Township of Halfmoon, PA Article III Stormwater Management

§ 200-13 General requirements.

A. All regulated activities in the municipality, which do not fall under the exemption criteria shown in § 200-10 of this chapter, shall submit a stormwater management plan to the governing municipality 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.

B. 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.

(1) Stormwater management facilities and related installations also shall be provided:

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

(b) 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).

(c) 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.

(d) To property 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 respective Municipal Engineer, with no impact on adjoining properties, unless an area specifically designed for stormwater detention is provided.

C. 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.

D. 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.

E. Where a development site is traversed by watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Also, maintenance, including mowing of vegetation within the easement, shall be required, except as approved by the appropriate governing authority.

F. 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.

G. Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PA DEP 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.

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

I. 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.

J. To promote overland 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 Municipal Engineer on a case-by-case basis.

K. Where deemed necessary by the respective Municipal 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 Municipal 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, flood-prone areas, etc.

L. 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 engineers for review and comment.

§ 200-14 Sensitive areas and developments.

Sensitive areas and water quality sensitive developments have been identified which require special consideration with regard to stormwater management.

A. Sensitive areas are defined as those areas that, if developed, have the potential to cause groundwater reservoir contamination to a water authority or association or Penn State University well field. These areas consist of the delineated one-year zone of contribution and direct upslope areas tributary to the wells (see Appendix B, Exhibit 1[1]). Municipalities may update the sensitive area boundaries based on new research or studies as required.

[1] Editor's Note: Appendix B is included at the end of this chapter.

B. "Water quality sensitive (WQS) development" is defined as a land development project that has a high potential to cause catastrophic loss to local water quality, and could potentially threaten groundwater reservoirs. The following is a provisional list of water quality sensitive developments. This list may be amended at the discretion of the local municipality.

(1) Vehicle fueling stations.

(2) Industrial manufacturing sites. (NOTE: The Municipal Engineer will make the determination relative to what constitutes these classifications 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.)

(3) Salvage yards.

(4) Recycling centers.

(5) Hazardous material storage areas. (NOTE: The Municipal Engineer will make the determination relative to what constitutes these classifications 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.)

(6) Interstate highways.

§ 200-15 Performance standards.

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

B. Sensitive area district (SAD) boundaries. The location of sensitive areas or sensitive area districts within the watershed are illustrated on a map, which is available for inspection at the municipal office. A reduced scale copy of this map is included as Exhibit 1 in Appendix B of this chapter.[1] The exact location of the boundaries of sensitive areas are set by drainage areas tributary to each of the points of interest as illustrated in Appendix B. The exact location of these boundaries as they apply to a given development site shall be determined using mapping at a scale which accurately defines the limits of the sensitive area. If the project site is within the sensitive area (in whole or in part), two-foot contour interval mapping shall be provided to define the limits of the sensitive area. If the project site is not adjacent to but within 500 linear feet of a defined sensitive area, a five-foot contour interval map defining the limits of the sensitive area shall be included in the stormwater management plan to document the site's location relative to the sensitive area.

[1] Editor's Note: Said map is on file in the Township office.

C. 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 postdevelopment runoff is directed towards the sensitive area.

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

E. 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.

F. 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:

(1) Natural or man-made channels or swales must be able to convey the postdevelopment runoff associated with a two-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.

(2) Natural or man-made channels or swales must be able to convey the postdevelopment twenty-five-year return period runoff without creating any hazard to persons or property.

(3) 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 (if applicable) and, at a minimum, pass the postdevelopment twenty-five-year return period runoff.

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

(5) Where the downstream conveyance channel or other facility is located within a special flood hazard area (as documented on the municipal Flood Insurance Rate Map), it must be demonstrated that the limits of said flood hazard area are not increased by the proposed activity.

(6) Stormwater management ponds that fall under the DEP Chapter 105 criteria of a dam must meet the criteria within Chapter 105.

§ 200-16 Calculation methodologies.

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; and 5) water quality standards. 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 stormwater management decisions are based more realistically on hydrologic processes. In addition, common sense should always be used as a controlling criteria. 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 section must be agreed to by the local Municipal Engineer prior to formal submission of plans for consideration by the municipality. A flow chart documenting the stormwater management design process is provided as Chart A-9, Stormwater Procedural Flow Chart, of this chapter.[1]

A. Peak runoff rate control.

(1) Exemption.

(a) Any site where the increase in postdevelopment peak runoff rates is determined to be negligible by the Municipal 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 plan and report documenting these design elements is also required. The Municipal Engineer shall use a five-percent increase as a general benchmark for defining "negligible." The final definition of "negligible" shall be at the Municipal 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 Municipal Engineer has the right to reject any plan which uses this assumption without prior approval of the Municipal 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 five acres) 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 Municipal 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 Municipal Engineer has the right to reject any plan that uses this exemption without prior approval of the Municipal Engineer.

(2) Stormwater management analysis models.

(a) 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.

[1] Up to 100 acres in size: NRCS's TR-55 or TR-20.

[2] Over 100 acres in size: NRCS's TR-20 or HEC-1 (HEC-HMS).

(b) The Rational Method or Modified Rational Method may be used for any site less than or equal to two acres in size without prior authorization from the Municipal Engineer. The Rational Method or Modified Rational Method may also be used for sites between two and 20 acres in size where the Municipal Engineer has approved the method's use. In this case, the design engineer must make a written request to the Municipal Engineer explaining why the use of the Rational Method is more appropriate than the NRCS's methods for the site in question. The design engineer should keep in mind that the 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 mode, their use is not appropriate if the Rational Method is used for runoff computations.

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

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

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

(f) 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, predevlopment and postdevelopment 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 postdevelopment scenarios is acceptable.

(g) 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 predevelopment to postdevelopment 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 predevelopment 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 postdevelopment nuisance flooding since the models overestimate the predevelopment runoff magnitude.

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

(4) Design storms.

(a) Predevelopment and postdevelopment stormwater management analysis shall be conducted using the following design storms:

[1] One-year.

[2] Two-year.

[3] Ten-year.

[4] One-hundred-year.

(b) For sites less than one acre in total area that connect directly to existing storm sewer systems, surface or subsurface (underground) stormwater detention facilities only need to be designed to control storm events up to the design return period of the existing pipes (usually 10 years). However, it must be demonstrated that adequate conveyance capacity (overland or within the existing storm sewer system) exists to ensure that flooding or damage from proposed releases will not exceed the existing potential for the system. If warranted by historic flooding in the tributary storm sewer system, the municipality may require more stringent criteria.

(c) The Municipal Engineer may waive the requirement to detain the one-hundred-year storm as long as the discharge is to a well defined, functioning conveyance system that does not currently exhibit flooding or other conveyance problems. The downstream conveyance system must be analyzed for the one-hundred-year event to ensure that the proposed development will not increase flooding or damage to existing buildings and/or infrastructure.

(5) The following twenty-four-hour precipitation depths shall be used for stormwater management analysis for the entire Spring Creek Basin. These values override the use of TP-40 (the basis of the NRCS twenty-four-hour precipitation maps).

	Return Period (years)	Precipitation Depth (inches)
	1	2.2
	2	2.6
	10	3.5
	25	4.2
	50	4.7
	100	5.3

(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 predevelopment and postdevelopment 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 predevelopment conditions. The only exception is areas that are actively in agricultural production (nonfallow). The designer may model the watershed using the actual agricultural land use/cover condition to show that increases from the site as compared to the all meadow condition are negligible.

(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 prepackaged 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 can 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 predevlopment and postdevelopment conditions regardless of the NRCS or Soil Survey's description, except for the following two conditions:

(a) Compacted structural fill areas shall use a minimum of HSG C for postdevelopment conditions regardless of the NRCS or Soil Survey's description. For most developments compacted structural fill areas are under impervious surfaces, but may include islands within parking areas, fringe land, etc. A HSG C shall also be applied to large projects that clear and compact building pad areas for later phases of development under an initial phase. The Municipal 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.

(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, Table A-8,[2] for a list of the soils.

[2] Editor's Note: Table A-8 is included at the end of this chapter.

(13) Soils not underlain by carbonate geology shall use the HSG as specified by the NRCS or 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 predevelopment conditions. Disturbed postdevelopment wooded areas shall carry the NRCS or 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 postdevelopment conditions regardless of the NRCS or 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 Municipal 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 predevelopment 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 postdevelopment 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 predevelopment peak flows. Assumptions that sinkholes spillover during some storm events must be supported by acceptable documentation (as determined by the Municipal 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 Municipal 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 comuptational methodologies.

(a) The following time of concentration (Tc) computational methodologies shall be used unless another method is preapproved by the Municipal Engineer:

[1] Predevelopment: NRCS's Lag Equation.

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

[3] Postdevelopment; 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 affect 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 postdevelopment Tc be greater than the predevelopment Tc for any watershed or subwatershed modeling purposes. This includes when the designer has specifically used swales to reduce flow velocities. In the event that the designer believes that the postdevelopment Tc is greater, it will still be set by default equal to the predevelopment Tc for modeling purposes.

(19) Postdevelopment minimum discharges for NRCS runoff model.

(a) The following postdevelopment minimum discharges are permitted for use with the NRCS (CN) runoff model [NOTE: Refer to Subsection A(28) regarding impervious area flashing (IAF).]:

	one-year return period	Qpmin = 0.018 (DA) + 0.2
	two-year return period	Qpmin = 0.03 (DA) + 0.4
	ten-year return period	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 predevelopment peak runoff rate.

(c) The minimum discharge criteria above are not appropriate for use with the 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 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 predevelopment 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 predevelopment 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 predevelopment condition, even if drainage areas are altered following development.

(20) All lined stormwater management ponds in carbonate and noncarbonate 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.

(22) Stormwater management ponds that have a capture depth for the purposes of water quality or volume capture shall assume that the pond water quality or capture volume is full at the beginning of design event routing.

(23) Stormwater management ponds must provide safe passage of the one-hundred-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 six-inch freeboard must also be maintained above the resulting maximum water surface elevations (W.S.E.). Any embankment emergency spillway can be assumed to be unclogged. SWM 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 predevelopment and postdevelopment 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 predevelopment and postdevelopment 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) can only be used for preliminary pond size estimates.

(26) Prepackaged 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. Pond 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 are limited to those delineated as USGS perennial streams. This should be treated as a guideline and not a steadfast rule. The final determination is at the discretion of the Municipal 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. 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) The capture volume is defined as a runoff depth of 0.25 inch 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. Supporting computations that show that 90% of the capture volume can dewater in a maximum of 72 hours must be provided. For surface ponds, the maximum depth of ponding for the capture volume shall be three feet (a health and safety precaution). However, in areas under karst influence, a limiting maximum ponding depth of 18 inches is recommended. Designers may always increase the capture volume to a value greater than the identified standard as long as the ponding depth criteria are met.

(4) 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 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.

(5) 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 Municipal Engineer.

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

(7) Stormwater management detention facilities that connect directly to storm drain pipe networks that discharge to natural well-defined channels do not require a capture volume.

C. Recharge volumes (Rv).

(1) The purpose of the recharge portion of this chapter 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 criteria 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 Centre County Conservation District, 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 inch 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 can 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 Municipal Engineer may require submission of supporting documentation prior to approving structural infiltration areas as pervious areas.

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

(a) Residential roof areas (detached, duplex, and townhome dwellings) and commercial industrial buildings with roof areas less than 5,000 square feet can 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 (one-half-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 should not exceed 24 inches. This roof sump standard shall apply unless the municipality has a separate roof sump standard for water quantity or peak control.

(b) All or portions of driveways, roadways, and parking areas can 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 nonerosive as they cross the impervious area/filter interface.

[1] To ensure proper infiltration characteristics the natural soil profile within natural buffer/filter areas can not be disturbed during construction. Completely undisturbed natural recharge areas serve this function best. However, minor surface scaring, seeding, and landscaping is permitted in these areas as long as natural grades are not altered. In special cases, when approved by the Municipal 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 1/2 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 Municipal Engineer.

[c] The total impervious area tributary to a natural buffer/filter area can not 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 1/2 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 can not 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 Municipal 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 Municipal Engineer prior to application. Reconstructed topsoil consists of soils augmented by tillage and the addition of soil amendments such as compost, lime, animal manures, crop residues, etc.

[e] To minimize erosion of the topsoil layer during construction, it is recommended that these areas be sodded. However, the Municipal 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 can 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 can not take advantage of this credit. A five-percent longitudinal sidewalk slope shall be used as the benchmark defining steep slopes.

(d) Impervious areas tributary to natural closed depressions can 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 pretreatment 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 can 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 can be created through the use of low head berms one-foot or less in height.

(f) The entire capture volume provided in a pond without a subsurface drain may be used as a credit towards the recharge volume requirement.

(g) Where a basin with an underdrain is approved in lieu of recharge, the volume treated through the underdrain must be twice the required recharge volume. A waiver of recharge requirements must be first requested and approved, see § 200-16C(11).[3]

[3] Editor's Note: Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I).

(h) 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 can 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, etc.).

(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 can 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, etc.).

(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 pretreatment BMP prior to recharge. Acceptable pretreated BMPs for these developments include BMPs that are based on filtering, setting, or chemical reaction processes such as chemical coagulation.

(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 can 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 Municipal Engineer may waive the recharge requirement in the following situations:

(a) The Municipal Engineer may waive the recharge requirement in highly developed areas or areas undergoing redevelopment where the Municipal Engineer has determined that forced recharge could have adverse impacts on adjacent landowner structures, property, or municipal 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 Municipal 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 item one above.

(c) The Municipal Engineer may waive the recharge requirement in areas where recharge can not 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 Municipal 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.

(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, etc.

(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. Explain how these limitations will not create the potential for undue harm to the environment and the Spring Creek Watershed when the site is developed.

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

(a) Soils should have a minimum percolation rate of 50 min/cm (0.47 inch/hour) for effective operation of subsurface exfiltration BMPs. If no site soils have percolation rates of 50 min/cm, subsurface exfiltration BMPs should not be used.

(b) A minimum of 30 inches of soil must be maintained between the bottom of a subsurface exfiltration BMP and the top of bedrock or seasonally high groundwater table. This is requirement may be modified subject to the recommendation of a qualified geotechnical professional.

(c) If the minimum percolation rate is not met and/or the minimum soil depth can not 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 can 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 Municipal Engineer.

D. 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:

(a) Up to 200 acres in size: Rational Method.

(b) Between 200 acres and 1.5 square miles: HEC-1; PSRM; TR-20.

(c) Over 1.5 square miles in size: PSU-IV with the carbonate adjustment factor at the discretion of the Municipal Engineer.

(d) Other methods as approved by the Municipal Engineer such as SWMM, SWIRM-ROUTE, etc.

(2) Rational coefficients used are to be from Rawls et al. (1981), PA DOT Design Manual 2-10 or using the Aron curves to convert CNs to C. If the Aron curves are used, all CNs must be applicable to the HSG as identified by the NRCS. 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 inches in diameter. The use of these conservative values shall fully be the choice of the design engineer.

(a) All impervious areas: C = 0.95.

(b) All pervious areas: C = 0.30.

(3) Storm drains shall be designed at a minimum using a ten-year runoff event without surcharging inlets. Storm drains tributary to a multiple site SWM facility across municipal roads or crossing other properties must convey, at a minimum, a twenty-five-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 Municipal Engineer may require the analysis of the one-hundred-year peak runoff rates for conveyance purposes in some instances where regional SWM facilities are employed.

(6) Any storm drain within state or federal rights-of-way or 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) can 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 postdevelopment condition), without needing to provide supporting documentation.

(9) Precipitation values applicable to the entire Spring Creek Drainage Basin are those reflected in the PA DOT's IDF curves for Region 2, regardless if the area was formerly considered in Region 3.

(10) Storm drain conveyance system stability (swales, open channels, and pipe discharge aprons) shall be computed using a ten-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 municipality, when parallel to the street within the right-of-way.

(12) When located in undedicated land, outside a public right-of-way, they shall be placed within a drainage easement not less than 20 feet wide as approved by the Municipal Engineer.

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

(a) Swales shall be designed to carry the ten-year storm discharge with one foot of freeboard without erosion to the floor and sidechannels of the swale.

(b) The maximum velocity of vegetated channels, as determined by Manning's Equation, shall not exceed the allowable velocities for specific types of vegetative material as specified in Table A-7 of Appendix A.[4]

[4] Editor's Note: Table A-7 is included at the end of this chapter.

(c) The side slope for any vegetated channel shall not exceed three horizontal to one vertical.

(d) All swale design shall conform to the design standards contained in the Erosion and Sediment Pollution Control Manual, latest edition, published by the Department of Environmental Protection.

(14) Inlets.

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

[1] Inlets shall, at a minimum, be located at the lowest point of street intersections to intercept the stormwater before it reaches 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 municipality 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 5, "Capacity of Waterway Areas."

(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 ten-year design storm of five 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 drainage plan approved by the municipality.

(16) When evidence available to the municipality indicates that existing storm sewers have sufficient capacity as determined by hydrograph summation and are accessible, the subdivider may connect their 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 other storm drain design methods are to be the same as specified in existing local ordinances.

(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) Culvert design criteria.

(a) Culvert design and construction shall be in accordance with PA DOT specifications and standards except where more stringent standards are contained within this chapter.

(b) Minimum grade of storm sewer invert shall be 0.5%.

(c) Minimum pipe diameter shall be 15 inches or the pipe shall have a cross-sectional area at least 176 square inches except that culvert pipes located under more than 25 feet of fill shall have a minimum diameter of 24 or a cross-sectional area at least 453 square inches and shall be reinforced concrete.

(d) Where storm sewers discharge into existing drainage channels at an angle greater than 30° from parallel to the downstream channel flow, the far side bank shall be stabilized by the use of riprap or concrete walls. The stabilization shall be designed to prevent erosion and frost heave under and behind the stabilizing materials.

E. Water quality standards.

(1) Water quality performance standards. To minimize adverse impacts to stream health resulting from stormwater nonpoint 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 nonstructural 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 nonstructural 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 nonstructural 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 Municipal Engineer should be consulted to clarify the design concept for meeting or exceeding the intent of this section.

(f) Where nonstructural 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 (WQv).

(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)[5] will require special consideration. In these instances the applicant shall provide water quality pretreatment (use of a filtering BMP and/or special structural design features) to prevent the discharge of stormwater contaminants to groundwater resources. In addition, hydrogeologic studies may be required to document potential karst-related impacts.

[5] Editor's Note: Appendix B is included at the end of this chapter.

(i) Prior to stormwater management and water quality design, applicants should consult with the Municipal Engineer to verify stormwater quality criteria and present proposed features and concepts for the treatment of stormwater runoff. Following this meeting, the Municipal 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 nonsensitive areas underlain by carbonate rock (see exhibits in Appendix B) within the Spring Creek Basin shall be computed as:

	WQdepth = 0.25 + (0.012)2.9(0.044(SIA))
	WQv = WQdepth(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

(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)2.9(0.044(SIA))
	WQv = WQdepth(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 Chapter 4 of the stormwater management plan.

(3) Water quality credits. Due to the karst nature of the Spring Creek Basin, the nonstructural 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 Municipal 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.

	Nonstructural 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 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 minimum width of 300 feet. The Municipal Engineer may modify the defined minimum width in cases where natural land forms 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:

[a] Drainageway protection areas must remain in an undisturbed condition during and after construction activities. There can 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 Municipal Engineer's discretion. However, the alignment of any such crossing must be perpendicular to the drainageway.

[b] These areas should be placed in a conservation easement or be permanently preserved through a similarly enforceable agreement with the municipality.

[c] The limits of the undisturbed DWP area and conservation easement must be shown on all construction plans.

[d] The DWP area must be located on the development site.

[e] The maximum total DWPA credit is 100% of the site impervious area.

[f] Water quality credits are not permitted for water quality sensitive (WQS) developments.

[2] Sensitive area and development restrictions:

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

[b] 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 predevelopment 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 municipality.

[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 NAC credit is 50% of the site impervious area. However, the combination of NAC VFRS 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) can 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 can be subtracted from the site impervious area (IA) on a one-to-one-half area ratio in the water quality volume computation.

[1] Restrictions on the credit:

[a] The maximum impervious area that can 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.

[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 Municipal Engineer is required prior to the use of all water quality credits for the reduction of the water quality treatment volume. The Municipal 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 culs-de-sac, 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 municipality before implementation into land development design.

[1] Editor's Note: Chart A-9 is included at the end of this chapter.

§ 200-17 Erosion and sedimentation requirements.

A. Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with 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 appropriate municipal government. The Centre County Conservation District has been delegated the authority by the Pennsylvania Department of Environmental Protection to administer the Erosion and Sediment Pollution Control Program in Centre County. It shall be the responsibility of the applicant to submit the erosion and sediment pollution control plan and all applicable materials and fees to the Centre County Conservation District. The applicant must provide proof of approval of the erosion and sediment pollution control plan prior to stormwater plan approval.

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

(1) Areas proposed for these BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.

(2) These BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the BMP has received final stabilization.

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

§ 200-18 Sinkhole protection.

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 subsections 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 § 200-18B below.

B. Filtration.

(1) Sinkholes capable of absorbing substantial amounts of stormwater shall be protected by diverting such runoff around the sinkhole (refer to Subsection F) or, upon recommended approval of the Municipal 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.

(2) 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%.

(3) 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.

(4) 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 Municipal Engineer determines that such poses a hazard to life, property or groundwater resources.

D. All sinkholes shall be posted by permanent on-site notices clearly visible at the sinkhole prohibiting any disposal of refuse, rubbish, hazardous wastes, organic matter or soil into the sinkhole. Rockfill may be permitted in the sinkhole for the purpose of preventing dumping of said materials.

E. To protect sensitive karst areas, the Municipal 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 Municipal Engineer.

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

G. If a sinkhole is to be utilized as part of the volume control of a stormwater management plan a note must be added to the plan stating: "If the sinkhole that is a component of this stormwater management plan closes or does not accept water, all owners of the stormwater management plan tributary to the closed sinkhole shall install volume control BMPs or provide safe conveyance to a suitable discharge point."

§ 200-19 Design criteria for stormwater management facilities.

Materials, workmanship and methods: All materials, workmanship and methods of work shall comply at a minimum with the Pennsylvania Department of Transportation Publication 408 specifications, as accepted and commonly used by the respective municipality, and shall be considered to be incorporated into this section as if copied in full. In the event a conflict arises between the requirements of this section and the Publication 408, Specifications, the Municipal Engineer shall resolve the difference, and his opinion shall be binding.

A. General.

(1) Facilities in state rights-of-way. Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PA DOT). Any stormwater facility that discharges directly onto state highway rights-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 twenty-five-year design storm with a minimum of one foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum of one 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 § 200-16. 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. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-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 predevelopment, one-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 shall have nonclogging trash racks over all openings six inches or smaller in any dimension. Design openings 12 inches or greater in any dimension shall be childproofed. 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 ten-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 antivortex 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 one-hundred-year postdevelopment 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 one-hundred-year postdevelopment 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 one-hundred-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 one-hundred-year postdevelopment 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, and therefore cannot provide a traditional berm emergency spillway, is not required to provide 0.5 foot of freeboard above the elevation required to safely pass the one-hundred-year postdevelopment inflow, but is required to show that the design is stable and no significant undermining, scour or erosion will occur.

(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 one-hundred-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 Municipal 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 County Conservation District 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 municipality.

(c) Where concrete, stone, brick walls are used with side slopes proposed to be steeper than three feet horizontal to one foot vertical, a fence or railing constructed of durable materials, at least 42 inches high, shall be installed above the wall.

(7) Width of berm. The minimum top width of detention basin berms shall be 10 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 0.5%.

(9) Basins designed for use as recreational facilities shall have a collecting swale or low flow channel and/or underdrain. The floor slope of the basin may be reduced to 1%.

(10) The lowest floor elevation of any structure constructed adjacent to a detention basin or other stormwater facility shall be two feet above the detention basin berm. The distance between any structure and any stormwater facility shall be a minimum of 50 feet.

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

(12) The maximum water depth shall not exceed five feet.[1]

[1] Editor's Note: Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I).

(13) 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 SWM 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:

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

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

(c) When property constructed in two or more eight-to-ten-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);

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

(e) 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. 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.

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 municipality must be provided for all embankments pursuant to § 200-19B of 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 Municipal Engineer upon request.

(4) 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 (for a manufactured liner, 24 to 30 inches). All exposed cracks and fissures are to be structurally filled.

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

(6) 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.

D. Construction inspection. Inspections may be conducted by the Municipal Engineer during the construction of the stormwater management basin and facilities. 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 parking facilities within basins and permanent wet ponds with stormwater management capabilities may also be appropriate. Prior approval and consultation with the municipality 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 municipality 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 Municipal Engineer.

F. Design of seepage pits and seepage trenches for infiltration of roof drainage:

(1) Criteria.

(a) These structures shall be designed to assimilate, in 72 hours, a volume of water equal to 0.2 cubic foot per square foot of roof coverage.

(b) The soils on which a seepage pit or trench are located shall have a minimum infiltration rate of 0.27 inch per hour based upon soils data obtained by direct testing methods in accordance with procedures outlined in the Technical Manual for Sewage Enforcement Officers, Pennsylvania Department of Environmental Protection.

(c) The porosity of the gravel or rock to be used in seepage pits must be specified in the plan. The rock or gravel shall be covered with a ground stabilization fabric [trade names: Mirafi 500 (Monsanto), Typer (DuPont), Bidim (trade name) and Supac (trade name)] or equivalent.

(d) Seepage pits or seepage trenches shall not be installed on slopes greater than 20%.

(e) Seepage pits or the drains to them must contain a sediment trap which can be maintained regularly. All downspouts should have leaf strains to prevent leaves from clogging the seepage pit.

(f) Seepage pits connected to roof drains shall be located at least 10 feet from basement walls and downhill from the building.

(g) The bottom of a seepage pit shall be at least two feet above seasonal high water table and bedrock or be shown to be otherwise capable of handling required design volumes.

(2) Guidelines.

(a) Runoff control capacity may be distributed among several seepage pits, trenches or runoff control berms so long as total assimilative capacity of all structures equals the required volume.

(b) Where adequate seepage pit capacity is difficult to achieve with a rock-filled pit, a concrete (or equivalent material) culvert pipe with a lid may be placed vertically over a stone bed to provide storage capacity; alternatively, a septic-tank-type structure operating as a cistern with discharge to the seepage pit may be used.

(c) The longer dimension of seepage pits or seepage trenches should parallel the slope where slopes exceed 5%.

(d) The use of a perforated or porous pipe leading to the seepage pit is encouraged.

(e) In all cases, an overflow system should be provided to accommodate heavy rains in excess of the design criteria.

§ 200-20 Easements.

Stormwater management facilities located outside of existing or proposed rights-of-way 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 one-hundred-year floodplain, in accordance with computed top widths for water surface elevations determined under § 200-16A of this chapter.

B. Access easements. Where proposed stormwater management facilities are not adjacent to proposed or existing public rights-of-way or are not accessible due to physical constraints, as determined by the Municipal 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. A note shall be added to the plan indicating the right of entry of the Township and its representatives for the purpose of inspection of the stormwater facilities.

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 one-hundred-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 municipality upon review by the Municipal Engineer. Upon approval of the Municipal 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 site plan. The format and content of the easement agreement shall be reviewed and approved by the Municipal 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 Municipal Engineer.