Borough of Lewisburg, PA Article III Stormwater Management

§ 290-15 Stormwater management areas.

The Lewisburg Borough Stormwater Management Areas are as follows and appear on a map in Appendix A[1] of this chapter.

A. Buffalo Creek Stormwater Area. Activities within the Buffalo Creek Watershed shall comply with the requirements of this chapter and the Buffalo Creek Act 167 Stormwater Management Plan, adopted by the County of Union and approved by the Pennsylvania Department of Environmental Protection.

B. Bull Run (Limestone Run) Stormwater Area. Activities occurring within the Bull Run Watershed shall comply with the requirements of this chapter and the original Bull Run Act 167 Stormwater Management Plan of 1993 and the Plan Update of 2002 adopted by the County of Union and approved by the Pennsylvania Department of Environmental Protection.

C. West Branch Susquehanna River Stormwater Area. Activities occurring within the West Branch Susquehanna River Watershed shall comply with the requirements of this chapter and the West Branch Susquehanna River Act 167 Stormwater Management Plan, adopted by the County of Union and approved by the Pennsylvania Department of Environmental Protection.

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

§ 290-16 Requirements.

A. The management of stormwater on the site, both during and upon the completion of construction, and the design of any temporary or permanent facilities or structures and the utilization of a natural drainage system shall be in full compliance with this section.

B. Site designs shall minimize impervious surfaces and shall promote the infiltration of runoff through seepage beds, infiltration trenches, etc., where soil conditions permit to reduce the size or eliminate the need for detention facilities.

C. Stormwater runoff generated from development discharged directly into a wetland or other waters of the Commonwealth shall be done in accordance with Federal and State regulatory requirements and shall be adequately treated to prevent degradation.

D. Annual groundwater recharge rates shall be maintained by promoting infiltration. At a minimum annual recharge from the post-development site shall mimic the annual recharge from the predevelopment site conditions.

E. Applicants may select runoff control techniques, or a combination of techniques, which are most suitable to control stormwater runoff from the development site. Lewisburg Borough encourages applicants to consider alternative solutions, including best management practices (BMP's) for stormwater management. Applicants are urged to consult the Pennsylvania Handbook of Best Management Practices for Developing Areas and with the Municipal Engineer and the County Conservation District. All stormwater designs are subject to the approval of the Municipal Engineer. The Borough may request specific information on design and/or operating features of the proposed stormwater controls in order to determine their suitability and adequacy in terms of the standards of this chapter.

F. All stormwater management facilities including detention basins, BMP's, sewers, and culverts shall be designed by an individual qualified and/or experienced in their design. These qualifications should be listed on the front cover of the plan narrative.

G. The anticipated peak rate of stormwater runoff from the site during and after full development shall not exceed the peak rate of runoff from the site prior to development activities, measured in accordance with the standards and criteria of this chapter.

H. Roof drains shall not be connected to streets, sanitary or storm sewers or roadside ditches but shall be allowed to drain to the land surface to promote overland flow and infiltration of stormwater.

I. Existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.

J. If existing diffused stormwater flow is proposed to be concentrated the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other harm will result from concentrated discharge.

K. Storm sewers, swales, culverts and bridges.

(1) Storm sewers, swales, culverts, bridges, and related facilities shall be provided to:

(a) Permit the unimpeded flow of natural watercourses;

(b) Insure the drainage of all points along the line of streets;

(c) Intercept stormwater runoff along streets at reasonable intervals related to the extent and grade of the area drained, and to prevent the flooding of intersections and the undue concentration of Stormwater; and

(d) Insure unrestricted flow of stormwater under driveways, and at natural watercourses or drainage swales.

(2) Consideration should be given to anticipated up slope development.

L. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing conditions unless an alteration is approved by the Borough due to topographic conditions.

M. Easements shall be provided conforming to the line of all natural streams, channels, swales, and drainage systems and other stormwater infrastructure installed to comply with this chapter. The terms of such easements shall prohibit excavation, the placement of fill or structures and any other alterations that may adversely affect the flow of stormwater.

N. All stream encroachment activities, including work in and adjacent to waters of the Commonwealth or wetlands, shall comply with applicable PA DEP requirements.

O. Any stormwater facility located on a state or Borough highway right-of-way shall be subject to the approval of the Pennsylvania Department of Transportation or Lewisburg Borough.

P. Applicants are encouraged to incorporate designs to take advantage of the stormwater credits presented in Appendix B[1] of this chapter.

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

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

R. The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Borough shall reserve the right to disapprove any design that would result in the occupancy or continuation of an adverse hydrologic or hydraulic condition.

§ 290-17 Water quality requirements.

A. For water quality, the objective is to provide adequate storage to capture and treat the runoff from 95% of the average rainfall in accordance with the following where P represents the depth of rain associated with 95% of the total rainfall events over 0.11 inches.

[Amended 11-15-2022 by Ord. No. 1069]

(1) The size of the water quality facility shall be based upon the following equation:

[Amended 11-15-2022 by Ord. No. 1069]

	WQv	=	(1.2) (Rv)(A)
				P = 1.53 inches of rainfall
			12

	Where:
		WQv = water quality volume (in ac-ft)
		Rv = 0.05 + 0.009(I) where I is percent impervious cover
		A = area in acres*

*Treatment of the WQv for off-site areas and areas not disturbed is not required.

(2) Treatment of the WQ_v shall be provided at all developments where stormwater management is required. A minimum WQ_v of 0.2 inches per acre shall be met at sites or in drainage areas that have less than 15% impervious cover.

(3) The WQ_v shall be based on the impervious cover for the proposed site. Off-site existing impervious areas may be excluded from the calculation of the water quality volume requirements.

(4) When a project contains or is divided by multiple drainage areas, the WQ_v shall be addressed for each drainage area.

(5) Drainage areas having no impervious cover and no proposed disturbance during development may be excluded from the WQ_v calculations. Designers are encouraged to use these areas as nonstructural practices for WQ_v treatment.

(6) Where structural practices for treating the recharge volume (Re_v) are employed upstream of a BMP, the Re_v may be subtracted from the WQ_v used for design.

(7) Where nonstructural practices are employed in the site design, the WQ_v can be reduced in accordance with the conditions outlined in Appendix B[1] of this chapter.

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

(8) The design of the facility shall consider and minimize the chances of clogging and sedimentation potential. Orifices smaller than three inches diameter are not recommended. However, if the design engineer can provide proof that the smaller orifices are protected from clogging by use of trash racks, etc., smaller orifices may be permitted.

(9) When designing flow splitters for off-line practices, consult the small storm hydrology method provided in Appendix L[2] of this chapter.

[2] Editor's Note: Appendix L is on file in the Borough offices.

B. To accomplish adequate water quality treatment the final WQ_v shall be treated by an acceptable BMP from the list presented in Appendix M[3] or an equivalent practice approved by the Municipal Engineer. The applicant may 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 BMPs.

[3] Editor's Note: Appendix M is on file in the Borough offices.

C. The water quality requirement can be met by providing a twenty-four-hour draw down of a portion of the (WQ_v) in conjunction with a stormwater pond or wetland system. Referred to as “extended detention” (ED), this is different than providing the extended detention of the one-year storm for the channel protection volume (Cp_v). The ED portion of the WQ_v may be included when routing the Cp_v.

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

(1) Total contributing area.

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

(3) Slope and topography.

(4) Seasonal high water table.

(5) Depth to bedrock.

(6) Proximity to building foundations and wellheads.

(7) Erodibility of soils.

(8) Subgrade stability and susceptibility to sinkhole formation

(9) Land availability and configuration of the topography.

(10) Peak discharge and required volume control.

(11) Stream bank erosion.

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

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

(14) The nature of pollutants being removed.

(15) Creation and protection of wildlife habitat.

(16) Enhancement of aesthetic and property value.

(17) Maintenance requirements.

E. Stormwater hotspots. If a site is designated as a stormwater hotspot as per Table 302-1, it has important implications for how stormwater is managed.

(1) A greater level of stormwater treatment is required at hotspot sites to prevent pollutant wash off after construction.

(2) For areas designated as hotspots, a Stormwater Pollution Prevention Plan may be required to be designed and implemented that contains operation practices at the site to reduce the generation of pollutants by preventing contact with rainfall.

(3) Stormwater Pollution Prevention Plans shall follow the requirements of the U.S. EPA NPDES stormwater program.

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

(a) Residential streets and rural highways, residential development, institutional development, commercial and office developments, non-industrial rooftops, pervious areas except for golf courses and nurseries. Large highways and retail gasoline outlet facilities are not designated as hotspots although it is important to ensure that stormwater plans for these facilities adequately protect groundwater.

	Table 302-1: Stormwater Hotspots
	Vehicle salvage yards and recycling facilities*
	Vehicle service and maintenance facilities
	Vehicle and equipment cleaning facilities*
	Fleet storage areas (bus, truck, etc.)*
	Industrial sites
	Marinas (service and maintenance)*
	Outdoor liquid container storage
	Outdoor loading/unloading facilities
	Public works storage areas
	Facilities that generate or store hazardous materials*
	Commercial container nursery
	Golf courses
	Other land uses and activities as designated

	NOTES:
	*Stormwater Pollution Plan implementation may be required for these land uses or activities under the U.S. EPA NPDES stormwater program.

§ 290-18 Groundwater recharge requirements.

A. Design of the infiltration/recharge stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is paved and roofed over. These measures are encouraged, particularly in hydrologic soil groups A and B and shall be utilized wherever feasible.

B. The criteria for maintaining recharge is based on the USDA average annual recharge volume per soil type divided by the annual rainfall in Union County (40 inches per year) and multiplied by 90%. This keeps the recharge calculation consistent with the WQ_v methodology. Thus, an annual recharge volume requirement shall be specified for a site as follows:

(1) Percent volume method.

Rev = [(S)(Rv)(A)]/12

	Where:
		Rv = 0.05 + 0.009(I) where I is percent impervious cover
		A = site area in acres

(2) Percent area method.

Rev = (S)(Ai)

	Where:
		Ai = the measured impervious cover

	Hydrologic Soil Group	Soil Specific Recharge Factor (S)
	A	0.40
	B	0.27
	C	0.14
	D	0.07

(3) The recharge volume is considered part of the total WQ_v that must be provided at a site and can be achieved either by a structural practice (e.g., infiltration, bioretention), a nonstructural practice (e.g., buffers, disconnection of rooftops), or a combination of both.

(4) Drainage areas having no impervious cover and no proposed disturbance during development may be excluded from the Re_v calculations. Designers are encouraged to use these areas as nonstructural practices for Re_v treatment.

(5) The Re_v and WQ_v are inclusive. When treated separately, the Re_v may be subtracted from the WQ_v when sizing the water quality BMP.

(6) Recharge/infiltration facilities may be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.

C. Basis for determining recharge volume.

(1) If more than one hydrologic soil group (HSG) is present at a site, a composite soil specific recharge factor shall be computed based on the proportion of total site area within each HSG. The recharge volume provided at the site shall be directed to the most permeable HSG available.

(2) The "percent volume" method is used to determine the Re_v treatment requirement when structural practices are used to provide recharge. These practices must provide seepage into the ground and may include infiltration and exfiltration structures (e.g., infiltration, bioretention, dry swales or sand filters with storage below the under drain). Structures that require impermeable liners, intercept groundwater, or are designed for trapping sediment (e.g., forbays) may not be used. In this method, the volume of runoff treated by structural practices shall meet or exceed the computed recharge volume.

(3) The "percent area" method is used to determine the Re_v treatment requirements when nonstructural practices are used. Under this method, the recharge requirements are evaluated by mapping the percent of impervious area that is effectively treated by an acceptable nonstructural practice and comparing it to the minimum recharge requirements.

(4) Acceptable nonstructural practices include filter strips that treat rooftop or parking lot runoff, sheet flow discharge to stream buffers, and grass channels that treat roadway runoff.

(5) The recharge volume criterion does not apply to any portion of a site designated as a stormwater hotspot or any project considered as redevelopment. In addition, the Municipal Engineer may alter or eliminate the recharge volume requirement if the site is situated on unsuitable soils (e.g., marine clays), karst or in an urban redevelopment area. In this situation, nonstructural practices (percent area method) shall be implemented to the maximum extent practicable and the remaining or untreated Re_v included in the WQ_v treatment.

(6) If Re_v is treated by structural or nonstructural practices separate and upstream of the WQ_v treatment, the WQ_v is adjusted accordingly.

D. Soils evaluation.

(1) A detailed soils evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified professional, and at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. Advanced testing methods such as the double ring test are encouraged. The municipal engineer reserves the right to require an additional soils evaluation when it is believed that test results are not reasonable.

(2) Extreme caution shall be exercised where infiltration is proposed in geologically susceptible areas such as strip mine or limestone areas. Extreme caution shall also be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. It is also extremely important that the design professional evaluates the possibility of groundwater contamination from the proposed infiltration/recharge facility and recommends a hydrogeologic justification study be performed if necessary. Whenever a basin will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent ground water contamination and, where necessary, sinkhole formation.

(3) The Borough may require the installation of an impermeable liner in detention basins underlain by limestone or in areas of karst topography. A detailed hydrogeologic investigation may be required by the Borough. The developer may also be required to provide safeguards against groundwater contamination for uses which may cause groundwater contamination, should there be an accident or spill.

E. All recharge/infiltration facilities shall be designed to completely drain within 72 hours of reaching maximum capacity.

§ 290-19 Channel protection storage volume (stream bank erosion).

A. Stream channel protection shall be considered in implementing the standards of § 290-20 of this chapter. If a stormwater storage facility needs to be constructed then, to protect channels from erosion, the outflow structure shall be designed to provide twenty-four-hour extended detention of the one-year twenty-four-hour storm event. The method for determining the Cp_v requirement is detailed in Appendix N[1] of this chapter.

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

B. For discharges to streams having verified naturally reproducing wild trout or that are stocked with trout (based upon the most recent resource classification or other appropriate documentation of the Pennsylvania Fish and Boat Commission or other appropriate agency), only 12 hours of extended detention shall be provided. The rationale for this criterion is that runoff will be stored and released in such a gradual manner that critical erosive velocities during bankfull and near-bankfull events will seldom be exceeded in downstream channels.

C. Basis for determining channel protection storage volume.

(1) The models HEC-HMS, TR-55 and TR-20 (or an equivalent approved by the Municipal Engineer) shall be used for determining peak discharge rates.

(2) The rainfall depth for the one-year twenty-four-hour storm event in Union County shall utilize the upper range of the 90% confidence interval provided by NOAA (refer to Appendix E).[2]

[Amended 11-15-2022 by Ord. No. 1069]

[2] Editor's Note: Appendix E is on file in the Borough offices.

(3) Off-site areas shall be modeled as present land use in good condition for the one-year storm event.

(4) The length of overland flow used in time of concentration (t_c) calculations is limited to no more than 150 feet.

(5) The Cp_v storage volume shall be computed using the detention lag time between hydrograph procedures outlined in Appendix N of this chapter. The detention lag time (T) for a one-year storm is defined as the interval between the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph.

(6) Cp_v is not required at sites where the one-year post-development peak discharge (q_i) is less than or equal to 2.0 cfs. A Cp_v orifice diameter (d_o) of less than 3.0 inches is subject to approval by the Municipal Engineer and is not recommended unless an internal control for orifice protection is used.

(7) Cp_v shall be addressed for the entire site. If a site consists of multiple drainage areas, Cp_v may be distributed proportionately to each drainage area.

(8) Extended detention storage provided for the Cp_v does not meet the WQ_v requirement (i.e., Cp_v and WQ_v shall be treated separately).

(9) The stormwater storage needed for the Cp_v may be provided above the WQ_v storage in stormwater ponds and wetlands, thereby meeting all storage criteria except Re_v in a single facility with appropriate hydraulic control structures for each storage requirement.

(10) Infiltration is not recommended for Cp_v control because of large storage requirements.

§ 290-20 Overbank and extreme event flood protection requirements.

[Amended 11-15-2022 by Ord. No. 1069]

For a site located within two or more districts the peak discharge rate from any sub-area shall be the predevelopment peak discharge for that sub-area. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by sub-area. Pre-development and post-development runoff for specific sites shall be computed using an approved method as per Table 307-1 for the two- , ten- , twenty-five- , fifty- , and one-hundred-year storm. In addition to the other requirements provided in this section, all sites must provide the following peak flow rate reductions for all subareas within the site:

Two-year post-development peak flow rate	<=	One-year pre-development peak flow rate
Five-year post-development peak flow rate	<=	Two-year pre-development peak flow rate

A. Bull run watershed. The Bull Run Watershed Release Rate Percentage Maps in Appendix I[1] of this chapter illustrate the release rate subareas and define the percentage of pre-development peak rate of runoff that can be discharged from an outfall on the site after development. The release rates are also shown in Table 305-1. The release rates apply uniformly to all land development or alterations within a subarea. Procedures for applying the release rate percentage are contained in Appendix J.[2]

	Table 305-1: Bull Run Watershed Release Rates
	Subareas		Release Rate (%)
	Bull Run Watershed	1	80
		2	50
		3	100
		4	100
		5	60
		6	60
		7	100
		8	70
		9	50
		10	60
	Miller Run	37-40	100
		42-45	100
		46	75
		47	100
		49-51	100

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

[2] Editor's Note: Appendix J is on file in the Borough offices.

B. Buffalo Creek Watershed. The Buffalo Creek Watershed contains three runoff control districts, "A," B," and "C." Development sites in the Buffalo Creek Watershed must control post-development runoff rates to pre-development runoff rates as shown in Table 305-2. Lewisburg Borough is completely within the Buffalo Creek Runoff Control District "C." See Appendix K[3] for the Buffalo Creek Runoff Control District and subwatersheds within the Borough.

	Table 305-2: Buffalo Creek
	Subareas		Post-Development Design Storm	Pre-Development Design Storm
	Runoff Control Districts
	A	1-4	2-year	1-year
		16	10-year	10-year
		28-39	50-year	50-year
		58-59
	B	5-15
		17-27	2-year	1-year
		40-55	10-year	5-year
		60-65	50-year	25-year
		70-75
	C	79-81
		56-57	2-year	1-year
		66-69	10-year	10-year
		76-78	50-year	50-year
		82-92

[3] Editor's Note: Appendix K is on file in the Borough offices.

C. West Branch Susquehanna River. The release rate within the West Branch Susquehanna River Watershed within Lewisburg Borough is 100%. For sites that discharge directly to the Susquehanna River the Municipal Engineer may waive the requirements of this section. However, safe conveyance of runoff must be demonstrated for the two- , ten- , twenty-five- , fifty- and one-hundred-year design storms. The Borough reserves the right to require that the release rates be met. Procedures for applying the release rates are in Appendix J.[4]

[4] Editor's Note: Appendix J is on file in the Borough offices.

§ 290-21 Design considerations.

A. All storm sewers shall be able to convey the post-development runoff from a ten-year design storm without surcharging inlets and shall be constructed using Penn DOT Form 408 Specifications, Standard Details, unless otherwise directed by the Municipal Engineer. When connecting to an existing storm sewer system the applicant must demonstrate that the proposed system will not exacerbate any existing stormwater problems.

B. Stormwater roof drains shall not discharge into any municipal sanitary sewer line or over a sidewalk.

C. Inlets shall be placed at the curb line where a curbed section is installed. Inlets required for parallel or cross drainage without a curbed section shall be set at the centerline of the ditch.

D. Structures shall be Penn DOT Type M pre-cast concrete or cast-in-place Class A concrete. Brick or block structures shall not be permitted. Solid concrete block or brick may be incorporated into a structure only for grade adjustment of the casting.

E. All water obstructions (bridges, culverts, outfalls or stream enclosures) shall have ample waterway opening to carry expected flows, based on a minimum post-development peak storm frequency of 25 years and shall have a minimum of one foot of freeboard measured below the lowest point along the top of the roadway.

F. Bridge and culvert construction shall be in accordance with the Pennsylvania Department of Transportation specifications and shall meet the requirements of the Pennsylvania Department of Environmental Protection.

G. Any drainage conveyance facility and/or channel that does not fall under PA DEP Chapter 105 Regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the ten-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure.

H. Roadway crossings located within designated floodplain areas shall be able to convey runoff from a one-hundred-year design storm.

I. Any stormwater management facility designed to store runoff and requiring an earthen berm or embankment shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year post-development conditions. The height of the embankment must be set as to provide a minimum of one foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year post-development inflow.

J. Stormwater management facilities that require a dam safety permit under PA DEP Chapter 105 shall meet the applicable dam safety requirements that may require the facility to pass storms larger than the one-hundred-year event.

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

L. Detention basins for stormwater peak discharge storage shall comply with the following criteria:

(1) Basins shall be installed prior to any earthmoving or land disturbance that they will serve. The phasing of their construction shall be noted in a narrative and on the plan.

(2) Basins located in an area underlain by limestone may require a geologic evaluation of the proposed location to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent ground water contamination and, where necessary, sinkhole formation. The Borough may require basins located over limestone to have an impermeable liner.

(3) Soils used for the construction of basins shall have low erodability factors ("K factors").

(4) Energy dissipators and/or level spreaders shall be installed at points where pipes or drainage ways discharge to or from basins. Discharge from basins shall be into a natural waterway or drainageway.

(5) Exterior slopes of compacted soil shall not exceed one foot vertical per three feet horizontal and may be further reduced if the soil has unstable characteristics.

(6) Interior slopes of the basin shall not exceed one foot vertical per three feet horizontal except with the approval of the Municipal Engineer. Where concrete, stone, or brick walls are used for steeper interior slopes, the basin shall be fenced with a permanent wire fence at least 42 inches in height and a ramp of durable, non-slip materials for maintenance vehicles shall be provided for basin access.

(7) Outlet structures within basins which will control peak discharge flows and distribute the flows by pipes to discharge areas shall be constructed of concrete, polymer-coated steel or aluminum and shall have childproof, non-clogging trash racks over all design openings exceeding 12 inches in diameter, except those openings used to carry perennial stream flows. Small outlet structures may be constructed of Schedule 40 PVC.

(8) Where spillways will be used to control peak discharges in excess of the ten-year storm, control weirs shall be constructed of concrete of sufficient mass and structural stability to withstand the pressures of impounded waters and outlet velocities.

(9) Concrete outlet aprons shall be designed as level spreaders and shall extend at a minimum to the toe of the basin slope. The incorporation of any large stone found on the site into the concrete apron to provide a more natural appearance is encouraged.

(10) Inlet and outlet structures shall be located at maximum distance from each other. The Borough may require a rock filter berm or rock-filled gabions between inlet and outlet areas when the distance is deemed insufficient for sediment trappings.

(11) Temporary and permanent grasses or stabilization measures shall be established on the sides of all earthen basins within 15 days of initial construction.

§ 290-22 Calculation methodology.

A. Stormwater calculations to determine runoff, peak flow rates, peak discharge, hydrographs and to design stormwater runoff rate reduction facilities shall use a generally accepted calculation technique that is based on the Natural Resource Conservation Service (NRCS) Soil-Cover Complex method. Table 307-1 summarizes acceptable methods.

B. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. The Borough may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain 200 acres or less; however, the Rational Method shall not be used to generate pseudo-hydrographs for drainage areas greater than 10 acres.

C. For predevelopment flow rate determination, it shall be assumed that all undeveloped and pervious land shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number or Rational "C" value (i.e., forest) as listed in Appendix C or Appendix D of this chapter.[1] Twenty percent of all existing impervious land that will be disturbed shall be considered meadow in good condition.

[Amended 11-15-2022 by Ord. No. 1069]

[1] Editor's Note: Appendixes C and D are on file in the Borough offices.

D. All calculations using the Soil Cover Complex method shall use the appropriate design rainfall depths for the various return period storms as presented in the table in Appendix E[2] of this chapter. If a hydrologic computer model such as PSRM or HEC-HMS is used for stormwater runoff calculations then the duration of rainfall shall be 24 hours. The SCS "S" curve shown in Appendix F[3] of this chapter shall be used for the rainfall distribution.

[2] Editor's Note: Appendix E is on file in the Borough offices.

[3] Editor's Note: Appendix F is on file in the Borough offices.

E. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the Design Storm Curves from PA Department of Transportation Design Rainfall Curves (1986) shown in Appendix G[4] of this chapter. 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.

[4] Editor's Note: Appendix G is on file in the Borough offices.

F. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex method shall be obtained from the table in Appendix C[5] of this chapter.

[5] Editor's Note: Appendix C is on file in the Borough offices.

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

[6] Editor's Note: Appendix D is on file in the Borough offices.

H. Where uniform flow is anticipated, the Manning's Equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Manning's Equation shall not be used for analysis of pipes under pressure flow or for analysis of culverts. Values for Manning's roughness coefficient (n) shall be consistent with Appendix H[7] of this chapter.

[7] Editor's Note: Appendix H is on file in the Borough offices.

I. When existing storm sewers, streets, roadside ditches or drainage swales are accessible, the applicant shall not connect the stormwater drainage system to the existing facilities provided without the approval of the Municipal Engineer and the facility owner.

J. Routing of hydrographs through detention/retention facilities for the purposes of designing those facilities shall be accomplished using the Storage-Indication method or other recognized routing method subject to approval of the Municipal Engineer. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Municipal Engineer may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

K. Any method approved by the Pennsylvania Department of Transportation or the Pennsylvania Department of Environmental Protection may be used to design the waterway areas of bridges.

	Table 307-1: Acceptable Stormwater Management Computation Methodologies
	Method	Method Developer	Applicability
	TR-20 (or commercial package based on TR-20)	USDA NRCS	Where use of full hydrologic computer model is desirable or necessary.
	TR-55 (or commercial package based on TR-55)	USDA NRCS	For plans within limitations described in TR-55.
	HEC-1, HEC-HMS	US Army Corps of Engineers	Where use of full hydrologic computer model is desirable or necessary.
	PSRM	Penn State University	Where use of full hydrologic computer model is desirable or necessary.
	Rational Method	Emil Kuichling (1889)	For sites less than 10 acres, or as approved by the Municipal Engineer.
	Other Methods	Varies	Other computations approved by Municipal Engineer.

§ 290-23 No-harm option.

A. For any proposed development site not located in a provisional direct discharge district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that "no harm" would be caused by discharging at a higher runoff rate than that specified by the Plan. The no-harm option is used when a developer can prove that the post-development hydrographs can match pre-development hydrographs, or if it can be proved that the post-development conditions will not cause increases in peaks at all points downstream. Proof of no-harm would have to be shown based upon the following Downstream Impact Evaluation which shall include a downstream hydraulic capacity analysis consistent with § 290-23B of this chapter to determine if adequate hydraulic capacity exists. The land developer shall submit to the Municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.

(1) The Downstream Impact Evaluation shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications due to the proposed development upon a dam, highway, structure, natural point of restricted streamflow or any stream channel section, established with the concurrence of the Municipal Engineer.

(2) The evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.

(3) The peak flow values to be used for downstream areas for the design return period storms (two- , ten- , twenty-five- , fifty- and one-hundred-year) shall be the values from the calibrated model used for the analysis and preparation of the particular Act 167 Stormwater Management Plan. These flow values can be obtained from the applicable watershed plan.

(4) Developer-proposed runoff controls that 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 § 290-21G.

(5) A financial distress shall not constitute grounds for granting a no-harm exemption.

(6) Capacity improvements may be provided by the developer as necessary to implement the no-harm option, which proposes specific capacity improvements to provide that a less stringent discharge control would not create any harm downstream.

(7) Any no-harm justifications shall be submitted by the developer as part of the drainage plan submission per Article IV of this chapter.

(8) Qualification of no-harm does not relieve the applicant of the other provisions of this chapter, including but not limited to the water quality, groundwater recharge and channel protection volume requirements of §§ 290-17, 290-18 and 290-19 of this chapter.

B. Downstream hydraulic capacity analysis. Any downstream capacity hydraulic 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. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual.

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

(3) 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.