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 provision must be agreed to by the local Township Engineer prior to formal submission of plans for consideration by the Township. A flow chart documenting the stormwater management design process is provided as Chart A-9, Stormwater Procedural Flow Chart, of this chapter.
A. Peak runoff rate control.
(1) Exemption.
(a) Any site where the increase in post-development peak runoff rates is determined to be negligible by the Township Engineer is exempt from the requirement to provide stormwater detention. In support of this exemption, it must be shown that the downstream conveyance systems have adequate capacity to convey the additional discharge without adversely affecting downstream properties. This does not exempt the requirement for implementation of designs for water quality, stormwater conveyance, and/or recharge as required. A stormwater management site plan and report documenting these design elements is also required. The Township Engineer shall use a 5% increase as a general benchmark for defining "negligible." The final definition of "negligible" shall be at the Township Engineer's discretion. Prior to using this exemption (and prior to any land development plan submission), the design engineer must provide written documentation and computations as to why no peak runoff control should be required. The Township Engineer has the right to reject any plan which uses this assumption without prior approval of the Township Engineer. The intent of this exemption is to eliminate the need for multiple or "piggyback" detention facilities as a result of minor changes in imperviousness or land use upstream of existing stormwater control facilities.
(b) Small sites (less than one acre) located directly adjacent to the main stem of creeks or within the floodplain are not required to provide stormwater detention unless directed to do so by the Township Engineer as a result of a documented drainage problem. All other stormwater management standards must be implemented, including water quality, adequate stormwater conveyance, and/or recharge as required. The Township Engineer has the right to reject any plan that uses this exemption without prior approval of the Township Engineer.
(2) Stormwater management analysis.
(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 Modified Rational Method using the Gert Aron Curves may be used for any site less than or equal to two acres in size without prior authorization from the Township Engineer. The Modified Rational Method may also be used for sites between two and five acres in size where the Township Engineer has approved the method's use. In this case, the design engineer must make a written request to the Township Engineer explaining why the use of the Modified Rational Method is more appropriate than the NRCS's methods for the site in question. Use of the Modified Rational methodology 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 Modified Rational Method is used for runoff computations.
(c) The Township Engineer has the right to reject any SWM design that uses hydrograph combinations with the Modified Rational Method where the designer has not validated that the effects of the timing differences are negligible. In addition, the Township 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 Township 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, pre- and post-development stormwater analysis should be conducted for watersheds that are as nearly natural as possible. Also, conducting stormwater analysis for a lot-by-lot comparison, such as within residential developments, is not permitted. Partitioning drainage areas into different subwatersheds for the post-development scenarios is acceptable.
(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 pre- to post-development runoff comparison. In these cases, a hypothetical drainage area defined by the property boundary should be used because it will allow for a better estimate of runoff changes directly downstream of the site. In addition, the designer should recognize that, within the Spring Creek Watershed, typical hypothetical drainage areas, in their 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 post-development 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 Township Engineer.
(4) Pre- and post-development stormwater management analysis shall be conducted using the following design storms:
(5) The twenty-four-hour precipitation depths as obtained from NOAA Atlas 14 shall be used for stormwater management analysis.
(6) The NRCS's Type II precipitation distribution is required for all stormwater management analyses.
(7) The NRCS's dimensionless unit hydrograph "k" factor shall be 484 for both pre- and post-development stormwater analyses.
(8) All undeveloped areas are to be modeled as meadow or woods in good hydrologic condition. Existing impervious areas may be modeled as being impervious for predevelopment conditions. The Township may require a percentage of the existing impervious area up to a maximum of 20% to be modeled as meadow in areas where there are known existing stormwater concerns downstream of the project area or where the site being developed has either deficient or nonexisting stormwater management facilities. Developers of sites with existing impervious areas are highly encouraged to set up a meeting with the Township prior to design so that any additional requirements are identified prior to plan submission.
(9) The NRCS's curve number (CN) shall be used as the rainfall to runoff transformation parameter for all stormwater management analyses.
(10) Curve numbers should be rounded to tenths for use in 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, CN's 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 pre- and post-development 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 post-development conditions, regardless of the NRCS or Centre County 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. An 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 Township 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 for a list of the soils.
(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 post-development 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 post-development 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. An HSG of C shall also be used for large projects that clear and grade land for later phases of development. The Township 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 post-development analysis. This allowance has been developed to entice designers to intentionally design or leave in place small closed depressions that can reduce the total volume required from a stormwater management pond. The site designer is responsible to document downstream impacts if the closed depression were removed.
(15) Drainage areas tributary to sinkholes shall be excluded from the modeled point-of-interest drainage areas defining predevelopment peak flows. Assumptions that sinkholes spill over during some storm events must be supported by acceptable documentation (as determined by the Township Engineer). In addition, the design professional must be aware that bypassing or sealing sinkholes will frequently result in downstream flooding and should not be done if existing downstream flooding already occurs. The site designer is responsible to document downstream impacts if the sinkhole were to stop taking stormwater runoff.
(16) Ponds or other permanent pools of water are to be modeled by the methods established in the NRCS's TR-55 Manual (1986). However, more rigorous documented methods are acceptable (as determined by the Township 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 computational methodologies.
(a) The following time of concentration (Tc) computational methodologies shall be used, unless another method is preapproved by the Township Engineer:
[1] Predevelopment: NRCS's Lag Equation.
[2] Post-development; commercial, industrial, or other areas with large impervious areas (greater than 20% impervious area): NRCS's Segmental Method.
[3] Post-development; residential, cluster, or other low-impact designs less than or equal to 20% impervious area: NRCS's Lag Equation.
(b) The time of concentration is to represent the average condition that best reflects the hydrologic response of the area. For example, large impervious areas bordered by small pervious areas may not consider the effect of the pervious areas in the Tc computation. If the designer wants to consider the effect of the pervious area, runoff from the pervious and impervious areas must be computed separately with the hydrographs being combined to determine the total runoff from the area.
(c) Under no circumstance will the post-development Tc be greater than the 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 post-development Tc is greater, it will still be set by default equal to the predevelopment Tc for modeling purposes.
(d) Refer to Subsection
A(28) regarding impervious area flashing (IAF).
(19) Post-development minimum discharges.
(a) The following post-development minimum discharges are permitted for use with the NRCS (CN) runoff model:
1-year return period | Qpmin = 0.018 (DA) + 0.2 |
2-year return period | Qpmin = 0.03 (DA) + 0.4 |
10-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. All subsurface drains are to be equipped with a restrictor plate with a one-inch opening in order to prevent the subsurface drain from functioning as a primary orifice.
(22) Stormwater management ponds that have a pond capture, recharge, or water quality component shall assume that the basin is full to the controlling component volume at the beginning of design event routing.
(23) Stormwater management ponds must provide safe passage of the 100-year return period peak runoff rate assuming that all of the principal spillway orifices are fully clogged, and the principal spillway overflow is 50% clogged. A minimum of a six-inch freeboard must also be maintained above the resulting maximum water surface elevations (WSE). 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 pre- and post-development comparisons of peak flows shall be rounded to tenths of a cfs. The intent here is to recognize the accuracy and precision limitations of hydrologic modeling procedures. Again, small differences between pre- and post-development discharge rates should be permitted when no negative downstream impacts will result.
(25) The full Modified Puls routing method must be used for stormwater management pond analyses. Simplified methods of determining pond size requirements such as those in TR-55 (1986) can only be used for preliminary pond size estimates. 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. Water volume controls shall be implemented using the Design Storm Method for sites one acre in size and larger.
(1) The Design Storm Method (CG-1 in the BMP Manual) shall be applied on all sites one greater than one acre. This method requires detailed modeling based on site conditions.
(a) Do not increase the post-development total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.
(b) The Managed Release Concept in lieu of infiltration shall be utilized at the discretion of the Township Engineer and consistent with the requirements of this chapter.
(c) For modeling purposes:
[1] Existing (predevelopment) non-forested pervious areas must be considered meadow in good condition.
[2] A minimum of 20% existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions.
[3] For the purposes of determining the existing impervious condition, any impervious added since 2006 without documented stormwater management shall be modeled as meadow in good condition. The existing impervious condition may utilize the 2006 Centre County aerials, previously recorded land development plans, or as-built stormwater management plans to establish the documented existing condition.
C. Pond capture volumes (Cv) for sites less than one acre cumulative disturbance.
(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 Township 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 Township 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.
D. Recharge volumes (Rv) for sites less than one acre.
(1) The purpose of the recharge portion of this chapter is twofold. 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 Township 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 Township 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 (0.5 inch rainfall depth). If sump stone has a voids ratio of 40%, the total sump volume will be 0.10 cubic feet per square foot of roof area. When designed only to meet this recharge criteria, the maximum size for a single sump is 100 cubic feet, and the minimum sump surface area (A) to depth (D) ratio (A/D) must be a minimum of 4/1. The sump depth less any freeboard should not exceed 24 inches.
[2] This roof sump standard shall apply unless the Township 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 cannot 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 Township 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 Township Engineer.
[c] The total impervious area tributary to a natural buffer/filter area cannot exceed twice the buffer/filter area.
[2] To qualify for a recharge volume credit, constructed filter strips shall be designed to the following standards:
[a] The minimum filter strip width shall be five feet or 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 cannot exceed twice the constructed filter strip area.
[c] The surface slope of the area tributary to the constructed filter strip area and the surface slope of the constructed filter strip area itself may not exceed 5% and 3%, respectively. In special cases, steeper slopes may be used if specifically authorized by the Township Engineer.
[d] The filter strip surface shall consist of a minimum of six inches of natural or reconstructed topsoil with a stable grass surface treatment. Reconstructed topsoil designs must be approved by the Township 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 Township 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 cannot take advantage of this credit. A 5% longitudinal sidewalk slope shall be used as the benchmark defining steep slopes.
(d) Impervious areas tributary to natural closed depressions 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) 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, settling, 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 Township Engineer may waive the recharge requirement in the following situations:
(a) The Township Engineer may waive the recharge requirement in highly developed areas or areas undergoing redevelopment where the Township Engineer has determined that forced recharge could have adverse impacts on adjacent landowner structures, property, or Township infrastructure. These waivers should be limited to small land areas (generally less than five acres in size), where the ability to place recharge beds may be limited or may hinder redevelopment.
(b) The Township Engineer may waive the recharge requirement in areas where a qualified soils scientist or geologist has determined that none of the site soils are suitable for recharge, or that the location of the suitable soils is such that harm to adjoining properties could occur as stated under Subsection
D(11)(a) above.
(c) The Township Engineer may waive the recharge requirement in areas where recharge cannot physically occur as documented by a qualified soil scientist, geologist, or hydrologist. These areas include:
[1] Exfiltration areas commonly found at the base of wooded hillsides where clay pans or fragipans exist; and
[2] Saturation areas near major streams or floodplains.
(12) As identified above, recharge analysis and/or waiver requests must be supported by a geotechnical report sealed by a qualified professional (soil scientist, geologist, hydrogeologist, or geotechnical engineer). The intent of this report will be to establish the suitability of a particular parcel of land or area for recharge and to identify areas on a development site appropriate for recharge. It is recommended that the geotechnical/soils consultant discuss the extent and approach to the analysis with the Township 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.
(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.
[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.
(f) Where it has been shown that recharge cannot be performed and a waiver of the recharge requirements is being requested, the Township shall require that the first one inch of runoff from all new impervious areas be treated through underdrained facilities. These facilities may include underdrained basins, rain gardens, and infiltration trenches. Treatment is to include use of an amended topsoil to provide filtration of the stormwater. All underdrain outlets are to include a restrictor plate to prevent the underdrain system from functioning as a primary outlet.
(13) The following guidelines 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 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 statement is subject to the recommendation of a qualified geotechnical engineer.
(c) If the minimum percolation rate is not met and/or the minimum soil depth cannot be maintained on a site, recharge should be accommodated by directing shallow sheet flow from impervious areas across surface filter strips and/or undisturbed natural areas, or some other innovative surface infiltration feature should be used. Limiting subsurface percolation rates and/or depth to bedrock shall not by themselves warrant a recharge waiver.
(14) In addition, since recharge is intended as a volume control, innovative or new methods that address the significant increase in the volume of runoff from sites having large impervious areas are encouraged. These volume control alternatives 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 Township Engineer.
E. 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 in size: 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 Township Engineer.
(d) Other methods as approved by the Township Engineer, such as SWMM, SWIRM-ROUTE, etc.
(2) Rational coefficients.
(a) Rational coefficients used are to be from Rawls et al. (1981), PADOT 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.
(b) The design engineer may choose to use the following Rational C coefficients without regard to soil HSG for small sites. However, it is recommended that they be used only for storm drains up to 24 inches in diameter. The use of these conservative values shall fully be the choice of the design engineer.
[1] All impervious areas: C = 0.95.
[2] 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 Township 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 Township Engineer may require the analysis of the 100-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 post-development 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 Township, when parallel to the street within the right-of-way.
(12) When located in undedicated land, they shall be placed within a drainage easement not less than 20 feet wide as approved by the Township Engineer.
(13) The use of properly designed, graded and turfed drainage swales is encouraged in lieu of storm sewers in commercial and industrial areas and, where approved by the Township 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.
(14) Inlet types and inlet assemblies.
(a) Inlet types and inlet assemblies shall conform to the Pennsylvania Department of Transportation Standards for Roadway Construction as approved by the Township 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 Township deems it necessary because of special land requirements, special inlets may be approved.
[3] The interval between inlets collecting stormwater runoff shall be determined in accordance with DM-2, Chapter
10, Section 3, "Capacity of Roadway Hydraulic Facilities."
(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 SWM site plan or area SWM site plan approved by the Township.
(16) When evidence available to the Township indicates that existing storm sewers have sufficient capacity as determined by hydrograph summation and are accessible, the subdivider may connect 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)].
F. 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 Township 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) 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.
(i) Prior to stormwater management and water quality design, applicants should consult with the Township Engineer to verify stormwater quality criteria and present proposed features and concepts for the treatment of stormwater runoff. Following this meeting, the Township 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 inch 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 Township 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 Township Engineer may modify the defined minimum width in cases where natural landforms define an appropriate alternate width.
(c) The impervious area used in the WQv equation for the development site may be reduced by twice the area of the preserved drainageway (two-to-one 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 Township 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 Township.
[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 WQv equation for the development site may be reduced by the natural area conserved in the water quality volume computations. Natural area conservation is different than vegetated filter strip/recharge area and drainageway protection in that in some cases surface runoff may never be directed over the natural area (i.e., if upslope wooded areas are conserved).
(a) Restrictions on the credit.
[1] Natural areas must remain in an undisturbed condition during and after construction activities. Temporary incidental land disturbance activities associated with utility construction may be permitted within the conservation area.
[2] These areas should be placed in a conservation easement or similarly enforceable agreement with the Township.
[3] The limits of the undisturbed area and conservation easement must be shown on all construction plans.
[4] The area must be located on the development site.
[5] Water quality credits are not permitted for water quality sensitive (WQS) developments.
[6] The maximum total 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 1:1/2 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 Township Engineer is required prior to the use of all water quality credits for the reduction of the water quality treatment volume. The Township 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 Township before implementation into land development design.