Borough of Dublin, PA STORMWATER MANAGEMENT

[Ord. 252, 5/24/2004, § 301]

All regulated activities in the Borough of Dublin which do not fall under the exemption criteria shown in § 25A-105 of this chapter shall submit a stormwater management site plan consistent with the Tohickon Creek Watershed Stormwater Management Plan to the Borough for review. This criteria shall apply to the total proposed development even if development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the waiver criteria.

Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.

The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.

Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the Developer must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other harm will result from the concentrated discharge.

Where a development site is traversed by watercourses drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Also, maintenance, including mowing of vegetation within the easement, shall be required, except as approved by the appropriate governing authority.

When it can be shown that, due to topographic conditions, natural drainage-ways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, through the general permit process.

Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the Developer or his agent to show that the land in question cannot be classified as wetlands, otherwise approval to work in the area must be obtained from PADEP.

Any stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PADOT).

Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities.

10.

Roof drains must not be connected to streets, sanitary or storm sewers or roadside ditches to promote overload flow and infiltration/percolation of stormwater where advantages to do so. When it is more advantageous to connect directly to streets or storm sewers, then it shall be permitted on a case by case basis by the municipality.

§ 25A-302 Stormwater Management Districts.

[Ord. 252, 5/24/2004, § 302]

Mapping of Stormwater Runoff Peak Rate Districts. In order to implement the provisions of this Dublin Borough Watershed Stormwater Management Plan, Dublin Borough is hereby divided into Stormwater Runoff Peak Rate Districts consistent with the plan. The boundaries of the districts are indicated on the runoff peak rate district map that is available for inspection at the Borough building. A large-scale boundary map is included as Appendix D for reference.

The exact location of the Stormwater Runoff Peak Rate District boundary as it applies to a given development site shall be determined by mapping the boundaries using the two-foot or five-foot topographic contours provided as part of the stormwater management plan developed for the site in accordance with the Borough subdivision and land development regulations. The district boundaries as originally drawn coincide with topographic divides or, in certain instances, are drawn from the intersection of the watercourse or a potential flow obstruction to the topographic divide consistent with topography. The locations determined on the stormwater management plan shall be reviewed and verified by the Borough Manager.

Description of Stormwater Runoff Hydrologic Peak Rate Districts.

Conditional No Detention Districts. Subareas included in this district are 2, 3, 8-10, 18, 20, 21, 42, 43, 52, 54, 56, 57, 59, 61, 62, 67, 70-73, 76, 77, 81-83. These subareas may discharge post-development runoff without detention facilities without adversely affecting the total watershed peak flow. These areas are located adjacent to the Tohickon Creek and Lake Nockamixon, which is capable of absorbing undetained runoff without affecting the watershed level control. In certain instances, the conveyance capabilities of the local receiving facilities may not be adequate to safely transport the increased peak flows from undetained runoff. In these cases, the developer shall assure that 100% release rate control is applied to the particular receiving stream(s), and/or the developer may provide increased capacity of those receiving facilities in order to insure safe passage of any undetained runoff.

One-hundred Percent Release Rate District. Subareas included in this district are 4-7, 11- 17, 19, 22-28, 31-34, 41, 44-51, 53, 55, 58, 60, 63, 64, 66, 68, 69, 74, 75, 84-98, 101, 109, 111-114, 116, 118, 119, 121-126. These subareas are not expected to incur a great deal of development growth due to location, topography, soils, or a combination of all three factors. Also, the location in the watershed of these subareas is of minor importance in supporting the overall watershed level runoff control. Therefore, these areas are allowed to release development runoff at a rate that does not exceed the existing rates of runoff.

Ninety Percent Release Rate District. Subareas included in this district are 30, 35-40, 226. These areas are located in developing areas that have adequate drainage capacity in the receiving waterways. A slight amount of reduction is necessary to preserve this integrity of the receiving waterways and increase water quality of the receiving waterways, at the request of the local municipality.

Seventy Percent Release Rate District. Subareas included in this district are 78-80, 99, 100, 102-108, 110, 115, 117, 120. Certain subareas require the control of stormwater runoff to a portion of the existing runoff equal to 75%. These areas are located in upper reaches of the watershed, specifically, areas around Quakertown Borough and Richlandtown Township which are projected to incur significant development impacts and have existing inadequate storm conveyance facilities. Some of these areas are expected to incur a relatively major increase in development pressure, while some areas may not see much development at all. In order to assure uniform watershed-level runoff control; however, the assignment of this release rate on a widespread basis will uniformly restrict the future runoff in a fashion that favors no particular subwatershed.

§ 25A-303 Stormwater Management District Implementation Provisions (Performance Standards).

[Ord. 252, 5/24/2004, § 303]

General. Post development rates of runoff from any regulated activity shall not exceed the specified hydrologic district peak release rates of runoff for the design storms specified on the Stormwater Runoff Peak Rate Districts Map, Appendix D and § 25A-302, of this chapter. Calculations must be provided to ensure that post-development runoff peak rates from storms including the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year frequency storm do not exceed pre-development peak rates, or the hydrologic district peak release rate for that district, for similar frequency storms.

Groundwater Recharge.

Developed areas shall maintain groundwater recharge consistent with predevelopment conditions, dependent on hydrologic soil groups and impervious cover. A minimum of one inch of runoff shall be infiltrated unless the developer can prove the inability of the site to achieve this specific volume based on existing site conditions. The maximum available recharge shall be calculated based on utilizing the most capable recharge areas of the site. This volume of runoff is termed the "recharge volume" and is calculated in accordance with § 25A-305, Subsection 1K.

Design of the stormwater management facilities shall provide for ground water recharge to compensate for the reduction in the percolation that occurs when the ground surface runoff characteristics have been altered. A detailed geologic evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified geologist and/or soil scientist, and shall, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. Where pervious pavement is permitted for parking lots, recreational facilities, nondedicated streets, or other areas, pavement construction specifications shall be noted on the plan.

Whenever a stormwater facility will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent ground water contamination and, where necessary, sinkhole formation. Soils used for the construction of basins shall have low-erodibility factors ("K" factors). The municipality may require the installation of an impermeable liner in detention basins. If the developer can prove through analysis that the site is in an area underlain by limestone, and such geologic conditions may result in sinkhole formations, then the site is exempt from recharge requirements. However, the site shall still be required to meet all other hydrologic and water quality management standards as found in this chapter.

It shall be the developer's responsibility to verify if the site is underlain by limestone. The following note shall be attached to all stormwater management site plans and signed and sealed by the developer's engineer/surveyor/landscape architect/architect "I, __________, certify that the proposed detention basin (circle one) is/is not underlain by limestone."

Water Quality. Developed areas will provide adequate storage and treatment facilities necessary to capture and treat the water quality volume (WQ) consistent with Parts 3 and 4 of this chapter. The "water quality volume" is calculated in accordance with § 25A-305, Subsection 10. The recharge volume may be a component of the water quality volume. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs.

District Boundaries. The boundaries of the Stormwater Management Districts are shown on an official map, which is available for inspection at the municipal office. A copy of the official map at a reduced scale is included in this chapter, Appendix D. The exact location of the Stormwater Management District boundaries as they apply to a given development site shall be determined by mapping the boundaries using topographic contours at an appropriate level of detail, but in no case less than two-foot intervals. The Borough may determine a more frequent contour interval is necessary to adequately delineate the district boundary. This information will be provided as part of the stormwater management site plan.

Sites located in more than one district for a proposed development site located within two or more release category subareas, the peak discharge rate from any subarea shall be the predevelopment peak discharge for that subarea multiplied by the applicable release rate. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas recombine in proximity to the site. In this case, peak discharge in any direction may be a 100% release rate provided that the overall site discharge meets the weighted average release rate.

Off-Site Areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.

Site Areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area shall be subject to the release rate criteria.

Stormwater Conveyance Corridor Protection (Riparian Corridor Preservation and Vegetation).

Runoff from developed areas of the site, including but not limited to areas of impervious surface, shall be managed through a series of riparian corridor vegetation facilities whenever possible. This will be accomplished in a manner satisfactory to the Borough, utilizing the "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998, Riparian Forested Buffer, and the priority goal of the riparian vegetation will be the reduction of thermal impacts on stormwater runoff associated with impervious areas, with a secondary goal being the protection of capacity of existing stormwater conveyance channels. These goals will be achieved through the use of design criteria in § 25A-304, Subsection 10, of this chapter, and shall be in addition to any other municipal ordinance provisions.

Within a particular district, the Borough may require the submission of a downstream hydraulic capacity analysis to verify stormwater management routing, timing, combined peak effects, etc., associated with unanticipated impervious cover and development, such as may result from changes in zoning or conditional uses which exceed projected growth for a particular watershed. Such analysis shall be conducted in accordance with this chapter and is subject to the review and approval of the Borough Engineer.

Regional Detention Alternatives. For certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the calibrated model as developed for the stormwater management plan.

§ 25A-304 Design Criteria for Stormwater Management Facilities.

[Ord. 252, 5/24/2004, § 304]

Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.

Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year post-development conditions. The height of embankment must be set as to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year post-development inflow. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than one-hundred-year event.

Any facilities that constitute water obstructions (e.g., culverts, bridges, outcall, or stream enclosures), and any work involving wetlands as directed in PADEP Chapter 105 regulations (as amended or replaced from time to time by PADEP), shall be designed in accordance with Chapter 105 and will require a permit from PADEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm within a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility that constitutes a dam as defined in PADEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PADOT right of way must meet PADOT minimum design standards and permit submission requirements.

Any drainage conveyance facility and/or channel that does not fall under Chapter 105 regulations, must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PADOT right-of-way must meet PADOT minimum design standards and permit submission requirements.

Storm sewers must be able to convey post-development runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.

Adequate erosion protection shall be provided along all open channels, and at all points of discharge.

The design of all stormwater management facilities shall incorporate sound engineering principles and practices. Guidelines established by the "Pennsylvania Handbook of Best Management Practices for Developing Areas" shall be utilized in determining stormwater management facility design. The Borough shall reserve the right to disapprove any design that would result in the occupancy or continuation of an adverse hydrologic or hydraulic condition within the watershed.

In selecting the appropriate BMPs or combinations thereof, the land developer shall take into account the following:

Total contributing area.

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 configuration of the topography.

The following additional factors should be considered when evaluating the suitability of BMPs used to control water quality at a given development site:

	A.	Peak discharge and required volume control.
	B.	Streambank erosion.
	C.	Efficiency of the BMPs to mitigate potential water quality problems.
	D.	The volume of runoff that will be effectively treated.
	E.	The nature of the pollutant being removed.
	F.	Maintenance requirements.
	G.	Creation/protection of aquatic and wildlife habitat.
	H.	Recreational value.
	I.	Enhancement of aesthetic and property value.

Pipe or artificial swale discharge shall be set back 75 feet from a receiving waterway, and the pipe discharge shall be immediately diffused or spread out to reduce and eliminate high-velocity discharges to the impacted ground surface. The conveyance mechanism shall minimize disturbance and minimize velocity of discharge.

All infiltration devices and groundwater recharge facilities shall be designed to completely drain all water in three days subsequent to any storm event.

10.

Riparian Corridor Preservation. The area up to 50 feet from top of streambank on either side of a stream shall be planted in accordance with Zone I and Zone II buffer planting requirements as depicted in "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998, Riparian Forested Buffer. Zone I will comprise, at a minimum, the first 15 feet from top of bank, with Zone II comprising the remaining 35 feet. This replanting may be waived by the local Borough along streambank areas which receive overland or shallow flow from upstream, undisturbed, meadow or other existing pervious surfaces.

11.

All developments which create impervious surface shall provide capacity for and treatment of the "Water Quality Volume" and "Recharge Volume," unless exempt from applicability under § 25A-104.

12.

Special requirements for areas falling within defined exceptional value and high-quality sub-watersheds: The temperature and quality of water and streams that have been declared as exceptional value or high quality is to be maintained as defined in Chapter 93, "Water Quality Standards," Title 25 of Pennsylvania Department of Environmental Protection Rules and Regulations. Temperature sensitive BMPs and stormwater conveyance systems are to be used and designed with storage pool areas and supply outflow channels and should be shaded with trees. This will require the modification of berms for permanent ponds and the relaxation of restrictions on planting vegetation within the facilities, provided that capacity for volume and rate controls is maintained. At a minimum, the southern half of pond shorelines shall be planted with shade or canopy trees within 10 feet of the pond shoreline. In conjunction with this requirement, the maximum slope allowed on the berm area to be planted is 10 to 1. This will lessen the de-stabilization of berm soils due to root growth.

A long-term maintenance schedule and management plan for the thermal control BMPs is to be established and recorded for all development sites.

§ 25A-305 Calculating Methodology.

[Ord. 252, 5/24/2004, § 305]

Stormwater runoff from all development sites shall be calculated using either the rational method or a soil-cover-complex methodology.

Any stormwater runoff calculations invoking drainage areas greater than 200 acres, including on-and off-site areas, shall use generally accepted calculation technique that is based on the NRCS soil cover complex method. Table VIII-1 summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. The Borough may approve the use of the rational method to estimate peak discharges from drainage areas that contain less than 200 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 presented in Table A-1 in Appendix A of this chapter. If a hydrologic computer model such as PSRM or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The NRCS 'S' curve shown in Figure A-1, Appendix A of this chapter, shall be used for the rainfall distribution.

For the purposes of predevelopment flow rate determination, undeveloped land shall be considered as "meadow" good condition, unless the natural ground cover generates a lower curve number or Rational "C" value (i.e., forest).

All calculations using the rational method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the Design Storm Curves from PA Department of Transportation Design Rainfall Curves (1986) (Figure A-2). 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 obtained from Table A-2 in Appendix A of this chapter.

Runoff coefficient (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table A-3 in Appendix A of this chapter.

Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table A-4 in Appendix A of this chapter.

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

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 20 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Borough 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.

If the designer can substantiate through actual physical calibration that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Borough Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.

Calculation of Water Quality Volume. The water quality volume (WQ_v) is the storage capacity needed to treat 90% of the average annual stormwater rainfall from the developed areas of the site. The following calculation is used to determine the storage volume, WQ_v, in acre-feet of storage:

	WQ_v = (1.95)(R_v)(A)
	12

	WQ_v = Water Quality Volume
	A = Area in acres
	R_v = 0.05 +)0.009(I) where I is the percent impervious cover (example: I = 50 for 50% impervious cover)
	1.95 = is a coefficient representing the 90% annual rainfall (Pa Handbook of Best Management Practices for Developing Areas)
	WQ_v shall be designed as part of a stormwater management facility which incorporates water quality BMPs as a primary benefit of using that facility, in accordance with design specifications contained in "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998.

Calculation of Recharge Volume. The recharge volume (Re_v) is the volume of stormwater runoff from a developed site which shall be required to maintain existing pre-development groundwater recharge at development sites. It may be part of the water quality volume, and is calculated on the basis of treatment and recharge by structural stormwater management practices, as follows:

	Re_v = (S) (R_v) (A)
	12

	Re_v = Recharge Volume
	A = Area in acres
	R_v = 0.05 + 0.009(I) where I is the percent impervious cover (example: I = 50 for 50% impervious cover)
	S is the soil specific recharge factor and varies according to the soil type, as follows:

	Hydrologic Soil Group	Soil Specific Recharge Factor (S)
	A	0.38
	B	0.26
	C	0.14
	D	0.07

Structural stormwater management facilities which provide treatment and recharge of the required recharge volume will be designed as part of a stormwater management facility which incorporates groundwater recharge BMPs as a primary benefit of using that facility, in accordance with design specifications contained in "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998.

Table VIII-1
Acceptable Computation Methodologies for Stormwater Management Plans
Method	Method Developed by	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 models limitations
HEC - 1	U.S. Army Corps of Engineers	When full model is desirable or necessary
PSRM	Penn State University	When full model is desirable or necessary
Rational Method or commercial package based on Rational Method	Emil Kuichling (1889)	For sites less than 200 acres
Other Methods	Various	As approved by the Borough Engineer

§ 25A-306 Erosion and Sedimentation Requirements.

[Ord. 252, 5/24/2004, § 306]

Whenever the vegetation and topography are to be disturbed such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the Bucks County Conservation District and the standards and specifications of the appropriate municipal government.

Additional erosion and sedimentation control design standards and criteria that must be or are recommended to be applied where infiltration BMPs are proposed and include the following:

Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.

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