Township of New Hanover, PA Part 4 STORMWATER MANAGEMENT

§ 23-401 General Requirements.

[Ord. No. 22-08, 12/1/2022[1]]

1. Applicants proposing regulated activities in New Hanover Township that do not fall under the exemption criteria shown in § 23-106 shall submit a PCSM plan consistent with this chapter to New Hanover 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.

2. No regulated activities shall commence until the municipality issues written approval of a PCSM plan which demonstrates compliance with the requirements of this chapter.

3. For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act. For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.

4. PCSM plans approved by New Hanover Township shall be on site throughout the duration of the regulated activity.

5. The applicant is required to find 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.

6. The PCSM plan must be designed consistent with the sequencing provisions of § 23-404 to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge, and to protect groundwater and surface water quality and quantity. The PCSM plan designer must proceed sequentially in accordance with Part 4 of this chapter.

7. Stormwater conveyance systems shall be designed in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.

8. Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise 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. Stormwater flows shall be conveyed to a municipally owned storm sewer or to a perennial stream located on the subject tract to be developed. Stormwater discharged off-site in a manner not referenced above shall be discharged to a flow regime consistent with predevelopment conditions, subject to the review and approval of the Municipal Engineer. If said flow regime cannot be met, an easement shall be required from the affected property(ies), in a form acceptable to the Township Solicitor, to an extent to contain the discharged flow, subject to review and approval of the Municipal Engineer.

9. 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 demonstrate to the satisfaction of the Municipal Engineer 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, in accordance with Subsection 8.

10. Where a development site is traversed by existing streams, easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the riparian buffer requirements contained in § 23-406, Subsection 1H, of this chapter.

11. Where a development contains a watercourse or a proposed drainage swale intended to convey stormwater through two or more lots or is intended to convey stormwater at a rate of four cfs or greater during the 100-year storm event, and provided that an easement for the swale or watercourse has not been provided under Subsection 10 above, a drainage easement shall be provided to the Township conforming with the line of the watercourse and of a width as will be adequate to preserve drainage but not less than 20 feet wide, the distance between the top-of-banks or the limit of 100-year storm water surface, whichever is greater.

12. Any stormwater management facilities regulated by this chapter that would be located adjacent to waters of the commonwealth or delineated wetlands shall be subject to approval by DEP through the joint permit application or the environmental assessment approval process, or where deemed appropriate, by the DEP general permit process, where required. When there is a question as to whether wetlands may be involved, it is the responsibility of the applicant or the applicant's agent to show that the land in question cannot be classified as wetlands; otherwise, approval to work in the area must be obtained from DEP.

13. Stormwater management facilities, including pipe outfalls and newly constructed swales, shall meet the Riparian Corridor Conservation District requirements, but shall not be permitted within 50 feet of the banks of any stream, waters of the commonwealth or wetland. Vegetative filter strips (with minimal grading activity), riparian buffer planting, and streambank restoration projects are not subject to this restriction.

14. Any proposed stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way shall be subject to approval by PennDOT.

15. 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.

16. All stormwater runoff shall be pretreated for water quality prior to discharge to surface water or groundwater.

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

A. Erosion and sediment control during earth disturbance activities (e.g., during construction).

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

C. Groundwater recharge measures after completion of earth disturbance activities (i.e., after construction), including operations and maintenance.

D. Nonstructural project design (sequencing to minimize stormwater impacts).

E. Stream bank erosion requirements.

F. Stormwater peak rate control.

G. Other requirements set forth in § 23-410.

18. No regulated activities within New Hanover Township shall commence until the requirements of this chapter are met.

19. Post-construction water quality protection shall be addressed as required by § 23-406.

20. Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Part 7.

21. 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 New Hanover Township.

22. Techniques described in Appendix 23-C, Low Impact Development Practices, of this chapter[2] shall be considered because they reduce the costs of complying with the requirements of this chapter and the state water quality requirements. All regulated activities shall include green infrastructure and LID methods to achieve the volume and rate controls required by this chapter. At a minimum, the applicant shall utilize at least one of the following PA BMP Manual specified LID BMPs, in order of preference, to meet the requirements of this chapter:

A. Infiltration basins.

B. Infiltration trenches.

C. Pervious/permeable pavement.

D. Runoff capture and reuse.

E. Bioretention facilities.

F. Extended dry detention basins.

G. Sand filter basins.

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

23. Incorporate methods described in the PA BMP Manual. The applicant shall follow all recommended design criteria and construction standards listed in the PA BMP Manual, unless otherwise specified herein. Any improvement recommended by the PA BMP Manual shall be considered a required improvement in New Hanover Township.

24. Managed release concept (MRC) facilities shall be permitted in accordance with the criteria set forth under § 23-405, Subsection 1B.

25. If methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required by this chapter, the PCSM plan must include a detailed justification demonstrating that the use of LID and green infrastructure is not practicable.

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

A. Total contributing area.

B. Permeability and infiltration rate of the site's soils.

C. Slope and depth to bedrock.

D. Seasonal high-water table.

27. Proximity to building foundations and wellheads. Existing drainage peak rate discharges up to and including the 100-year storm and the volume of runoff up to and including the two-year storm onto or through adjacent property(ies) or downgradient property(ies), including diffuse drainage discharge, shall not be altered in any manner by regulated activities under this chapter without written permission from, and, where applicable as determined by New Hanover Township, an easement and agreement with the affected landowner(s) for conveyance of discharges onto or through their property(ies). Altered stormwater discharges shall be subject to any applicable discharge criteria specified in this chapter.

A. Erodibility of soils.

B. Land availability and configuration of the topography.

C. Peak discharge and required volume control.

D. Stream bank erosion.

E. Effectiveness of the BMPs to mitigate potential water quality problems.

F. The volume of runoff that will be effectively treated.

G. The nature of the pollutant being removed.

H. Maintenance requirements.

I. Creation/protection of aquatic and wildlife habitat.

J. Recreational value.

K. 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 23-A.[3]

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

[1] Editor's Note: This ordinance superseded former Part 4, Stormwater Management, adopted by Ord. No. 07-07, 7/23/2007, as amended by Ord. No. 18-04, 7/23/2018; by Ord. No. 18-05, 10/4/2018; and by Ord. No. 2022-01, 1/3/2022.

§ 23-402 Permit Requirements by Other Governmental Entities.

[Ord. No. 22-08, 12/1/2022]

1. 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 DEP under regulations at 25 Pa. Code Chapter 102.

B. Work within wetlands, watercourses, and floodways subject to permit by DEP under 25 Pa. 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 Pa. Code Chapter 105.

D. Any stormwater management facility or conveyance that would be located on a state highway right-of-way or require access 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 DEP under 25 Pa. Code Chapter 105.

§ 23-403 Erosion and Sediment Control During Regulated Earth Disturbance Activities.

[Ord. No. 22-08, 12/1/2022]

1. For earth disturbance activities greater than or equal to one acre or other activities requiring erosion and sediment control plan approval by DEP, no regulated earth disturbance activities within New Hanover Township shall commence until New Hanover Township is in receipt of an approval from the Conservation District of an erosion and sediment control plan for construction activities.

2. 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 Pa. Code § 102.4(b).

3. In addition, under 25 Pa. Code Chapter 92a, a DEP permit for stormwater discharges associated with construction activities is required for earth disturbances greater than or equal to one acre.

4. Evidence of any necessary permit(s) for regulated earth disturbance activities from the DEP or the Conservation District must be provided to New Hanover Township.

5. 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.

6. Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:

A. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity. Additional measures, such as placement of orange construction fencing around proposed infiltration BMPs during construction to minimize or eliminate traffic overtop of these areas, and temporary sealing off of pipes and inlet connections to infiltration BMPs to prevent sediment clogging should be given consideration.

B. Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.

7. Peak discharges and discharge volumes occurring during construction from the site shall comply with the appropriate sections above, with the following additions:

A. For purposes of calculating required detention storage during land disturbance, peak discharges and discharge volumes shall be calculated based upon the runoff coefficients for bare soils and maximum regulated impervious surfaces during the maximum period and extent of disturbance. Controls shall ensure that the "during construction" peak discharge rate and discharge volume shall not exceed those peak discharges and discharge volumes before development. It should be understood that detention storage during the period of land disturbance and prior to establishment of permanent cover may require additional facilities on a temporary basis. Such measures shall be located so as to preserve the natural soil infiltration capacities of the planned infiltration bed areas.

B. During land disturbance, wherever soils, topography, cut and fill or grading requirements, or other conditions, suggest substantial erosion potential, the Township shall require that the entire volume of all storms up to a two-year storm for the disturbed areas be retained on-site and that special sediment trapping facilities (such as check dams, etc.) be installed.

8. Areas of the site to remain undisturbed shall be protected from encroachment by construction equipment/vehicles to maintain the existing infiltration characteristics of the soil. These areas shall be delineated on the plans and shall be separated from the development areas by a forty-eight-inch-high snow fence, such as super silt fence, mounted on steel posts located eight feet on center or as approved by the Township Engineer.

9. Permanent erosion and sedimentation control facilities (i.e., sediment basins, sediment traps, grass diversion swales, terraces, etc.) shall be provided for all proposed agricultural activities designed in conjunction with the development plans (farmland preserve areas, agricultural open space areas). Erosion and sedimentation control measures and facilities for agricultural activities must be designed to meet the same requirements and standards as nonagricultural activities. Likewise, agricultural erosion and sedimentation control plans must address all of the relative factors which contribute to erosion and sediment pollution as specified in § 102.5(b) of the PADEP Chapter 102 regulations. However, an additional requirement is placed on agricultural erosion and sediment control plans in that they must consider, and thereafter develop, appropriate control measures and facilities for each year's operation. If for example, operations on a farm are based on a four-year cycle of crop rotation, a separate erosion and sediment control subplan must be developed for each year's specific amount and configuration of earthmoving. These subplans, when added together, constitute that farm's overall erosion and sediment control plan. The agricultural erosion and sedimentation control plans shall be incorporated into the farm conservation plan, which shall be reviewed and approved by the Board of Supervisors with the subdivision and land development application.

§ 23-404 Nonstructural Project Design (Sequencing to Minimize Stormwater Impacts).

[Ord. No. 22-08, 12/1/2022]

1. For subdivisions and land developments, the nonstructural project design sequencing requirements shall be the first item discussed with all reviewing boards and commissions. No discussion of the proposed subdivision or land development shall occur until the applicant has provided a satisfactory analysis of the site, a plan to protect the existing natural resources, and a plan to provide stormwater management facilities that meet the objectives of this chapter. The design of all regulated activities shall include the following to minimize stormwater impacts.

A. The applicant shall find practicable alternatives to the surface discharge of stormwater, such as those listed in Appendix 23-D, Table 23-D-4,[1] and Chapter 5 of the PA BMP Manual. The creation of regulated impervious surfaces and the degradation of waters of the commonwealth shall be minimized as much as possible to correspond to the predevelopment natural hydrologic regime of the site.

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

B. 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 municipal requirements.

C. 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.

2. The applicant shall demonstrate that the regulated activities were designed in 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:

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, riparian buffers, hydrologic soil groups, wooded areas, woodlands, and potential infiltration areas. Land development, any existing recharge areas, and other requirements outlined in Chapter 22, Subdivision and Land Development, shall also be included.

B. Establish a riparian buffer according to § 23-406, Subsection 1H, and a Riparian Corridor Conservation District according to § 27-2111, Subsection 2B. In cases where the requirements of § 23-406 and § 27-2111 differ, the requirement that causes the preservation of the most extensive area of riparian buffer area shall control.

C. Prepare a draft project layout avoiding sensitive areas identified in § 23-404, Subsection 2A.

D. Identify site-specific existing conditions drainage areas, discharge points, recharge areas, and hydrologic soil groups A and B (areas conducive to infiltration). Infiltration should still be considered in well-draining soils listed as hydrologic soil group C, but additional soils testing should be performed to verify on-site conditions and placement of these BMPs.

E. Evaluate nonstructural stormwater management alternatives:

(1) Minimize earth disturbance.

(2) Minimize impervious surfaces.

(3) Break up large impervious surfaces.

F. Satisfy the groundwater recharge (infiltration) objective (§ 23-405) and provide for stormwater pretreatment prior to infiltration.

G. Provide for water quality protection in accordance with § 23-406, Water Quality Requirements.

H. Provide stream bank erosion protection in accordance with § 23-407, Stream Bank Erosion Requirements.

I. Determine the watershed in which development is proposed to verify applicable stormwater peak rate control criteria as set forth in § 23-408.

J. 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.

K. Conduct a proposed conditions runoff analysis based on the final design that meets the requirements set forth in § 23-408.

L. Manage any remaining runoff prior to discharge through detention, bioretention, or through the utilization of other structural BMPs as referenced in the PA BMP Manual.

§ 23-405 Groundwater Recharge.

[Ord. No. 22-08, 12/1/2022]

1. 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 regulated impervious surface is created. These measures are required to be consistent with § 23-103. 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 that this cannot be physically accomplished. Soils infiltration testing shall be performed in accordance with Appendix C of the PA BMP Manual, if it can be physically accomplished, and then alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the managed release concept, may be used to manage this volume with approval from the Municipal Engineer.

A. Infiltration BMPs shall meet the following minimum requirements:

(1) 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:

(a) A minimum depth of 24 inches, preferably 36 inches, between the bottom of the BMP and the top of the limiting zone (e.g., SHWT, groundwater, bedrock, etc.).

(b) An infiltration rate sufficient to dewater completely the volume of runoff specified in Subsection 1A(1)(c) as determined by field tests conducted by the applicant's design professional.

(c) The infiltration facility shall be capable of completely infiltrating the recharge (infiltration) volume (Re_v) within three days (72 hours) or less. Infiltration BMPs using commercially available turf grasses shall be designed for a maximum ponding time of two days (48 hours).

(d) Pretreatment shall be provided prior to infiltration utilizing a BMP in accordance with Chapter 6 of the PA BMP Manual.

(e) Infiltration BMPs shall be dispersed over the entirety of the site, constructed as shallow as supporting testing allows, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.

(2) The size of the infiltration facility shall be based upon the Net Two-Year Volume Approach, where the recharge (infiltration) volume (Re_v) to be captured and infiltrated shall be the volume difference between the predevelopment two-year, twenty-four-hour storm event and post-development two-year, twenty-four-hour storm event. The recharge volume calculated using this section 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 recharge can be achieved, the applicant is encouraged to infiltrate as much of the stormwater runoff from the site as possible.

(3) As an alternative to the Net Two-Year Volume Approach, for residential development projects that create impervious surfaces between 1,001 square feet up to and including 3,000 square feet, and the total earth disturbance is less than 1.0 acre, the applicant may utilize the Simplified Method to determine the recharge volume (Rev). Stormwater facilities shall be provided to capture 20 cubic feet (150 gallons) of runoff from every 100 square feet of impervious surface area created by the regulated activity. This volume of runoff is to be considered the "recharge volume" (Rev) and shall be managed using the small project SWM site plan guidelines as follows:

[Added by Ord. No. 2023-01, 9/7/2023]

(a) A minimum of 75% of the recharge volume shall be infiltrated/treated by an infiltration-type structural BMP (e.g., infiltration basin, infiltration trench, bioretention basin, rain garden, managed release concept basin) following the guidelines presented in Appendix 23-H.

(b) A maximum of 25% of the recharge volume may be reused, evapotranspirated or infiltrated through nonstructural BMPs identified in Worksheet 3, Nonstructural BMP Credits of the PA BMP Manual, provided in Appendix 23-H of this chapter (e.g., minimum soil compaction, protect existing trees, disconnect roof leaders to vegetated areas, disconnect non-roof impervious to vegetated areas).

B. Managed release concept (MRC) BMPs shall meet the following requirements:

(1) Subject to the approval of the Municipal Engineer, MRC facilities may be utilized to comply with the criteria set forth in §§ 23-405 (Groundwater Recharge), 23-406 (Water Quality Requirements), and 23-408 (Stormwater Peak Rate Control) if the following criteria is met:

(a) The 100-year design water surface elevation shall result in a maximum depth of two feet, from the top of the soil media.

(b) Maximum contributary acreage to facility shall not exceed 1.0 acre.

(c) MRC BMPs shall incorporate a multi-stage detention basin with the upper portions of the facility providing flow attenuation for the storm events greater than a two-year, twenty-four-hour storm to ensure that the flow depths are not exceeded.

(2) MRC facilities that do not comply with the criteria set forth in Subsection 1B(1) shall only be permitted to manage the volume requirements as set forth in §§ 23-405 (Groundwater Recharge) and 23-406 (Water Quality Requirements). MRC facilities that do not meet the requirements of said section shall be prohibited from being utilized to meet the requirements of § 23-408 (Stormwater Peak Rate Control) and shall be implemented as separate facilities from those proposed to provide peak rate control.

(3) Predevelopment soil and geologic testing shall provide, at a minimum, one infiltration test for every 40,000 square feet of disturbed acreage and shall provide at least four infiltration tests per site. Only those BMPs listed in § 23-401, Subsection 22A through G, shall be utilized to meet the groundwater recharge and water quality requirements set forth in §§ 23-405 and 23-406 if testing indicates that the applicable infiltration and limiting zones set forth in Subsection 1A can be met. MRC facilities shall only be permitted where it can be demonstrated that this criterion cannot be met at any other testing location, or that insufficient volume control to meet the requirements set forth in this section cannot be provided.

(4) Areas utilizing MRC facilities shall require additional testing to include a minimum of two test pits and a minimum of four infiltration tests per facility. Separation distance shall be in accordance with Subsection 1B(9), below.

(5) All predevelopment soil and geologic testing shall be performed in accordance with Appendix C of the PA BMP Manual and the requirements of this chapter.

(6) Predevelopment soil and geologic testing intended to support the use of the MRC BMPs shall be provided within areas of the site that would allow for the construction of the MRC BMP facility. Testing shall avoid the sensitive areas identified in § 23-404, Subsection 2A. Testing performed in hydric soils, floodplains, stream buffers, steep slope areas, and any other area where construction activities would be in violation of any Township ordinance requirement shall not be included in the tested area intended to meet Subsection 1B(1) above.

(7) The total drainage area to any individual MRC BMP shall not exceed 3.0 acres.

(8) The total impervious surface area to any individual MRC shall not exceed 1.5 acres.

(9) The total amount of impervious surface area that can be managed by the MRC BMP shall be limited to 10 acres per development site. For development sites that can be developed with more than 20 acres of impervious surface (based on the maximum impervious surface ratio or building coverage requirements), no more than 50% of the impervious surface area may be managed by MRC BMPs.

(10) For all MRC BMPs, at least two feet of separation distance shall be maintained between the groundwater elevation or the seasonal high-water table (SHWT) elevation and the bottom footprint of the MRC. For MRC facilities with a footprint that exceeds 2,500 square feet, the highest identified groundwater or SHWT elevation from the BMP testing shall be used to establish the minimum separation distance (average elevations shall not be utilized) for the MRC.

(11) The maximum ponding time (i.e., the time after the end of the storm event for stored surface water to lower to soil surface) shall not exceed 72 hours for any storm event. Along with the testing of soil media referenced in Subsection 1B(12), supporting calculations shall be provided demonstrating that the perforated underdrain has capacity to meet said dewatering requirement.

(12) A maximum ponding depth (i.e., storage depth above the amended soil surface) shall not exceed one foot at the peak of the two-year/twenty-four-hour storm event for above-surface MRC BMPs. MRC BMPs shall incorporate a multi-stage detention basin with the upper portions of the facility providing flow attenuation for the storm events greater than a two-year/twenty-four-hour storm to ensure that the flow depths are not exceeded.

(13) Prior to placement of the MRC soil media, the applicant shall submit appropriate testing to confirm that the soil media that is proposed to be installed provides an infiltration rate between 2.0 and 6.0 inches per hour. Prior to the acceptance of the MRC by the Township, the applicant shall submit additional testing of the soil media to confirm that the material placed in the MRC continues to provide the 2.0- to 6.0-inch-per-hour infiltration rate. The in-place testing shall be performed following Appendix C Protocol 1, Site Evaluation and Soil Infiltration Testing and Appendix C Protocol 2, Infiltration Systems Guidelines per the Stormwater BMP Manual (as revised).

(14) All runoff being discharged to an MRC BMP shall be filtered to prohibit sediment-laden runoff from entering the MRC BMP utilizing pretreatment as included in the Pennsylvania BMP Manual. All MRC BMPs that receive runoff from a drainage area exceeding 0.25 acre shall be provided with a sediment forebay. A shallow forebay shall be provided adjacent to all inflow areas unless the inlet provides less than 10% of the total inflow to the MRC. The forebay shall be planted as a marsh with emergent wetland vegetation. The forebay serves to enhance sediment trapping and pollutant removal, as well as concentrating accumulated sediment in an area where it can be readily removed. To create a forebay, a baffle can be introduced to restrict hydraulic communication between the inlet and the remainder of the basin. The minimum standards for forebays are as follows:

(a) The minimum forebay length is 10 feet.

(b) Sufficient storage volume shall be provided to trap sediment between cleanout intervals (10 years minimum). At a minimum, the forebay shall be sized to contain 0.25 inch per impervious acre of contributing drainage.

(c) The forebay shall be constructed at a depth equal to the depth of the MRC facility in which the forebay is providing pretreatment.

(d) The height of the berm or baffle that separates the forebay from the MRC shall be one foot below the ponding depth associated with the one-year water quality design storm.

(e) The forebay shall be accessible and stabilized to accommodate equipment used in removing accumulated sediment.

(15) Surface MRC BMPs shall be vegetated in accordance the PA BMP Manual's guidance for rain garden/bioretention type facilities, including the placement of at least three species of shrubs and trees, planted at a rate of 700 shrubs and 300 trees per acre.

C. 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 shall be performed in accordance with Appendix C of the PA BMP Manual. 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 in accordance with PA BMP Manual, Appendix C.

(3) The location of the test pit or boring shall correspond to each BMP location. One test pit/boring shall be provided for each 2,500-square-foot (fifty-foot by fifty-foot grid) area of basin/filter area. The test pit boring location and number shall be identified on the PCSM plan. A report of the findings for all test pits shall be included with the stormwater management report.

(4) Testing for infiltration/percolation rates shall be performed at the bottom design elevation of the proposed BMP.

(5) A minimum of 0.2 inch per hour (in/hr) shall be utilized for the design of all infiltration facilities based on field testing. A safety factor of 50% shall be applied for design purposes (e.g., for soil which measured 0.4 in/hr, the BMP design shall use 0.2 in/hr to ensure safe infiltration rates after construction).

(6) All field testing shall be performed in the presence of the Municipal Engineer. The Municipal Engineer shall be notified two business days in advance of any field testing being performed. Any field testing performed without the Municipal Engineer on-site will not be acceptable for use in the preparation of the final PCSM plan. In cases where testing is desired prior to the submission of the stormwater management plan application, the applicant shall contact the Township at least two weeks prior to testing to establish a professional services agreement to allow for the Municipal Engineer to witness the testing.

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

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

D. Stormwater Hotspots. Below is a list of examples of designated hotspots. If a site is designated as a hotspot, it has important implications for how stormwater is managed. First and foremost, 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 water quality volume must still be treated). Second, a greater level of stormwater treatment shall be considered at hotspot sites to prevent pollutant washoff after construction. The Environmental Protection Agency's (EPA's) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.

(1) 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 codes.

(g) Marinas (service and maintenance).

(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) 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 large highways [average daily traffic volume (ADT) greater than 30,000] are not designated as stormwater hotspots, it is important to ensure that highway stormwater management plans adequately protect groundwater.

E. Extreme caution shall be exercised where infiltration is proposed in SWPAs as defined by the local municipality or water authority.

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

G. Extreme caution shall be exercised where salt or chloride (municipal salt storage) would be a pollutant since soils do little to filter this pollutant, and it may contaminate the groundwater. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary. Specific consideration should be given to the particular type of salt or deicing material to be used within this watershed in regard to its potential long-term effects on the soils, especially in areas that contain clay soil.

H. The infiltration requirement in HQ or EV waters shall be subject to the Department's 25 Pa. Code Chapter 93 antidegradation regulations.

I. An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by New Hanover Township.

J. New Hanover Township shall require the applicant to provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.

§ 23-406 Water Quality Requirements.

[Ord. No. 22-08, 12/1/2022]

1. The applicant shall comply with the following water quality requirements of this Part, regardless of whether or not the affected property is located in the Swamp Creek Watershed.

A. No regulated earth disturbance activities within New Hanover Township shall commence until approval by New Hanover Township of a plan that demonstrates compliance with post-construction state water quality requirements.

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

C. To control post-construction 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 post-construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. As described in the DEP Comprehensive Stormwater Management Policy (#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.

(3) Stream Bank and Streambed Protection. Management of volume and rate of post-construction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).

D. Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. If site conditions allow for infiltration, the water quality volume and the recharge volume are the same volume and may be managed in a single facility. If infiltration cannot be physically accomplished, the water quality volume shall be calculated using the Net Two-Year Volume Approach described in § 23-405, Subsection 1A(2). In this case, the water quality volume may be captured and treated by methods other than infiltration BMPs.

(1) This volume requirement can be accomplished by the permanent volume of a wet pond/retention basin (per BMP Manual 6.6.2) or the detained volume from other BMPs. Wet ponds/retention basins shall be utilized for water quality control and shall follow the guidelines set forth in the PA BMP Manual, BMP 6.6.2.

(2) The water quality volume shall take a minimum of 24 hours to be discharged from a BMP 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 the water quality volume 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.

(3) The water quality volume shall be provided below the primary outlet.

E. For areas within defined special protection subwatersheds that include EV and HQ waters, the temperature and quality of water and streams shall be maintained through the use of temperature-sensitive BMPs and stormwater conveyance systems, as may be determined acceptable by the Municipal Engineer.

F. For regulated activities that create greater than 1,000 square feet of impervious surface area, the applicant shall reduce the post-construction pollutant loads for total suspended solids (TSS), total phosphorus (TP), and total nitrogen (TN) to preconstruction pollutant loads through the use of structural and nonstructural BMPs. The applicant shall utilize the PA DEP PCSM Spreadsheet, as amended, to verify compliance with this requirement. The applicant may utilize other methods to show compliance with this requirement, as may be determined to be acceptable by the Municipal Engineer.

G. To accomplish the above, the applicant shall submit original and innovative designs to the Municipal Engineer for review and approval. Such designs may achieve the water quality objectives through a combination of different BMPs.

H. Except as required by 25 Pa. Code Chapter 102, if a perennial or intermittent stream passes through 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. The buffer area shall be maintained with and encouraged to use appropriate native vegetation (refer to Appendix A of the PA BMP Manual, latest version, for plant lists). If the applicable rear or side yard setback is less than 50 feet or a stream traverses the site, the buffer width may be reduced to 25% of the setback and/or to a minimum of 10 feet. 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. This does not include lakes or wetlands. The applicant shall comply with the applicable criteria set forth in § 27-2111 of the New Hanover Township Zoning Ordinance as it pertains to uses within the Riparian Corridor Conservation District.

I. Evapotranspiration may be quantified and credited towards meeting volume requirements according to the PADEP Post-Construction Stormwater Management (PCSM) Spreadsheet and Instructions (December 2020) or the most recent guidance from PADEP.

J. Evidence of any necessary permit(s) for regulated earth disturbance activities from DEP and/or the Conservation District must be provided to New Hanover Township.

§ 23-407 Stream Bank Erosion Requirements.

[Ord. No. 22-08, 12/1/2022]

1. In addition to the control of water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement 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, and the criteria set forth in § 23-408, Subsection 1. Additionally, 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).

2. The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter, 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, supporting calculations shall be demonstrating that requirement cannot be met. Orifice sizes less than three inches can be utilized, upon determination by the Municipal Engineer.

3. BMPs that have a point discharge proposed must be conveyed directly to a municipally owner storm sewer or to a perennial stream located within the subject tract being developed. If this is not possible, compliance with the provisions of § 23-401, Subsection 8, shall be required.

§ 23-408 Stormwater Peak Rate Control.

[Ord. No. 22-08, 12/1/2022]

1. The criteria for peak runoff control are designed to reduce the post-development peak flow to 50% of the predevelopment peak flow. Development sites must control proposed conditions runoff rates to 50% of the existing conditions runoff rates for the two-, five-, ten-, twenty-five-, fifty-, and 100-year storm events and the criteria set forth in § 23-407, Subsection 1.

2. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas recombine in proximity to the site. In this case, peak discharge in any direction may be a 100% release rate, provided that the overall site discharge meets the weighted average release rate.

3. 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 convey 100-year-frequency off-site flows through the development site.

4. 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. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the peak rate control standards.

5. Alternate Criteria for Redevelopment Sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by New Hanover Township:

A. Meet the full requirements specified by Subsections 1 through 4.

B. Reduce the total impervious surface on the site by at least 20%, based upon a comparison of existing impervious surface to proposed impervious surface.

§ 23-409 Calculation Methodology.

[Ord. No. 22-08, 12/1/2022]

1. Table 23-409.1 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. Note that successors to the methods listed in Table 23-409.1 are also acceptable, such as WinTR55 for TR-55 and WinTR20 for TR-20. New Hanover Township may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Soil Cover Complex Method shall be used for drainage areas greater than 200 acres.

Table 23-409.1 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 where limitations described in TR-55
HEC-1/HEC-HMS	U.S. Army Corps of Engineers	Applicable where use of a full hydrologic 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
Rational Method (or commercial computer package based on Rational Method)	Emil Kuichling (1889)	For sites approved by New Hanover Township and/or Municipal Engineer
Other Methods	Varies	Other computation methodologies approved by New Hanover Township and/or Municipal Engineer

2. 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 the region in which they are located as presented in Table D-1 in Appendix 23-D of this chapter.[1] Rainfall depths used shall be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland (NOAA Atlas 14) values consistent with a partial duration series. If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.

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

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

A. For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:

(1) Wooded sites shall use a ground cover of "woods in good condition." A site shall be considered to be a wooded site where a biological community dominated by trees and other woody plants exists that covers an area of 10,000 square feet or more and contains at least 100 trees with at least 50% of those trees having a dbh of two inches or greater. (Duerksen, Christopher J., with Suzanne Richman, Tree Conservation Ordinances, Planning Advisory Service Report Number 446, American Planning Association, Chicago, Illinois, and Scenic America, Washington, D.C., August, 1993.)

(2) 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 number (CN) or Rational "c" value (i.e., woods) as listed in Table 23-D-2 or 23-D-3 in Appendix 23-D of this chapter.[2]

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

(3) If farmland area is preserved with the development plan and the farmland area is intended to be farmed during and after development, the pre- and post-development ground cover for that portion of the development site may be considered as "Fallow - Crop residue cover (CR) - Good condition" if the post development area is tributary to either a sedimentation basin or a stormwater management facility.

(4) Off-site land use conditions used to determine storm flows for designing storm facilities shall be based on existing land uses assuming winter or poor land cover conditions.

B. For development considered redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.

4. 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 appropriate curves from the PennDOT Region Four Storm-Duration-Frequency Chart. 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.

5. Runoff curve numbers (CNs) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table 23-D-2 in Appendix 23-D of this chapter.[3]

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

6. Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table 23-D-3 in Appendix 23-D of this chapter.[4]

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

7. 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 accepted published values.

8. 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 as deemed acceptable by the Municipal Engineer.

9. 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 the Stage Storage Method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. New Hanover Township 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.

10. The design of all stormwater management facilities and BMPs shall be based on the maximum permitted impervious surface ratio, as defined by the Township's Zoning Ordinance, for the development site or lot, whichever creates the greatest amount of impervious surface area. In cases where stormwater management facilities are being provided for new regulated impervious surface areas on an existing residential lot (where subdivision and/or land development approval is not required) or for nonresidential land development applications where complete buildout of the lot is not anticipated within five years of the submission of the land development application, the stormwater management facilities and BMPs may be based on the regulated impervious surface area if the PCSM plan clearly states the maximum impervious surface area permitted to be constructed on the lot is based on the submitted PCSM plan and calculations.

11. 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. The maximum length of sheet flow over unpaved surfaces shall be 50 to 100 feet in calculating the time of concentration for a watershed. The theoretical maximum length of 300 feet shall be utilized only in unique situations such as uniformly sloped, smoothly paved parking areas. If the drainage area is small (less than one acre) and sheet flow length is greater than 10% of the total hydraulic travel distance, the designer shall:

A. Recheck sheet flow Tt before accepting the calculated Tc value.

B. If sheet flow Tt is greater than 80% of Tc, then sheet flow area may be noncontributing area, especially if the sheet flow is from very flat slopes (1% to 2%).

C. The calculations may need to be revised to compute the peak discharge using a smaller drainage area and total runoff volume from the total drainage area.

D. In no cases shall the post-development time of concentration be equal to or greater than the predevelopment time of concentration, unless approved by the Municipal Engineer.

12. In performing the stormwater calculations, all those areas to be disturbed during construction shall be assumed to be reduced one Hydrologic Soil Group (HSG) category level during post-development runoff calculations (i.e., HSG B is reduced to HSG C, and so forth).

13. The Township has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the designer can substantiate through actual physical calibration that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.

14. Downstream Hydraulic Capacity Analysis. It shall be the applicant's responsibility to prepare a downstream capacity hydraulic analysis for all points of discharge. The downstream capacity hydraulic analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting the post-development peak flow rates:

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

B. Natural or man-made channels or swales must be able to convey the post-development 100-year return period runoff without creating any hazard to persons or property.

C. Culverts, bridges, separate storm sewer systems or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP Chapter 105 regulations (if applicable) and, at a minimum, pass the post-development 100-year return period runoff.

D. The downstream hydraulic capacity analysis shall be performed in accordance with the floodplain delineation requirements listed in the Township Zoning Ordinance (i.e., HEC-2 or HEC-RAS Study).

E. The analysis shall be performed to the nearest "natural" outfall (stream, watercourse, etc.) for each point of concentrated or dispersed discharge, which shall not be less than 1,000 feet downstream from the development boundary or to the "downstream" end of the next major off-site stormwater facility, as defined by this chapter (i.e., road culvert, wet pond, and detention/retention basin, etc.), whichever is less. However, in no event shall the analysis be performed less than 1,000 feet downstream from the development boundary. The analysis shall document the depth of flow, duration of peak flow, and flow velocities at critical locations along the downstream channel. The analysis shall also identify any locations where the peak rate exceeds the capacity of the downstream facility. Development activities shall not adversely impact any off-site drainage facility.

§ 23-410 Other Requirements.

[Ord. No. 22-08, 12/1/2022]

1. 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 100-year proposed conditions. The upstream edge of the emergency spillway shall provide a minimum of six inches of freeboard to the crest of the emergency spillway and an additional one foot of freeboard above the flow through of the emergency spillway to the top of the embankment when the facility functions for the 100-year proposed conditions inflow. Should any stormwater management facility require a dam safety permit under DEP, 25 Pa. Code Chapter 105, the facility shall be designed in accordance with 25 Pa. Code Chapter 105, and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may be required to pass storms larger than the 100-year event.

2. Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands governed by DEP, 25 Pa. Code 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.

3. Any other conveyance that does not fall under 25 Pa. Code Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the 100-year design storm. Calculations shall demonstrate that all conveyance facilities have full capacity to convey the 100-year frequency storm. 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.

4. Any conveyance 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 100-year design storm. Conveyances to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a 100-year design storm without raising the 100-year water surface elevation in accordance with the floodplain requirements. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

5. Storm sewers must be able to convey proposed conditions runoff from a 100-year design storm and shall demonstrate that all conveyance facilities have the ability, utilizing the Manning Equation, to fully convey the storm.

6. Adequate erosion protection shall be provided along all open channels and at all points of discharge. Calculations shall be provided to support the stabilization proposed.

7. The design of all stormwater management BMPs shall incorporate sound engineering principles and practices. New Hanover Township reserves the right to withhold approval for any design that would result in construction of or continuation of a stormwater problem area.

8. Any stormwater management BMP which proposes aboveground storage required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide the following unless determined otherwise by the Municipal Engineer based upon the contributing drainage area to the facility:

A. The maximum water depth shall not exceed six feet.

B. The minimum top width of all dams/embankments/berms utilized for peak rate control that store stormwater volumes of 10,000 cubic feet or greater at the 100-year water surface elevation shall be 10 feet. For basins with a storage volume less than 10,000 cubic feet, the minimum berm width shall be five feet.

C. Sides slopes of facilities to be utilized for peak rate control shall contain embankment slopes of not steeper than five horizontal to one vertical.

D. All detention/retention basin embankment material shall be placed in maximum of eight-inch lifts compacted to a minimum of 95% of Modified Proctor Density as established by ASTM D-1557. Prior to proceeding to the next lift, the compaction shall be checked by a soils engineer. Compaction test shall be run on the leading and the trailing edge of the berm as well as along the top of the berm. Results shall be submitted to the Township for review and approval.

E. An emergency spillway shall be provided for the basin and shall be capable of discharging the 100-year peak rate of runoff that enters the basin after development, in a manner that will not damage the integrity of the facility and will not create a downstream hazard. Emergency spillways shall be designed to accommodate the 100-year post-development flow conveyed to the basin. The bottom of the basin shall be constructed flat at an elevation above the 100-year design water surface elevation within the basin. A minimum of one foot of freeboard shall be maintained between the water surface elevation within the spillway and the top of the basin berm. Emergency spillways discharging over embankment fill shall be constructed of reinforced concrete checkerblock to protect the berm against erosion. The Checker Blocks^® shall be backfilled with topsoil and seeded. The Checker Block^® lining shall extend to the toe of the fill slope on the outside of the berm and shall extend to an elevation three feet below the spillway crest on the inside of the berm. The spillway material shall also be provided on the side slopes of the spillway and extend to the top of the berm. Where practical, the emergency spillway shall be constructed in undisturbed ground. An easement for inspection and repair shall be provided when the conveyance crosses property boundaries.

F. Vegetated spillways may be utilized for spillways constructed entirely on undisturbed ground (i.e., not discharging over fill) if the designer can demonstrate that flow velocities and sheer stresses through the spillway will not cause erosion of the spillway. A dense cover of vegetation shall be rapidly established in such spillways by sodding or seeding with a geotextile anchor. Such a vegetated spillway must be stabilized before runoff is directed to the basin.

G. All basins not including groundwater recharge and/or water quality storage shall include an outlet structure to permit draining the basin to a completely dry position within 24 hours following the end of the design rainfall. All basins that do include groundwater recharge and/or water quality storage shall include an outlet structure to permit draining the basin to the level of the groundwater recharge and/or water quality storage within 24 hours following the end of the design rainfall.

H. A cutoff trench of impervious material shall be provided within all basin embankments which propose a berm consisting of greater than two feet of impounding height (measured from floor of basin to crest of emergency spillway) that are constructed utilizing fill material. The cutoff trench shall be a minimum of eight feet wide, two feet deep and have side slopes of 1:1. Anti-seep collars shall be installed around the pipe barrel within the normal saturation zone of the detention basin berms. The anti-seep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel or provide a minimum of 15% increased flow length, whichever is greater.

I. All stormwater conveyance facilities passing through a stormwater management facility embankment which propose a berm consisting of greater than two feet of impounding height (measured from floor of basin to crest of emergency spillway) shall have properly spaced concrete cutoff collars, and all piping must be watertight. All structures passing through dam embankments shall have seepage diaphragms and drains.

J. All discharge conveyance facilities shall be constructed of reinforced concrete, in accordance with the applicable specifications set forth in the PennDOT Publication 408 specifications, latest edition.

K. Design storms for the computation of retention basins (where approved) volumes shall be based upon a twenty-four-hour storm with 100-year return period (a storm with a 1% chance of occurrence each year).

L. The effect on downstream areas if the basin embankment fails shall be considered in the design of all basins. Where possible, the basin shall be designed to minimize the potential damage caused by such failure of the embankment.

M. All structures (detention basins, cisterns, etc.), other than those used for groundwater recharge volume and water quality volume, must completely drain within 24 hours after the end of the design storm.

N. Soils used for the construction of basins containing a depth from floor to spillway crest of greater than two feet shall have a minimum erodibility factor of K-0.3.

O. Detention and retention basins shall be in place and functioning prior to the creation of any additional impervious surfaces on the site. As-built drawings of the basin(s) shall be submitted to the Township for review. The basin shall not be considered functional, and no applicable financial security shall be released, until it is proved by the applicant that the basin meets the volume requirements and the outflow characteristics of the original design of the basin(s). Runoff shall not be directed to an infiltration structure until all tributary drainage areas are stabilized.

P. Pipe outlets shall permit complete drainage of all detained water, unless the stormwater management facility is designed as a retention basin or wet pond, or provides for stormwater renovation with constructed wetlands. In no case shall retained water be held in storm sewer piping that is located within a roadway subgrade.

Q. Stone gabion baskets shall not be used in the construction of stormwater management facilities, retaining walls, etc.

R. Retaining walls shall not be specified for use within the 100-year water surface elevation area of any embankment or cut slope that is appurtenant to the construction of the stormwater management or best management practice facility.

S. All outlet structures shall be located within the embankment for purposes of maintenance, access, safety and aesthetics. Below-grade structures (standard inlet boxes, manholes, etc.) shall not project above the ground surface.

T. All inflow headwalls and outlet structures shall be provided with slanted trash racks with a maximum grid opening of four feet by four feet, constructed of aluminum. The trash rack shall be slanted at 45°, be able to withstand loading of 500 pounds and be hinged at the top for cleaning.

U. Water wells shall not be permitted to be used to contribute to, or to maintain water surfaces in, any constructed treatment wetland, extended detention wetland systems, retention basin (wet pond) or any other designed permanent pool unless such use is specifically approved by the Township.

V. All stormwater management facilities and BMPs shall be capable of being completely drained by gravity flow. When equipped with a drain, all drains shall be provided with a manually operated drain that can be secured against unauthorized operation. Drains shall be accessible during all times (emergency spillway flow conditions) and must be sized to pass a flood having a one-year recurrence interval with limited ponding in the BMP, with a pipe diameter of not less than eight inches.

W. In order to promote the safety of the residents of the Township, the Township may determine that fencing is required around all or portions of the stormwater management facilities and BMPs, including temporary sediment basins/traps which contain an impounding height, measured from basin floor to crest of emergency spillway, of greater than two feet. When required by the Township, the fencing shall provide a suitable barrier at least four feet in height of material approved by the Township, such as split-rail fencing with wire backing.

X. Inspection ports shall be provided for all below-grade stormwater management facilities (e.g., infiltration trenches, bioretention, porous paving storage areas, etc.). Inspection ports shall consist of a four- to six-inch diameter, perforated polyvinyl chloride (PVC) pipe with a locking cap, designed to ensure that runoff does not bypass the primary outlet control device.

Y. Underdrains and cleanouts shall be provided for all stormwater management facilities and BMPs, excluding constructed wetland and wet pond type facilities, where the coefficient of permeability of the existing soils is less than 0.5 inch per hour. Underdrains shall consist of eight-inch smooth bore, perforated high density polyethylene pipe (HDPEP-SB). Cleanouts shall be provided to fully flush the underdrain system, with the cleanouts located outside of the proposed 100-year water surface elevation of the proposed BMP.

Z. All stormwater management facilities and BMPs which propose an impounding volume of 10,000 cubic feet or greater at the 100-year-frequency stage shall be provided with a landscaping plan that is designed to effectively naturalize the area to become an integral and harmonious element in the natural landscape. For each proposed stormwater management facility or BMP, the applicant shall prepare a landscaping plan that follows the landscaping guidelines for stormwater management facilities and BMPs provided in the Appendix 23-G of this chapter.[1]

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

9. Minimum building and basement floor elevations for all structures that would be affected by a basin, other temporary impoundments, or open conveyances where ponding may occur shall be located a minimum of two feet above the 100-year water surface.

10. All storm sewer pipes, culverts and bridges (excluding detention and retention basin outfall structures), gutters and swales conveying water originating only from within the boundaries of the development site shall be designed for a 100-year storm event. All storm sewer pipes, culverts and bridges (excluding detention and retention basin outfall structures) conveying water originating from off-site shall be designed for a 100-year storm event. Easements shall be provided to contain and convey the 100-year-frequency flood throughout the development site. Easements shall begin at the furthest upstream property line of the proposed development site in a watershed.

11. A concentrated discharge of stormwater shall not be permitted to discharge to an adjacent property. The following discharges shall be considered acceptable:

A. Connection of discharge to a municipally owned storm sewer. Supporting calculations shall be provided to demonstrate that the flow within the storm sewer is decreased from its predevelopment condition.

B. Discharge to a perennial stream, provided the downstream analysis as required by this chapter indicates, to the satisfaction of the Municipal Engineer, that flow to the stream is reduce from its predevelopment condition.

C. Discharge via a perforated level spreader, provided that the tributary area to the discharge facility is less than one acre, and that supporting calculations are provided, in accordance with the criteria set forth in the Pennsylvania BMP Manual, BMP 6.8.1. It shall be demonstrated, to the satisfaction of the Municipal Engineer, that the post-development flow regime is identical to predevelopment conditions.

12. Stormwater roof drains, sump pumps, and pipes shall not directly discharge water into a street right-of-way or discharge into a sanitary sewer.

13. All existing and natural watercourses, channels, drainage systems, wetlands and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by New Hanover Township and any other necessary approving body.

14. Flow velocities from any storm sewer may not result in erosion of the receiving channel.

15. Energy dissipators shall be placed at the outlets of all storm sewer pipes, culverts, and bridges where flow velocities exceed maximum permitted channel velocities as specified below:

A. Three feet per second where only sparse vegetation can be established and maintained because of shade or soil condition.

B. Four feet per second where normal growing conditions exist and vegetation is to be established by seeding.

C. Five feet per second where a dense, vigorous sod can be quickly established or where water can be temporarily diverted during establishment of vegetation. Netting and mulch or the equivalent methods for establishing vegetation shall be used.

D. Six feet per second where there exists a well-established sod of good quality.

16. The following conditions shall be met for all swales:

A. Capacities and velocities shall be computed using the Manning Equation. The design parameters shall be as follows:

(1) Vegetated swales shall meet the following two design considerations:

(a) The first shall consider swale stability based upon a low degree of retardance ("n"=0.03).

(2) The second shall consider swale capacity based upon a high degree of retardance ("n"=0.05). All vegetated swales shall have a minimum slope of 1% unless approved by the Municipal Engineer.

(3) The "n" factors to be used for paved or rip-rap swales or gutters shall be based upon accepted engineering design practices as approved by New Hanover Township.

B. All swales shall be designed to concentrate low flows to minimize siltation and meandering.

17. Manning "n" values used for design of pipes and culverts shall be in accordance with accepted published values.

18. All storm sewer crossings of streets shall be perpendicular or nearly perpendicular to the street center line.

19. Storm facilities not located within a public right-of-way shall be contained in and centered within an easement that shall be a minimum of 20 feet wide. Easements shall follow property boundaries where possible.

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

21. All groundwater recharge facilities shall be designed to dewater in not less than 24 hours and not more than three days (72 hours) subsequent to any storm event in excess of two-year frequency. All water quality facilities shall be designed so that water is released slowly for a minimum of 24 hours subsequent to any storm event. All infiltration, detention or retention facilities the volume of which will be used for stormwater management (predevelopment vs. post-development) shall be designed so that the volume of runoff in excess of the volume referenced in § 23-406 will empty within 24 hours subsequent to any storm event. Volumes that will not be available within 24 hours subsequent to any storm event shall not be used for stormwater management (predevelopment vs. post-development).

22. In order to protect against the adverse consequences of uncontrolled sump pump and pipe drain discharges, no person shall construct, install, improve, maintain or permit to be operated on the person's owned or occupied property any device which allows any point discharge of water by either forced pumping such as a sump pump or gravity such as a pipe drain (including but not limited to roof downspouts) onto a street, sidewalk or public right-of-way except as provided for below.

A. New sump pump and pipe drain installations.

(1) Sump pumps and pipe drains initially installed on or after October 9, 2018, require a permit issued by the Township.

(2) In order to obtain a permit, the owner or occupier of a property must show compliance with the following conditions:

(a) The sump pump or pipe drain discharge shall be directed onto an absorbent surface such as grass, mulch, riprap or soil so that the discharge will be dissipated and not immediately drain to the Township right-of-way or adjacent properties. The discharge location shall be as approved by the Municipal Engineer or the Engineer's designee. In the event that it is not feasible to directly discharge onto an absorbent surface, the discharge may be directed to a storm sewer, swale, stormwater management facility or other drainage facility or location so long as the applicant proves to the satisfaction of the Municipal Engineer that the stormwater management facility is designed to accommodate the discharge. For all required computations, the applicant shall use the following figures to account for sump discharge, in the design of the receiving stormwater conveyance and stormwater management facilities:

1) Residential units with subgrade elements: 0.25 cubic foot per second (cfs) per unit.

2) Nonresidential buildings with subgrade elements: 0.25 cfs for each 2,500 square feet of building coverage as defined in the Zoning Ordinance, § 27-202.

(b) The permit shall provide for inspections as deemed necessary by the Municipal Engineer once the sump pump is operational, to determine if the discharge has been adequately addressed.

(c) The discharge location shall not create a public nuisance, which is any thing, condition or act which is or may become injurious or hazardous to the public, as determined by the Township.

(d) The discharge shall not create a buildup of icing, standing water or algae growth on the street, sidewalk, public right-of-way, or adjacent properties.

(e) The discharge shall not be directed to adjacent property in such a manner as to cause damage to the adjacent property or create a nuisance and shall be no closer than 20 feet from the property line unless permission is granted in writing by the applicable adjacent property owner(s).

(f) No person shall connect, or cause or permit to be connected, any sump pump, pipe drain, floor drain, surface drain, subsoil drain or leader pipe with the house sanitary sewer, or to use the sanitary sewer connection or any pipe or drain connected therewith for the purpose of receiving and discharging drainage of any kind other than from plumbing fixtures.

(g) Sump pump and pipe drain discharges may not be directed into a Township street or right-of-way unless permission is granted in writing by the Municipal Engineer or the Engineer's designee.

(h) The applicant for the permit must agree to allow inspection by the Township of the connections being made for the sump pump or pipe drain discharge and allow the Township to inspect such connection(s) at any time in the future, upon reasonable notice to the owner or occupier. The permit issued under this section does not affect the rights and responsibilities of the permit holder in regard to claims or defenses by a neighboring property owner, and the permit holder shall indemnify the Township for any easement disputes.

B. Existing sump pump and pipe drain installations. Sump pumps and pipe drains installed prior to October 9, 2018, shall comply with the following conditions:

(1) An existing sump pump or pipe drain discharge may continue at the location where the discharge was made on October 9, 2018, as long as such discharge does not:

(a) Create a public nuisance, which is any thing, condition or act which is or may become injurious or hazardous to the public, as determined by the Township; and

(b) Create a buildup of icing, standing water, or algae growth or cause erosion on the street, sidewalk, public right-of-way, or adjacent properties.

(2) An existing sump pump or pipe drain discharge that, in the opinion of the Township Code Officer, is causing a public nuisance or is creating a public concern due to a buildup of icing, standing water, or algae growth, causing erosion, or if it is deemed necessary by the Municipal Engineer to protect the health, safety and welfare of the public, shall be corrected by the owner to bring the sump pump or pipe drain discharge into compliance with the requirements with Subsection 22A(2)(a) through (h).

C. Violations and Penalties. The requirements of this Subsection 22 shall be enforced by the Township Code Enforcement Officer or the Code Enforcement Officer's designee, consistent with this Chapter 23. Whenever there is a violation of this subsection, the Township shall provide written notice to the person that owns or occupies the property, consistent with the notification section of this chapter. The failure of the owner or occupier of property to comply with the provisions of the written notification shall be a violation of this chapter, for which the Code Officer or its designee shall issue a civil or summary enforcement action. Each violation and each day of each violation shall constitute a separate and distinct violation thereof, subjecting the owner or occupier to the penalties and enforcement available in this chapter. If the owner or occupier of a property is found by a court of competent jurisdiction to have committed a violation, the owner or occupier must thereafter obtain a permit as required under Subsection 22A herein.