A. 
All regulated activities in Antrim Township not granted an exemption in accordance with § 126-5 shall submit a stormwater management plan to Antrim Township for review.
B. 
Design standards. The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Township 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. Computations for determining stormwater runoff and for the design of stormwater management facilities shall be based upon the NRCS Soil-Cover-Complex Methods (Technical Release 55 or Technical Release 20), the Modified Rational Method, or the Standard Rational Method. Use of the Standard Rational Method shall be acceptable for analysis of watersheds up to 20 acres in size. The Modified Rational Method shall be acceptable for watersheds up to 200 acres in size. NRCS methods shall be acceptable for watershed analyses of all sizes. For each area or subarea analyzed, use the same analytical method for predevelopment and postdevelopment conditions. The Township may require that computed existing runoff rates be reconciled with field observations and conditions. 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 recommendation of the Township Engineer. The following general requirements apply to all regulated activities in Antrim Township.
C. 
The stormwater management plan shall consider and provide provisions for management of all stormwater runoff affecting the development site, including safe conveyance of the one-hundred-year design storm runoff from off-site watersheds, if any.
D. 
All predevelopment calculations shall be based upon existing land uses except existing agricultural uses, which shall be based on cultivated land with conservation treatment, pasture in good condition, meadow in good condition, and farmstead, unless the actual ground cover generates a lower curve number or Rational "C" value.
E. 
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 Pennsylvania Department of Transportation Design Rainfall Curves (1986) (Figure B-3) in Exhibit 2 of this chapter.[1]
[1]
Editor's Note: Figure B-3 is included at the end of this chapter.
F. 
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 Exhibit 2 of this chapter.[2]
[2]
Editor's Note: Exhibit 2, containing Figures B-1. B-2 and B-3 and Tables B-1 through B-6, is included at the end of this chapter.
G. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table B-3 in Exhibit 2 of this chapter.
H. 
Storm sewers, swales and other stormwater conveyance structures shall be designed to convey postdevelopment runoff from a twenty-five-year design storm without surcharging inlets, and with adequate freeboard in open drainageways. Six inches of freeboard shall be provided in swales carrying 14 cubic feet per second or less. One foot of freeboard shall be provided for in drainageways designed to carry flows of 15 cubic feet per second or greater.
I. 
The minimum pipe diameter permitted for use in storm sewer systems and/or stormwater-carrying culverts shall be 15 inches.
J. 
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.
K. 
The existing points of diffused or concentrated drainage that discharge onto adjacent property shall not be altered without written permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.
L. 
If existing diffused drainage discharge will be concentrated in the postdevelopment condition and discharged onto an 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.
M. 
Where a development site is traversed by watercourses, drainage easements shall be provided conforming to the line of such watercourses. Easements shall be centered on the watercourse and have a minimum width of 20 feet. 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.
N. 
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 PA DEP.
O. 
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 PA DEP and, if applicable, the U.S. Army Corps of Engineers. When there is a question whether wetlands may be involved, it is the responsibility of the developer or his agent to demonstrate to the applicable regulatory agencies that the land in question cannot be classified as wetlands.
P. 
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.
Q. 
Where soil conditions permit, minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc. are encouraged strongly to reduce the size or eliminate the need for detention facilities.
R. 
Cleanout structures for storm sewers shall be installed at the following maximum spacing:
Size of Pipe
(inches)
Maximum Allowable Spacing
(feet)
15
200
18 to 36
300
42 to 60
500
66 or larger
Unlimited
S. 
The stormwater management plan shall include calculations indicating velocities of flow, grades, sizes, and capacities of water-carrying structures and retention and detention structures as well as sufficient design information to construct such facilities.
T. 
Proposed lots or buildings adjacent to basins and significant channels shall have a finished first floor elevation of two feet above the one-hundred-year storm event level calculated for these facilities. The plans shall specify the minimum allowable finished first floor elevation for these lots or buildings.
U. 
Stormwater runoff channels shall be designed and installed to avoid trapping excess sediment.
V. 
Erosion and sediment pollution control measures shall be required for all plans submitted and shall be in accordance with the applicable standards and specifications set forth in the latest edition of the DEP Erosion and Sediment Pollution Control Program Manual and all other aspects of DEP Chapter 102 regulations.
W. 
Special provisions may be required for watersheds draining to high quality (HQ) and exceptional value (EV) waters in accordance with PA DEP regulations.
X. 
Plans approved by Antrim Township shall be on site throughout the duration of the regulated activity.
Y. 
Adequate erosion protection shall be provided along all open channels at all points of discharge.
A. 
Antrim Township is divided into stormwater management districts as shown on the Watershed Map (Exhibit 1).[1]
[1]
Editor's Note: Exhibit 1 is included at the end of this chapter.
B. 
In addition to the requirements specified below, the groundwater recharge, § 126-10; water quality, § 126-11; and streambank erosion, § 126-12, requirements shall be implemented.
C. 
Standards for managing runoff in each stormwater management district are shown below. Development sites in Antrim Township must control postdevelopment runoff rates to predevelopment runoff rates for the each specified design storm as follows:
District
Design Storm Post Development
Design Storm Predevelopment
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
(See below)
(See below)
District C, Provisional Direct Discharge District: Development sites which can discharge directly to the Conococheague Creek main channel or indirectly to the main channel through an existing or proposed man-made stormwater drainage system (i.e., storm sewer or conveyance swale) may, at the discretion of the Township Engineer and Supervisors, do so without control of postdevelopment peak rate of runoff. Sites in District C will still have to comply with the criteria for groundwater recharge, § 126-10); water quality, § 126-11; and streambank erosion, § 126-12. If the postdevelopment 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. When adequate capacity of downstream system does not exist and will not be provided through improvements, the postdevelopment peak rate of runoff must be controlled to the predevelopment peak rate as required in District A provisions for the specified design storms.
A. 
General. Postdevelopment 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 (Exhibit 1)[1] and this section.
[1]
Editor's Note: Exhibit 1 is included at the end of this chapter.
B. 
District boundaries. The boundaries of the stormwater management districts are shown on an official map that is available for inspections at the municipal office. A copy of the official map at a reduced scale is included in the chapter (Exhibit 1). 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.
C. 
Sites located in more than one district. For proposed development located in two or more stormwater management districts, the allowable postdevelopment peak discharge rate shall be determined based on the location of the point(s) of discharge from the site. Stormwater discharge shall be detained according to the provisions of the stormwater management district in which the point of discharge is located. Existing district boundaries shall not be altered by development beyond that which is required to construct the proposed structures and/or facilities and provide proper access to and from the site given the natural topographic limitations and constraints of the site.
A. 
Release rates from storage structures shall be based on the runoff from the one-, two-, five- and ten-, twenty-five- and fifty-, and one-hundred-year predevelopment storm events according to the Watershed District Map in (Exhibit 1).
B. 
All stormwater storage facilities (i.e., detention and retention basins) shall be designed in accordance with the following minimum standards.
(1) 
Embankment material used for basin construction shall be comprised of either on-site or imported fill which meets the following criteria:
(a) 
Free of organic material, ash, cinders and demolition debris.
(b) 
Particle size distribution that is well graded.
(c) 
Plasticity index less than 10; liquid limit less than 30.
(d) 
Less than 15% by weight of rock fragments larger than three inches; less than 30% by weight larger than 3/4 inch; and less than 30% smaller than No. 200 sieve.
(2) 
Embankment slopes shall not be steeper than one foot vertical in three feet horizontal.
(3) 
The top width of basin embankments shall not be less than six feet.
(4) 
Emergency spillways in berms or earthen embankments shall be designed to convey the peak discharge from a one-hundred-year design storm event while maintaining a minimum 1.0 foot of freeboard, assuming that the principal outlet structure is completely blocked. Proper erosion control measures shall be provided to protect the spillway and embankment against the erosive effects of accelerated discharge. Calculations for erosion protection shall be provided.
(5) 
All embankments shall incorporate a compacted clay core and cutoff key trench meeting the criteria for embankment material as well as the following additional criteria:
(a) 
Soil shall be relatively impermeable and meet the following USCS classification groups as determined by ASTM D2487/D2488: CL and/or CL-ML.
(6) 
Top width of compacted clay core shall be a minimum of two feet with a top elevation equal to the twenty-five-year storm peak water storage elevation or higher. The bottom width of clay key trench shall be a minimum of four feet, and the key shall extend a minimum of two feet into virgin, undisturbed soil below the topsoil layer.
(7) 
Anti-seep collars shall be installed on all basin outlet pipes. The required size and spacing of the collars shall be confirmed through calculations.
(8) 
Basins shall be kept in a maintainable condition with a minimum bottom slope of 1% sloped toward the principal outlet structure. If paved low-flow channels are used, then the basin bottom slope can be reduced to 0.5%. Paved low-flow channels shall have a minimum width of four feet and be constructed of four-inch-thick concrete over four inches of compacted PennDOT No. 2A stone.
A. 
The ability to retain and maximize the groundwater recharge capacity of the area being developed is encouraged. Design of the infiltration/recharge stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is paved and roofed over. These measures are encouraged, particularly in hydrologic soil groups A and B and should be utilized wherever feasible. Soils used for the construction of basins shall have low-erodibility factors ("K" factors). The Township reserves the right to waive the requirements of this section if the Township receives documentation from a qualified design professional which demonstrates the implementation of infiltration practices would pose a threat to public health, safety, welfare and the protection of the people of the municipality, their resources and the environment, or if site-specific soils testing demonstrates that suitable infiltration rates cannot be achieved. At a minimum, a site's suitability for infiltration practices shall be determined using the criteria in Subsections C and D.
B. 
Infiltration best management practices (BMPs) shall meet the following minimum requirements:
(1) 
The size of the recharge facility shall be based upon the following equation:
Rev = [(S) (Rv) (A)]/12
Where:
Rev =
Recharge Volume (acre-feet)
S =
Soil specific recharge factor (inches)
Rv =
Volumetric runoff coefficient
A =
Site area contributing to the recharge facility (acres)
And:
Rv = 0.05 + 0.009(I)
Where:
I =
Percent impervious area
And:
S may be obtained based upon hydrologic soil group based upon the table below recommended as general values by DEP. More site specific data may be utilized to determine recharge if justified by the design engineer and approved by the Municipal Engineer.
Hydrologic Soil Group
Soil Specific Recharge Factor (S)
(inches)
A
0.38
B
0.25
C
0.13
D
0.06
If more than one hydrologic soil group (HSG) is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.
(2) 
The recharge volume provided at the site shall be directed to the most permeable HSG available, if feasible.
C. 
Soils evaluation.
(1) 
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 design professional, and at a minimum, address soil permeability, depth to bedrock and normal or seasonal high-water table, susceptibility to sinkhole formation, and subgrade stability. The general process for designing the infiltration BMP shall be:
(a) 
Analyze hydrologic soil groups as well as natural and man-made features within watershed to determine general areas of suitability for infiltration practices.
(b) 
Provide field tests to determine the presence of appropriate soil percolation rates between 0.55 and 8.25 inches per hour. Testing shall be performed at the location of each proposed infiltration BMP at the bottom elevation of the proposed facility. A plan including the frequency and locations of soil tests shall be submitted to the Township for review and approval.
(c) 
Infiltration BMPs shall be capable of completely infiltrating the collected runoff volume within two days (48 hours).
(d) 
Infiltration BMP bottoms shall be separated by a minimum 36 inches vertically from the seasonal high-water table and/or bedrock layer, as documented by on-site soil testing. For infiltration BMPs receiving only roof runoff from a single-family residential dwelling, a twenty-four-inch vertical separation distance may be permitted at the discretion of the Township.
(e) 
Infiltration BMPs shall not be located within 50 feet of carbonate bedrock formations.
(f) 
Design infiltration BMPs for the required storm volume based on field-determined capacity at the level of the proposed infiltration surface.
(g) 
Infiltration BMPs shall be constructed in virgin soil, not fill, after all site work is completed and the contributing drainage area has received final stabilization. Recharge/infiltration facilities shall not be used as sediment basins at any time. This requirement shall be provided in note form on the plan.
(h) 
Specific requirements shall be included on the plans to protect infiltration BMPs from compaction by equipment and to prevent sediment from entering infiltration BMPs during construction. This requirement shall be provided in note form on the plan.
(i) 
Infiltration BMPs shall be at least 20 feet downgradient or 100 feet upgradient from on-site and/or off-site building foundations.
(j) 
Infiltration BMPs shall be lined with a geotextile filter fabric, having a replaceable top liner surface.
(k) 
Infiltration BMPs shall be a minimum of 100 feet from any water supply well where the runoff is from commercial or industrial pervious parking areas. For all other applications a minimum distance of 50 feet shall be used.
(l) 
Infiltration BMPs shall not be located within 50 feet of septic systems and/or septic system drain fields.
(m) 
All infiltration BMP's shall incorporate a conveyance and control for overflow runoff.
(2) 
Waiver of any or all of these requirements will be evaluated on an individual basis and may be granted at the discretion of the Township only upon written request and explanation of the reason(s) that these requirements should not apply provided by a qualified design professional.
D. 
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. It is also extremely important that the qualified design professional evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and recommend a hydrogeologic justification study be performed if necessary. Whenever a basin 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 groundwater contamination and, where necessary, sinkhole formation. The infiltration requirement in the high quality/exceptional waters shall be subject to the PA DEP's Chapter 93 and Antidegradation Regulations. The municipality may require the installation of an impermeable liner in detention basins. A detailed hydrogeologic investigation may be required by the Township. All prevention provisions shall be stated and shown on the plan with details provided for each.
(1) 
In carbonate areas, BMPs shall be designed to disperse stormwater over the largest feasible area (e.g., subtle berms and level spreaders) so as not to significantly modify the natural hydrologic regime. Use of infiltration BMPs which result in significant increases (more than a one-hundred-percent increase) in the predevelopment rate of infiltration per unit area should be avoided in carbonate areas, unless a detailed geologic evaluation (see below) demonstrates that the potential for sinkhole formation is minimal.
(2) 
Stormwater runoff from significant pollutant producing sources (so-called "hot spots" such as industrial uses, gas stations, fast-food and other commercial uses generating large numbers of vehicle trips, and other uses at the determination of the Township) shall be filtered and/or pretreated using a water quality BMP before being discharged in carbonate areas.
(3) 
In these cases, the Township may require that a detailed geologic evaluation of the project site be performed to determine the suitability for recharge, including both the potential for groundwater contamination and potential for sinkhole formation. The evaluation shall be performed by a registered professional geologist licensed in the Commonwealth of Pennsylvania and/or any other Township-approved professional, and at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. It shall be the developer's responsibility to verify if the site is underlain by limestone. The following note shall be attached to all drainage plans and signed and sealed by the developer's engineer or geologist:
"I, _____________________, hereby certify that the proposed detention basin/infiltration BMP (circle one) is/is not underlain by limestone."
E. 
The municipality may require the developer to provide safeguards against groundwater contamination for uses which may cause groundwater contamination should there be a mishap or spill.
F. 
Where pervious pavement is permitted for parking lots, recreational facilities, nondedicated streets, or other areas, pavement construction specifications and maintenance schedules shall be noted on the plan.
G. 
Recharge/infiltration facilities may be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.
A. 
In addition to the performance standards and design criteria requirements of Article III of this chapter, the land developer shall comply with the following water quality requirements of this article unless otherwise exempted by provisions of this chapter.
(1) 
The Township reserves the right to waive the requirements of this section if the Township believes that the implementation of infiltration practices would pose a threat to public health, safety, welfare, property and the protection of the people of the municipality, their resources and the environment.
(2) 
Developed areas will provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The recharge volume computed under § 126-10B 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.
(3) 
The water quality volume (WQv) is the storage capacity needed to treat stormwater runoff produced by "P" inch of rainfall (90% Rule) from the developed areas of the site (for "P" values, see Exhibit 2, Table B-5[1]). The following calculation formula is used to determine the storage volume, WQv, in acre-feet of storage:
WQv = [(P)(Rv)(A)]/12
WQv =
Water Quality Volume
P =
Rainfall amount (90% of events producing this rainfall (Exhibit 2, Table B-5)
A =
Area in acres
Rv =
0.05 + 0.009(I), where I is the impervious surface percentage
[1]
Editor's Note: Exhibit 2 is included at the end of this chapter.
B. 
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. Design of infiltration BMPs proposed to comply with the water quality requirements of this section shall be in accordance with the design requirements outlined in the previous § 126-10C, D and E.
A. 
Storage facilities shall be designed to release the one-year postdevelopment storm runoff volume over a minimum duration of 24 hours.