Chadds Ford Township, PA Article III Stormwater Management

§ 105-301 General requirements.

A. Applicants proposing regulated activities in the municipality which do not fall under the exemption criteria shown in § 105-106 shall submit a stormwater management site plan consistent with this chapter and the applicable watershed stormwater management plan to the municipality 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. No regulated activity within the municipality shall commence until the municipality issues approval of an SWM plan, which demonstrates compliance with the requirements of this chapter.

(1) For any site with proposed regulated earth disturbance equal to or greater than one acre where, after a close evaluation of alternative site designs, it proves to be impracticable to meet the mandatory minimum volume and infiltration control standards of this chapter on site, the municipality may approve measures other than those in this chapter after consultation with and evaluation by PADEP that the alternate site design meets state water quality requirements and does not conflict with state law, including, but not limited to, the Clean Streams Law.

(2) For any site with proposed regulated earth disturbance that is less than one acre where, after a close evaluation of alternative site designs, it proves to be impracticable to meet any one or more of the mandatory minimum standards of this chapter on site, the municipality may approve measures other than those in this chapter.

(3) "Regulated earth disturbance" is defined to mean any activity involving earth disturbance subject to regulation under 25 Pa. Code, Chapter 92; 25 Pa. Code, Chapter 102; or the Clean Streams Law.

C. The applicant is required to design the site to minimize surface discharge of stormwater and the creation of impervious surfaces in order to maintain, as much as possible, the natural hydrologic regime.

D. The SWM site plan must be designed consistent with the sequencing provisions of § 105-304 to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity. The SWM site plan designer must proceed sequentially in accordance with Article III of this chapter.

E. Stormwater drainage systems shall be designed in order to preserve natural flow conditions to the maximum extent practicable.

F. Existing drainage discharge onto adjacent property shall not be increased in any manner without written permission/notification from/to the affected property owner(s). Such discharge shall be subject to any applicable discharge criteria specified in this chapter and still must meet the requirements of Act 167.

G. 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 erosion, sedimentation, flooding, or other impacts will result from the concentrated discharge.

H. Where a development site is traversed by a stream, drainage easements of 10 feet shall be provided on either side of, and conform to the line of, such streams.

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

J. All stormwater runoff from new development or redevelopment shall be pretreated for water quality prior to discharge to surface water or groundwater. Rooftop runoff may go directly to an infiltration BMP or be evapotranspirated.

K. All regulated activities within the municipality 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 (e.g., during construction); and

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

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

M. Post-construction water quality protection shall be addressed as required by § 105-306.

N. Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.

O. All BMPs used to meet the requirements of this chapter shall conform to the state water quality requirements and any more-stringent requirements as set forth by the municipality.

P. Techniques described in Appendix D (Low-Impact Development) of this chapter shall be considered because they reduce the costs of complying with the requirements of this chapter and the state water quality requirements.[1]

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

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

(1) Total contributing drainage 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 and wellheads.

(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.

(16) Enhancement of aesthetic and property values.

R. The design of all stormwater management facilities shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems. The municipality reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.

S. The applicant may meet the stormwater management criteria through off-site stormwater management measures as long as the proposed measures are in the same subwatershed as shown in Appendix A.[2]

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

T. Stormwater hotspots. Stormwater runoff from hotspots shall be pretreated prior to surface water or groundwater infiltration to prevent pollutant runoff. Industrial sites referenced in 40 CFR 125 are examples of hotspots.

(1) Below is a list of examples of hotspots:

(a) Vehicle salvage yards and recycling facilities.

(b) Vehicle fueling stations.

(c) Vehicle service and maintenance facilities.

(d) Vehicle and equipment cleaning facilities.

(e) Fleet storage areas (bus, truck, etc.).

(f) Industrial sites based on Standard Industrial Classification Codes.

(g) Marinas (service and maintenance areas).

(h) Outdoor liquid container storage.

(i) Outdoor loading/unloading facilities.

(j) Public works storage areas.

(k) Facilities that generate or store hazardous materials.

(l) Commercial container nursery.

(m) Contaminated sites/brownfields.

(n) Other land uses and activities as designated by an appropriate review authority.

(2) The following land uses and activities are not normally considered hotspots:

(a) Residential streets and rural highways.

(b) Residential development.

(c) Institutional development.

(d) Office developments.

(e) Nonindustrial rooftops.

(f) Pervious areas, except golf courses and nurseries [which may need an integrated pest management (IPM) plan].

(3) While streets and highways [average daily traffic volume (ADT) greater than (30,000)] are not considered stormwater hotspots, it is important to ensure that highway stormwater management facilities are designed to adequately protect receiving streams and/or groundwater.

(4) The Environmental Protection Agency's (EPA) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.

U. The following standards for protection of downgradient properties from off-site conveyance must be accomplished:

(1) For any location where a new concentrated discharge of stormwater from any frequency rainfall event, up to and including the one-hundred-year, twenty-four-hour event, will flow onto a downgradient property, the following are required:

(a) A drainage easement (or other legal agreement/approval) must be obtained for conveyance of discharges onto or through adjacent properties.

(b) The conveyance must be designed to avoid erosion, flooding, or other damage to the properties through which it is being conveyed.

§ 105-302 Permit requirements by other governmental entities.

The following permit requirements may apply to certain regulated earth disturbance activities and must be met prior to commencement of regulated earth disturbance activities, as applicable:

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

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

D. Any stormwater management facility that would be located on or discharging to a state highway right-of-way or require access to or from a state highway shall be 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 PADEP under Title 25, Pennsylvania Code, Chapter 105.

§ 105-303 Erosion and sediment control during regulated earth disturbance activities.

A. No regulated earth disturbance activities within the municipality shall commence until the municipality receives an approval from the PADEP in compliance with Title 25, Chapter 102, of the Pennsylvania Code of an erosion and sediment control plan for construction activities, if applicable.

B. PADEP has regulations regarding erosion and sediment control under Title 25, Pennsylvania Code, Chapter 102.

C. In addition, under Title 25, Pennsylvania Code, Chapter 92, a PADEP NPDES construction activities permit is required for regulated earth disturbance activities.

D. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or County Conservation District must be provided to the municipality. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of § 105-403A.

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

F. Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part 1, Commonwealth Pennsylvania, DEP, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Delaware County Conservation District, as appropriate, and the standards and specifications of the appropriate municipal government.

G. Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following:

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

(2) In order to ensure compliance with Chapter 102, the timing of the installation and operation of the infiltration BMP shall be at the discretion of the Municipal Engineer.

H. Soil erosion and sedimentation control; general regulations.

(1) All subdivisions and land developments are required to comply with the Clean Streams Law of Pennsylvania and Chapter 102 of the Rules of Regulations of the Pennsylvania Department of Environmental Protection, as amended from time to time.

(2) The landowner or developer shall be responsible for the submission and approval of an application to the regional office of the Pennsylvania Department of Environmental Protection for a permit to allow any earthmoving activity where more than 25 acres are disturbed at one time. All landowners or developers are required to obtain such permit except as otherwise provided in Section 102.41, Permits and Plans, of Chapter 102 of the Rules and Regulations of the Pennsylvania Department of Environmental Protection (DEP), as amended from time to time.

(3) The soil erosion and sedimentation control plan must be available at all times at the construction site. The permit allowing earthmoving activity shall be obtained by the landowner or developer before any construction on the site shall begin.

(4) The sequence of soils stabilization to control soil erosion and sedimentation shall be planned so as to compensate for seasonal effects such as the inability to establish vegetation during winter months, the influence of freezing, and the like.

(5) Measures shall be taken to preclude the tracking of mud, soil and the like from construction vehicles and equipment onto streets which serve the site. Such measures shall include temporary scrub pads with collector sumps and cleaning devices, which construction vehicles shall pass through prior to leaving the site.

(6) Erosion and sedimentation control measures.

(a) Measures used to control erosion and reduce sedimentation shall, as a minimum, meet the latest revised standards, specifications and/or regulations of:

[1] The United States Department of Agriculture Soil Conservation Service as adopted by the Delaware County Conservation District in its Erosion and Sediment Control Handbook.

[2] The Commonwealth of Pennsylvania Department of Environmental Protection in its Soil Erosion and Sedimentation Control Manual.

[3] "Standards and Specifications for Soil Erosion and Sediment Control in Developing Areas" by the United States Department of Agriculture, Soil Conservation Service, College Park, Maryland.

(b) Erosion and sedimentation control measures shall be required on all erosion and sedimentation control and stormwater management plans for all subdivisions and/or land developments within the Township. The Township or its designated representative shall ensure and enforce compliance with the appropriate standards.

(7) All soil erosion and sedimentation control and grading activities shall be performed in such a manner so as not to endanger or damage public or private property or to cause physical damage or personal injury. A landowner or developer shall be responsible for any property damage or personal injury caused by his activities.

(8) There shall be no increase in discharge of sediment or other solid materials from the site as a result of stormwater runoff; and, in the event of any increase, the landowner and/or developer shall be responsible.

(9) Soil erosion and sedimentation control devices, such as temporary vegetation and mulch, temporary earthen berms, interceptor dikes, ditches, diversion terraces, rock filter berms, crushed stone tire scrubbers, silt basins, silt fences, and the like, appropriate to the scale of operations, shall be installed concurrent with earthmoving activities and whenever any situation is created which would contribute to increased erosion.

(10) Earthmoving operations shall be minimized where possible and practicable to preserve desirable natural features and the topography of the site.

(11) Stripping of vegetation, regrading or other development shall be done in such a way that will minimize erosion.

(12) To the maximum extent possible, mature, healthy trees of 12 inches or greater in caliper and other significant existing vegetation shall be retained and protected. Such trees shall not be removed, except as provided on the approved subdivision and/or land development plan. The filling of soil more than five inches is presumed to extend out from the tree as far as the tree's branches extend outward.

(13) Land disturbance shall be limited to the actual construction site and an access strip. The amount of disturbed area and the duration of exposure shall be kept to a practical minimum. Disturbed areas shall be stabilized with vegetation, mulch, erosion control fabric, and the like as soon as possible after earthmoving procedures.

(14) Provisions shall be made to effectively accommodate the increased runoff caused by changed soil and surface conditions during and after development. Water runoff shall be minimized and retained on site wherever possible to facilitate groundwater recharge.

(15) Temporary vegetation and/or mulching shall be used to protect critical areas during development. ("Critical areas" shall be construed to mean those portions of the site which are extremely vulnerable to soil erosion.)

(16) The permanent final vegetation and structural erosion control and drainage measures shall be installed as soon as practical in the development in accordance with the approved plans.

(17) Sediment in the runoff water shall be trapped until the disturbed area is stabilized by the use of debris and sediment basins, silt fences or other approved measures. Sediment deposits in basins, silt fences, and the like shall be removed at periodic intervals during the construction period, as directed by the Township.

(18) Procedures shall be established for protecting soils and rock or geologic formations with water supply potential from contamination by surface water or other source or disruption caused by construction activity.

(19) Silt fences shall be utilized in lieu of straw bale silt barriers downhill of all construction areas. In general, straw bale silt barriers will be allowed only on projects with a construction period of less than 60 days and where the uphill drainage area is less than 1/2 acre. In all applications, silt fences and straw bale silt barriers shall be securely anchored in place and embedded into the soil.

(20) Silt fences or silt traps shall be placed at all inlets, headwalls, basin outlets and similar drainage structures during the construction period in order to prevent sediment from entering any watercourse, storm drainage system, or other areas downstream.

(21) Crushed-stone tire scrubbers shall be placed at all entrances to construction areas. Tire scrubbers shall be sufficient width and length to prevent the transportation of sediment off of the construction site.

(22) Temporary and permanent seeding and mulch specifications shall be noted on all plans. The specifications shall include lime and fertilizer rates of application, as well as other provisions regarding procedures and materials. In critical areas, the Township may require hydroseeding.

(23) During roadway grading, interceptor dikes shall be installed on all roadway subgrades with slopes in excess of 5% to prevent erosion of the subgrade. The interceptor dikes shall divert stormwater runoff into silt traps or silt fences.

(24) The crushed stone base course for driveways, roadways and parking areas shall be applied as soon as possible after grading procedures, in order to prevent erosion of the subgrade.

(25) Drainage swales and ditches, and all slopes greater than 3:1, shall be protected against erosive velocities with erosion control measures such as erosion control fabric and other material as approved by the Township.

(26) Energy dissipators and/or stilling basins shall be installed at the outlet end of all storm drainage facilities.

(27) Whenever sedimentation is caused by stripping vegetation, regrading or other development, it shall be the responsibility of the person causing such sedimentation to remove the accumulated sediment from all adjoining or downstream properties, surfaces, drainage systems and watercourses and to repair any damage at his expense as quickly as possible.

(28) All necessary soil erosion and sediment control measures installed under this chapter shall be adequately maintained by the developer after completion of the approved plan or until such measures are permanently stabilized as determined by the Township.

(29) In any construction in the Township, no construction trash, trees or parts of trees or any foreign material shall be buried at the site. Each contractor shall maintain a facility wherein all trash shall be deposited and thereafter removed from the site to an approved landfill. In the case of placement of fill, all fill must be approved by the Code Enforcement Officer prior to placement of the same. Any contractor desiring to place fill other than the natural earth on the site shall notify the Code Enforcement Officer and request an inspection.

[Amended 9-7-2016 by Ord. No. 146]

(a) Whenever fill other than clean soil or earth is utilized, clean soil shall be placed over the top of the fill to a depth sufficient as determined by the Township to conceal all materials other than clean soil or earth;

(b) All wooden materials shall be excluded from clean fill.

(c) If anything other than clean soil or earth is identified in the clean fill material, prior to delivery onto the construction site, the developer must document and certify the following information to the Township:

[1] Origin of the material, location and prior use of clean fill material;

[2] Name, address, phone contact of hauler/supplier of clean fill material;

[3] Date and location of delivery of clean fill material;

[4] Statement of compliance with DEP criteria for noncontamination of clean fill material, signed, dated and notarized;

[5] All pieces of concrete and/or used asphalt to be broken into pieces no larger than four-inch to six-inch pieces, and is mixed with clean granular material. Asphalt and concrete must be free of all construction debris (including steel, wood, insulation, etc.) or other questionable materials as determined by the Township; and

(d) A CBR (compaction rate) 95% to 98% must be achieved, and six-inch to eight-inch lifts must be adhered to during compaction of the "clean fill." Compaction reports are required for all structural fill.

§ 105-304 Nonstructural project design process (sequencing to minimize stormwater impacts).

The design of all regulated activities shall include the following to minimize stormwater impacts to reduce the surface discharge of stormwater, reduce the creation of unnecessary impervious surfaces, prevent the degradation of waters of the commonwealth, and maintain as much as possible the natural hydrologic regime of the site:

A. The applicant may apply low-impact development (LID) methods such as those listed in Appendix D, provided that use of this method does not conflict with other local codes.[1]

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

B. The applicant shall demonstrate that the design process follows the sequence noted below. 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) The following items in this subsection shall be addressed prior to development of other stormwater management site plan design elements:

(a) 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, stream buffers, and hydrologic soil groups. Land development, any existing recharge areas, and other requirements outlined in the municipal SALDO shall also be included.

(b) Establish a stream buffer according to § 105-306D.

(c) Prepare a draft project layout avoiding sensitive areas identified in § 105-304B(1)(a).

(d) Identify site-specific existing conditions drainage areas, discharge points, recharge areas, and Hydrologic Soil Groups A and B (areas conducive to infiltration).

(e) Evaluate nonstructural stormwater management alternatives:

[1] Minimize earth disturbance.

[2] Minimize impervious surfaces.

[3] Break up large impervious surfaces.

(f) Determine into what management district the site falls (Appendix A), and conduct an existing conditions' runoff analysis.

(2) The following items in this subsection may be addressed in any order, provided that all items in § 105-304B(1) have been completed.

(a) Satisfy the infiltration objective (§ 105-305) and provide for stormwater pretreatment prior to infiltration.

(b) Provide for water quality protection in accordance with § 105-306, Water quality requirements.

(c) Provide stream bank erosion protection in accordance with § 105-307, Stream bank erosion requirements.

(d) Prepare a 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.

(e) Conduct a proposed conditions' runoff analysis based on the final design that meets the management district requirements (§ 105-308).

(f) Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge, or other structural control.

§ 105-305 Infiltration volume requirements.

A. Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created.

B. If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the municipality that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc., vs. a financial hardship as defined in § 105-202). If it can be physically accomplished, the volume of runoff to be infiltrated shall be determined from § 105-305C(2) depending on demonstrated site conditions and shall be the greatest volume that can be physically infiltrated. For example:

(1) Any applicant (developer or redeveloper) shall first attempt to infiltrate the volume required in § 105-305C(2)(a).

(2) If the § 105-305C(2)(a) requirement cannot be physically accomplished, then the applicant is required to attempt to infiltrate the volume required in § 105-305C(2)(b).

(3) Finally, if the § 105-305C(2)(b) infiltration volume cannot be physically accomplished, the applicant must, at a minimum, infiltrate the maximum volume the site can accommodate.

C. Infiltration BMPs shall meet the following minimum requirements:

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

(a) A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone.

(b) An infiltration rate sufficient to accept the additional stormwater volume and dewater completely as determined by field tests conducted by the applicant's design professional.

(c) The infiltration facility shall be capable of completely draining the retention (infiltration) volume (Re_v) within three days (72 hours) from the end of the design storm.

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

(a) Modified Control Guideline One (MCG-1) of the Pennsylvania Stormwater Best Management Practices (PA BMP) Manual — 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 post-development runoff volume and the predevelopment runoff volume. The post-development 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 20% of existing impervious area, when present, shall be considered meadow in good condition.

(b) Infiltrating the entire Re_v volume in § 105-305C(2)(a) (above) may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:

[1] At least the first one inch of runoff from new or replacement impervious surfaces shall be infiltrated.

Re_v = 1 (inch) * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)

An asterisk (*) in equations denotes multiplication.

[2] The retention volume value derived from the methods in § 105-305C(2)(a) or (b) is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.

[3] Only when the minimum infiltration requirement cannot be physically accomplished, a waiver from § 105-305, Infiltration volume requirements, is required from the municipality.

D. 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 minimum address soil permeability, depth to at least two feet below the stormwater management facility, and subgrade stability. The general process for designing the infiltration BMP shall be:

(1) Analyze hydrologic soil groups as well as natural and man-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.

(2) Provide 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.

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

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

E. Infiltration facilities should, to the greatest extent practicable, be located to avoid introducing contaminants via groundwater and be in conformance with an approved source water protection assessment or source water protection plan.

F. Roadway drainage systems should provide an opportunity to capture accidental spills. Road de-icing material storage facilities shall be designed to avoid salt and chloride runoff from entering waterways and infiltration facilities. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.

G. The antidegredation analysis found in Chapter 93 shall be applied in HQ or EV streams.

H. An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. The municipality may require a detailed hydrogeologic investigation.

I. The applicant should provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.

§ 105-306 Water quality requirements.

The applicant shall comply with the following water quality requirements of this article:

A. To control post-construction stormwater impacts from regulated activities and conform to state water quality requirements, BMPs which replicate predevelopment stormwater infiltration and runoff conditions must be provided in the site design such that post-construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. This may be achieved by the following:

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

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

(3) Stream bank and stream bed protection: management of volume and rate of post-construction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).

B. Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 105-305 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the calculated water quality volume (WQ_v) is greater than the volume required to be infiltrated as described in § 105-305C(2), then the difference between the two volumes shall be treated for water quality by an acceptable stormwater management practice(s). 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.

(1) To achieve this requirement, the following criterion is established:

(a) From Control Guideline 1 (CG-1) in the PA BMP Manual, the water quality volume shall be the net two-year, twenty-four-hour volume. The net volume is the difference between the post-development runoff volume and the predevelopment runoff volume. The post-development 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 20% of existing impervious area, when present, shall be considered meadow in good condition.

(2) This volume requirement can be managed by the permanent volume of a wet basin or the detained volume from other BMPs. Where appropriate, wet basins shall be utilized for water quality control and shall follow the guidelines of the PA BMP Manual referenced in Appendix B.[1]

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

(3) 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.

C. The temperature of receiving waters shall be protected through the use of BMPs that moderate temperature.

D. If a perennial or intermittent stream passes through, or a water body (i.e., lake, pond, and wetland) is present on the site, the applicant shall create a riparian buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel, lake, or wetland. The buffer area shall be planted with native vegetation and maintained in a vegetated state (Refer to Appendix B, Pennsylvania Native Plant List, contained in the PA BMP Manual).

(1) The following provisions also apply to riparian buffers on lots in existence at the time of adoption of this chapter:

(a) If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback or 25 feet, whichever is greater.

(b) If a stream traverses a site in a manner that significantly reduces the use of the site, the buffer may be either:

[1] Reduced to 25 feet on either side, with municipal approval; or

[2] Reduced to 10 feet with municipal waiver; or

[3] If an existing buffer is legally prescribed (e.g., deed covenant, easement, etc.) and it exceeds the requirement of this chapter, the existing buffer shall be maintained.

(2) Permitted uses within the buffer include the following, subject to municipal approval and provided that they comply with all federal, state, and local regulations:

(a) Recreational trails. See Appendix H, Riparian Buffer Trail Guidelines.[2]

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

(b) Utility rights-of-way.

(c) Bridges.

(d) Other uses subject to municipal approval.

E. 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.

§ 105-307 Stream bank erosion requirements.

A. In addition to controlling the water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement to control stream bank erosion is to 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. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) to release the proposed conditions' one-year storm for a minimum of 24 hours from a point in time when the maximum volume of water from the one-year storm is stored in a proposed BMP (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).

B. 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. When the calculated orifice size is below three inches, gravel filters (or other methods) are recommended to discharge low-flow rates subject to the Municipal Engineer's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function. All facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.

§ 105-308 Stormwater peak rate control.

A. Each watershed has been divided into either stormwater management districts or release rate districts as shown on the respective Management District or Release Rate Maps in Appendix A.[1]

(1) In addition to the watershed-specific requirements specified in Tables 308.1 and 308.2 below, the erosion and sedimentation control (§ 105-303), the nonstructural project design (§ 105-304), the infiltration (§ 105-305), the water quality (§ 105-306), and the stream bank erosion (§ 105-307) requirements shall be implemented.

(2) Standards for managing runoff from each subarea in a watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms are shown in Tables 308.1 and 308.2. Development sites located in each of the management/release rate districts must control proposed conditions' runoff rates to existing conditions' runoff rates for the design storms in accordance with the table.

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

B. General. Proposed conditions' rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Appendix A) and this section of this chapter.

C. District boundaries. The boundaries of the stormwater management districts are shown on an official map that is available for inspection at the municipal and county planning offices. A copy of the official map at a reduced scale is included in Appendix A. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most-accurate data required) provided as part of the SWM site plan.

D. Sites located in more than one district or watershed. For a proposed development site located within two or more stormwater management district subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located. The natural hydrology of each respective subarea shall be maintained, and drainage shall not be redirected from one subarea to another. Under circumstances where the applicant shows this cannot be accomplished, a waiver is required by the municipality.

	Table 308.1
	Control Criteria for Chester Creek Watershed Stormwater Management Districts
	District	Control Criteria
	100%	Post-development peak discharge for all design storms must be no greater than predevelopment peak discharges
	75%	Post-development peak discharge for all design storms must be no greater than 75% of the predevelopment peak discharges.
	50%	Post-development peak discharge for all design storms must be no greater than 50% of the predevelopment peak discharges.

	Table 308.2
	Peak Rate Control Standards in the Brandywine Creek Watersheds
	Proposed Condition Design Storm	Reduce to	Existing Condition Design Storm
	2 - year		50% of the pre 1 - year
	5 - year		50% of the pre 5 - year
	10 - year		50% of the pre 10 - year
	25 - year		50% of the pre 25 - year
	50 - year		50% of the pre 50 - year
	100 - year		50% of the pre 100 - year

Post-development peak discharge for all design storms must be no greater than 50% of the predevelopment peak discharges.

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. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.

F. Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the peak rate control standards noted above. Unimpacted areas for which the discharge point has not changed are not subject to the peak rate control standards.

G. Downstream hydraulic capacity analysis. Downstream hydraulic capacity analysis conducted at the direction of Chadds Ford Township shall use the following criteria for determining adequacy for accepting peak flow rates:

(1) Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year storm event within their banks at velocities consistent with protection of the channels from erosion. Velocities shall be based upon criteria and methodologies acceptable to the municipality.

(2) Natural or man-made channels or swales must be able to convey increased twenty-five-year storm event runoff without creating any increased hazard to persons or property.

(3) Culverts, bridges, storm sewers or any other hydraulic facilities which must pass or convey flows from the tributary area must be designed in accordance with PADEP Chapter 105 regulations (if applicable) and, at a minimum, pass the increased twenty-five-year storm event runoff.

(4) Water quality requirements defined in § 105-306 must be met.

(5) Post-construction peak rates shall not exceed the existing peak rates for the respective subarea.

(6) Meet the full requirements specified by Tables 308.1 and 308.2 and § 105-308A through G.

§ 105-309 Calculation methodology.

A. Stormwater runoff from all development sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 309.1 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval of the municipality.

	Table 309.1
	Acceptable Computation Methodologies for SWM Site Plan
	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 where limitations are described in TR-55
	HEC-1/HEC-HMS	United States Army Corps of Engineers	Applicable where use of a full hydrologic computer model is desirable or necessary
	Rational Method (or commercial computer package based on Rational Method)	Emil Kuichling (1889)	For sites up to 5 acres, or as approved by the municipality and/or Municipal Engineer
	Other methods	Varies	Other computation methodologies approved by the municipality and/or Municipal 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. Rainfall depths shall be according to NOAA Atlas 14 values consistent with a partial duration series. When stormwater calculations are performed for routing procedures or water quality functions, the duration of rainfall shall be 24 hours.

C. The following criteria shall be used for peak rate 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 "woods in good condition." Portions of a site having more than one viable tree measuring a diameter at breast height (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) The undeveloped portion of the site, including agriculture, bare earth, and fallow ground, shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Table F-1 or F-2 in Appendix F of this chapter.[1]

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

(2) For redevelopment sites, the ground cover used in determining the existing conditions' flow rates for the developed portion of the site shall be based on the following ground cover assumptions which shall be used:

(a) For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."

(b) For areas that are not woods or not impervious surfaces, predevelopment calculations shall assume ground cover of "meadow."

(c) For areas that are impervious surfaces, predevelopment calculations shall assume at least 20% of the existing impervious surface area to be disturbed as "meadow" ground cover.

D. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584). Times-of-concentration shall be calculated based on the methodology recommended in the respective model used. 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 F-1 in Appendix F of this chapter.

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

G. Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114 HEC 15). Values for Manning's roughness coefficient (n) shall be consistent with Table F-3 in Appendix F 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 municipality 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.

§ 105-310 Other requirements.

A. All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H, PADEP document 363-0300-001, "Design Criteria - Wetlands Replacement/Monitoring," or contact the Pennsylvania State Cooperative Wetland Center (www.wetlands.psu.edu/) or the Penn State Cooperative Extension Office (www.extension.psu.edu/extmap.html).

B. Any stormwater 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 of 2.0 feet 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 PADEP 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 require the passing of storms larger than the one-hundred-year event.

C. 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 storm event. The larger the events (fifty-year and one-hundred-year) must also be safely conveyed in the direction of natural flow without creating additional damage to any drainage structures, nearby structures, or roadways.

D. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from the facility.

E. Roadway crossings or structures located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program - floodplain management requirements.

F. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

G. Adequate erosion protection and energy dissipation shall be provided along all open channels and at all points of discharge. Design methods shall be consistent with the Federal Highway Administration Hydraulic Engineering Circular Number 11 (Publication No. FHWA-IP-89-016) and the PADEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008).

§ 105-311 Design criteria for stormwater management.

A. Detention/retention basins.

(1) All outlet control structures shall be constructed of galvanized steel, aluminum or concrete, properly anchored to prevent flotation, and equipped with childproof, nonclogging trash racks with overall design openings of 12 inches or greater in diameter, except those openings designed to carry perennial stream flows.

(2) Temporary sedimentation controls shall be provided during construction to prevent the flow of sediment through the basin outlet pipe. Such measures may include temporary riser pipes, rock-filled gabions, plywood stand-boxes, silt fences, and the like.

(3) Emergency spillways. Whenever possible, the emergency spillway for basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of reinforced concrete, vegetated earth, concrete rubble, and the like. All emergency spillways shall be constructed so that the basin berm is protected against erosion. The minimum capacity of all emergency spillways shall be such that the combined capacity of the emergency spillway and the principal from the one-hundred-year design storm after development. Emergency spillways shall extend along the upstream and downstream berm embankment slopes. The emergency spillway shall not discharge stormwater over earthen fill and/or easily erodible material without adequate protection against erosion.

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

(5) Basin outlets. Energy-dissipating devices (rip-rap, stilling basin, concrete aprons, and the like) shall be placed at all basin outlets. Rock-level spreader berms shall be required where basins do not discharge into an existing drainage swale, ditch or channel. Concrete endwalls shall be placed at all basin outlets. All basin outlet pipes 12 inches in diameter or greater shall be equipped with childproof devices.

(6) Slope of detention basin embankment. The maximum slope of earthen basin embankments shall be 3:1. The top or toe of any slope shall be located a minimum of 15 feet from adjacent property lines, with the exception of the downstream property line, where the toe of the embankment shall be placed a sufficient distance to allow for energy-dissipating devices but in no case less than 30 feet unless approved otherwise by the Township. Whenever possible, the side slopes and basin shape shall blend with the natural topography. Straight side slopes and rectangular basins shall be avoided whenever possible.

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

(8) Construction specifications. The plans shall indicate the construction specifications and compaction requirements for all detention/retention basins.

(9) Slope of basin bottom. In order to ensure proper drainage of detention basins, a minimum grade of 2% shall be maintained for all sheet flow. A minimum grade of 1.0% shall be maintained for all channel flow.

(10) Cut-off trench. A cut-off trench shall be excavated along the center line of the dam on earth fill embankments. The minimum depth shall be three feet. The minimum bottom width shall be 10 feet or wide enough to permit operation of compaction equipment. The side slopes shall be no steeper than 1:1. The trench shall be kept free from standing water during the backfilling operations.

(11) Overflow system. An overflow system shall be provided to carry flow to the detention basin when the capacity of the storm drain pipe system is exceeded. The overflow system shall be of sufficient capacity to carry the difference between the one-hundred year and the ten-year peak flow rates.

B. Grading and landscaping of basins.

(1) Cuts and fills. No excavation or fill shall be made with a cut or fill slope steeper than three horizontal to one vertical, except where the excavation or fill is sufficiently stable to prevent sliding or erosion and will not result in property damage or personal injury. A written statement shall be required from a civil engineer licensed by the Commonwealth of Pennsylvania having experience in soils engineering certifying that he has inspected the site and that any proposed deviation from the slope specified above should not endanger any property or result in personal injury. Retaining walls will be required if a stable slope cannot be maintained. Any retaining wall design must be designed by an experienced structural engineer licensed by the Commonwealth of Pennsylvania and approved by the Township. The toe of any cut or fill slope must be located a minimum of 15 feet from adjacent property lines, with the exception stated above.

(2) Retention/detention basins shall be designed to utilize the natural contours of the land whenever possible. When such design is impracticable, the construction of the basin shall utilize slopes as shallow as possible to blend the structures into the terrain.

(3) A minimum of six inches of topsoil shall be placed on all areas affected by the basin construction (bottom of basin, side slopes, top of berm, and the like).

(4) All earthen basins shall be stabilized with temporary and permanent grasses or other approved ground covers within 15 days after initial construction.

(5) Fencing and/or a suitable vegetation screening shall be provided around all detention/retention basins unless the Board of Supervisors determines that such screening is not necessary.

(6) Basins shall be installed prior to any earthmoving or land disturbances which they will serve.

C. Subsurface disposal of stormwater.

(1) The design and construction of all subsurface facilities shall provide proper procedures to prevent silt from clogging the aggregate backfill.

(2) The following procedures and materials shall be required for all subsurface facilities:

(a) Excavation for the infiltration facility shall be performed with equipment which will not compact the bottom of the seepage bed/trench or like facility.

(b) The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.

(c) Only clean aggregate, free of fines, shall be allowed.

(d) The top and sides of all seepage beds, trenches, or like facilities shall be covered with drainage filtration fabric.

(e) Perforated distribution pipes connected to centralized catch basins and/or manholes with provisions for the collection of debris shall be provided in all facilities. The perforated pipes shall distribute stormwater throughout the entire seepage bed/trench or like facility.

(f) A positive outlet pipe placed at or near the bottom of the seepage bed and/or trench, or like facility, shall be provided.

(g) The landowner or developer shall be responsible for the proper installation, operation and maintenance of all subsurface stormwater management facilities. If, in the opinion of the Township, the underground system is not functioning properly, the landowner or developer shall be required to make the necessary improvements/corrections to the system or provide an alternate stormwater management facility which is functional.

D. Storm sewer system design.

(1) Design flow rate. The storm sewer system shall be designed to carry a fifty-year peak flow rate, with a one-hundred-year peak flow rate at all low points. The peak flow rate into each inlet shall be indicated on the stormwater drainage stormwater management plan. The design flow rate shall be determined by the rational formula, Q=CIA.

(2) Pipe material and gage thickness. All storm sewers shall be either reinforced cement concrete, corrugated aluminum, corrugated galvanized steel pipe or high-density polyethylene pipe (HDPE). Storm sewers shall be of the proper class and thickness to support the above fill material. Pipe class and gage or thickness shall be noted on the plans.

(3) Allowable headwater depth. At all inlets or manholes, the maximum allowable headwater depth shall be one foot below the top of the inlet grate or the manhole cover.

(4) Horizontal pipe deflections. A manhole or inlet shall be provided at all horizontal deflections in the storm pipe system exceeding 5°.

(5) Minimum and maximum cover. A minimum of 18 inches of cover shall be maintained over all storm drain pipes. The top of storm drain pipes shall be at least 0.5 foot below subgrade elevation. The maximum cover over storm drain pipes shall be 10 feet unless approved otherwise by the Township.

(6) Storm sewer system outlets. Storm sewer system outlet pipes shall extend to proposed stormwater management facilities, natural watercourses, and the like. A concrete endwall shall be required on all storm sewer system outlet pipes.

(7) Roof drains. Stormwater roof drains shall not discharge water directly over a sidewalk, into any sanitary sewer line, or into a street or paved area without a straight curbed gutter.

(8) The storm sewer system shall be designed to the more-restrictive of the following: to collect stormwater at any point where three to five cubic feet per second is accumulated during the design storm; and/or inlets/manholes shall not be spaced more than 300 feet apart on pipe sizes up to 24 inches in diameter and not more than 400 feet apart on greater sizes.

(9) Inlets, manholes, grates, covers, frames, and the like shall conform to the Pennsylvania Department of Transportation Roadway Construction Standards (Publication No. 72) and Form 408 Specifications, and all amendments, revisions or updates thereto.

(10) All drainage channels shall be designed to carry a flow rate equal to a one-hundred-year, twenty-four-hour storm.

(11) All drainage channels shall be designed to prevent the erosion of the bed and bank areas. The flow velocity in all vegetated drainage channels shall not exceed the maximum permissible velocity to prevent erosion. Suitable bank stabilization shall be provided where required to prevent erosion of the drainage channels. Where storm sewers discharge into existing drainage channels at an angle greater than 30° from parallel with the downstream channel flow, the far side bank shall be stabilized by the use of rip-rap, masonry, and/or concrete walls. The stabilization shall be designed to prevent erosion and frost heave under and behind the stabilizing media.

(12) Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum slope of three horizontal to one vertical on those areas to be mowed.

(13) Inlet capacity. All inlets must be designed to accommodate the ten-year peak flow rate, except at low points, where they shall accommodate the twenty-five-year peak flow rate.

(14) Straight pipe sections. Wherever possible, all storm sewers shall be designed to follow straight courses. No angular deflections of storm sewer pipe sections in excess of 5° shall be permitted. No vertical curves shall be permitted in the storm sewer system.

(15) Minimum grade and size. All storm drain pipes shall be designed to maintain a minimum grade of 1.0%. All storm sewer pipes shall have a minimum inside diameter of 15 inches.

(16) Pipe arches. Where headroom is restricted, equivalent pipe arches may be used in lieu of circular pipes.