Town of McCandless, PA Stormwater Management Design Standards

§ 913.14 Design goals, principles, and standards.

[Ord. No. 1528, 9-26-2022]

Design goals. Applicants shall adhere to a holistic design process incorporating the goals listed below. The objective is to achieve post-development hydrologic conditions that are consistent with the predevelopment ground cover assumption for new development [Refer to § 913.15(b).] and improve the runoff conditions for redevelopment. [Refer to § 913.14(c)(3).] The design goals are:

(1)

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

(2)

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

(3)

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

(4)

Provide for peak flow attenuation, as needed; and

(5)

Attenuate runoff to protect the tributaries of the receiving stream.

(b)

General principles. The following general principles apply to all applicable activities pursuant to § 913.03:

(1)

Incorporate conservation design practices to minimize the amount of stormwater generated on a site, encourage the disconnection of impervious land cover, and maximize the use of pervious areas for stormwater treatment and on-site rainfall infiltration.

(2)

Infiltration of surface water runoff at its source is to be a mechanism for stormwater management based on hydrologic soil group (or infiltration testing). Infiltration practices include, but are not limited to, those referenced in § 913.16(b)(1) and as outlined in the publications listed in § 913.16. Infiltration practices shall adhere to the following criteria:

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

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, porous paving and perforated pipe storage.

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

Discourage concentration of flows;

Encourage disconnection of flows;

Infiltrate as close to the source of runoff as possible; and

Reduce visual impact.

Where high water tables, subsurface contamination, slope stability concerns, or other 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 § 913.16(a)(1).

Infiltration areas should be designed to maintain a broad and even infiltration pattern which existed prior to development. 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 Town's satisfaction that alternative approaches would be more effective and more harmonious with their existing environment and as easily maintained.

Aboveground stormwater infiltration facilities should be as shallow as possible while still achieving the requirements of this article.

(3)

Water quality improvement shall be achieved in conjunction with or as part of infiltration practices. 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.

(4)

To reduce the need for large retention and/or detention basins designed to satisfy the peak flow attenuation and extended detention requirements, other innovative stormwater management practices located close to the source of runoff generation shall be considered, including a combination of practices (e.g., rooftop storage, open vegetated channels, bioretention, pervious pavement systems and infiltration trenches).

(5)

When designing stormwater management facilities to satisfy the peak flow attenuation and extended detention requirements [Refer to § 913.14(c)(2)B.], the effect of structural and nonstructural stormwater management practices implemented as part of the overall site design may be taken into consideration when calculating total storage volume and release rates.

(6)

Site hydrology and natural infiltration patterns shall guide site design, construction, and vegetation decisions. All 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, whether they are new development or redevelopment, pursuant to § 913.03(a):

Water quality treatment of stormwater runoff shall be provided for all discharges prior to release to a receiving water body. If a stormwater management practice does not provide water quality treatment, then water quality best management practices shall be utilized prior to the runoff entering the stormwater management practice.

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 the site. It is presumed that a stormwater management practice complies with this requirement if it is:

Designed according to the specific performance criteria outlined in § 913.08(b);

Constructed in accordance with all permits and approved plans and specifications; and

Maintained per an approved operation and maintenance plan or agreement or, in lieu of that, in accordance with customary practices.

Stormwater discharges from land uses or activities with higher potential for pollutant loadings (hotspots) may require the use of specific structural stormwater management practices and pollution prevention practices. In addition, stormwater from a hotspot land use shall be provided with proper pretreatment prior to infiltration. For the purpose of this article, the sites/facilities listed in § 913.14(c)(1)D below are considered hotspots.

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

Vehicle salvage yards and recycling facilities;

Vehicle and equipment cleaning facilities;

Fleet storage areas for buses, trucks, etc.;

Marinas (service and maintenance);

Facilities that generate or store hazardous materials.

Conveyance structures/channels shall be designed and adequately sized so as 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, or any amendments thereto, a drainage easement 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.

All stormwater management practices shall have an operation and maintenance plan pursuant to § 913.19(c) of this article, and if to be privately owned, an enforceable operation and maintenance agreement per § 913.19(d) of this article to ensure the system functions as designed and to provide remedies for system failure. A sample agreement may be found in Appendix C of this article.[2]

[2]

Editor's Note: Appendix C is included as an attachment to this article.

Stormwater runoff generated from development and 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 into a natural wetland and shall not be used to meet the minimum 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. In such instances, 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. 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. In addition, it shall be demonstrated that such an approach will not cause erosion.

All stormwater discharges from a project during the one- and two-year frequency storms shall be reduced by at least 50% of the allowable rate for those storm frequencies as determined in the predevelopment hydrologic analysis for the regulated activity.

(2)

The following minimum performance standards shall apply to all applicable new development activities, pursuant to § 913.03(a):

Water quality improvement shall be achieved in conjunction with or as part of infiltration practices (if used). 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. Stormwater quality management practices shall be designed to capture and treat stormwater runoff generated by the one-inch rainfall event. Refer to § 913.16(a)(2) for water quality volume design standards and assumptions. Stormwater quality management practice selection, design and implementation shall be based upon appropriate reference materials, as provided in § 913.08(b).

The post-development peak discharge rate shall not exceed the predevelopment peak discharge rate multiplied by the subbasin release rate percentage (where determined in Act 167 watersheds) for the one-, two-, five-, ten-, twenty-five-, fifty- and 100-year twenty-four-hour storm events pursuant to the predevelopment cover assumption described in § 913.15(b). Refer to Appendix A for release rate percentages information.[3]

[3]

Editor's Note: Appendix A is included as an attachment to this article.

Facilities capable of attenuating rainfall runoff shall be provided and shall be designed to attenuate the runoff volume from the one-year twenty-four-hour storm event for at least 24 hours. When applying the use of green infrastructure BMPs (for example, rain garden/bioretention) that rely on existing soil infiltration rates, if it is identified that this standard cannot be met, a modification request may be submitted following the procedures in § 913.05.

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.

Structural and nonstructural stormwater management practices that make the best possible use of infiltration on-site shall be considered in all site designs, when appropriate.

(3)

The following minimum performance standards shall apply to all applicable redevelopment activities, pursuant to § 913.03(b):

One of the following minimum performance standards shall be accomplished. Selection of the performance standard shall be whichever is most appropriate for the given site conditions:

Reduce the total impervious cover on the site (e.g., by replacement of pavement with pervious planting areas or green roof systems) by at least 25%, based on a comparison of existing impervious cover to proposed impervious cover; or

Provide facilities designed to attenuate the runoff volume from the one-year twenty-four-hour post-development storm event for at least 24 hours; or

Provide facilities to ensure that the post-development peak discharge rate shall not exceed the predevelopment peak discharge rate multiplied by the subbasin release rate percentage (where determined in Act 167 watersheds) for the two-year and ten-year twenty-four-hour storm events. A predevelopment cover CN of 71 and a time of concentration of not less than 10 minutes shall be assumed.

In addition to the minimum performance standards for redevelopment areas in § 913.14(c)(3) above, water quality improvements shall 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. Stormwater quality management facilities shall be designed to capture and treat 0.25 inch of runoff from all pavement areas (i.e., parking lots, pavements and noncovered sidewalks). Roof area may be excluded from this calculation.

(d)

Volume controls.

(1)

The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities whenever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection (d)(1)A or the Simplified Method in Subsection (d)(1)B below, or alternative design criteria as allowed by 25 Pa. Code Chapter 102.

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, at a minimum, shall be incorporated into the CG-1 Method:

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.

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.

For modeling purposes:

(A)

Existing (predevelopment) non-forested 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.

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:

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

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.

Whenever 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.

This method is exempt from the requirements of § 913.15.

§ 913.15 Stormwater runoff calculation criteria.

[Ord. No. 1528, 9-26-2022]

In addition to the infiltration and water quality requirements of this article, peak flow from those activities resulting in increases in impervious surface and/or regrading and compaction shall be attenuated consistent with the following stormwater calculation methods:

(a)

The precipitation values used for each design storm shall be 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) or the latest version. The one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year return periods shall be analyzed. The design storm values are applicable to the Soil Cover Complex Method.

(b)

The following assumptions shall be used for runoff calculations:

(1)

For new development sites, the ground cover used as the predevelopment assumption for runoff calculations shall be as follows:

Wooded sites shall use a ground cover of woodland in good condition. Portions of a site having more than one viable tree of a DBH [diameter at breast height (DBH) is the diameter of the tree stem 4 1/2 feet above the ground] of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within 10 years of application. If there is evidence of logging within the ten-year period, the logged area shall be considered as woodland in good condition.

Agricultural sites shall use a ground cover of pasture in good condition.

All other portions of a site shall use a ground cover of meadow in good condition.

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

For redevelopment sites, see § 913.14(c)(3).

(2)

Runoff curve numbers used in developing runoff calculations shall be obtained from the Urban Hydrology for Small Watersheds Technical Release No. 55 (USDA, 1986) or the latest version or standard. Aggregate areas shall be considered impervious in post-development conditions.

(3)

A Type II distribution storm (for the Soil Cover Complex Method only for example, TR-55, TR-20).

(4)

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

(5)

Manning's roughness coefficients for sheet flow shall be taken from Urban Hydrology for Small Watersheds, Technical Release No. 55 (USDA, 1986) or the latest version.

(c)

In all plans and designs for stormwater management systems and facilities submitted to the Engineer for approval, stormwater peak discharge and runoff shall be determined through the use of the NRCS Soil Cover Complex Method as set forth in Urban Hydrology for Small Watersheds, Technical Release No. 55 (USDA, 1986), with specific attention given to antecedent moisture conditions, flood routing, time of concentration, and peak discharge specifications included therein and in the Hydrology National Engineering Handbook, Section 4, (USDA, 1985), both by the U.S. Department of Agriculture, Natural Resources Conservation Service. Note that when TR-55 is used for natural system-based approaches and practices encouraged herein, calculations must be performed on a detailed small subarea basis. Use of Technical Release No. 20 and other methods listed in Table 1 is also acceptable. The design professional's selection of a specific 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 1 herein summarizes the computational methods available.

Table 1 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
PSRM	Penn State University	When full model is desirable or necessary
VT/PSUHM	Virginia Polytechnic Institute and Penn State University	When full model is desirable or necessary
HEC-1	U.S. Army Corps of Engineers	When full model is desirable or necessary
SWMM or commercial package based on SWMM	U.S. EPA	Most applicable in urban areas
Small Storm Hydrology Method (as included in SLAMM)	PV & Associates, or the website www.winslamm.com	Calculation of runoff volume from urban and suburban areas

(d)

A Modified Rational Method analysis may be used for drainage areas and/or development sites smaller than one acre when permitted by the 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. The methodology and model chosen for use shall be well-documented as being appropriate for use in this region, and all relevant assumptions, methodologies, calculations and data used shall be provided to the Engineer for review. Information on the Modified Rational Method is presented in the Recommended Hydrologic Procedures for Computing Urban Runoff from Small Watersheds in Pennsylvania (PADEP, 1982).

(e)

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

(f)

The Rational Method (that is, 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. This methodology is generally limited to drainage areas less than 10 acres in size.

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)

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.

(h)

Impervious Area Flash (IAF) analysis. An Impervious Area Flash (IAF) analysis shall be conducted for all sites. The analysis requires that the watershed impervious area be modeled without the pervious areas within the watershed. The time of concentration for the watershed should also be determined from the impervious areas only. If the IAF results in a higher post-development peak runoff rate than the post-development watershed analysis, then this higher value must be used for the final design of the stormwater management facility.

§ 913.16 Standards for stormwater management practices.

[Ord. No. 1528, 9-26-2022]

The Pennsylvania Stormwater Best Management Practices Manual shall serve as a guide for the design of stormwater management practices. Additional design guidance may also 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. A list of references is provided with this article.[1] The water quality volume design measures used herein are partially based on the methodology expressed in the Maryland Manual referenced above. Pursuant to the design options recommended in the above documents, the following standards shall be adhered to:

(a)

Extended detention, water quality volume, infiltration and nonstructural BMP credits criteria. The following sizing criteria shall be followed at all sites required to meet the standards of this article:

(1)

Extended detention.

Detain the one-year, twenty-four-hour design storm using the SCS Type II distribution. The design of the facility shall consider and minimize the chances of clogging and sedimentation potential.

Detention basins shall detain the one-year storm event and allow it to naturally infiltrate and recharge the groundwater table if possible. All subsequent orifices and control structures for the two-, ten-, twenty-five- and 100-year storm events shall be placed above the maximum water surface elevation of the one-year storm.

Flow from off-site areas must be considered as pass-through flow if it is conveyed through the BMP and should be modeled as predeveloped conditions for the one-year storm event only.

(2)

Water quality volume.

Treatment of the water quality volume (WQv) of stormwater prior to its release to receiving waters or water bodies shall be provided at all developments where stormwater management is required. The WQv equals the storage volume needed to capture and treat the runoff from storms of one inch or less. Runoff from the first one inch of rainfall transports most of the total pollutant load. The WQv is based on the following equation:

WQv = [(P)(Rv)(A)]/12 (acre-feet)
Where:
P	=	Rainfall depth in inches (set to one inch).
Rv	=	Volumetric runoff coefficient, 0.05 + 0.009(1), where 1 is percent impervious cover.
A	=	Site area (acres).

The formula assumes approximately 5% runoff from pervious surfaces and 90% runoff from impervious surfaces. A minimum of 0.2 inch per acre of runoff volume shall be met at sites or in drainage areas that have less than 15% impervious cover.

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.

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 Town.

For new developments and redevelopments, infiltration is considered an acceptable method of satisfying part or all of the water quality volume.

For new developments, the WQv requirements of this section shall be sized and designed in conjunction with the standards under § 913.16(a)(1).

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

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

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. Where possible, all of the water quality volume should be treated using infiltration BMPs. The following calculation shall be used to determine the minimum recharge goal for the site:

Recharge Volume (Rev), (acre-feet)
Fraction of WQv, depending on soil hydrologic group. Rev = (S)(Ai)
Where:
S	=	Soil-specific recharge factor in inches.
Ai	=	The measured impervious cover.

Hydrologic Soil Group	Soil-Specific Recharge Factor (S) (inch of runoff)
A	0.40
B	0.25
C	0.10
D	0.05

Infiltrated volume may be subtracted from the total site WQv.

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.

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 downgradient areas.

If more than one hydrologic soil group is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each hydrologic soil group.

All infiltration facilities shall be set back at least 20 feet from all structures with subgrade elements (e.g., basements, foundation walls).

All infiltration facilities shall be set back at least 20 feet from all property lines and road rights-of-way.

A detailed, on-site infiltration test is required at the location of all infiltration BMPs.

(4)

Credits for use of nonstructural BMPs. The developer may obtain credits for the use of nonstructural BMPs using the procedures outlined below. Examples of nonstructural credit calculations are provided in Appendix E.[2]

Volume Reduction Method No. 1: Natural Area Conservation.

A water quality volume reduction can be taken when undisturbed natural areas are conserved on a site, thereby retaining their predevelopment hydrologic and water quality characteristics. Under this method, a designer would be able to subtract the conservation areas from the total site area when computing the water quality protection volume. An added benefit is that the post-development peak discharges will be smaller, and hence, water quantity control volumes will be reduced due to lower post-development curve numbers or Rational Formula "C" values.

Rule. Subtract conservation areas from total site area when computing water quality protection volume requirements.

Criteria.

(A)

Conservation area cannot be disturbed during project construction and must be protected from sediment deposition. The conservation area shall be protected with a safety fence until construction has been completed. After construction, the area shall be posted with signage indicating that it is a conservation area.

(B)

Shall be protected by limits of disturbance clearly shown on all construction drawings.

(C)

Shall be located within an acceptable conservation easement instrument that ensures perpetual protection of the proposed area. The easement must clearly specify how the natural area vegetation shall be managed and boundaries will be marked. [Note: Managed turf (e.g., playgrounds, regularly maintained open areas) is not an acceptable form of vegetation management.]

(D)

Shall have a minimum contiguous area requirement of 10,000 square feet.

(E)

Rv is kept constant when calculating WQv.

(F)

Must be forested or have a stable, natural ground cover.

Volume Reduction Method No. 2: Stream Buffers.

This reduction can be taken when a stream buffer effectively treats stormwater runoff. Effective treatment constitutes treating runoff through overland flow in a naturally vegetated or forested buffer. Under the proposed method, a designer would be able to subtract areas draining via overland flow to the buffer from total site area when computing water quality protection volume requirements. The design of the stream buffer treatment system must use appropriate methods for conveying flows above the annual recurrence (one-year storm) event.

Rule. Subtract areas draining via overland flow to the buffer from total site area when computing water quality protection volume requirements.

Criteria.

(A)

The minimum undisturbed buffer width shall be 50 feet from top of bank.

(B)

The maximum contributing length shall be 150 feet for pervious surfaces and 75 feet for impervious surfaces.

(C)

The average contributing slope shall be 3% maximum unless a flow spreader is used. In no case shall the average contributing slope be greater than 10%.

(D)

Runoff shall enter the buffer as overland sheet flow. A flow spreader can be installed to ensure this.

(E)

Buffers shall remain as naturally vegetated or forested areas and will require only routine debris removal or erosion repairs.

(F)

Rv is kept constant when calculating WQv.

(G)

Not applicable if overland flow filtration/groundwater recharge reduction is already being taken.

Volume Reduction Method No. 3: Enhanced Swales.

This reduction may be taken when enhanced swales are used for water quality protection. Under the proposed method, a designer would be able to subtract the areas draining to an enhanced swale from total site area when computing water quality protection volume requirements. An enhanced swale can fully meet the water quality protection volume requirements for certain kinds of low-density residential development. (See Volume Reduction Method No. 5.) An added benefit is the post-development peak discharges will likely be lower due to a longer time of concentration for the site.

Rule. Subtract the areas draining to an enhanced swale from total site area when computing water quality protection volume requirements.

Criteria.

(A)

This method is typically only applicable to moderate- or low-density residential land uses (three dwelling units per acre maximum).

(B)

The maximum flow velocity for the water quality design storm shall be less than or equal to 1.0 foot per second.

(C)

The minimum residence time for the water quality storm shall be five minutes.

(D)

The bottom width shall be a maximum of six feet. If a larger channel is needed, use of a compound cross section is required.

(E)

The side slopes shall be 3:1 (horizontal:vertical) or flatter.

(F)

The channel slope shall be 3% or less.

(G)

Rv is kept constant when calculating WQv.

Volume Reduction Method No. 4: Overland Flow Filtration/Groundwater Recharge Zones.

This reduction can be taken when overland flow filtration/infiltration zones are incorporated into the site design to receive runoff from rooftops or other small impervious areas (e.g., driveways, small parking lots, etc.). This can be achieved by grading the site to promote overland vegetative filtering or by providing infiltration or rain garden areas. If impervious areas are adequately disconnected, they can be deducted from total site area when computing the water quality protection volume requirements. An added benefit will be that the post-development peak discharges will likely be lower due to a longer time of concentration for the site.

Rule. If impervious areas are adequately disconnected, they can be deducted from total site area when computing the water quality protection volume requirements.

Criteria.

(A)

Relatively permeable soils (Hydrologic Soil Groups A and B) should be present.

(B)

Runoff shall not come from a designated hotspot.

(C)

The maximum contributing impervious flow path length shall be 75 feet.

(D)

Downspouts shall be at least 10 feet away from the nearest impervious surface to discourage reconnections.

(E)

The disconnection shall drain continuously through a vegetated channel, swale, or filter strip to the property line or structural stormwater control.

(F)

The length of the disconnection shall be equal to or greater than the contributing length.

(G)

The entire vegetative disconnection shall be on a slope less than or equal to 3%.

(H)

The surface impervious area tributary to any one discharge location shall not exceed 5,000 square feet.

(I)

For those areas draining directly to a buffer, reduction can be obtained from either overland flow filtration or stream buffers. (See Method No. 2.)

(J)

Rv is kept constant when calculating WQv.

Volume Reduction Method No. 5: Environmentally Sensitive Large-Lot Subdivisions.

This reduction can be taken when a group of environmental site design techniques are applied to low- and very-low-density residential development [e.g., one dwelling unit per two acres (du/ac) or lower]. The use of this method can eliminate the need for structural stormwater controls to treat water quality protection volume requirements. This method is targeted towards large-lot subdivisions and will likely have limited application.

Rule: targeted towards large-lot subdivisions (e.g., two- acre lots and greater). The requirement for structural facilities to treat the water quality protection volume may be waived.

Criteria.

(A)

For single-lot development:

Total site impervious cover is less than 15%.

Lot size shall be at least two acres.

Rooftop runoff is disconnected in accordance with the criteria in Method No. 4.

Grass channels are used to convey runoff versus curb and gutter.

(B)

For multiple lots:

Total impervious cover footprint shall be less than 15% of the area.

Lot areas should be at least two acres, unless clustering is implemented. Open space developments should have a minimum of 25% of the site protected as natural conservation areas and shall be at least a half-acre average individual lot size.

Grass channels should be used to convey runoff versus curb and gutter (see Method No. 3).

Overland flow filtration/infiltration zones should be established (see Method No. 4).

[2]

Editor's Note: Appendix E is included as an attachment to this article.

(b)

Stormwater infiltration practices.

(1)

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

Permeability and infiltration rate of the site soils.

Slope and depth to bedrock.

Seasonal high water table.

Proximity to building foundations and wellheads.

Erodibility of soils.

Land availability and topography.

Slope stability.

Effects on nearby properties and structures.

(2)

Loading ratios.

The loading ratio of impervious surface to infiltration bed bottom size must be considered in all designs. The loading ratio describes the ratio of impervious drainage area to infiltration area, or the ratio of total drainage area to infiltration area. The following loading ratios may not be exceeded:

Maximum impervious loading ratio of 5:1 relating impervious surface draining to a particular infiltration area.

Maximum total loading ratio of 8:1 relating total drainage area to infiltration area.

Maximum impervious loading ratio of 3:1 relating impervious drainage area to infiltration area for Karst topography.

If a BMP is proposed to exceed any of these values, a detailed geotechnical analysis prepared by a licensed geotechnical engineer will be required to determine the overall effects to downstream properties should the load ratios be exceeded. Items such as groundwater mounding and the potential for subsurface flooding of downstream structures should be clearly addressed. The Town Engineer will review this analysis and render an opinion on the viability of the property design.

(3)

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 and, at a minimum, address soil permeability, depth to bedrock and slope stability. The general process for designing the infiltration BMP shall be:

Analyze hydrologic soil groups as well as natural and man-made features within the watershed to determine general areas of suitability for infiltration BMPs.

Provide field testing data to determine appropriate percolation rate and/or hydraulic connectivity.

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

(4)

Soil characteristics are subject to the specific considerations below:

Infiltration BMPs are particularly appropriate in Hydrologic Soil Groups A and B, as described in the Natural Resources Conservation Manual TR-55.

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

There must be a minimum of 24 inches between the bottom of any facility (i.e., basin bottom for noninfiltration facilitates or infiltration media bottom for facilities) and the seasonal high water table and/or bedrock (limiting zones) for facilitates located outside floodplains. There must be a minimum of 48 inches between the bottom of any facility (i.e., basin bottom for noninfiltration facilitates or infiltration media bottom for facilities) and the seasonal high water table and/or bedrock (limiting zones) for facilitates located within floodplains.

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.

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

Infiltration rates shall not be used for computing the storage volume of the infiltration system.

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

(5)

The recharge volume provided at the site shall be directed to the most permeable hydrologic soil group available, except where other considerations apply, such as in limestone geology.

(6)

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.

(7)

The Town may require additional analyses for stormwater management facilities proposed for susceptible areas such as:

Strip mines.

Storage areas for salt, chloride, other materials for winter deicing.

Unstable and steep slopes.

(8)

During the period of land disturbance, runoff shall be controlled prior to entering any proposed infiltration area. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase so as to maintain their maximum infiltration capacity.

(9)

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

(10)

Infiltration facilities shall be selected based on suitability of soils and site conditions. Acceptable infiltration includes but is not limited to: filter strips or stormwater filtering systems (for example, bioretention facilities, sand filters), open vegetated channels (that is, dry swales and wet swales), infiltration trenches, dry wells, infiltration basins, porous paving systems, retention basins, wet extended detention ponds, riparian corridor management, riparian forested buffers, rooftop runoff management systems, and sand filters (closed or open).

(11)

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.

(12)

Large infiltration facilities, such as retention basins or detention ponds, shall be set back at least 20 feet from all habitable structures with subgrade elements (e.g., basements, foundation walls). Small infiltration facilities, such as rain gardens or vegetated swales, may be set back 10 feet from habitable structures with subgrade elements.

(13)

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

(14)

If detailed infiltration study is required, the following guidelines shall be followed:

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, licensed geologist, geotechnical/civil engineer, or soil scientist and, at a minimum, address soil types, soil permeability, depth to bedrock, limitations of soils, presence/absence of carbonate geology, susceptibility to subsidence and/or sinkhole formation, and subgrade stability. The testing and evaluation should be completed at the preliminary design stage.

Infiltration requirements shall be based on the portions of the site that are permeable prior to disturbance and the degree to which development will reduce the permeability of the site. Permeability of the site shall be determined based on the detailed evaluations described herein. 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.

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 or Appendix C, Site Evaluation and Soil Testing, in the PA Stormwater BMP Manual.

The testing shall include a test pit and percolation test holes. The test hole 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 performed. All test holes used for evaluating the percolation rate shall be presoaked in accordance with the procedures established by the ACHD. The location and number of test pits and percolation holes shall be determined based on the type(s) of stormwater management facilities being designed. 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 Engineer.

(15)

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

The lowest elevation of the infiltration area, when located outside a floodplain, shall be at least two feet above the seasonal high water table and bedrock. The lowest elevation of the infiltration area, when located within a floodplain, shall be at least four feet above the seasonal high water table and bedrock.

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

All infiltration facilities shall have appropriate positive overflow controls to prevent storage within one foot of the finished surface or grade unless a specific amount of surface storage away from pedestrian and vehicular traffic is provided and such areas infiltrate the stored volume within 48 hours after the end of the twenty-four-hour design storm.

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

All surface inflows shall be treated to prevent the direct discharge of sediment into the infiltration facility. Accumulated sediment reduces stormwater storage capacity and ultimately clogs the infiltration mechanism. No sand, salt or other particulate matter may be applied to a porous (pervious) surface for winter ice conditions.

During site construction, all infiltration practice components shall be protected from compaction due to heavy equipment operation or storage of fill or construction material. Infiltration areas shall also be protected from sedimentation. 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 Engineer. All areas designated for infiltration shall not receive runoff until the contributory drainage area has achieved final stabilization.

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

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

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

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

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.

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.

(c)

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:

The channel shall be designed to safely convey the ten-year frequency storm event with a freeboard of at least 12 inches. "Freeboard" is the difference between the elevation of the design flow in the channel and the top elevation of the channel.

The peak velocity of the runoff from the ten-year storm shall be nonerosive for the soil and ground cover provided in the channel.

The longitudinal slope shall be no greater than 6%. Check dams or turf reinforcement matting is recommended for slopes between 3% and 6%.

Channels shall be trapezoidal in cross section.

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

The maximum allowable ponding time in the channel shall be less than 48 hours.

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

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.

Landscape specifications shall address the grass species, wetland plantings (if applicable), soil amendment and hydric conditions present along the channel.

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% loss level.

Check dams along the channel length may be warranted.

The bottom of dry swales 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, HEC 15, FHWA, September 2005.

(d)

Retention basins.

(1)

Retention basins shall be designed to create a healthy ecological community with sufficient circulation of water to prevent the growth of unwanted vegetation and mosquitoes or other vectors. If circulation cannot be provided via natural means, then artificial aeration and circulation shall be provided. Care shall be taken to landscape retention basins in accordance with § 913.17.

(2)

The retention basin shall be of sufficient size to allow the appropriate aquatic community needed to maintain healthy pond ecology and avoid mosquitoes capable of carrying West Nile virus and other diseases. The Allegheny County Health Department, the Pennsylvania Fish and Boat Commission, the Natural Resources Conservation Service, the Pennsylvania Extension Service, or other qualified professional consultant 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.

(3)

An outlet structure shall be designed to allow complete drainage of the pond for maintenance.

(4)

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 and required rate and quality of dry weather inflow, the stormwater inflow, seasonal and longer-term variations in groundwater table, and impacts of potential pollutant loadings.

(5)

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

(6)

A sediment forebay with a hardened bottom shall be provided at each inlet into the retention basin. The forebay storage capacity shall, at a minimum, be 10% of the permanent pool storage. The forebay shall be designed to allow for access by maintenance equipment for periodic cleaning. A permanent concrete cleanout maker shall be installed in the forebay to indicate the level where 25% for the forebay storage has been used.

(7)

Emergency spillways shall be sized and located to permit the safe passage of stormwater flows from an unattenuated, 100-year, post-development storm with one foot of freeboard. The maximum velocities in vegetated spillways excavated in otherwise undisturbed soil shall be analyzed based upon the velocity of peak flow in the emergency spillway during 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. All emergency spillways placed on fill materials shall be lined. Lining for emergency spillways shall incorporate native colors and materials where possible, including mono slab revetments, grass pavers, riprap and native stone.

(8)

Basin and pond embankments must be designed by a professional engineer registered in the Commonwealth of Pennsylvania. The design 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. The submittal of a geotechnical report from a geotechnical engineer for any embankment over 10 feet in effective height or posing a significant hazard to downstream property or life is required. The selection of fill materials must be subject to approval of the design engineer. Fill must be free of frozen soil, rocks over six inches, sod, brush, stumps, tree roots, wood, or other perishable materials. Embankment fills less than 10 feet in fill height must be compacted using compaction methods that would reasonably guarantee that the fill density is at least 90% of the maximum density as determined by standard proctor (ASTM-698). 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. A PADEP dam permit is required for 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; and/or for ponds having contributory drainage area of greater than 100 acres; and/or for impoundments of greater than 50 acre-feet.

(9)

The embankment's interior and exterior slopes may not be steeper than 3:1 (three horizontal to one vertical).

(10)

The minimum embankment width shall be five feet for embankments less than three feet in height, 10 feet if the embankment is between 3.1 feet and 9.9 feet in height, and 15 feet if the embankment is higher than 10 feet.

(11)

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.

(12)

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

(13)

Retention basins shall be designed to provide a length-to-width ratio of at least 2L:1W as measured in plan view (for example, a ratio of 4L:1W is too narrow).

(14)

The retention basin depth shall average three to six feet.

(15)

A five-foot-wide bench around the pond perimeter is required at an elevation of one foot below the permanent water surface elevation. A structural or vegetative barrier may be required by the Town.

(16)

Stabilization. Proper stabilization structures, including stilling basins, energy dissipaters, and channel lining, shall be constructed at the outlets of all retention basins and emergency spillways. The stabilization structures shall control water to: avoid erosion, reduce velocities of released water and direct water so that it does not interfere with downstream activities.

(17)

Energy dissipaters shall be installed to prevent erosion and/or initiate sheet flow at points where pipes or drainageways discharge to or from basins. Energy dissipaters shall comply with criteria in Hydraulic Design of Energy Dissipaters for Culverts and Channels, HEC 14, FHWA, July 2006. Such facilities shall be both functional and harmonious with the surrounding environment; for example, native rock shall be used in constructing dissipaters where practical.

(18)

Discharge points. 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 20 feet. 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. The Engineer may require that the setback distance be increased based upon factors such as topography, soil conditions, the size of structures, the location of structures, and discharge rates. A drainage easement may also be required.

(19)

Outlet structures. Outlet structures shall meet the following specifications:

To minimize clogging and to facilitate cleaning and inspecting, outlet pipes shall have an internal diameter of at least 15 inches and a minimum grade of 1%.

Bentonite plugs shall be provided on all outlet pipes within a constructed berm.

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

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.

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.

Basin outlet structures shall have childproof, nonclogging trash racks overall design openings exceeding 12 inches in diameter except those openings designed to carry perennial stream flows. Periodic cleaning of debris from trash racks shall be included in the operation and maintenance plan.

Anti-vortex devices, consisting of a thin vertical plate normal to the basin berm, shall be provided at the top of all circular risers or standpipes.

(20)

An easement 25 feet in width must be provided around the top perimeter of all retention facilities. This easement shall remain clear of any structures, sheds, retaining walls, swing sets, stored debris, etc. The entire twenty-five-foot easement, unless otherwise approved by the Town, must remain on the applicant's property.

(21)

All detention facilities shall be provided with an access road (with a legal easement) for maintenance purposes. The design criteria for such access roads shall be as follows:

Access roads shall be a minimum of 10 feet wide and have a maximum grade of 15%.

Access must be provided from a public street to the berm of the facility so that equipment can obtain access to the outlet control works without entering the water impoundment area.

Access roads with gradients of 3% or less are permitted to be constructed with a stabilized base material and natural surface or equal as approved by the Town.

Access roads with gradients of 10% or less are permitted to be constructed with a stabilized base material and bituminous seal coat surface in accordance with PennDOT Publication 408, Section 470, or equal as approved by the Town.

A permanent easement must be provided for the access roadway from the public right-of-way to the stormwater detention facility. This easement shall be a minimum of 25 feet in width.

Access roads with gradients exceeding 10% shall be constructed with full depth bituminous pavement or concrete surfacing as approved by the Town.

Where deemed necessary by the Town, a permanent gate shall be provided for the access roadway. The design for the gate must be submitted to and approved by the Town.

(e)

Detention basins.

(1)

The landscape standards of § 913.17 shall apply.

(2)

The maximum interior and exterior side slopes shall not exceed three horizontal to one vertical (3H:1V). The minimum required slope for the basin bottom is 2%. A level bottom is acceptable, provided that the designer demonstrates to the Town's satisfaction that the basin bottom will be landscaped with appropriate wetland vegetation pursuant to § 913.17. In addition, detention basins of sufficient size and slope may serve other functions as well, including recreational uses, which do not hinder or conflict with the function of the detention basin.

(3)

Inlet structures. The inlet pipe invert into a basin shall be set above the one-year quality storm elevation or a minimum of 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)

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

(5)

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. The minimum slope of a low-flow channel shall be 1%.

(6)

Outlet structures. Outlet structures shall meet the following specifications:

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

Bentonite plugs shall be provided on all outlet pipes within a constructed berm.

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

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.

Energy-dissipation facilities that convert concentrated flow to uniform shallow sheet flow shall be used where appropriate.

Basin outlet structures shall have childproof, nonclogging trash racks overall design opening exceeding 12 inches in diameter except those openings designed to carry perennial stream flows.

Anti-vortex devices, consisting of a thin vertical plate normal to the basin berm, shall be provided at the top of all circular risers or standpipes.

(7)

Emergency spillways shall be sized and located to permit the safe passage of stormwater flows from an unattenuated, 100-year, post-development storm with one foot of freeboard. The maximum velocities in vegetated spillways excavated in otherwise undisturbed soil shall be analyzed based upon the velocity of peak flow in the emergency spillway during 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. In general, emergency spillways should not be located in fill areas; all such facilities placed on fill materials shall be lined. Lining for emergency spillways shall incorporate native colors and materials where possible, including mono slab revetments, grass pavers, riprap, and native stone.

(8)

(9)

Except where special erosion-protection measures are provided, all disturbed areas shall be graded evenly, topped with four inches of topsoil, fertilized, seeded, and mulched by methods approved by the Town. Seed mixes including crown vetch shall not be permitted.

(10)

The minimum embankment width shall be five feet for embankments less than three feet in height, 10 feet if the embankment is between 3.1 feet and 9.9 feet in height and 15 feet if the embankment is higher than 10 feet.

(11)

A structural or vegetative barrier may be required by the Town.

(12)

Energy dissipaters shall be installed to prevent erosion and/or initiate sheet flow at points where pipes or drainageways discharge to or from basins. Level spreaders shall be used only where the maximum slope between the discharge point and the waterway does not exceed 5%. Energy dissipaters shall comply with criteria in Hydraulic Design of Energy Dissipaters for Culverts and Channels, HEC 14, FHWA, July 2006. Such facilities shall be both functional and attractive; for example, native rock shall be used in constructing dissipaters where practical.

(13)

Stabilization. Proper stabilization structures, including stilling basins, energy dissipaters, and channel lining, shall be constructed at the outlets of all basins and emergency spillways. The stabilization structures shall control water to avoid erosion, reduce velocities of released water and direct water so that it does not interfere with downstream activities.

(14)

(15)

A sediment forebay with a hardened bottom shall be provided at each inlet into the detention 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 for periodic cleaning.

(16)

A private easement, 25 feet in width, must be provided around the top perimeter of all detention facilities. This easement shall remain clear of any structures, sheds, retaining walls, swing sets, stored debris, etc. The entire twenty-five-foot easement must remain on the applicant's property.

(17)

All detention facilities shall be provided with an access road (with a legal easement) for maintenance purposes. The design criteria for such access roads shall be as follows:

Access roads shall be a minimum of 10 feet wide and have a maximum grade of 15%.

Access must be provided from a public street to the berm of the facility so that equipment can obtain access to the outlet control works without entering the water impoundment area.

Access roads with gradients of 3% or less are permitted to be constructed with a stabilized base material and natural surface, or equal as approved by the Town.

A permanent easement must be provided for the access roadway from the public right-of-way to the stormwater detention facility. This easement shall be a minimum of 25 feet in width.

Access roads with gradients exceeding 10% shall be constructed with full depth bituminous pavement or concrete surfacing as approved by the Town.

Where deemed necessary by the Town, a permanent gate shall be provided for the access roadway. The design for the gate must be submitted to and approved by the Town.

(18)

The volume and rate control capacities must drain in a period between 24 and 72 hours after the design storm.

(f)

Conveyance systems (open channels, drainageways, and storm sewers).

(1)

Applicants are encouraged to design conveyance systems that encourage infiltration and improve water quality wherever practicable.

(2)

All conveyance systems shall convey stormwater to the nearest established stream channel as approved by the Engineer or storm sewer system.

(3)

All conveyance systems shall have the appropriate erosion and/or energy-dissipation controls installed.

(4)

Conveyance systems should not be installed parallel and near the top or bottom of major embankments to avoid the possibility of slope failure.

(5)

Wherever conveyance channels are necessary, drainage shall be maintained by an open channel with landscaped banks designed to carry the 100-year stormwater runoff from upstream contributory areas. The Engineer may increase the design storm, as conditions require. All open channels shall be designed with one foot of freeboard above the design water surface elevation of the design runoff condition.

(6)

Flood relief channels shall be provided and designed to convey the runoff from the 100-year, twenty-four-hour storm, such that a positive discharge of this runoff to an adequate receiving stream or conveyance system occurs without allowing this runoff to encroach upon other properties.

(7)

Storm sewers and their appurtenant facilities shall be designed to convey the runoff from the 100-year storm. At no time shall the hydraulic grade line or energy grade line exceed the top of any inlet or manhole. Supporting calculations to document compliance with this requirement must be provided to the Engineer for review and approval.

(8)

All storm inlets, manholes, endwalls, and headwalls shall be constructed in accordance with the Town Standard Specifications, Design Standards, and Details (latest edition).

(9)

Storm inlets shall be placed at all street intersections and should be placed on the tangent and not the curved portions of any street. If possible, inlets shall be placed at lot lines to avoid potential conflicts with driveways.

(10)

Storm inlets which are deeper than five feet in depth shall have ladder bars installed.

(11)

A subarea drainage area map delineating the area draining to each inlet point of the storm sewer system is required to be part of the stormwater management plan.

(12)

All storm sewer pipe proposed for public dedication, either to the Town or a homeowners' association, shall be greater than or equal to 15 inches in diameter.

(13)

All storm sewer pipe proposed for public dedication under the cartway must be constructed of a minimum Class III reinforced concrete pipe (RCP) or as approved by the Town.

(14)

No corrugated metal pipe may be used for storm sewers to be dedicated to the Town or a homeowners' association.

(15)

The minimum grade on any storm sewer pipe shall be 1%, unless otherwise approved by the Engineer.

(16)

Concrete anchors, per the Town Standard Specifications, Design Standards, and Details (latest edition), shall be required where the storm sewer pipe slope exceeds 20%.

(17)

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. Inlet capacity for inlets in paved or other impervious areas shall be based on the design standards provided by the latest edition of PennDOT Publication 584 and 13M. If acceptable information is not available, inlets in nonponding areas shall be designed for a maximum capacity of four cubic feet per second (cfs). When ponding occurs, inlet capacity shall be based on accepted engineering design practices.

(18)

Where drainage swales are used in lieu of, or in addition to, storm sewers, they shall be designed to carry the required runoff without erosion and in a manner not detrimental to the properties they cross. Drainage swales shall provide a minimum grade of 2% but shall not exceed a grade of 9%. Drainage swales used strictly for conveyance are not the same as open vegetated channels. Design standards for open vegetated channels are provided under § 913.16(c) of this article.

(19)

On streets that must contain curbing, storm sewers shall be placed in front of 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.

(20)

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

(21)

Roof drains and sump pumps shall discharge to infiltration or vegetated BMPs where feasible.

(22)

The developers shall obtain or grant a minimum twenty-foot-wide, private drainage easement over all storm sewers, drainage swales, channels, etc., that are a component of the stormwater management system when located within undedicated land. 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, private drainage easement that allows proper legal access and maintenance vehicle access.

(23)

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.

(24)

Water quality inlets and/or bioretention areas. Storm drainage systems that collect runoff from parking areas and/or loading areas exceeding 10,000 square feet of impervious coverage and discharge to stormwater management systems, including surface or subsurface infiltration systems, shall have a minimum of one water quality inlet or bioretention area per each acre of drainage area. The purpose of water quality inlets is to remove oil, grease, and heavy particulates or total suspended solids, hydrocarbons, and other floating substances from stormwater runoff. Methods other than water quality inlets or bioretention areas may be permitted if the applicant demonstrates to the Town's satisfaction that any such alternative will be as effective and as easily maintained. Periodic cleaning of these systems shall be addressed in the operation and maintenance plan submitted to the Town. All bioretention areas should be designed as per the guidelines set forth in the PA Stormwater BMP Manual.

(g)

Underground detention facilities.

(1)

Underground detention tanks shall utilize the largest practicable pipe diameter in order to provide ease of access and maintenance.

(2)

Design measures must be implemented to prevent pipe flotation and allow for visual inspection capabilities, adequate pipe ventilation, and access maintenance.

(3)

Underground facilities shall be designed parallel to existing and/or proposed contours.

(4)

The underground facilities shall be designed to include overflow controls able to pass the unattenuated 100-year peak storm without overtopping the access point.

(5)

A minimum forty-eight-inch diameter manhole with steps must be provided at either end of the facility and at all terminal ends and junctions for maintenance purposes. A minimum of two accesses must be provided.

(6)

Maximum life expectancy of the underground system shall also be a design consideration for pipe material specifications. The use of corrugated metal pipe (CMP) for underground facilities proposed for ownership and maintenance by the Town is prohibited.

(7)

For underground rock sumps, refer to Appendix F of this article.[3]

[3]

Editor's Note: Appendix F is included as an attachment to this article.

(h)

All other stormwater management BMPs not discussed as part of this article shall adhere to the design requirements listed as part of the PA Stormwater BMP Manual.

[1]

Editor's Note: See Appendix D, List of References Cited and Additional Sources of Information, included as an attachment to this article.

§ 913.17 Landscaping of stormwater management facilities.

[Ord. No. 1528, 9-26-2022]

Stormwater management facilities shall be landscaped in accordance with the following standards:

(a)

Landscaping shall be required in and around all constructed stormwater management facilities with a minimum surface area of 1,000 square feet for the purposes of:

(1)

Assisting in the management of stormwater;

(2)

Stabilizing the soil within such facilities to minimize and control erosion;

(3)

Enhancing the visual appearance of such facilities; and

(4)

Mitigating maintenance problems commonly associated with the creation of such facilities.

(b)

A planting plan and planting schedule shall be submitted in accordance with the following:

(1)

Wet meadows, including floors of stormwater management facilities.

Wet meadows and floors of stormwater management facilities shall be planted with noninvasive plants native to western Pennsylvania, such as wildflowers and noninvasive grasses, the intent being to create a mixed meadow of such plantings, where appropriate. Selection of plantings shall be based on whether the area in question is usually well-drained or permanently wet and whether the area will be used for recreation purposes. No woody plants shall be planted within the saturated zone (phreatic line) of a stormwater management facility or on a berm constructed for impounded water.

Seeding by drills, corrugated rollers, cyclone or drop seeders or hand-seeding of such areas is preferred; however, hydroseeding followed by hydromulching can be used on wet ground and steep slopes.

Fertilizers, as a nutrient supplement, shall not be used unless it is documented that soil conditions warrant such use and the nutrient applied does not exceed plant uptake. Soil for planting of wildflowers shall contain not less than 3% nor more than 10% organic matter, as determined by an agricultural chemist, with certification of the test before planting.

Seeding shall take place either between April 1 and May 15 or between September 1 and October 15. Planting areas shall be soaked to maintain a consistent level of moisture for at least four to six weeks after planting. For seeding recommendations, reference the DEP's E&S Pollution Control Program Manual.

Once established, a single annual mowing when plants are dormant should be sufficient to maintain a wet meadow and/or floor of a stormwater management practice.

(2)

Wet edges that remain wet all or most of the year shall be planted with wildflowers, grasses, and shrubs. Plants to be located on rims or banks, which remain dry most of the year, shall be planted with species tolerant of dry soil conditions.

(3)

Wooded areas.

Where stormwater management facilities adjoin wooded areas, trees and shrubs shall be selected and planted outside the practice so as to blend with existing surroundings.

Plantings in such areas shall be of sufficient density to eliminate the need for mowing.

It is recommended that clusters of trees and shrubs be planted around stormwater management facilities but well away from outfalls and any constructed berms, where applicable, to provide for wildlife habitat, wind control and buffering and screening.

Vegetation shall be planted during appropriate times of the year, predominantly between late March and mid-May or from early October until evidence of ground freezing, depending upon the species selected. Most deciduous trees and shrubs can be planted in either spring or fall. Evergreens are best planted in late summer or early fall.

(4)

Slopes.

Where slopes are gentle, a mixture of meadow grasses and wildflowers (for wet meadows) shall be planted.

On steep slopes, as defined by the Town's Code of Ordinances, dense spreading shrubs (shrubs tolerant of dry soils) shall be planted. Heavy mat mulch shall be used during the period of establishment.

No woody plant materials or trees shall be located on a constructed or natural berm acting as the impoundment structure of a stormwater management practice. Trees shall be located downstream of an impoundment berm a sufficient distance from the toe of the constructed slope to assure that the toe of the slope is outside the dripline of the species planted at maturity but in no case less than 15 feet.

(5)

In cases where stormwater management facilities are to be located in proximity to wetlands or waterways, the applicant's planting plan and schedule shall consider the sensitive conditions existing therein and be modified accordingly to reflect existing flora.

(6)

Stormwater management facilities shall be screened in a manner which complements the existing landscape and provides sufficient access for maintenance.

§ 913.18 Stream buffer requirements.

[Ord. No. 1528, 9-26-2022]

Stream buffers shall be provided as per the following requirements:

(a)

A minimum stream buffer width of 50 feet landward in each direction from the top of stream banks is required for all waterways having both a defined bank and a contributing watershed area of greater than 100 acres.

(b)

A minimum stream buffer width of 35 feet landward in each direction from the center line of the waterway is required for smaller waterways having a contributing watershed area of less than 100 acres and greater than 10 acres.

(c)

The stream buffer area should be maintained in a natural state.

(d)

Trails in the buffer are permitted and are preferred to be permeable and for nonmotorized use only. Impermeable trails are permitted, provided that they have adequate drainage.

(e)

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.

(f)

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.

(g)

When wetland(s) extend beyond the edge of the required buffer width, the buffer shall be adjusted so that the buffer consists of the extent of the wetland plus a twenty-five-foot zone extending beyond the wetland edge.

(h)

Stream buffer averaging may be applied to account for encroachments, such as road crossings. The following criteria must be met in order to utilize buffer averaging on a development site:

(1)

All waterways with defined bed and banks must be mapped, and the total waterway length on the property proposed for buffer averaging must be calculated.

(2)

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

(3)

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

(4)

The average width must be calculated based upon the entire length of stream bank that is located within the boundaries of the property to be developed. When calculating the buffer length, the natural stream channel should be followed.

(5)

Stream 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.

(6)

On each stream bank, the total width of the buffer shall not be less than 25 feet at any location, except at approved stream crossings. Those areas of the buffer having a minimum width of 25 feet (or less at approved stream crossings) can comprise no more than 50% of the buffer length.

(i)

Stream buffer locations and widths should be illustrated on all subdivision plans with notations requiring that they be maintained in a natural state.

(j)

Stream 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.