A. 
All regulated activities in East Branch Perkiomen Creek watershed that do not fall under the exemption criteria shown in § 129-52 shall submit a drainage plan consistent with the East Branch Perkiomen Creek Watershed Stormwater Management Plan to the municipality for review. This criterion 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 exemption criteria.
B. 
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 Part 2.
C. 
The drainage plan must be designed consistent with the sequencing provisions of § 129-42 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article XI of this Part 2.
D. 
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 Part 2.
E. 
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 Part 2. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the applicant 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.
F. 
Whenever a watercourse is located within a development site, it shall remain open in its natural state and location and should not be piped, impeded, or altered (except for road crossings). It is the responsibility of the developer to stabilize existing eroded stream/channel banks.
G. 
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.
H. 
When it can be shown that, due to topographic conditions, natural drainageways 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 the municipality and the DEP through the Joint Permit Application process, or, where deemed appropriate by DEP, through the General Permit process.
I. 
Any stormwater management facilities regulated by this Part 2 that would be located in or adjacent to waters of the Commonwealth or wetlands shall be subject to approval by DEP through the Joint Permit Application process, or, where deemed appropriate by DEP, the General Permit process. When there is a question whether wetlands may be involved, it is the responsibility of the applicant 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 DEP.
J. 
Any stormwater management facilities regulated by this Part 2 that would be located on or discharge into state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
K. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are required to reduce the size or eliminate the need for detention facilities.
L. 
Roof drains must not be discharged to streets or roadside ditches or connected to sanitary or storm sewers. Overland flow and infiltration/percolation of stormwater shall be promoted where site conditions allow. If a developer wishes to connect directly to streets or storm sewers, it shall it be permitted on a case-by-case basis only after review and approval by the municipality.
M. 
Special requirements for watersheds draining to high-quality (HQ) and exceptional value (EV) waters. The temperature and quality of water and streams that have been declared as exceptional value and high-quality are 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 modification of berms for permanent ponds and the relaxation of restrictions on planting vegetation within the facilities, provided that capacity for volumes and rate control is maintained. At a minimum, the southern half on 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 one. This will lessen the destabilization 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.
N. 
All stormwater runoff shall be pretreated for water quality prior to discharge to surface or groundwater as required by § 129-43 of this Part 2.
A. 
The design of all regulated activities shall include the following steps in sequence to minimize stormwater impacts.
(1) 
The applicant is required to find practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime of the site.
(2) 
An alternative is practicable if it is available and capable of being done after taking into consideration cost, existing technology and logistics in light of overall project purposes, and other municipal requirements.
(3) 
All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.
B. 
The applicant shall demonstrate that they designed the regulated activities in the following sequence to minimize the increases in stormwater runoff and impacts to water quality:
(1) 
Prepare an existing resource and site analysis map (ERSAM), showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, stream buffer zones, hydrologic soil groups A, B, C, and D, any existing recharge areas and any other requirements outlined in Ch. 135, Subdivision and Land Development. Establish stream buffer according to recommended criteria or applicable ordinances.
(2) 
Prepare a draft project layout avoiding sensitive areas identified in Subsection B(1) and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.
(3) 
Identify site specific existing conditions drainage areas, discharge points, recharge areas, and hydrologic soil groups A and B.
(4) 
Evaluate nonstructural stormwater management alternatives (See Appendix B, Table B-6[1]).
(a) 
Minimize earth disturbance.
(b) 
Minimize impervious surfaces.
(c) 
Break up large impervious surfaces.
[1]
Editor's Note: Appendix B is on file in the Borough offices.
(5) 
Satisfy water quality objective (§ 129-43).
(6) 
Satisfy groundwater recharge (infiltration) objective (§ 129-44) and provide for stormwater treatment prior to infiltration.
(7) 
Satisfy streambank erosion protection objective (§ 129-45).
(8) 
Determine what management district the site falls into (Appendix D[2]) and conduct a predevelopment runoff analysis.
[2]
Editor's Note: Said appendix is on file in the Borough offices.
(9) 
Prepare final project design to maintain predevelopment drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and to reduce runoff to the maximum extent possible, the use of surface or point discharges.
(10) 
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 129-46).
(11) 
Manage any remaining runoff through treatment prior to discharge, as part of detention, bioretention, direct discharge or other structural control.
In addition to the performance standards and design criteria requirements of this article, the applicant shall comply with the following water quality requirements of this article.
A. 
Adequate storage and treatment facilities will be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under § 129-44 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. 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. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P). To achieve this goal, the following criterion is established:
(1) 
The following calculation formula is to be used to determine the water quality storage volume, (WQv), in acre-feet of storage for the East Branch Perkiomen Creek watershed:
WQv = [(P)(Rv)(A)]/12
Equation: § 129-43.1
Where:
WQv
=
Water Quality Volume (acre-feet)
P
=
Rainfall Amount equal to 90% of events producing this rainfall (in)
A
=
Area of the project contributing to the water quality BMP (acres)
Rv
=
0.05 + 0.009(I) where I is the percent of the area that is impervious surface (impervious area/A*100)
(2) 
The P value for the five PennDOT rainfall regions is shown in Figure B-2 in Appendix B of the Model Ordinance within this plan and as shown in Appendix Table B-5.[1] Since the East Branch Perkiomen Creek is in PennDOT Region 4, the P value to be utilized to meet this requirement is 1.95 inches.
[1]
Editor's Note: Appendix B is on file in the Borough offices.
B. 
Design of BMPs used for water quality control shall be in accordance with design specifications outlined in the Pennsylvania Handbook of Best Management Practices for Developing Areas or other applicable manuals. The following factors shall be considered when evaluating the suitability of BMPs used to control water quality at a given development site:
(1) 
Total contributing drainage area.
(2) 
Permeability and infiltration rate of the site soils.
(3) 
Slope and depth to bedrock.
(4) 
Seasonal high water table.
(5) 
Proximity to building foundations and well heads.
(6) 
Erodibility of soils.
(7) 
Land availability and configuration of the topography.
(8) 
Peak discharge and required volume control.
(9) 
Stream bank erosion.
(10) 
Efficiency of the BMPs to mitigate potential water quality problems.
(11) 
The volume of runoff that will be effectively treated.
(12) 
The nature of the pollutant being removed.
(13) 
Maintenance requirements.
(14) 
Creation/protection of aquatic and wildlife habitat.
(15) 
Recreational value.
(16) 
Enhancement of aesthetic and property value.
C. 
To accomplish the above, the applicant shall submit original and innovative designs to the municipality for review and approval. Such designs may achieve the water quality objectives through a combination of BMPs (best management practices).
A. 
Infiltration BMPs shall meet the following minimum requirements. Regulated activities will be required to recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from § 129-44A(2)(a) or (b) depending upon demonstrated site conditions.
(1) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(a) 
A minimum depth of 24 inches between the bottom of the BMP and the limiting zone.
(b) 
An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the applicant's design professional.
(c) 
The recharge facility shall be capable of completely infiltrating the recharge volume within four days (96 hours).
(d) 
Pretreatment shall be provided prior to infiltration.
(e) 
The requirements for recharge are applied to all disturbed areas, even if they are ultimately to be an undeveloped land use such as grass, since studies have found that compaction of the soils during disturbance reduces their infiltrative capacity.
(2) 
The recharge volume (Re) shall be computed by first obtaining the infiltration requirement using methods in either § 129-44A(2)(a) or (b) then multiplying by the total proposed impervious area. The overall required recharge volume for a site is computed by multiplying total impervious area by the infiltration requirement.
(a) 
NRCS curve number equation. The following criteria shall apply. The NRCS runoff shall be utilized to calculate infiltration requirements (P) in inches.
[1] 
Equation: § 129-44.1
For zero runoff: P = I (Infiltration) = (200/CN) - 2
Where:
P = I
=
infiltration requirement (inches)
CN
=
SCS(NRCS) curve number of the existing conditions contributing to the recharge facility
This equation can be displayed graphically in, and the infiltration requirement can also be determined from Figure 129-44-1.
[2] 
Equation: § 129-44.2
The recharge volume (Rev) required would therefore be computed as:
Rev
=
I* impervious area (SF)/12 = Cubic Feet (CF)
(b) 
Annual recharge water budget approach.
[1] 
It has been determined that infiltrating 0.6 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 129-44A(2)(a) cannot be achieved, then 0.6 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 77. Above a curve number of 77, Equation § 129-44.1 or the curve in Figure § 129-44.1 should be used to determine the Infiltration requirement.
Where:
I
=
0.6 inches
The recharge volume (Rev) required would therefore be computed as:
Rev
=
I* percent impervious area (SF)/12 = (CF)
[2] 
The recharge values derived from these methods are the minimum volumes the Applicant must control through an infiltration/recharge BMP facility. However, if a site has areas of soils where additional volume of infiltration can be achieved, the applicant is encouraged to recharge as much of the stormwater runoff from the site as possible.
Figure § 129-44-1. Infiltration Requirement Based upon NRCS Curve Number
Table 304_1 Infiltration Req.tif
B. 
The general process for designing the infiltration BMP shall be: a detailed soils evaluation of the project site shall be required to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified applicant and, at a minimum, address soil permeability, depth to bedrock, and subgrade stability.
(1) 
Analyze hydrologic soil groups as well as natural and man-made features within the watershed to determine general areas of suitability for infiltration practices.
(2) 
Provide field tests, such as double ring infiltration tests at the level of the proposed infiltration surface to determine the appropriate hydraulic conductivity rate.
(3) 
Design the infiltration structure for the required storm volume based on field determined capacity at the level of the proposed infiltration surface.
(4) 
Where the recharge volume requirement cannot be physically accomplished due to the results of the field soils testing, supporting documentation and justification shall be supplied to the municipality with the drainage plan.
(5) 
If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.
C. 
Extreme caution shall be exercised where infiltration is proposed in geologically susceptible areas such as strip mine or limestone areas. Extreme caution shall also be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. Extreme caution shall be exercised where infiltration is proposed in source water protection areas. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and perform a hydrogeologic justification study if necessary. The infiltration requirement in high-quality/exceptional value waters shall be subject to the DEP's Title 25, Chapter 93, Antidegradation Regulations. The municipality may require the installation of an impermeable liner in BMP and/or detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.
D. 
The municipality shall require the applicant to provide safeguards against groundwater contamination for uses which may cause groundwater contamination, should there be a mishap or spill.
E. 
Recharge/infiltration facilities shall be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.
A. 
In addition to the water quality volume, to minimize the impact of stormwater runoff on downstream streambank erosion, the requirement is to design a BMP to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured. (i.e., the maximum water surface elevation is achieved in the facility.)
B. 
Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall consider and minimize the chances of clogging and sedimentation. Orifices smaller than three inches diameter are not recommended. However, if the Design Engineer can provide proof that the smaller orifices are protected from clogging by use of trash racks, etc., smaller orifices may be permitted. Trash racks are required for any primary orifice.
C. 
In "no detention" areas (District C) only, the objective is not to attenuate the larger storms. This can be accomplished by configuration of the outlet structure not to control the larger storms, or by a bypass or channel to divert only the two-year flood into the basin or divert flows in excess of the two-year storm away from the basin.
A. 
The East Branch Perkiomen Creek watershed has been divided into stormwater management districts as shown on the Watershed Map in Appendix D.[1]
[1]
Editor's Note: Said appendix is on file in the Borough offices.
B. 
In addition to the requirements specified below, the water quality (§ 129-43), groundwater recharge (§ 129-44) and streambank erosion (§ 129-45) requirements shall be implemented.
C. 
Standards for managing runoff from each subarea in the East Branch Perkiomen Creek watershed is shown below. Development sites located in each of the A, B, or C Districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms as follows:
District
Design Storm Proposed Conditions
Design Storm Existing Conditions
A
2-year
1-year
5-year
5-year
10-year
10-year
25-year
25-year
100-year
100-year
B
2-year
1-year
5-year
2-year
10-year
5-year
25-year
10-year
100-year
50-year
C-1
2-year
1-year
5-year
2-year
10-year
10-year
25-year
25-year
C-2*
2-year
1-year
5-year
2-year
*
EXPLANATION OF DISTRICT C-2: Development sites which can discharge directly to the East Branch Perkiomen Creek main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) may do so without control of proposed conditions peak rate of runoff greater than the five-year storm. Sites in District C will still have to comply with the water quality criteria (§ 129-43), the groundwater recharge criteria (§ 129-44), and streambank erosion criteria (§ 129-45). If the proposed conditions runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the increased peak flows or will be provided with improvements to furnish the required capacity. If storms greater than the two-year storm cannot be conveyed to the stream or watercourse is a safe manner, the proposed conditions peak rate of runoff must be controlled to the existing conditions peak rate as required in District C-1 provisions (i.e., twenty-five-year proposed conditions flows to twenty-five-year existing conditions flows) for the specified design storms.
A. 
General. Proposed conditions peak rates of runoff from any regulated activity shall meet the peak release rates of runoff prior to development for the design storms specified on the Stormwater Management District Watershed Map (Appendix D[1]) and § 129-46 of this Part 2.
[1]
Editor's Note: Said appendix is on file in the Borough offices.
B. 
District boundaries. The boundaries of the stormwater management districts are shown on an official stormwater district map that is available for inspections at the municipal office. A copy of the map at a reduced scale is included in Appendix D.[2] 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 the two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.
[2]
Editor's Note: Said appendix is on file in the Borough offices.
C. 
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall be the existing conditions peak discharge for that subarea as indicated in § 129-46. 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 one-hundred-percent release rate provided that the overall site discharge meets the weighted average release rate.
D. 
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.
E. 
Site areas. Where the area of a site being impacted by a proposed development activity differs significantly from the total site area, only the proposed disturbed area utilizing stormwater management measures shall be subject to the management district criteria. Unimpacted or undisturbed areas that do flow into or are bypassing the stormwater management facilities would not be subject to the management district criteria.
F. 
"No harm" option. For any proposed development site not located in District C1 or C2, the applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. The "no harm" option is used when a applicant can prove that the proposed conditions hydrographs can match existing conditions hydrographs, or if it can be proved that the proposed conditions will not cause increases in peaks at all points downstream. Proof of "no harm" would have to be shown based upon the following downstream impact evaluation which shall include a downstream hydraulic capacity analysis consistent with § 129-47G to determine if adequate hydraulic capacity exists. The applicant shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1) 
The downstream impact evaluation shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications due to the proposed development upon a dam, highway, structure, natural point of restricted streamflow, or any stream channel section, established with the concurrence of the municipality.
(2) 
The evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.
(3) 
The peak flow values to be used for downstream areas for the design return period storms (two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year) shall be the values from the calibrated model for the East Branch Perkiomen Creek watershed. These flow values can be obtained from the watershed plan.
(4) 
Applicant-proposed runoff controls that would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove "no harm," except in conjunction with proposed capacity improvements for the problem areas consistent with § 129-43G.
(5) 
Financial considerations shall not constitute grounds for granting a "no harm" exemption.
(6) 
Capacity improvements may be provided as necessary to implement the "no harm" option which proposes specific capacity improvements to provide that a less stringent discharge control would not create any harm downstream.
(7) 
Any "no harm" justifications shall be submitted by the applicant as part of the drainage plan submission per Article X.
G. 
Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this Part 2 shall use the following criteria for determining adequacy for accepting increased peak flow rates:
(1) 
Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the Department of Environmental Protection's Erosion and Sediment Pollution Control Program Manual.
(2) 
Natural or man-made channels or swales must be able to convey increased twenty-five-year return period runoff without creating any hazard to persons or property.
(3) 
Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with the Department of Environmental Protection's Chapter 105 regulations (if applicable) and, at minimum, pass the increased twenty-five-year return period runoff.
H. 
Regional stormwater management facilities 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 applicants. The design of any regional control facility must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional control facility 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. It is a requirement that, even if regional facilities are proposed for the water quantity control, that the water quality, streambank erosion, and recharge criteria be accomplished on-site, or as close to the source of the runoff as possible.
A. 
Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
B. 
Any stormwater management facility (i.e., detention basin BMP) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Part 2 shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed 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 proposed conditions peak inflow. Should any stormwater management facility require a dam safety permit under Title 25, Environmental Protection, Chapter 105, Dam Safety and waterway management, 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.
C. 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed in DEP Chapter 105 regulations (as amended or replaced from time to time by DEP), shall be designed in accordance with Chapter 105 and will require a permit from DEP. 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 with a minimum 1.0 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 the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
D. 
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 ten-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 PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
E. 
Storm sewers must be able to convey proposed conditions runoff from a twenty-five-year design storm without surcharging inlets, where appropriate. (Note: Municipality must select a design storm appropriate to its situation.)
F. 
Adequate erosion protection shall be provided along all open channels, and at all points of discharge.
G. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the creation of, exacerbation or continuation of an adverse hydrologic or hydraulic condition within the watershed.
Stormwater runoff from all development sites shall be calculated using either the rational method or a soil cover complex methodology.
A. 
Any stormwater runoff calculations shall use generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 129-49-1 summarizes acceptable computation methods. It is assumed that all methods will be selected by the applicant based on the individual limitations and suitability of each method for a particular site.
(1) 
The municipality may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Rational Method is recommended for drainage areas under 100 acres.
B. 
All calculations consistent with this Part 2 using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms according to the region for which they are located as presented in Table B-1 in Appendix B of this Part 2.[1] If a hydrologic computer model such as HEC-1 or HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The SCS 'S' curve shown in Figure B-1, Appendix B of this Part 2 shall be used for the rainfall distribution.
[1]
Editor's Note: Said appendix is on file in the Borough offices.
C. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table B-2 in Appendix B of this Part 2.[2] For the purposes of existing conditions flow rate determination, undeveloped land shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number or Rational 'C' value (i.e., forest), as listed in Table B-2 or B-3 in Appendix B of this Part 2. For areas of prior mining disturbance (i.e., strip mining, mine spoil areas, etc.), the designer must first locate in which of mining affect area the site is located, using the Management District Map in Appendix D. The appropriate curve number or Rational 'C' value from Table B-2 or Table B-3 should then be used.
[2]
Editor's Note: Said appendix is on file in the Borough offices.
D. 
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) (Figures B-2 to B-3[3]). 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.
[3]
Editor's Note: Figures B-2 and B-3 of Appendix D are on file in the Borough offices.
E. 
The designer shall consider that the runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions even if topsoiled and seeded. The designer may increase their proposed condition CN or C value to reflect proposed soil conditions.
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table B-3 in Appendix B of this Part 2.[4]
[4]
Editor's Note: Said appendix is on file in the Borough offices.
G. 
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 B-4 in Appendix B of this Part 2.[5] Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Part 2 using any generally accepted hydraulic analysis technique or method.
[5]
Editor's Note: Said appendix is on file in the Borough offices.
H. 
The design of any stormwater detention facilities intended to meet the performance standards of this Part 2 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.
Table 129-49-1. 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 200 acres, or as approved by the Municipality and/or Municipal Engineer
Other methods
Varies
Other computation methodologies approved by the Municipality and/or Municipal Engineer
A. 
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.
B. 
Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization.