Township of Salisbury, PA C Stormwater Management Requirements.

§ 26-421 General Requirements.

[Ord. 07-2005-523, 7/28/2005, § 301]

1. All regulated activities in the municipality shall be subject to the stormwater management requirements of this Part.

2. Storm drainage systems shall be provided to permit unimpeded flow in natural watercourses except as modified by stormwater detention facilities, pipe systems or open channels consistent with this Part.

3. The existing locations of concentrated drainage discharge onto adjacent property shall not be altered without written approval of the affected property owner(s).

4. Areas of existing diffused drainage discharge onto adjacent property shall be managed such that, at minimum, the peak diffused flow does not increase in the general direction of discharge, except as otherwise provided in this Part. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer must document that there are adequate downstream conveyance facilities to safely transport the concentrated discharge to the point of predevelopment flow concentration, to the stream reach or otherwise prove that no harm will result from the concentrated discharge. Areas of existing diffused drainage discharge shall be subject to any applicable release rate criteria in the general direction of existing discharge whether they are proposed to be concentrated or maintained as diffused drainage areas.

5. Where a site is traversed by watercourses other than those for which a one-hundred-year floodplain is defined by the municipality, there shall be provided drainage easements conforming substantially with the line of such watercourses. The width of any easement shall be adequate to provide for unimpeded flow of storm runoff based on calculations made in conformance with § 26-427 for the one-hundred-year return period runoff and to provide a freeboard allowance of 1/2 foot above the design water surface level. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations which may adversely affect the flow of stormwater within any portion of the easement. Also, periodic maintenance of the easement to ensure proper runoff conveyance shall be required. Watercourses for which the one-hundred-year floodplain is formally defined are subject to the applicable Municipal floodplain regulations.

6. When it can be shown that, due to topographic conditions, natural drainage swales on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainage swales. Capacities of open channels shall be calculated using the Manning equation.

7. Post-construction BMPs shall be designed, installed, operated and maintained to meet the requirements of the Clean Streams Law and implementing regulations, including the established practices in 25 Pa. Code Chapter 102 and the specifications of this Part as to prevent accelerated erosion in watercourse channels and at all points of discharge.

8. No earth disturbance activities associated with any regulated activities shall commence until approval by the municipality of a plan which demonstrates compliance with the requirements of this Part.[1]

[1] NOTE: Throughout this Part, these provisions are from the DEP Guidance on MS4 Ordinance Provisions and are not required for municipalities not subject to the NPDES Phase II regulations.

9. Techniques described in Appendix F (Low Impact Development) of this Part are encouraged because they reduce the costs of complying with the requirements of this Part and the state water quality requirements.[2]

[2] NOTE: Throughout this Part, these provisions are from the DEP Guidance on MS4 Ordinance Provisions and are not required for municipalities not subject to the NPDES Phase II regulations.

10. Infiltration for stormwater management is encouraged where soils and geology permit, consistent with the provisions of this Part and, where appropriate, the recommendation chart for Infiltration Stormwater Management BMPs in carbonate bedrock in Appendix H. Infiltration is encouraged for capturing and treating the water quality volume (as calculated in § 26-424), any part of the water quality volume or for otherwise meeting the purposes of this Part.

§ 26-422 Permit Requirements by Other Government Entities.

[Ord. 07-2005-523, 7/28/2005, § 302]

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

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

B. Work within natural drainageways subject to permit by DEP under 25 Pa. Code Chapter 102.

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 that would be located on a state highway right-of-way or require access from a state highway shall be subject to approval by the Pennsylvania Department of Transportation (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.

§ 26-423 Erosion and Sediment Control During Regulated Earth Disturbance Activities. [1]

[Ord. 07-2005-523, 7/28/2005, § 303]

1. No regulated earth disturbance activities within the municipality shall commence until approval by the municipality of an Erosion and Sediment Control Plan for construction activities. Written approval by DEP or a delegated County Conservation District shall satisfy this requirement.

2. An Erosion and Sediment Control Plan is required by DEP regulations for any earth disturbance activity of 5,000 square feet or more under Pa. Code § 102.4(b).

3. A DEP NPDES Stormwater Discharges Associated with Construction Activities Permit is required for regulated earth disturbance activities under Pa. Code Chapter 92.

4. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the municipality before the commencement of an earth disturbance activity.

5. A copy of the Erosion and Sediment Control Plan and any permit, as required by DEP regulations, shall be available at the project site at all times.

[1] NOTE: Throughout this Part, these provisions are from the DEP Guidance on MS4 Ordinance Provisions and are not required for municipalities not subject to the NPDES Phase II regulations.

§ 26-424 Post-Construction Water Quality Criteria.

[Ord. 07-2005-523, 7/28/2005, § 304]

1. [1]No regulated earth disturbance activities within the municipality shall commence until approval by the municipality of a plan which demonstrates compliance with this Part. DEP has determined that this Part meets state water quality requirements. Therefore, any approvals under this Part would satisfy the post-construction stormwater management requirements associated with an NPDES permit for stormwater discharges associated with construction activities.

[1] NOTE: Throughout this Part, these provisions are from the DEP Guidance on MS4 Ordinance Provisions and are not required for municipalities not subject to the NPDES Phase II regulations.

2. The water quality volume (WQv) shall be captured and treated. The WQv shall be calculated two ways. First, WQv shall be calculated using the following formula:

WQv = (c)(P)(A)\12

	Where:
		WQv	=	Water quality volume in acre-feet
		c	=	Rational Method post-development runoff coefficient for the two-year storm
		P	=	1.25 inches
		A	=	Area in acres of proposed regulated activity

Second, the WQv shall be calculated as the difference in runoff volume from predevelopment to post-development for the two-year-return-period storm. The effect of closed depressions on the site shall be considered in this calculation. The larger of these two calculated volumes shall be used as the WQv to be captured and treated, except that in no case shall the WQv be permitted to exceed 1.25 inches of runoff over the site area.

3. The WQv shall be calculated for each post-development drainage direction on a site for sizing BMPs. Site areas having no impervious cover and no proposed disturbance during development may be excluded from the WQv calculations and do not require treatment.

4. If an applicant is proposing to use a wet pond, constructed wetland or other BMP that ponds water on the land surface and may receive direct sunlight, the discharge from that BMP must be treated by infiltration, a vegetated buffer, filter strip, bioretention, vegetated swale or other BMP that provides a thermal benefit to protect the high quality waters of the Little Lehigh Creek from thermal impacts.

5. Any stormwater runoff from the site as a result of the regulated activities must either be treated with infiltration or two acceptable BMPs such as those listed in § 26-424, Subsection 14.

6. Infiltration BMPs shall not be constructed on fill.

7. The applicant shall document the bedrock type(s) present on the site from published sources. Any apparent boundaries between carbonate and noncarbonate bedrock shall be verified through more detailed site evaluations by a qualified geotechnical professional.

8. For each proposed regulated activity in the watershed, the applicant shall conduct a preliminary site investigation on the portion of the site that is judged to be the best candidate hydrogeologically for possible infiltration, including gathering data from published sources, a field inspection of the site, a minimum of one test pit and a minimum of two percolation tests, as outlined in Appendix G. This investigation will determine depth to bedrock, depth to the seasonal high water table, soil permeability and location of special geologic features, if applicable. The location(s) of special geologic features shall be verified by a qualified geotechnical professional. The requirements of this section are not required for the carbonate areas shown on the Salisbury Township Carbonate Overlay Map if no infiltration is proposed.

9. For entirely noncarbonate sites, the WQv shall be infiltrated unless the applicant demonstrates that it is infeasible to infiltrate the WQv for reasons of seasonal high water table, permeability rate, soil depth or isolation distances; or except as provided in § 26-424, Subsection 20. The preliminary site investigation described in § 26-424, Subsection 8, shall continue on different areas of the site until a suitable infiltration location is found or the entire site is determined to be infeasible for infiltration. For proposed infiltration areas, the additional site investigation and testing as outlined in Appendix G shall be completed. The municipality may determine infiltration to be infeasible if there are known existing conditions or problems that may be worsened by the use of infiltration. The following conditions are suitable for infiltration in noncarbonate areas:

A. Depth to bedrock below the invert of the BMP greater than or equal to two feet.

B. Depth to seasonal high water table below the invert of the BMP greater than or equal to three feet. (If the depth to bedrock is between two and three feet and the evidence of the seasonal high water table is not found in the soil, no further testing to locate the depth to seasonal high water table is required.)

C. Soil permeability greater than or equal to 0.5 inch/hour and less than or equal to 12 inches per hour.

D. Setback distances or buffers as follows:

(1) One hundred feet from water supply wells.

(2) Ten feet downgradient or 100 feet upgradient from building foundations.

(3) Fifty feet from septic system drainfields.

(4) Fifty feet from a geologic contact with carbonate bedrock unless a preliminary site investigation is done in the carbonate bedrock to show the absence of special geologic features within 50 feet of the proposed infiltration area.

(5) One hundred feet from the property line unless documentation is provided to show that all setbacks from wells, foundations and drainfields on neighboring properties will be met.

If it is not feasible to infiltrate the full WQv, the applicant shall infiltrate that portion of the WQv that is feasible based on the site characteristics.

10. In entirely carbonate areas, if infiltration is proposed, in addition to the testing required in § 26-424, Subsection 8, the preliminary site investigation shall include an assessment of the remainder of the site for possible infiltration based on required isolation distances from special geologic features and the likely soil depth and permeability based on published data or other site data available. Where infiltration BMPs are proposed, the applicant shall conduct the additional site investigation and testing as outlined in Appendix G. The soil depth, percolation rate and proposed loading rate, each weighted as described in § 26-427, along with the buffer from special geologic features shall be compared to the recommendation chart for infiltration stormwater management BMPs in carbonate bedrock in Appendix D to determine if the site is recommended for infiltration. If at any point in the preliminary site investigation the data (e.g., location of Karst features on the site or the published soils data for the site) indicates that the entire site will not be recommended for infiltration based on the ordinance standards, then no further investigation is required. In addition to the recommendation from Appendix D, the following conditions are required for infiltration in carbonate areas:

A. Depth to bedrock below the invert of the BMP greater than or equal to two feet.

B. Depth to seasonal high water table below the invert of the BMP greater than or equal to three feet. (If the depth to bedrock is between two and three feet and the evidence of the seasonal high water table is not found in the soil, no further testing to locate the depth to seasonal high water table is required.)

C. Soil permeability greater than or equal to 0.5 inch/hour and less than or equal to 12 inches per hour.

D. Setback distances or buffers as follows:

(1) One hundred feet from water supply wells.

(2) Ten feet downgradient or 100 feet upgradient from building foundations.

(3) Fifty feet from septic system drainfields.

(4) One hundred feet from the property line unless documentation is provided to show that all setbacks from wells, foundations and drainfields on neighboring properties will be met.

Applicants are not required to use infiltration BMPs on a carbonate site even if the site falls in the recommended range on the chart in Appendix D. If infiltration is not proposed, the WQv shall be treated by two acceptable BMPs, as specified in § 26-424, Subsection 15.

11. If a site has both carbonate and noncarbonate areas, the applicant shall investigate the ability of the noncarbonate portion of the site to fully meet this Part to control runoff for the whole site through infiltration. If that proves infeasible, the applicant shall perform the preliminary site investigation for the carbonate area to determine the appropriate design strategy. No infiltration structure in the noncarbonate area shall be located within 50 feet of a boundary with carbonate bedrock, except when a preliminary site investigation has been done showing the absence of special geologic features within 50 feet of the proposed infiltration area.

12. If infiltration BMPs are proposed in carbonate areas, the post-development two-year-runoff volume leaving the site shall be 80% or more of the predevelopment runoff volume for the carbonate portion of the site to prevent infiltration of volumes far in excess of the predevelopment infiltration volume.

13. Site areas proposed for infiltration shall be protected from disturbance and compaction except as necessary for construction of infiltration BMPs.

14. If infiltration of the entire WQv is not proposed, the remainder of the WQv shall be treated by two acceptable BMPs in series for each discharge location. Sheet flow draining across a pervious area can be considered as one BMP. Sheet flow across impervious areas and concentrated flow shall flow through two BMPs. If sheet flow from an impervious area is to be drained across a pervious area as one BMP, the length of the pervious area must be equal to or greater than the length of impervious area. In no case, may the same BMP be employed consecutively to meet this requirement. Acceptable BMPs are listed below along with the recommended reference for design.

	Best Management Practice	Design Reference
	Bioretention	Low Impact Development Design Strategies, Prince George's County, Md., June 19992
	Capture/Reuse1	Texas Guide to Rainwater Harvesting, 2nd Edition. Texas Water Development Board, Center for Maximum Potential Building Systems, 19973
	Constructed Wetlands	2000 Maryland Stormwater Design Manual, Maryland Department of the Environment4
	Minimum Disturbance/Minimum Maintenance Practices	Conservation Design for Stormwater Management. Delaware Dept. of Natural Resources and Brandywine Conservancy, September 19975
	Oil/Water Separators	Georgia Stormwater Management Manual Volume 2 Technical Handbook, August 20016
	Sediment Traps/Catch Basin Sumps	US Environmental Protection Agency's Post-Construction Storm Water Management in New Development & Redevelopment BMP Fact Sheet for "Catch Basins/Catch Basin Insert"7
	Significant Reduction of Existing Impervious Cover	N/A
	Stormwater Filters (Sand, Peat, Compost, etc.)	Design of Stormwater Filtering Systems. Claytor, R. and Schueler, T., Center for Watershed Protection, December 19968
	Trash/Debris Collectors in Catch Basins	Pennsylvania Handbook of BMPs for Developing Areas9 or Latest PA Dept. of Environmental Protection Manual
	Vegetated Buffers/Filter Strips	Pennsylvania Handbook of BMPs for Developing Areas9 or Latest PA Dept. of Environmental Protection Manual
	Vegetated Roofs	Roof Gardens: History, Design, and Construction. Osmundson. T., W.W. Norton & Co., 199810
	Vegetated Swales/Filter Strips	2000 Maryland Stormwater Design Manual, Maryland Department of the Environment4
	Water Quality Inserts for Inlets	Pennsylvania Handbook of BMPs for Developing Areas9 or Latest PA Dept. of Environmental Protection Manual
	Wet Detention Ponds	Pennsylvania Handbook of BMPs for Developing Areas9 or Latest PA Dept. of Environmental Protection Manual

	1	If this BMP is used to treat the entire WQv then only one BMP is required because of this BMPs superior water quality performance.
	2	Available at www.co.pg.md.us/Government/AgencyIndex/DER/PPD/LiDNatl.pdf as of January 2004
	3	Available at www.twdb.state.tx.us/publications/reports/RainHarv.pdf as of January 2004
	4	Available at www.mde.state.md.us as of January 2004
	5	Available at www.dnrec.state.de.us/dnrec2000/Divisions/Stormwater/ Apps/DesignManualRequest.htm as of January 2004
	6	Available at www.georgiastormwater.com as of January 2004
	7	Available at http://cfpub.epa.gov/npdes/stormwater/menuofbmps/post_7.cfm as of January 2004
	8	Available from the Center for Watershed Protection (www.cwp.org) as of January 2004
	9	Available at www.dep.state.pa.us (keyword Stormwater) as of January 2004
	10	Available at www.wwnorton.com of January 2004

15. Stormwater runoff from hot spot land uses shall be pretreated. In no case, may the same BMP be employed consecutively to meet this requirement and the requirement in Subsection 14. Acceptable methods of pretreatment are listed below.

	Hot Spot Land Use	Pretreatment Method(s)
	Vehicle Maintenance and Repair Facilities, Including Auto Parts Stores	-Oil/Water Separators
		-Use of Drip Pans and/or Dry Sweep Material Under Vehicles/Equipment
		-Use of Absorbent Devices to Reduce Liquid Releases
		-Spill Prevention and Response Program
	Vehicle Fueling Stations	-Oil/Water Separators
		-Water Quality Inserts for Inlets
		-Spill Prevention and Response Program
	Storage Areas for Public Works	-Oil/Water Separators
		-Sediment Traps/Catch Basin Sumps
		-Water Quality Inserts for Inlets
		-Use of Drip Pans and/or Dry Sweep Material Under Vehicles/Equipment
		-Use of Absorbent Devices to Reduce Liquid Releases
		-Spill Prevention and Response Program
		-Diversion of Stormwater away from Potential Contamination Areas
	Outdoor Storage of Liquids	-Spill Prevention and Response Program
	Commercial Nursery Operations	-Vegetated Swales/Filter Strips
		-Constructed Wetlands
		-Stormwater Collection and Reuse
	Salvage Yards and Recycling Facilities*	-BMPs that are a part of a Stormwater Pollution Prevention Plan under an NPDES Permit
	Fleet Storage Yards and Vehicle Cleaning Facilities*	-BMPs that are a part of a Stormwater Pollution Prevention Plan under an NPDES Permit
	Facilities that Store or Generate Regulated Substances*	-BMPs that are a part of a Stormwater Pollution Prevention Plan under an NPDES Permit
	Marinas*	-BMPs that are a part of a Stormwater Pollution Prevention Plan under an NPDES Permit
	Certain Industrial Uses (listed under NPDES)*	-BMPs that are a part of a Stormwater Pollution Prevention Plan under an NPDES Permit

*

Regulated under the NPDES Stormwater Program.

Design references for the pretreatment methods, as necessary, are listed below. The applicant may demonstrate that due to the site characteristics the land use is not a hot spot land use.

	Pretreatment Method	Design Reference
	Constructed Wetlands	2000 Maryland Stormwater Design Manual, Maryland Department of the Environment1
	Diversion of Stormwater away from Potential Contamination Areas	Pennsylvania Handbook of BMPs for Developing Areas2 or Latest PA Dept. of Environmental Protection Manual
	Oil/Water Separators	Georgia Stormwater Management Manual Volume 2 Technical Handbook, August 20013
	Sediment Traps/Catch Basin Sumps	US Environmental Protection Agency's Post-Construction Storm Water Management in New Development & Redevelopment BMP Fact Sheet for "Catch Basins/Catch Basin Insert"4
	Stormwater Collection and Reuse (especially for irrigation)	Texas Guide to Rainwater Harvesting, 2nd Edition, Texas Water Development Board, Center for Maximum Potential Building Systems, 19975
	Stormwater Filters (Sand, Peat, Compost, etc.)	Design of Stormwater Filtering Systems. Claytor, R. and Schueler, T., Center for Watershed Protection, December 19966
	Trash/Debris Collectors in Catch Basins	Pennsylvania Handbook of BMPs for Developing Areas2 or Latest PA Dept. of Environmental Protection Manual
	Vegetated Swales/Filter Strips	2000 Maryland Stormwater Design Manual, Maryland Department of the Environment1
	Water Quality Inserts for Inlets	Pennsylvania Handbook of BMPs for Developing Areas2 or Latest PA Dept. of Environmental Protection Manual

	1	Available at www.mde.state.md.us as of January 2004
	2	Available at www.dep.state.pa.us (keyword Stormwater) as of January 2004
	3	Available at www.georgiastormwater.com as of January 2004
	4	Available at http://cfpub.epa.gov/npdes/stormwater/menuofbmps/post_7.ctm as of January 2004
	5	Available at www.twdb.state.tx.us/publications/reports/RainHarv.pdf as of January 2004
	6	Available from the Center for Watershed Protection (www.cwp.org) as of January 2004

16. The use of infiltration BMPs is prohibited on hot spot land use areas.

17. Stormwater infiltration BMPs shall not be placed in or on a special geologic feature(s). Additionally, stormwater runoff shall not be discharged into existing on-site sinkholes.

18. Applicants shall request, in writing, public water suppliers to provide the Zone I Wellhead Protection radius, as calculated by the method outlined in the Pennsylvania Department of Environmental Protection Wellhead Protection regulations, for any public water supply well within 400 feet of the site. In addition to the setback distances specified in § 26-424, Subsections 9 and 10, infiltration is prohibited in the Zone I radius as defined and substantiated by the public water supplier in writing. If the applicant does not receive a response from the public water supplier, the Zone I radius is assumed to be 100 feet.

19. The volume and rate of the net increase in stormwater runoff from the regulated activities must be managed to prevent the physical degradation of receiving waters from such effects as scour and streambank destabilization, to satisfy state water quality requirements.

20. The municipality may, after consultation with DEP, approve alternative methods for meeting the state water quality requirements other than those in this section, provided that they meet the minimum requirements of and do not conflict with state law, including, but not limited to, the Clean Streams Law.

§ 26-425 Stormwater Management Districts.

[Ord. 07-2005-523, 7/28/2005, § 305]

1. Mapping of Stormwater Management Districts. To implement the provisions of the Little Lehigh Creek Watershed Stormwater Management Plan, the municipality is hereby divided into Stormwater Management Districts consistent with the Little Lehigh Creek Release Rate Map presented in the Plan. The boundaries of the Stormwater Management Districts are shown on an official map which is available for inspection at the municipal office. A copy of the official map at a reduced scale is included in Appendix A for general reference.[1]

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

2. Description of Stormwater Management Districts. Two types of Stormwater Management Districts may be applicable to the municipality, namely Conditional No Detention Districts and Dual Release Rate Districts as described below.

A. Conditional No Detention Districts. Within these districts, the capacity of the local runoff conveyance facilities (as defined in Subpart B) must be calculated to determine if adequate capacity exists. For this determination, the developer must calculate peak flows assuming that the site is developed as proposed and that the remainder of the local watershed is in the existing condition. The developer must also calculate peak flows assuming that the entire local watershed is developed per current zoning and that all new development would use the runoff controls specified by this Part. The larger of the two peak flows calculated will be used in determining if adequate capacity exists. If adequate capacity exists to safely transport runoff from the site to the main channel (as defined in Subpart B), these watershed areas may discharge post-development peak runoff without detention facilities. If the capacity calculations show that the local runoff conveyance facilities lack adequate capacity, the developer shall either use a 100% release rate control or provide increased capacity of downstream elements to convey increased peak flows consistent with § 26-426, Subsection 16. Any capacity improvements must be designed to convey runoff from development of all areas tributary to the improvement consistent with the capacity criteria specified in § 26-426, Subsection 4. By definition, a storm drainage problem area associated with the local runoff conveyance facilities indicates that adequate capacity does not exist.

B. Dual Release Rate Districts. Within this district, the two-year post-development peak runoff must be controlled to 30% of the predevelopment two-year runoff peak. Further, the ten-year, twenty-five-year and one-hundred-year post-development peak runoff must be controlled to the stated percentage of the predevelopment peak. Release rates associated with the ten- through one-hundred-year events vary from 50% to 100% depending upon location in the watershed.

§ 26-426 Stormwater Management District Implementation Provisions.

[Ord. 07-2005-523, 7/28/2005, § 306]

1. Applicants shall provide a comparative pre- and post-construction stormwater management hydrograph analysis for each direction of discharge and for the site overall to demonstrate compliance with the provisions of this Part.

2. Any stormwater management controls required by this Part and subject to a dual release rate criteria shall meet the applicable release rate criteria for each of the two-, ten-, twenty-five- and one-hundred-year return period runoff events consistent with the calculation methodology specified in § 26-427.

3. 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 provided as part of the drainage plan. The District boundaries as originally drawn coincide with topographic divides or, in certain instances, are drawn from the intersection of the watercourse and a physical feature such as the confluence with another watercourse or a potential flow obstruction (e.g., road, culvert, bridge, etc.). The physical feature is the downstream limit of the subarea and the subarea boundary is drawn from that point up slope to each topographic divide along the path perpendicular to the contour lines.

4. Any downstream capacity analysis conducted in accordance with this Part shall use the following criteria for determining adequacy for accepting increased peak flow rates:

A. Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year-return-period event within their banks at velocities consistent with protection of the channels from erosion.

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

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

5. For a proposed development site located within one release rate category subarea, the total runoff from the site shall meet the applicable release rate criteria. For development sites with multiple directions of runoff discharge, individual drainage directions may be designed for up to a 100% release rate so long as the total runoff from the site is controlled to the applicable release rate.

6. For a proposed development site located within two or more release category subareas, the peak discharge rate from any subarea shall be the predevelopment peak discharge for that subarea multiplied by the applicable release rate. 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 re-combine 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.

7. For a proposed development site located partially within a release rate category subarea and partially within a conditional no detention subarea, a significant portion of the site area subject to the release rate control may not be drained to the discharge point(s) located in the no detention subarea except as part of a no harm or hardship waiver procedure.

8. No portion of a site may be regraded between the Little Lehigh Creek Watershed and any adjacent watershed except as part of a no harm or hardship waiver procedure.

9. Within a release rate category area, for a proposed development site which has areas which drain to a closed depression(s), the design release from the site will be the lesser of (a) the applicable release rate flow assuming no closed depression(s) or (b) the existing peak flow actually leaving the site. In cases where (b) would result in an unreasonably small design release, the design discharge of less than or equal to the release rate will be determined by the available downstream conveyance capacity to the main channel calculated using § 26-426, Subsection 4, and the minimum orifice criteria.

10. Off-site areas which drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site using the capacity criteria in § 26-426, Subsection 4, and the detention criteria in § 26-427.

11. For development sites proposed to take place in phases, all detention ponds shall be designed to meet the applicable release rate(s) applied to all site areas tributary to the proposed pond discharge direction. All site tributary areas will be assumed as developed, regardless of whether all site tributary acres are proposed for development at that time. An exception shall be sites with multiple detention ponds in series where only the downstream pond must be designed to the stated release rate.

12. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area shall be subject to the release rate criteria. The impact area includes any proposed cover or grading changes.

13. Development proposals which, through groundwater recharge or other means, do not increase either the rate or volume of runoff discharged from the site compared to predevelopment are not subject to the release rate provisions of this Part.

14. No Harm Water Quantity Option.

A. For any proposed development site not located in a conditional no detention district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that special circumstances exist for the proposed development site and that no harm would be caused by discharging at a higher runoff rate than that specified by the Plan. Special circumstances are defined as any hydrologic or hydraulic aspects of the development itself not specifically considered in the development of the Plan runoff control strategy. Proof of no harm would have to be shown from the development site through the remainder of the downstream drainage network to the confluence of the creek with the Lehigh River. Proof of no harm must be shown using the capacity criteria specified in § 26-426, Subsection 4, if downstream capacity analysis is a part of the no harm justification.

B. Attempts to prove no harm based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:

(1) The peak flow values to be used for downstream areas for the design return period storms (two-, ten-, twenty-five- and one-hundred-year) shall be the values from the calibrated watershed model for the Little Lehigh Creek or as calculated by an applicant using an alternate method acceptable to the municipality. The flow values from the watershed model would be supplied to the developer by the municipality upon request.

(2) Any available capacity in the downstream conveyance system as documented by a developer may be used by the developer only in proportion to his development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if his site is 10% of the upstream undeveloped acreage, he may use up to 10% of the documented downstream available capacity).

(3) Developer-proposed runoff controls which would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove no harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 26-426, Subsection 16.

C. Any no harm justifications shall be submitted by the developer as part of the drainage plan submission per Subpart D.

15. Regional Detention Alternatives. For certain areas within the study area, it may be more cost effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined based on the required release rate at the point of discharge.

16. Capacity Improvements.

A. In certain instances, primarily within the conditional no detention areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Peak flow calculations shall be done assuming that the local watershed is in the existing condition and then assuming that the local watershed is developed per current zoning and using the specified runoff controls. Any capacity improvements would be designed using the larger of the above peak flows and the capacity criteria specified in Subsection 4. All new development in the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.

B. Capacity improvements may also be provided as necessary to implement any regional detention alternatives or to implement a modified no harm option which proposes specific capacity improvements to provide that a less stringent discharge control would not create any harm downstream.

§ 26-427 Calculation Methodology.

[Ord. 07-2005-523, 7/28/2005, § 307]

1. Stormwater runoff from all development sites shall be calculated using either the rational method or the soil-cover-complex methodology.

2. Infiltration BMP loading rate percentages in the recommendation chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix D shall be calculated as follows:

(Area Tributary to infiltration BMP\Base area of infiltration BMP) *100%

	The area tributary to the infiltration BMP shall be weighted as follows:
		All disturbed areas to be made impervious:	Weight at 100%
		All disturbed areas to be made pervious:	Weight at 50%
		All undisturbed pervious areas:	Weight at 0%
		All existing impervious areas:	Weight at 100%

3. Soil thickness is to be measured from the bottom of any proposed infiltration system. The effective soil thickness in the recommendation chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix H is the measured soil thickness multiplied by the thickness factor based on soil permeability, as follows:

	Permeability Range	Thickness Factor
	6.0 to 12.0 inches/hour	0.8
	2.0 to 6.0 inches/hour	1.0
	1.0 to 2.0 inches/hour	1.4
	0.75 to 1.0 inches/hour	1.2
	0.5 to 0.75 inches/hour	1.0

*

If the permeability rate falls on a break between two thickness factors, the smaller thickness factor shall be used.

Sites with soil permeability greater than 12.0 in./hr. or less than 0.5 in./hr. are not recommended for infiltration.

4. The design of any detention basin intended to meet the requirements of this Part shall be verified by routing the design storm hydrograph through the proposed basin using the storage indication method or other methodology demonstrated to be more appropriate. For basins designed using the rational method technique, the design hydrograph for routing shall be either the Universal Rational Hydrograph or the modified rational method trapezoidal hydrograph which maximizes detention volume. Use of the modified rational hydrograph shall be consistent with the procedure described in Section "PIPE.RAT" of the Users' Manual for the Penn State Urban Hydrograph Method (1987).

5. BMPs designed to store or infiltrate runoff and discharge to surface runoff or pipe flow shall be routed using the storage indication method.

6. BMPs designed to store or infiltrate runoff and discharge to surface runoff or pipe flow shall provide storage volume for the full WQv below the lowest outlet invert.

7. Wet detention ponds designed to have a permanent pool for the WQv shall assume that the permanent pool volume below the primary outlet is full at the beginning of design event routing for the purposes of evaluating peak outflows.

8. All stormwater detention facilities shall provide a minimum one-foot freeboard above the maximum pool elevation associated with the two- through twenty-five-year-runoff events. A 0.5 foot freeboard shall be provided above the maximum pool elevation of the one-hundred-year-runoff event. The freeboard shall be measured from the maximum pool elevation to the invert of the emergency spillway. The two- through one-hundred-year-storm events shall be controlled by the primary outlet structure. An emergency spillway for each basin shall be designed to pass the one-hundred-year-return frequency storm peak basin inflow rate with a minimum 0.5 foot freeboard measured to the top of basin. The freeboard criteria shall be met considering any off-site areas tributary to the basin as developed, as applicable. If this detention facility is considered to be a dam as per DEP Chapter 105, the design of the facility must be consistent with the Chapter 105 regulations, and may be required to pass a storm greater than the one-hundred-year event.

9. The minimum circular orifice diameter for controlling discharge rates from detention facilities shall be three inches. Designs where a lesser size orifice would be required to fully meet release rates shall be acceptable provided that as much of the site runoff as practical is directed to the detention facilities.

10. Runoff calculations using the soil-cover-complex method shall use the Natural Resources Conservation Service Type II twenty-four-hour rainfall distribution. The twenty-four-hour rainfall depths for the various return periods to be used consistent with this Part may be taken from NOAA Atlas 14, Volume 2 or the PennDOT Intensity - Duration - Frequency Field Manual ("PDT-IDF") (May 1986) for Region 4. The following values are taken from the PDT-IDF Field Manual:

	Return Period	24-Hour Rainfall Depth
	1-year	2.40 inches
	2-year	3.00 inches
	5-year	3.60 inches
	10-year	4.56 inches
	25-year	5.52 inches
	50-year	6.48 inches
	100-year	7.44 inches

A graphical and tabular presentation of the Type II twenty-four hour distribution is included in Appendix C.

11. Runoff calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration and return periods and the Intensity-Duration-Frequency curves as presented in Appendix C.

12. Runoff curve numbers (CNs) to be used in the soil-cover-complex method shall be based upon the matrix presented in Appendix C.

13. Runoff coefficients for use in the Rational Method shall be based upon the table presented in Appendix C.

14. All time of concentration calculations shall use a segmental approach which may include one or all of the flow types below:

A. Sheet flow (overland flow) calculations shall use either the NRCS average velocity chart (Figure 3-1, Technical Release-55, 1975) or the modified kinematic wave travel time equation (equation 3-3, NRCS TR-55, June 1986). If using the modified kinematic wave travel time equation, the sheet flow length shall be limited to 50 feet for designs using the Rational Method and limited to 150 feet for designs using the soil-cover-complex method.

B. Shallow concentrated flow travel times shall be determined from the watercourse slope, type of surface and the velocity from Figure 3-1 of TR-55, June 1986.

C. Open channel flow travel times shall be determined from velocities calculated by the Manning equation. Bankfull flows shall be used for determining velocities. Manning 'n' values shall be based on the table presented in Appendix C.

D. Pipe flow travel times shall be determined from velocities calculated using the Manning equation assuming full flow and the Manning 'n' values from Appendix C.

15. If using the Rational Method, all predevelopment calculations for a given discharge direction shall be based on a common time of concentration considering both on-site and any off-site drainage areas. If using the Rational Method, all post-development calculations for a given discharge direction shall be based on a common time of concentration considering both on-site and any off-site drainage areas.

16. The Manning equation shall be used to calculate the capacity of watercourses. Manning 'n' values used in the calculations shall be consistent with the table presented in Appendix C or other appropriate standard engineering 'n' value resources. Pipe capacities shall be determined by methods acceptable to the municipality.

17. The Pennsylvania DEP, Chapter 105, Rules and Regulations, apply to the construction, modification, operation or maintenance of both existing and proposed dams, water obstructions and encroachments throughout the watershed. Criteria for design and construction of stormwater management facilities according to this Part may not be the same criteria that are used in the permitting of dams under the Dam Safety Program.