[Amended 6-17-2025 by Ord. No. 426]
A.
For all regulated activities, unless preparation of a stormwater management plan is specifically exempted, activities shall be designed, implemented, operated and maintained to meet the purposes of this Part 1, through these elements:
(1)
Preparation and implementation of an approved stormwater management plan is required which shall include erosion and sediment controls during earth disturbance activities (e.g., during construction).
(2)
Water quality protection measures after completion of earth disturbance activities (e.g., after construction), including operations and maintenance.
(3)
No regulated activities shall commence until the municipality issues written approval of a stormwater management plan which demonstrates compliance with the requirements of this Part.
(4)
The municipality may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this Part, provided that they meet the minimum requirements of, and do not conflict with, state law, including, but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
B.
Erosion and sediment control during regulated earth disturbance activities shall be addressed as required by § 125-11.
D.
All best management practices (BMPs) used to meet the requirements of this Part 1 shall conform to the state water quality requirements, and any more stringent requirements as determined by Whitpain Township.
E.
Techniques for low-impact development, as hereinafter set forth, are encouraged to be implemented because they reduce the costs of complying with the requirements of this Part 1 and the state water quality requirements.
F.
At the Township's discretion, the applicant may be required to provide written notification to the adjacent property owner(s) for any stormwater flows onto the adjacent property that will be created, increased, decreased, relocated, or otherwise altered. Such stormwater flows shall be subject to the requirements of this Part.
G.
Impervious areas:
(1)
The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.
(2)
For development taking place in stages, the entire development plan must be used in determining conformance with this Part.
(3)
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this Part, except that the volume controls in § 125-9O and the peak rate controls of § 125-9P do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
H.
Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual). If methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required under this Part, the stormwater management plan must include a detailed justification demonstrating that the use of LID and green infrastructure is not practicable.
I.
Low-impact development practices alternative approach for managing stormwater runoff.
(1)
Natural hydrologic conditions may be altered radically by poorly planned development practices, such as introducing unneeded impervious surfaces, destroying existing drainage swales, constructing unnecessary storm sewers, and changing local topography. A traditional drainage approach of development has been to remove runoff from a site as quickly as possible and capture it in a detention basin. This approach leads ultimately to the degradation of water quality as well as expenditure of additional resources for detaining and managing concentrated runoff at some downstream location.
(2)
The recommended alternative approach is to promote practices that will minimize postdevelopment runoff rates and volumes, which will minimize needs for artificial conveyance and storage facilities. To simulate predevelopment hydrologic conditions, forced infiltration is often necessary to offset the loss of infiltration by creation of impervious surfaces. The ability of the ground to infiltrate depends upon the soil types and its conditions.
(3)
Preserving natural hydrologic conditions requires careful alternative site design considerations. Site design practices include preserving natural drainage features, minimizing impervious surface area, reducing the hydraulic connectivity of impervious surfaces, and protecting natural depression storage. A well-designed site will contain a mix of all those features. The following describes various techniques to achieve the alternative approach:
(a)
Preserving natural drainage features. Protecting natural drainage features, particularly vegetated drainage swales and channels, is desirable because of their ability to infiltrate and attenuate flows and to filter pollutants. However, this objective is often not accomplished in land development. In fact, commonly held drainage philosophy encourages just the opposite pattern; streets and adjacent storm sewers typically are located in the natural headwater valleys and swales, thereby replacing natural drainage functions with a completely impervious system. As a result, runoff and pollutants generated from impervious surfaces flow directly into storm sewers with no opportunity for attenuation, infiltration, or filtration. Developments designed to fit site topography also minimizes the amount of grading on site.
(b)
Protecting natural depression storage areas. Depressional storage areas have no surface outlet, or drain very slowly following a storm event. They can be commonly seen as ponded areas in farm fields during the wet season or after large runoff events. Traditional development practices eliminate these depressions by filling or draining, thereby obliterating their ability to reduce surface runoff volumes and trap pollutants. The volume and release-rate characteristics of depressions should be protected in the design of the development site. The depressions can be protected by simply avoiding the depression or by incorporating its storage as additional capacity in required detention facilities.
(c)
Avoiding introduction of impervious areas. Careful site planning should consider reducing impervious coverage to the maximum extent possible. Building footprints, sidewalks, driveways and other features producing impervious surfaces should be evaluated to minimize impacts on runoff.
(d)
Reducing the hydraulic connectivity of impervious surfaces. Impervious surfaces are significantly less of a problem if they are not directly connected to an impervious conveyance system (such as storm sewer). Two basic ways to reduce hydraulic connectivity are routing of roof runoff over lawns and reducing the use of storm sewers. Site grading should promote increasing travel time of stormwater runoff, and should help reduce concentration of runoff to a single point in the development.
(e)
Routing roof runoff over lawns. Roof runoff can be easily routed over lawns in most site designs. The practice discourages direct connections of downspouts to storm sewers or parking lots. The practice also discourages sloping driveways and parking lots to the street. By routing roof drains and crowning the driveway to run off to the lawn, the lawn is essentially used as a filter strip.
(f)
Reducing the use of storm sewers. By reducing use of storm sewers for draining streets, parking lots, and back yards, the potential for accelerating runoff from the development can be greatly reduced. The practice requires greater use of swales and may not be practical for some development sites, especially if there are concerns for areas that do not drain in a reasonable time. The practice requires educating local citizens and public works officials, who expect runoff to disappear shortly after a rainfall event.
(g)
Reducing street widths. Street widths can be reduced by either eliminating on-street parking or by reducing roadway widths. Municipal planners and traffic designers should encourage narrower neighborhood streets which ultimately could lower maintenance.
(h)
Limiting sidewalks to one side of the street. A sidewalk on one side of the street may suffice in low-traffic neighborhoods. The lost sidewalk could be replaced with bicycle/recreational trails that follow back-of-lot lines. Where appropriate, backyard trails should be constructed using pervious materials.
(i)
Using permeable paving materials. These materials include permeable interlocking concrete paving blocks or porous bituminous concrete. Such materials should be considered as alternatives to conventional pavement surfaces, especially for low-use surfaces such as driveways, overflow parking lots, and emergency access roads.
(j)
Reducing building setbacks. Reducing building setbacks reduces driveway and entry walks and is most readily accomplished along low-traffic streets where traffic noise is not a problem.
(k)
Constructing cluster developments. Cluster developments can also reduce the amount of impervious area for a given number of lots. The biggest savings is in street length, which also will reduce costs of the development. Cluster development clusters the construction activity onto less sensitive areas without substantially affecting the gross density of development.
(4)
In summary, a careful consideration of the existing topography and implementation of a combination of the above-mentioned techniques may avoid construction of costly stormwater control measures. Other benefits include reduced potential of downstream flooding, water quality degradation of receiving streams/water bodies and enhancement of aesthetics and reduction of development costs. Beneficial results include more stable base flows in receiving streams, improved groundwater recharge, reduced flood flows, reduced pollutant loads, and reduced costs for conveyance and storage.
J.
The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.
K.
Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this Part.
(1)
All sites utilizing infiltration stormwater facilities must perform soil infiltration testing in compliance with the soil infiltration testing requirements as defined in Attachment 3, Appendix C.[2] Results from the soil testing must be submitted to the Township for review.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
All infiltration BMPs should be designed with a positive overflow that discharges excess volume in a non-erosive manner and allows for controlled discharge during rainfall events.
L.
Stormwater BMPs should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.
M.
The design storm volumes to be used in the analysis of peak rates of discharge should be obtained per the latest NOAA Atlas precipitation data.
N.
O.
Volume Controls. The green infrastructure and low impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection O(1) or the Simplified Method in Subsection O(2) below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this Part establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors.
(1)
The Design Storm Method (CG-1 in the BMP Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions.
(a)
Do not increase the post-development total runoff volume for all storms equal to or less than the two-year, 24-hour duration precipitation.
(2)
The Simplified Method (CG-2 in the BMP Manual) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre, or for projects that require design of stormwater storage facilities. For new impervious surfaces:
(a)
Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces. [Note: An asterisk (*) in equations denotes multiplication.]
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
(b)
At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow (i.e., it shall not be released into the surface waters of the commonwealth). Removal options include reuse, evaporation, transpiration, and infiltration.
Volume (cubic feet) = (1/12) * Impervious Surfaces (square feet)
(c)
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 0.5 inch of the permanently removed runoff should be infiltrated.
P.
Rate controls.
(1)
For areas not covered by a release rate map from an approved Act 167 stormwater management plan: Post-development discharge rates shall not exceed the pre-development discharge rates for the one-, two-, five-, 10-, 25-, 50-, and 100-year, 24-hour storm events. If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the pre-development analysis for one-, two-, five-, 10-, 25-, 50-, and 100-year, 24-hour storms, then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.
(2)
For areas covered by a release rate map from an approved Act 167 stormwater management plan: For the one-, two-, five-, 10-, 25-, 50-, and 100-year, 24-hour storm events, the post-development peak discharge rates will follow the applicable approved release rate maps. For any areas not shown on the release rate maps, the post-development discharge rates shall not exceed the pre-development discharge rates.
Q.
Riparian buffers.
(1)
In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer.
(2)
Except as required by 25 Pa. Code Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 10 feet from the top of the streambank (on each side).
(3)
Minimum management requirements for riparian buffers.
(a)
Existing native vegetation shall be protected and maintained within the riparian buffer easement.
(b)
Whenever practicable invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.
(4)
The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the appropriate County Recorder of Deeds office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area a required by zoning, unless otherwise specified in the municipal Zoning Ordinance.
(5)
Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
(7)
Septic drainfields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.