A. 
Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channel consistent with this Part 1.
B. 
The existing points of concentrated drainage that discharge onto adjacent property shall not be relocated and shall be subject to any applicable discharge criteria specified in this Part 1.
C. 
Where a development site is traversed by watercourses other than permanent streams, a drainage easement shall be provided conforming substantially 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 affect adversely the flow of stormwater within any portion of the easement. Also, maintenance and mowing of vegetation within the easement shall be required.
D. 
The PADEP rules and regulations, 25 Pa. Code Chapter 105, Dam Safety and Waterway Management, apply to the construction, modification, operation or maintenance of both existing and proposed water obstructions and encroachments throughout the watershed, including work in wetlands. Inquiries on permit requirements or other concerns shall be addressed to PADEP's Regional Office. Permit requirements or inquiring on dam safety should be addressed to the PADEP, Bureau of Waterways Engineering - Harrisburg.
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
E. 
When it can be shown that, due to topographic conditions, natural drainage ways on the development 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.
F. 
Carbonate geology. In areas of carbonate geology, a registered professional geologist shall certify the following:
(1) 
No stormwater facilities shall be placed in, over or immediately adjacent to the following features:
(a) 
Sinkholes.
(b) 
Closed depressions.
(c) 
Lineaments in carbonate areas.
(d) 
Fracture traces.
(e) 
Caverns.
(f) 
Intermittent lakes.
(g) 
Ephemeral streams.
(h) 
Bedrock pinnacles (surface or subsurface).
(2) 
Stormwater management basins shall not be located closer than 100 feet from the rim of sinkholes or closed depressions, nor within 100 feet from disappearing streams, nor shall these basins be located closer than 50 feet from lineaments or fracture traces, nor shall these basins be located closer than 25 feet from surface or identified subsurface pinnacles.
(3) 
Stormwater resulting from regulated activities shall not be discharged into sinkholes.
(4) 
If the developer can prove through analysis that the development site is in an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the development site is exempt from recharge requirements as described in §§ 410-12D and 410-14F. However, the development site shall still be required to meet all other hydrologic and water quality management standards as found in this Part 2.
(5) 
It shall be the developer's responsibility to verify if the development site is underlain by carbonate geology. The following note shall be attached to all stormwater management site plans and signed and sealed by the developer's qualified professional:
"I, _____, certify that the proposed detention basin is/is not [circle one] underlain by carbonate geology."
(6) 
Whenever a stormwater facility will be located in an area underlain by carbonate geology, a geological evaluation of the proposed location by a registered professional geologist shall be conducted to determine susceptibility to sinkhole formation. The evaluation may include the use of impermeable liners to reduce or eliminate the separation distances listed in Subsection F(1) and (2) above.
A. 
General. The following general standards shall be applied to all development within the Conestoga River Watershed to promote flow attenuation, erosion and sediment control and flood control.
(1) 
The developer of any development site in the Conestoga River Watershed which does not fall under the exemption criteria in Appendix A[1] shall submit a stormwater management site plan consistent with the Conestoga River Watershed Act 167 Plan to the municipality for review. The exemption criteria in Appendix A shall apply to the total proposed development on a parent tract 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.
[1]
Editor's Note: Appendix A, Act 167 Stormwater Management Exemption Criteria, is included as an attachment to this chapter.
(2) 
All stormwater management site plans shall be designed and certified by individuals registered in the Commonwealth of Pennsylvania and qualified to perform such duties based on education and training in hydrology and hydraulics.
(3) 
Runoff from impervious areas shall be drained to pervious areas of the development site.
(4) 
Roof drains shall not be connected to streets, sanitary or storm sewers or roadside ditches.
(5) 
Stormwater management facilities which involve a state highway shall be subject to the approval of the PennDOT.
(6) 
Stormwater management facilities located within or affecting the floodplain or any watercourse shall also be subject to the requirements of Section 306 (Floodplain) of this Part 1,[2] Chapter 500, Zoning, of the Code of Caernarvon Township, Chapter 425, Subdivision and Land Development, of the Code of Caernarvon Township, any ordinance which regulates construction and development within areas of Caernarvon Township subject to flooding, and any other applicable requirements of the Flood Plain Management Act.[3]
[2]
Editor's Note: So in original; see now Ch. 280, Floodplain Management.
[3]
Editor's Note: See 32 P.S. § 679.101 et seq.
(7) 
Stormwater runoff from a development site shall flow directly into a natural drainageway, watercourse, or into an existing storm sewer system, or onto adjacent properties in a manner similar to the runoff characteristics of the predevelopment flow. Maximum use shall be made of the existing on-site natural and man-made stormwater management facilities.
(8) 
Stormwater runoff shall not be transferred from one watershed to another unless they are subwatersheds of a common watershed which join together within the perimeter of the development site or the effect of the transfer does not alter the peak discharge (in conformance with the requirements of the Act 167 Plan) onto adjacent lands or drainage easements from the affected landowners are provided.
(9) 
All stormwater runoff flowing over the development site shall be considered in the design of the stormwater management facilities.
(10) 
In accordance with 25 Pa. Code Chapter 102, Erosion and Sediment Control, temporary facilities shall be included in the submitted plans for a phased section where the following conditions are met:
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
(a) 
A regulated activity constitutes a subdivision or land development.
(b) 
The final plan applications are submitted in sections.
(c) 
Temporary facilities are required for construction of a section.
B. 
Match preexisting hydrograph. Developers and/or landowners are encouraged to provide infiltration facilities or utilize other techniques which will allow the post-development hydrograph to match the preexisting hydrograph, along all parts of the hydrograph, for the development site. To match the preexisting hydrograph means that it is not to be exceeded at all points in time. This option is most feasible for small subdivisions in areas of noncarbonate geology. Groundwater recharge and water quality volumes as given in § 410-12D and E below can be used as part of this option.
C. 
Detention/infiltration standards (only if existing hydrographs cannot be matched)
(1) 
Post-development rates of runoff from any regulated activity shall not exceed a given percentage of the peak rate of runoff prior to development for all design storms unless the preexisting hydrograph is not exceeded at all points in time. The percentage of the predevelopment peak rate which may be released is known as the "release rate." The areas of the watershed for which a certain release rate applies can be found in Appendix D, Subwatershed Boundary Map.[4]
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2) 
Innovative methods for the control of stormwater runoff are encouraged. Various combinations of methods should be tailored to suit the particular requirements of the type of development and the topographic features of the development site. The following is a partial listing of detention and control methods which can be utilized in stormwater management systems where appropriate:
(a) 
Detention basins.
(b) 
Retention basins (subject to prior municipal approval).
(c) 
Rooftop detention.
(d) 
Parking lot storage.
(e) 
Seepage pits, seepage trenches or other infiltration structures.
(f) 
Concrete lattice block surfaces.
(g) 
Grassed channels and vegetated strips.
(h) 
Cisterns and underground reservoirs.
(i) 
Routed flow over grass.
(j) 
Decreased impervious surface coverage.
(k) 
Bioretention areas (rain gardens).
(l) 
Other methods as may be found in the "Pennsylvania Handbook of Best Management Practices for Developing Areas," the "Pennsylvania Stormwater Best Management Practices Manual," or equivalent.
(3) 
The following principles shall be applied to the erosion and sediment pollution control plan and construction schedule to minimize soil erosion and sedimentation:
(a) 
Stripping of vegetation, grading, or other soil disturbance shall be done in a manner which will minimize soil erosion.
(b) 
Whenever feasible, natural vegetation shall be retained and protected.
(c) 
The extent of the disturbed area and the duration of its exposure shall be kept to a minimum, within practical limits.
(d) 
Either temporary seeding, mulching, or other suitable stabilization measures shall be used to protect exposed critical areas during construction.
(e) 
Drainage provisions shall accommodate the stormwater runoff both during and after construction.
(f) 
Soil erosion and sedimentation facilities shall be installed prior to any on-site grading.
D. 
Groundwater recharge.
(1) 
Developed areas shall maintain groundwater recharge consistent with predevelopment conditions, dependent on hydrologic soil groups and impervious cover, unless the developer can prove the inability of the development site to achieve recharge based on existing development site conditions. This volume of runoff is termed the "recharge volume" and is calculated in accordance with § 410-14F. The recharge volume must be infiltrated within 48 hours after the end of the design storm. Development sites where the post-developed impervious area is equal to or less than the predeveloped impervious area shall not be required to provide groundwater recharge volume.
(2) 
Design of the stormwater management facilities shall provide for groundwater 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 development site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified design professional 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.
(3) 
If the developer can prove through analysis that the development site is in an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the development site is exempt from recharge requirements. However, the development site shall still be required to meet all other hydrologic and water quality management standards as found in this Part 1.
E. 
Water quality. Developed areas will provide adequate storage and treatment facilities necessary to capture and treat a minimum of the runoff from the first 1.2 inches of rainfall. This volume of storage is the "water quality volume" and is calculated in accordance with § 410-14G. 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. The water quality volume must take a minimum of 24 hours to be discharged. Development sites where the post-developed impervious area is equal to or less than the predeveloped impervious area shall not be required to provide water quality volume unless required by NPDES Part II.
F. 
Stormwater conveyance corridor protection (riparian corridor preservation and vegetation). Runoff from developed areas of the development 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 municipality, 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 § 410-13 of this Part 1 and shall be in addition to any other municipal ordinance provisions.
G. 
Subregional (combined development site) storage. Runoff can be managed regionally by one or more developers, either on-site or off-site. The design and release rate shall be consistent with the Conestoga River Act 167 Plan. Groundwater recharge and water quality volumes as described in § 410-12D and E will be a part of this option.
H. 
The no harm option does not apply to the water quality requirement described in Subsection E above. The no harm option does not apply to the groundwater recharge requirement described in Subsection D above, unless it can be shown that the development site is underlain by carbonate geology and infiltration can not be safely accomplished.
(1) 
For any proposed development, the developer has the option of using a less restrictive runoff control if the developer can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. Proof of no harm would have to be shown from the development site through the remainder of the downstream drainage network until there is no additional flow increase. Proof of no harm must be shown using the capacity criteria specified in § 410-13C if downstream capacity analysis is a part of the no harm justification.
(2) 
Attempts to prove no harm based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:
(a) 
The peak flow values to be used for downstream areas for the design return period storms (two-, five-, ten-, twenty-five-, fifty-, and 100-year) shall be the values from the calibrated TR-20 Model for the Conestoga River Watershed. These flow values would be supplied to the developer by the county upon request.
(b) 
At peak flow, any available capacity in the downstream conveyance system (as documented by a developer) may be used only in proportion to the proposed development site acreage relative to the total upstream undeveloped acreage (i.e., if the development site is 10% of the upstream undeveloped acreage, the developer may use up to 10% of the documented downstream available capacity at peak flow).
(c) 
Developer-proposed runoff controls which would generate increased peak flow rates at documented 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 § 410-13C and S of this Part 1.
(3) 
Any no harm justifications shall be submitted by the developer as part of the drainage plan submission per Article IV.
A. 
Any stormwater management facility designed to store stormwater runoff and requiring a berm or earth embankment (i.e., detention or retention basin) shall be designed to provide an emergency spillway to handle the 100-year, post-development peak flow rate. The height of embankment must be set to provide a minimum 1.0 foot of freeboard above the maximum elevation computed when the entire 100-year peak flow passes through the spillway. However, criteria for design and construction of stormwater management facilities are not the same criteria that are used in the permitting of dams under the PADEP Dam Safety Program. Depending upon the physical characteristics of a dam, a dam permit may be required and the design will have to meet the provisions of 25 Pa. Code Chapter 105. Depending on the physical characteristics of a dam, the design could require that anywhere from a 100-year to a probable maximum flood (PMF) storm event be considered. The following minimums shall be required:
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
(1) 
The maximum water depth shall not exceed six feet, unless approved by a modification granted in conformance with § 410-38 by the municipal officials upon recommendation of the Municipal Engineer.
(2) 
The minimum top width of all dams/embankments/berms shall be five feet.
(3) 
The interior side slopes shall not be greater than five horizontal to one vertical.
(4) 
All basins shall be structurally sound and shall be constructed of sound and durable materials. The completed structure and the foundation of all basins shall be stable under all probable conditions of operation. An emergency spillway shall be provided for the basin and shall be capable of discharging the 100-year peak rate of runoff which enters the basin after development, in a manner which will not damage the integrity of the facility and will not create a downstream hazard. Where practical, the emergency spillway shall be constructed in undisturbed ground. An easement for inspection and repair shall be provided when the conveyance structure crosses property boundaries.
(5) 
All basins not including groundwater recharge and/or water quality storage shall include an outlet structure to permit draining the basin to a completely dry position within 24 hours following the end of the design rainfall. All basins that do include groundwater recharge and/or water quality storage shall include an outlet structure to permit draining the basin to the level of the groundwater recharge and/or water quality storage within 24 hours following the end of the design rainfall.
(6) 
A cutoff trench of relatively impervious material shall be provided within all basin embankments.
(7) 
All structures passing through detention basin embankments (as defined in Article II above) shall have properly spaced concrete cutoff collars and all piping must be watertight. All structures passing through dam embankments (as defined in Article II above) shall have seepage diaphragms and drains.
(8) 
All discharge control devices with appurtenances (except discharge pipes) shall be made of reinforced concrete and stainless or hot-dip galvanized steel. Bolts/fasteners are to be stainless or galvanized steel. Discharge pipes shall conform to the requirements of Subsection E below.
(9) 
Low-flow channels shall be provided from each water-carrying facility to the outlet structure for all basins that do not include groundwater recharge and/or water quality storage. Low-flow channels shall be 1% minimum slope and shall be designed to enable ease of maintenance. All basins that do include groundwater recharge and/or water quality storage shall not be required to have a low-flow channel.
(10) 
Minimum slope within a basin that does not include groundwater recharge and/or water quality storage shall be 2% positive grade to the low-flow channel.
(11) 
Design storms for the computation of retention basins (where approved) volumes shall be based upon a twenty-four-hour storm with 100-year return period (a storm with a 1% chance of occurrence each year).
(12) 
The effect on downstream areas if the basin embankment fails shall be considered in the design of all basins. Where possible, the basin shall be designed to minimize the potential damage caused by such failure of the embankment.
(13) 
All structures (detention basins, cisterns, etc.), other than those used for groundwater recharge volume and water quality volume, must completely drain within 24 hours after the end of the design storm.
(14) 
Soils used for the construction of basins shall have low erodibility factors ("K" factors).
B. 
Minimum floor elevations for all structures that would be affected by a basin, other temporary impoundments, or open conveyance systems where ponding may occur shall be two feet above the 100-year water surface. If basement or underground facilities are proposed, detailed calculations addressing the effects of stormwater ponding on the structure and waterproofing and/or floodproofing design information shall be submitted for approval.
C. 
All storm sewer pipes, culverts and bridges (excluding detention and retention basin outfall structures), gutters and swales conveying water originating only from within the boundaries of the development site shall be designed for a twenty-five-year storm event. All storm sewer pipes, culverts and bridges (excluding detention and retention basin outfall structures) conveying water originating from off-site shall be designed for a fifty-year storm event. Drainage easements shall be provided to contain and convey the 100-year frequency flood throughout the development site. Easements shall begin at the furthest upstream property line of the proposed development site in a watershed.
D. 
A concentrated discharge of stormwater to an adjacent property shall be within an existing natural drainageway or watercourse or otherwise an easement shall be required.
E. 
Storm sewer pipes, other than those used as roof drains, detention basin underdrains, and street subbase underdrains, shall have a minimum diameter of 15 inches and be made of reinforced concrete pipe, corrugated galvanized metal pipe, smooth-lined corrugated polyethylene pipe, or approved equivalent. Where installation conditions merit, structural calculations that address the actual design requirements will be required.
F. 
Storm sewer pipes and culverts shall be installed on sufficient slopes to provide a minimum velocity of three feet per second when flowing full.
G. 
All storm sewer pipe and culverts shall be laid to a minimum depth of one foot from finished subgrade to the crown of pipe in paved areas and one foot from finished grade to the crown of pipe in grassed areas.
H. 
Curves in pipes or box culverts without an inlet or manhole are prohibited. T-joints, elbows and wyes are also prohibited.
I. 
Manholes, inlets, headwalls and endwalls proposed for dedication or located along streets or subject to vehicular traffic shall conform to the requirements of the PennDOT, Bureau of Design, Standards for Roadway Construction, in effect at the time the design is submitted, or as otherwise modified by the municipality.
J. 
Headwalls and endwalls shall be used where stormwater runoff enters or leaves the storm sewer horizontally from a natural or man-made channel. PennDOT Type "DW" headwalls and endwalls shall be utilized.
K. 
Stormwater roof drains, sump pumps, and pipes shall not directly discharge water into a street right-of-way or discharge into a sanitary sewer or storm sewer.
L. 
All existing and natural watercourses, channels, drainage systems, wetlands and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the municipality and any other necessary approving body.
M. 
Flow velocities from any storm sewer may not result in erosion of the receiving channel.
N. 
Energy dissipaters shall be placed at the outlets of all storm sewer pipes, culverts, and bridges where flow velocities exceed maximum permitted channel velocities as specified below:
(1) 
Three feet per second where only sparse vegetation can be established and maintained because of shade or soil condition.
(2) 
Four feet per second where normal growing conditions exist and vegetation is to be established by seeding.
(3) 
Five feet per second where a dense, vigorous sod can be quickly established or where water can be temporarily diverted during establishment of vegetation. Netting and mulch or the equivalent methods for establishing vegetation shall be used.
(4) 
Six feet per second where there exists a well-established sod of good quality.
O. 
The following conditions shall be met for all swales:
(1) 
Capacities and velocities shall be computed using the Manning Equation. The design parameters shall be as follows:
(a) 
Vegetated swales shall meet the following two design considerations:
[1] 
The first shall consider swale stability based upon a low degree of retardance ("n" = 0.03);
[2] 
The second shall consider swale capacity based upon a high degree of retardance ("n" = 05).
(b) 
All vegetated swales shall have a minimum slope of 1% unless approved by the Municipal Engineer.
(c) 
The "n" factors to be used for paved or rip-rap swales or gutters shall be based upon accepted engineering design practices as approved by the municipality.
(2) 
All swales shall be designed to concentrate low flows to minimize siltation and meandering.
P. 
Manning "n" values used for design of pipes and culverts shall be in accordance with Appendix B-7.[1]
[1]
Editor's Note: Appendix B, Runoff Examples, is included as an attachment to this chapter.
Q. 
All storm sewer crossings of streets shall be perpendicular to the street center line.
R. 
Storm facilities not located within a public right-of-way shall be contained in and centered within an easement. Easements shall follow property boundaries where possible.
S. 
Stormwater conveyance improvements. If the developer can prove under the no harm option (see § 410-12H) that it would be feasible to provide conveyance improvements (install or upgrade pipes, bridges, swales, etc.) to relieve existing deficiencies as defined by § 410-13C and any local, state, or federal regulations, the conveyance improvements could be provided by the developer in lieu of stormwater management facilities on the development site. Any conveyance improvements would be designed based on the eventual development of all areas tributary to the improvements and the conveyance criteria specified in this Part 1. The eventual development of all tributary areas that the developer must consider shall either be based on the current zoning or be established by the municipality, whichever results in a greater amount of impervious surface. It shall be assumed that all new development upstream of a proposed conveyance improvement would implement applicable stormwater management techniques, consistent with this Part 1.
T. 
Adequate erosion protection shall be provided along all open channels and at all points of discharge.
U. 
All groundwater recharge facilities shall be designed to empty in 48 hours subsequent to any storm event. All water quality facilities shall be designed so that water is released slowly for a minimum of 24 hours subsequent to any storm event. All infiltration, detention or retention facilities the volume of which will be used for stormwater management (predevelopment versus post-development) shall be designed to empty within 24 hours subsequent to any storm event. Volumes which will not be available within 24 hours subsequent to any storm event shall not be used for stormwater management (predevelopment versus post-development).
Stormwater runoff from all development sites shall be calculated using either the Modified Rational Method, a Soil Cover Complex Methodology, or other method acceptable to the municipality or its designee.
A. 
Any stormwater runoff calculations involving drainage areas generally greater than 200 acres and TC greater than 60 minutes, including on- and off-site areas, shall use generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 410-14A 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 development site.
Table 410-14A
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 TR-55.
HEC-1/HEC-HMS
US Army Corps of Engineers
Applicable where use of full hydrologic computer model is desirable or necessary.
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For development sites less than two acres, TC<60 min. or as approved the municipal designee.
Other methods
Varies
Other methodologies approved by municipal designee.
B. 
If the Soil Cover Complex Method is used, stormwater runoff shall be based on the following twenty-four-hour storm events published in "Urban Hydrology for Small Watersheds," by USDA NRCS Engineering Division, also known as TR-55. The original source was the U.S. Department of Commerce, Weather Bureau Technical Paper No. 40 (TP-40), "Rainfall Frequency Atlas of the United States," May 1961.
Storm Event
(years)
Rainfall
(inches)
2
3.1
5
4.1
10
5.0
25
5.5
50
6.2
100
7.0
(1) 
If the NRCS method is used, Antecedent Moisture Condition 1 is to be used in areas of carbonate geology and Antecedent Moisture Condition 2 is to be used in all other areas.
(2) 
If the Rational Method is used, the Region 5, PennDOT Storm Intensity - Duration - Frequency Chart (PDT-IDF), dated May 1986, shall be used to determine the rainfall intensity in inches per hour. See chart in Appendix B-1.[1] [NOTE: The Rational Method is not to be used for the design of dams (as defined in Article II above).]
[1]
Editor's Note: Appendix B, Runoff Examples, is included as an attachment to this chapter.
C. 
Runoff calculations shall include a hydrologic and hydraulic analysis indicating volume and velocities of flow and the grades, sizes, and capacities of water carrying structures, sediment basins, retention and detention structures and sufficient design information to construct such facilities. Runoff calculations shall also indicate both predevelopment and post-development rates for peak discharge of stormwater runoff from the development site.
D. 
For the purpose of calculating predevelopment peak discharges, all runoff coefficients, both on-site and off-site, shall be based on actual land use assuming summer or good land conditions. Runoff coefficients for off-site discharges used to design facilities shall be based on actual land use assuming winter or poor land conditions.
E. 
Criteria and assumptions to be used in the determination of stormwater runoff and design of management facilities are as follows:
(1) 
Runoff coefficients shall be based on the information contained in Appendix B-2 and B-3 if the actual land use is listed in those appendices.[2] If the actual land use is not listed in these appendices, runoff coefficients shall be chosen from other published documentation, and a copy of said documentation shall be submitted with the stormwater management report.
[2]
Editor's Note: Appendix B, Runoff Examples, is included as an attachment to this chapter.
(2) 
Times of concentration shall be based on the following design parameters:
(a) 
Sheet flow. The maximum length for each reach of sheet or overland flow before shallow concentrated or open channel flow develops is 150 feet. Flow lengths greater than 100 feet shall be justified based on the actual conditions at each development site. Sheet flow may be determined using the nomograph in Appendix B-4 or the Manning's Kinematic Solution shown in the sheet flow section of Worksheet No. 1 in Appendix B-5.[3]
[3]
Editor's Note: Appendix B, Runoff Examples, is included as an attachment to this chapter.
(b) 
Shallow concentrated flow. Travel time for shallow concentrated flow shall be determined using Figure 3-1 from TR-55, Urban Hydrology for small watersheds, as shown in Appendix B-6. A sample worksheet for calculating times of concentration is provided in Appendix B-5.[4]
[4]
Editor's Note: Appendix B, Runoff Examples, is included as an attachment to this chapter.
(c) 
Open channel flows. At points where sheet and shallow concentrated flows concentrate in field depressions, swales, gutters, curbs, or pipe collection systems, the travel times and downstream end of the development site between these design points shall be based upon Manning's Equation and/or acceptable engineering design standards as determined by the Municipal Engineer.
F. 
Groundwater recharge requirements:
(1) 
The groundwater recharge volume (Rev) is the volume of stormwater runoff from a developed site which shall be required to maintain existing predevelopment 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:
Rev = (S)(Rv)(A) / 12
Rev = recharge volume in acre-feet
A = area of watershed in acres
Rv = 0.05 + 0.009(I)
Where I = net increase in impervious area / area of watershed (A)
(2) 
S is the soil specific recharge factor and varies according to soil type:
Hydrologic Soil Group
Soil Specific Recharge Factor (S)
A
0.32
B
0.22
C
0.10
D
0.05
(3) 
Each specific recharge factor (S) is based on the USDA average annual recharge volume per soil type divided by the annual rainfall in Lancaster County[5] (41 inches per year) and multiplied by 90% (to model a volume which captures 90% of the runoff). This keeps the recharge volume calculation consistent with the WQv methodology. The USDA average annual recharge volume per soil type is 18 inches for HSG A, 12 inches for HSG B, six inches for HSG C, and three inches for HSG D (Rawls, Brakensiek and Saxton, 1982).
(a) 
If more than one hydrologic soil group (HSG) is present at a development site, a composite recharge volume shall be computed based upon the proportion of total development site area within each HSG.
(b) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and development site conditions and shall be constructed on soils that have the following characteristics:
[1] 
A minimum depth of 48 inches between the bottom of the facility and the seasonal high-water table and/or bedrock (limiting zones).
[2] 
An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the owner's professional designer.
(c) 
Infiltration BMPs receiving only roof runoff may be placed in soils having a minimum depth of 24 inches between the bottom of the facility and the limiting zone.
(d) 
The recharge volume provided at the development site shall be directed to the most permeable HSG available.
(e) 
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, or the most recent version thereof.
(f) 
The groundwater recharge volume shall be infiltrated within 48 hours after the end of the design storm.
(g) 
Development sites where the post-developed impervious area is equal to or less than the predeveloped impervious area shall not be required to provide groundwater recharge volume.
[5]
Editor's Note: So in original.
G. 
Calculation of water quality volume:
(1) 
The water quality volume (WQv) is the storage capacity needed to treat stormwater runoff equivalent to a minimum of the first 1.2 inches of runoff from the developed areas of the development site. The following calculation is used to determine the storage volume, WQv, in acre-feet of storage:
WQv = (1.2)(Rv)(A) / 12
WQv = water quality volume in acre-feet
A = area of watershed in acres
Rv = 0.05 + 0.009(I)
Where I = net increase in impervious area / area of watershed (A)
(2) 
WQv 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. The water quality volume shall take a minimum of 24 hours to be discharged from the basin. Development sites where the post-developed impervious area is equal to or less than the predeveloped impervious area shall not be required to provide water quality volume, unless required by NPDES part II.
The methodology for determining required stormwater controls for a regulated activity is shown in Table 410-14A in § 410-14, and is outlined below.
A. 
Compute:
(1) 
Predevelopment hydrograph at the development site discharge point for the required design storm.
(2) 
Post-development hydrograph at the development site discharge point incorporating best management practices such as groundwater recharge volume and water quality volume, pervious areas, grass swales, infiltration trenches, etc. [NOTE: Hydrographs may be obtained from NRCS methods such as TR-55, TR20, or from use of the Modified or Unit Hydrograph Rational Methods.]
B. 
Compare post-development hydrographs with predevelopment hydrographs. If the peak rate of runoff does not exceed the predevelopment runoff at all points in time, stormwater management has been achieved. Detention will not be required. If not, proceed to the item in Subsection C.
C. 
Design. Detention/retention facilities, in conjunction with any nondetention techniques, such that post-development peak rates from the development site will not exceed permissible levels for required design storms.