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Chapter 3 - Site Design and Stormwater Management Integration

3.1 Site Assessment

3.1 Site Assessment

This Section guides the designer in performing a site assessment – the necessary first step in designing a project that complies with the Philadelphia Water Department (PWD) Stormwater Regulations (Stormwater Regulations). The designer must know the site location and general plan for development before beginning the site assessment process.

A site assessment is an investigation of the administrative and physical factors that shape the development and stormwater management plan for a proposed site. The assessment consists of three components: the collection of background site factors, a site factors inventory, and a site factors analysis. Site assessment must be completed in the early stages of project design, before the submission of an Existing Resources and Site Analysis (ERSA) Application Submission Package (Section 2.1.1) to PWD.

A properly completed site assessment helps the designer understand a site’s existing condition and natural systems. The assessment aids the designer in determining the most appropriate site layout and crafting a stormwater management approach and design for a site. Documentation is required for many of the site factors as part of the PWD Stormwater Plan Review Submission Packages (Chapter 2) and Pennsylvania Department of Environmental Protection (PA DEP) National Pollutant Discharge Elimination System (NPDES) Permit Applications, as applicable.

3.1.1 Background Site Factors and Site Factors Inventory

Background site factors consist of macro- or watershed-scale project site characteristics that describe how a site functions within its watershed. These factors include a project’s watershed and sewershed, and factors that influence flooding.

Site factors are smaller, site-scale features including property/land use boundaries and physical features that may affect the site layout or stormwater compliance strategy. These factors assist the designer in developing a stormwater management plan for the project that reduces the impacts of proposed earth disturbance and directly connected impervious area (DCIA).

Project Watershed

A watershed is defined as an area of land that contains a common set of drainage pathways, streams, and rivers that all discharge to a single, larger body of water, such as a large river, lake, or ocean. A project’s location within a watershed and its proximity to the watershed’s final discharge point determines in part how changes in the quantity, rate, and quality of stormwater runoff from the site will affect receiving waters (Section 3.4). A project’s location will also impact its Review Path (Chapter 2) and applicable Stormwater Regulations (Chapter 1). For example, a redevelopment project within the Lower Schuylkill River or Delaware Direct Watersheds is exempt from the Channel Protection requirement.

The nature of stormwater-related issues differs for waters receiving runoff via direct discharge (e.g., a project site discharging directly to the Schuylkill or Delaware Rivers via sheet flow or separate sewer) than for waters located in the headwater areas. The frequency and magnitude of flooding in headwater streams is affected to a far greater degree by unmanaged stormwater than in larger receiving bodies of water such as the Schuylkill or Delaware Rivers. Additionally, increases in runoff from sites discharging to a combined sewer where the sewer system has limited capacity may lead to greater occurrences of combined sewer overflows (CSOs).

Watershed Maps in Appendix D, as well as PWD's "Find Your Watershed" website, are available to assist the designer in determining a site's watershed location.

Project Sewershed

A sewershed is a defined area of land, or catchment, which drains via storm drain infrastructure to a common outlet point. As opposed to natural watersheds, the boundaries of which are defined by natural ridges and high points and that drain to a single point in a stream network, sewershed boundaries are determined by stormwater infrastructure such as curbs, storm drains, pipes, and stream outfalls. Sewershed boundaries often differ from watershed boundaries because stormwater infrastructure may cross watershed boundaries that predate urbanization.

It is necessary to determine the means by which runoff exits a site and a project’s sewershed boundary early in the design process. Runoff may leave a site through a combined sewer system, separate sewer system, or via surface runoff. Some project sites may span multiple sewersheds, and runoff may leave different portions of the site via different methods. Discharge to different sewersheds will not only affect the stormwater management strategy, but also the requirements associated with the Water Quality requirement for the site.

Appendix D contains Collection System Maps for use in determining the project sewershed. Detailed guidance on Regulatory compliance requirements based on the method by which runoff leaves a project site is available in Section 3.4.


An evaluation of existing flooding issues on a project site, on adjacent properties, and in the receiving storm sewer network and/or receiving waterbody must be performed. For example, it is important to know whether floodwaters flow via an overland flow path on the site, and whether runoff from off-site properties is a component of these flows. It is also crucial to understand how flooding impacts the conveyance capacity of storm sewer infrastructure if the design proposes overflow connections from stormwater management practices (SMPs). This is especially important for private or semi-private systems. For example, there may be a high tailwater condition at the outfall or connection point during a relatively small rainfall event. The designer must account for these conditions during SMP design.

Although Federal Emergency Management Agency (FEMA) flood maps and related studies show flood-prone areas along the City’s streams and rivers, these resources do not adequately address this issue at site scale. Prior property owners, neighbors, and other local sources may be able to indicate anecdotally the extent to which on-site or downstream flooding is already a problem. The Public Health and Safety (PHS) Release Rate requirement (if applicable to the project site) is focused on specific capacity limitations in the combined sewer system, but PHS areas are not the only flood-prone areas of the City. The designer cannot rely solely on PHS applicability to determine flooding potential. The designer is referred to Chapter 1 for an overview of the PHS Release Rate requirement. An applicant with a project believed to be located within a designated PHS boundary, or who wants to learn more about whether PHS release rates apply to the project, is advised to contact PWD prior to submission.

Published FEMA Flood Maps are available at the Philadelphia City Planning Commission, which can be reached by phone at 215-683-4615, or online at FEMA’s Flood Map Service Center.

Property/Land Use Boundaries

Property/land use boundaries refer to the parcel’s non-physical boundaries, such as zoning classification and/or overlays, setbacks, and any existing easements. Suitable locations for SMPs must be identified by mapping existing property/land use features. These features often leave large spaces of undevelopable land available for non-structural design opportunities and/or structural SMPs. Boundaries must be depicted on the Existing Conditions Plan submitted during the Conceptual Review Phase. The designer is referred to Section 2.1.1 and Appendix E, Table E-2 for specific Existing Condition Plan requirements.

Physical Features

It is necessary to assess important physical features within the project site to minimize impacts to these features and to identify opportunities to use existing natural areas and drainage patterns for stormwater management. Table 3.1-1 lists critical physical features that are required to be inventoried and understood, methods and data sources for assessing these features, and notes whether each feature is required for an Existing Conditions Plan. Care must be taken to conserve and protect, or avoid, these areas, as appropriate.

Table 3.1-1: Physical Site Features Required in Existing Conditions Plan

Physical Feature Action Source


Determine the location and extent of woodlands, riparian areas, or other special habitat areas (e.g., meadows) as defined by the Pennsylvania Stormwater Best Management Practices (BMP) Manual (2006 or latest version).

Topographic survey, aerial photography, Geographic Information Systems (GIS) mapping, local and regional natural resources inventories, Pennsylvania Natural Diversity Inventory (PNDI) surveys

Soils and Geology

Determine existing soil conditions, expected permeability, hydrologic soil groups, depths to high seasonal groundwater table/bedrock, and presence of hydric soils or special geologic formations (e.g., carbonate). Document whether the site has native soils or if past development has led to fill conditions.

United States Department of Agriculture (USDA) Soil Surveys, hydrologic soil maps, existing geotechnical reports, existing soil investigation or infiltration reports, United States Geological Survey (USGS) Quadrangle Maps, USGS historic fill maps, historical aerial photography, local or regional groundwater studies or well data (Note: Usefulness of soil survey data for soils in urban settings may be limited)

Wetlands, Waterways, Floodplains, and Drainageways

Note location and type of on-site waterbodies, waterways, and floodplains. Determine existing drainage pathways and patterns, both on-site and for site runoff to off-site receiving waters.

Topographic survey, FEMA Flood Maps, aerial photography, GIS mapping

Existing Structures and Paved Areas

Determine on-site location of buildings, sheds, loading docks, parking lots, driveways, sidewalks, trails, etc.

Topographic survey, aerial photography

Existing Stormwater Infrastructure

Determine on-site locations of stormwater pipes, manholes, inlets, catch basins, outfalls, etc.

Topographic survey, utility records

Existing Utilities, Sewer, and Water Lines (Within 50 Feet of Property Lines)

Determine on-site locations of sewer pipes, manholes, force mains, water lines, water manholes, valve box covers, gas service lines, gas transmission mains, electric lines, and telephone/cable/fiber optic lines.

Topographic survey, utility records, utility locator services (PA One Call, private contractors)

Steep Slopes

Determine location of slopes of 15% or greater. Determine whether site is located in Steep Slope Protection Area as per §14-704 (2) of the Philadelphia Zoning Code.

Topographic survey, GIS topographic data, Philadelphia Zoning Code

Hotspot Investigation and Historic Fill Assessment

Understanding the presence, extent, and location of potential soil, groundwater, or surface water contamination and potentially unstable fill is an important component of characterizing existing site conditions. Infiltration of stormwater through contaminated soils has the potential to negatively impact groundwater and downstream surface water bodies. Additionally, concentrated infiltration of stormwater in areas of unstable fill can increase the potential for soil stability issues such as differential settlement and sinkhole formation. Both the presence of contamination and unstable fill can present significant risks to public health and public safety and can damage public and private property.

During this phase of the site assessment, the designer collects important information on both of these factors that may ultimately inform the placement of SMPs as described in Section 3.2.4 and whether SMPs can be designed as infiltrating facilities or must instead be designed as slow-release facilities. If a project can comply with the Stormwater Regulations solely through non-structural design techniques and/or disconnected impervious cover (DIC) (Section 3.2.2 and Section 3.2.3), a hotspot and historic fill assessment may not be needed for stormwater management purposes. However, if a development site has significant amounts of DCIA that are unlikely to be managed through non-structural techniques and DIC, the designer should proceed with the hotspot investigation and historic fill assessment procedures outlined in this Section before proceeding with the development of an integrated stormwater management plan as detailed in Section 3.2. The procedures outlined in this Section are not intended to be specific to particular SMP location. Rather, these procedures are intended to inform the eventual selection and layout of SMPs, should they be needed.

The designer must use the following hotspot investigation and historic fill assessment procedure to identify soil contamination and unstable fill risks and to evaluate the potential for implementing infiltration SMPs if these conditions are present.

Step 1 - Determine the prior land use at the site where development is proposed, and review all available data on soil and groundwater quality.

For larger development sites, a formal Phase I site assessment is often required by the lender in order to determine if any environmental hazards exist on the site. A determination of prior land use is part of this assessment.

On sites where a formal Phase I is not conducted, methods to determine prior land use may include a title search; review of aerial photographs, soil surveys, topographic maps, City and State regulatory databases; and a review of local and State records. Historic maps, records of previous construction, local knowledge or test pit data can also be used to determine whether contamination is present on-site if the site has a history of hotspots or the presence of unstable fill.

Step 2 - Determine the potential for groundwater or surface water contamination through infiltrating SMPs based on available data and prior land use (determined in Step 1), history of hotspots, and suspected/known presence of unstable fills.

The following land uses are considered to have a potential for contaminated soil, which may adversely affect the quality of groundwater discharging to surface water. These uses may qualify a project site, or portions of a project site, as a hotspot.

  • Sites designated as Comprehensive Environmental Response, Compensation, and Liability Act sites, also known as Superfund Sites,
  • Auto recycler facilities and junk yards,
  • Commercial laundry and dry cleaning facilities,
  • Commercial nurseries,
  • Vehicle fueling stations, service and maintenance areas,
  • Toxic chemical manufacturing and storage facilities,
  • Petroleum storage and refining facilities,
  • Public works storage areas,
  • Airports and deicing facilities,
  • Railroads and rail yards,
  • Marinas and ports,
  • Heavy manufacturing and power generation facilities,
  • Landfills and hazardous waste material disposal facilities, and
  • Sites located on subsurface material such as fly ash known to contain mobile heavy metals and toxins.

Infiltration is required on all sites unless the designer can show that it is not feasible. A common factor in the preclusion of infiltration is the potential for contaminant migration (Step 3). Hotspot usage and historic fill sites could contain fill material, such as fly ash, which may contain mobile metals and toxins, as well as being a potentially unstable soil. When concentrated infiltration is present within regions with known hotspot usage or fly ash fill, infiltration can lead to extensive erosion and subsidence of infill containing very fine material. The designer is responsible for investigating the presence of contaminated or unstable soils.

If Steps 1 and 2 reveal that the presence of hotspots or unstable fill is known or anticipated, the designer must proceed to the detailed testing procedures in Step 3. Before starting Step 3, the designer is encouraged to identify initial areas that could be used for stormwater management so that testing can be focused on potential SMP areas.

Step 3 (if necessary) - Conduct field investigations to further evaluate contamination and/or historic fill.

For project sites that qualify as hotspots, due diligence must be performed to determine whether any contamination is present on-site. It is not sufficient to rule out infiltration based on historical site uses alone. Testing must be performed to determine whether the site is contaminated and, if so, in what areas contamination is concentrated. Even if a site is contaminated, there may be some areas where infiltration is still feasible. Contamination must be evaluated per PA DEP guidelines, including, but not limited to, comparing testing results to PA DEP Direct Contact Medium Specific Concentration (MSC) thresholds and Soil-to-Groundwater MSC thresholds, evaluating contaminant solubility, and conducting Synthetic Precipitation Leachate Procedure (SPLP, EPA Method SW-846-1312) testing.

For project sites that anticipate the presence of unstable fill, the designer must work with a geotechnical professional to create a plan of action to identify if unstable fill exists and whether the fill is suitable for infiltration. Field testing may include, but is not limited to, soil sampling, the use of ground penetrating radar (GPR), and electromagnetic induction (EMI) scanning.

For projects that require the use of SMPs for Stormwater Regulation compliance, information on historic fill may be used as justification for the use of non-infiltrating SMPs. Many sites in Philadelphia are constructed on fill; however, the presence of fill alone does not preclude a site from installing infiltrating practices. An infiltration waiver can be requested if sufficient proof of soil instability or soil contamination is provided based on soil sampling results (Section 3.3.6). If an infiltration waiver is requested due to contamination, environmental reports for any testing completed, as well as a justification letter from the geotechnical engineer or environmental professional clearly stating why infiltration is not recommended, must also be submitted. If appropriate justification and documentation is provided to demonstrate that contamination or soil stability precludes the site from infiltrating, an impervious liner may be necessary for SMPs where stormwater is concentrated.

3.1.2 Site Factors Analysis

The final step in the site assessment is to review the information obtained in the background and site factors inventories and perform a stormwater management constraints and opportunities analysis. A stormwater constraints and opportunities analysis identifies areas where stormwater management may or may not be appropriate and assists the designer in making preliminary determinations regarding the size and layout of any development features.

Stormwater Management Constraints

Stormwater management constraints are areas on the project site where stormwater management may be difficult, infeasible, or inadvisable. These are not necessarily the same as site development constraints such as existing utilities, wetlands, riparian buffers, steep slopes, and soils with high permeability, although some of these conditions can result in stormwater management constraints. Stormwater management constraints consist of existing conditions that preclude the implementation of SMPs to mitigate stormwater quantity, rate, or quality per the Stormwater Regulations. The designer is referred to Section 3.2 for discussion of constraints related to specific design strategies and to Chapter 4 for constraints related to different SMP types.

Stormwater Management Opportunities

Once constrained areas have been determined, the designer must identify site characteristics that are favorable to stormwater management, such as soils with desirable permeability or locations for proposed discharge points (e.g., connections to existing storm sewers). Likewise, site development should be focused as much as possible in areas that provide poor opportunities for stormwater management, maximizing the areas conducive to stormwater. Certain types of critical natural areas can present both constraints to land development and significant opportunities for stormwater management. For example, riparian areas, which are not prime development areas, can sometimes be used to disconnect impervious cover (Section 3.2.3). However, the environmental impacts of implementing stormwater management in natural areas must be carefully considered.

Recognizing opportunities to reduce proposed 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 3.2 for additional guidance on stormwater management design strategies and to Section 3.5 for examples of integrated stormwater management strategies for different project types.