) selection, layout, and design strategies (Section 3.2.4) to meet PWD Stormwater Regulations. Often, a combination of non-structural design, DIC, and SMP implementation will be required to meet the Stormwater Regulations.
Section 3.2 contains a significant amount of design guidance that the designer should use to integrate robust and cost-effective stormwater management into site designs in ways that achieve PWD’s key stormwater management goals of minimizing the harmful effects of flooding and maintaining the health of Philadelphia’s streams and rivers. Additionally, this Section contains general requirements and standards of which the designer must be aware.
PWD has developed an integrated design approach through which developers can meet the Stormwater Regulations for proposed development projects. The intent of the approach is to promote the use of stormwater management solutions that protect receiving waters in a cost-effective manner. The integrated design approach presented here is based on recommendations found within the Pennsylvania Department of Environmental Protection (PA DEP) Pennsylvania Stormwater Best Management Practice (BMP) Manual, with minor modifications for adaptation to the urban conditions in Philadelphia. For example, non-structural design, one of three major design strategies discussed in this Section, may be challenging to implement in cases where higher densities/intensities are proposed on small sites in densely developed areas. However, DIC opportunities, such as green roofs, may be more cost-effective in the highly dense areas of Philadelphia because of energy savings, retail value, and other factors. Additional informational resources on the economic benefits of incorporating green features into an urban environment can be found on the PWD Stormwater Plan Review website.
The process of integrating site development and stormwater management design begins with a comprehensive understanding of existing site conditions per a site assessment, as described in Section 3.1. The site assessment process allows the designer to identify key site and stormwater management design constraints and opportunities. For example, the designer may desire to locate a proposed building to preserve an existing large and mature tree or an area of existing native vegetation in good condition in order to obtain credits for preserving existing trees under §14-705(g) of the Philadelphia Zoning Code. In addition, low-lying areas on a site can be used for SMPs in order to minimize conveyance costs.
With an integrated design approach, the designer uses a combination of three primary strategies (non-structural design, DIC, and SMPs) to meet the Stormwater Regulations, as applicable, outlined in Chapter 1. These strategies are implemented initially in sequence, then in an iterative approach leading to formulation of a comprehensive site and stormwater management design as illustrated in Figure 3.2-1.
PWD places a high value on protecting sensitive and special value resources and preserving the natural systems and hydrologic functions that may be present on a site. Non-structural strategies, a primary characteristic of low-impact development, promote the treatment, infiltration, evaporation, and transpiration of precipitation close to where it falls, and are a primary means by which the designer works to preserve and protect high-value natural features. PWD recommends that the designer use non-structural design practices early in the site planning process to reduce the size and cost of stormwater management facilities. Implementing these practices involves the careful consideration of the project site’s predevelopment condition, topography, natural drainage systems, and landscaping to arrange site development features in ways that minimize the use of impervious cover and the disruption of existing natural features, and the use of construction staging strategies that limit disturbance and soil compaction.
When used in combination, non-structural strategies can result in a variety of environmental and financial benefits. In the designer’s interest, the use of non-structural design practices can reduce land clearing and grading costs, reduce the size and cost of stormwater management facilities, reduce the cost and scope of operations and maintenance, and increase property values. In some cases, these strategies can result in the preservation of open space and working lands, protection of natural systems, and the incorporation of existing site features, such as wetlands and stream corridors, which provide natural hydrologic and water quality functions in addition to fish and wildlife habitat.
While most development sites within the City of Philadelphia do not generally possess extensive natural systems, more modest natural systems and features may be of sufficient value to warrant preservation and integration within the development plan. These features may include mature trees or flowering shrubs, natural topography or rock outcroppings, or plant communities that protect slopes from erosion or act as buffers for streams or drainage ways. The designer must complete a site assessment, as described in Section 3.1, to better understand the physical features of an existing property before exploring non-structural design strategies.
Following the completion of the site assessment, the first step in the stormwater design process is to thoroughly consider the use of non-structural strategies, finding creative ways of incorporating built features around existing natural areas. Recommended non-structural strategies fall within three categories: protecting sensitive and special value resources, clustering and concentrating, and minimizing disturbance and maintenance.
Protect Sensitive and Special Value Resources
To minimize stormwater impacts, land development activities must avoid encroaching on areas that provide important natural stormwater functions, such as floodplains, wetlands, and riparian areas, and on areas that are especially sensitive to stormwater impacts, such as steep slopes. These features may not be widespread in the urban environment, but where they do exist, they must be identified and protected. By protecting sensitive and special value resources, the designer can make existing natural features an important and integral part of a development site, enhancing the development’s role in the landscape and the community and providing attractive amenities for future tenants or owners. Protecting these features can also reduce the amount of stormwater runoff discharged from the site.
Within Philadelphia, most development sites do not have extensive sensitive and special value resources due to the density and history of development in the region. Many of the features that provide hydrologic functions within the landscape have been removed, covered, or buried, and most native soils have been removed, compacted, contaminated, or replaced with low-value fill material and debris. For these reasons, it may be difficult to identify substantial resources or features for protection. This relative scarcity of existing resources, however, prompts PWD to recognize the value and function of less extensive natural areas, even to the extent of valuing an individual tree. PWD urges the designer to consider the preservation and enhancement of natural features present at any scale, as well as enhancements that may help to protect natural features adjacent to the site, such as creating buffer zones or stabilizing steep slopes.
Special Value Features
Trees and shrubs are highly effective at retaining precipitation through interception, and all plants further reduce runoff through evapotranspiration. Well-developed root systems help keep soil ecosystems healthy, enhance infiltration, and limit erosion. Naturally-occurring bioretention areas - small, sometimes saturated areas that sustain plant communities such as pocket wetlands and vernal pools - are effective filters that sequester contaminants and support microbes that decompose organic pollutants. These existing vegetated features should be strongly prioritized for preservation. On larger sites, existing drainage pathways, such as natural draws or swales, should be identified and used whenever possible to convey stormwater in the post-development condition. By identifying these features and integrating their preservation within the development plan, sites can benefit from improved quality and reduced volume of off-site stormwater discharges, while simultaneously providing the many benefits of natural vegetation including wildlife habitat, improved air quality, and reductions in the urban heat island effect.
When development sites are adjacent to streams or rivers, riparian buffer systems can protect and enhance streams by limiting erosion, filtering and sequestering pollutants, and providing habitat for wildlife. Buffers can be especially important along steep banks that are vulnerable to erosion, and serve to separate waterbodies from decorative landscape areas where fertilizers are applied and runoff carries nutrients to the open water. Streambeds, the disturbance of which is regulated by State and Federal regulations, support a variety of life and must be protected from trampling or other abuse. In urban areas where riparian habitat is limited, protecting and enhancing remaining streamside corridors is critical to avoiding further impacts to water quality and ecological health.
Natural Flow Pathways
Where natural flow pathways or depressions exist, the designer should consider using these systems to help manage site runoff. Planting or protecting existing, deep-rooted plant cover within these existing features can limit erosion. Most larger sites, unless highly disturbed, will possess natural drainage features that, when conditions allow, will sustain and support a diverse plant community while also slowing and filtering runoff before it reaches larger bodies of water. These flow pathways can be attractively integrated within the site’s landscaping, reducing irrigation demands, and providing valuable site amenities that require minimal maintenance. Plant choices should be selected from native species that are adapted to the hydrologic conditions expected within the channel. The designer should assess whether existing drainage features are regulated by State or Federal statutes prior to conducting planting within these areas.
Cluster and Concentrate
When assessing the programming needs of the development, the designer should make an effort to cluster and concentrate structures in order to build on the smallest area possible and minimize extensive directly connected impervious area (DCIA), reserving as much area as possible for “green” cover. By limiting the footprints of buildings, parking areas, and other DCIA, either through stacking or clustering structures on the site, the designer can leave larger areas open for green space programming without reducing gross density. This practice not only improves the ability of the site to manage stormwater, but also increases the opportunity for green amenities and enhances long-term property values. Multi-story buildings also have lower energy consumption per square foot of floor space, fetch higher rent compared with low-rise buildings, and retain the urban character of the city.
This practice is not highly applicable to small or single parcel developments, but is more conducive to larger master planning for neighborhoods, campuses for hospitals or educational institutions, or redevelopment of large brownfield sites. In these environments, designation of open spaces can provide enhanced access to shared amenities and promote community cohesion. Concentrating buildings can also reduce per unit construction costs and the cost of providing infrastructure and site circulation.
Minimize Disturbance and Maintenance
Builders and contractors must minimize unnecessary land disturbance in order to limit the movement and compaction of in situ soils and preserve existing vegetation. When planning and staging construction, the designer should work with contractors to avoid trampling or stockpiling where unnecessary, and to stay clear of special value and environmentally sensitive areas. Disturbed or compacted soils are less effective in supporting plant growth and promoting infiltration. Heavy equipment paths must be well marked to avoid unnecessary compaction of in situ soils in areas specified for open spaces, especially areas where infiltration is intended, and tree guards must be erected to prevent damage from construction vehicles. Site planners should also seek to conform to the existing topography to the greatest extent possible, limiting the cost of grading and planting, reducing soil compaction, and assuring that healthy topsoil remains on the surface. These practices will provide for more robust plant growth, speed the recovery of green spaces following construction, and require less maintenance in the long term.
Disturbed areas must be restored with native plant species that do not require chemical maintenance and are selected for the appropriate hydrologic regime. In some cases, it will be necessary to protect re-vegetated areas during the establishment period by erecting fences and limiting access.
Other Considerations Beyond Stormwater Regulations
Beyond the PWD Stormwater Regulations, applicants may want to consider other factors in the stormwater management design to meet immediate development and long term site needs. This may include designing the site such that it complies with both the Stormwater Regulations and Stormwater quantity and/or quality control requirements for LEED certification. Eligible projects may also choose to take advantage of various stormwater management based Zoning Bonuses to increase a building’s height and/or density. If there are large swaths of existing impervious area that will not be disturbed during construction the applicant may want to consider capturing these areas as well to maximize potential stormwater billing credits (Section 2.5)
How to Use Non-Structural Strategies to Help Comply With the Stormwater Regulations
The designer can use non-structural strategies to help comply with the Stormwater Regulations described in Chapter 1 in the following ways:
Water Quality and Channel Protection
Non-structural practices encourage minimizing the use of DCIA, thus reducing the volume of stormwater required to be managed. Additionally, Redevelopment projects that reduce impervious area within the limits of earth disturbance (excluding public right-of-way) by at least 20%, based on a comparison of predevelopment impervious area to post-development DCIA, are exempt from the Channel Protection requirement.
The use of non-structural practices will generally increase on-site stormwater retention and time of concentration, thus reducing the amount and peak flow rate of stormwater required to be managed. Additionally, Redevelopment projects that reduce impervious area within the limits of earth disturbance (excluding public right-of-way) by at least 20%, based on a comparison of predevelopment impervious area to post-development DCIA, are exempt from the Flood Control requirement.
How Non-Structural Design Strategies Influence the PWD Review and Approval Process
As described in Chapter 2, characteristics of a project will determine the Review Path required for stormwater management compliance. The amount of earth disturbance associated with a proposed project is an important characteristic that can be influenced by non-structural design. By minimizing the amount of earth disturbance, the designer can potentially change Review Paths. For example, a project that is outside of the Darby and Cobbs Creeks or Wissahickon Creek Watersheds and that is able to reduce the amount of earth disturbance to less than 15,000 square feet will be eligible for a Development Exemption Review. After using all possible non-structural strategies to minimize earth disturbance, the designer should refer back to Chapter 2 to confirm the Review Path for the project.
This Section includes guidance for discharging stormwater runoff from impervious surface and discusses techniques for reducing DCIA through disconnection. Depending on the configuration, all, or a portion, of DIC may be deducted from the post-development impervious cover on a site, leading to an elimination of, or reduction in, total site DCIA. Further, by incorporating DIC into the design of a Redevelopment project, developers may be eligible for an Expedited PCSMP Review. Section 2.4 details the criteria for Expedited PCSMP Review eligibility. The Online Technical Worksheet guides the designer through this stage of the design process and assists in analysis of post-development impervious area, DIC, and ultimate calculation of total site DCIA. All proposed DIC must be documented in the PCSMP Submission Package (Section 2.3.1).
PWD distinguishes between impervious cover from which runoff is directed toward pervious areas for management within the landscape (DIC) and impervious cover from which runoff is directed toward SMPs with discharge/overflow connections to the sewer (DCIA). Disconnection opportunities depend on incorporating sufficient pervious areas into a site layout. Completing a site assessment (Section 3.1) will help to characterize the nature and extent of existing pervious areas on a site that can be used for impervious area disconnections. Disconnection strategies are described in the following Sections.
A reduction in DCIA is permitted when a roof downspout is directed to a vegetated area that allows for infiltration, filtration, and increased time of concentration. PCSMP approval issued by PWD Stormwater Plan Review may support the designer in his or her request to obtain relevant and necessary City of Philadelphia Plumbing Code variances for approved rooftop disconnections. The designer is advised to contact the City of Philadelphia Department of Licenses and Inspections (L&I) to confirm the Plumbing Code requirements associated with the disconnection of roof leaders. Under certain circumstances, drainage to an approved point of disposal, SMP, or open space is allowed under the Plumbing Code.
A rooftop is considered to be completely, or partially, disconnected if it meets all of the following requirements:
- The contributing area of rooftop to each disconnected discharge must be 500 square feet or less.
- The soil of the pervious area must not be designated as a hydrologic soil group “D” or equivalent.
- The overland flow path of the pervious area must have a slope of 5% or less.
For designs that meet these requirements, the portion of the roof that may be considered disconnected depends on the length of the overland path as designated in Table 3.2-1.
|Length of Pervious Flow Path*
|Roof Area Treated as Disconnected
(% of contributing roof area)
|75 or more||100|
*Flow path cannot include DCIA, must be at least 15 feet from any ground-level impervious surfaces, and must be continuous starting from the point of roof leader discharge. Two roof leaders cannot discharge to the same flow path for disconnection credit.
For example, consider a 1,000-square foot roof with two roof leaders, each draining an area of 500 square feet (Figure 3.2-2). Both roof leaders discharge to a lawn. The lawn has type B soils and a slope of 3%. The distance from the downspout discharge point to the street is 65 feet. Therefore, based on Table 3.2-1, 80% of the roof area may be considered disconnected and treated as pervious cover when calculating stormwater management requirements. Disconnecting the roof leaders will significantly reduce the size and cost of stormwater management facilities at this site.
A reduction in DCIA is permitted when pavement runoff is directed to a vegetated area that allows for infiltration, filtration, and an increased time of concentration. This method is generally applicable to small or narrow pavement structures such as driveways and narrow pathways through otherwise pervious areas (e.g., a trail through a park).
Pavement is considered to be completely, or partially, disconnected if it meets all of the following requirements:
- The contributing flow path over impervious pavement must be no more than 75 feet.
- The length of overland flow over pervious areas must be greater than, or equal to, the length of the contributing flow path over impervious pavement.
- The overland flow must be non-concentrated sheet flow over a vegetated area (flow through a swale is not eligible for pavement disconnection credit).
- The soil of the pervious area must not be designated as a hydrologic soil group “D” or equivalent.
- The contributing impervious area must have a slope of 5% or less.
- The overland flow path of the pervious area must have a slope of 5% or less.
- If discharge is concentrated at one or more discrete points, no more than 1,000 square feet may discharge to any one point. In addition, an erosion control measure, such as a gravel strip, is required for concentrated discharges. Erosion control measures are not required for non-concentrated discharges along the entire edge of pavement; however, there must be provision for the establishment of vegetation along the pavement edge and temporary stabilization of the area until vegetation becomes established.
When choosing pavement disconnections, the designer should consider the impact of directing runoff from adjacent impervious areas on the pervious area. Disconnecting larger areas of pavement along stream banks and other potentially erosive or sensitive areas may necessitate additional measures to be taken beyond meeting the minimum requirements.
Tree Disconnection Credit
A reduction in DCIA is permitted when existing or newly proposed tree canopy from an approved species list extends over, or is in close proximity to, impervious area. Trees planted in vegetated practices, such as bioinfiltration/bioretention areas, and that meet the requirements set forth in this Section can be used toward tree disconnection credit.
Existing tree disconnection credit may be applied for a reduction in DCIA if it meets the following requirements:
- The existing tree species cannot be one of the invasive species included in Table I-2: Common Invasive Species of the Mid-Atlantic Region (Appendix I).
- The existing tree must be at least four-inch caliper.
- Existing tree canopies must be field measured, and tree location, size, and species must be indicated on submitted plans. Alternatively, an annotated aerial photo clearly showing the existing tree canopy limits must be submitted.
- Only impervious area located directly under the tree canopy area can be considered disconnected.
- Overlapping existing tree canopy area cannot be counted twice toward disconnection credit.
New tree disconnection credit may be applied for a reduction in DCIA if it meets the following requirements:
- The proposed tree species must be chosen from the approved plant list (Appendix I).
- New trees must be planted within ten feet of ground-level impervious area, within the limits of earth disturbance, and outside of the public right-of-way.
- New deciduous trees must be at least two-inch caliper.
- New evergreen trees must be at least six feet tall.
- A 100-square foot DCIA reduction is permitted for each new tree. This credit may only be applied to the impervious area adjacent to the tree.
- Overlapping 100-square foot DCIA reduction areas corresponding to adjacent new trees cannot be counted twice toward disconnection credit.
The maximum reduction permitted for both new and existing trees is 25% of ground-level impervious area within the limits of earth disturbance, unless the width of the impervious area is less than ten feet. Up to 100% of narrow impervious areas (e.g., sidewalks and trails) may be disconnected through the application of tree credits.
A reduction in DCIA is permitted when a green roof is installed on a proposed building and when the design, construction, and maintenance plans meet the minimum requirements specified in Section 4.3. To encourage the use of this technology, the entire extent of the green roof area may be considered DIC. However, since a green roof is not a zero discharge system, the remaining site design must safely convey roof runoff from larger storm events to an approved point of discharge. When performing calculations for Flood Control and Public Health and Safety (PHS) Release Rate requirements, green roof discharge (i.e., overflows) must be modeled using appropriate Natural Resources Conservation Service (NRCS) runoff Curve Number (CN) values for green roof areas as described in Section 3.4.3. The designer is referred to Section 4.3 for more information on green roofs.
PWD recognizes two types of porous pavement systems that can be used to achieve compliance with the Stormwater Regulations: porous pavement DIC areas receiving direct rainfall only; and porous pavement over a structural SMP, which is designed to manage direct rainfall and concentrated runoff from adjacent DCIA.
Porous pavement can be considered DIC when it does not create any areas of concentrated infiltration and does not receive runoff from any adjacent impervious areas.
Porous pavement over structural SMPs is not considered DIC, and therefore must be designed pursuant to the requirements of either a subsurface infiltration (Section 4.4) or subsurface detention (Section 4.8) SMP, depending upon the feasibility of infiltration.
For disconnection credit, the design, construction, and maintenance plan must meet the minimum requirements for porous pavement DIC, as specified in Section 4.2. When performing calculations for Flood Control and PHS Release Rate requirements, appropriate CN values must be used for porous pavements, as described in Section 3.4.3.
There is a broad range of additional applications, including proprietary products, which may be suitable for receipt of disconnection credits. Many of these products will require the use of an appropriate sub-base to allow for storage and infiltration and must generally be installed above non-compacted soil. In most cases, underdrain systems are not required for DIC. The designer must consult with PWD Stormwater Plan Review for specific performance or installation parameters. Potential applications include, but are not limited to, the following:
- Trails (Section 3.5.4);
- Synthetic turf surfaces for athletic fields (Section 3.5.5);
- Porous safety surfaces as found in play lots;
- Geogrid systems or other similar soil reinforcements;
- Pervious decking installed over a porous surface; and/or
- Paving tiles with porous grout or gaps.
How to Use Disconnection Strategies to Help Comply With the Stormwater Regulations
The designer can use DIC to help comply with the Stormwater Regulations described in Chapter 1 in the following ways:
Water Quality and Channel Protection
Impervious area that meets the disconnection criteria for the various strategies described above is considered DIC and is therefore no longer subject to the Water Quality and Channel Protection requirements for treatment of on-site DCIA. Implementing DIC can be an excellent strategy for managing small areas of DCIA for which routing the runoff to the proposed SMP is not feasible, such as porches, steps, concrete pads, walkways, or impervious cover atop utility trenching, etc. Additionally, Redevelopment projects that reduce impervious area within the limits of earth disturbance (excluding public right-of-way) by at least 20%, based on a comparison of predevelopment impervious area to post-development DCIA, are exempt from the Channel Protection requirement.
The use of some disconnection strategies such as green roofs and porous pavements will generally increase on-site stormwater retention, thus reducing the amount and peak flow rate of stormwater required to be managed. Additionally, the use of disconnection strategies reduces DCIA. Redevelopment projects that reduce impervious area within the limits of earth disturbance (excluding public right-of-way) by at least 20%, based on a comparison of predevelopment impervious area to post-development DCIA, are exempt from the Flood Control requirement.
How Disconnection Design Strategies Influence the PWD Review and Approval Process
Reducing DCIA through the implementation of DIC can influence the project Review Path, as described in Chapter 2. By incorporating DIC into the design of a Redevelopment project, developers may be eligible for an Expedited PCSMP Review (Section 2.4) to obtain PCSMP approval faster and meet tighter construction schedules. Disconnection Green Review projects (Section 2.4.1) are those that incorporate at least 95% DIC in the stormwater management design in order to meet the Stormwater Regulations, and Surface Green Review projects (Section 2.4.2) use a combination of bioinfiltration/bioretention SMPs and DIC to meet the Stormwater Regulations. Disconnection Green Reviews benefit from a shorter (five-day) PCSMP Review Phase, and exemption from the infiltration testing requirement. Surface Green Review projects benefit from a shorter (five-day) PCSMP Review Phase and the option to delay infiltration testing until construction to provide flexibility and potential cost savings.
The designer will often need to use SMPs to meet the Stormwater Regulations. PWD expects that the designer will first consider maximizing the use of non-structural design and DIC strategies outlined earlier in this Chapter, but also recognizes that, for many sites, stormwater management compliance will rely strongly on the use of SMPs.
This Section provides detailed guidance to the designer in the selection and design of SMPs to meet the Stormwater Regulations. The first portion of this Section focuses on approaches for selecting SMPs, while the latter portions of the Chapter focus on design strategies. In selecting and designing SMPs, the designer will draw strongly from the site assessment, stormwater opportunities, and constraints work described in Section 3.1.
SMPs are systems that use physical, chemical, and biological processes to provide a certain level of stormwater control and treatment. This level of control typically includes a required storage volume, a volume to be infiltrated, and an acceptable release rate. These requirements are met through five principal hydraulic functions of SMPs, described below.
Figure 3.2-3 illustrates a variety of design elements available to provide these functions. Depending on the configuration, physical, chemical, and biological processes lead to removal of pollutants during these processes. By combining design components in a variety of ways, the designer can identify alternative systems that achieve a given function. The SMP functions are not mutually exclusive and certain SMPs may perform multiple functions.
Storage can be provided through surface ponding, enclosed surface storage, or subsurface storage. Subsurface stone storage beds provide storage in stone pore spaces, or voids. Some SMPs, such as bioinfiltration/bioretention basins, can provide a combination of both surface and subsurface storage.
A rough estimate of surface storage can be obtained by averaging the surface area and bottom area of a basin and multiplying by the average depth. For irregular shapes, volume can be estimated by finding the area inside each contour, multiplying each area by the contour interval, and adding the results.
Storage in stone pores is equal to the volume of the crushed stone bed times the porosity. A design porosity of 40% can be assumed for the stone if specifications for the crushed stone meet those provided in Chapter 4.
Storage available in porous media is equal to the initial moisture deficit, the portion of total porosity that is not already occupied by moisture. This portion varies at the beginning of every storm; acceptable design values are 30% for sand and 20% for growing soil.
Not all physical space in a given SMP is active. The maximum elevation that is considered active storage is the overflow elevation. In tanks draining by gravity whose bottoms do not infiltrate, any volume below the invert of the orifice or control structure cannot be considered active storage.
Infiltration of stored water into the underlying soil is desired because stormwater runoff is eliminated from the City’s drainage system, and natural hydrology is restored. Surface vegetation helps prolong design life because the growth of plant roots helps to keep the soil pore structure open over time. This effect is greatest with vegetation that has a deeper root structure (e.g., trees, shrubs, and native herbaceous species rather than turf grass). Using such attractive landscaping practices improves the quality of life in the urban landscape.
3. Evaporation and Transpiration
Evaporation and transpiration are minor SMP functions when measured over the course of one storm, but they are significant when measured over time. Surface SMPs will provide the greatest evaporation and transpiration benefit, particularly if they are vegetated. Some water that infiltrates the surface will evaporate. For this reason, vegetated systems provide both water quality treatment and volume reduction.
4. Slow Release
When stored water cannot be infiltrated or evaporated, it must be released at a slow rate to a sewer or receiving water body. This allows the runoff to slowly drain into the City’s system, preventing environmental issues stemming from large amounts of water entering the sewer system or receiving water all at once. For volumes in excess of the SMP’s infiltrated static storage, and for non-infiltrating SMPs, the SMP may release the volume slowly through an outlet control device. The outlet control structure may require design and maintenance measures to avoid clogging.
5. Controlled Positive Overflow
All designs must have a mechanism for water to overflow, or bypass, the system unimpeded during events larger than the design event. A riser or other overflow structure can be incorporated into the design to achieve this, or the flow capacity of some SMPs themselves can act as a bypass mechanism.
Overview of Technologies/Uses
This Section provides general technical design guidance for managing stormwater using SMPs. The designer is encouraged to seek compliance with Stormwater Regulations using non-structural design and DIC strategies, discussed in Section 3.2.2 and Section 3.2.3, respectively.
Table 3.2-2 presents a list of acceptable pollutant-reducing practices to be used for projects where infiltration is found to be infeasible. The designer is referred to the Chapter 4 Section referenced in the table for detailed design information concerning each type of SMP. Additional information on SMP types is provided later in this Section in the SMP Selection and Conceptual Design Section. If a particular practice is listed as “not acceptable” within separate sewer or direct discharge areas, it does not imply that this practice cannot be used; it simply means that that particular practice does not qualify as pollutant-reducing when used in those areas.
|Section||Combined Sewer Area||Separate Sewer Area or Direct Discharge|
|Porous Pavement DIC||4.2||Yes||Yes|
|Ponds and Wet Basins||4.7||Yes||Yes|
|Vegetated Media Filters||4.9||Yes||Yes|
|Roof Runoff Isolation*||3.2.4||Yes||No|
*Roof runoff isolation is the routing of runoff from non-vehicular roof area that is not commingled with untreated runoff. The designer is referred to Section 3.2.4 for more information.
SMPs contained in this Manual are by no means exclusive. PWD encourages the development of innovative practices that meet the intent of the Stormwater Regulations. PWD recognizes that new stormwater management products are continuously being developed and introduced into the marketplace and is in support of innovative approaches to management. If an applicant is interested in utilizing technologies not discussed in this Manual, he or she is encouraged to contact PWD Stormwater Plan Review.
How to Use SMPs to Help Comply With the Stormwater Regulations
A well-designed SMP will use combinations of the five principal hydraulic functions described above to achieve compliance with Stormwater Regulations. As noted previously, SMPs are one tool available to the designer to meet the Stormwater Regulations. PWD encourages the designer to first consider non-structural design and DIC to meet the Stormwater Regulations prior to considering SMPs (Section 3.2.2 and Section 3.2.3). Specific suggestions for using SMPs for compliance are discussed below. The designer should also consult the guidance on designing SMPs in series and Stormwater Management Trading presented later in this Section for additional options in using SMPs to help comply with the Stormwater Regulations.
Where infiltration is feasible, SMPs must provide adequate static storage for the entire Water Quality Volume (WQv) below the lowest outlet. (For SMPs in series, the series as a whole must comply with this requirement.) Additionally, the designer must ensure a drain down time of no more than 72 hours. Drain down time compliance is typically achieved by varying the storage area dimensions.
Where infiltration is not feasible in a combined sewer area, the WQv must be treated and released at a controlled release rate and routed through an acceptable pollutant-reducing practice. The designer is referred to Section 3.4.1 for detailed information on how to comply with the Water Quality requirement.
For gravity systems, the target controlled release rate is a function of head on the outlet structure orifice/weir and the orifice/weir characteristics. Compliance is typically achieved by varying storage area dimensions and outlet structure configuration to meet the target slow release rate.
Channel Protection (if applicable)
Compliance with the Channel Protection requirement is typically achieved by varying storage area dimensions and outlet structure configuration to reduce the peak outflow rate during the one-year storm. Additionally, the designer must ensure a drain down time of no more than 72 hours. Controlled positive overflow must be provided, typically in the form of a riser or other overflow structure, to safely pass events larger than the one-year design storm. The designer is referred to Section 3.4.1 for detailed information on how to comply with the Channel Protection requirement.
Flood Control (if applicable)
Compliance with the Flood Control requirement is also typically achieved by varying storage area dimensions and outlet structure configuration to reduce the peak outflow rates for the post-development condition. Peak runoff in the proposed condition must be no greater than the peak runoff in the predevelopment condition for design storms specific to a project’s given Flood Management District and discharge point. Controlled positive overflow must be provided, typically in the form of a riser or other overflow structure, to safely pass large storms. The designer is referred to Section 3.4.1 for detailed information on how to comply with the Flood Control requirement.
Major SMP Types
Infiltrating SMPs, such as porous pavement, subsurface infiltration, and bioinfiltration practices, manage stormwater by infiltrating it into the ground. The designer is required to use infiltrating practices to meet the Water Quality requirement unless infiltration is found to be infeasible. All infiltrating practices are considered pollutant-reducing.
Slow release SMPs
Slow release SMPs detain and slowly release stormwater over time. Some slow release practices are inherently pollutant-reducing practices (if stormwater is passed through a soil/vegetation/media complex) while others may need to be in series with an additional pollutant-reducing SMP.
On sites where infiltration is not feasible, DCIA must be routed to an acceptable pollutant-reducing practice. Table 3.2-2 above presents the non-infiltrating SMPs that PWD currently accepts as pollutant-reducing practices. (For detailed information and design guidelines for individual SMPs, the designer is referred to Chapter 4.) Alternative pollutant-reducing practices may be proposed and will be reviewed on a case-by-case basis. Pollutant-reducing practices include all infiltrating practices and some slow release practices.
Vegetated practices include vegetation as a significant or dominant component within the storage area and include bioinfiltration/bioretention basins, ponds and wet basins, green roofs, and vegetated media filters.
Non-vegetated practices include all subsurface practices, blue roofs, porous pavement, media filters, and cisterns, and do not have significant vegetative components.
SMP Selection and Conceptual Design
The process of selecting the right SMPs for a site is complex and can be challenging, particularly for constrained sites. PWD accepts many different SMPs and offers approaches such as SMPs in series and Stormwater Management Trading that provide the designer flexibility in fitting SMPs into challenging project sites. During the SMP selection and conceptual design process, the designer will select and perform an initial layout of SMPs, incorporating site assessment data, an understanding of remaining stormwater management requirements (after accounting for non-structural design and DIC strategies); PWD’s SMP preferences; and other factors such as aesthetics, cost, and maintenance requirements. This SMP selection and initial layout process should be performed prior to the finalization of the development site layout, such that the site layout can be revised, if needed, based on SMP requirements. Typically, the designer will perform initial SMP selection and conceptual design prior to the submission of the Conceptual Review Phase Submission Package (Section 2.3).
PWD requires that infiltrating SMPs be used to meet the Water Quality requirement unless the designer demonstrates that infiltration is not feasible. Infiltration testing and soil characterization procedures are outlined in Section 3.3. In many cases, infiltration testing will not be performed until the initial layout of SMPs has been completed. Infiltration testing does not need to be performed during SMP selection and conceptual design. In fact, it is generally better not to conduct this testing until after SMP footprints and depths have been estimated. By performing a site assessment and stormwater management opportunities and constraints analysis in accordance with Section 3.1, the designer can reduce the likelihood that a properly conducted infiltration and soil characterization plan (Section 3.3.1) will uncover non-infiltrating subsurface conditions at the SMPs footprints laid out during SMP selection and conceptual design. A site assessment and opportunities/constraints analysis will do so by screening out locations, such as areas with documented high seasonal groundwater, shallow bedrock, clay, or other limiting soil layers that may preclude infiltration, and steering the conceptual SMP layout toward areas more likely to support infiltration.
PWD recommends a three-step process for selecting and advancing SMP design through the conceptual design phase.
Step 1 - Understanding the Options: The SMP Hierarchy
SMPs can differ greatly from each other in terms of cost, function, and applicability to different types of sites. The designer is encouraged to thoroughly review the SMP-specific guidance provided in Chapter 4 when selecting SMPs. The SMP One-Sheets at the beginning of each SMP Chapter should help in understanding the potential for using each SMP type to meet the various Stormwater Regulations.
The SMP Hierarchy is a tool developed to help PWD understand and communicate the order of PWD’s preference for all SMPs. This tool has allowed PWD to formulate incentive-based policies that promote the use of high-performance and cost-effective stormwater management approaches that more effectively achieve the goals of the Green City, Clean Waters program. Similarly, the Hierarchy provides a clear reference point for the private development community to understand which SMPs are most preferred by PWD. Specifically, the Hierarchy seeks to promote practices that do the following:
- Reduce stormwater and pollutants entering and leaving the PWD collection system;
- Are likely to be maintained and have indicated longevity in previous installations; and
- Provide vegetation to create a greener city.
The criteria used to rank the SMPs reflect a wide range of characteristics, such as water quality and quantity performance, space requirements, construction and maintenance costs, likeliness of failure, and triple bottom line performance. As a result, the Hierarchy reflects preferences based on stormwater management performance, constructability, and longevity. Table 3.2-3 outlines the main criteria considered when ranking SMPs in order of their relative weight. The SMP One-Sheet at the beginning of each SMP Section in Chapter 4 displays its relative performance level for each attribute.
|SMP Hierarchy Ranking Criteria|
Infiltration and Volume Reduction: The SMP’s ability to infiltrate or reduce the WQv
Effluent Pollutant Load: The typical annual mass of total suspended solids (TSS) in the SMP’s effluent runoff (total annual mass of TSS, accumulated at a point of analysis downstream of the SMP). Annual TSS mass is computed by considering the SMP to be managing one acre of DCIA with effluent event mean concentrations of TSS based on data from the International Stormwater BMP Database.
Likeliness of Failure: The relative likelihood that the SMP will fail to operate and will fail to be repaired so that it functions as designed over a unit period of time based on observations at a program level
Construction Costs: The marginal redevelopment implementation costs associated with the construction of the SMP per acre of DCIA treated. As defined in the Long Term Control Plan Update (LTCPU), marginal redevelopment cost is considered the cost beyond traditional measures to implement an SMP approach assuming that redevelopment is already taking place. SMP costs were derived from the construction cost analysis and reference cost assessment prepared for the LTCPU, with updated unit costs.
Evapotranspiration: The SMP’s ability to manage stormwater runoff via evapotranspiration (ET). Each SMP is evaluated based on the characteristics of the surface area available for ET and any enhancement factors (vegetation). These vary by typical vegetation cover type and density, as well as any non-vegetative evaporation pathways, i.e., surface water and void spaces.
Triple Bottom Line: The SMP’s ability to provide social, environmental, and economic benefits (land value, energy efficiency, etc.)
Water Quality Rate Control: The ability of an SMP to reduce the release rate of the WQv to not exceed the maximum release rate
Large Storm Rate Control: The ability of an SMP sized for Water Quality compliance to reduce the discharge rate of large runoff events and to be resized to manage large storm events, which is helpful in complying with the Flood Control and PHS Release Rate requirements
Operation and Maintenance Costs: The annual costs associated with operation and maintenance activities for the SMP. They were derived from the maintenance cost analysis prepared for the LTCPU.
Building Footprint Encroachment: Encroachment onto site area that could otherwise be used for building footprint
Ground-Level Encroachment: Encroachment onto potential usable, open space on the ground-level surface of the site
The SMP Hierarchy is shown below in Table 3.2-4. All SMPs are classified as one of three preference levels: Highest, Medium, and Lowest.
The highest-ranking SMPs include bioinfiltration, bioretention, porous pavement, and green roofs. Bioinfiltration is ranked highest for its ability to infiltrate stormwater and provide triple bottom line benefits while being cost effective and long-lasting. Similarly, bioretention is ranked very high, reflecting its ability to settle suspended solids and cycle nutrients via plant uptake.
The designer is encouraged to incorporate SMPs from this Hierarchy tier into his or her stormwater management design. As discussed in Section 2.4, projects that manage stormwater with SMPs only in this category are eligible for a Surface Green Review. Advantages of a Surface Green Review include a shorter (five-day) PCSMP Review Phase and the option to postpone infiltration testing until construction.
SMPs considered to have medium preference (subsurface infiltration, cisterns, blue roofs, and ponds and wet basins) tend to efficiently manage stormwater via infiltration, volume reduction, or detention. These SMPs often provide fewer triple bottom line benefits and may not last as long as more highly preferred SMPs.
The least-preferred SMPs in the Hierarchy (subsurface detention with vegetated media filters, subsurface detention with roof runoff isolation, subsurface detention with media filters, vegetated media filters, and media filters) are non-infiltrating and generally provide little, to no, triple bottom line benefits. Additionally, the SMPs in this tier tend to have relatively high operations and maintenance costs and may malfunction more frequently than other SMPs.
Step 2 –Determining Residual Management Requirements
The designer may be able to satisfy some or all of the Stormwater Regulations using non-structural design or DIC strategies. Prior to considering the use of SMPs, the designer must develop a quantitative understanding of the remaining stormwater management needs with respect to each of the Post-Construction Stormwater Management Criteria: Water Quality, Channel Protection, and Flood Control. Following the evaluation of non-structural and disconnection options, the designer must determine the following prior to proceeding to the SMP design stage:
- Total remaining DCIA to be treated and associated WQv;
- Peak flow attenuation required for all site DCIA, for the Channel Protection requirement, if applicable; and
- Total peak flow comparison from predevelopment to post-development conditions for each point of interest, for the Flood Control requirement, if applicable.
Step 3 - SMP Placement and Layout
Some sites will offer numerous options for locating SMPs (on rooftops, on the ground surface, or underground), while other sites, particularly “full build-out” sites (where ground-level open space is not available in the proposed site layout), will have fewer options for SMP placement. PWD encourages the designer to incorporate ground-level vegetated SMPs on sites wherever possible, resorting to subsurface SMPs only when other options have been exhausted. The designer should approach the SMP placement and layout process after becoming thoroughly familiar with the characteristics, advantages, limitations, and appropriate uses of acceptable SMPs. The designer should choose SMPs per the SMP Hierarchy presented above, exhausting opportunities for preferred practices prior to considering lower priority practices.
The following guidelines and suggestions are provided to assist the designer with selecting and arranging SMPs.
- Assessing Space Constraints – SMPs rely on storage volume to achieve performance. The availability of space for SMPs will often dictate the location and type of SMPs that can work on a site. Considering SMP placement early in the design process is critical to ensuring that sufficient space for incorporating SMPs, particularly ground-level SMPs, is present. The designer should calculate approximate design requirements (e.g., total required storage volume) to allocate space for stormwater management within the site layout. If insufficient space is available to incorporate surface-vegetated practices, the designer may need to consider alternatives such as porous pavement, or other SMPs, proceeding down the SMP Hierarchy. The use of SMPs in series, Stormwater Management Trading, and/or adding subsurface storage to a bioretention system can help the designer maximize the use of surface-vegetated SMPs, even on constrained sites.
- Creating On-Site Amenities – SMPs such as green roofs and bioinfiltration/bioretention basins can provide on-site greening, as vegetated features, which can act as an aesthetic amenity, particularly for residential and commercial retail sites. Bioinfiltration/bioretention SMPs should be designed in conjunction with other desired and required landscaping.
- Choosing Areas with Infiltration Potential – Although the exact infiltration rate at a particular location within a site is not generally known during the Conceptual Review Phase, the designer should use existing information to locate SMPs in areas that have a strong potential for infiltration. Much of this information, such as United States Department of Agriculture (USDA) Hydrologic Soil Maps, existing geotechnical reports, existing soil investigation reports, drainage feature mapping, topographic mapping, information on existing site drainage issues, and data on high seasonal groundwater, will have been compiled during initial site assessment activities as described in Section 3.1, and must be used for this purpose.
- Prioritizing Low-Lying Areas – Surface-level SMPs should be located on lower portions of a site, where stormwater can be gravity-fed from DCIA to the SMPs without making the SMPs excessively deep. These low-lying areas should be prioritized for stormwater management early in the site design process.
- Providing Downstream Points of Relief – SMPs need to provide gravity drainage for both overflow structures and underdrains. SMP elevations must not be too low to preclude tying in underdrains and overflow structures to a downstream point of relief (e.g., sewer or receiving water)
- Minimizing Conveyance Requirements – SMPs are less costly and easier to maintain if the designer reduces the amount of collection and distribution piping. Opportunities to sheet flow stormwater from DCIA to SMPs, or to use surface conveyance systems like swales to bring stormwater into SMPs, should be sought. In some cases, the designer may be able to use natural drainage features to convey stormwater with little additional cost.
- Avoiding Utilities – Careful mapping of surface and subsurface utilities on-site is necessary to reduce conflicts and the potential for relocating of existing utilities. A designer can view PWD utility records by contacting PA One Call and PWD Water Transport Records Unit (Section 2.5).
- Avoiding Sensitive Features – SMPs should be placed in locations that avoid sensitive features, such as mature tree stands, wetlands, steep slopes, and floodplains, and constraints, such as shallow bedrock. These areas will have been mapped during the site assessment process in Section 3.1. Many of these areas are regulated by State and Federal agencies and/or City ordinances.
- Providing Maintenance Access – Locating SMPs in areas where they can be easily accessed for maintenance is an important design consideration. Vehicular access routes, if needed for sediment removal, should be considered.
- Avoiding Hotspots and Contamination – Locating SMPs away from hotspots and areas of known contamination is always a good idea. Location of infiltrating SMPs within contaminated areas is not permitted. The designer is referred to the hotspot investigation procedures in Section 3.1 for more information. During this phase, a preliminary investigation of likely hotspots is suggested. During detailed design, more exhaustive characterization of soil contamination issues may be required for individual SMP sites to determine infiltration feasibility.
- Avoiding Unstable Fill – Many areas of Philadelphia are underlain by historic fill, which can be loose or unstable. The designer is advised to identify areas of unstable fill through geophysical methods, exploratory geotechnical testing, or historic mapping to avoid these areas where possible.
- Maintaining Sight Lines – Clear lines of sight are critical for pedestrian and vehicular safety. SMPs should be placed so as not to impair lines of sight, and the designer must consider full grow-out condition for vegetation when assessing sight line issues.
- Ensuring Safety – Many SMPs contain features such as ponded water that could be unsafe, particularly for vulnerable populations, such as young children. The designer should consider locating SMPs with ponded water away from play-yards, playgrounds, or other areas where children are playing, or installing fencing or other features to limit interaction with the system.
- Considering Appropriate Conditions for Vegetated SMPs - Some variables to consider include amount of sunlight received and solar orientation, wind speed and direction, temperature gain, and surface character. For example, sites facing northeast receive morning sun and tend to be cooler and wetter than those facing southwest and runoff from asphalt will be hotter than that from concrete. Combinations of these variables create different micro-climates and should be taken into account when placing the SMP and selecting plants.
Roof Runoff Isolation
Recognizing that runoff from some areas is cleaner than others, PWD has identified roof runoff isolation as an acceptable non-infiltrating pollutant-reducing practice in combined sewer areas. Roof runoff isolation is the practice of segregating clean roof runoff from other untreated runoff. Here, clean roof runoff is defined as runoff from roof areas that are not exposed to vehicular activity (e.g., a roof-level parking deck). The designer can incorporate roof runoff isolation into site layout and design by providing dedicated stormwater conveyance piping from roof areas to SMPs designed to meet the combined sewer area Water Quality slow release requirement. Runoff from isolated roofs must not commingle with roof runoff exposed to vehicular activity or other untreated runoff until a point in the system after which such runoff has been treated by another pollutant-reducing practice. The designer is referred to Section 3.5.6 for an example of integrated stormwater management using roof runoff isolation.
Placing SMPs in Series
Many of the SMPs discussed in this Manual provide both Water Quality treatment and rate control. Some SMPs provide only rate control. The designer must keep in mind that some SMPs cannot fully meet all applicable Stormwater Regulations on their own, and a network of SMPs can be used to meet the Stormwater Regulations for a given site. For example, peak rate control for Flood Control compliance could be progressively achieved through flow attenuation in a series of smaller, linked SMPs. Many of these SMPs could also be used to meet the Water Quality requirement by providing cumulative static storage equal to the contributing WQv. In addition, non-pollutant-reducing practices, such as subsurface detention systems, can be used to meet the Water Quality slow release rate requirement, Channel Protection, and Flood Control requirements, but they cannot be used to meet the Water Quality pollutant-reduction requirement. In other cases, space constraints may preclude the ability to comply with the Stormwater Regulations using only one SMP.
While it is generally more cost effective, efficient, and easier to meet the Stormwater Regulations using as few SMPs as possible, to provide more flexibility, PWD allows the designer to use approaches that achieve compliance through the use of multiple SMPs connected in series. Placing SMPs in series allows the designer to minimize the disrupted space, limit the construction or maintenance costs of a system, or meet the Stormwater Regulations on a crowded or complex site. Particular approaches will vary by site, and the designer is encouraged to use creativity to combine SMPs in ways that achieve site-wide compliance. Some examples of these approaches are discussed below.
Multiple Small Bioinfiltration/Bioretention SMPs
A series of smaller bioinfiltration/bioretention SMPs can be placed within small landscaped areas in lieu of a single large bioinfiltration/bioretention SMP. This approach can be effective for promoting vegetated surface SMPs within constrained sites. Figure 3.2-4 illustrates this approach.
Bioretention with Subsurface Detention
Bioretention systems are particularly effective for managing the WQv. They provide treatment and rate control, but may not provide enough storage to meet the Flood Control or PHS Release Rate requirements, if applicable. A bioretention basin installed directly over a subsurface detention basin provides a number of benefits. The bioretention basin is relatively easy to maintain and is a pollutant-reducing practice. The subsurface detention basin provides effective rate control for small and large storms. This combination allows the subsurface detention basin to act as an overflow chamber for large runoff volumes generated by large storms. The bioretention and subsurface detention basin in series can reduce the amount of usable surface area disrupted while meeting the Stormwater Regulations. Figure 3.2-5 illustrates this approach.
Vegetated Media Filter with Subsurface Detention
A subsurface detention basin with an upstream vegetated media filter is a combination of SMPs that can be used to meet the Water Quality slow release and pollutant-reduction requirements on sites that cannot infiltrate in the combined sewer area. The subsurface detention basin is a compact SMP that can be installed below a parking lot to limit the amount of usable surface that is disrupted. With a site employing the pollutant-reducing practice of roof runoff isolation, runoff from rooftop DCIA can be sent directly to the subsurface detention basin without any filtering treatment. A vegetated media filter can then be installed on-site to capture the WQv from surface-level DCIA and treat the runoff before discharging the treated volume to the subsurface detention basin. Figure 3.2-6 illustrates this approach.
The following requirements apply to SMPs placed in series:
- SMPs can be placed in series to achieve rate control for the Stormwater Regulations. The designer does not have to demonstrate compliance with rate control requirements at the discharge point of each SMP, as long as rate control can be provided at the downstream-most point of the SMP series, prior to discharge to PWD sewer or receiving water.
- When complying with the Water Quality requirement, cumulative static storage volume may be provided within a connected series of SMPs, rather than any single SMP.
- Individual SMPs within a series must be designed in full accordance with design requirements provided in Chapter 4. For example, each bioretention system in a series must individually meet loading ratio and drain down time requirements.
- When using SMP in series, upstream flow splitters may be used to direct larger events around Water Quality SMPs, such as bioretention systems, to larger Flood Control SMPs.
Stormwater Management Trading
In most cases, PWD requires full compliance with the Stormwater Regulations for each point at which stormwater leaving the site is discharged to either a receiving water or PWD sewer. SMPs must be provided as appropriate to achieve compliance at each of these locations. However, if site constraints or existing conditions prevent the designer from complying with the Stormwater Regulations, or if placement of a SMP could result in a potential environmental or safety hazard, Stormwater Management Trading may provide relief.
Stormwater Management Trading allows developers to shift placement of SMPs from a location that directly treats runoff from a proposed improvement to a nearby location that may or may not be hydraulically connected. Trades are considered by PWD on a case-by-case basis. A pre-application meeting with PWD is highly recommended if this option is being considered for Regulatory compliance. In general, there are three types of stormwater management trades currently allowed by PWD.
- Same Parcel Trading: Siting SMPs on a parcel that will manage DCIA not associated with the proposed improvement (outside the project’s limit of disturbance (LOD));
- Same Owner Trading: Siting SMPs on a different parcel (owned by the same owner) than the proposed improvement; and
- Same Owner Banking: Over-sizing SMPs to be used toward compliance with the Stormwater Regulations associated with future improvements on the SMPs’ parcel.
Stormwater Management Trading Standards
PWD will review trade applications on a case-by-case basis. Property owners must expect to comply with the following general criteria:
- An SMP associated with a trade must achieve the same Regulatory standard (Water Quality and Channel Protection) as if it were directly managing stormwater from the proposed improvement.
- Trades must occur within the same sewershed.
- Trades between two parcels are allowed only when the parcels are owned by the same entity or person.
- Trades may not be used for Flood Control compliance.
- Once a trade is approved, the parcel(s) containing the regulated improvement and the SMP will be subject to post-construction requirements, including applicable deed restrictions and Operations and Maintenance Agreements.
- SMPs constructed on a separate parcel must be constructed and operating prior to the start of construction on the development parcel.
- Applicant must provide sufficient written justification in their PCSMP Report (Section 2.3.1) for the trade, including reasons why management of the required areas is not feasible or least preferred. A short explanation should also be included in the ERSA Application.
- Area proposed for trade must be unmanaged in the pre-development condition unless the area has been previously identified as part of a Same Owner Banking agreement.
- Trade area must be equal to or greater than the unmanaged area and produce an equivalent pollutant load. For example, PWD will not approve a trade of unmanaged impervious parking lot with existing roof area because the total pollutant load from the trade surfaces is not equivalent.
Submission Package components for Stormwater Management Trading are no different from typical submissions. The designer, however, must clearly identify the Stormwater Management Trade on all plans and reports in the Submission Package. This information can be easily conveyed as a table; an example of which is provided below:
|Total LOD||18,000 SF|
|On-site LOD||17,000 SF|
|Impervious Area Within On-site LOD||16,500 SF|
Managed DCIA (i.e. DCIA routed to SMP) Within On-site LOD
|DIC Area Within On-site LOD||600 SF|
|Remaining Unmanaged DCIA Within On-site LOD (e.g. parking lot runoff)||3,500 SF|
Acceptable Trade Area (i.e. managed impervious area) outside of LOD
≥ 3,500 SF of surface-level cover
Additionally, if the designer believes that a Stormwater Management Trade will be necessary to meet the Stormwater Regulations, he or she is encouraged to discuss this during the Conceptual Review Phase.
Understanding the limit of disturbance is key to proposing a trade approach for regulatory compliance. The existing impervious area to be managed for trade must remain outside the LOD throughout construction. For example, depaving or otherwise converting an existing impervious surface to pervious cover (such as converting a parking lot to porous pavement) cannot be used as trade as this activity increases the LOD, and the LOD boundary is used to determine the area applicable to the Stormwater Regulations. While this approach may help achieve an exemption from the Flood Control requirement (Section 1.2.1), it cannot be used for trade. Instead, the applicant should look at low impact options that will minimize the amount of existing impervious area to be disturbed, thus maximizing the available trade. The next section presents examples of how this may be achieved.
Stormwater Management Trading Examples
Example of Same Parcel Trading – Food Distribution Facility
A property owner sought approval from the City to construct a new loading dock (Figure 3.2-7, in red) at an existing food distribution facility. The only on-site area large enough on which to place an SMP was adjacent to the food warehouse, and the property owner had concerns about food contamination from wildlife attracted to a surface SMP. Therefore, the property owner considered subsurface SMPs that could be installed adjacent to the new loading dock; however, the disadvantages and constraints of subsurface SMPs in this application included the following:
- Relatively high cost to construct and maintain;
- Large space requirements to achieve controlled release standards, since soils near the loading dock were significantly compacted, precluding infiltration; and
- The need for the subsurface SMP design to accommodate heavy truck traffic, balancing SMP access points with heavy load-bearing surfaces.
The property owner instead proposed an SMP (shown in blue) elsewhere on-site to manage existing undisturbed impervious area in the same sewershed. The benefits from this trade included the following:
- Less expensive SMP installation cost;
- Less disruption to distribution center’s operations during construction;
- Smaller SMP footprint located in better-infiltrating soils; and
- An above-ground SMP that can be more easily inspected and maintained.
Example of Same Owner Trading – Pharmacy and Parking Lot
A single party owned two parcels separated by the public right-of-way (ROW). The developer proposed construction of a pharmacy and a parking lot on one undeveloped parcel. The same developer owns an already developed parking lot across the street (presented post-development in Figure 3.2-8).
The site designer was able to manage all new impervious area proposed for Parcel A with a subsurface detention facility (shown in blue), except for a portion of the pharmacy roof area (shown in red). To meet Stormwater Regulations on Parcel A, the designer proposed to manage the existing parking lot on Parcel B with a surface infiltration SMP (shown in blue). The unmanaged pharmacy roof area discharges directly to the sewer system. The benefits from this trade included the following:
- Increased ability to fully use Parcel A;
- Less expensive SMP installation cost;
- An above-ground SMP that can be more easily inspected and maintained; and
- Limited reliance on underground SMPs.
Example of Same Owner Banking – Shopping Mall
A shopping mall owner proposed a series of improvements planned in phases. These included expansions to existing mall buildings, new standalone restaurants, and additional parking areas and driveways. Instead of designing and constructing SMPs for each individual improvement, the designer proposed a stormwater management banking scenario to construct a single SMP to serve all improvements.
The designer first proposed an existing building expansion and additional parking areas and driveways under Phase 1 (shown in yellow on Figure 3.2-9). Upon approval of this first phase, as well as a conceptual design of future standalone restaurant buildings under Phase 2 (shown in orange), PWD permitted the owner to install an oversized SMP (shown in blue) to manage these impervious surfaces. The owner then installed the remainder of the proposed improvements in Phase 2. The site designer directed all runoff to the single SMP that was constructed in Phase 1.
A benefit of this scenario was that the owner was able to obtain approvals quickly for the second phase of construction as the SMP was sized to meet the Stormwater Regulations for the entire project.
When Same Owner Banking is proposed, PWD will acknowledge the bank amount in terms of additional cubic feet capacity remaining in the SMP. This type of banking approach works best for projects where phases are planned in rapid succession, as each phase is held to the Stormwater Regulations in place at the time of its ERSA submission. Property owners who are interested in long term site master planning (which may occur over several years or decades) are encouraged to discuss with PWD prior to implementation.
SMP Design Guidance and General Requirements
Once the initial selection of SMPs is complete, and PWD has approved the conceptual design, detailed design of SMP systems can be performed. Detailed design of SMPs and associated documentation will be submitted as part of the designer’s PCSMP Review Phase Submission Package to PWD. The designer is referred to Chapter 2 for details on preparing this Submission Package.
This Section provides guidance to the designer in the design of SMPs, outlining general requirements that apply to all SMPs. The designer is also referred to Chapter 4, which provides detailed guidance and requirements for specific SMPs.
Infiltration Testing and Waiver Requirements
A designer using SMPs to comply with the Water Quality requirement must use infiltration unless they can demonstrate that infiltration is infeasible. The designer must exhaust all possibilities for implementing infiltrating practices on proposed sites, including exploring alternative locations for infiltration facilities if initial locations are not found to be suitable for infiltration or over-excavating poorly infiltrating soils. The designer is referred to Section 3.3 for detailed information on performing infiltration tests, assessing infiltration feasibility, and preparing requests for infiltration waivers. If appropriate justification that contamination will preclude the site from infiltration is provided, an impervious liner must be incorporated into the SMP design.
Pretreatment is critical for extending the design life and maximizing the performance of SMPs. The designer must provide adequate pretreatment for all SMPs. Appropriate pretreatment is based on a number of factors including SMP type, loading ratios, and drainage area characteristics. The designer is referred to Chapter 4 for more information on the design of pretreatment systems for specific SMPs and general pretreatment options.
Conveyance and Inlet and Outlet Control Requirements
Conveyance systems, including piping conveying stormwater to and from an SMP, and inlet and outlet control systems, which regulate the flow into and out of an SMP, are important aspects of SMP design. All storm sewer pipes must be designed to have adequate capacity to safely convey the ten-year storm without surcharging the crown of the pipe. Section 3.4.2 contains detailed guidance on storm sewer design and pipe capacity calculations, while Section 4.11 and Section 4.12 provide guidance on the design of inlet and outlet controls, respectively.
Appropriate sizing is critical for SMP performance. The designer must incorporate several factors, including SMP type, function, maximum loading ratio requirements, release rate requirements, ponding depth, static storage requirements, media characteristics, freeboard requirements, and space limitations in determining appropriate SMP sizing. The designer is referred to the loading ratio requirements later in this Section and the SMP-specific sizing requirements in Chapter 4 to aid in determining appropriate SMP sizing.
Safe Overflow Requirements
Safe overflow must be provided for all SMPs. Runoff that overflows from an SMP (runoff that is not infiltrated or slow released) must be conveyed to receiving waters or sewers in a controlled manner that does not cause flooding, endanger public safety, or produce erosive conditions. Positive overflow for large storm events, up to and including the 100-year storm, must be provided.
Release Rate Requirements
For non-infiltrating practices in combined sewer areas, the designer must meet slow release rate requirements prior to discharge into PWD sewers or receiving waters. Typically, release rates for slow release systems are met using small orifices or other rate control devices. The designer is referred to Chapter 4 for specific information on designing outlet control systems.
Loading Ratio Requirements
Loading ratio is defined as the area of contributing DCIA divided by the bottom surface footprint of vegetated surface SMPs and the bottom footprint of infiltrating subsurface SMPs. The loading ratio is a tool that is used for sizing an SMP with consideration of acceptable sediment loading. It is a balancing point between maintenance requirements, performance requirements, and safety considerations. PWD’s loading ratios are used as maximum acceptable SMP sizes for stabilized sites that are appropriately maintained; they are not necessarily the recommended loading ratios. The maximum loading ratio for vegetated surface SMPs is 16:1. The maximum loading ratio for infiltrating subsurface SMPs is 8:1.
Maximum Loading Ratios
Surface vegetated SMPs: 16:1
Subsurface infiltrating SMPs: 8:1
Subsurface SMPs are inherently more difficult to maintain because they are buried. If construction sediment or some other sediment source discharges to the subsurface basin it can become clogged. Repairing the basin could require a complete removal and replacement of the system. This is one reason why PWD requires lower loading ratios for subsurface SMPs.
When considering SMPs that receive runoff from a likely sediment source, the designer must factor into his or her design the likelihood of clogging, and therefore the need for increased maintenance frequency; the cost of maintenance/replacement; and the likelihood of this occurring when determining the appropriate sizing of the system.
Planting and Vegetation Guidance
Vegetated SMPs are among the most preferred SMP types, as indicated in the SMP Hierarchy. They can often be integrated within planned landscape areas, with minor modifications to conventional landscape design. It is essential that impervious surfaces be graded toward the vegetated areas that are used as SMPs and that these SMPs are depressed to allow for flow and/or surface ponding.
Landscaping is a critical element to improve both the function and appearance of vegetated SMPs. Integrated stormwater landscapes can provide many benefits, such as construction cost savings, reduced maintenance, aesthetic enhancement, and improved long-term functionality. A well-designed and established landscape will also prevent post-construction soil erosion. Additionally, these approaches can help mitigate urban heat island effects, improve air quality, and reduce atmospheric carbon levels. Since these design approaches are still relatively new to many construction contractors, it is advisable to clearly show planting details in cross-sectional and plan view drawings.
The designer should adhere to the following general planting guidelines and is referred to Chapter 4 for detailed planting requirements and guidance for specific SMPs. The planting recommendations shown under this Section are based on research, local experience, and/or standard landscape industry methods for design and construction.
- Selected plant materials must be appropriate for soil, hydrologic, and other site conditions.
- Vegetated SMPs must use appropriate native and recommended non-invasive species from Appendix I.
- The design for planting must minimize the need for herbicides, fertilizers, pesticides, or soil amendments at any time before, during, and after construction and on a long-term basis.
- Plantings must be designed to minimize the need for mowing, pruning, and irrigation.
- Grass or wildflower seed must be applied at the rates specified by the suppliers. If plant establishment cannot be achieved with seeding by the time of substantial completion of the SMP portion of the project, the contractor must plant the area with wildflower sod, plugs, container plants, or some other means to complete the specified plantings and protect against erosion.
- The designer should select a diversity of tree species and avoid overused species to reduce the risk of disease or insect infestation.
- The designer should avoid combinations of plants that will harm one another.
- The designer should consider the mature size of any plant and ensure that it has enough space to grow to this size.
- Plant Selection and Arrangement
- Existing native and non-invasive vegetation should be preserved where possible.
- Noxious weeds must not be specified or used (see list, this Section). Aggressive species should be used carefully to avoid spreading to other areas.
- Stream and water buffers should be planted with trees, shrubs, ornamental grasses, and herbaceous materials, where possible, to stabilize banks and provide shade. This will help to reduce thermal warming, reduce erosion, increase roughness, and protect habitat.
- Plantings that will require routine or intensive chemical applications (e.g., turf areas) should be avoided. Low-maintenance ground cover should be used as an alternative to turf.
- The designer should consider stressors (e.g., wind, exposure, exposure to deicing salt, salt tolerance, insects, drought and inundation tolerance, and disease), micro-climates, and sunlight conditions when laying out the planting plan.
- Aesthetics and visual characteristics should be a prime consideration when developing planting plans. Plant form, texture, color, bloom time, and fragrance are important to the overall feel of the site. Plants can be used to enhance and frame desirable views or screen undesirable views. Care should be taken to not block views at entrances, exits, or along difficult road curves.
- Where such conditions exist, trees and shrubs should be placed in a manner that restricts pedestrian access to steep pools or slopes without blocking maintenance access.
- Existing and proposed utilities must be identified and considered.
|Prohibited noxious weeds, as identified in Pennsylvania Code Section 110.1: Noxious Weed Control List|
- Maintenance Considerations
- The designer should carefully consider the long-term vegetation management strategy for the SMP, keeping in mind the maintenance legacy for the future owners. The SMP maintenance agreement must include requirements to ensure vegetation cover in perpetuity.
- When appropriate, the designer should provide signage to help educate the public about SMPs and designate limits of mowing (wildflower areas, meadows, etc.).
- The edge of the basin may be designated by woody vegetation to further designate limits of mowing and foot traffic.
- Planting in massings (each group consisting of one to three individuals of the same species) may support maintenance efforts by simplifying plant identification.
- Embankments, Spillways, and Dams
- Planting of trees, shrubs, and/or any type of woody vegetation is not allowed on structural embankments.
- All emergency spillways should be stabilized with plant material that can withstand strong flows. Root material should be fibrous and substantial, but lack a taproot.
- Trees or shrubs known to have long taproots should not be planted within the vicinity of an earthen dam or subsurface drainage facilities.
- Trees and shrubs should be planted at least 25 feet away from a principal spillway structure and at least 15 feet away from the toe of slope of a dam.
- SMP soils should provide adequate infiltration rates and be suitable for healthy tree and vegetation growth. Soil analysis must be conducted within the SMP area to determine appropriate levels and types of soil amendments. The designer is referred to Section 3.3 for guidance and requirements for soil amendment installation.
- If topsoil exists on-site and is stockpiled for re-use, appropriate erosion control measures, as required by the latest edition of the PA DEP Erosion and Sediment Pollution Control Program Manual, must be used.
- Site Selection, Preparation, and Grading
- When selecting a location for the SMP, the designer should take into consideration the physical variables of the site and the effects they will have on the SMP.
- Some variables to consider include amount of sunlight received and solar orientation, wind speed and direction, temperature gain, and surface character. For example, sites facing northeast receive morning sun and tend to be cooler and wetter than those facing southwest. Also, runoff from asphalt will be hotter than that from concrete because asphalt’s dark color absorbs more solar energy. Combinations of these variables create different micro-climates and should be taken into account when placing the SMP and selecting plants.
- Unwanted vegetation in the SMP area should be removed during site preparation with equipment appropriate for the type of material encountered and site conditions. It is recommended that the maximum amount of pre-existing native vegetation be retained and protected.
- No material storage or heavy equipment is allowed within the SMP area after site clearing and grading has been completed, except to excavate and grade as needed to build the SMP. No compaction of infiltration areas must occur during this excavation.
- After the SMP area is cleared and graded, any necessary soil amendments should be added and tilled into the existing soil to the depth specified for each SMP. No tilling should occur within the drip line of existing trees. After tilling is complete, no other construction traffic must be allowed in the area, except for planting and related work. Where topsoil is needed, it should be spread to a depth of four to eight inches and lightly compacted to minimum thickness of four inches. This provides organic matter and important nutrients for the plant material. The use of topsoil allows vegetation to become established faster and roots to penetrate deeper. This ensures quicker and more complete stabilization, making it less likely that the plants will wash out during a heavy storm.
- The mulch layer helps maintain soil moisture and avoid surface sealing that reduces permeability. Mulch helps prevent erosion, and provides a micro-environment suitable for soil biota at the mulch/soil interface. It also serves as a pretreatment layer, trapping the finer sediments that remain suspended after the primary pretreatment.
- Approved mulching materials include organic materials such as compost, bark mulch, leaves, as well as small river gravel, pumice, or other inert materials. Grass clippings should not be used as mulch.
- For ground cover plantings, mulch must be applied to cover all soil between plants.
- Care should be exercised to use the appropriate amount of mulch – any more than three to four inches can negatively impact growing conditions and cause excessive nutrients to leach into the SMP.
- Mulch must be weed-free. Manure mulching and high-fertilizer hydroseeding are prohibited in a SMP area during and after construction.
- Mulch should be kept three inches away from tree trunks, woody vegetation, and the base of herbaceous vegetation.
- Newly installed plant material requires water in order to recover from the shock of being transplanted. A source of water should be provided during establishment of the SMP, especially during dry periods. This will reduce plant loss and provide the new plant materials with a chance to establish root growth.
- Permanent irrigation systems are allowed, but the designer is encouraged to minimize the need for permanent irrigation. Innovative methods for watering vegetation are encouraged, such as the use of cisterns and air conditioning condensate.
- Pollution Prevention
- Stormwater pollution prevention practices related to landscaping can be categorized into two broad categories: Toxic Substance Use Reduction and Pollutant Source Reduction
- Toxic Substance Use Reduction – Projects must be designed to minimize the need for toxic or potentially polluting materials such as herbicides, pesticides, fertilizers, or petroleum-based fuels within the SMP area before, during, and after construction. Use of these materials creates the risk of spills, misuse, and future draining or leaching of pollutants into facilities or the surrounding area.
- Pollutant Source Reduction - Materials that could leach pollutants or pose a hazard to people and wildlife must not be used as components of a SMP. Some examples of these materials are chemically treated railroad ties and lumber and galvanized metals. Many alternatives to these materials are available.
- Stormwater pollution prevention practices related to landscaping can be categorized into two broad categories: Toxic Substance Use Reduction and Pollutant Source Reduction
- SMP Establishment and Maintenance
- Establishment procedures must include: control of invasive weeds, prevention of damage from animals and vandals, use of erosion control mats and fabrics in channels, temporary diversion of flows from seeded areas until stabilized, mulching, re-staking, watering, and mesh or tube protection replacement, to the extent needed to ensure plant survival.
- To ensure landscape plant survival and overall stormwater facility functional success, the design and construction documents must include elements that help achieve these results.
- Construction specifications and details must include staking, irrigation schedule, soil amendments, plant protection, overplanting, and potentially mycorrhizal inoculation.
|Sequence of Construction||Describe site preparation activities, soil amendments, etc.; address erosion and sediment control procedures; specify step-by-step procedure for plant installation through site clean-up.|
|Contractor’s Responsibilities||Specify the contractors responsibilities, such as watering, care of plant material during transport, timeliness of installation, repairs due to vandalism, etc.|
|Planting Schedule and Specifications||Specify the materials to be installed, the type of materials (e.g., B&B, bare root, containerized); time of year of installations, sequence of installation of types of plants; fertilization, stabilization seeding, if required; watering and general care.|
|Maintenance||Specify inspection periods; mulching frequency (annual mulching is most common); removal and replacement of dead and diseased vegetation; treatment of diseased trees; watering amount and schedule after initial installation (once per day for 14 days is common); repair and replacement of staking and wires.|
|Warranty||All systems should contain a two-year warranty. Specifications should contain the warranty period, the required survival rate, and expected condition of plant species at the end of the warranty period.|
- Screening and Aesthetics
- SMP elements such as chain link fences, concrete bulkheads, outfalls, riprap, gabions, large steel grates, steep side slopes, manhole covers/vault lids, berm embankments planted only with grasses, exposed pipe, banks, retaining walls greater than two feet high, and access roads are generally not aesthetically pleasing. When these elements face public right-of-way or other private property, these elements should be screened with plant materials.
- The designer is strongly encouraged to integrate aesthetically pleasing landscape design into SMP design.
Operations and Maintenance
An Operations and Maintenance (O&M) Agreement, discussed in detail in Section 6.1.2, is a required component of the Stormwater Regulations. Decisions made in the design phase can affect operations and maintenance and can extend the design life of stormwater facilities. Key factors to consider are ownership, access, maintenance tasks, and frequency.
Designing to Minimize Maintenance
- Use of pretreatment systems should be maximized, particularly for infiltration systems. Reducing velocities and pollutant loads entering SMPs will extend their design lives. The designer is referred to Section 4.10 for guidance on appropriate pretreatment design.
- For infiltration, surface-vegetated SMPs with deeper-rooted vegetation (e.g., trees, shrubs, and native herbaceous species) should be used whenever possible. Root growth helps to keep the soil’s pore structure open and maximizes the life of infiltration SMPs. Routine landscaping tasks are the primary maintenance required.
- On smaller sites, SMPs that do not require slow release control structures should be chosen. These structures can clog and require periodic inspection and maintenance.
- Vehicle access from a public right-of-way can help to minimize the difficulty of maintenance.
- A 15-foot wide vehicle access path leading from a public right-of-way to all stormwater controls is strongly recommended.
- Post-construction ownership
- The owner of the land where the SMP is located is responsible for performing long-term maintenance.
- In the case of a single property owner, that owner is responsible for maintenance. In cases of common ownership, a homeowners’ or condominium association may assume responsibility for maintenance.
- Considering the type of ownership and owner preference can help the designer choose between smaller, distributed SMPs and a single, centralized SMP.