Township of East Allen, PA Article III Stormwater Management Requirements

§ 204-12 General requirements.

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

B. Storm drainage systems shall be designed to preserve natural watercourses except as modified by stormwater detention facilities, recharge facilities, water quality facilities, pipe systems or open channels consistent with this chapter.

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

D. 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 chapter. 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.

E. Where a site is traversed by watercourses other than those for which a 100-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 § 204-19 for the 100-year return period runoff and to provide a freeboard allowance of 0.5 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 100-year floodplain is formally defined are subject to the applicable municipal floodplain regulations.

F. Post-construction BMPs shall be designed, installed, operated and maintained to meet the requirements of the Clean Streams Law[1] and implementing regulations, including the established practices in 25 Pa. Code Chapter 102 and the specifications of this chapter as to prevent accelerated erosion in watercourse channels and at all points of discharge.

[1] Editor's Note: See 35 P.S. § 691.1 et seq.

G. 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 chapter.

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

[2] Editor's Note: Appendix F is included as an attachment at the end of this chapter.

I. Infiltration for stormwater management is encouraged where soils and geology permit, consistent with the provisions of this chapter and, where appropriate, the Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix D.[3]

[3] Editor's Note: Appendix D is included as an attachment at the end of this chapter.

§ 204-13 Permit requirements.

Other regulations contain independent permit requirements that apply to certain regulated and earth disturbance activities eligible for authorization by the municipality in accordance with the permitting requirements in this chapter. Permit requirements pursuant to those other regulations must be met prior to commencement, and during the conduct, of such 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 Chapters 102 and 105.

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

D. 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.

E. Projects that involve use of PennDOT right-of-way, or that involve new discharges onto or toward PennDOT right-of-way, are subject to the requirements, including the permitting requirements, of Title 67 of Chapter 441 of the Pennsylvania Code.

§ 204-14 Erosion and sediment control.

A. 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.

B. A written erosion and sediment control plan is required by DEP regulations for any earth disturbance activity under Pa. Code § 102.4(b).

C. A DEP NPDES stormwater discharges associated with construction activities permit is required for regulated earth disturbance activities of one acre or greater under Pa. Code Chapter 92.

D. 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.

E. 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.

§ 204-15 Post-construction water quality criteria.

A. No regulated earth disturbance activities within the municipality shall commence until approval by the municipality of a drainage plan which demonstrates compliance with this chapter.

B. The water quality volume (WQV) shall be captured and treated with vegetated/surface and/or direct recharge/subsurface BMPs. The WQV shall be calculated as the difference in runoff volume from predevelopment to post-development for the twenty-four-hour, two-year return period storm. This may be calculated using either the Soil-Cover-Complex Method or Rational Method using the two-year rainfall depth as noted in § 204-19I. The effect of closed depressions on the site shall be considered in this calculation. The WQV shall be captured and treated in a manner consistent with the standards outlined in § 204-16 of the chapter.

C. 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.

D. 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.

E. For each proposed regulated activity in the watershed where an applicant intends to use infiltration BMPs, the applicant shall conduct a preliminary site investigation, 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.[1] This investigation will determine depth to bedrock, depth to the seasonal high water table, soil permeability and location of special geologic features, if applicable. This investigation may be done by a certified Sewage Enforcement Officer (SEO) except that the location(s) of special geologic features shall be verified by a qualified geotechnical professional.

[1] Editor's note: Appendix G is included as an attachment at the end of this chapter.

F. Sites where applicants intend to use infiltration BMPs must meet the following criteria:

(1) Depth to bedrock below the invert of the BMP greater than or equal to two feet.

(2) Depth to seasonal high water table below the invert of the BMP greater than or equal to two feet; except for infiltration of residential roof runoff where the seasonal high water table must be below the invert of the BMP.

(3) Soil permeability (as measured using the standards listed in Appendix C of the Pennsylvania Stormwater Best Practices Manual) greater than or equal to 0.1 inches/hour and less than or equal to 10 inches per hour.

(4) Setback distances or buffers as follows:

(a) One hundred feet from water supply wells or 50 feet in residential development.

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

(5) Fifty feet from septic system drainfields.

(6) 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.

G. In entirely carbonate areas, where the applicant intends to use infiltration BMPs, the preliminary site investigation described in Appendix G[2] shall be conducted. For infiltration areas that appear feasible based on the preliminary site investigation, 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 § 204-19, 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[3] to determine if the site is recommended for infiltration. In addition to the recommendation from Appendix D, the conditions listed in § 204-15F are required for infiltration in carbonate areas.

[2] Editor's Note: Appendix G is included as an attachment at the end of this chapter.

[3] Editor's Note: Appendix D is included as an attachment at the end of this chapter.

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

I. If infiltration of the entire WQV is not proposed, the remainder of the WQV shall be treated by acceptable BMPs for each discharge location. Acceptable BMPs are listed in Appendix H.[4]

[4] Editor's Note: Appendix H is included as an attachment at the end of this chapter.

J. Stormwater runoff from hot spot land uses shall be pretreated. Suggested methods of pretreatment are listed in Appendix H.

K. The use of infiltration BMPs is prohibited on hot spot land use areas unless the applicant can demonstrate that existing and proposed site conditions, including any proposed runoff pretreatment, create conditions suitable for runoff infiltration under this chapter.

L. 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.

M. Stormwater drainage wells may only be used for runoff from roof areas.

N. 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 § 204-15F, 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.

O. The municipality may, after consultation with DEP, approve alternative methods for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law including but not limited to the Clean Streams Law.[5]

[5] Editor's Note: See 32 P.S. § 680.1 et seq.

§ 204-16 Green infrastructure; existing water balance preservation.

A. The entire WQV as calculated in § 204-15B of this chapter shall be captured and treated by either direct recharge/subsurface and/or vegetated/surface BMPs.

B. As much proposed impervious area as practical shall be directed to water quality BMPs.

C. Existing impervious area that is not proposed to be treated by direct recharge/subsurface BMPs should be excluded from all water balance calculations.

D. Vegetated/surface BMPs shall be employed first for the site to capture the equivalent of a minimum of 0.38 inches of runoff for each square foot of impervious area, unless proven not feasible by the applicant. For proposed impervious cover directed to multiple BMPs, the vegetated/surface BMP capture volume chart in Appendix C[1] shall be used to determine overall site compliance. Direct recharge/subsurface BMPs may be used first for portions of the impervious cover provided the overall vegetated/surface BMP first standard is met.

[1] Editor's Note: Appendix C is included as an attachment at the end of this chapter.

E. A maximum of 30% of the total annual rainfall for a site may be directly recharged to groundwater using direct recharge/subsurface BMPs, for runoff from impervious areas.

(1) For development sites with greater than 33% proposed impervious cover:

(a) If all impervious cover is directed to vegetated/surface BMPs to capture the entire two-year, twenty-four-hour event, the direct recharge standard is met.

(b) Up to 33% of the site as impervious cover may be directed to direct recharge/Subsurface BMPs designed to capture the entire two-year, twenty-four-hour event provided the overall vegetated/surface BMP first standard is met. All remaining impervious cover shall be directed to vegetated/surface BMPs designed to capture the remainder of the WQV.

(c) For vegetated/surface and/or direct recharge/subsurface BMPs designed for runoff from impervious areas designed to capture less than the entire two-year, twenty-four-hour event, Appendix C[2] shall be used to assure that the maximum direct recharge standard is met.

[2] Editor's Note: Appendix C is included as an attachment at the end of this chapter.

(2) The maximum 30% direct recharge standard applies on an overall site basis, rather than in each drainage direction.

§ 204-17 Stormwater management districts.

A. Mapping of stormwater management districts. To implement the provisions of the Monocacy Creek Watershed Stormwater Management Plan Update, the municipality is hereby divided into stormwater management districts consistent with the Monocacy Creek Release Rate Map presented in the plan update. 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 included as an attachment at the end of this chapter.

B. Release rate districts. There are six single release rate districts that differ in the extent to which the post-development runoff must be controlled. The release rate districts are 50%, 60%, 70%, 80%, 90% and 100%. Within a given district, the post-development peak rate of storm runoff must be controlled to the stated percentage of the predevelopment peak rate of runoff for each of the ten-, twenty-five-, fifty- and 100-year return period storms to protect downstream watershed areas.

(1) There is one dual release rate district. Within this district, the ten-year return period event needs to meet a 30% release rate, and the twenty-five-year and higher return period events need to meet a 100% release rate.

C. Conditional no detention districts. These watershed areas peak very early with respect to the total watershed peak flow and contribute very minimal flow to the watershed peak flow. For that reason, these watershed areas may discharge post-development peak runoff without detention for the ten- through 100-year return periods without adversely affecting the total watershed peak flow. These areas are designated as conditional no detention areas because in certain instances the local runoff conveyance facilities, which transport runoff from the site to the main channel, may not have adequate capacity to safely transport the peak flows associated with no detention for a proposed development. In those instances, a 100% release rate control would have to be provided or, alternately, the capacity deficiency(ies) would have to be corrected.

§ 204-18 District implementation provisions.

A. Applicants shall provide a comparative preconstruction 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 chapter.

B. Any stormwater management controls required by this chapter and subject to release rate criteria shall meet the applicable release rate criteria for each of the two-, ten-, twenty-five-, fifty- and 100-year return period runoff events consistent with the calculation methodology specified in § 204-19.

C. 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.

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

(1) 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.

(2) Natural or man-made channels, swales, culverts, bridges, storm sewers or any other facilities which must convey flows from the tributary area must be able to convey the increased twenty-five-year return period runoff.

E. 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.

F. For a proposed development site located within two or more release rate category subareas, the peak discharge rate from any subarea shall be the pre-development 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 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.

G. For sites straddling major watershed divides (e.g., Monocacy Creek and Bushkill Creek), runoff volumes shall be managed to prevent diversion of runoff between watersheds, as practicable.

H. 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 § 204-18D and the minimum orifice criteria.

I. 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 § 204-18D and the detention criteria in § 204-19. In addition to the criteria in § 204-18D, on-site conveyance systems designed to carry runoff to a detention basin must be able to transport the basin's 100-year tributary flow either in-system, in-gutter or overland.

J. 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 areas 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.

K. 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.

L. 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 chapter.

M. "No harm" water quantity option. For any proposed development site, the developer has the option of using a less restrictive runoff control 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 this chapter. Special circumstances are defined as any hydrologic or hydraulic aspects of the development itself not accommodated by the runoff control standards of this chapter. 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 Monocacy Creek with the Lehigh River. Proof of "no harm" must be shown using the capacity criteria specified in § 204-18D if downstream capacity analysis is a part of the "no harm" justification.

(1) Attempts to prove "no harm" based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:

(a) 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).

(b) 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".

(c) Any downstream capacity improvements proposed by the developer as part of a "no harm" justification would be designed using the capacity criteria specified in § 204-18D. Peak flow contributions to the proposed improvements shall be calculated as the larger of: (1) assuming the local watershed is in the existing condition, or (2) assuming that the local watershed is developed per current zoning and using the specified runoff controls.

(2) Any "no harm" justifications shall be submitted by the developer as part of the drainage plan submission per Article IV. Developers submitting "no harm" justifications must still meet all of the water quality requirements in § 204-15. The municipality will process all eligible "no harm" requests in accordance with § 204-15O.

N. Capacity improvements. In certain instances, 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 § 204-18D. 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.

O. Release rates need to be met year round. Designs involving BMPs that function differently in winter versus nonwinter conditions (e.g., capture/reuse with spray irrigation shut off for the winter) must still meet release rates during the winter.

§ 204-19 Calculation methodology.

A. Stormwater runoff from all development sites shall be calculated using either the Rational Method or the Soil-Cover-Complex methodology.

B. Infiltration BMP loading rate percentages in the Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix D[1] shall be calculated as follows:

Area tributary to infiltration BMP * 100%

__________________________________

Base area of infiltration BMP

(1) The area tributary to the infiltration BMP shall be weighted as follows:

(a) All disturbed areas to be made impervious: Weight at 100%.

(b) All disturbed areas to be made pervious: Weight at 50%.

(c) All undisturbed pervious areas: Weight at 0%.

(d) All existing impervious areas: Weight at 100%.

[1] Editor's Note: Appendix D is included as an attachment at the end of this chapter.

C. The design of any detention basin intended to meet the requirements of this chapter 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 another rational hydrograph that closely approximates the volume of the Universal Rational Hydrograph.

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

E. 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.

F. 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.

G. All aboveground stormwater detention facilities shall provide a minimum 0.5 feet of freeboard above the maximum pool elevation associated with the two- through 100-year runoff events, or an additional 10% of the 100-year storage volume as freeboard volume, whichever is greater. All below-ground stormwater detention and infiltration facilities shall have an additional 10% of the 100-year storage volume available within the storage medium, as well as a minimum of 0.5 feet of freeboard. The freeboard shall be measured from the maximum pool elevation to the invert of the emergency spillway for aboveground facilities, and from the maximum pool elevation to the lowest overflow elevation for below-ground facilities. The two- through 100-year storm events shall be controlled by the primary outlet structure. An emergency spillway for each above-ground basin shall be designed to pass the 100-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.

(1) Exceptions to the freeboard requirements are as follows:

(a) Bioretention BMPs with a ponded depth less than or equal to 0.5 feet are exempt from the freeboard requirements.

(b) Small detention basins, with a ponded depth less than or equal to 1.5 feet or having a depth to the top of the berm less than or equal to 2.5 feet, may provide 20% additional storage volume measured from the maximum ponded depth to the invert of the emergency spillway in lieu of the above requirements. The depth of the emergency spillway must be sufficient to pass either two times the 100-year peak or the 100-year peak with 0.2 feet of freeboard to the top of berm, whichever is greater.

(c) Small infiltration basins, with a ponded depth less than or equal to 1.5 feet or having a depth to the top of the berm less than or equal to 2.5 feet, may provide 20% additional storage volume measured from the maximum ponded depth to the top of the berm in lieu of the above requirements. In this case, an emergency spillway is only necessary if runoff in excess of the basin volume would cause harm to downstream owners. If a spillway is necessary, it must be sufficiently sized to pass the 100-year peak inflow.

(2) 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 100-year event.

H. 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 with a three-inch orifice provided that as much of the site runoff as practical is directed to the detention facilities. The minimum three-inch diameter does not apply to the control of the WQV.

I. 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 chapter may be taken from NOAA Atlas 14, Precipitation Frequency Atlas of the United States, current volume, or the Pennsylvania Department of Transportation Drainage Manual, 2015 Edition for Region 4. The following values are taken from the Drainage Manual:

Return Period	24-Hour Rainfall Depth
One-year	2.64 inches
Two-year	3.16 inches
Five-year	3.91 inches
Ten-year	4.57 inches
Twenty-five-year	5.60 inches
Fifty-year	6.53 inches
100-year	7.63 inches

(1) A graphical and tabular presentation of the Type II twenty-four-hour distribution is included in Appendix C.[2]

[2] Editor's Note: Appendix C is included as an attachment at the end of this chapter.

J. Runoff calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration and return periods and NOAA Atlas 14, Precipitation Frequency Atlas of the United States Precipitation and Intensity Charts, current volume, as presented in Appendix C.

K. Runoff Curve Numbers (CN's) to be used in the Soil-Cover-Complex method shall be based upon the table presented in Appendix C.

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

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

(1) 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.

(2) 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.

(3) 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.

(4) 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.

N. 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.

O. When conditions exist such that a proposed detention facility may experience a tailwater effect, the basin shall be analyzed without any tailwater effect for all storm events for comparison against the required release rates. An additional routing of the 100-year storm with the full tailwater effect shall be performed to check that the basin has sufficient storage to contain the 100-year tributary flow and meet freeboard requirements.

P. 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.

Q. 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 chapter may differ from the criteria that are used in the permitting of dams under the Dam Safety Program.