Municipality of Monroeville, PA Article IV Permanent Stormwater Management Design Standards

§ 319-19 Design goals, principles, and standards.

A. Design goals.

(1) Achieve post-development hydrologic conditions that are consistent with the requirements of this chapter. The design goals are:

(a) Minimize the volume of runoff that must be collected, conveyed, treated and released by stormwater management facilities;

(b) Maintain the natural infiltration process and rate, and infiltrate runoff at its source when appropriate;

(c) Remove and/or treat pollutants at the source or during conveyance;

(d) Provide for peak flow attenuation, as needed;

(e) Attenuate runoff to protect the channel of the receiving stream;

(f) Conform to all requirements of the Turtle Creek Watershed Act 167 Stormwater Management Plan. The location and boundaries of all subareas are shown on the current "Assigned Release Rate Percentages Map," which is hereby adopted as a part of this chapter.

B. General principles.

(1) The following general principles apply to all applicable activities pursuant to § 319-5:

(a) Incorporate design practices to minimize the amount of stormwater generated on a site. Encourage the disconnection of impervious land cover. Maximize the use of pervious areas for stormwater treatment and on-site rainfall infiltration.

(b) Infiltration of surface water runoff at its source can be a mechanism for stormwater management based on infiltration testing results. Infiltration practices include, but are not limited to, those referenced in § 319-21 and as outlined in the publications listed in § 319-21. Infiltration practices shall adhere to the following criteria:

[1] In choosing methods of infiltration, preference shall be given to a combination of surface and subsurface infiltration methods.

[2] Applicants shall first consider minimum disturbance/minimum maintenance techniques combined with site grading that distributes runoff to reduce concentration. Next, applicants shall consider depression areas combined with subsurface infiltration practices followed by other subsurface measures, including, but not limited to, pervious pavers and perforated pipe storage.

[3] The use of multiple infiltration features and facilities that provide for the following is encouraged:

[a] Discourage concentration of flows;

[b] Encourage disconnection of flows;

[c] Infiltrate as close to the source of runoff as possible; and

[d] Reduce visual impact.

[4] Where site constraints preclude achieving the required infiltration volume, additional conservation design practices and alternative stormwater management practices should be implemented to reduce to the maximum extent practicable the total volume of stormwater released to streams. The applicant shall follow the stormwater runoff hierarchy of § 319-21.

[5] Infiltration areas should be designed to maintain any broad and even infiltration pattern which existed prior to earth disturbance. Such facilities should use the natural topography and vegetation in order to blend in with the site. Infiltration designs which do not provide this may be used if the applicant demonstrates to the Municipality's satisfaction that alternative approaches would be more effective, more harmonious with their existing environment and as easily maintained.

[6] Surface stormwater infiltration facilities should be as shallow as possible while still achieving the requirements of this chapter.

(2) Water quality improvements shall also be provided for drainage areas not otherwise addressed by infiltration practices either at the source of runoff and/or during conveyance away from the source of runoff.

(3) To reduce the need for large retention and/or detention basins, innovative stormwater management practices located close to the source of runoff generation shall be considered, including rooftop storage, open vegetated channels, bioretention, and infiltration trenches.

(4) All existing channels, drainageways, swales, natural streams and other surface water concentrations shall be considered and, where possible, incorporated into design decisions.

C. Minimum performance criteria.

(1) The following minimum performance standards shall apply to all applicable activities, pursuant to § 319-5:

(a) Water quality treatment of stormwater runoff shall be provided for all discharges prior to release to a receiving water body.

(b) The Municipality requires regulated activities to provide a sediment reduction of a minimum of 20% for the project site. Sediment loading calculations and BMP effectiveness values shall be determined using methodologies acceptable to PADEP and to the Municipal Engineer. Applicants are encouraged to exceed the 20% minimum reduction and may choose to take advantage of the Municipality's "Sediment Reduction Credit" towards reduction of their pollution control and flood reduction fee in accordance with the Pollution Control and Flood Reduction Fee Credit Manual. The post-construction stormwater management report shall include a specific section entitled "Sediment Reduction," which shall provide sediment loading calculations for existing conditions, a narrative of sediment loading reduction methods, and sediment loading calculations for the proposed conditions.

(c) Water quality management shall be provided through the use of structural and/or nonstructural stormwater management practices. Water quality stormwater management practices shall be designed to reduce or eliminate solids, sediment, nutrients, and other potential pollutants from leaving the site. It is presumed that a stormwater management practice complies with this requirement if it is:

[1] Designed according to the specific performance criteria outlined in § 319-7;

[2] Constructed in accordance with all permits and approved plans and specifications; and

[3] Maintained per an approved operation and maintenance plan or agreement.

(d) Stormwater discharges from land uses or activities with higher potential for pollutant loadings ("hot spots") may require the use of specific structural stormwater management and pollution prevention practices. In addition, stormwater from a hot spot land use shall be provided with proper pretreatment prior to infiltration. For the purpose of this chapter, the sites/facilities listed below are considered hot spots.

(e) Certain industrial sites may be required to prepare and implement a stormwater pollution prevention plan and file notice of intent as required under the provision of the EPA Industrial Stormwater NPDES permit requirements. Industrial sites storing significant quantities of chemicals/wastes should also prepare a prevention plan. Sites that are required by EPA to prepare a plan include, but are not limited to:

[1] Vehicle salvage yards and recycling facilities;

[2] Vehicle and equipment cleaning facilities;

[3] Fleet storage areas for buses, trucks, etc.;

[4] Marinas (service and maintenance);

[5] Facilities that generate or store hazardous materials;

[6] Metal scrapyards and salvage yards;

[7] Light manufacturing, including food processing, printing and publishing, electronic/electrical equipment manufacturing, and public warehousing/storage.

(f) Conveyance structures/channels shall be designed and adequately sized to protect the properties receiving runoff from impacts of flooding and erosion. Where necessary, and to the maximum extent permitted under the Municipalities Planning Code[1] and Act 167,[2] or any amendments thereto, drainage easements from adjoining properties shall be obtained to ensure the drainageway and the property and shall also establish the operation and maintenance requirements for the drainageway.

[1] Editor's Note: See 53 P.S. § 10101 et seq.

[2] Editor's Note: See 32 P.S. § 680.1 et seq.

(g) All stormwater management practices shall have an operation and maintenance plan pursuant to § 319-24 of this chapter, and, if to be privately owned, an enforceable operation and maintenance agreement per § 319-25 of this chapter to ensure the system functions as designed and to provide remedies for system failure.

(h) Stormwater runoff discharged directly into a jurisdictional wetland, or waters of the United States and their adjacent wetlands, shall be treated by an approved stormwater management practice prior to release. This flow may not be used to meet the minimum calculated design requirements for stormwater management or stormwater runoff quality treatment, except when used as part of a treatment train that incorporates a portion of the outer zone (filter strip) of the wetland's riparian buffer as a stormwater outfall.

[1] The discharge velocity from the terminal end of a pipe or associated energy dissipation practice shall not exceed two feet per second for the two-year frequency storm event.

[2] Where such a management strategy is used, all feasible methods shall be used to convert concentrated flow to uniform, shallow sheet flow before entering the outer zone of the wetland's riparian buffer.

[3] It shall be demonstrated that such an approach will not cause erosion.

(2) The following minimum performance standards shall apply to all applicable regulated activities, pursuant to § 319-5.

(a) Water quality improvements shall be provided at the source of runoff and/or during conveyance away from the source of runoff. Stormwater quality management practices shall be designed to capture and treat the stormwater runoff volume difference between pre- and post-development generated by the two-year, twenty-four-hour rainfall event. Refer to § 319-21 for water quality volume design standards and assumptions.

(b) For areas covered by a release rate map from an approved Act 167 stormwater management plan:

[1] The post-development peak discharge rate shall not exceed the predevelopment peak discharge rate multiplied by the subbasin release rate percentage for the one-year, two-year, five-year, ten-year, twenty-five-year, fifty-year, and 100-year, twenty-four-hour storm events pursuant to the predevelopment cover assumption described in § 319-20. Refer to the current release rate percentage information.

(c) Facilities shall be designed to attenuate the runoff volume increase for the two-year, twenty-four-hour storm event for at least 24 hours.

(d) Stormwater shall be infiltrated and/or discharged within the same drainage area of the stream receiving the runoff from the development site prior to development.

(3) The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection C(3)(a) or the Simplified Method in Subsection C(3)(b) below. Water volume controls shall be implemented using the Design Storm Method in Subsection C(3)(a) or the Simplified Method in Subsection C(3)(b) below, or alternative design criteria as allowed by Pa. Code Title 25, Chapter 102.

(a) The Design Storm Method ("CG-1" in the BMP Manual) is applicable as a method to any size of regulated activity. This method requires detailed modeling based on site conditions. The following shall be incorporated into the CG-1 method:

[1] Do not increase the post-development total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.

[2] At least the first one inch of runoff from the net increase in impervious surfaces shall be permanently removed from the runoff flow; i.e., it shall not be released into the surface waters of this commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration. If the developer provides justification that the listed removal options are not feasible, and the designated plan reviewer agrees, runoff shall be detained in a facility designed for a twenty-four-hour to seventy-two-hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a seventy-two-hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.

[3] For modeling purposes:

[a] Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.

[b] Twenty percent of existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions.

(b) The Simplified Method ("CG-2" in the BMP Manual) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require design of stormwater storage facilities. For new impervious surfaces:

[1] Stormwater facilities shall capture at least the first two inches of runoff from the net increase in impervious surfaces.

[2] At least the first one inch of runoff from the net increase in impervious surfaces shall be permanently removed from the runoff flow; i.e., it shall not be released into the surface waters of this commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration. If the developer provides justification that the listed removal options are not feasible, and the designated plan reviewer agrees, runoff shall be detained in a facility designed for a twenty-four-hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a seventy-two-hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.

[3] Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases, at least the first 0.5 inch of the permanently removed runoff should be infiltrated.

[4] This method is exempt from the requirements of § 319-20, Stormwater runoff calculation criteria.

§ 319-20 Stormwater runoff calculation criteria.

A. Determination of runoff and peak flows shall be consistent with the following stormwater calculation methods:

(1) The following design storms shall be used for analysis of the pre- and post-development conditions. These values are applicable to the Soil-Cover-Complex Method:

Table A Design Rainfall Depths
Return Period (years)	24-Hour Storm (inches)
1	2.00
2	2.50
10	3.61
25	4.31
100	5.71

(2) The precipitation values for each frequency storm listed above were abstracted from the precipitation frequency estimates developed by the National Oceanic and Atmospheric Administration as set forth in NOAA Atlas 14, Volume 2 (NOAA June 2004). The NOAA data are available from the Hydrometeorological Design Studies Center of the National Weather Service.

(3) The following assumptions shall be used for runoff calculations and modeling purposes:

(a) The ground cover used as the predevelopment assumption for runoff calculations shall be as follows:

[1] Predevelopment conditions shall be those which exist on the date of the application.

[2] Wooded sites shall use a ground cover of woodland in good condition (see Curve Number Table).

[3] Agricultural sites shall use a ground cover of pasture in good condition (see Curve Number Table).

[4] All other pervious portions of a site shall use a ground cover of meadow in good condition (see Curve Number Table).

[5] All watershed area(s) contributing to the point of interest, including off-site area, shall be considered.

[6] If there are existing impervious areas on the site, the predevelopment calculations shall be based on the requirement that 20% of the existing impervious area must be considered as meadow in good condition. This requirement is only applicable to calculation of the two-year, twenty-four-hour attenuation and water quality treatment volume and does not apply to calculation of predevelopment peak runoff rates.

[7] If there are existing stormwater management facilities on the site that the applicant is proposing to use for the project, the predevelopment calculations shall include the impact of such facilities, along with the requirement that the applicant must certify the condition and operation of the existing facilities as discussed in § 319-7C(4) of this chapter.

(b) The runoff curve numbers listed in the table below shall be used in developing the runoff calculations for the ground covers noted above. These values are referenced from the Urban Hydrology for Small Watersheds Technical Release No. 55 (USDA, 1986). Coefficients for equivalent ground cover conditions shall be used if a runoff method other than the Soil Cover Complex Method is used.

Table B Runoff Curve Numbers
Ground Cover	Curve Numbers
Woodland	70
Meadow	71
Pasture	74
Lawn	74

[1] Impervious cover shall have a curve number of 98.

[2] Gravel pavement shall have a curve number of 89.

(4) Average antecedent moisture conditions, or AMC II, shall be used (for the Soil Cover Complex Method; for example, TR-55, TR-20).

(5) A Type II distribution storm shall be used (for the Soil Cover Complex Method; for example, TR-55, TR-20).

(6) For time of concentration calculations, sheet flow lengths shall not exceed 100 feet and shallow concentrated flow lengths shall not exceed 1,000 feet.

B. Note that when TR-55 is used, calculations must be performed on a detailed small subarea basis.

C. The design professional's selection of a specific runoff calculation method shall be based on the suitability of the method for the given project site conditions with due consideration to the limitations of the method chosen. Table C summarizes the computational methods available.

(1) The Municipal Engineer reserves the right to stipulate the runoff calculation method appropriate for any project site.

D. The release rate percentage defines the percentage of the predevelopment peak rate of runoff that can be discharged from the site after development. It applies to all regulated activities within a designated subarea. The subareas and required release rate percentages are delineated on the current Release Rate Percentage Map.

Table C Acceptable Computation Methodologies for Stormwater Management Plans
Method	Source	Applicability
TR-20 or commercial package based on TR-20	USDA-NRCS	When use of full model is desirable or necessary
TR-55 or commercial package based on TR-55	USDA-NRCS	Applicable for plans within the model's limitations
HEC-HMS	U.S. Army Corps of Engineers	When full model is desirable or necessary
Modified Rational Method or commercial package based on this method	Emil Kuiching (1889)	Sites less than 2 acres
Small Storm Hydrology Method (as included in SLAMM)	PV and Associates, or the website www.winslamm.com	Calculation of runoff volume from urban and suburban areas

E. The Modified Rational Method analysis may be used for drainage areas smaller than two acres when permitted by the Municipal Engineer. The term "Modified Rational Method" used herein refers to a procedure for manipulation of the basic rational method techniques to reflect the fact that storms with durations greater than the normal time of concentration for a basin will result in a larger volume of runoff even though the peak discharge is reduced. Information on the Modified Rational Method is presented in the Recommended Hydrologic Procedures for Computing Urban Runoff from Small Watersheds in Pennsylvania (PADEP, 1982).

F. The Rational Method (Q = CIA) shall be used for calculations of the peak rate of runoff for the design of storm sewers and drainage swales but not for the design of stormwater management facilities where a full hydrograph is needed. The equation representing the Rational Method is comprised of the following (in English units):

Q	=	Peak flow rate, cubic feet per second (CFS)
C	=	Runoff coefficient, dependent on land use/cover
I	=	Design rainfall intensity, inches per hour
A	=	Drainage area, acres

G. Rainfall intensities used for the Rational and Modified Rational Methods shall be based on the precipitation frequency estimates developed by the National Oceanic and Atmospheric Administration as set forth in NOAA Atlas 14.

H. Runoff characteristics of off-site areas that drain through a proposed development shall be considered and be based on the existing conditions in the off-site area.

§ 319-21 Standards for stormwater management practices.

A. The Pennsylvania Stormwater Best Management Practices Manual shall serve as a guide for the design of stormwater management practices. Additional design guidance may be obtained from other related sources, including the 2000 Maryland Stormwater Design Manual Volumes I and II (MDE 2000), Design of Stormwater Filtering Systems (CWP 1996), and the American Society of Civil Engineers Manual and Report on Engineering Practice, No. 87, Urban Runoff Quality Management (ASCE 1998) for the design of stormwater runoff quality control features for site development.

B. The Municipality may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law, including, but not limited to, the Clean Streams Law.[1]

[1] Editor's Note: See 35 P.S. § 691.1 et seq.

C. Pursuant to the design options recommended in the above documents, the following standards shall apply:

(1) Extended detention, water quality volume, infiltration and nonstructural BMP criteria.

(a) The following sizing criteria shall be followed at all sites required to meet the standards of this chapter:

[1] Extended detention.

[a] Detain the volume difference between the pre- and post-development two-year, twenty-four-hour design storm using the SCS[2] Type II distribution. Provisions shall be made so that the volume takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water is captured or is infiltrated. (i.e., the maximum water surface elevation achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall consider and minimize the chances of clogging and sedimentation potential.

[2] Editor's Note: The SCS is now the USDA Natural Resources Conservation Service (NRCS).

[b] All subsequent orifices for the two-, ten-, twenty-five-, and 100-year storm events shall be placed above the maximum water surface elevation of the extended detention.

[c] Flow from off-site areas must be considered as pass-through flow if it is conveyed through the BMP and should be modeled as "present condition" for the two-year storm event.

[2] Water quality volume.

[a] Treatment of the water quality volume (WQv) of stormwater prior to its release to receiving waters or water bodies shall be provided for all regulated activities. The WQv equals the extended detention volume.

[b] Drainage areas having no impervious cover and no proposed disturbance during development may be excluded from the WQv calculations. However, designers are encouraged to incorporate water quality treatment practices for these areas.

[c] Stormwater quality treatment. The final WQv shall be treated by an acceptable stormwater management practice(s) from those described in this section or as approved by the Municipality.

[d] Infiltration is considered an acceptable method of satisfying part or all of the water quality volume, if site testing results indicate appropriate capacity.

[e] As a basis for design, the following assumptions may be made:

[f] Multiple drainage areas. When a project contains or is divided by multiple drainage areas, the WQv volume shall be addressed for each drainage area.

[g] Off-site drainage areas. The WQv shall be based on the impervious cover of the proposed site. Off-site existing impervious areas may be excluded from the calculation of the water quality volume requirements.

[3] Infiltration volume.

[a] Where possible, all of the water quality volume should be treated using infiltration BMPs.

[b] Infiltrated volume may be subtracted from the total site WQv.

[c] Infiltration should not be considered for sites or areas of sites that have activities that may allow pollution to be infiltrated. For example, the use of infiltration for the runoff of a service station's paved lot would not be appropriate, although roof water from the service station may be infiltrated.

[d] Infiltration should only be used when, in the opinion of a professional engineer, it will not contribute to slope instability or cause seepage problems into basements or developed down-gradient areas.

[e] All infiltration facilities shall be set back at least 15 feet from all structures with subgrade elements (e.g., basements, foundation walls).

D. Stormwater infiltration practices.

(1) In selecting the appropriate infiltration BMPs, the applicant shall consider the following:

(a) Permeability and infiltration rate of the site soils.

(b) Slope and depth to bedrock.

(c) Seasonal high-water table.

(d) Proximity to building foundations and well heads.

(e) Erodibility of soils.

(f) Land availability and topography.

(g) Slope stability.

(h) Effects on nearby properties and structures.

(2) A detailed soils evaluation of the project site shall be performed to determine the suitability of infiltration BMPs. The evaluation shall be performed by a qualified professional. The general process for designing the infiltration BMP shall be:

(a) Provide field-testing data to determine appropriate percolation rate and/or hydraulic connectivity.

(b) Design infiltration BMPs for required stormwater volume based on field-determined capacity at the level of the proposed infiltration surface.

(3) Soil characteristics, as subject to the specific considerations below:

(a) Low-erodibility factors ("K" factors) are preferred for the construction of basins.

(b) There must be a minimum depth of 48 inches between the bottom of any facility and the seasonal high-water table and/or bedrock (limiting zones).

(c) Infiltration BMPs receiving only roof runoff shall be placed in soils having a minimum depth of 24 inches between the bottom of the facility and the limiting zone.

(d) There must be an infiltration and/or percolation rate sufficient to accept the additional stormwater load and to drain completely as determined by field tests.

(e) The infiltration system shall have positive overflow controls to prevent storage within one foot of the finished surface or grade.

(f) Infiltration rates shall not be used in computing the storage volume of the infiltration system.

(g) Surface inflows shall be designed to prevent direct discharge of sediment into the infiltration system.

(4) Any infiltration BMP shall be capable of completely infiltrating the impounded water within 48 hours. The forty-eight-hour period is to be measured from the end of the twenty-four-hour design storm.

(5) The Municipality may require additional analyses for stormwater management facilities proposed for susceptible areas such as:

(a) Strip mines.

(b) Storage areas for salt, chloride, other materials for winter deicing.

(c) Unstable slopes.

(6) During land disturbance, runoff shall be controlled prior to entering any proposed infiltration area. Areas proposed for infiltration BMPs shall also be protected from sedimentation and compaction during the construction phase.

(7) Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization.

(8) Acceptable infiltration facilities include, but are not limited to: filter strips or stormwater filtering systems (bioretention facilities), open vegetated channels (dry swales and wet swales), retention basins, wet extended detention ponds, riparian, corridor management, riparian forested buffers, and rooftop runoff management systems.

(9) Where sediment transport in the stormwater runoff is anticipated to reach the infiltration system, appropriate permanent measures to prevent or collect sediment shall be installed prior to discharge to the infiltration system.

(10) The applicant shall propose the distance infiltration facilities shall be set back from all structures with subgrade elements (e.g., basements, foundation walls), subject to review by the Municipal Engineer.

(11) All infiltration facilities that serve more than one lot and are considered a common facility shall have a drainage easement. The easement shall provide to the Municipality the right of access.

(12) If detailed infiltration study is required, the following guidance shall be followed:

(a) Soil evaluations shall be performed to determine the feasibility and extent to which infiltration systems can be used. The evaluation shall be performed by a qualified, professional geologist, geotechnical/civil engineer or soil scientist registered in the Commonwealth of Pennsylvania. The testing and evaluation should be completed at the preliminary design stage.

(b) Use of stormwater management facilities to retain stormwater for infiltration should be applied to all areas where the soils evaluation indicates favorable conditions. Areas generally not favorable for infiltration shall still be provided with an appropriate water quality practice.

(c) Soil infiltration tests shall be performed to an equivalent depth or elevation of the bottom of the proposed infiltration areas. These tests shall follow the procedures of percolation test holes as established by the Allegheny County Health Department (ACHD) for on-lot septic systems.

[1] The testing shall include a test pit and percolation test holes. The test pit shall be excavated to a depth so that the presence or absence of bedrock and/or seasonal high-water table can be determined. A soil log describing the soils present in each test pit shall be completed.

[2] All test holes used for evaluating the percolation rate shall be presoaked in accordance with the procedures established by the ACHD.

[3] The location and number of test pits and percolation holes shall be determined based on the type(s) of stormwater management facilities being designed.

[4] Acceptability of infiltration rates shall be based on sound engineering judgment and recommended design considerations described in the design manuals listed in the references or other source material acceptable to the Municipal Engineer.

(13) The following design and construction standards shall be followed when planning and constructing infiltration BMPs:

(a) The lowest elevation of the infiltration area shall be at least four feet above the seasonal high-water table and bedrock.

(b) Where roof drains are designed to discharge to infiltration facilities, they shall have appropriate measures to prevent clogging by unwanted debris such as silt, leaves and vegetation. Such measures shall include, but are not limited to, leaf traps, gutter guards and cleanouts.

(c) All infiltration facilities shall have appropriate positive overflow controls to prevent storage within one foot of the finished surface or grade, unless:

[1] A specific amount of surface storage away from pedestrian and vehicular traffic is provided; and

[2] Such areas infiltrate the stored volume within 48 hours after the end of the twenty-four-hour design storm.

(d) All infiltration facilities shall be designed to infiltrate the stored volume within 48 hours after the end of the twenty-four-hour design storm.

(e) All surface inflows shall be treated to prevent the direct discharge of sediment into the infiltration practice. No sand, salt or other particulate matter may be applied to a porous (pervious) surface for winter ice conditions.

(f) During site construction, areas that are accidentally compacted or graded shall be remediated to restore soil composition and porosity. Adequate documentation to this effect shall be submitted for review by the Municipal Engineer.

(g) The following procedures and materials shall be required during the construction of all subsurface facilities:

[1] Excavation for the infiltration facility shall be performed with equipment that will not compact the bottom of the seepage bed/trench or like facility.

[2] The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.

[3] Only clean aggregate with documented porosity, free of fines, shall be allowed.

[4] The tops and sides of all seepage beds, trenches, or like facilities shall be covered with drainage fabric. Fabric shall meet the specifications of PennDOT Publication 408, Section 735, Construction Class 1.

[5] Perforated distribution pipes connected to centralized catch basins and/or manholes with the provision for the collection of debris shall be provided in all facilities. Where perforated pipes are used to distribute stormwater to the infiltration practice, stormwater shall be distributed uniformly throughout the entire seepage bed/trench or like facility.

E. Open vegetated channels.

(1) "Open vegetated channels" are conveyance systems that are engineered to also perform as water quality and infiltration facilities. Such systems can be used for the conveyance, retention, infiltration and filtration of stormwater runoff.

(2) Open vegetated channels primarily serve a water quality function (WQv). They also have the potential to augment infiltration. Examples of such systems include, but are not limited to: dry swales, wet swales, grass channels, and biofilters. Open vegetated channels are primarily applicable for land uses such as roads, highways, residential developments (dry swales only) and pervious areas.

(3) Open vegetated channels shall be designed to meet the following minimum standards:

(a) The channel shall be designed to safely convey the twenty-five-year frequency storm event with a freeboard of at least 12 inches.

(b) The peak velocity of the runoff from the twenty-five-year storm shall be nonerosive for the soil and ground cover provided in the channel.

(c) The longitudinal slope shall be no greater than 4%.

(d) Channels shall be trapezoidal in cross section.

(e) Channels shall be designed with moderate side slopes of four horizontal to one vertical. Flatter side slopes may be necessary under certain circumstances.

(f) The maximum allowable ponding time in the channel shall be 48 hours.

(g) Channels (for example, dry swales) may require an underdrain in order to function and dewater.

(h) Channels shall be designed to temporarily store the WQv within the system for a maximum period of 48 hours and a minimum period of one hour.

(i) The designer shall provide landscape specifications addressing the grass species, lining material, wetland plantings (if applicable), soil amendment and hydric conditions present along the channel.

(j) Accumulated sediment within the channel bottom shall be removed when 25% of the original WQv volume has been exceeded. The channel shall be provided with a permanent concrete cleanout marker that indicates the 25% level.

(k) Check dams along the channel length may be warranted.

(l) The bottom of the open vegetated channel shall be situated at least two feet above the seasonal high-water table.

(4) Additional design information for open vegetated channels is available in Design of Roadside Channels with Flexible Linings, I-IEC 15, FHWA, September 2005.

F. Detention and retention basins.

(1) Basin definitions.

(a) "Detention basins" are designed to be empty during dry weather and drain completely within a certain time period following a storm.

(b) "Retention basins" are designed to maintain a permanent pool of water and allow for the addition of the required stormwater management volume during a storm.

(2) Retention basin requirements.

(a) The retention basin shall be of sufficient size to maintain healthy pond ecology and avoid mosquitoes capable of carrying West Nile virus and other diseases. The Allegheny County Health Department, Pennsylvania Fish and Boat Commission, the Natural Resources Conservation Service, the Pennsylvania Extension Service, or other qualified professional shall be consulted during the design of these facilities in order to ensure the health of aquatic communities and minimize the risk of creating mosquito breeding areas.

(b) A dewatering mechanism shall be designed to allow complete drainage of the pond for maintenance.

(c) The design of a retention basin shall include the determination of the proposed site's ability to support a viable permanent pool. The design shall take into account such factors as the available rate and quality of dry-weather inflow, the stormwater inflow, seasonal and longer-term variations in groundwater table, and impacts of potential pollutant loadings.

(d) Sediment storage volume equal to at least 20% of the volume of the permanent pool shall be provided.

(e) Existing ponds or permanent pool basins can be used for stormwater management, provided that it can be demonstrated that the ponds are structurally sound and meet the design requirements herein.

[1] Runoff from the proposed regulated activity must, at a minimum, flow through a water quality BMP prior to entering an existing pond.

[2] The use of an existing pond or permanent pool basin will not be permitted where the water quality may be degraded by the addition of runoff from a regulated activity.

(f) Retention basins shall be designed to provide a length-to-width ratio of at least 3L:1W as measured in plan view (for example, a ratio of 4L:1W is too narrow).

(g) The retention basin depth shall average three feet to six feet.

(h) Fencing of the facility is not required if the interior slope of the pond is 4H:1V or flatter and the design also includes a five-foot-wide bench around the pond perimeter at an elevation one foot below the permanent water surface elevation.

(i) Any side slopes below the permanent water surface level shall not exceed 3H:1V. Interior side slopes above the permanent water surface level shall not exceed 3H:1V.

(j) Inlet structures and outlet structures shall be separated to the greatest extent possible in order to maximize the flow path through the retention basin.

(3) Detention basin requirement.

(a) Ground surface detention ponds.

[1] The exterior shall be landscaped and vegetated in an aesthetically pleasing manner acceptable to the Municipality.

[2] The maximum inside side slopes shall not exceed 2 1/2 horizontal to one vertical. The minimum required slope for the basin bottom is 2%. A level bottom is acceptable, provided the designer demonstrates to the Municipality's satisfaction that the basin bottom will be landscaped with appropriate wetland vegetation.

[3] Inlet structures. The inlet pipe invert into a basin shall be six inches above the basin floor or lining so that the pipe can adequately drain after rainstorms. Inlets shall discharge into areas of the basin that slope toward the outlet structure.

[4] Low-flow channels. Low-flow channels constructed of concrete or asphalt are not permitted. Where low-flow channels are necessary, they shall be composed of a natural or bioengineered material. Low-flow channels shall be designed to promote water quality and slow the rate of flow through the basin. Low-flow channels may also be designed to infiltrate where practical.

(b) Underground detention facilities.

[1] Pipes/tanks, etc., shall be a minimum of 48 inches in diameter, or provide an equivalent area if not circular, for case of inspection and maintenance.

[2] Access shall be provided through means of an inlet or manhole at both the uppermost end of the facility and at the orifice plate or other outlet control. Manholes are preferred.

[3] The designer is encouraged to include manholes at intersections of tank barrels and headers for ease of inspection and maintenance.

[4] The total volume of an underground detention facility must be 110% of the storage volume required for the 100-year storm, to allow for sediment accumulation.

[5] Orifice plates shall be a minimum of one-fourth-inch-thick painted steel and shall not be covered by trash racks or screens.

[6] Orifice plates shall be bolted securely and watertight to the surrounding walls and floor.

(4) Common requirements for detention and retention basins.

(a) Shared-storage facilities, which provide detention or retention of runoff for more than one development site within a single subarea may be considered and are encouraged. Such facilities shall meet the criteria contained in this section.

[1] Runoff from the development sites involved shall be conveyed to the facility in a manner that avoids adverse impacts to channels and properties located between the development site and the shared storage facilities.

(b) Where detention or retention basins are proposed, multiple use facilities, including recreational and open space uses, are encouraged wherever feasible, subject to the approval of the Municipality.

(c) Outlet structures shall meet the following specifications:

[1] Outlet pipes shall have an internal diameter of at least 15 inches and a minimum grade of 1% to minimize clogging and to facilitate cleaning and inspection.

[2] Antiseep collars shall be provided on all outlet pipes within a constructed berm.

[3] All principal outlet structures shall be built using reinforced concrete with watertight construction joints.

[4] The use of architecturally treated concrete, stucco, painted surface or stone facade treatment shall be considered for enhancing the outlet structure. Such facilities shall be both functional and harmonious in design with the surrounding environment.

[5] Outlet pipes shall be constructed of reinforced concrete with rubber gaskets in conformance with AASHTO M170, M198 and M207, or smooth interior HDPE pipe in conformance with AASHTO M252 or M294.

[6] Energy dissipation facilities that convert concentrated flow to uniform shallow sheet now shall be used where appropriate.

[7] Basin outlet structures shall have childproof, nonclogging trash racks over all openings exceeding 12 inches in diameter except those openings designed to carry perennial stream flows. The minimum size opening in the trash rack shall be four inches in each direction.

[8] Antivortex devices, consisting of a thin vertical plate normal to the basin berm, shall be provided at the top of all circular risers or standpipes.

[9] Buoyancy calculations shall be submitted to certify that the outlet structure will not become buoyant or to specify concrete footing dimensions required to counteract buoyancy. Outlet structure box sections shall be bolted together with steel straps of sufficient thickness to prevent separation.

(d) Emergency spillways:

[1] Shall be sized and located to permit the safe passage of stormwater flows from the unattenuated 100-year post-development storm with one foot of freeboard.

[2] The maximum velocities in vegetated spillways shall be based upon an assumed clogged primary outlet condition. Where maximum velocities exceed design standards contained in the Engineering Field Manual for Conservation Practices (USDA, SCS, July 1984), suitable lining shall be provided.

[3] Emergency spillways should not be located in fill areas. Facilities placed on fill materials shall be lined.

[a] Lining for emergency spillways shall incorporate native colors and materials where possible, including mono slab revetments, grass pavers, riprap and native stone.

[4] Emergency spillways may be either incorporated into the principal outlet structure or designed as a separate pipe or channel.

(e) Basin and pond embankments:

[1] Must be designed and scaled by a professional engineer with relevant experience registered in the Commonwealth of Pennsylvania.

[2] Must include an investigation of the subsurface conditions at the proposed embankment location to evaluate settlement potential, groundwater impacts, and the need for seepage controls.

[3] A geotechnical report prepared by a geotechnical engineer licensed in the Commonwealth of Pennsylvania must be submitted for any embankment over 10 feet in effective height or posing a significant hazard to downstream property or life.

[4] The selection of fill materials is subject to approval of the Municipal Engineer. Fill must be free of frozen soil, rocks over six inches, sod, brush, stumps, tree roots, wood, or other perishable materials.

[5] Embankment fills less than 10 feet in fill height must be compacted using methods that would reasonably guarantee that the fill density is at least 90% of the maximum density as determined by standard proctor (ASTM-698).

[6] All embankment fills more than 10 feet in fill height must be compacted to at least 90% of the maximum density as determined by standard proctor (ASTM-698) and must have their density verified by field density testing.

[7] A PADEP dam permit is required for:

[a] Embankments having a maximum depth of water, measured from the upstream toe of the dam to the top of the dam at maximum storage elevation, of greater than 15 feet;

[b] Ponds having a contributory drainage area of greater than 100 acres;

[c] Impoundments of greater than 50 acre-feet.

[8] The embankment's interior slope may not be steeper than 2 1/2 horizontal to one vertical. The exterior slope of the embankment may not exceed two horizontal to one vertical.

[9] The embankment width shall be in conformance with the requirements of the PADEP and subject to the approval of the Municipal Engineer.

(f) Fencing of the facility may not be required if the interior slope of the pond is 4H:1V or flatter. The Municipality reserves the right to require fencing.

(g) Energy dissipators and/or level spreaders shall be installed to prevent erosion and/or initiate sheet flow at points where pipes or drainageways discharge to or from basins.

[1] Level spreaders shall be used only where the maximum slope between the discharge point and the waterway does not exceed 5%.

[2] Energy dissipators shall comply with criteria in Hydraulic Design of Energy Dissipators for Culverts and Channels, 1-IEC 14, FHWA, July 2006, or current edition.

[3] Such facilities shall be both functional and attractive; for example, native rock shall be used in constructing dissipators where practical.

(h) Proper stabilization structures, including stilling basins, energy dissipaters, and channel lining, shall be constructed at the outlets of all basins and emergency spillways.

(i) The minimum distance between a proposed basin discharge point (including the energy dissipater, etc.) and a downstream property boundary shall in no case be less than 15 feet.

[1] Where there is discharge onto or through adjacent properties prior to release to a stream, designers shall demonstrate how downstream properties are to be protected.

[2] The Municipal Engineer may require that the setback distance is increased based upon factors such as topography, soil conditions, the size of structures, the location of structures, and discharge rates.

[3] A drainage easement must be obtained, where required.

(j) A sediment forebay shall be provided at each inlet into a surface basin. The forebay storage capacity shall at minimum be 10% of the permanent pool storage. The forebay shall be designed to allow for access by maintenance equipment and to facilitate periodic cleaning, including an access road and gate in conformance with the Municipality's current standards for construction.

G. Conveyance system.

(1) Applicants are encouraged to design conveyance systems that facilitate infiltration and improve water quality wherever practicable.

(2) Open channels with landscaped banks shall be designed to carry the twenty-five-year, twenty-four-hour stormwater runoff. The Municipal Engineer may increase the design storm as conditions require. All open channels shall be designed with one foot of freeboard above the water surface elevation of the design runoff condition.

(3) Flood relief channels shall be designed to convey the runoff from the 100-year, twenty-four-hour storm, such that its discharge to an adequate receiving stream or conveyance system occurs without allowing it to encroach upon other properties.

(4) Where drainage swales are used in lieu of or in addition to storm sewers, they shall be designed to carry the required runoff with a minimum grade of 2% and a maximum grade of 9%.

(a) Drainage swales used strictly for conveyance are not the same as open vegetated channels. Design standards for open vegetated channels are provided under § 319-21E of this chapter.

(5) Use of grassed swales or open vegetated swales in lieu of curbing to convey, infiltrate and/or treat stormwater runoff from roadways is encouraged.

(a) Inlets receiving water from the swale shall be placed at the center of the shoulder swale draining the street and shall be located no closer than four feet from the edge of the cartway.

(b) The invert of the swale shall be at least one foot lower in elevation than the adjacent roadway subgrade.

(6) When requested by the Municipality, the applicant shall obtain or grant a minimum twenty-foot-wide easement over all storm sewers, drainage swales, channels, etc., that are a component of the stormwater management system when located within undedicated land.

(a) All permanent detention basins and/or other stormwater management facilities providing stormwater control for other than a single residential lot shall be located within a defined drainage easement that allows proper legal and maintenance access.

(7) No property owner shall obstruct or alter the flow, location or carrying capacity of a stream, channel or drainage swale to the detriment of any other property owner, whether upstream or downstream. All subdivision and/or land development plans containing streams, channels, drainage swales, storm sewers or other conveyance systems that cross property boundaries, existing or proposed, or whose discharge crosses such boundaries shall contain a note stating the above.

H. Water quality inlets.

(1) Water quality inlets shall be utilized in parking areas and/or loading areas exceeding 5,000 square feet of impervious coverage that discharge to stormwater management systems.

(a) The total capacity of the water quality inlets installed shall be based upon the calculated runoff rates expected to be tributary to the inlets.

(b) In no case shall the number of water quality inlets installed be less than one per each acre of drainage area.

(2) The purpose of water quality inlets is to remove oil, grease, heavy particulates, total suspended solids, hydrocarbons and other floating substances from stormwater runoff.

(3) Methods other than water quality inlets may be permitted if the applicant demonstrates to the Municipality's satisfaction that any such alternative will be as effective and as easily maintained.

(4) Periodic cleaning of these systems shall be addressed in the operation and maintenance plan submitted to the Municipality.

I. Pervious pavement.

(1) Pervious pavements, including pervious portland cement concrete, pervious or permeable asphalt concrete, pervious pavers, etc., are encouraged to be utilized as an environmentally conscious surfacing material.

(2) Designers cannot, however, use pervious pavements as infiltration credit for water quality requirements nor infiltration volume deductions from detention requirements due to the potential that pervious pavements will become clogged with sediments and winter deicing materials.

J. Rock sumps.

(1) The use of rock sumps or other similar measures for runoff infiltration will not be permitted in the Municipality due to the potential for clogging and the need for significant maintenance.

K. Storm sewer criteria.

(1) Manholes and/or inlets shall not be spaced more than 300 feet apart for pipe sizes up to 24 inches in diameter and not more than 450 feet apart for larger pipe sizes.

(2) Street curbing for the purpose of stormwater conveyance is discouraged. On streets that must contain curbing, storm sewers shall be placed behind the curbing. To the greatest extent possible, storm sewers shall not be placed directly under curbing. At curbed street intersections, storm inlets shall be placed in the tangent section of the road.

(3) Where practicable, sewers shall be designed to traverse under seeded and planted areas. If constructed within 10 feet of roads, sidewalks, or other paved surfaces, storm sewers shall be installed within the narrowest trench possible and backfilled with select material to prevent surface settlement.

(4) Storm sewers will be designed with a concrete cradle when traversing fill areas of questionable stability.

(5) Storm sewers shall be designed with pipe anchors when the pipe slope exceeds 20%.

(6) The minimum storm sewer size shall be 15 inches in diameter.

(7) Pipe material, trenching, bedding, and backfilling requirements shall conform to the requirements of the Municipal Engineer and/or applicable sections of PennDOT Publication 408.

(8) Storm sewer shall be either reinforced concrete or high-density polyethylene (HDPE) pipe, subject to cover requirements and the approval of the Municipal Engineer.

(9) Where a proposed sewer or other stormwater management facility connects with an existing storm sewer system, the applicant shall demonstrate that sufficient capacity exists in the downstream system to convey the additional flow.

L. Properties on the high side of streets shall have downspouts or other private drainage facilities discharge to a lawn or other open space to provide overland flow to the area collection/conveyance system.

M. Foundation drains from high-side properties may discharge directly to the area collection/conveyance system if surface discharge/overland flow is not feasible.

N. Properties on the low side of streets shall discharge downspouts and foundation drains to provide overland flow to the area collection/conveyance system or natural watercourse in accordance with the approved stormwater management plan for the development site.

O. Collection/conveyance facilities should not be installed parallel or close to the top or bottom of an embankment to avoid the possibility of failure of the facility or the embankment.

P. Riparian buffer requirements.

(1) The purpose of establishing buffer requirements is to ensure that antidegradation standards are met, thereby resulting in elevated water quality. A riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer. The riparian buffer is intended to slow overland flow to the stream through the presence of native grasses, trees and shrubs, allowing infiltration/groundwater recharge; causing deposition of sediment, nutrients, pesticides, and other pollutants in the buffer rather than in the stream; and reducing erosion by providing stream bank stabilization. The trees provide shade for streams, keeping waters cooler and reducing evaporation.

(2) Except as required by Pa. Code Title 25 Chapter 102, the riparian buffer easement shall be required for all streams (as defined in Article VII) with a contributing watershed area of greater than 10 acres. The riparian buffer easement shall be measured to be a minimum of 50 feet from the top of the stream bank (on each side).

(3) Riparian buffers shall be provided as per the following requirements:

(a) Effect on existing lots.

[1] Preexisting lots or parcels/development in buffers. In the case of legally preexisting lots or parcels (approved prior to the effective date of this chapter) where the usable area of a lot or parcel lies within a buffer area, rendering the lot or parcel unable to be developed in accordance with this chapter, the development may only be permitted by variance in accordance with Chapter 359, Zoning.

[2] Improvements to existing structures in buffers. The provisions of the buffer requirements do not require any changes or improvements to be made to lawfully existing structures in buffers. However, when any substantial improvement to a structure is proposed which results in a horizontal expansion of that structure, the improvement may only be permitted by variance in accordance with Chapter 359, Zoning.

(b) Riparian buffer considerations.

[1] A fifty-foot buffer measured perpendicular to and horizontally from the edge of a delineated wetland, vernal pond, pond, lake, or along a stream shall be maintained.

[2] The buffer area should be maintained in a natural state to the maximum extent practical.

[3] Buffer averaging may be applied to account for required encroachments such as road crossings. The following criteria must be met in order to utilize buffer averaging on a development site:

[a] Buffer averaging is required for water quality buffers that have stream crossings.

[b] An overall average buffer width of at least 50 feet must be achieved within the boundaries of the property to be developed. Buffer corridors on adjoining properties cannot be included with buffer averaging on a separate property, even if owned by the same property owner.

[c] The average width must be calculated based upon the entire length of stream bank or perimeter of the wetland, pond or lake that is located within the boundaries of the property to be developed. When calculating the buffer length for a stream, the natural stream channel should be followed.

[d] Buffer averaging shall be applied to each side of a stream independently. If the property being developed encompasses both sides of a stream, buffer averaging can be applied to both sides of the stream, but must be applied to both sides of the stream independently.

[e] Riparian buffer locations and widths should be illustrated on all subdivision plans, with notations requiring that they be maintained in a natural state.

[f] Riparian buffers should be illustrated on all grading and erosion and sedimentation control plans. The defined stream buffer location should be properly recorded. The recording should provide a plan illustrating the stream buffer location, width and the requirement that it be maintained in a natural state.

[g] No use or construction within the riparian buffer shall be permitted that is inconsistent with the intent of the riparian buffer as described in § 319-21P.

[h] Whenever practicable, invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.

(c) Permitted activities/development.

[1] The following activities are permitted within riparian buffers, provided that the area of the buffer altered by such activities shall be minimized to the greatest extent practicable. Existing native vegetation shall be protected and maintained within the riparian buffer easement.

[a] Stormwater conveyance required by the Municipality.

[b] Maintenance of infrastructure by government entities and utilities permitted under the PUC.

[c] Buffer maintenance and restoration.

[d] The correction of hazardous conditions.

[e] Wetland or stream crossings, boat docks/launches permitted by DEP.

[f] Fish hatcheries.

[g] Wildlife sanctuaries.

[h] Passive, unpaved, stable trails shall be permitted.

[i] No other earth disturbance, grading, filling, buildings, structures, new construction, or development shall be permitted.

(d) The riparian buffer easement shall be enforceable by the Municipality and shall be recorded in the appropriate County Recorder of Deeds Office so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area required by zoning, unless otherwise specified in Chapter 359, Zoning.

(e) Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.

(f) Stormwater drainage pipes shall be permitted within the riparian buffer easement, but they shall cross the easement in the shortest practical distance. Other structural stormwater management facilities are not permitted within the riparian buffer easement.

(g) The following conditions shall apply when public and/or private recreation trails are permitted by the Municipality within riparian buffers:

[1] It is preferred that trails be designed to be permeable and for nonmotorized use only; however, impermeable trails are permitted, provided they have adequate drainage.

[2] Trails shall be designed to have the least impact on native plants species and other sensitive environmental features.

(h) Septic drain fields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.

(i) Underground utilities shall be permitted within the riparian buffer easement; however, work shall be performed to minimize disturbance area and removal of trees. Restoration within the riparian buffer easement shall be with native species of trees, grasses, and other plantings. One tree shall be planted for each tree removed and the restoration shall be designed by a registered professional with the requisite experience. Aboveground utilities shall only be permitted to cross the easement perpendicular to the easement or in the shortest practical distance. Existing utilities may remain and be maintained as required.