Township of Tredyffrin, PA Article V Stormwater Management Provisions

§ 174-16 General requirements.

A. Applicants proposing regulated activities in the Township which do not fall under the exemption criteria shown in § 174-8 shall submit a drainage plan consistent with this chapter to the Township for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.

B. The applicant is required to explore and submit practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth and must maintain as much as possible the natural hydrologic regime.

C. The drainage plan must be designed consistent with the sequencing provisions of § 174-19 to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge, and to protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article V of this chapter.

D. Stormwater drainage systems shall be designed in order to permit unimpeded flow along natural watercourses.

E. Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without the permission of the adjacent property owner(s). Such stormwater flows shall be subject to the requirements of this chapter. Existing points of concentrated drainage that discharge onto adjacent property shall not be altered in any manner which could cause property damage without permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter. Proof of permission from all affected property owners shall be submitted in writing to the Township prior to issuance of a permit.

F. Areas of existing diffused drainage discharge, whether proposed to be concentrated or maintained as diffused drainage areas, shall be subject to any applicable discharge criteria in the general direction of existing discharge, except as otherwise provided by this chapter. If diffused drainage discharge is proposed to be concentrated and discharged onto adjacent property, the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge or otherwise prove that no accelerated erosion, sedimentation, flooding, or other impacts will result from the concentrated discharge. Proof of permission from all affected property owners shall be submitted to the Township prior to issuance of a stormwater and grading permit.

G. Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. At a minimum, the drainage easement shall include the streambed, banks, and 10 feet extending from the top of each bank. It is recommended that the drainage easement also includes the limits of the one-hundred-year floodway.

H. Minimization of impervious surfaces and the use of green roofs, and infiltration of runoff through seepage beds, infiltration trenches, rain gardens, etc., is encouraged where soil conditions permit in order to reduce the size or eliminate the need for detention facilities or other structural BMPs.

I. All stormwater runoff shall be pretreated for water quality prior to discharge to surface or groundwater. Pretreatment devices may consist of a variety of BMPs, such as vegetative swales, water quality inlet devices, bioretention areas or rain gardens, etc., and are subject to review and approval by the Township Engineer. Pretreatment devices should be examined for pollutant removal efficiency for specific types of pollutants generated by proposed land development activities.

J. All regulated activities within the Township shall be designed, implemented, operated, and maintained to meet the purposes of this chapter, through these two elements:

(1) Erosion and sediment control during earth disturbance activities (i.e., during construction); and

(2) Water quality protection measures after completion of earth disturbance activities (i.e., after construction), including operations and maintenance.

K. No regulated earth disturbance activities within the Township shall commence until the requirements of this chapter are met.

L. Postconstruction water quality protection shall be addressed as required by § 174-21.

M. Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VIII.

N. All BMPs used to meet the requirements of this chapter shall conform to the state water quality requirements, the provisions in Appendix B of this chapter,[1] and any more stringent requirements as set forth by the Township.

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

O. In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:

(1) Total contributing area.

(2) Permeability and infiltration rate of the site's soils.

(3) Slope and depth to bedrock.

(4) Seasonal high water table.

(5) Proximity to building foundations, wellheads, and on-site wastewater disposal systems.

(6) Erodibility of soils.

(7) Land availability and configuration of the topography.

(8) Peak discharge and required volume control.

(9) Stream bank erosion.

(10) Efficiency of the BMPs to mitigate potential water quality problems.

(11) The volume of runoff that will be effectively treated.

(12) The nature of the pollutant being removed.

(13) Maintenance requirements.

(14) Creation/protection of aquatic and wildlife habitat.

(15) Recreational value.

P. The stormwater management criteria are to be met on site to the maximum extent possible to comply with the requirements of this chapter. In specific cases where the applicant has demonstrated, to the satisfaction of the Township Engineer, that the requirements of this chapter cannot be achieved on site with the use of nonstructural and structural stormwater BMPs, the applicant may, with Township approval, meet the stormwater management criteria through off-site stormwater management measures as long as the proposed measures are in the same subwatershed. It is strongly recommended that such off-site area be located upstream of the proposed developed site. It will be the applicant's responsibility to locate adequate off-site areas and obtain all easements, agreements and permits to construct such facilities on property other than that which is proposed to be developed and currently owned by the applicant. All easements, agreements and permits must be obtained and provided along with the stormwater plan submission to the Township for review and approval prior to the start of any construction activities. Furthermore, these agreements and plans will be recorded at the office of the Recorder of Deeds for Chester County and two copies of those dated/signed plans provided to the Township.

§ 174-17 Permit requirements by other governmental entities.

Where any of the following permit requirements apply, they must be met prior to commencement of regulated earth disturbance activities, as applicable:

A. All regulated earth disturbance activities subject to permit requirements by DEP under regulations at 25 Pennsylvania Code Chapter 102.

B. Work within natural drainageways subject to permit by DEP under 25 Pennsylvania Code Chapter 105.

C. Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by DEP under 25 Pennsylvania Code Chapter 105.

D. Any stormwater management facility that would be located on a state highway right-of-way or require access from a state highway subject to approval by PennDOT.

E. Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by DEP under 25 Pennsylvania Code Chapter 105.

§ 174-18 Erosion and sediment control during regulated earth disturbance activities.

A. No regulated earth disturbance activities within the Township shall commence until the applicant receives an approval from the Chester County Conservation District of an erosion and sediment control plan for construction activities. Township approval for earth disturbances is still required for all sites containing earth disturbances exceeding 1,000 square feet on steep slopes, and within 50 feet of other sensitive environmental features, such as streams, ponds, lakes and wetlands, as well as for all earth disturbances greater than 5,000 square feet.

B. DEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pennsylvania Code § 102.4(b).

C. In addition, under 25 Pennsylvania Code Chapter 92, a DEP NPDES construction activities permit is required for regulated earth disturbance activities. All earth disturbances exceeding one acre require NPDES permits submittal and approval by the Chester County Conservation District and/or Pennsylvania Department of Environmental Protection.

D. Evidence of any necessary permit(s) for regulated earth disturbance activities from the DEP Southeast Regional Office or Chester County Conservation District must be provided in writing to the Township.

E. A copy of the erosion and sediment control plan and any required permit, as required by DEP regulations, shall be available on the project site at all times.

F. All erosion and sediment control measures shall be inspected daily, as well as after each rainfall event during construction. Maintenance of all erosion and sediment control measures shall be performed at a minimum on a weekly basis, unless sediment or other pollutants are observed leaving the site onto roadways, adjacent properties and/or environmentally sensitive areas, such as streams, ponds, lakes, wetlands, etc., in which case maintenance shall be performed immediately. Failure to maintain sites in accordance with approved erosion and control plans will result in temporary shut down of all site construction and disturbance activities until repairs and maintenance is performed to the satisfaction of the Township Engineer. An inspection and maintenance log shall be maintained on site and available for inspection by Township staff during the full extent of site construction.

G. Additional erosion and sediment control design standards and criteria shall be applied where infiltration BMPs are proposed. They shall include but are not limited to the following:

(1) Proposed infiltration areas shall not be used as temporary sediment control BMPs during the construction phases of the project, unless such a proposal is approved by the Township Engineer. Sedimentation and compaction of soils can greatly reduce the permeability of underlying soils and negatively impact the ability of infiltration systems to function. In cases where proposed infiltration areas are proposed for dual use as temporary sedimentation basins or traps, the following conditions, at a minimum, shall apply:

(a) A minimum of 18 inches or greater of separation shall be maintained between the bottom of any proposed sedimentation BMP and the proposed infiltration bed bottom;

(b) Additional soil testing shall be performed by the applicant prior to final installation and conversion of the BMP to ensure previous designs and soil testing values still apply;

(c) A soil remediation plan shall be submitted to show a minimum of 18 inches of soil amendments, including incorporation of sand, organic matter, and tilling, to further improve site soils and permeability rates; and

(d) An approved erosion and sedimentation inspection and maintenance plan shall be followed to reduce the risk of soil clogging and negative effects to proposed infiltration BMPs.

(2) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity. Such protection measures should include measures, such as installation of orange construction fencing around proposed infiltration areas, protection from vehicular traffic and material laydown during construction, and erosion and sediment controls that minimize to the maximum extent practical sedimentation and uncontrolled runoff from unstabilized site areas to the proposed infiltration areas on site.

(3) If at all possible, infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.

(4) Infiltration BMPs shall be constructed with equipment outside the limits of the proposed system or that will not compact the bottom of proposed beds or trenches. Equipment shall not be allowed on excavated areas of infiltration BMPs at any time during construction.

(5) Infiltration BMPs shall be protected during and after excavation so as to eliminate and reduce to the maximum extent practicable sediment backwash into the BMP while the bottom and sides of the bed or trench is open, and once stone and filter fabric is in place.

(6) Clean stone aggregate to be placed into infiltration BMPs shall be kept out of contact with site soils and disturbed areas, and shall be absent of fines. Stone aggregate for all proposed infiltration BMPs shall be delivered and placed directly into the BMP or stored in an appropriate manner to prevent sediment and soil contamination.

(7) All contributing drainage to infiltration BMPs shall be protected with appropriate erosion and sediment control measures, in conformance with the Pennsylvania DEP Chapter 102 regulations, as well as the PA Stormwater BMP Manual. At a minimum, inlet protection and other measures shall remain until the contributory drainage area has achieved full stabilization.

(8) Contributing drainage areas to infiltration BMPs shall be fully stabilized prior to allowance of stormwater runoff into the BMPs.

§ 174-19 Nonstructural project design (sequencing to minimize stormwater impacts).

A. The design of all regulated activities shall include the following to minimize stormwater impacts:

(1) The applicant shall explore and submit practicable alternatives to the surface discharge of stormwater (such as those listed in Appendix C, Table C-4),[1] the creation of impervious surfaces, and the degradation of waters of the commonwealth and must strive to maintain as much as possible the natural hydrologic regime of the site.

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

(2) An alternative is practicable if it is available and capable of implementation after taking into consideration existing technology and logistics in light of overall project purposes and other Township requirements.

(3) All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.

B. The applicant shall design the proposed project according to the following sequence. The goal of the sequence is to minimize the increases in stormwater runoff and impacts to water quality resulting from the proposed regulated activity.

(1) Prepare an existing resource and site analysis map (ERSAM) showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools (springs), stream buffers, hydrologic soil groups, and geology. Land development, any existing recharge areas, and other requirements outlined in the Township SALDO shall also be included.

(2) Establish a stream buffer according to § 174-21J.

(3) Prepare a sketch plan showing avoidance of sensitive areas identified in § 174-19B(1).

(4) Identify site-specific existing conditions' drainage areas, discharge points, recharge areas, and hydrologic soil groups A and B (areas conducive to infiltration).

(5) Evaluate nonstructural stormwater management alternatives, including:

(a) Minimize earth disturbance.

(b) Minimize impervious surfaces.

(c) Break up large impervious surfaces.

(d) Minimize clearing operations (vegetation removal).

(6) Satisfy the groundwater recharge (infiltration) objective in accordance with § 174-20 and provide for stormwater pretreatment prior to infiltration.

(7) Provide for water quality protection in accordance with § 174-21 water quality requirements.

(8) Provide stream bank erosion protection in accordance with § 174-22 stream bank erosion requirements.

(9) Determine the peak rate control standard in accordance with § 174-23 and conduct an existing and proposed conditions runoff analysis.

(10) Prepare final project design to maintain existing conditions' drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and, to the maximum extent possible, to ensure that the remaining site development has no surface or point discharge.

(11) Conduct a proposed conditions runoff analysis based on the final design that meets the peak rate control standard.

(12) Manage any remaining runoff prior to discharge through infiltration, detention, bioretention, green roof, retention, capture and reuse, direct discharge, or other structural control.

§ 174-20 Groundwater recharge and volume control standards.

A. The applicant shall comply with the following groundwater recharge and volume control requirements of this chapter.

B. Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration facilities shall consider groundwater recharge to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. These measures are required to be consistent with § 174-3 and to take advantage of utilizing any existing recharge areas.

C. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, then the design professional shall be responsible to show to the satisfaction of the Township Engineer that this cannot be physically accomplished (e.g., shallow depth to bedrock or limiting zone, etc.). In situations where it has been adequately demonstrated that the recharge requirement cannot be fully accomplished, the applicant shall examine other alternative BMPs, such as green roofs, capture and reuse systems, wet ponds, and low-impact design measures, employed to the maximum extent possible to achieve the maximum reduction in volume identified in this chapter. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 174-20C(1)(b) depending on demonstrated site conditions and shall be the greater of the volumes.

(1) Infiltration BMPs shall meet the following minimum requirements:

(a) Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:

[1] A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone.

[2] An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests conducted by the applicant's design professional.

[3] The infiltration facility shall be capable of completely infiltrating the retention (infiltration) volume (Re_v) within three days (72 hours).

[4] Pretreatment shall be provided prior to infiltration.

[5] The design professional is required to follow hotspot investigation, subsurface stability, and suitability of infiltration procedures as set forth in the PA Stormwater BMP Manual, dated December 2006, and as amended, to determine whether the proposed infiltration on the development site is appropriate.

[6] If soil and/or geotechnical analysis and testing reports demonstrate that the soil is unsuitable for infiltration, the design professional shall be responsible for providing written documentation showing that the required volume cannot physically be infiltrated within the required time period. Alternative BMPs, such as green roofs, stormwater capture and reuse systems, bioretention areas, etc., shall be incorporated into the proposed design to maximize stormwater runoff volume reduction and achieve water quality protection in accordance with §§ 174-20 and 174-21 of this chapter.

(b) The size of the infiltration facility and Re_v shall be based upon the following volume criteria:

[1] Net two-year volume approach. The retention (infiltration) volume (Re_v) to be captured and infiltrated shall be the net two-year twenty-four-hour volume. The net volume is the difference between the postdevelopment runoff volume and predevelopment runoff volume. The postdevelopment total runoff volume for all storms equal to or less than the two-year twenty-four-hour-duration precipitation shall not be increased. For modeling purposes existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent, and existing impervious areas must be considered as meadow in good condition.

[a] In all plans and designs for stormwater management systems and facilities submitted to the Township Engineer for approval, stormwater peak discharge and runoff volume shall be determined through the use of the NRCS Soil Cover Complex Method as set forth in Urban Hydrology for Small Watersheds, Technical Release No. 55 (USDA, 1986), with specific attention given to antecedent moisture conditions, flood routing, time of concentration, and peak discharge specifications included therein and in Hydrology National Engineering Handbook, Section 4 (USDA, 1985), both by the U.S. Department of Agriculture, Natural Resources Conservation Service.

[b] Note that when TR-55 is used for natural system-based approaches and practices encouraged herein, calculations must be performed on a detailed-small-subarea basis.

[c] Use of Technical Release No. 20 and other methods listed in Table 1[1] are also acceptable, provided the Township Engineer provides written approval to the methods prior to submission.

[1] Editor's Note: See § 174-24.

[d] The design professional's selection of a specific method shall be based on the suitability of the method for the given project site conditions with due consideration to the limitations of the method chosen. Table 1 herein summarizes the computational methods available.[2]

[2] Editor's Note: See § 174-24.

[e] Calculations shall be based on average antecedent moisture conditions (for the Soil Cover Complex Method only for example, TR-55, TR-20)

[f] Calculations shall be based on a Type II distribution storm (for Soil Cover Complex Method, TR-55, TR-20).

[g] Calculations shall be based on the entire disturbed area (including disturbed pervious area) and must be performed for detailed subdrainage areas for the project site.

[h] The averaging or weighting of a composite curve number (CN) or runoff coefficient is prohibited. Runoff shall be referenced from the Urban Hydrology for Small Watersheds Technical Release No. 55 (USDA, 1986). Coefficients for equivalent ground cover conditions shall be used if a runoff method other than the Soil Cover Complex Method is used.

[i] Predevelopment runoff calculations for wooded areas shall use a CN for ground cover of "woodland in good condition." Portions of a site having more than one viable tree of DBH of six inches or greater per 1,500 square feet shall be considered wooded were such trees existed within three years of application.

[j] Predevelopment runoff calculations for all other portions of a site shall use a CN for ground cover of "meadow in good condition," even if the area is impervious.

[k] The following design storms shall be analyzed for the peak discharge and volumes from the predevelopment and postdevelopment conditions. These values are applicable to the Soil-Cover Complex Method:

[i] A one-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2);

[ii] A two-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2);

[iii] A ten-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2);

[iv] A twenty-five-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2);

[v] A fifty-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2);

[vi] A one-hundred-year, twenty-four-hour storm (rainfall per NOAA Atlas 14, Volume 2).

[l] The Township Engineer may permit the use of the Rational Method for calculation of runoff on land developments of five acres or less and for the design of storm sewers.

[2] Site-specific limitations.

[a] Infiltrating the entire Re_v volume in § 174-20C(1)(b)[1] (above) may not be feasible on every site due to site-specific limitations such as soil type or shallow depth to bedrock. If it cannot be physically accomplished, then the design professional shall be responsible for showing that this cannot be physically accomplished. If it cannot be physically accomplished, then the retention (infiltration) volume Re_v required shall be as much as can be physically accomplished with a minimum of the first one inch of runoff from all proposed impervious and disturbed pervious areas. Control of the first one inch of runoff by alternative BMP designs, such as green roofs, capture and reuse systems, etc., may be allowed on sites where it has been demonstrated to the satisfaction of the Township Engineer that infiltration of the first one inch of runoff cannot be achieved.

[b] The minimum recharge volume (Re_v) required would, therefore, be computed as:

	Rev = I (inches) * [impervious area (square feet) + disturbed pervious area (square feet)] / 12 (inches/foot) = cubic feet (cf)
	An asterisk (*) in equations denotes multiplication.
	Where: I = The maximum equivalent infiltration amount (inches) that the site can physically accept or 1.0 inch, whichever is greater.

[c] The retention volume values derived from the methods in § 174-20C(1)(b)[1] and/or § 174-20C(1)(b)[2] is the minimum volume the applicant must control through an infiltration BMP facility. However, if a site has areas of soils where additional volume of retention can be achieved, the applicant is encouraged to infiltrate as much of the stormwater runoff from the site as possible.

[3] The ground cover assumptions used for selection of runoff coefficient for calculation of the predevelopment runoff for both redevelopment and development shall be as follows:

[a] Woodland areas shall use a ground cover of woodland in good condition. Portions of a site having more than one viable tree of a DBH of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.

[b] Agricultural sites shall use ground cover of pasture in good condition.

[c] All other portions of a site shall use ground cover of meadow in good condition even if the area is impervious.

[d] Weighted averaging of ground cover for calculation purposes is not permitted.

(2) Soils. A detailed soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified design professional and at a minimum address soil permeability, depth to bedrock, depth to groundwater and/or seasonal high water, and subgrade stability. The general process for designing the infiltration BMP shall be:

(a) Analyze hydrologic soil groups as well as natural and human-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.

(b) Conduct field tests such as double-ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.

(c) Design the infiltration structure for the required retention (Re_v) volume based on field-determined capacity at the level of the proposed infiltration surface.

(d) If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the Township that the soils are conducive to infiltrate by actual field tests on the lots identified.

(3) Karst and/or carbonate geology areas. In areas underlain by karst and/or carbonate geology, the viability and specific design standards of infiltration BMPs at a given site must be determined on a site-specific basis to avoid groundwater contamination and formation and/or expansion of solution channels, sinkholes, and other potentially dangerous karst features. A site evaluation shall be conducted by a qualified professional geologist, geotechnical engineer, or other qualified professional, licensed by the Commonwealth of Pennsylvania, to ascertain the subsurface conditions of soil, rock and groundwater relevant to formation of karst features. Such an evaluation shall include, but not be limited to:

(a) Soil thickness, gradation, anisotropy, and permeability (from existing soil data, borings, and on-site testing) to determine the capacity and rate of infiltration of the soil, and relative depth of soil necessary to protect against sinkhole formation.

(b) Karst characteristics of geologic units underlying the site (from current publications, maps and information of U.S. Geological Survey, PA Geological Survey, PA Department of Transportation, etc.).

(c) Inventory of existing karst landforms, visual indications and/or surface manifestations of subsurface features or other karst features (from interviews with Township representatives familiar with known problem areas, review of aerial photography, and site reconnaissance.).

(d) Geophysical survey of the site to identify locations and extent of existing subsurface karst features.

(e) Effectiveness of soil mantle to remove pollutants from infiltrating water to determine whether or not the need exists for removal of pollutants from stormwater runoff prior to infiltration (for example, soil thickness and soil cation exchange capacity, etc.).

(f) Depth to groundwater and vertical location of water table to carbonate geologic unit (from existing information and/or borings).

(g) Other appropriate site-specific parameters affecting infiltration.

(4) Stormwater hotspots.

(a) Untreated stormwater runoff from hotspots shall not be allowed to recharge into groundwater where it may contaminate water supplies. Therefore, the Re_v requirement shall not be applied to development sites that fit into the hotspot category (the entire WQ_v must still be treated) without adequate prior pretreatment suitable to address the pollution or contaminant(s) in question.

(b) A greater level of stormwater treatment shall be considered at hotspot sites to prevent pollutant washoff after construction.

(c) Residential streets are not normally considered hotspots.

(d) While large highways (average daily traffic volume greater than 30,000) are not designated as stormwater hotspots, highway stormwater management plans shall include BMPs to adequately protect groundwater.

(e) The design of all facilities at hotspots over karst shall include an evaluation of measures to minimize adverse effects.

(5) Extreme caution shall be exercised where infiltration is proposed in SWPAs as defined by the Township or the Chester County Water Resources Authority.

(6) Infiltration facilities shall be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.

(7) Extreme caution shall be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant, and it may contaminate the groundwater. A qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.

(8) The infiltration requirement in high quality (HQ) or exceptional value (EV) waters shall be subject to DEP's Chapter 93 antidegradation regulations.

(9) A detailed hydrogeologic investigation shall be required by the Township. An impermeable liner may be required in detention basins where the possibility of groundwater contamination exists.

(10) The applicant shall provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill. A narrative shall be provided describing the proposed material to be used and the possibility of contamination occurring.

§ 174-21 Water quality requirements.

The applicant shall comply with the following water quality requirements.

A. Developed and redeveloped areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The retention volume computed under § 174-20 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the retention volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. The required water quality volume (WQ_v) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site.

B. The first one inch of runoff from all disturbed areas must be captured and treated prior to release.

C. WQ_v.

(1) Any volume captured and not released from the site as part of the volume control may be subtracted from the volume required to be managed under this standard. To achieve this goal, the following calculation formula is to be used to determine the water quality storage volume (WQ_v) in acre-feet of storage required by this chapter:

	WQv = [(P)(Rv)(A)] / 12
	Where:	WQv	=	Water quality volume (acre-feet).
		P	=	1 inch.
		A	=	Area of the project contributing to the water quality BMP (acres).
		Rv	=	0.05 + 0.0009 (I), where I is the percent of the area that is impervious surface [(impervious area/A)*100].

(2) This volume requirement can be accomplished by the permanent volume of a wet basin or the detained volume from other BMPs, such as bioretention. Where appropriate, wet basins shall be utilized for water quality control and shall follow the guidelines of the PA Stormwater BMP Manual.

D. Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall provide for protection from clogging and unwanted sedimentation.

E. No regulated earth disturbance activities within the Township shall commence until approval by the Township of a plan which demonstrates compliance with postconstruction state water quality requirements.

F. The BMPs shall be designed, implemented, and maintained to meet state water quality requirements and any other more stringent requirements as determined by the Township.

G. To control postconstruction stormwater impacts from regulated earth disturbance activities, state water quality requirements can be met by BMPs, including site design, which provide for replication of preconstruction stormwater infiltration and runoff conditions so that postconstruction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. As described in the DEP Comprehensive Stormwater Management Policy (No. 392-0300-002, September 28, 2002), this may be achieved by the following:

(1) Infiltration: replication of preconstruction stormwater infiltration conditions;

(2) Treatment: use of water quality treatment BMPs to ensure filtering out of the chemical and physical pollutants from the stormwater runoff; and

(3) Stream bank and streambed protection: management of volume and rate of postconstruction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).

H. The temperature and quality of waters and streams shall be maintained through the use of temperature-sensitive BMPs and stormwater conveyance systems.

I. To accomplish the above, the applicant shall submit designs to the Township Engineer for review and approval. Such designs shall achieve the water quality objectives through a combination of different BMPs.

J. If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 10 feet to either side of the top-of-bank of the channel. The buffer shall be maintained with appropriate native vegetation [refer to Appendix H of the Handbook of Best Management Practices for Developing Areas and Section 6.7 of the PA Stormwater BMP Manual (December 2006)]. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained.

K. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Township prior to any site activity.

§ 174-22 Stream bank erosion requirements.

A. The applicant must design a BMP to detain the proposed conditions' two-year, twenty-four-hour design storm to the existing conditions' one-year flow using the SCS Type II distribution.

B. Provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions' one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility).

C. The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake.

§ 174-23 Stormwater peak rate control and management standards.

A. Watershed criteria. Developments must control proposed condition peak runoff rates to predeveloped condition peak runoff rates for the design storms in accordance with Table 23.1. The criteria for peak runoff control are designed to reduce postdevelopment peak flow to predeveloped conditions.

	Table 23.1
	Peak Rate Control Standards for Tredyffrin Township Watersheds
	Proposed Condition Design Storm	Reduce to	Predeveloped Condition Design Storm
	2-year		1-year
	5-year		1-year
	10-year		10-year
	25-year		25-year
	50-year		50-year
	100-year		100-year

B. The ground cover assumptions used for selection of the runoff coefficient for calculation of the predevelopment runoff for both redevelopment and new development shall be as follows:

(1) Woodland areas shall use a ground cover of woodland in good condition. Portions of a site having more than one viable tree of a DBH of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.

(2) Agricultural sites shall use ground cover of pasture in good condition.

(3) All other portions of a site shall use ground cover of meadow in good condition, even if the area is impervious.

(4) Weighted averaging of ground cover for calculation purposes is not permitted.

C. General. Proposed conditions' rates of runoff from any regulated activity shall not exceed the predeveloped peak rate of runoff as specified in § 174-23A and the criteria identified in Table 23.1.

D. Watershed boundaries. The exact location of the stormwater management watershed boundaries as they apply to a given site development shall be determined by mapping the boundaries using two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.

E. Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.

F. Site areas. Where site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the watershed criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management criteria.

G. Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is the most appropriate for the given site conditions as determined by the Township Engineer:

(1) Meet the full requirements specified by Table 23.1 and § 174-23A through G; or

(2) Reduce the total impervious surface coverage on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.

H. A Modified Rational Method analysis may be used for drainage areas smaller than five acres when permitted by the Township Engineer. The term "Modified Rational Method" used herein refers to a procedure for manipulation of the basic Rational Method techniques to reflect the fact that storms with a duration greater than the normal time of concentration for a basin will result in a larger volume of runoff even though the peak discharge is reduced. The methodology and model chosen for use shall be well documented as being appropriate for use in this region, and all relevant assumptions, methodologies, calculations and data used shall be provided to the Township Engineer for review. Information on the Modified Rational Method is presented in the Recommended Hydrologic Procedures for Computing Urban Runoff from Small Watersheds in Pennsylvania (DEP, 1982).

I. Rainfall intensities used for the Modified Rational Method shall be based on the current PennDOT Storm Intensity-Duration-Frequency chart appropriate to the specific site.

J. The Rational Method (that is, Q=CIA) shall be used for calculations of the peak rate of runoff for the design storm sewers and drainage swales but not for the design of stormwater management facilities where a full hydrograph is needed. The equation representing the Rational Method is comprised of the following (in English units):

	Q	=	Peak flow rate, cubic feet per second (CFS).
	C	=	Runoff coefficient, dependent on land use/cover.
	I	=	Design rainfall intensity, inches per hour.
	A	=	Drainage area, acres.

§ 174-24 Calculation methodology.

A. Stormwater runoff from all development sites with a drainage area of greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 24.1 summarizes acceptable computation methods, and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site as approved by the Township Engineer. The Township Engineer may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than five acres. The Soil Cover Complex Method shall be used for drainage areas greater than five acres.

	Table 24.1
	Acceptable Computation Methodologies for Stormwater Management Plans
	Method	Developed By	Applicability
	TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary.
	TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans within the limitations described in TR-55.
	HEC-1/HEC-HMS	US Army Corps of Engineers	Applicable where use of a full hydrologic model computer is desirable or necessary.
	PSRM	Penn State University	Applicable where use of a hydrologic model is desirable or necessary; simpler than TR-20 or HEC-1.
	Modified Rational Method (or commercial computer package based on this method)	Emil Kuichling (1889)	For sites less than five acres, or as approved by the Township and/or Township Engineer.
	Other methods	Varies	Other computation methodologies approved by the Township Engineer.

B. All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to NOAA Atlas 14. If a hydrologic computer model such as HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.

C. The following criteria shall be used for runoff calculations:

(1) For development sites not considered redevelopment, the ground cover used in determining the existing conditions' flow rates shall be as follows:

(a) Wooded sites shall use a ground cover of woodlands in good condition. Portions of a site having more than one viable tree of a DBH of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.

(b) For all other development and redevelopment sites, the ground cover used in determining the existing conditions' flow rates for the developed portion of the site shall be considered as meadow in good condition, even if the area is impervious.

(c) Weighted averaging of ground cover for calculation purposes is not permitted.

D. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods presented in the NOAA Atlas 14, or the PADOT Storm Duration Frequency Charts. Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's equation.

E. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table C-1 in Appendix C.[1]

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

F. Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table C-2 in Appendix C.[2]

[2] Editor's Note: Appendix C is located at the end of this chapter.

G. Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table C-3 in Appendix C.[3]

[3] Editor's Note: Appendix C is located at the end of this chapter.

H. Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.

I. The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using an acceptable method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Township Engineer may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

§ 174-25 Other requirements .

A. Any stormwater facility located on state highway rights-of-way shall be subject to approval by PennDOT.

B. All wetland treatment systems, wet ponds, and wet basin designs shall incorporate the following biologic controls to deter mosquitoes that can cause West Nile Virus:

(1) If a wetland is planned, proper hydrologic soil conditions and the establishment of hydrophytic vegetation should be included to promote the population of the wetland by amphibians and other mosquito predators. Species of emergent vegetation with little submerged growth are preferable. By limiting the vegetation growing below the water surface, larvae lose protective cover, and there is less chance of anaerobic conditions occurring in the water.

(2) Stormwater ponds should include aeration fountains and stocked fish to keep larval mosquito populations in check.

C. Any stormwater management facility (i.e., detention basin) required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions' inflow. Should any stormwater management facility require a dam safety permit under DEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may be required to pass storms larger than the one-hundred-year event.

D. Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands governed by DEP Chapter 105 regulations (as amended or replaced from time to time by DEP) shall be designed in accordance with Chapter 105 and will require a permit from DEP.

E. 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 one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

F. Any drainage conveyance facility and/or channel not governed by Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm. Conveyance facilities to or exiting from stormwater management facilities (e.g., 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 PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

G. Storm sewers must be able to convey proposed conditions' runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.

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

I. The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Township reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.

J. Underground stormwater management systems must be designed to store the required recharge volume below the invert of any positive overflow pipe.

K. Roof drains shall discharge to infiltration areas or vegetative BMPs to the maximum extent practicable.

L. Roof drains shall not be connected to streets, sanitary sewer or storm sewers, roadside ditches, or into the legal ROW of streets.

M. When it is more advantageous to connect directly to streets or storm sewers, connections of roof drains to streets or roadside ditches may be permitted on a case-by-case basis as determined by the Township Engineer.

N. No person shall modify, remove, fill, landscape, or alter any existing stormwater control or BMP without written approval by the Township unless it is part of an approved maintenance program.

O. No person shall place any structure, fill, landscaping, or vegetation into a stormwater control or BMP or within a drainage easement which would limit or alter the functioning of the stormwater control or BMP without the written approval of the Township.