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Create Account Forgot Password? Back to Main Nav. Guidance Manual Introduction CH. However, the environmental impacts of implementing stormwater management in natural areas must be carefully considered.
Recognizing opportunities to reduce proposed directly connected impervious area DCIA to be managed and protecting and using existing site features during the site assessment can lower project costs associated with meeting the Stormwater Regulations. Additionally, proposed site features that are conducive to stormwater management should be identified. For instance, areas such as parking lot islands can be used for surface management of stormwater.
The designer is referred to Section 3. Stormwater management constraints are areas on the project site where stormwater management may be difficult, infeasible, or inadvisable. These can include constraints such as contamination, existing utilities, wetlands, riparian buffers, steep slopes, and soils with high permeability. For development projects where all the impervious area within the limit of disturbance cannot be disconnected and constraints persist that prohibit stormwater management of the remaining DCIA on-site, the applicant should investigate opportunities to provide off-site management as described in Section 3.
If no such opportunities exist, the applicant may consider making a one-time fee in lieu payment for the unmanaged DCIA. All requests for fee in lieu payment must be approved by PWD. More information on the criteria for fee in lieu is provided in Section 3. 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.
However, non-structural DIC opportunities 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 Private Development Services 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.
The site assessment process allows the designer to identify key site and stormwater management design opportunities and constraints. In addition, low-lying areas on a site can be used for SMPs in order to minimize conveyance costs. 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.
Figure 3. 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. When used in combination, non-structural strategies can result in a variety of environmental and financial benefits.
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. 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. To minimize stormwater impacts, land development activities should 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.
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.
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. 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.
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.
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.
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.
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. 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 credits Section 6.
The designer can use non-structural strategies to help comply with the Stormwater Regulations described in Chapter 1 in the following ways:. Non-structural practices encourage minimizing the use of DCIA, thus reducing the volume of stormwater required to be managed. 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.
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, 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. Section 2. The Online Technical Worksheet Section 3. Disconnection opportunities depend on incorporating sufficient pervious areas into a site layout. Completing a site assessment Section 3. 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. 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:.
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. Two roof leaders cannot discharge to the same flow path for disconnection credit.
For example, consider a 1,square foot roof with two roof leaders, each draining an area of square feet Figure 3. Both roof leaders discharge to a lawn. The distance from the downspout discharge point to the street is 65 feet. Therefore, based on Table 3. Disconnecting the roof leaders will significantly reduce the size and cost of stormwater management facilities at this site. Roof leader disconnection can be incorporated into Stormwater Retrofit designs and is eligible for stormwater credit.
Updates to the real estate tax abatement will take effect January 1, Refer to Covid Service Updates version 6 and the newsfeed for the latest information. This is an alert about something very important! Please read this important alert about this very important thing!! With every development in Philadelphia, we have the opportunity to improve stormwater management in the City.
Development projects that disturb more than 15, square feet 5, square feet in certain watersheds of earth trigger the PWD Stormwater Regulations, which require management of the first inch and a half of stormwater runoff. Learn more about Stormwater Management on the Philadelphia Water website.
Download our Stormwater Management Fact Sheet. Guidance Manual Introduction Ch.
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