City of Philadelphia

The City of Philadelphia has enacted a Stormwater Ordinance in order to maintain a healthy environment for the citizens of Philadelphia, prevent flooding, protect drinking water sources, and comply with the Federal Clean Water Act. The City of Philadelphia is the permitting authority for all land disturbing activities and requires the land owner to maintain all on-site stormwater control facilities and all open space areas (e.g. parks or “green” areas) required by the approved stormwater control plan. The City of Philadelphia will only provide construction permits to projects that establish a plan to manage stormwater runoff occurring during the construction process. Stormwater fees will be calculated and collected by the city. Stormwater credits and grants are available to non-residential properties that engage in stormwater best management practices and/or maintain stormwater management facilities. The City of Philadelphia, under the NPDES program, also has the authority to inspect properties for noncompliance and can issue a notice of violation (NOV) for any deficiency or infraction onsite. Property owners are responsible for the maintenance of any stormwater facilities or practices located on the property. The City of Philadelphia has the authority to inspect stormwater facilities and practices in order to ascertain that they properly maintained and functioning.

 

Stormwater Fee

Non‐residential customers’ fees are based on the measured gross area and impervious area on their parcels.  The rates for each of these rate components are: $0.526 per 500 square feet of gross area and $4.145 per 500 square feet of impervious area.  The stormwater charge for a parcel is equal to: (gross area rate * gross area of the property) + (impervious area rate * impervious area of property). 

Gross Area (GA): All of the parcel area within the legally described boundaries except streets, medians, and sidewalks in the public right-of-way.

Impervious Area (IA): A surface, which is compacted or is covered with material that restricts infiltration of water, including semi-pervious surfaces such as compacted clay, most conventionally hard-scaped surfaces such as streets, driveways, roofs, sidewalks, parking lots, attached and detached structures, and other similar surfaces. 

 

Credits

 

Eligibility

Customers on a Non-residential or Condominium parcel with at least five-hundred (500) square feet of gross area and whose water, sewer, and SWMS accounts(s) are not past due are eligible to apply for credits.

Impervious Area Reductions (IARs) Credit

PWD currently offers IARs for the following five categories:

  1. Tree Canopy Cover
  2.  Roof Leader/Downspout Disconnections
  3. Pavement Disconnections
  4. Green Roofs
  5. Porous Pavement

IARs result in a direct reduction of the billable IA on a parcel. The customer must demonstrate that the IARs calculated on the parcel meet the requirements defined below.

Note: IARs require ongoing maintenance in order to maintain their effectiveness and are subject to renewal requirements.

Tree Canopy

A property is eligible for a Tree Canopy Cover IAR when new or existing tree canopy (from the approved species list) extends over or is in close proximity to ground level impervious area on a parcel. Under these circumstances, a portion of impervious area may be deducted from total billable IA.

The IAR is calculated for new trees as follows:

  • The tree species must be chosen from the approved list (see “Section 8: Landscape Guidance” of the Stormwater Management Guidance Manual).
  • New trees planted must be planted within 10 feet of ground level IA within the parcel boundary.
  • New deciduous trees must be at least 2-inch caliper, and new evergreen trees must be at least 6 feet tall to be eligible for the reduction.
  • A 100 square foot IA reduction is permitted for each new tree. This reduction may only be applied to the impervious area adjacent to the tree.

The IAR is calculated for existing trees as follows:

  • An IA reduction equal to half of the overlapping canopy area is permitted. The reduction may only be applied to the IA adjacent to the tree.

Roof Leader/Downspout Disconnection

A property is eligible for a Roof Leader/ Downspout Disconnection IAR when a downspout is disconnected and then directed to a pervious area which allows for infiltration, filtration, and increased time of concentration of stormwater runoff. A roof is considered to be completely or partially disconnected if it meets the following requirements:

  • The contributing area of roof to each disconnected downspout is 500 square feet or less.
  • The soil is not designated as a hydrologic soil group “D” or equivalent.
  • The overland flow path has a positive slope of 5% or less.

The IAR is calculated as the area of the roof that meets the Roof Leader/Downspout Disconnection requirements. The maximum allowable IA reduction is 500 square feet per disconnected roof leader.

Note: Disconnection of roof leaders/downspouts may require additional approvals from PWD as well as from the Office of Licenses and Inspections (L&I). PWD will support the customer in its request to obtain necessary Plumbing Code variances for approved downspout disconnections.

Pavement Disconnection

A property is eligible for a Pavement Disconnection IAR when pavement runoff is directed to a pervious area which allows for infiltration, filtration and increase time of concentration. This method is generally applicable to small or narrow pavement structures such as driveways and narrow pathways. For impervious areas that meet the requirements, all of the disconnected impervious area may be deducted from the total IA. Pavement is considered disconnected if it meets the following requirements:

  • The contributing flow path over impervious cover is no more than 75 feet
  • The width of the receiving pervious area is greater than or equal to the contributing impervious area’s width
  • The soil is not designated as a hydrologic soil group “D” or equivalent
  • The slope of the contributing impervious area is 5% or less
  • The slope of the overland flow path is 5% or less

Green Roofs

A property is eligible for a Green Roof IAR when a green roof is installed on a building. The design, construction, and maintenance plan must meet the minimum requirements specified by PWD in “Section 7: Stormwater Management Practices Design Guidelines” of the Stormwater Management Guidance Manual. Since a green roof is not a zero discharge system, the roof must be able to safely convey excess runoff to the storm sewer, combined sewer, or receiving water body.

The IAR is calculated based on the entire area of the green roof in square feet. Normal roof areas directed to a green roof may also be eligible for a Green Roof IAR. Please contact PWD’s Stormwater Credits team directly for further information.

Porous Pavement

A property is eligible for a Porous Pavement IAR when a porous pavement system is installed on a customer’s property such that it does not create any areas of concentrated infiltration. Porous pavement systems, including porous asphalt, porous concrete, porous/permeable pavers, and other approved porous surfaces, can be considered disconnected if they receive direct rainfall only and are underlain by a crushed stone infiltration bed that is at least 8 inches deep.

Porous pavement systems must meet the minimum requirements specified by the PWD in “Section 7: Stormwater Management Practices Design Guidelines” of the Stormwater Management Guidance Manual.

If the porous surface receives runoff from adjacent conventional pavement surfaces, or if roof drains or other sources of runoff are directed into the subsurface storage bed, the porous pavement/infiltration bed system will be considered a structural SMP and therefore not eligible for an IAR. However, these systems may be eligible for credit as defined in Section 4.2.5.

Calculation of SWMS Charge with Credits

In the case of parcels that are eligible for one or more classes of credits and/or IARs, the total monthly IA and GA charges are calculated in the following sequence:

- Step 1: Determine any areas that meet the IAR criteria and subtract the total IAR area from the billable IA.

- Step 2: Determine any applicable credits. The approach used in calculating the IA and GA credits varies depending on a property’s drainage characteristics and existence of an active NPDES permit for stormwater industrial discharge activities. The IA and GA credits are calculated as follows:

i. NON-SURFACE DISCHARGE Areas without NPDES Credits: For areas of a property that do not discharge stormwater to a surface water body, applicable credits are calculated as follows:

• The IA Credit is calculated by applying an 80% maximum credit factor to the parcel’s IA square footage that is managed (IA Managed).

• For the GA underneath the IA Managed, the GA Credit is calculated by applying an 80% maximum credit factor to the square footage of IA Managed.

• For any open space areas (GA minus IA), a GA Credit up of 80% may be applied, as applicable

ii. NON-SURFACE DISCHARGE Areas with NPDES Credits: For areas of a property that do not discharge stormwater to a surface water body and the property has an active NPDES permit for industrial stormwater discharge activities, applicable credits are calculated as follows:

• The IA Credit is calculated by applying an 87% maximum credit factor to the parcel’s IA square footage that is managed (IA Managed).

• For the GA underneath the IA Managed, the GA Credit is calculated by applying an 87% maximum credit factor to the square footage of IA Managed.

• For any open space areas (GA minus IA), a GA Credit maximum of 87% may be applied, as applicable.

• If the property is not eligible for any GA or IA credits, then the property will receive a 7% credit towards both the IA and GA square footage.

iii. SURFACE DISCHARGE Areas without NPDES Credits: For areas of a property that discharge stormwater to a surface water body, applicable credits are calculated as follows:

• The IA Credit is calculated by applying a 90% maximum credit factor to the parcel’s IA square footage that is managed (IA Managed).

• For the GA underneath the IA Managed, the GA Credit is calculated by applying a 90% maximum credit factor to the square footage of IA Managed.

• For any open space areas (GA minus IA), a GA Credit maximum of 90% may be applied, as applicable.

iv. SURFACE DISCHARGE Areas with NPDES Credits: For areas of a property that discharge stormwater to a surface water body and the property has an active NPDES permit or industrial stormwater discharge activities, applicable credits are calculated as follows:

• The IA Credit is calculated by applying a 97% maximum credit factor to the parcel’s IA square footage that is managed (IA Managed).

• For the GA underneath the IA Managed, the GA Credit is calculated by applying a 97% maximum credit factor to the square footage of IA Managed.

• For any open space areas (GA minus IA), a GA Credit maximum of 97% may be applied, as applicable.

- Step 3: The final billable IA and GA (Final IA and Final GA) are calculated as the total IA and GA minus any applicable IA and GA credits, determined in Step 2.

- Step 4: The IA and GA charges are then calculated based on the Final IA and GA square footage (calculated in Step 2) and the IA and GA rates defined in PWD Regulations, Section 304.0.

 

Impervious Area Stormwater Credit

To be eligible for IA Credit, the customer must demonstrate applicable management of the first inch of runoff from impervious areas on a property. As defined in the PWD Regulations, Section 304.5(c)(1), management of the first inch of runoff can be achieved by employing SMPs that manage runoff in one of the three following ways:

1. Infiltration

2. Detention and slow release

3. Volume reduction and filtration

Not all SMPs are effective in every area of the City. The types of SMPs that qualify for credits are based on the ultimate discharge location of a parcel’s runoff.

Table 1 presents the IA stormwater management options that are available to customers based on the discharge location of the parcel:

 

Combined Sewer Areas

Separate Sewer Areas

Infiltrate first 1” of runoff

Detain and slow-release the first 1” of runoff

 

Route the first 1” of runoff through an approved volume-reducing practice

 

 

Properties with Surface Discharge: Parcels with areas that discharge directly to surface water bodies may satisfy the management of the first inch of run-off by virtue of the property’s drainage characteristics.

For areas that are routed to existing SMPs before being discharged to a surface water body, customers must demonstrate that the SMPs are constructed according to the original design plans and are being maintained in a functional condition.

Gross Area Stormwater Credit

Two options are available to customers to receive GA Credit.

- Option 1: Management of the First-Inch of Runoff (Impervious Area Only)

GA Credit is automatically achieved when a parcel is approved for IA Credit through management of the first inch of runoff. The area that receives IA Credit will also receive an equivalent amount of GA Credit for the land area underneath the IA.

- Option 2: GA Credit Based on NRCS-CN (Open Space Only)

This option is only applicable to the open space of a parcel. Under this option, the customer must demonstrate a Natural Resource Conservation Service Curve Number (NRCS-CN) that meets one of the values contained in the Curve Number Scale found in Appendix 1. The CN represents the runoff potential for a particular soil and ground cover.

A percent reduction is applied for each whole number below a CN of 75. A CN of 55 may yield the maximum applicable GA Credit. The percent reduction that is applied varies depending on a property’s discharge characteristics and the existence of a NPDES permit for industrial stormwater discharge activities. The CN Scale in Appendix 1 provides the GA Credit associated with the different scenarios. More information on NRCS-CN is available in the Stormwater Management Guidance Manual.

Credits Application Rejections

If the customer disagrees with PWD’s credits decision, he or she may request a Stormwater Hearing by submitting the Stormwater Hearing Request Form. Stormwater Hearing Request Forms are available upon request from PWD.

Credits Application Fee, Cancellation

The credits application fee is non-refundable. Incomplete credits applications expire one year from the date of the original submission. Once an application expires, the customer must submit a new credits application with all supporting documentation and fee.

Credits Effective Date

Credits are effective the date that PWD receives a complete credits application, including all required supporting documentation and the application fee.

Credits Expiration

Credits expire four (4) years from their effective date. The customer must submit a credits renewal application (FORM C) at least thirty (30) days before the expiration date to renew the stormwater credits. The approved credits renewal is effective on the expiration date of the original credit.

If the customer fails to submit a renewal application at least thirty (30) days before the expiration date, then the credits may expire, as PWD cannot guarantee the renewal will be processed prior to the next billing period. Note: Should the customer fail to reapply within 30 days of the expiration date, the customer will not receive credit towards the SWMS Charge during the months in which the renewal application was past due and the credits had lapsed.

Credits Application Processing

PWD will conduct technical review of credits only after receipt of a complete application, including the fee and all required supporting documentation. PWD will review a credits application only if the customer’s water, sewer, and stormwater charge payments pertaining to the parcel are current.

Site Inspections

After a customer submits a credits or credits renewal application, PWD may inspect the parcel to verify the information provided in the application and in the supporting documentation. It is the responsibility of the customer to allow PWD access to the parcel.

Following submission of a credits or credits renewal application, the customer shall grant the City, its employees, or authorized agents permission to enter the parcel on providing forty-eight (48) hour written notice and, in any case, at reasonable times and without unreasonable disruption, to inspect the parcel to ensure that the information provided in the application accurately represents the current parcel conditions.

Termination of Credits

PWD may review and terminate one or more classes of approved credits at any time if SMPs associated with those credits are found to be improperly maintained and/or not adequately functional. Customers may from time to time be asked to submit documentation and/or grant access to the parcel receiving credit. Failure to comply with such requests may result in the termination of credit.

SWMS Charge Payment

A pending credits application shall not constitute a valid reason for non-payment of the SWMS charge.

Documentation Costs

The customer is solely responsible for costs incurred in the preparation of the required documentation and/or the submission of the credits application.

Application Review Priority

If a customer submits an adjustment appeals and a credits application at the same time, the adjustment appeals application will be reviewed and processed first, followed by the credits application.

Stormwater Credits Application Requirements

Application

FORM B: The customer must submit a complete FORM B - Stormwater Credits Application when applying for an initial credit or additional credits. The customer may request one or more classes of credit in the application.

FORM C: The customer must submit a complete FORM C - Stormwater Credits Renewal Application when renewing existing credits. Each application must be submitted with the appropriate fee and supporting documentation.

Supporting Documentation

The following supporting documentation is required for each of the three classes of credits:

Impervious Area Stormwater Credit

- Post-2006 Development: Parcels that have redeveloped since 2006 and were subject to the PWD Stormwater Regulations must be compliant in order to be eligible for stormwater credit. Appropriate documentation, including as-built plans and a construction certification package, must be on file with PWD Plan Review before credits can be evaluated. Please contact PWD Plan Review (pwd.planreview@phila.gov) for questions concerning post-construction documentation requirements.

- Pre-2006 Development: Parcels with stormwater facilities constructed prior to 2006 must demonstrate compliance with current stormwater credits regulations in order to be eligible. Appropriate documentation, including as-built plans and associated engineering calculations, must be submitted with a completed application (FORM B). Please contact the PWD Credits Program (pwd.stormwatercredits@phila.gov) for questions concerning submission requirements.

- Stormwater Retrofits: Customers that have implemented stormwater retrofits must also provide appropriate post-construction documentation to PWD before credits can be evaluated. At a minimum, this includes as-built drawings and a construction certification package. Please contact the PWD Credits Program (pwd.stormwatercredits@phila.gov) for questions concerning post-construction stormwater management documentation requirements.

- Surface Discharge: Customers with areas that discharge to a surface water body must provide a plan indicating drainage conditions and topography. Areas that discharge to a surface water body should be delineated on the plan.

Impervious Area Stormwater Credit Renewal

The customer must provide all of the following along with a completed credits renewal application (FORM C):

- Recent photos of the SMP.

- For areas receiving credit because of SMPs, record of inspections and maintenance activity for each SMP and a letter certified by a Registered Professional verifying that the SMP has been observed following a runoff event meeting the original stormwater management requirements.

- For areas receiving credit because of surface water discharge, a letter certified by a Registered Professional that drainage conditions have not changed since the date of the original credits application.

Gross Area Stormwater Credit

- Option 1: Management of the First-Inch of Runoff (Impervious Area Only)

If a customer is granted an IA Credit, a GA Credit for the same square footage of the IA managed will also be granted. No additional documentation is required from the customer to support approval of the GA Credit.

- Option 2: GA Credit Based on NRCS-CN (Non-Impervious Area Only)

The customer must provide the following along with a completed application (FORM B):

• Plan delineating cover types and hydrologic soil groups

• Calculations supporting computed CN

Gross Area Stormwater Credit Renewal

- Option 1: Management of the First-Inch of Runoff (Impervious Area Only)

Renewal of GA Credits approved under Option 1 will be processed in conjunction with the associated IA Credit renewal requirements. No additional documentation is required for the GA Credit renewal.

- Option 2: GA Credit Based on NRCS-CN (Non-Impervious Area Only)

The customer must provide the following along with a completed renewal application (FORM C):

• Recent site aerial image or photos of the site.

Fees

Stormwater Credits Application: $150

Stormwater Credits Renewal Application: $50

Fees are non-refundable and may be paid by check or money order to The City of Philadelphia Water Department.

 

 

Grants

 

Stormwater Management Incentives Program Grant

 

Eligibility

Only non-residential properties are eligible for SMIP grant funds. Applicants must be owners of the property or have permission from the property owner(s). The applicant cannot be an agency with the City of Philadelphia, the Commonwealth of Pennsylvania or any United States Department or Federal Agency. Use of funds is restricted to projects that support the design and construction of stormwater mitigation measures. These may include, but are not limited to: detention and retention basins, green roofs, porous paving, and rain gardens.

Evaluation Requirements

Projects will be evaluated based on a variety of criteria, including, but not limited to, the total volume of runoff managed, cost-competiveness, environmental and educational benefits. Competitive applications will limit grant requests to $100,000 per impervious acre managed or less and manage at least the first one inch of stormwater runoff. All recipients will be expected to execute an Economic Opportunity Plan and Operations & Maintenance Agreement as part of the subgrant agreement.

 

Greened Acre Retrofit Program Grant

 

Eligibility

Funding for the Greened Acre Retrofit Program (GARP) is reserved for stormwater retrofit projects on private property in the combined sewer area only. Properties undergoing redevelopment are not eligible for GARP funding and must comply with PWD’s Stormwater Regulations. Recipients of the grant funds are limited to companies and project aggregators that can assemble large areas, often over multiple properties, for stormwater management projects. The recommended minimum project size is 10 acres.

Evaluation Requirements

GARP applications will be evaluated based on a variety of criteria including total area managed, cost to PWD, quality of long-term maintenance plan and availability of matching funds. Competitive applications will limit grant requests to $90,000 per impervious acre managed or less. Agreements or contracts with any participating property owners must be included in the application.

 

Stormwater BMPs

 

Green Roofs

Green roofs consist of a layer of vegetation that completely covers an otherwise conventional flat or pitched roof. The hydrologic response of a green roof bears closer resemblance to a lawn or meadow than impervious surface. The green roof system is composed of multiple layers including waterproofing, a drainage layer, engineered planting media, and specially selected plants. Vegetated roof covers can be optimized to achieve water quantity and quality benefits. Through the appropriate selection of materials, vegetated covers can provide rainfall retention and detention functions.

Key Elements:

• Extensive green roofs with engineered media at least 3 inches in depth can be considered pervious in stormwater design calculations.

• Vegetated roof covers intended to achieve water quality benefits should maintain a soluble nitrogen level of 4ppm.

•Internal drainage, including provisions to cover and protect deck drains or scuppers, must anticipate the need to manage large rainfall events without inundating the cover.

• Provide urban green space and aesthetically pleasing views.

• Act as heat sink to reduce heating and cooling costs.

• Can extend roof life by two to three times.

•Improve air quality by filtering dust particles.

Maintenance Guidelines

All facility components, including plant material, growing medium, filter fabric, drainage layer, waterproof membranes, and roof structure should be inspected for proper operations, integrity of the waterproofing, and structural stability throughout the life of the green roof.

Activity

Schedule

• Roof drains should be cleared when soil substrate, vegetation, debris or other materials clog the drain inlet. Sources of sediment and debris may be identified and corrected.

• Plant material should be maintained to provide 90% plant cover. Weeding should be manual with no herbicides or pesticides used. Weeds should be removed regularly.

• Irrigation can be accomplished either through hand watering or automatic sprinkler systems if necessary during the establishment period.

As needed

• Growing medium should be inspected for evidence of erosion from wind or water. If erosion channels are evident, they can be stabilized with additional growth medium similar to the original material.

Quarterly

• Inspect drain inlet pipe and containment system.

• Test growing medium for soluble nitrogen content. Fertilize as needed.

Annually

• Maintain a record of all inspections and maintenance activity.

Ongoing

 

• Fertilization should be minimized. Fertilization should be applied according to soil test in order to maintain soluble nitrogen (nitrate and ammonium ion) levels between 1 and 4 ppm. The best source of nutrients for fertilization is mature compost.

• During the plant establishment period, maintenance staff should conduct 3-4 visits per year to conduct basic weeding, fertilization, and in-fill planting. Thereafter, only two annual visits for inspection and light weeding should be required (irrigated assemblies will require more intensive maintenance).

• Spill prevention measures from mechanical systems located on roofs should be exercised when handling substances that can contaminate stormwater.

 

Rain Barrels

Rain barrels, cisterns, and tanks are structures designed to intercept and store runoff from rooftops. Rain barrels are used on a small scale, while cisterns and tanks may be larger. These systems may be above or below ground, and they may drain by gravity or be pumped. Stored water may be slowly released to a pervious area. This technique only serves an effective stormwater control function if the stored water is emptied between most storms, freeing up storage volume for the next storm. Irrigation as a use for runoff stored in a cistern is not an acceptable strategy for meeting the Regulations.

The design recommendations within this Section serve as guidance only. Site-specific parameters, such as anticipated water demand, may dictate alternative designs and calculations which will be reviewed accordingly.

Key Elements:

• Storage devices designed to capture a portion of small, frequent storm events.

• Storage techniques may include rain barrels, underground concrete or prefabricated tanks, above ground vertical storage tanks, or other systems.

• Systems must provide for overflow or bypass of large storm events.

• Placement of storage elements higher than areas where water will be reused may reduce or eliminate pumping needs.

• For effective stormwater control, water must be used or discharged before the next storm event.

• Most effective when designed to meet a specific water need for reuse.

Maintenance Guidelines

As with other stormwater management practices (SMPs), these stormwater storage systems require regular maintenance to ensure a prolonged life. The following table suggests maintenance activities to perform on rain barrels, cisterns, or vertical storage.

Activity

Schedule

• Occasional cleaning may be necessary to remove debris, such as leaves, coming off the drainage area.

As needed

• Flush cisterns to remove sediment.

• Brush the inside surfaces and thoroughly disinfect.

• To avoid structural damage, the rain barrel should be drained prior to freezing weather.

Annually

• Maintain records of all inspections and maintenance activity

Ongoing

 

Filter Strips

Filter strips are densely vegetated lands that treat sheet flow stormwater from adjacent pervious and impervious areas. They function by slowing runoff, trapping sediment and pollutants, and in some cases infiltrating a portion of the runoff into the ground. Filter strips are a sensible and cost-effective stormwater management pretreatment option applicable to a variety of development sites including roads and highways.

Key Elements:

• Filters strips are only considered a viable pretreatment option for other SMPs.

• Sheet flow across the vegetated filter strip is mandatory for proper filter strip function.

• Filter strip length is a function of slope, vegetation type, soil type, drainage area, and desired amount of pretreatment.

• Level spreading devices are recommended to provide uniform sheet flow conditions at the interface of the filter strip and the adjacent land cover.

• The longest flow path to a filter strip, without the installation of energy dissipaters and/or flow spreaders, is 75 feet for impervious ground covers and 150 feet for pervious ground covers.

• Filter strip slope should never exceed 8%. Slopes less than 5% are generally preferred.

• Maximum contributing drainage area is less than 5 acres, and should also never exceed a drainage area to filter strip area ratio of 6:1.

• Maximum contributing drainage area slope is generally less than 5%, unless energy dissipation and/or flow spreaders are provided.

• Construction of filter strips shall entail as little disturbance to existing vegetation at the site as possible.

Maintenance Guidelines

All areas of the filter strip should be inspected after significant storm events for ponding that exceeds maximum depth and duration guidelines. Corrective measures should be taken when excessive ponding occurs.

Activity

Schedule

• Mowing and/or trimming of vegetation (not applicable to all filter strips). Filter strips that need mowing are to be cut to a height no less than 4 inches. Greater than 5 inches is preferred.

As needed

• Inspect all vegetated strip components expected to receive and/or trap debris and sediment for clogging and excessive debris and sediment accumulation; remove sediment during dry periods.

Quarterly

• Vegetated areas should be inspected for erosion, scour, and unwanted growth. This should be removed with minimum disruption to the planting soil bed and remaining vegetation.

• Inspect all level spreading devices for trapped sediment and flow spreading abilities. Remove sediment and correct grading and flow channels during dry periods.

Biannually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

When correcting grading of a flow spreading device, use proper erosion and sediment control precautions in the concentrated area of disturbance to ensure protection of the remaining portion of the filter.

• Disturbance to filter strips should be minimal during maintenance. At no time should any vehicle be driven on the filter strip. In addition, foot traffic should be kept to a minimum.

• If the filter strip is of the type that needs mowing (i.e., turf grass and possibly other native grasses), the lightest possible mowing equipment (i.e., push mowers, not riding mowers) should be used. The filter strip should be mowed perpendicular to the flow path (however not exactly the same path every mowing) to prevent any erosion and scour due to channeling of flow in the maintenance depressions.

• When establishing or restoring vegetation, biweekly inspections of vegetation health should be performed during the first growing season or until the vegetation is established.

• Bi-weekly inspections of erosion control and flow spreading devices should be performed until soil settlement and vegetative establishment has occurred.

 

Filters

Filters are structures or excavated areas containing a layer of sand, compost, organic material, peat, or other filter media. They reduce pollutant levels in stormwater runoff by filtering sediments, metals, hydrocarbons, and other pollutants. Filtered stormwater may be infiltrated or released to a sewer or receiving water. Depending on design, the filter media may provide significant detention time or may be combined with an outlet control.

Key Elements:

• Acceptable technique on sites where vegetated systems are impractical.

• Surface ponding that drains down in no more than 72 hours.

• Filter medium (typically sand, peat, or a mixture) removes pollutants and provides some travel time.

• Underdrain allowed on sites where infiltration is infeasible, or where a filter is used in combination with other practices.

• Flow splitter or positive overflow bypasses large storms.

• Maintenance required to maintain capacity of system.

Maintenance Guidelines

For filters located entirely underground, unobstructed access must be provided over the entire sand filter, including inlet and outlet pipe structures, by either doors or removable panels. Ladder access is required for vault heights greater than 4 feet.

Activity

Schedule

• Rake filter media surface for the removal of trash and debris from control openings.

• Repair of leaks from the sedimentation chamber or deterioration of structural components.

As needed

• Inspect filter for standing water (filter drainage is not optimal) and discoloration (organics or debris have clogged filter surface).

Quarterly

• Removal of the top few inches of filter media and cultivation of the surface when filter bed is clogged.

• Clean out accumulated sediment from filter bed chamber.

• Clean out accumulated sediment from sedimentation chamber.

Annually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

In areas where the potential exists for the discharge and accumulation of toxic pollutants (such as metals), filter media removed from filters must be handled and disposed of in accordance with all State and Federal Regulations.

Winter concerns

Pennsylvania’s low temperature dips below freezing for about four months out of every year, and surface filtration may not take place as well in the winter. Peat and compost may hold water, freeze, and become impervious on the surface. Design options that allow direct sub-surface discharge into the filter media during cold weather may help overcome this condition.

 

Bioinfiltration/Bioretention

Bioinfiltration/Bioretention systems use surface storage, vegetation, a select growing medium, flow controls, and other components to meet stormwater management goals. These systems may be referred to by a variety of names such as bioinfiltration areas, biofilters, rain gardens, or recharge gardens. On a small scale, these systems may be contained inside planter boxes. This section will refer to all these systems as bioretention.

Key Elements:

• Preferred stormwater management design that replicates natural hydrologic processes.

• Flexible in size and configuration; can be used for a wide variety of applications.

• Water Quality volume that drains down in no more than 72 hours.

• Modified soil that provides temporary stormwater storage and enhances plant growth.

• Native plantings that provide evapotranspiration of stormwater, remove pollutants, and enhance the landscape.

• Positive overflow limits inundation depth.

• Maintenance of vegetation is required.

Maintenance Guidelines

Properly designed and installed bioretention systems require little maintenance. During periods of extended drought, bioretention systems may require watering approximately every 10 days.

Activity

Schedule

• Water vegetation at the end of each day for two weeks after planting is completed.

• Water vegetation regularly to ensure successful establishment.

First year after installation

• Remulch void areas.

• Treat diseased trees and shrubs.

• Keep overflow free and clear of leaves.

As needed

• Inspect soil and repair eroded areas.

• Remove litter and debris.

• Clear leaves and debris from overflow.

Monthly

• Inspect trees and shrubs to evaluate health, replace if necessary.

• Inspect underdrain cleanout.

• Verify drained out time of system.

Biannually

• Add additional mulch.

• Inspect for sediment buildup, erosion, vegetative conditions, etc.

Annually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

 

Detention Basins

Detention basins are constructed to provide temporary surface or subsurface storage of runoff and function hydraulically to attenuate stormwater runoff peaks. Traditional detention basins function primarily to provide water quantity control. The designer should note that detention basins can also be configured to provide water quality treatment. These designs are referred to as dry extended detention basins. More information on dry extended detention basins can be found in the Pennsylvania Stormwater BMP Manual.

Key Elements:

• Detention basins should completely drain in 72 hours.

• Most basins are designed to provide Channel Protection and Flood Control only.

• A sediment forebay helps decrease maintenance and prolong design life of the basin.

• Vegetation stabilizes the soil in the basin.

• Outlet structure design is critical and determines how the basin meets stormwater control requirements.

Maintenance Guidelines

• Maintenance is required for the proper operation of detention basins.

Activity

Schedule

• Remove trash and debris.

• Remove invasive plants.

• Grassed areas require periodic prudent fertilizing, dethatching and soil conditioning.

• Trees, shrubs, and other vegetative cover will require periodic maintenance such as fertilizing, pruning, and pest control.

• Mow / trim detention basin vegetation.

As needed

• Sediment should be removed from the basin.

As needed

(at least once every 5 to 25 years)*

* The frequency of sediment removal depends on site conditions such as soil type and maintenance of site stabilization which influence the sediment load on the basin.

• Inspect outlet control structure for clogging.

Quarterly and after every storm greater than 1 inch

• Inspect detention basin for potential problems including: subsidence, erosion, cracking or tree growth on the embankment; damage to the emergency spillway; sediment accumulation around the outlet; inadequacy of the inlet/outlet channel erosion control measures; changes in the condition of the pilot channel; and erosion within the basin and banks.

Annually

• Maintain records of all inspections and maintenance activity.

Ongoing

                                          

• In most cases, no specific limitations have been placed on disposal of sediments removed from detention basins. Studies to date indicate that pond sediments are likely to meet toxicity limits and can be safely landfilled. On-site sediment disposal is always preferable as long as the sediments are deposited away from the shoreline to prevent their re-entry into the pond and away from recreation areas where people could inhale resulting dust. Information regarding sediment disposal should be provided to the property owner by the design engineer.

• Sediments should be tested for toxicants in compliance with current disposal requirements if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed.

 

Berms

Berms and retentive grading techniques use a site’s topography to manage stormwater and avoid erosion. They may function alone in grassy areas or may be incorporated into the design of other stormwater control facilities such as bioretention and constructed wetlands. They are landscaped features placed parallel to existing contours that direct runoff while promoting retention and infiltration of stormwater.

Key elements:

• High quality topsoil in outer layer of berm that provides growing medium for plants (minimum 4 inches).

• Inner layer of berm constructed of a stable fill material.

• Established vegetation to prevent erosion and improve appearance.

• An overflow weir or runoff bypass mechanism.

Maintenance Guidelines

Infiltration berms have low to moderate maintenance requirements, depending on the design.

Activity

Schedule

• Remove trash and debris.

• Remove invasive plants.

• If desired, mow grass to maintain 2 – 4 inch height.

As needed

• Inspect soil for erosion and repair eroded areas.

Monthly

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Swales

A swale is an open channel vegetated with a combination of grasses and other herbaceous plants, shrubs, and trees. A traditional swale reduces peak flow at the discharge point by increasing travel time and friction along the flow path. A swale provides some infiltration and water quality treatment; these functions can be enhanced by adding check dams periodically along its length. Swales planted with turf grass provide some of these functions but turf grass is not as effective as deeper-rooted vegetation at decreasing peaks, allowing infiltration, and decreasing erosion. A swale can be more aesthetically pleasing than a concrete or rock-lined drainage system and is generally less expensive to construct.

Key Elements:

• Open channel design that balances storage, treatment, and infiltration with peak flow conveyance needs

• Check dams often used to increase storage, dissipate energy, and control erosion

• Native vegetation increases friction and stabilizes soil

• Designed to fit into many types of landscapes in an aesthetically pleasing manner

Maintenance Guidelines

The following schedule of inspection and maintenance activities is recommended:

Activity

Schedule

• Remulch void areas.

• Treat or replace diseased trees and shrubs.

• Keep overflow free and clear of leaves.

As needed

• Inspect soil and repair eroded areas.

• Remove litter and debris.

• Clear leaves and debris from overflow.

Monthly

• Inspect trees and shrubs to evaluate health.

Biannually

• Add additional mulch.

• Inspect for sediment buildup, erosion, vegetative conditions, etc.

Annually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Constructed Wetland

Constructed Wetlands are shallow marsh systems planted with emergent vegetation that are designed to treat stormwater runoff.

Key Elements:

· Adequate drainage area (usually 5 to 10 acres minimum) or proof of sustained base flow

· May require investigation of water supply to ensure a sustained baseflow to maintain the wetland

· Maintenance of permanent water surface

· Multiple vegetative growth zones through varying depths

· Robust and diverse vegetation

· Relatively impermeable soils or engineered liner

· Sediment collection and removal

· Adjustable permanent pool and dewatering mechanism

· Maintenance - periodic sediment removal from the forebay and vegetation maintenance

Maintenance Issues

Constructed Wetlands must have a maintenance plan and privately owned facilities should have an easement, deed restriction, or other legal measure to prevent neglect or removal. During the first growing season, vegetation should be inspected every 2 to 3 weeks. During the first 2 years, CWs should be inspected at least 4 times per year and after major storms (greater than 2 inches in 24 hours). Inspections should access the vegetation, erosion, flow channelization, bank stability, inlet/outlet conditions, and sediment/debris accumulation. Problems should be corrected as soon as possible. Wetland and buffer vegetation may require support – watering, weeding, mulching, replanting, etc. – during the first 3 years. Undesirable species should be removed and desirable replacements planted if necessary.

Once established, properly designed and installed Constructed Wetlands should require little maintenance. They should be inspected at least semiannually and after major storms as well as rapid ice breakup. Vegetation should maintain at least an 85 percent cover of the emergent vegetation zone. Annual harvesting of vegetation may increase the nutrient removal of CWs; it should generally be done in the summer so that there is adequate regrowth before winter. Care should be taken to minimize disturbance, especially of bottom sediments, during harvesting. The potential disturbance from harvesting may outweigh its benefits unless the CWs receives a particularly high nutrient load or discharges to a nutrient sensitive waterbody. Sediment should be removed from the forebay before it occupies 50 percent of the forebay, typically every 3 to 7 years.

 

Wet Pond/Retention Basin

Wet Ponds/Retention Basins are stormwater basins that include a substantial permanent pool for water quality treatment and additional capacity above the permanent pool for temporary runoff storage.

Key Elements:

· Adequate drainage area (usually 5 to 10 acres minimum) or proof of sustained baseflow

· Natural high groundwater table

· Maintenance of permanent water surface

· Should have at least 2 to 1 length to width ratio

· Robust and diverse vegetation surrounding wet pond

· Relatively impermeable soils

· Forebay for sediment collection and removal

· Dewatering mechanism

Maintenance Issues

Wet Ponds should have a maintenance plan and privately owned facilities should have an easement, deed restriction, or other legal measure to prevent neglect or removal. During the first growing season or until established, vegetation should be inspected every 2 to 3 weeks. WPs should be inspected at least 4 times per year and after major storms (greater than 2 inches in 24 hours) or rapid ice breakup. Inspections should access the vegetation, erosion, flow channelization, bank stability, inlet/outlet conditions, embankment, and sediment/debris accumulation. The pond drain should also be inspected and tested 4 times per year. Problems should be corrected as soon as possible. Wet Pond and buffer vegetation may need support (watering, weeding, mulching, replanting, etc.) during the first 3 years. Undesirable species should be carefully removed and desirable replacements planted if necessary.

Once established, properly designed and installed Wet Ponds should require little maintenance. Vegetation should maintain at least an 85 percent cover of the emergent vegetation zone and buffer area. Annual harvesting of vegetation may increase the nutrient removal of WPs; if performed it should generally be done in the summer so that there is adequate regrowth before winter. Care should be taken to minimize disturbance, especially of bottom sediments, during harvesting. The potential disturbance from harvesting may outweigh its benefits unless the WP receives a particularly high nutrient load or discharges to a nutrient sensitive waterbody. Sediment should be removed from the forebay before it occupies 50 percent of the forebay, typically every 5 to 10 years.

 

Subsurface Vaults

Subsurface vaults are underground structures designed primarily to reduce peak stormwater flows, although in some cases they may allow infiltration. They are usually constructed of either concrete or corrugated metal pipe (CMP) and must account for the potential loading from vehicles. Pretreatment structures can be used at the inlet to treat stormwater runoff and remove debris. A permanent pool can also be incorporated to dissipate energy and improve the settling of particulate stormwater pollutants. Dry systems are primarily used for volume control or in combination with pretreatment, whereas wet systems include a permanent pool and provide water quality treatment.

Key Elements:

• Effective for urban areas with limited space for SMPs.

• More effective in areas of combined sewer than in areas of separate sewers.

• Provides peak rate control.

• Pretreatment may be included to remove sediment and pollutants associated with sediment.

• Traffic loading capabilities.

• Maintenance required periodically to remove sediment and debris.

Maintenance Guidelines

The systems must be designed so that the vault can have easy access for inspection and maintenance. Vault maintenance procedures must meet OSHA confined space entry requirements, which include clearly marking entrances to confined space areas. This may be accomplished by hanging a removable sign in the access riser(s), just under the access lid.

Activity

Schedule

• Removal of sediment and debris from subsurface vault sedimentation chamber when the sediment zone is full. Sediments should be tested for toxicants in compliance with current disposal requirements if land uses in the catchment include commercial or industrial zones, or if indications of pollution are noticed.

As needed

• Inspection of subsurface vault and control structures.

• Floating debris and accumulated petroleum products should be removed.

Quarterly

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Subsurface Infiltration

Subsurface infiltration systems are designed to provide temporarily below grade storage infiltration of stormwater as it infiltrates into the ground. Dry wells, infiltration trenches and beds are a few examples of these types of systems.

Key Elements:

• Infiltration testing is required for this SMP.

•Reduce volume of runoff from a drainage area by promoting infiltration though uncompacted subgrade.

• Flexible design can be sited beneath lawns, parking areas, and recreational areas.

• Maintain minimum distance of 10 feet from building foundations and property lines not abutting Public Right-of-Way.

• Open-graded aggregate or other approved material provides storage.

• System must be designed to drain down in less than 72 hours.

• Greater than 2 feet from any limiting zone such as groundwater or bedrock.

• Pre-treatment is required.

• Positive overflow required for large storms.

• Areas of soil contamination or areas of unstable soils should be avoided.

Maintenance Guidelines

As with all infiltration practices, subsurface infiltration systems require regular and effective maintenance to ensure prolonged functioning. The following table describes minimum maintenance requirements for subsurface infiltration systems.

Activity

Schedule

• Regularly clean out gutters and catch basins to reduce sediment load to infiltration system. Clean intermediate sump boxes, replace filters, and otherwise clean pretreatment areas in directly connected systems.

As needed

• Inspect and clean as needed all components of and connections to subsurface infiltration systems.

• Evaluate the drain-down time of the subsurface infiltration system to ensure the drain-down time of 24-72 hours.

Biannually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Porous Pavement

Porous pavement provides the structural support of conventional pavement, but allows stormwater to drain directly through the surface into the underlying base and soils, thereby reducing stormwater runoff. There are porous varieties of asphalt, concrete, and interlocking pavers. Porous pavements are designed with an open graded subbase that allows water to pass through to the native soil and provides temporary storage.

Key Elements:

• Pervious structural surface with high infiltration rate.

• Porous surface and stone sub-base suitable for design traffic loads. Can be used on most travel surfaces with slopes less than 5%.

• Uncompacted, level sub-grade allows infiltration of stormwater.

• Open-graded aggregate sub-base provides storage.

• Additional storage and control structures can be incorporated to meet channel protection and flood control.

• Positive overflow prevents system flooding.

Maintenance Guidelines

As with most SMPS, porous pavement systems require regular maintenance to extend their life. The following table displays maintenance recommendations for porous pavement systems.

Activity

Schedule

• Mow grass in paver or grid systems that have been planted with grass.

As needed

• Vacuum porous asphalt or concrete surface with commercial cleaning unit (pavement washing systems and compressed air units are not recommended).

• Clean out inlet structures within or draining to the subsurface bedding beneath porous surface.

Biannually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Sediment Control

Superficial dirt does not necessarily clog the voids in porous surfaces. However, dirt that is ground in repeatedly by tires can lead to clogging. Therefore, trucks or other heavy vehicles should be prevented from tracking or spilling dirt onto the pavement. Furthermore, all construction or hazardous materials carriers should be prohibited from entering a porous pavement lot.

Winter Maintenance

Winter maintenance for a porous parking lot may be necessary, but is usually less intensive than that required for a standard asphalt lot. By its very nature, a porous pavement system with subsurface aggregate bed has better snow and ice melting characteristics than standard pavement. Once snow and ice melt, they flow through the porous pavement rather than refreezing. Therefore, ice and light snow accumulation are generally not as problematic. However, snow will accumulate during heavier storms. Abrasives such as sand or cinders shall not be applied on or adjacent to the porous pavement. Snow plowing is acceptable, provided it is done carefully (i.e. by setting the blade about one inch higher than usual). Salt is acceptable for use as a deicer on the porous pavement, though non-toxic, organic deicers, applied either as blended, magnesium chloride-based liquid products or as pretreated salt, are preferable. Any deicing materials should be used in moderation.

Repairs

Potholes are not common; though settling might occur if a soft spot in the subgrade is not removed during construction. Damaged areas that are smaller than 50 square feet can be patched with a porous or standard asphalt mix, depending on the location within the porous area. In many cases the loss of porous surface will be insignificant. If an area greater than 50 square feet is in need of repair, approval of patch type must be sought from either the engineer or owner. Porous pavement must never be seal coated under any circumstances. Any required repair of drainage structures should be done promptly to ensure continued proper functioning of the system.

Inlet & Outlet Controls

Inlet and outlet controls are the structures or landscape features that manage the flow into and out of a stormwater management facility. Flow splitters, level spreaders, curb openings, energy dissipaters, traditional inlets, and curbless design are all examples and elements of inlet controls. Outlet controls regulate the release of stormwater from a management facility. Examples of outlet controls include risers and orifices, underdrains, permeable weirs, positive overflows, and impervious liners. Outlet control structures limit flow to meet release rate requirements and bypass larger flows to prevent re-suspension of sediment, hydraulic overload, or erosion of management practices.

Key Elements:

• Inlet Controls: Flow splitters divert a portion of the storm hydrograph to a management facility, while allowing the remainder of the flow to bypass the facility.

• Inlet Controls: Curbless roads, streets, and parking lots allow stormwater to sheet flow into a SMP.

• Inlet Controls: Curb openings allow water to flow through a curb that would otherwise block the flow.

• Inlet Controls: Level spreaders spread out concentrated flow and release it as low-velocity, non-erosive diffuse flow.

• Inlet Controls: Large-scale energy dissipaters slow down and spread flow from culverts and steeper slopes.

• Outlet Controls: Risers and orifices release ponded water at a reduced rate.

• Outlet Controls: Positive overflows allow stormwater to safely flow out of an SMP.

• Outlet Controls: Underdrains collect water that has filtered through a porous medium and convey it to an outlet.

• Outlet Controls: Impervious liners prevent water from infiltrating the soil where infiltration is not desirable.

• Outlet Controls: Permeable weirs allow water to flow slowly through smaller openings and more quickly over the top of the weir.

Maintenance Concerns for Inlet Controls

Activity

Schedule

• Inlet control devices should be inspected after several storms to ensure that they are functioning properly and that there are no erosion problems developing.

• Source of sediment contamination should be identified and controlled when native soil is exposed or erosion channels are present.

As needed

• Inspected for sediment and debris buildup. Sediment buildup exceeding 2 inches in depth or that begins to constrict the flow path should be removed.

• Clean out leaves, trash, debris, etc.

Biannually

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Maintenance Concerns for Outlet Controls

Activity

Schedule

• Outlet control devices should be inspected after several storms to ensure that they are functioning properly and that there are no erosion problems developing.

• Source of sediment contamination should be identified and controlled when native soil is exposed or erosion channels are present.

As needed

• Inspected for sediment and debris buildup. Sediment buildup exceeding 2 inches in depth or that begins to constrict the flow path should be removed.

• Clean out leaves, trash, debris, etc.

Biannually

(Quarterly for small orifice designs)

• Maintain records of all inspections and maintenance activity.

Ongoing

 

Sediments should be tested for toxicants in compliance with current disposal requirements if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed.

 

LINKS:

Stormwater Incentives Grant Manual

Stormwater Management Incentives Program Grant Fact Sheet

Greened Acre Retrofit Program Grant Fact Sheet

Form A: Stormwater Management Service Charge Adjustment Appeals Application

Form B: Stormwater Credits Application

Form C: Stormwater Credits Renewal Application

Stormwater Management Service Charge Revised Charge Allocation

Storm Water Management Service Charge Credits and Adjustment Appeals Manual

Stormwater Management Guide Chapter 1 - Introduction

Stormwater Management Guide Chapter 5 - Post Construction Stormwater Management Plans

Philadelphia Water Department Stormwater Management Regulations