Township of Ferguson, PA Part 3 STORMWATER MANAGEMENT

§ 26-301 General Requirements.

[Ord. No. 1102, 9/3/2024]

1. All regulated activities in the Township which do not fall under the exemption criteria shown in § 26-110 of this chapter shall submit a stormwater management site plan to the Township for review. This plan must be consistent with the Spring Creek Watershed Stormwater Management Plan. These criteria shall apply to the total proposed development even if development is to take place in phases. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas, and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the waiver criteria. The DEP Stormwater BMP Manual, as amended and updated, shall be used for design of stormwater management facilities. No disturbance may be performed prior to approval of a stormwater management site plan. All activities (especially earth disturbance activities) must comply with this chapter, Title 25 of the Pa. Code and the Clean Streams law.[1]

[1] Editor's Note: See 35 P.S. § 691.1 et seq.

2. 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.

A. Stormwater management facilities and related installations also shall be provided:

(1) To ensure adequate drainage of all low points along the curbline of streets.

(2) To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained, and to prevent substantial flow of water across intersections or flooded intersections during storms, in accordance with the procedures contained in Design Manual Part 2 (DM-2), Chapter 10, of the Pennsylvania Department of Transportation (PA DOT).

(3) To ensure adequate and unimpeded flow of stormwater under driveways in, near, or across natural watercourses or drainage swales. Suitable pipes or other waterways shall be provided as necessary.

(4) To properly drain stormwater runoff from all land development projects, except as required by recharge criteria. All lot and open areas shall be designed to drain to the nearest practical street or drainage system, existing or proposed, as defined by the Township Stormwater Engineer, with no impact on adjoining properties, unless an area specifically designed for stormwater detention is provided.

3. The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the altered property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.

4. Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto 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. If, in the opinion of the Township Stormwater Engineer, the approved analysis identifies there will be an impact on the downstream adjacent property, the Township may require that the developer obtain a stormwater easement. Where a development site is traversed by watercourses or significant drainageways, drainage easements shall be provided conforming to the line of such watercourses or significant drainageways. 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. Developers are encouraged to allow stormwater easements to revert to a natural condition. Mowing or similar types of vegetative control are discouraged while removal of invasive species is encouraged.

5. 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 through the joint permit application process, or, where deemed appropriate by PA DEP, through the general permit process.

6. Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth (any and all rivers, streams, creeks, rivulets, ditches, watercourses, storm sewers, lakes, dammed water, wetlands, ponds, springs, and all other bodies or channels of conveyance of surface and underground water, or parts thereof, whether natural or artificial, within or on the boundaries of this commonwealth) or wetlands shall be subject to approval by PA DEP through the joint permit application process, or, where deemed appropriate by PA DEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the developer or his agent to show that the land in question cannot be classified as wetlands; otherwise approval to work in the area must be obtained from PA DEP.

7. Any stormwater management facilities regulated by this chapter that would discharge to or be located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PA DOT).

8. Low impact development (LID) is to be used to the maximum extent practicable. Minimization of impervious surfaces and infiltration of runoff through seepage beds, recharge trenches, etc., are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities.

9. To promote over-land flow and infiltration/percolation of stormwater, roof drains should not be connected to streets, sanitary or storm sewers, or roadside ditches unless approved by the Township Stormwater Engineer on a case-by-case basis.

10. Where deemed necessary by the Township Stormwater Engineer, the applicant shall submit an analysis of the impacts of detained stormwater flows on downstream areas within the watershed. These impacts shall be identified with concurrence from the Township Stormwater Engineer. The analysis shall include hydrologic and hydraulic calculations necessary to determine the impact peak discharge modifications from the proposed development have on critical locations such as dams, tributaries, existing developments, undersized culverts, floodprone areas, etc. The analysis shall include the impact of the modified quantity of discharged water to developed downstream areas that experience flooding. The applicant must demonstrate that the additional flow does not adversely impact downstream properties or structures and the limits of said flooded area are not increased by the proposed activity. Any regulated activity that adversely impacts downstream properties, structures, or public facilities, or threatens the public health or safety, shall be prohibited by the Township.

11. When stormwater management facilities are proposed within 1,000 feet of a downstream municipality, the analysis of downstream impacts shall be submitted to the downstream municipality's engineer for review and comment concurrent with the Township review. Proof of written notification from the applicant to the downstream municipality's engineer shall be provided with the submission.

12. When stormwater flows onto an adjacent property may be altered, proof of written notification from the developer to the adjacent property owner shall be provided. The notification shall be mailed by certified mail, return receipt, to document delivery of the notification. The notification shall advise the adjacent property owner of the specific proposal and provide appropriate contact information for the developer's engineer and Township Stormwater Engineer.

13. Approved stormwater management site plans must be on site at all times during construction.

14. Use of alternate and/or new stormwater management controls will be considered by the Township. A description of the proposed alternate controls must be submitted to the Township, the Centre County Conservation District, and to DEP. The Township will coordinate any approvals with the CCCD and/or DEP.

15. The stormwater management site plan must contain a proper long term ownership, operation and maintenance plan in accordance with Part 7 of this chapter.

16. No habitable structure shall be built within 50 feet of the 100-year floodplain. All floors of any living space shall be at least one foot above the 100-year floodplain water surface elevation. Any access, including doors, windows, and openings, to an uninhabited basement or floor shall be at least one foot above the 100-year floodplain water surface elevation. Plans shall clearly state if the basement or lowest floor is habitable.

17. Landscaping and grading of lots shall provide positive drainage away from all structures.

§ 26-302 Water Quality Sensitive Area Districts and Developments.

[Ord. No. 1102, 9/3/2024]

1. Water quality sensitive area districts (sensitive area districts) and water quality sensitive developments (sensitive developments) have been identified which require special consideration with regard to stormwater management.

A. Water quality sensitive area districts are defined as those areas that, if developed, have the potential to cause catastrophic loss to a Water Authority well field. These areas consist of all Zone 2 wellhead protection areas defined by DEP within Township boundaries (see Appendix B, Exhibit 1[1]). The Township may update the sensitive area district boundaries based on new research or studies as required.

[1] Editor's Note: Said appendix is included as an attachment to this chapter.

B. Water quality sensitive (WQS) developments (sensitive developments) are defined as a land development project that has a high potential to cause loss to local water quality, and could potentially threaten groundwater reservoirs as determined by the Township. Water quality sensitive developments include, but are not limited to:

(1) Vehicle fueling stations.

(2) Industrial manufacturing sites.*

(3) Salvage yards.

(4) Recycling centers.

(5) Hazardous material storage areas.*

(6) Interstate highways.

* The Township Stormwater Engineer will determine if a development is a sensitive development on a case-by-case basis. The Pennsylvania DEP wellhead protection contaminant source list shall be used as a guide in these determinations. Industrial manufacturing site and hazardous material storage areas must provide NPDES SIC codes.

2. In sensitive area districts and developments, the capture volume shall be managed in a stormwater facility separate from the peak rate control facility. Peak rate control facilities in sensitive areas and developments shall be lined with an impervious liner. For sites less than two acres in size, or in cases where BMPs are limited to a total depth of 24 inches, separate peak rate control and capture volume facilities are not required. In addition, the developer of any project located within a defined Zone II wellhead protection area, as defined by PA DEP, and/or the owner of a public water supply shall invite the governing body of the water provider (or appointed representative) to a pre-application meeting at the conceptual design stage to solicit input relative to providing appropriate protection for the associated groundwater resource.

§ 26-303 Performance Standards.

[Ord. No. 1102, 9/3/2024]

1. General. Post-development rates of runoff from any regulated activity shall not exceed the peak release rates of runoff prior to development for the design storms specified.

2. Sensitive Area District Boundaries. The location of sensitive areas or sensitive area districts (SAD) within the watershed are illustrated on a map included as Exhibit 1 in Appendix B of this chapter.[1]

[1] Editor's Note: Said appendix is included as an attachment to this chapter.

3. Sites Located in More Than One District. For a proposed development site which is traversed by a SAD boundary, the design criteria for sensitive areas must be applied if post-development runoff is directed towards the sensitive area.

4. Off-Site Areas. Off-site areas that drain from sensitive areas through a proposed development site that is located entirely in a non-sensitive area are not required to use or apply the sensitive area criteria.

5. Site Areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area shall be subject to the design criteria.

6. Downstream Hydraulic Capacity Analysis. Any downstream or off-site hydraulic capacity analysis conducted in accordance with these standards shall use the following criteria for determining adequacy for accepting increased peak flow rates:

A. Natural or man-made channels or swales must be able to convey the post-development runoff associated with a 2-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual.

B. Natural or man-made channels or swales must be able to convey the post-development 25-year return period runoff without creating any hazard to persons or property.

C. Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP Chapter 105 regulations [2] (if applicable) and, at a minimum, pass the post-development 25-year return period runoff.

[2] Editor's Note: See 25 Pa. Code § 105.1 et seq.

D. It must be demonstrated that the downstream conveyance channel, other stormwater facilities, roadways, or overland areas are capable of safely conveying the 100-year design storm without causing damage to buildings or other infrastructure.

E. Where the downstream conveyance channel or other facility is located within a special flood hazard area as documented on the Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Maps for Ferguson Township, it must be demonstrated that the limits of said flood hazard area are not increased by the proposed activity.

F. Stormwater management ponds that fall under the DEP Chapter 105 criteria of a "dam" must meet the criteria within Chapter 105.

§ 26-304 Calculation Methodologies.

[Ord. No. 1102, 9/3/2024]

1. Design criteria and calculation methodologies have been classified by functional group for presentation as follows: 1) peak runoff rate discharge requirements; 2) stormwater pond capture volumes; 3) recharge volumes; 4) storm drain design, including conveyance, channel protection, and stability; 5) water quality standards; and 6) infiltration test requirements.

2. These criteria and calculation methodologies have been developed to simplify stormwater management designs, unify methods, remove model parameter subjectivity, remove improperly used methods, and to ensure that stormwater management decisions are based more realistically on hydrologic processes. In addition, common sense should always be used as a controlling criteria.

3. These standards provide consistent and process oriented design procedures for application by land development professionals. It is recognized that in an attempt to generalize the computational procedures, assumptions have been made which on some occasions may be violated. If such a violation is identified, alternate standards and procedures may be applied. Both the violation and the alternate procedures to be applied must be documented by a hydrologist or hydrogeologist. Any request for use of alternate standards or procedures under this provision must be agreed to by the Township Stormwater Engineer prior to formal submission of plans for consideration by the Township.

A. Peak Runoff Rate Control.

(1) Exemptions.

(a) Any site where the increase in post-development peak runoff rates is determined to be negligible by the Township Stormwater Engineer is exempt from the requirement to provide stormwater detention. In support of this exemption, it must be shown that the downstream conveyance systems have adequate capacity to convey the additional discharge without adversely affecting downstream properties. This does not exempt the requirement for implementation of designs for water quality, stormwater conveyance, and/or recharge as required. A stormwater management site plan and report documenting these design elements is also required. The Township Stormwater Engineer shall use a 5% increase as a general benchmark for defining "negligible." The final definition of "negligible" shall be at the Township Stormwater Engineer's discretion. Prior to using this exemption (and prior to any land development plan submission), the design engineer must provide written documentation and computations as to why no peak runoff control should be required. The Township Stormwater Engineer has the right to reject any plan that uses this assumption without prior approval of the Township Stormwater Engineer. The intent of this exemption is to eliminate the need for multiple or "piggyback" detention facilities as a result of minor changes in imperviousness or land use upstream of existing stormwater control facilities.

(b) Small sites (less than one acre) located directly adjacent to the main stem of creeks, or within the floodplain, are not required to provide stormwater detention unless directed to do so by the Township Stormwater Engineer as a result of a documented drainage problem. All other stormwater management standards must be implemented, including water quality, adequate stormwater conveyance, and/or recharge as required. The Township Stormwater Engineer has the right to reject any plan that uses this exemption without prior approval of the Township Stormwater Engineer.

(2) Stormwater management analysis must be performed using the following models. The size criteria are based on drainage area size, including site area and all off-site area draining across the development.

Drainage Area Size (acres)	Model
Up to 100	NRCS's TR-55 or TR-20
Over 100	NRCS's TR-20 or HEC-1 (HEC-HMS)

(a) The Modified Rational Method using the Gert Aron Curves may be used for any site less than or equal to two acres in size without prior authorization from the Township Stormwater Engineer. The Modified Rational Method may also be used for sites between two and five acres in size where the Township Stormwater Engineer has approved the method's use. In this case the design engineer must make a written request to the Township Stormwater Engineer explaining why the use of the Modified Rational Method is more appropriate than the NRCS's (Natural Resource Conservation Service's) methods for the site in question. The design engineer should keep in mind that the Modified Rational Formula methodology was not calibrated to account for the karst nature of the Spring Creek Drainage Basin, and therefore, its use should be limited to the special cases identified above. In addition, since the minimum discharge criteria are based on a calibration of the NRCS runoff model, their use is not appropriate if the Modified Rational Method is used for runoff computations.

(b) The Township Stormwater Engineer has the right to reject any stormwater management design that uses hydrograph combinations with the Modified Rational Method where the designer has not validated that the effects of the timing differences are negligible. In addition, the Township Stormwater Engineer has the right to reject any stormwater management design that improperly uses the method for determining runoff volumes or does not properly apply the method.

(c) More intensive physically based models may be used at the discretion of the Township Stormwater Engineer, but only for sites greater than 100 acres in size.

(d) Commercial software packages that use the basic computational methods of TR-55 or TR-20 are permitted.

(e) The NRCS models and methods recommended above are based on data collected from actual watersheds. In contrast to this, stormwater management analysis for land development activities is often conducted using property lines to define drainage boundaries. Drainage areas based on property boundaries are not true watersheds and are referred to here as "hypothetical" drainage areas. It is known that these hypothetical drainage areas do not respond like natural watersheds. Peak runoff rates from hypothetical drainage areas are much smaller than comparable runoff rates from natural watersheds of the same size. Therefore, wherever possible, pre- and post-development stormwater analysis should be conducted for watersheds that are as nearly natural as possible. Also, conducting stormwater analysis for a lot-by-lot comparison, such as within residential developments, is not permitted. Partitioning drainage areas into different subwatersheds for the post-development scenarios is acceptable.

(f) It is noted that natural watershed boundaries should not be used where the relative size of the watershed compared to the site size would inappropriately distort the pre- to post-development runoff comparison. In these cases a hypothetical drainage area defined by the property boundary should be used because it will allow for a better estimate of runoff changes directly downstream of the site. In addition, the designer should recognize that, within the Spring Creek Watershed, typical hypothetical drainage areas, in their pre-development or natural condition, do not produce surface runoff during minor to moderate rainfall events. Available hydrologic models do not accurately reflect this condition. This often results in post-development nuisance flooding since the models overestimate the pre-development runoff magnitude.

(3) Major natural drainage divides may not be altered without the prior consent of the Township Stormwater Engineer.

(4) Pre- and post-development stormwater management analysis shall be conducted using the following design storms: 1-year; 2-year; 5-year; 10-year; 25-year; 50-year; 100-year.

(5) The 24-hour precipitation depths as obtained from NOAA Atlas 14 shall be used for stormwater management analysis for all of Ferguson Township. PennDOT IDF curves may be used only if required by PennDOT.

(6) The NRCS's Type II precipitation distribution is required for all stormwater management analyses.

(7) The NRCS's dimensionless unit hydrograph "k" factor shall be 484 for both pre-and post-development stormwater analyses.

(8) All undeveloped areas are to be modeled as meadow or woods in good hydrologic condition. Existing impervious areas may be modeled as being impervious for pre-development conditions. The Township may require a maximum of 20% of the existing impervious be modeled as meadow in areas where there are known existing stormwater concerns downstream of the project area or where the site being developed has either deficient or non-existing stormwater management facilities. Developers of sites with existing impervious are highly encouraged to set up a meeting with the Township prior to design so that any additional requirements are identified prior to plan submission.

(9) The NRCS's curve number (CN) shall be used as the rainfall to runoff transformation parameter for all stormwater management analyses.

(10) Curve numbers should be rounded to tenths for use in pre-packaged hydrologic models. It should be recognized that the CN is only a design tool with a large degree of statistical variability. For large sites, CNs should realistically be rounded to the nearest whole number.

(11) The NRCS's method to determine unconnected impervious area adjustments for CN may be used for distinctly defined impervious land areas that flow onto pervious areas in a dispersed manner. The method may only be used to calculate runoff from site impervious areas that actually flow across pervious areas. The method cannot be applied to the entire site using average weighted CN values.

(12) Soils underlain by carbonate geology (limestone or dolomite) shall have a hydrologic soil group (HSG) B used for both pre- and post-development conditions regardless of the NRCS or Centre County Soil Survey's description, except for the following two conditions:

(a) With the exception of minimally disturbed areas, the HSG for all disturbed pervious areas shall be reduced by a minimum of one HSG level to reflect an increase in runoff associated with the loss of natural soil structure. No reduction in post-construction HSG level is required in areas where soil restoration techniques are applied. Acceptable soil restoration techniques include those defined in the current version of the Pennsylvania Stormwater Best Management Practices Manual. Other techniques may be accepted if approved by the Township Stormwater Engineer.

(b) Soils identified as "on floodplains" or "on terraces above floodplains" in the Centre County Soil Survey will use the HSG as designated in the Soil Survey. Refer to Appendix A for a list of the soils.[1]

[1] Editor's Note: Said appendix is included as an attachment to this chapter.

(13) Soils not underlain by carbonate geology shall use the HSG as specified by the NRCS or Centre County Soil Survey's description, except for the following two conditions:

(a) Wooded areas on HSG C and D soils shall be treated as HSG B for pre-development conditions. Disturbed post-development wooded areas shall carry the NRCS or Centre County Soil Survey's defined HSG with a minimum HSG of B.

(b) Highly compacted structural fill areas shall use a minimum of HSG C for post-development conditions regardless of the NRCS or Centre County Soil Survey's description. For most developments these areas are normally covered with impervious surfaces, but may include islands within parking areas, fringe land, etc. A HSG of C shall also be used for large projects that clear and grade land for later phases of development. The Township Stormwater Engineer shall make the final determination as to what areas of a land development site constitute compacted structural fill. The intent is to account for large compacted areas, and not minor grading within lawn areas or small areas around buildings, etc.

(14) Areas draining to closed depressions must be modeled by removing the storage volume from the pre-development condition. The designer may assume that infiltration in the closed depression does not occur during a design runoff event. Areas draining to closed depressions may also be used to adjust peak runoff rates to stormwater management ponds for the post-development analysis. This allowance has been developed to entice designers to intentionally design or leave in place small closed depressions that can reduce the total volume required from a stormwater management pond. The site designer is responsible to document downstream impacts if the closed depression were removed.

(15) Drainage areas tributary to sinkholes shall be excluded from the modeled point-of-interest drainage areas defining pre-development peak flows. Assumptions that sinkholes spill over during some storm events must be supported by acceptable documentation as determined by the Township Stormwater Engineer. In addition, the design professional must be aware that bypassing or sealing sinkholes will frequently result in downstream flooding and should not be done if existing downstream flooding already occurs. The site designer is responsible to document downstream impacts if the sinkhole were to stop taking stormwater runoff.

(16) Ponds or other permanent pools of water are to be modeled by the methods established in the NRCS's TR-55 Manual (1986). However, more rigorous documented methods are acceptable as determined by the Township Stormwater Engineer.

(17) The NRCS antecedent runoff condition II (ARC II, previously AMC II) must be used for all simulations. The use of continuous simulation models that vary the ARC are not permitted for stormwater management purposes. In addition, prior to any continuous simulation model being used in the Spring Creek Basin for any other purposes, the model unit hydrograph must be modified for common events in additional to extreme events based on an in-depth analysis of historical data from the basin.

(18) Time of Concentration.

(a) The following time of concentration (Tc) computational methodologies shall be used unless another method is pre-approved by the Township Stormwater Engineer:

1) Pre-development: NRCS's Lag Equation.

2) Post-development; commercial, industrial, or other areas with large impervious areas (greater than 20% impervious area): NRCS's Segmental Method.

3) Post-development; residential, cluster, or other low impact designs less than or equal to 20% impervious area: NCRS's Lag Equation.

(b) The time of concentration is to represent the average condition that best reflects the hydrologic response of the area. For example, large impervious areas bordered by small pervious areas may not consider the effect of the pervious areas in the Tc computation. If the designer wants to consider the effect of the pervious area, runoff from the pervious and impervious areas must be computed separately with the hydrographs being combined to determine the total runoff from the area.

(c) Under no circumstance will the post-development Tc be greater than the pre-development Tc for any watershed or sub-watershed modeling purposes. This includes when the designer has specifically used swales to reduce flow velocities. In the event that the designer believes that the post-development Tc is greater, it will still be set by default equal to the pre-development Tc for modeling purposes.

1) Refer to Subsection 3A(28) regarding impervious area flashing (IAF).

(19) Post-Development Minimum Discharges.

(a) The following post-development minimum discharges are permitted for use with the NRCS (CN) runoff model:

Return Period	Minimum Discharge
1-year	Qpmin = 0.018 (DA) + 0.2
2-year	Qpmin = 0.03 (DA) + 0.4
5-year	Qpmin = 0.054 (DA) + 0.6
10-year	Qpmin = 0.09 (DA) + 1.0

Where:
DA	=	the drainage area in acres
Qpmin	=	minimum allowable peak runoff rate in cfs

(b) For return periods greater than 10 years, the minimum discharge shall be equal to the computed pre-development peak runoff rate.

(c) The minimum discharge criteria above are not appropriate for use with the Modified Rational Method. This is because these values were developed based on NRCS model corrections and do not actually represent a true physical process or discharge. However, common sense should be used by both the designer and reviewer in the evaluation of acceptable minimum discharges for use with the Modified Rational Method.

(d) The intent of the minimum discharge is to allow reasonable runoff release from a site when a hydrologic model has produced a pre-development runoff rate close to zero. The method is NOT permitted for areas that previously drained completely to sinkholes in order to bypass the sinkhole after development.

(e) These minimum discharge values include the total of all stormwater management facilities discharges and undetained area discharges. Peak runoff rates for undetained fringe areas (where the designer has made a realistic effort to control all new impervious areas) will be computed using the pre-development time of concentration for the drainage areas tributary to them. Undetained areas are those portions of the site that cannot be routed to a stormwater management facility due to topography and typically include lower pond berms, or small areas around entrance drives. The site drainage areas used shall represent the pre-development condition, even if drainage areas are altered following development.

(20) All lined stormwater management ponds in carbonate and non-carbonate areas must be considered impervious and may not be used as pervious areas for stormwater management computations. "Lined" here means lined with synthetic liners or bentonite. All other compacted soil liners will be considered to be HSG D for hydrologic computations.

(21) Stormwater management ponds that have a capture depth for the purposes of water quality or volume capture shall assume a negligible discharge from these structures during design event routing. Only discharges from the primary principal spillway or emergency spillway need to be considered. Discharges from subsurface drains that tie into a principal spillway should not be considered during design event routing. All subsurface drains are to be equipped with a restrictor plate with a 1" opening in order to prevent the subsurface drain from functioning as a primary orifice. A restrictor plate orifice may be increase if necessary to the smallest size required to drain the facility within the required 72 hours.

(22) Stormwater management ponds that have a pond capture, recharge, or water quality component shall assume that the basin is full to the controlling component volume at the beginning of design event routing.

(23) Stormwater management ponds must provide safe passage of the 100-year return period peak runoff rate assuming that all of the principal spillway orifices are fully clogged, and the principal spillway overflow is 50% clogged. A minimum of a 6-inch freeboard must also be maintained above the resulting maximum water surface elevations (W.S.E.). Any embankment emergency spillway may be assumed to be unclogged. Stormwater management ponds with embankments completely made up of natural undisturbed soils (fully in "cut") or where roadways act as the emergency spillway are permitted. However, the design engineer must verify downstream stability and control.

(24) All pre- and post-development comparisons of peak flows shall be rounded to tenths of a cfs. The intent here is to recognize the accuracy and precision limitations of hydrologic modeling procedures. Again, small differences between pre- and post-development discharge rates should be permitted when no negative downstream impacts will result.

(25) The full Modified Puls routing method must be used for stormwater management pond analyses. Simplified methods of determining pond size requirements such as those in TR-55 (1986) may only be used for preliminary pond size estimates. The full Modified Puls routing method must be used for stormwater management pond analyses.

(26) Pre-packaged hydraulic programs are not approved for the analysis of underground stormwater management facilities unless it can be verified that the program rounding subroutines used for the stage/storage data do not affect the results. This is because, for very small storage volumes, the program may round off the volume to a significant percentage.

(27) Full supporting documentation must be provided for all stormwater management designs.

(28) Designs must be checked for impervious area flash (IAF). This check is used to determine if flooding may occur due to poor modeling choices specifically related to the time of concentration. This analysis requires that the watershed impervious area be modeled without the pervious areas. The time of concentration should also be determined from the impervious areas only. If the IAF analysis results in a higher peak runoff rate at a culvert or discharge from a pond, this higher rate must be used for the final design/comparison. The check will frequently yield higher values if a watershed's impervious area is located primarily near the watershed outlet or point of interest.

B. Storm Drain Conveyance System Design. Storm drainage conveyance systems consist of storm sewer pipes, swales, and open channels. Computational methods for design of storm drain conveyance systems shall be as follows:

(1) Recommended computational methods (models) for storm drain design are based on site or watershed drainage area as follows:

Size	Model
Up to 200 acres	Rational Method
Between 200 acres and 1.5 square miles	HEC-1
	PSRM
	TR-20
Over 1.5 square miles	PSU-IV with the carbonate adjustment factor at the discretion of the Township Stormwater Engineer
	Other methods as approved by the Township Stormwater Engineer such as SWMM, SWIRM-ROUTE, etc.

(2) Rational Coefficients.

(a) Rational Coefficients used are to be from Rawls et al. (1981), PA DOT Design Manual 2-10 or using the Aron curves (Appendix A, Table 3[2]) to convert CNs to C. For previously disturbed or newly disturbed pervious areas, runoff coefficients should be appropriately adjusted to reflect an increase in runoff associated with the loss of natural soil structure. If the Aron curves are used, CNs must also be adjusted to reflect the loss of soil structure in disturbed areas by reducing the HSG as identified by the NRCS by a minimum of one HSG level. For example, if the natural HSG designation is B, an HSG of C should be used for disturbed soils. These disturbed area adjustments to HSG levels do not apply to minimally disturbed areas.

[2] Editor's Note: Said appendix is included as an attachment to this chapter.

(b) The Design Engineer may choose to use the following Rational C coefficients without regard to soil HSG for small sites. However, it is recommended that they be used only for storm drains up to 24" in diameter. The use of these conservative values shall fully be the choice of the design engineer.

All impervious areas: C = 0.95

All pervious areas: C = 0.30

(3) Storm drains shall be designed at a minimum using a 10-year runoff event without surcharging inlets. Storm drains tributary to a multiple-site stormwater management facility across Township roads or crossing other properties must convey, at a minimum, a 25-year runoff event without surcharging inlets. Runoff events in excess of the indicated design event must be conveyed safely downstream.

(4) Inlets on grade cannot assume a sumped condition for hydraulic modeling (i.e., top of inlet casting set below pavement surface in parking areas).

(5) The Township Stormwater Engineer may require the analysis of the 100-year peak runoff rates for conveyance purposes in some instances where regional stormwater management facilities are employed.

(6) Any storm drain within state or federal rights-of-ways or one that falls under the design criteria of any higher authority must meet the requirements of that agency in addition to the minimum requirements of this chapter.

(7) The time of concentration (Tc) may be computed by any method which best represents the subject watershed. However, the NRCS's segmental method is not recommended for use with drainage areas that are predominately undeveloped and are greater than 100 acres in size. The NRCS Lag Equation or another more appropriate method should be used under these conditions.

(8) For any drainage area smaller than five acres in size, a Tc of five minutes may always be assumed at the discretion of the design engineer (for the post-development condition), without needing to provide supporting documentation.

(9) Precipitation values shall be taken from the current version of NOAA Atlas 14 at a station closest to the project site and/or most representative of the project site. The current version of the PennDOT IDF curves can be used only if required by PennDOT. Appropriate documentation supporting the selected precipitation values used shall be provided.

(10) Storm drain conveyance system stability (swales, open channels, and pipe discharge aprons) shall be computed using a 10-year return period peak runoff rate.

(11) Storm sewers, where required by zoning and land use densities, shall be placed under or immediately adjacent to the roadway side of the curb, or as directed by the Township, when parallel to the street within the right-of-way.

(12) When located in undedicated land, storm sewers shall be placed within a drainage easement not less than 20 feet wide as approved by the Township Stormwater Engineer.

(13) Drainage Swales.

(a) The use of properly designed, graded and turfed drainage swales is encouraged in lieu of storm sewers in commercial and industrial areas and, where approved by the Township Stormwater Engineer, in residential areas.

(b) Such swales shall be designed not only to carry the required discharge without excessive erosion, but also to increase the time of concentration, reduce the peak discharge and velocity, and permit the water to percolate into the soil, where appropriate.

(14) Inlets.

(a) Inlet types and inlet assemblies shall conform to the Pennsylvania Department of Transportation Standards for Roadway Construction as approved by the Township Stormwater Engineer.

1) Inlets shall, at a minimum, be located at the lowest point of street intersections to intercept the stormwater before it reaches a pedestrian crossing; or at sag points of vertical curves in the street alignment which provide a natural point of ponding of surface stormwater.

2) Where the Township deems it necessary because of special land requirements, special inlets may be approved.

3) The interval between inlets collecting stormwater runoff shall be determined in accordance with DM-2, Chapter 10, Section 3, "Capacity of Roadway Hydraulic Facilities." Flank inlets in low areas may only be required on higher order streets at the discretion of the Township Stormwater Engineer.

(b) In curbed sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through traffic lane or one inch less than the depth of curb during the 10-year design storm of 5-minute duration. Inlets shall be provided to control the encroachment of water on the pavement. When inlets are used in a storm system within the right-of-way limits of a street in lieu of manholes, the spacing of such inlets shall not exceed the maximum distance of 450 feet.

(15) Accessible drainage structures shall be located on a 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 influent storm sewer mains. The construction locations of accessible drainage structures shall be as indicated on the land development stormwater management plan or area stormwater management plan approved by the Township.

(16) When evidence available to the Township indicates that existing storm sewers have sufficient capacity as determined by hydrograph summation and are accessible, the subdivider may connect its stormwater facilities to the existing storm sewers so long as the peak rate of discharge does not exceed the amount permitted by this chapter.

(17) All storm drain culvert pipes shall be designed to maintain a minimum grade of 1/2%. All storm pipes shall have a minimum inside diameter of 15 inches or a cross-sectional area of 176 square inches, except that pipes under 20 feet or greater fill shall not be less than 24 inches or a cross-sectional area of 453 inches and shall consist of reinforced concrete.

(18) Computational procedures other than those indicated here should follow the methods of the Federal Highway Administration's Urban Drainage Design Manual [Hydraulic Engineering Circular No 22 (HEC-22)].

(19) Swales provided in cut areas shall be designed such that stormwater will not encroach upon the shoulder during a 25-year-frequency storm. Flooding of the subbase shall be avoided.

C. Water Quality Requirements. Disturbed area size shall be used to determine applicability of water quality volumes. For sites less than one acre, the requirements described in Subsections 3D (Capture Volume), 3E (Recharge Volumes), and3F(2) (Water Quality Volume) shall apply. Sites one acre or greater shall meet the larger of the cumulative volume requirements set out by Subsections 3D (Capture Volume), 3E (Recharge Volumes), and3F(2) (Water Quality Volume) or Subsection 3G [Design Storm Method (CG-1)].

D. Capture Volumes (Cv).

(1) To minimize nuisance flooding from small precipitation events, a runoff capture volume is required for all stormwater management ponds that do not discharge directly to natural, well-defined (with bed and banks) perennial streams. In general, natural well-defined streams in the Spring Creek Basin and all of Ferguson Township are limited to those delineated as USGS (U.S. Geological Survey) perennial streams. This should be treated as a guideline and not a steadfast rule. The final determination is at the discretion of the Township Stormwater Engineer. The pond capture volume is a volume of runoff that will be retained in a pond below the elevation of any free surface principal spillway orifice. In addition to the minimum required capture volume identified within this section, the capture volume may also include all or portions of the recharge volume and water quality volume (see Subsection 3E and 3F(2) below). No principal spillway orifice (except those connected to subsurface drains), regardless of how small, shall be below the pond elevation equivalent to this volume.

(2) The Centre County Conservation District (CCCD) receives numerous complaints regarding ponds that are located at the downslope edge of a property that result in discharging runoff onto downstream properties in an uncontrolled manner or where no existing defined outlet channel exists. This is a very common problem in areas underlain by carbonate rock. These discharges can cause erosion and flooding downstream. While the pond capture volume is intended to minimize some of these negative effects, it cannot deter or reduce the impacts from poor design practices. Therefore, whenever possible, the CCCD recommends that the designer consider the downstream morphological changes that may occur and, when possible, consider constructing conveyance systems to a stable natural channel. In some cases this may require cooperation between landowners.

(3) A minimum capture volume is required to mitigate nuisance flooding from small precipitation events. The minimum required capture volume is defined as a runoff depth of 0.25 inches from all impervious areas tributary to the stormwater management facility. This volume will be allowed to infiltrate, evaporate, or dewater from a subsurface drain system connected directly to the facility's principal spillway.

(4) The maximum capture depth shall be 24 inches.[3] The Township Stormwater Engineer reserves the right to require that the capture and peak rate control be managed in separate facilities in cases where potential sinkhole damage could cause impacts to public facilities and/or off-site areas. Special consideration with regard to allowable capture depth and dewatering time shall be given to stormwater designs relying on total infiltration, or where capture of the 10-year event or larger is proposed to be infiltrated due to site constraints (onsite or offsite). In these cases a pre-application meeting shall be held with the Township Stormwater Engineer to identify an appropriate design capture depth and dewatering time. Any increase in capture depth above 24 inches must be supported by a detailed geotechnical report in support of the increased capture depth.

[3] Editor's Note: Said appendix is included as an attachment to this chapter.

(5) Supporting computations that show that 90% of the capture volume can dewater in a maximum of 72 hours must be provided. The dewatering time must also provide for sufficient passive water quality treatment. Infiltration rates are to be maintained at less than six inches per hour unless it is demonstrated that the soils cation exchange rate is greater than 5 meg/100 g in which case the maximum infiltration rate shall be 10 inches per hour. Basins outfitted with underdrains for dewatering shall incorporate an orifice at the underdrain outlet to maintain dewatering times as specified above.

(6) To simplify computational requirements for design event analysis, designers do not need to calculate discharges from subsurface drains related to the capture volumes if the filter media is sand, or smaller than AASHTO (American Association of State Highway and Transportation Officials) 57 stone. The capture volume is to control runoff rates from impervious areas and is not related to water quality. However, pond designs that include a water quality volume that is greater than the required capture volume are assumed to have also met the required capture volume as long as it dewaters as required.

(7) Designs that rely on the natural infiltration of in situ soils must provide documentation supporting the infiltration rates used for analysis. Infiltration rates reported in the Soil Survey of Centre County or other published rates may be used at the discretion of the Township Stormwater Engineer.

(8) The pond capture volume should always be used when up-slope areas are developed where the pond's design creates a point discharge that did not previously exist.

(9) Stormwater management detention facilities that connect directly to storm drainpipe networks discharging to natural well-defined channels do not require a capture volume.

E. Recharge Volumes (Rv).

(1) The purpose of the recharge portion of the Part is two-fold. First, the recharge requirement is to mitigate the loss of groundwater recharge associated with the creation of impervious surfaces. In addition, the recharge criterion is to mitigate the increase in runoff volume associated with the creation of impervious surfaces. This increase in runoff volume has significant impacts on downstream landowners. These impacts are most often exhibited in the form of increased nuisance flooding and channel or drainageway erosion and instability. According to local Municipal Engineers and representatives of the CCCD, these problems are of significant local concern. The magnitude of these problems increases with the percentage of impervious coverage created on a site.

(2) Recharge mitigation shall be provided for runoff from all proposed impervious areas. The required recharge volume shall be computed as 0.5" of runoff from all proposed impervious areas. It is noted that lined detention ponds and compacted fill areas are considered to be impervious when calculating site impervious area for recharge considerations. In addition, land areas covered by paver blocks, pervious pavement, and other structural surface treatments which permit surface infiltration may be treated as pervious areas when calculating the site impervious area for recharge considerations as long as the structural infiltration practice is supported by sound design and appropriate construction specifications. The Township Stormwater Engineer may require submission of supporting documentation prior to approving structural infiltration areas as pervious areas. It must also be demonstrated that the runoff from the area directly tributary to recharge BMPs will satisfy the recharge volume requirement during the 1-year, 24-hour storm.

(3) The following design practices may be used as credits to reduce the recharge volume requirement:

(a) Residential roof areas (detached, duplex, and townhouse dwellings) and commercial/industrial buildings with roof areas less than 5,000 square feet may be removed from the computed impervious area when these roof areas are sumped to dry wells designed in accordance with the following minimum standard:

1) Sump Design Criteria: To meet the recharge criteria, sump storage or voids volume shall be equal to 0.04 cubic feet per square foot of roof area (0.5-inch rainfall depth). If sump stone has a voids ratio of 40%, the total sump volume will be 0.10 cubic feet per square foot of roof area. When designed only to meet this recharge criteria, the maximum size for a single sump is 100 cubic feet, and the minimum sump surface area (A) to depth (D) ratio (A/D) must be a minimum of 4/1. The sump depth less any freeboard shall not exceed 24 inches. This roof sump standard shall apply unless the Township adopts a separate roof sump standard for water quantity or peak control.

(b) 

All or portions of driveways, roadways, and parking areas may be removed from the impervious area calculation when sheet flow from these areas is directed to undisturbed natural buffer/filter areas or constructed filter strips. This flow must be dispersed as sheet flow as it crosses the buffer/filter area. Sheet flow velocities should be non-erosive as they cross the impervious area/filter interface.

1) To ensure proper infiltration characteristics, the natural soil profile within natural buffer/filter areas cannot be disturbed during construction. Completely undisturbed natural recharge areas serve this function best. However, minor surface scarring, seeding, and landscaping is permitted in these areas as long as natural grades are not altered. In special cases, when approved by the Township Stormwater Engineer, minor grading, combined with soil profile reconstruction, may be permitted in natural buffer/filter areas. In addition, the following standards apply to natural filter/buffer areas:

a) Natural filter/buffer areas must have a minimum width of five feet or one-half of the impervious area drainage length immediately tributary to the buffer area, whichever is greater. This width is measured parallel to the direction of sheet flow.

b) To qualify for a recharge volume credit, the surface slope of natural filter/buffer areas must be conducive to recharge, and not result in flow concentration or erosion. To meet this intent, the surface slope of the area tributary to the natural buffer/filter area, and the surface slope of the natural buffer/filter area itself, may not exceed 5%. In special cases steeper slopes may be used if specifically authorized by the Township Stormwater Engineer.

c) The total impervious area tributary to a natural buffer/filter area cannot exceed twice the buffer/filter area.

2) To qualify for a recharge volume credit, constructed filter strips shall be designed to the following standards:

a) The minimum filter strip width shall be five feet or one-half of the impervious area drainage length immediately tributary to the constructed filter strip, whichever is greater. This width is measured parallel to the direction of sheet flow.

b) The total impervious area tributary to a constructed filter strip area cannot exceed twice the constructed filter strip area.

c) The surface slope of the area tributary to the constructed filter strip area, and the surface slope of the constructed filter strip area itself, may not exceed 5% and 3%, respectively. In special cases steeper slopes may be used if specifically authorized by the Township Stormwater Engineer.

d) The filter strip surface shall consist of a minimum of six inches of natural or reconstructed topsoil with a stable grass surface treatment. Reconstructed topsoil designs must be approved by the Township Stormwater Engineer prior to application. Reconstructed topsoil consists of soils augmented by tillage and the addition of soil amendments such as fertilizers and organic material. Fertilizers include any organic or inorganic plant nutrients or soil amendments. Organic material is any nutrient or compound containing animal or plant materials such as manure or compost.

e) To minimize erosion of the topsoil layer during construction, it is recommended that these areas be sodded. However, the Township Stormwater Engineer may permit the use of an acceptable erosion control seeding application. In this later case, any loss of topsoil and seed must be replaced until a permanent vegetative stand is achieved.

(c) Sidewalks separated from roadways and/or other impervious surfaces by a grass strip of equal or greater width than the sidewalk itself may be removed from the impervious area calculation when the sidewalks are graded so that sheet flow from the walk is directed to the grass strip. Sidewalks with steep longitudinal slopes that themselves would act as channels during runoff events cannot take advantage of this credit. A 5% longitudinal sidewalk slope shall be used as the benchmark defining steep slopes.

(d) Impervious areas tributary to natural closed depressions may be subtracted from the total site impervious area used in the recharge volume calculation as long as a qualified geotechnical engineer or soil scientist certifies to the soundness of these site-specific applications. Water quality pre-treatment may be necessary prior to the direct discharge of runoff to existing closed depressions or sinkholes.

(e) Impervious areas tributary to man-made closed depressions may be subtracted from the total site impervious area as long as a qualified geotechnical engineer or soil scientist certifies to the soundness of these site-specific applications. Man-made closed depressions may be created through the use of low head berms one foot or less in height.

(f) Additional credits may apply for undisturbed land areas that are known to have high infiltration capacity and that are maintained or enhanced. These areas must be defined and quantified from actual site data collection.

(4) After credits, the remaining recharge volume shall be directed to a recharge BMP such as infiltration trenches, beds, etc. These facilities may be located in open areas or under pavement structures. The appropriateness of the particular infiltration practice proposed, as well as the design parameters used, shall be supported by a geotechnical report certified by a qualified professional (soil scientist, geologist, hydrogeologist, geotechnical engineer).

(5) Stormwater recharge requirements or credits affect stormwater management design requirements. For stormwater management computations, the reduction of site CNs based only on a weighting type analysis, as is sometimes done for cluster type developments, is not permitted. However, for stormwater management purposes, the CN for recharged areas may be computed using the NRCS method for disconnected impervious areas. The actual hydrologic process that occurs within the basin must be stressed in all recharge situations.

(6) These recharge requirements must be met on all sites unless it can be demonstrated that recharge would be inappropriate. Any request for such a waiver from these recharge requirements must be accompanied by a supporting report certified by a qualified professional (soil scientist, geologist, hydrogeologist, geotechnical engineer).

(7) Developers and site design professionals are encouraged to use a higher standard for recharge volume on sites where local site conditions do not restrict a higher standard.

(8) Water quality sensitive (WQS) developments must use an acceptable pre-treatment BMP prior to recharge. Acceptable pre-treated BMPs for these developments include BMPs that are based on filtering, settling, or chemical reaction processes such as chemical coagulation. If infiltration devices, including subsurface exfiltration devices, are to be utilized as a part of the SWM design for a development in the WQS district, the infiltration rate should be a maximum 6 in./hr. to allow proper soil filtration/passive treatment of the runoff. A maximum infiltration rate of 10 in./hr. may be used for soils with demonstrated cation exchange rates greater than five. This infiltration rate may be achieved through soil amendments or engineered soil media.

(9) Accounting for recharge within lined stormwater management ponds is not permitted. However, if unlined, uncompacted ponds and/or depressed lawn areas are used to satisfy water quality or capture volume criteria, these areas and volumes may also be used to meet recharge requirements as previously defined. Additional recharge volume may be credited to these areas as long as it is demonstrated by a qualified professional that recharge processes can naturally occur in these areas.

(10) Finally, because this analysis is concerned with trying to adequately represent real processes that occur within the watershed, there will be areas that cannot physically recharge stormwater. These areas include exfiltration areas that are commonly found at the base of wooded hillsides where clay pans exist, and saturation areas near major streams or floodplains. These areas may not accept recharge during most runoff events. These areas are exempt from recharge requirements when these conditions are documented and certified by a qualified professional (soil scientist, geologist, hydrogeologist, or geotechnical engineer). In addition, stormwater management techniques relying on infiltration techniques are not permitted in these areas.

(11) The Township Stormwater Engineer may waive the recharge requirement in the following situations:

(a) The Township Stormwater Engineer may waive the recharge requirement in highly developed areas or areas undergoing redevelopment where the Township Stormwater Engineer has determined that forced recharge could have adverse impacts on adjacent landowner structures, property, or Township infrastructure. These waivers should be limited to small land areas (generally less than five acres in size), where the ability to place recharge beds may be limited or may hinder redevelopment.

(b) The Township Stormwater Engineer may waive the recharge requirement in areas where a qualified soils scientist or geologist has determined that none of the site soils are suitable for recharge, or that the location of the suitable soils is such that harm to adjoining properties could occur as stated under Subsection 3E(11)(a) above.

(c) The Township Stormwater Engineer may waive the recharge requirement in areas where recharge cannot physically occur as documented by a qualified soil scientist, geologist, or hydrologist. These areas include:

1) Exfiltration areas commonly found at the base of wooded hillsides where clay pans or fragipans exist; and

2) Saturation areas near major streams or floodplains.

(12) As identified above, recharge analysis and/or waiver requests must be supported by a geotechnical report sealed by a qualified professional (soil scientist, geologist, hydrogeologist, or geotechnical engineer). The intent of this report will be to establish the suitability of a particular parcel of land or area for recharge, and to identify areas on a development site appropriate for recharge. It is recommended that the geotechnical/soils consultant discuss the extent and approach to the analysis with the Township Stormwater Engineer prior to initiating the field investigation. At a minimum, this report should include the following information:

(a) A description of the geotechnical site investigation performed, including the methods and procedures used;

(b) Data presentation;

(c) Analysis results, including the following minimum information:

1) A map identifying site areas inappropriate for recharge along with supporting justification. In addition to illustrating topographic features, significant geologic and hydrologic features should be identified (rock outcrops, sinkholes, closed depressions, etc).

2) Determination of the permeability coefficient for potential recharge areas.

3) Determination of the infiltration capacity of natural site soils.

4) Location, depth, and permeability coefficient for any restrictive layers identified.

5) Soil uniformity.

6) Depth to bedrock in potential recharge areas, and a statement reflecting the uniformity of the depth to bedrock across the site.

7) A statement relating to the site's proximity to fracture zones within the bedrock.

8) Additional information deemed pertinent by the geotechnical engineer.

9) 

In sensitive area districts, projects shall also comply with the following requirements for a preliminary geologic investigation:

a) A qualified professional (soil scientist, geologist, hydrogeologist, or geotechnical engineer) shall review aerial photographs, soils, geologic and other available related data, including the local public water supplier's Source Water Protection Study, as the data relates to the subject property. The qualified professional shall also conduct a comprehensive field view and inspection of the property, and consult with the public water supplier's consulting hydrogeologist. Based on the above information, the professional shall prepare a map of the site indicating the following:

i. Closed depressions.

ii. Open sinkholes.

iii. Outcrops of bedrock.

iv. Surface drainage into ground.

v. "Ghost lakes" after rainfall.

vi. Lineaments, faults and fracture traces.

vii. Limonite excavations and quarries.

b) Based on the map prepared above, and in conjunction with other mapping and available information, the geologist shall prepare a report on project suitability and the impact potential on the groundwater resource of the water quality sensitive areas. The report shall demonstrate compliance with the requirements of this chapter, including any recommended mitigating measures designed to ensure compliance.

c) In addition, infiltration facilities shall be located greater than 50 feet from any critical geologic features identified in the geotechnical report or geophysical testing report unless the reports provide for approved mitigating measures that will produce no negative impact on groundwater recharge or quality.

10) If the preliminary geologic investigation identifies a significant potential for sinkhole formation, or where soil and subsurface conditions may not be suitable, additional geologic investigations may be determined to be necessary. This determination will be at the discretion of the Township Stormwater Engineer. The additional investigations and analysis may include electrical resistivity tomography or other similar types of geophysical testing. A detailed report of the geophysical testing along with recommendations for avoidance of sinkhole-prone areas shall be included in the stormwater management site plan.

(d) Recommendations for any special design considerations necessary for the design of recharge systems on the site. For example, required soil depth over bedrock, appropriate surface grades over recharge areas, appropriate hydraulic head over recharge areas, alternative siting, increased use of decentralized and non-structural SWM techniques and/or remediation recommendations must be included in the report.

(e) Justification as to why the site should be developed to a high impervious density if the site has adverse soil and geotechnical limitations, which prohibit the ability to induce natural recharge. Explanation of how these limitations will not create the potential for undue harm to the environment and the Spring Creek Watershed or other any area in Ferguson Township when the site is developed.

(f) Where it has been shown that recharge cannot be performed and a waiver of the recharge requirements is being requested, the Township shall require that the first one inch of runoff from all new impervious areas be treated through underdrained facilities. These facilities may include underdrained basins, rain gardens, and infiltration trenches. Treatment is to include use of an amended topsoil to provide filtration of the stormwater. All underdrain outlets are to include a restrictor plate to prevent the underdrain system from functioning as a primary outlet.

(13) The following guidelines apply to the use of subsurface exfiltration BMPs (often incorrectly referred to as engineered infiltration BMPs):

(a) Soils shall have minimum infiltration rates as specified in Subsection 3H. If no site soils have percolation rates meeting these standards, subsurface exfiltration BMPs shall not be used.

(b) A minimum of 30 inches of soil shall be maintained between the bottom of a subsurface exfiltration BMP and the top of bedrock or seasonally high groundwater table (the upper surface of a layer of saturated material in the soil), unless a qualified geotechnical engineer provides documentation supporting a design depth less than 30 inches and the minimum depth is approved by the Township Stormwater Engineer.

(c) If the minimum percolation rate is not met and/or the minimum soil depth cannot be maintained on a site, recharge should be accommodated by directing shallow sheet flow from impervious areas across surface filter strips and/or undisturbed natural areas, or some other innovative surface infiltration feature should be used. Limiting subsurface percolation rates and/or depth to bedrock shall not by themselves warrant a recharge waiver.

(14) In addition, since recharge is intended as a volume control, innovative or new methods that address the significant increase in the volume of runoff from sites having large impervious areas are encouraged. These volume control alternatives may be used only if they can be shown to function with the original intent through sound engineering and science. The final determination of "original intent" shall always be the right of the Township Stormwater Engineer.

F. Water Quality Standards.

(1) Water Quality Performance Standards. To minimize adverse impacts to stream health resulting from stormwater non-point source (NPS) pollution, standards are provided for the implementation of water quality best management practices (BMPs) to reduce NPS pollutant loadings resulting from land development activities. The following performance standards and guidelines shall be addressed at all sites where stormwater management is required.

(a) Site designs shall minimize the generation of stormwater runoff through the use of low-impact design techniques.

(b) Stormwater runoff from all land development activities should be treated through the use of non-structural and structural BMPs to effectively treat the adverse impacts of stormwater runoff, including NPS pollutants.

(c) Water quality BMPs shall be incorporated into site designs to treat the required water quality volume as defined below.

(d) The use of non-structural BMPs shall always take priority over the use of structural BMPs. The use of innovative BMPs and low-impact site planning is encouraged to reduce the generation of stormwater runoff and effectively treat pollutants transported in stormwater from the site.

(e) The use of multiple non-structural water quality techniques along with new, emerging, and innovative techniques is encouraged to improve the quality of stormwater runoff to receiving areas and reduce and/or eliminate the need for structural BMPs. The Township Stormwater Engineer should be consulted to clarify the design concept for meeting or exceeding the intent of this section

(f) Where non-structural BMPs are unable to effectively treat all of the stormwater runoff generated from land development activities, structural BMPs shall be designed to capture and treat the computed water quality volume (WQ_v).

(g) The priority pollutant source areas to be treated with BMPs are streets, parking lots, driveways, and roof areas.

(h) Due to the karst nature of the watershed, stormwater discharges from water quality sensitive developments and discharges to sensitive wellhead protection areas (defined in Appendix B, Exhibit-1[4]) will require special consideration. In these instances, the applicant shall provide water quality pre-treatment (use of a filtering BMP and/or special structural design features) to prevent the discharge of stormwater contaminants to groundwater resources. In addition, preliminary hydrogeologic studies shall be required to document potential karst-related impacts.

[4] Editor's Note: Said appendix is included as an attachment to this chapter.

(i) In water quality sensitive area districts, groundwater quality monitoring wells may be required for development sites greater than 10 acres in size. The need for water quality monitoring wells will be determined on a case-by-case basis by the Township Stormwater Engineer based on the results of the geologic investigations.

(j) In water quality sensitive area districts where rock excavation is anticipated as part of the project construction, the following note shall be added to the plan:

"This project is located within a source water protection area for the (insert name of water provider). Blasting should be used as a last resort method of rock excavation. If blasting techniques are to be employed, the contractor shall provide the Township and the water provider with information supporting the use of blasting techniques along with copies of blasting permits."

(k) Prior to stormwater management and water quality design, applicants should consult with the Township Stormwater Engineer to verify stormwater quality criteria and present proposed features and concepts for the treatment of stormwater runoff. Following this meeting, the Township Stormwater Engineer shall define any needed support studies or documentation.

(2) Water Quality Volume (WQv).

(a) The required water quality volume that must be treated for non-sensitive areas underlain by carbonate rock (see exhibits in Appendix B[5]) within the Township shall be computed as:

WQ_depth = 0.25+(0.012)2.9^(0.044(SIA))

WQ_v = WQ_depth(A)/12

Where:	WQv	=	water quality volume in acre-feet
	WQdepth	=	depth in inches that must be captured for impervious areas
	SIA	=	percent of site impervious area (all paved areas and roof with asphalt-based roofs)
	A	=	total of all paved areas and asphalt-based roofs on site in acres

[5] Editor's Note: Said appendix is included as an attachment to this chapter.

(b) The required water quality volume that must be treated for any WQS development, on sites in sensitive areas underlain by carbonate rock, and all areas not underlain by carbonate rock is to be computed within the entire Spring Creek Basin as:

	WQdepth = the larger of 0.5 inches or 0.25+(0.012)2.90.044(SIA))
	WQv = WQv depth(A)/12
Where:	WQv	=	water quality volume in acre-feet
	WQdepth	=	depth in inches that must be captured for impervious areas
	SIA	=	percent of site impervious area (all paved areas and roof with asphalt-based roofs)
	A	=	total of all paved areas and asphalt-based roofs on site in acres

(c) For designs in which the final roof material is unknown, the design engineer must assume an asphalt-based roof.

(d) The water quality volume must be captured and treated through a water quality BMP over an extended period of time as per the specific requirements of each structure. Credits to reduce the effective impervious area are applicable as presented in this section.

(3) Water Quality Credits. Due to the karst nature of the Spring Creek Basin and areas of Ferguson Township, the non-structural water quality credits and techniques identified below may be limited for suitability and use based on development type and location. These limitations for use are specified in the restrictions section for each credit. The Township Stormwater Engineer may require additional documentation or investigation prior to use of each specific credit to reduce the risks of sinkhole development or groundwater contamination for sensitive areas and development types. No area may be double counted for use with credits. The combined credits of natural area conservation and vegetated filter strips is limited to 50% of the site's impervious area. The drainageway credit is limited to 50% of the site's impervious area. The drainageway protection credit is limited to 50% of the site's impervious area. The maximum total water quality credit for any site may therefore be 100% of the site's impervious area.

Non-Structural Technique	Water Quality Credit
Drainageway Protection (DWP)	Subtract drainageway protection areas from impervious site area in WQv computation
Natural Area Conservation (NAC)	Subtract conserved natural areas from impervious site area in WQv computation
Filter/Buffer Area	Subtract impervious areas discharged over pervious areas from impervious site area in WQv computation

(4) Drainageway Protection.

(a) A water quality credit is given for the protection of natural drainageways on a development site. Natural karst drainageways within the Spring Creek Watershed and Ferguson Township often do not exhibit a defined channel bed and banks. More often, these drainageways appear as wide, shallow parabolic swales. These drainageways are an integral part of the natural drainage system, and often exhibit significant infiltration capacity. Protection of these drainageways is critically important to the health of the watershed.

(b) The drainageway protection (DWP) area is defined as an area centered on the drainageway and having a width equivalent to the 100-year flow spread. The Township Stormwater Engineer may modify the defined minimum width in cases where natural landforms define an appropriate alternate width.

(c) The impervious area used in the WQ_v equation for the development site may be reduced by twice the area of the preserved drainageway (2:1 ratio).

1) Restrictions on the Credit:

2) Drainageway protection areas must remain in an undisturbed condition during and after construction activities. There shall be no construction activity within these areas, including temporary access roads or storage of equipment or materials. Temporary access for the construction of utilities crossing this protection area may be permitted at the Township Stormwater Engineer's discretion. However, the alignment of any such crossing must be perpendicular to the drainageway.

3) These areas should be placed in a conservation easement or be permanently preserved through a similarly enforceable agreement with the Township.

4) The limits of the undisturbed DWP area and conservation easement must be shown on all construction plans.

5) The DWP area must be located on the development site.

6) The maximum total DWPA credit is 100% of the site impervious area.

7) Water quality credits are not permitted for water quality sensitive (WQS) developments.

8) Sensitive Area and Development Restrictions:

9) DWP areas may not be counted as a credit in sensitive areas unless the impervious area actually flows across the area as sheet flow.

10) Untreated urban runoff from sensitive development types may not be directed to DWP areas without pretreatment.

(5) Natural Area Conservation. A water quality credit is given for natural areas that are conserved at the development site, thereby maintaining pre-development water quality characteristics. The impervious area used in the WQ_v equation for the development site may be reduced by the natural area conserved in the water quality volume computations. Natural area conservation is different than vegetated filter strip/recharge area and drainageway protection in that in some cases surface runoff may never be directed over the natural area (i.e., if upslope wooded areas are conserved).

(a) Restrictions on the Credit:

1) Natural areas must remain in an undisturbed condition during and after construction activities. Temporary incidental land disturbance activities associated with utility construction may be permitted within the conservation area.

2) These areas should be placed in a conservation easement or similarly enforceable agreement with the Township.

3) The limits of the undisturbed area and conservation easement must be shown on all construction plans.

4) The area must be located on the development site.

5) Water quality credits are not permitted for water quality sensitive (WQS) developments.

6) The maximum total natural area credit is 50% of the site impervious area. However, the combination of natural area credit and vegetated filter strip credit is also 50%.

(b) Sensitive Area and Development Restrictions:

1) NAC areas may not be counted as a credit in sensitive areas unless the impervious area actually flows across the area as sheet flow.

2) Untreated urban runoff from sensitive development types may not be directed to natural areas without pretreatment.

(6) Filter/Buffer Area.

(a) A water quality credit is given when stormwater runoff is effectively treated via a filter/buffer area or strip. A filter/buffer area is a vegetated boundary characterized by uniform mild slopes. Filter strips may be forested or vegetated with turf grass. Effective treatment is achieved when impervious area runoff is directed as sheet flow across vegetative filter or buffer areas (i.e., concentrated flow discharged to a filter strip does not meet water quality reduction criteria).

(b) The area draining via overland sheet flow to an undisturbed, natural, vegetated filter strip (natural unmaintained meadow or forested area) may be subtracted from the site impervious area (IA) on a 1:1 area ratio in the water quality volume computation. Impervious areas draining across constructed (disturbed or regarded) pervious areas may be subtracted from the site impervious area (IA) on a 1:1/2 area ratio in the water quality volume computation.

1) 

Restrictions on the Credit:

a) The maximum impervious area that may be included in this credit shall be computed as follows:

	IAc = WIA LIA
Where:	IAc	=	Impervious area recharge credit (L2).
	LIA	=	Length of impervious area measured perpendicular to the sheet flow direction (L).
	WIA	=	Width of impervious area (L). Maximum width permitted for credit is the smaller of 100 feet or twice the width of the vegetated filter strip.

b) To qualify for a water quality credit, natural and constructed filter areas or strips must meet the same restrictions identified for natural or constructed recharge areas with regard to width, length, slope, tributary drainage length, and construction. These restrictions are presented in Chapter 3.[6]

[6] Editor's Note: So in original.

c) Runoff shall enter the filter/buffer strip as overland sheet flow.

d) Filter/buffer areas shall remain undisturbed/unmanaged other than to remove accumulated trash and debris.

e) Water quality credits are not permitted for water quality sensitive (WQS) developments.

f) The maximum total water quality credit for vegetative filter/buffer areas is 50% of the site impervious area. However, the combination of NAC and filter/buffer areas is also 50%.

2) 

Sensitive Area and Development Restrictions:

a) Untreated urban runoff from WQS developments may not be directed to filter/buffer areas without pretreatment.

(7) Comments Related to Water Quality Credits.

(a) Concurrence of the Township Stormwater Engineer is required prior to the use of all water quality credits for the reduction of the water quality treatment volume. The Township Stormwater Engineer may approve the use of additional credits based upon sufficient documentation regarding suitability for sensitive development types and areas, pollutant removal effectiveness, and maintenance criteria. Multiple water quality credits cannot be claimed for the identical area of the site (i.e., a stream buffer credit and disconnecting roof recharge area cannot both be claimed for the same area).

(b) Additional impervious coverage reduction using low impact development techniques (development practices which reduce the impact of urban runoff such as narrower residential road sections, smaller cul-de-sacs, smaller parking stalls, smaller building setbacks to reduce driveway lengths, etc.) will also reduce the required water quality treatment volume. Many of these techniques require prior approval by the Township before implementation into land development design.

G. Design Storm Method (CG-1). This method is based on the method described in the current PA DEP BMP Manual. This method is applicable to all development sites in the Township one acre or greater in size. The Design Storm Method requires the applicant to demonstrate that for all storms equal to or less than the 2-year, 24-hour rainfall event, runoff volume being discharged from the site does not increase.

(1) Calculation Methodology.

(a) Existing (predevelopment) non-forested pervious areas must be considered meadow in good condition.

(b) Existing (predevelopment) forested or emergent forest (unmaintained brush or growth) must be considered forest in good condition.

(c) 20% of existing impervious area shall be considered meadow in good condition. If it is determined that any site impervious was added since 2006 without a documented stormwater management plan, this impervious shall be modeled as 100% meadow in good condition. This impervious shall be documented using the 2006 Centre County aerials, previously recorded land development plans, or as-built stormwater management plans.

(2) Volume methods guidance. Due to the unique karst geology of the Township and the Centre Region as a whole, standard SCS computations may deliver inaccurate results. To account for this, the conditions described in Subsection 3A(12), (13), (14), and (15) shall apply as applicable to the existing and proposed runoff volume computation.

H. Infiltration Test Requirements. Infiltration testing shall be performed in accordance with the current version of the Pennsylvania Stormwater Best Management Practices Manual and as outlined here.

(1) Double ring infiltrometer methods are the preferred method for establishing design infiltration rates. Borehole tests may be used in cases where proposed infiltration surfaces are at a depth not reasonably accessible using infiltrometer methods. Other test methods may be used with prior approval by the Township Stormwater Engineer.

(2) Double ring infiltrometers shall have inner rings six inches or more in diameter.

(3) Design infiltration rates shall be based only on infiltration tests performed less than 15 feet horizontally and one foot vertically from the infiltration surface. If a more restrictive soil layer is identified at a depth greater than one foot below the basin, the more restrictive layer shall be used.

(4) A minimum of three infiltration tests shall be performed for infiltration BMPs having footprints less than or equal to 0.5 acres. For basins larger than 0.5 acres, a minimum of six infiltration tests shall be performed per acre of basin footprint. For borehole tests using inner casings less than six inches in diameter, the number of infiltration test locations shall be doubled. In all cases, test locations shall be evenly distributed across the basin. Infiltration test locations shall be documented on the stormwater management site plan drawing.

(5) The infiltration rate shall be consistent with rates recommended in the current version of the Pennsylvania Stormwater BMP Manual and as specified in this chapter, including limitations on rates associated with soils having low cation exchange rates.

§ 26-305 Erosion and Sedimentation Requirements.

[Ord. No. 1102, 9/3/2024]

1. Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion and Sediment Control, and in accordance with the Centre County Conservation District and the standards and specifications of the Township.

2. Additional erosion and sedimentation control design standards and criteria that must be applied where recharge or water quality BMPs are proposed include the following:

A. Areas proposed for these BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity. They shall not be used as sediment traps unless there are no other options on the site. If used as sediment traps, they must have two feet of existing soil remain for use during the sediment trap stage that is excavated just prior to conversion to an infiltration BMP.

B. If possible, these BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the BMP has received final stabilization.

3. Adequate erosion protection shall be provided along all open channels and at all points of discharge.

§ 26-306 Sinkhole Protection.

[Ord. No. 1102, 9/3/2024]

1. The use of sinkholes for stormwater management must be carefully planned, because discharging runoff directly into existing sinkholes is not an engineered stormwater solution. Aside from potential water quality effects, cover collapse sinkholes that exist throughout the watershed can be unstable, and it should be assumed that they could stop taking water at any time. Numerous sinkholes throughout the region already flood during larger runoff events. Nonetheless, in the watershed there are large drainage areas that completely drain to existing sinkholes and all upslope development tributary to them cannot be realistically stopped. Therefore, the following sections have been developed.

A. Stormwater from roadways, parking lots, storm sewers, roof drains, or other concentrated runoff paths shall not be discharged directly into sinkholes without prior filtration in accordance with Subsection 1B, below.

B. Sinkholes capable of absorbing substantial amounts of stormwater shall be protected by diverting such runoff around the sinkhole (refer to Subsection 1F) or, upon recommended approval of the Township Stormwater Engineer, by planting and maintaining a dense filter path of suitable vegetative material in such a manner and location to disperse and slow the runoff to a sheet flow condition to promote the maximum possible filtration and sedimentation of impurities.

(1) The filter path must be at least 100 feet in length and 20 feet in width. Ten-foot-wide filter paths are acceptable if land slope is less than 2%.

(2) Filter paths shall be designed and installed so that they filter sheet flow rather than concentrated flow. If concentrated flow occurs, grading and shaping or the use of best management practices such as grass waterways or drop structures may be required.

(3) Sedimentation basins designed to DEP Chapter 102 standards or permanent stormwater storage criteria, whichever is larger, and proposed vegetative filter paths, in conjunction with temporary stone filter check dams, shall be installed prior to subdivision or land development construction activities, where sinkholes are used to accept stormwater discharges.

C. If increased runoff is to be discharged into a sinkhole, even in filtered conditions, a hydrogeologic assessment of the effects of such runoff on the increased risk of land subsidence and adverse impacts to existing sinkhole floodplains and groundwater quality shall be made by a qualified professional and submitted with the stormwater management plan. Such discharge shall be prohibited if the Township Stormwater Engineer determines that such poses a hazard to life, property or groundwater resources.

D. Sinkholes that exist in natural drainageways shall be protected and should be utilized to accept that flow of water currently discharging to the sinkhole. All sinkholes shall be posted by clearly visible permanent on-site notices prohibiting any disposal of refuse, rubbish, hazardous wastes, organic matter or soil into the sinkhole. A uniform grade select rock fill may be permitted in the sinkhole for the purpose of preventing dumping of said materials, subject to approval of the Township Stormwater Engineer.

E. To protect sensitive karst areas, the Township Stormwater Engineer may require basins to contain an impervious liner. The liner may be of the impervious membrane type, placed in accordance with the manufacturer's recommendations, or may be constructed by mixing bentonite, or an approved alternative, with existing soil available at the site as approved by the Township Stormwater Engineer.

F. If it is determined that runoff from upslope developing areas should be diverted around a sinkhole due to existing problems, the Township Stormwater Engineer may require additional upstream volume controls as required to protect downstream areas.

§ 26-307 Design Criteria for Stormwater Management Facilities.

[Ord. No. 1102, 9/3/2024]

1. Materials, Workmanship and Methods: All materials, workmanship and methods of work shall comply at a minimum with the Pennsylvania Department of Transportation Form 408 specifications, as accepted and commonly used by the Township, and shall be considered to be incorporated into this chapter as if copied in full. In the event a conflict arises between the requirements of this chapter and the Form 408 specifications, the Township Stormwater Engineer shall resolve the difference, and his opinion shall be binding.

A. General.

(1) Facilities in State Right-of-Ways. Any stormwater facility located on a state highway right-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PA DOT). Any stormwater facility that discharges directly onto a state highway right-of-way shall be subject to review by the PA DOT.

(2) Water Obstructions. Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed in PA DEP Chapter 105 regulations (as amended or replaced from time to time by PA DEP), shall be designed in accordance with Chapter 105 and will require a permit from PA DEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the 25-year design storm with a minimum of 1.0 foot of freeboard measured below the lowest point along the top of the roadway for local streets, runoff from the 50-year design storm measured below the lowest point along the top of the roadway for collector streets, and runoff from the 100-year design storm measured below the lowest point along the top of the roadway for arterial streets. Roadway crossings located within designated floodplain areas must be able to convey runoff from a 100-year design storm with a minimum of 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility that constitutes a dam as defined in PA DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PA DOT right-of-way must meet PA DOT minimum design standards and permit submission requirements.

(3) Conveyance Facilities. Any drainage conveyance facility and/or channel that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the return period design storm as specified in § 26-304, Subsection 3B. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. If the conveyance facility connects to an existing stormpipe or conveyance facility, the design engineer must show that the existing facility has the capacity to accept the additional water flow. Roadway crossings located within designated floodplain areas must be able to convey runoff from a 100-year design storm. Any facility located within a PA DOT right-of-way must meet PA DOT minimum design standards and permit submission requirements.

B. Stormwater Basin Design Considerations. Stormwater management basins for the control of stormwater peak discharges shall meet the following minimum requirements.

(1) The design of all facilities over limestone formations shall include measures to prevent groundwater contamination and, where required, sinkhole formation. Soils used for the construction of basins shall have moderate to low erodibility factors (i.e., "K" factors of 0.32 or less). Any basin greater than four feet in height, measured from the top of berm to the downslope toe of the abutment, must also contain:

(a) Berm soil specifications;

(b) A determination if site soils are available for the construction of the berm or cutoff trench;

(c) An impervious cutoff trench, which extends the full length of the downstream berm located in fill.

(2) Energy dissipators and/or level spreaders shall be installed at points where pipes or drainageways discharge to or from basins. Generally, outlet pipes designed to carry the pre-development, 1-year storm flow will be permitted to discharge to a stream with only an energy dissipator. Discharges to drainage swales shall be spread with a level spreader or piped to an acceptable point.

(3) Outlet structures:

(a) Outlet structures within detention/retention basins shall be constructed of reinforced concrete or an approved alternate. With the exception of those openings designed to carry perennial stream flows, design openings 12 inches or less shall have nonclogging trash racks, and all openings over 12 inches shall have childproof nonclogging trash racks. Outlet aprons shall be designed and shall extend at a minimum to the toe of the basin slope. Where spillways will be used to control peak discharges in excess of the 10-year storm, the control weirs shall be constructed to withstand the pressures of impounded waters and convey flows at computed outlet velocities without erosion.

(b) All metal risers, where approved for use, shall be suitably coated to prevent corrosion. A trash rack or similar appurtenance shall be provided to prevent debris from entering the riser. All metal risers shall have a concrete base attached with a watertight connection. The base shall be sufficient weight to prevent flotation of the riser. An anti-vortex device, consisting of a thin vertical plate normal to the basin berm, shall be provided on the top of all metal risers.

(4) Emergency Spillway:

(a) Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the 100-year post-development conditions. The height of embankment must be set as to provide a minimum 0.5 foot of freeboard above the elevation required to safely pass the 100-year post-development inflow. Should any stormwater management facility require a dam safety permit under PA DEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than a 100-year event.

1) Any underground stormwater management facility (pipe storage systems) must have a method to bypass flows higher than the required design (up to a 100-year post-development inflow) without structural failure or causing downstream harm or safety risks.

2) Any stormwater management facility that has a paved roadway as the lower berm should safely convey the 100-year storm under the paved roadway.

(b) Emergency spillways shall be constructed of reinforced concrete, vegetated earth, or riprap in accordance with generally accepted engineering practices. All emergency spillways shall be constructed so that the detention basin berm is protected against erosion. The minimum capacity of all emergency spillways shall be the peak flow rate from the 100-year design storm. The dimensions of the emergency spillways can be determined from the Centre County Erosion and Sediment Control Handbook. Emergency spillways shall extend along the upstream and downstream berm embankment slopes. Protection should be provided on the upstream embankment a minimum of three feet below the spillway crest elevation. Protection at the downstream slope of the spillway shall, as a minimum, extend to the toe of the berm embankment. The emergency spillway shall not be located on or discharge over uncompacted earthen fill and/or easily erodible material.

(c) Rock-filled gabions may be used where combination berm and emergency spillway structures are required to prevent concentrated flows. The Township Stormwater Engineer may require the use of open concrete lattice blocks, stone riprap, or concrete spillways when slopes would exceed four feet horizontal to one foot vertical and spillway velocities might exceed Soil Conservation Service standards for the particular soils involved.

(5) Antiseep Collars: Antiseep collars shall be installed around the principal pipe barrel within the normal saturation zone of the detention basin berms. The antiseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe.

(6) Slope of Detention Basin Embankment: The top or toe of any slope shall be located a minimum of 10 feet from any property line. Whenever possible, the side slopes and basin shape shall be amenable to the natural topography. Straight side slopes and rectangular basins shall be avoided whenever possible.

(a) Exterior slopes of compacted soil shall not exceed three feet horizontal to one foot vertical, and may be further reduced if the soil has unstable characteristics.

(b) Interior slopes of the basin shall not exceed three feet horizontal to one foot vertical, except with approval of the Township. Retaining walls will be required if a stable slope cannot be maintained. All retaining walls shall be designed as per the procedures outlined in the American Association of State Highway Officials, Standard Specifications for Highway Bridges, 1973. Details and calculations prepared and stamped and signed by a registered professional engineer shall be submitted to the Township Stormwater Engineer for any retaining walls greater than 4 1/2 feet in height.

(c) Where concrete, stone or brick walls are used with side slopes proposed to be steeper than three feet horizontal to one foot vertical, the basin shall be fenced by a permanent wire (or other material approved by the Township) fence 42 inches in height, and a ramp constructed of durable, non-slip material to a grade of less than 10% for maintenance vehicles shall be provided for access into the basin.

(7) Width of Berm. The minimum top width of detention basin berms shall be 10 feet. For shallow rain gardens or basins less than two feet in depth, the minimum berm width shall be three feet.

(8) Slope of Basin Bottom. In order to ensure proper drainage of the detention basin, a minimum grade of 2% shall be maintained for all basins used exclusively for peak runoff control. Water quality or recharge basins with filtration systems incorporated into them may have a minimum grade of 0%.

(9) The lowest floor elevation of any structure constructed within 50 feet of a detention basin or other stormwater facility shall be two feet above the detention basin berm. The distance between any residential structure and any stormwater facility shall be a minimum of 50 feet. The distance between any non-residential structure and any stormwater facility shall be a minimum of 25 feet.

(10) Landscaping and planting specifications must be provided for all stormwater management basins and be specific for each type of basin.

(11) Liners.

(a) Basins should be lined with impervious liners only in areas with a high risk of sinkhole formation or potential groundwater contamination as determined by a geotechnical engineer. However, where a liner is deemed necessary or appropriate, the use of controlled, compacted natural clay liners for stormwater management basins should be considered. Locally available clay, when properly installed, can provide near impervious conditions (approximately E-6 cm/s or less). Some of the advantages of using controlled, compacted, natural clay soil liners are:

1) Can offer better long-term solution as a basin liner versus geosynthetics because of greater thickness and the ability to withstand settlement;

2) Can be constructed to allow relatively uniform leakage rates to facilitate groundwater recharge but not to an excessive degree that overloads karst bedrock;

3) When properly constructed in two or more 8- to 10-inch thick lifts, rapid movement of surface water through the clay liner is eliminated (rapid leaks can occur in geosynthetic lined basins due to poor seaming, punctures, or other factors);

4) Cleaning/maintenance of clay-lined stormwater basins will be easier/safer versus geosynthetic liners which could easily be damaged during maintenance operations; and

5) The abundance of clayey soils (derived from limestone residuum) within the Spring Creek Watershed can provide adequate, cost effective, soil resources for construction of clay liner systems at most development projects.

(b) However, the installation of any low permeability clay liner system needs to be carefully controlled, and the designer needs to ensure that specifications meet standards to ensure integrity. A quality control plan identifying the specific clay source material and construction techniques shall be submitted to the Township Stormwater Engineer for approval prior to the placement of the clay liner.

C. Construction of Basins.

(1) Basins shall be installed prior to or concurrent with any earthmoving or land disturbances which they will serve. The phasing of their construction shall be noted in the narrative and on the plan. Basins that include water quality or recharge components shall have those components installed in such a manner as to not disturb or diminish their effectiveness.

(2) Construction specifications in accordance with the minimum criteria of the Township must be provided for all embankments pursuant to this chapter.

(3) Compaction test reports shall be kept on file at the site and be subject to review at all times with copies being forwarded to the Township Stormwater Engineer.

(4) A quality control program is critical for embankment fills. Therefore, whenever embankment fill material in excess of three feet is to be used, each layer of compacted fill shall be tested to determine its density per ASTM 2922 or ASTM 3017. The density of each layer shall be 98% of a Standard Proctor Density analysis per ASTM 698.

(5) When rock is encountered during the excavation of a pond, it shall be removed to an elevation of at least 12 inches below the proposed basin floor (or 24 to 30 inches for a manufactured liner). All exposed cracks and fissures are to be structurally filled.

(6) Temporary and permanent grasses or stabilization measures shall be established on the sides and base of all earthen basins within 15 days of construction.

(7) A minimum of six inches of topsoil material shall be placed on all areas affected by the basin construction. The materials must meet the requirements of PennDOT Form 408 specifications.

(8) All areas proposed for recreational use, whether active of passive, shall be planted to effectively naturalize areas so as to become an integral and harmonious part of the landscape by contour and type of plant material employed.

(9) All disturbed areas shall be planted with grass in accordance with PennDOT Form 408 specifications.

(10) A fence or suitable vegetative screening may be provided, as required by the Township, around all detention basins. All fencing shall be at least 42 inches in height, and the material type is subject to approval by the Township. A vegetative screening of suitable landscaping plant material in or around a detention basin may also be required. Vegetative screenings should generally provide a barrier to prevent entrance to, and effectively naturalize the appearance of, the detention basin area. Combinations of grassed areas and densely planted shrub areas consisting of species suited to use in the highway environment are encouraged. Species of raspberry (Rubus spp.) are recommended.

D. Construction Inspection. Inspections may be conducted by the Township Stormwater Engineer during the construction of the stormwater management basin and facilities. A preconstruction meeting between the Township Stormwater Engineer, owner, and owner's contractor is required prior to any construction. Such inspections do not constitute approval of construction methods or materials.

E. Special Use Basins.

(1) The design and construction of multiple-use stormwater detention facilities are strongly encouraged. In addition to stormwater management, facilities should, where appropriate, allow for recreational uses, including ball fields, play areas, picnic grounds, etc. Provision for permanent wet ponds with stormwater management capabilities may also be appropriate. Prior approval and consultation with the Township are required before design. Multiple-use basins should be constructed so that potentially dangerous conditions are not created. Water quality basins or recharge basins that are designed for a slow release of water or other extended detention ponds are not permitted for recreational uses, unless the ponded areas are clearly separated and secure.

(2) Multiple Development Basins. Stormwater management facilities designed to serve more than one property or development in the same watershed are encouraged. Staged construction of existing or proposed multiple-use detention facilities by several developers in conjunction with watershed development is encouraged. Each developer shall be responsible for the incremental increase in runoff generated by the respective development and incremental construction improvements necessary for the overall detention facility. Prior approval and consultation with the Township is required before design of such facilities.

(3) Alternative Detention Facilities. Alternative stormwater detention facilities, including rooftop, subsurface basins or tanks and in-pipe detention storage, or other approved alternative designs are permitted as determined by the Township Stormwater Engineer.

§ 26-308 Easements.

[Ord. No. 1102, 9/3/2024]

1. Stormwater management facilities located outside of existing or proposed right-of-ways shall be located within and accessible by easements as follows:

A. Drainage Easements. Where a tract is traversed by a watercourse, drainageway, channel or stream, there shall be provided a drainage easement paralleling the line of such watercourse, drainageway, channel or stream. The width of the drainage easement will be adequate to preserve the unimpeded flow of natural drainage in the 100-year floodplain, in accordance with computed top widths for water surface elevations determined under Part 3 of this chapter.

B. Access Easements. Where proposed stormwater management facilities are not adjacent to proposed or existing public right-of-ways or are not accessible due to physical constraints, as determined by the Township Stormwater Engineer, a twenty-foot-wide passable access easement specifying rights of entry shall be provided. Access easements shall provide for vehicle ingress and egress on grades of less than 10% for carrying out inspection or maintenance activities.

C. Maintenance Easements. A maintenance easement shall be provided which encompasses the stormwater facility and appurtenances and provides for access for maintenance purposes. The maintenance easement must be located outside of 100-year surface elevation and the stormwater facility and appurtenances.

D. Easements shall stipulate that no trees, shrubs, structures, excavation or fill be placed and no regrading be performed within the area of the easement without written approval from the Township upon review by the Township Stormwater Engineer. Upon approval of the Township Stormwater Engineer, such landscaping may be placed in maintenance easements, provided it does not impede access.

E. Whenever practicable, easements shall be parallel with and conjunctive to property lines of the subdivision.

F. All easement agreements shall be recorded with a reference to the recorded easement indicated on the plan. The format and content of the easement agreement shall be reviewed and approved by the Township Stormwater Engineer and Solicitor.

G. When stormwater conveyance pipes or channels are located in undedicated land, they shall be placed within a drainage easement not less than 20 feet wide as approved by the Township Stormwater Engineer.

§ 26-309 Prohibited Discharges and Connections.

[Ord. No. 1102, 9/3/2024]

1. Non-stormwater discharges are to be regulated as noted below. In general, non-stormwater discharges are prohibited from entering any portion of the municipal separate storm sewer system or any waters of the commonwealth except as noted below:

A. Prohibited Discharges.

(1) No person in the Township shall allow, or cause to allow, stormwater discharges into the Township's storm sewer system which are not composed entirely of stormwater, except as provided in Subsection 1B below and discharges allowed under a state or federal permit.

(2) Discharges that may be allowed, based on a finding by the Township Stormwater Engineer that the discharges do not significantly contribute to pollution of surface waters of the commonwealth, are:

(a) Discharges or flows from firefighting activities.

(b) Discharges from potable water sources, including water line flushing and fire hydrant flushing, if such discharges do not contain detectable concentrations of total residual chlorine (TRC).

(c) Non-contaminated irrigation water, water from lawn maintenance, landscape drainage and flows from riparian habitats and wetlands.

(d) Diverted stream flows and springs.

(e) Non-contaminated pumped groundwater and water from foundation and footing drains and crawl space pumps.

(f) Non-contaminated HVAC condensation and water from geothermal systems.

(g) Residential (i.e., not commercial) vehicle wash water where cleaning agents are not utilized.

(h) Non-contaminated hydrostatic test water discharges, if such discharges do not contain detectable concentrations of TRC.

(3) In the event that the Township determines that any of the discharges identified in Subsection 1A(2) above significantly contribute to pollution of waters of the commonwealth, or is so notified by the Pennsylvania Department of Environmental Protection (PA DEP), the Township will notify the responsible person to cease the discharge.

(4) Upon notice provided by the Township under Subsection 1A(3) above, the discharger will have a reasonable time, as determined by the Township, to cease the discharge consistent with the degree of pollution caused by the discharge.

(5) Nothing in this section shall affect a discharger's responsibilities under state law.

B. Prohibited Connections. The following connections are prohibited, except as provided in Subsection 1A(2) above.

(1) Any drain or conveyance, whether on the surface or subsurface, which allows any non-stormwater discharge, including sewage, process wastewater, and wash water, to enter the storm sewer system, and any connections to the storm drain system from indoor drains and sinks.

(2) Any drain or conveyance connected from a commercial or industrial land use to the storm sewer system which has not been documented in plans, maps, or equivalent records, and approved by the Township.