For all regulated activities not eligible for exemptions pursuant to this chapter, the applicant shall submit a Stormwater Management (SWM) site plan and report prepared and sealed by a professional engineer or landscape architect licensed in the Commonwealth of Pennsylvania.
A. 
Appropriate sections from Chapter 385, Subdivision and Land Development, of the Wilkins Township Code, and other applicable ordinances, shall be followed in preparing the SWM site plans. These include, but are not limited to the requirements relating to mapping and plan preparation and submittal.
B. 
The Township shall not approve any SWM site plan that is deficient in meeting the requirements of this chapter. At its sole discretion and in accordance with this article, when a SWM site plan is found to be deficient, the Township may either disapprove the submission and require a resubmission, or in the case of minor deficiencies, the Township may accept submission of modifications.
C. 
Provisions for permanent access or maintenance easements must be made for all physical SWM BMPs, such as ponds and infiltration structures, as necessary to implement the operation and maintenance (O&M) plan discussed in § 374-21E(9) below. All such easements and access points shall be clearly delineated in the SWM site plan.
D. 
The following signature block shall be included in the SWM site plan: "[The Wilkins Township Official or Designated Plan Reviewer (§ 374-23)], on this date (date of signature), has reviewed and hereby certifies that the SWM site plan appears to meet all design standards and criteria of Chapter 374, Stormwater Management, of the Code of the Township of Wilkins, except where waivers have been noted on the plan. The review is based on a survey and SWM site plan prepared by others and assumes that all information is correct and valid as submitted."
E. 
The SWM site plan shall provide the following information:
(1) 
The overall stormwater management concept for the project.
(a) 
A grading plan, indicating all areas of earth disturbance for the proposed activity.
(b) 
A listing of all permits and other authorizations that will be required for the project, including the status of permit applications and approvals.
(c) 
Drainage area maps for all watersheds, subwatersheds, inlets, etc., for both predevelopment and post-development conditions.
(2) 
A determination of site conditions in accordance with the BMP Manual.[1] A detailed site evaluation shall be completed for projects proposed in areas of carbonate geology or karst topography, and other environmentally sensitive areas, such as brownfields.
(a) 
Hydrologic (watershed) and water feature boundaries, including all areas flowing to the proposed project, existing streams, springs, lakes, ponds, or other bodies of water within the project area.
(b) 
Existing and proposed topographical information with contours and elevations.
(c) 
Locations of existing standing water, seepage, wetlands, and hydrologically sensitive areas.
(d) 
One-hundred-year flood elevations for any floodplains on or within 100 feet of the property.
(e) 
Existing and proposed ground cover and land use. The total area of and percent impervious cover shall be noted.
(f) 
Wetlands delineation report, if required.
[1]
Editor's Note: See § 374-42A.
(3) 
Stormwater runoff design computations and documentation as specified in this chapter, or as otherwise necessary to demonstrate that the maximum practicable measures have been taken to meet the requirements of this chapter, including the recommendations and general requirements in Article III.
(4) 
A plan of the proposed stormwater drainage system, stormwater management practices to be applied both during and after construction, and the expected project time schedule.
(5) 
A soil erosion and sediment control plan, where applicable, as prepared for and submitted to the Allegheny County Conservation District.
(6) 
The effect of the project (in terms of runoff volumes, water quality, and peak flows) on surrounding properties and aquatic features and on any existing stormwater conveyance system that may be affected by the project.
(7) 
Plan and profile drawings of all existing and proposed SWM BMPs, including drainage structures, pipes, open channels, and swales.
(a) 
For any sites with existing stormwater management facilities or BMPs, the nature and condition of such facilities must be discussed in the SWM site plan narrative. If the existing facilities are proposed to remain intact and function as all or part of the facilities required for the project, the following information must be included in the SWM site plan:
[1] 
Inspections and/or certifications that the existing facilities are sized and capable of operating as required, including:
[a] 
Surveys of all pertinent elevations associated with the facility, including but not limited to inverts or channel bottoms, outlet controls, embankments, and receiving streams or structures;
[b] 
Internal conditions of all underground pipes and structures, including the use of photographs or video for documentation;
[c] 
Design calculations from the facilities' original construction, if available;
[d] 
Operation and maintenance records, if available.
[2] 
Plans and descriptions of any alterations proposed for the facilities.
[3] 
Runoff and design calculations.
(b) 
If proposing infiltration BMPs, the applicant shall consider the following:
[1] 
Permeability and infiltration rate of the site soils.
[2] 
Slope and depth to bedrock.
[3] 
Seasonal high water table.
[4] 
Proximity to building foundations and well heads.
[5] 
Erodibility of soils.
[6] 
Land availability and topography.
[7] 
Slope stability.
[8] 
Effects on nearby properties and structures.
(c) 
A detailed soils evaluation of the project site must be performed to determine the suitability of infiltration BMPs. The evaluation shall be performed by a qualified professional.
(8) 
The SWM site plan shall show the locations of existing and proposed on-lot wastewater facilities and water supply wells.
(9) 
The SWM site plan shall include an O&M plan for all existing and proposed physical stormwater management facilities. This plan shall address long-term ownership and responsibilities for O&M as well as schedules and costs for O&M activities.
(10) 
A justification, acceptable to the designated plan reviewer, must be included in the SWM site plan if BMPs other than green infrastructure methods and LID practices are proposed to achieve the volume, rate and water quality controls under this chapter.
(11) 
General data, including:
(a) 
Project name.
(b) 
Project location map; address of the project site. The location map shall show the project site and relation to release rate districts.
(c) 
Name, address, and telephone number of the applicant and, if different, the property owner.
(d) 
Name, address, telephone number, email address, and seal of the individual preparing the SWM site plan.
(e) 
Date of submission and revisions.
(f) 
Total site area of predevelopment and post-development conditions, which must be equal or include justification of differences.
(g) 
Total site impervious area, both predevelopment and post-development.
(12) 
Post-construction stormwater management report, including, but not limited to:
(a) 
Project narrative which clearly discusses the project, including a statement of the proposed regulated activity and overall stormwater management concept.
(b) 
Summary tables.
[1] 
Predevelopment conditions, including hydrologic soil group (HSG) assumptions, curve numbers and/or runoff coefficients, slopes, Tc, etc.
[2] 
Pre-development conditions runoff volume and peak rate.
[3] 
Post-development conditions runoff volume and peak rate, with and without the impact of proposed BMPs.
[4] 
Allowable post-development runoff based upon release rate district, if applicable.
[5] 
Infiltration testing results, if applicable.
(c) 
Complete hydrologic and hydraulic computations, calculations, assumptions, and BMP design criteria.
A. 
The applicant shall submit 12 copies of the SWM site plan as follows:
(1) 
Nine copies to the Township of Wilkins.
(2) 
One copy to the Wilkins Township municipal engineer.
(3) 
One copy to the Allegheny County Conservation District (when requested by the District).
(4) 
One copy to the Allegheny County Sanitary Authority (in areas with combined sewer systems).
B. 
Additional copies shall be submitted as requested by the Township or DEP.
A. 
The Township has designated the Township Engineer as the designated plan reviewer of SWM site plans for the Township, and shall be understood to be the reviewer where indicated as the Township within this chapter.
B. 
SWM site plans shall be reviewed by the Township for consistency with the provisions of this chapter.
C. 
The Township shall notify the applicant in writing within 45 days whether the SWM site plan is approved or disapproved. If the SWM site plan involves a subdivision and land development plan, the notification shall occur within the time period allowed by the Municipalities Planning Code (90 days). If a longer notification period is provided by other statute, regulation, or ordinance, the applicant will be so notified by the Township.
D. 
For any SWM site plan that proposes to use any BMPs other than green infrastructure and LID practices to achieve the volume and rate controls required under this chapter, the municipality will not approve the SWM site plan unless it determines that green infrastructure and LID practices are not practicable.
E. 
If the Township disapproves the SWM site plan, the Township will state the reasons for the disapproval in writing. The Township also may approve the SWM site plan with conditions and, if so, shall provide the acceptable conditions for approval in writing.
A modification to a submitted SWM site plan that involves a change in SWM BMPs or techniques, or that involves the relocation or redesign of SWM BMPs, or that is necessary because soil or other conditions are not as stated on the SWM site plan as determined by the Township shall require a resubmission of the modified SWM site plan in accordance with this article.
A disapproved SWM site plan may be resubmitted, with the revisions addressing the Township's concerns, to the Township in accordance with this article. The applicable review fee must accompany a resubmission of a disapproved SWM site plan.
The Township's approval of an SWM site plan authorizes the regulated activities contained in the SWM site plan for a maximum term of validity of five years following the date of approval. The Township may specify a term of validity shorter than five years in the approval for any specific SWM site plan. Terms of validity shall commence on the date the Township signs the approval for an SWM site plan. If an approved SWM site plan is not completed according to § 374-27 within the term of validity, then the Township may consider the SWM site plan disapproved and may revoke any and all permits. SWM site plans that are considered disapproved by the Township shall be resubmitted in accordance with § 374-25 of this chapter.
A. 
The developer shall be responsible for providing record drawings of all SWM BMPs included in the approved SWM site plan. The record drawings plans and an explanation of any discrepancies with the construction plans shall be submitted to the Township.
B. 
The record drawings submission shall include a certification of completion signed by a professional engineer licensed in the Commonwealth of Pennsylvania verifying that all permanent SWM BMPs have been constructed according to the approved plans and specifications. The latitude and longitude coordinates for all permanent SWM BMPs must also be submitted, at the central location of the BMPs. If any licensed qualified professionals contributed to the construction plans, then a licensed qualified professional must sign the completion certificate.
C. 
The municipality may conduct inspections during construction as it deems appropriate. If inspections performed by the municipality reveal deficiencies from the submitted and approved SWM site plan, the municipality may request corrective actions. Any corrective action shall be at the cost of the stormwater facility owner.
D. 
After receipt of the completion certification by the Township, the Township may conduct a final inspection.
The following paragraphs identify technical and design standards that must be utilized in all SWM site plans submitted to the Township.
A. 
General design standards.
(1) 
No outlet structure from a stormwater management facility or swale shall discharge directly onto a municipal or state roadway without approval from the municipality or PennDOT.
(2) 
The top, or toe, of any slope shall be located a minimum of 10 feet from any property line.
(3) 
The minimum horizontal distance between any stormwater holding facility shall be 25 feet. The lowest floor elevation of any structure constructed immediately adjacent to a detention basin or other stormwater facility shall be a minimum of two feet above the 100-year water surface elevation.
(4) 
Stormwater management facility bottom (or surface of permanent pool) elevations must be greater than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If no floodplain is defined, bottom elevations must be greater than existing ground elevations 50 feet from top of stream bank in the facilities' vicinity.
(5) 
Energy dissipaters and/or level spreaders shall be installed at points where pipes or drainageways discharge to or from basins. Discharges to drainage swales shall be dissipated, or piped, to an acceptable point.
(6) 
Landscaping and planting specifications must be provided for all stormwater management basins and be specific for each type of basin.
(7) 
Stormwater roof drains and pipes shall not discharge onto impervious areas without approval by the Township Engineer.
B. 
Conveyance facility design standards.
(1) 
Where practicable, storm sewers shall be designed to traverse under seeded and planted areas. If constructed within 10 feet of roads, sidewalks, or other paved surfaces, storm sewers shall be installed within the narrowest trench possible and backfilled with select material to prevent surface settlement.
(2) 
Storm sewers shall be designed with a concrete cradle when traversing fill areas of questionable stability.
(3) 
Storm sewers shall be designed with pipe anchors when the pipe slope exceeds 20%.
(4) 
The minimum storm sewer size shall be 15 inches in diameter. Pipes shall be designed to provide a minimum velocity of 2 1/2 feet per second when flowing full, but in all cases, the slope shall be no less than 0.5%. Arch pipe of equivalent cross-sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.
(a) 
All storm sewer pipes shall be laid to a minimum depth of one foot from subgrade to the crown of pipe.
(5) 
Pipe material, trenching, bedding, and backfilling requirements shall conform to the requirements of the Township Engineer and/or applicable PennDOT Standards, latest version.
(6) 
Storm sewers shall be either reinforced concrete or high density polyethylene (HDPE) pipe, subject to cover requirements and the approval of the Township Engineer.
(7) 
Collection/conveyance facilities should not be installed parallel or close to the top or bottom of an embankment to avoid the possibility of failure of the facility or the embankment.
(8) 
All collection/conveyance facilities shall be designed to convey the twenty-five-year frequency storm peak flow rate and carry it to the nearest suitable outlet. The conveyance capacities of any downstream facilities to which this flow is tributary must be verified.
(9) 
All developments shall include provisions that allow for the overland conveyance and flow of the post-development 100-year storm event without damage to public or private property.
(10) 
Stormwater conveyance, including a cutoff trench/berm, storm sewer pipes, inlets, and stormwater easement, shall be provided along the toe of all cut and fill slopes that are adjacent to residential lots.
(11) 
Inlets.
(a) 
In curbed roadway sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through travel lane or one inch less than the depth of curb during the ten-year design storm of five-minute duration. In curbed sections of super-elevated roadways, the maximum encroachment of water on the roadway shall not exceed one inch less than the depth of curb during the ten-year design storm of five-minute duration. Gutter depth shall be verified by inlet capture/capacity calculations that account for road slope and opening area.
(b) 
Inlets shall be placed at a maximum of 400 feet apart.
(c) 
Inlets shall be placed so drainage cannot cross intersections or street center lines.
(d) 
For inlets containing a change in pipe size, the elevation for the crown of the pipes shall be the same or the smaller pipe's crown shall be at a higher elevation.
(e) 
All inlets shall provide a minimum two-inch drop between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.
(f) 
On curbed sections, a double inlet shall be placed at the low point of sag vertical curves, or an inlet shall be placed on each side of the low point at a distance not to exceed 100 feet, or at an elevation not to exceed 0.2 feet above the low point.
(g) 
At all roadway low points, swales and easements shall be provided behind the curb or swale and through adjacent properties to channelize and direct any overflow of stormwater runoff away from dwellings and structures.
(h) 
All inlets in paved areas shall have heavy duty bicycle safe grating. A note to this effect shall be added to the SWM site plan or inlet details therein.
(i) 
Inlets must be sized to accept the specified pipe sizes without knocking out any of the inlet corners. All pipes entering or exiting inlets shall be cut flush with the inside wall of the inlet. A note to this effect shall be added to the SWM site plan or inlet details therein.
(j) 
Inlets shall have weep holes covered with geotextile fabric placed at appropriate elevations to completely drain the subgrade prior to placing the base and surface course on roadways.
(k) 
Inlets, junction boxes, or manholes greater than five feet in depth shall be equipped with ladder rungs and shall be detailed on the SWM site plan.
(l) 
Inlets shall not have a sump condition in the bottom (unless designed as a water quality BMP or specifically approved by the Township). Pipe shall be flush with the bottom of the box or concrete channels shall be poured.
(m) 
Accessible drainage structures shall be located on continuous storm sewer system at all vertical dislocations, at all locations where a transition in storm sewer pipe sizing is required, at all vertical and horizontal angle points exceeding 5°, and at all points of convergence of two or more storm sewer pipes.
(n) 
All storm drainage piping shall be provided with either reinforced concrete headwalls or end sections of compatible material as the pipe involved at its entrance and discharge.
(o) 
Outlet protection and energy dissipaters shall be provided at all surface discharge points in order to minimize erosion consistent with the E&S Manual.
(p) 
Flow velocities and volumes from any storm sewer shall not result in a degradation of the receiving channel.
C. 
Stormwater management facility design standards.
(1) 
If proposing underground detention facilities, the design must incorporate appropriate access features and means to inspect, maintain, and ensure the proper operation of the facilities and their ability to control discharge rates to the levels mandated in the SWM site plan.
(2) 
The design of BMPs incorporating embankments must be completed and sealed by a professional engineer with relevant experience licensed in the Commonwealth of Pennsylvania.
(3) 
Detention facilities and impoundments must provide a total storage volume allowance equal to 110% of the maximum required 100-year storm storage volume to allow for the accumulation of sediment. Appropriate means of access and the ability to maintain the BMP shall be incorporated into the design.
(4) 
Principal outlet structures:
(a) 
The primary outlet structure shall be designed to pass all design storms (up to and including the 100-year event) without discharging through the emergency spillway. All principal outlet structures shall:
(b) 
Be constructed of reinforced concrete or an alternative material approved by the Township Engineer. When approved for use, all metal risers shall:
[1] 
Be suitably coated to prevent corrosion.
[2] 
Have a concrete base attached with a watertight connection. The base shall be sufficient weight to prevent flotation of the riser.
[3] 
Provide a trash rack or similar appurtenance to prevent debris from entering the riser.
[4] 
Provide an antivortex device, consisting of a thin vertical plate normal to the basin berm.
[5] 
Provide trash racks to prevent clogging of primary outflow structure stages for all orifices.
[6] 
Provide outlet aprons and shall extend to the toe of the basin slope at a minimum.
(5) 
Emergency spillways.
(a) 
The top of embankment elevation shall provide a minimum one foot of freeboard above the maximum water surface elevation. This is to be calculated when the spillway functions for the 100-year post-development inflow, with a blocked outlet structure.
(b) 
Avoid locating on fill areas, whenever possible.
(c) 
The spillway shall be armored to prevent erosion during the 100-year post-development flow, with a blocked primary outlet structure.
(d) 
Synthetic liners or riprap may be used, and calculations sufficient to support proposed armor must be provided. An earthen plug must be used to accurately control the spillway invert if riprap is the proposed armoring material.
(e) 
Spillway armor must extend up the sides of the spillway, and continue full width to a minimum of 10 feet past the toe of the slope.
(f) 
Township Engineer may require the use of additional protection when slopes exceed 4:1 and spillway velocities might exceed NRCS standards for the particular soils involved.
(g) 
Detention facilities must include an emergency spillway or outlet configuration sized and located to permit the safe passage of stormwater flows from the unattenuated 100-year post-development storm with one foot of freeboard, and assuming that all other facility outlets are clogged or otherwise out of service.
(6) 
All ground surface, open detention facilities, BMPs. or other facilities where water depths could exceed 24 inches must incorporate fencing in accordance with Chapter 160, Construction Codes, Uniform, and Chapter 360, Property Maintenance, of the Code of the Township of Wilkins.
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
(7) 
Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the 100-year floodplain elevation must be used for all design storms, or the applicant may elect to determine flood elevations of the adjacent watercourse for each design storm. The floodplain is assumed to be 50 feet from top of stream bank in areas where a floodway is not designated, or no other evidence is provided.
(8) 
The invert of all stormwater management facilities and underground infiltration/storage facilities shall be located a minimum of two feet above the seasonal high groundwater table. The invert of stormwater facilities may be lowered if adequate subsurface drainage is provided. Flows from underdrains need not be accounted for in volume or rate control calculations.
(9) 
Whenever possible the side slopes and basin shape shall be amenable to the natural topography. Vertical side slopes and rectangular basins shall be avoided whenever possible.
(10) 
Exterior slopes of compacted soil shall not exceed 3:1, and may be further reduced if the soil has unstable characteristics.
(11) 
Interior slopes of the basin shall not exceed 3:1.
(12) 
Unless specifically designed as a volume control facility, all stormwater management facilities shall have a minimum slope of 2% extending radially out from the principal outlet structure. Facilities designed as water quality/infiltration BMPs may have a bottom slope of zero.
(13) 
Impervious low-flow channels are not permitted within stormwater management facilities.
(14) 
Unless specifically designed as a volume control or water quality facility, all stormwater management facilities must empty over a period of time not less than 24 hours and not more than 72 hours from the end of the facility's inflow hydrograph. Infiltration tests performed at the facility locations and proposed basin bottom depths, in accordance with the BMP Manual, must support time-to-empty calculations if infiltration is a factor.
(15) 
Water quality inlets shall be utilized in parking areas and/or loading areas that discharge to stormwater management systems. At least one water quality inlet shall be installed on any project proposing storm inlets, with no less than one installed for each acre of drainage area. The purpose of such inlets is to remove oil, grease, heavy particulates, total suspended solids, hydrocarbons, and other floating substances from stormwater runoff. Methods other than water quality inlets may be permitted if the applicant demonstrates to the Township's satisfaction that any such alternative will be as effective and as easily maintained. Periodic cleaning of these systems shall be addressed in the operation and maintenance plan for the facility.