Borough of New Britain, PA Stormwater Management Standards

The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.

For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.

For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter.

Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s). Such stormwater flows shall be subject to the requirements of this chapter.

All regulated activities shall include such measures as necessary to:

Protect health, safety, and property;

Meet the water quality goals of this chapter by implementing measures to:

Minimize disturbance to floodplains, wetlands, and wooded areas.

Create, maintain, repair or extend riparian buffers.

(c)

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of this commonwealth.

(e)

Disconnect impervious surfaces (i.e., disconnected impervious areas/DIAs) by directing runoff to pervious areas, wherever possible. See Appendix F for detail on DIAs.

To the maximum extent practicable, incorporate the techniques for low-impact development practices (e.g., protecting existing trees, reducing area of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual, Pennsylvania Department of Environmental Protection (PA BMP Manual) No. 363-0300-002(2006), as amended and updated. See Appendix E for a summary description.

Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize the use of natural on-site infiltration features while still meeting the other requirements of this chapter.

The design of all facilities over karst shall include an evaluation of measures to minimize the risk of adverse effects.

Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.

The design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service Hydrometeorological Design Studies Center, Silver Spring, Maryland, using data from the Doylestown station (36-2221). NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html

For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Stormwater Management Act.

Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual).

§ 375-302 Exemptions.

Regulated activities that create impervious surfaces less than or equal to 1,000 square feet are exempt from the peak rate control requirements, volume control, and the SWM site plan preparation located in Article IV of this chapter unless the activity is found to be a significant contributor of pollution to the waters of this Commonwealth.

Regulated activities that create impervious surfaces between 1,001 square feet up to and including 5,000 square feet are exempt only from the peak rate control requirements of this chapter. Regulated activities that create impervious surfaces between 1,001 square feet up to and including 5,000 square feet shall be subject to Appendix I hereof.

Agricultural activity is exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Ch. 102.

Forest management and timber operations are exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Ch. 102.

Any aspect of BMP maintenance to an existing SWM system made in accordance with plans and specifications approved by the Municipality is exempt.

The use of land for gardening for home consumption is exempt from the requirements of this chapter.

Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in § 375-301D through L.

Additional exemption criteria:

The Municipality may deny or revoke any exemption pursuant to this chapter at any time for any project that the Municipality believes may pose a threat to public health and safety or the environment.

Exemption responsibilities. An exemption shall not relieve the applicant from implementing such measures as are necessary to protect public health, safety, and property.

Drainage problems. Where drainage problems are documented or known to exist downstream of or is expected from the proposed activity, the Municipality may deny exemptions.

(4)

Exemptions are limited to this chapter.

(5)

HQ and EV streams. The Municipality may deny exemptions in high-quality (HQ) or exceptional value (EV) waters and source water protection areas (SWPA).

§ 375-303 Volume control.

Volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. Stormwater runoff volume control methods are based on the net change in runoff volume for the two-year storm event.

The green infrastructure and low-impact development practices provided in the PA BMP Manual shall be utilized for all regulated activities wherever possible.

Volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activities equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations of the procedures associated with each methodology, and other factors. All regulated activities greater than one acre and those that require hydrologic routing to design the stormwater facilities must use the Design Storm Method.

The Design Storm Method (CG-1 in the PA BMP Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions. For modeling assumptions, refer to § 375-305.

Post-development total runoff cannot be increased from pre-development total runoff for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.

The following applies in order to estimate the increased volume of runoff for the two-year, twenty-four-hour duration precipitation event:

To calculate the runoff volume (cubic feet) for existing site conditions (pre-development) and for the proposed developed site conditions (post-development), it is recommended to use the Soil Cover Complex method as shown below. The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means. Runoff volume must be calculated for each land use type and soil. The use of a weighted CN value for volume calculations is not acceptable. Table B-3 in Appendix B is available to guide a qualified professional and/or an applicant to calculate the stormwater runoff volume.

Soil Cover Complex Method:
Step 1: Runoff (in) = Q = (P-0.2S)²/(P + 0.8S)
Where:
P	=	Two-year rainfall (inches)
S	=	(1,000/CN) — 10; the potential maximum retention (including initial abstraction, la)
Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12
Where:
Q	=	Runoff (inches)
Area	=	Stormwater management area (square feet)

The Simplified Method (CG-2 in the PA BMP Manual) 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 the runoff volume from at least the first two inches of runoff from all new impervious surfaces.

Volume (cubic feet) = (2 inches runoff/12 inches)* impervious surface (square feet)

At least the first one inch of runoff volume from new impervious surfaces shall be permanently removed from the runoff flow; i.e., it shall not be released into the surface waters of the Commonwealth. The calculated volume shall be removed through reuse, evapotranspiration, or infiltration through structural or nonstructural means.

Volume (cubic feet) = (1 inch runoff/12 inches)* impervious surface (square feet)

Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases where soils are suitable for infiltration based on the criteria of § 375-303C(5) and (6), at least the first 1/2 inch of the permanently removed runoff should be infiltrated.

(4)

No more than one inch of runoff volume from impervious surfaces shall be released from the site. The release time must be over 24 to 72 hours.

Stormwater control measures. The applicant must demonstrate how the required volume is controlled through stormwater best management practices (BMPs) which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the total runoff volume.

If natural resources exist on the site, the applicant who is required to submit a SWM site plan shall determine the total acreage of protected area where no disturbance is proposed. The acreage of the protected area should be subtracted from the total site area and not included in the stormwater management site area acreage used in determining the volume controls.

Stormwater Management Site Area =

Total Site Area (for both pre and post development conditions) minus Protected Area

Natural resource areas should be calculated based upon the municipality's own natural resource protection ordinance requirements in § 450-38. See Table B-2 in Appendix B for guidance to assess the protected area. For additional reference, see Chapter 5 Section 5.4.1 of the PA BMP manual.

Calculate the volume controls provided through nonstructural BMPs. Table B-5 in Appendix B is recommended as guidance.

Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.

Required )		Nonstructural		Structural Volume
Volume Control (feet³	minus	Volume Control (feet³)	=	Requirement (feet³)

(4)

Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix B is recommended as guidance. See PA BMP Manual Chapter 6 for description of the BMPs.

(5)

Infiltration BMPs intended to receive runoff from developed areas shall be selected based on the suitability of soils and site conditions (see Table B-6 in Appendix B for a list of Infiltration BMPs). Infiltration BMPs shall be constructed on soils that have the following characteristics:

A minimum soil depth of 24 inches between the bottom of the infiltration BMPs and the top of bedrock, seasonally high water table, or other limiting zone.

An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inches/hour should be utilized and for acceptable rates a safety factor of 50% should be applied for design purposes (e.g., for soil which measured 0.4 inch/hour, the BMP design should use 0.2 inch/hour to ensure safe infiltration rates after construction).

(c)

All open-air infiltration facilities shall be designed to completely infiltrate runoff volume within three days (72 hours) from the start of the design storm.

(6)

Soils. A soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. All regulated activities are required to perform a detailed soils evaluation by a qualified design professional which at minimum address soil permeability, depth to bedrock, and subgrade 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 site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.

Provide field tests such as double-ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.

(c)

Design the infiltration structure based on field-determined capacity at the level of the proposed infiltration surface and based on the safety factor of 50%.

(d)

If on-lot infiltration structures are proposed, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.

(e)

An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.

§ 375-304 Stormwater peak rate control and management districts.

Peak rate controls for large storms, up to the 100-year event, is essential in order to protect against immediate downstream erosion and flooding. The following peak rate controls have been determined through hydrologic modeling of the Neshaminy Creek Watershed.

Standards for managing runoff from each subarea in the Neshaminy Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms are shown in Table 304.1. All of New Britain Borough is located with District A. Development sites must control proposed development conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 304.1 below:

Table 304.1 Peak Rate Runoff Control Standards in the Neshaminy Creek Watershed (includes Little Neshaminy Creek)
District	Design Storm Post-development (Proposed Conditions)	Design Storm Pre-development (Existing Conditions)
A	2-year	1-year
	5-year	5-year
	10-year	10-year
	25-year	25-year
	50-year	50-year
	100-year	100-year

General. Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Appendix D) and in this section.

District boundaries. The boundaries of the Stormwater Management Districts are shown on official maps and are available for inspection at the municipal office and county planning offices. A copy of the map at a reduced scale is included in Appendix D.

Off-site areas. When calculating the allowable peak runoff rates, developers do not have to account for runoff draining into the subject development site from an off-site area. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.

Site areas. The stormwater management site area is the only area subject to the management district criteria. Non-impacted areas or nonregulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.

Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the Borough.

Meet the full requirements specified by Table 304.1 and § 375-304A through E; or

Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.

§ 375-305 Calculation methodology.

The following criteria shall be used for runoff calculations:

For development sites not considered redevelopment, the ground cover used to determine the existing conditions runoff volume and flow rate shall be as follows:

Wooded sites shall use a ground cover of "woods in good condition." Wooded sites is defined as an area of trees which occupies at least 1/4 acre, containing any tree or trees either three inches in diameter or 20 feet tall, and including the natural layers of vegetation beneath the canopy and understory plants. Diameter shall be measured at 4.5 feet above ground level.

Other pervious portions of the site, including agriculture, bare earth, and fallow ground, shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve number (CN) or Rational "c" value (i.e. woods) as listed in Tables B-4 or B-7 in Appendix B of this chapter.

For development and redevelopment sites, the ground cover used to determine the existing conditions runoff volume and flow rate for the developed portion of the site shall be based upon actual land cover conditions, except that 20% of the existing impervious surface area shall be considered meadow in good condition in the model for existing conditions.

Stormwater runoff peak discharges from all development sites with a drainage area equal to or greater than one acre shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 305.1 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The municipality may allow the use of the Rational Method (Q=CIA) to estimate peak discharges from drainage areas that contain less than one acre.

Where:
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

All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the latest version of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Doylestown rain gage. This data may also be directly retrieved from the NOAA Atlas 14 website: http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.

Table 305.1 Acceptable Computation Methodologies for Stormwater Management Plans
Method	Method Developed By	Applicability
TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans within limitations described in TR-55
HEC-1/HEC-HMS	U.S. Army Corps of Engineers	Applicable where use of full hydrologic computer model is desirable or necessary
PSRM	Penn State University	Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1
Rational Method (or commercial computer package based on Rational Method)	Emil Kuichling (1889)	For sites less than 1 acre, or as approved by the municipality and/or municipal engineer
Other methods	Varies	Other computation methodologies approved by the municipality and/or municipal engineer

All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from NOAA Atlas 14, Volume 2, Version 3 corresponding to the Doylestown rain gage. This data may also be directly retrieved from the NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's equation.

Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be based on Table B-4 in Appendix B.

Runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method shall be consistent with Table B-7 in Appendix B.

Runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions because of soil compaction, even after topsoiling or seeding. The proposed condition "CN" or "C" shall increase by 5% to better reflect proposed soil conditions.

The Manning equation is preferred for one-dimensional, gradually varied, open channel flow. In other cases, appropriate applicable methods should be applied; however, early coordination with the municipality is necessary.

Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using the generally accepted hydraulic analysis technique or method of the municipality.

The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

§ 375-306 Other requirements.

Hot spots.

The use of infiltration BMPs is prohibited on hot spot land use areas. Examples of hot spots are listed in Appendix G.