A. 
Applicants proposing regulated activities in the Borough of Narberth which are not exempt under this chapter shall submit a SWM site plan consistent with this chapter and the respective stormwater management plan to the Borough for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages. No regulated activities shall commence until the Borough issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter. SWM site plans approved by the Borough shall be on site throughout the duration of the regulated activity.
B. 
The applicant is required to find practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth and must maintain as much as possible the natural hydrologic regime.
C. 
The SWM site plan must be designed consistent with the sequencing provisions of this chapter to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge, and to protect groundwater and surface water quality and quantity. The SWM site plan design must proceed sequentially in accordance with this chapter.
D. 
Stormwater drainage systems shall be designed in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.
E. 
Stormwater flows, drainage, or discharges onto adjacent properties shall not be created, increased, decreased, relocated, or otherwise altered without written permission from all such adjacent property owners affected by such drainage, flow or discharge. Such stormwater flows/drainage/discharges shall be subject to the requirements of this chapter.
F. 
Areas of existing diffused drainage discharge, whether proposed to be concentrated or maintained as diffused drainage areas, shall be subject to any applicable discharge criteria in the general direction of existing discharge, except as otherwise provided by this chapter. If diffused drainage discharge is proposed to be concentrated and discharged onto adjacent property, the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge or otherwise prove that no erosion, sedimentation, flooding, or other impacts will result from the concentrated discharge.
G. 
Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the riparian buffer requirements contained in this chapter.
H. 
Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or delineated wetlands shall be subject to approval by DEP through the joint permit application or the environmental assessment approval process, or where deemed appropriate, by the DEP general permit process. When there is a question as to whether wetlands may be involved, it is the responsibility of the applicant to show that the land in question cannot be classified as wetlands; otherwise, approval to work in the area must be obtained from DEP.
I. 
Any proposed stormwater management facilities regulated by this chapter that would be located on commonwealth highway rights-of-way shall be subject to approval by PennDOT. If the Borough is required to be a permittee or co-permittee for any such facilities, the applicant shall execute an agreement with the Borough, drafted by the Borough Solicitor, that indemnifies the Borough and requires the applicant, or the applicant's assigns, to be responsible for the operation, maintenance, inspection, and replacement of such facilities.
J. 
The design of any proposed stormwater management facilities over karst shall include an evaluation of measures to minimize adverse effects.
K. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., is encouraged where soil conditions permit in order to reduce the size or eliminate the need for detention facilities or other structural BMPs.
L. 
All stormwater runoff shall be pretreated for water quality prior to discharge to surface or groundwater.
M. 
All regulated activities within the Borough of Narberth shall be designed, implemented, operated, and maintained to meet the purposes of this chapter and all other applicable laws, through these two elements:
(1) 
Erosion and sediment control during earth disturbance activities (e.g., during construction). Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual), No. 363-2134-008, as amended and updated.
(2) 
Water quality protection measures after completion of earth disturbance activities (i.e., after construction), including inspection, repair, reporting, operations, and maintenance.
N. 
No regulated earth disturbance activities within the Borough shall commence until the requirements of this chapter are met.
O. 
Postconstruction water quality protection shall be addressed as required by this chapter.
P. 
For all regulated activities, SWM BMPs shall be designed, implemented, inspected, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law,[1] and the Storm Water Management Act.[2]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
[2]
Editor's Note: See 32 P.S. § 680.1 et seq.
Q. 
All BMPs used to meet the requirements of this chapter shall conform to the commonwealth water quality requirements and any more stringent requirements as set forth by the Borough. The Borough may, after consultation with DEP and in the sole discretion of Borough Council, approve measures for meeting the commonwealth water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, commonwealth law including, but not limited to, the Clean Streams Law.[3]
[3]
Editor's Note: See 35 P.S. § 691.1 et seq.
R. 
Techniques described in Appendix E[4] (Low-Impact Development) of this chapter shall be considered because they reduce the costs of complying with the requirements of this chapter and the commonwealth's water quality requirements.
[4]
Editor's Note: Appendix E is included as an attachment to this chapter.
S. 
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's soils.
(3) 
Slope and depth to bedrock.
(4) 
Seasonal high-water table.
(5) 
Proximity to building foundations and wellheads.
(6) 
Erodibility of soils.
(7) 
Land availability and configuration of the topography.
(8) 
Peak discharge and required volume control.
(9) 
Stream bank erosion.
(10) 
Efficiency of the BMPs to mitigate potential water quality problems.
(11) 
The volume of runoff that will be effectively treated.
(12) 
The nature of the pollutant being removed.
(13) 
Maintenance requirements.
(14) 
Creation/protection of aquatic and wildlife habitat.
(15) 
Recreational value.
T. 
All regulated activities shall include such measures as necessary to:
(1) 
Protect health, safety, and property.
(2) 
Meet the water quality goals of this chapter by implementing measures to:
(a) 
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b) 
Maintain or extend riparian buffers.
(c) 
Avoid erosive flow conditions in natural flow pathways.
(d) 
Minimize thermal impacts to waters of the commonwealth.
(e) 
Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.
(3) 
Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual. If methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required under this chapter, the SWM site plan must include a detailed justification demonstrating to the satisfaction of Borough Council that the use of LID and green infrastructure is not practicable.
U. 
If approved by Borough Council, the applicant may meet the stormwater management criteria through off-site stormwater management measures as long as the proposed measures are in the same watershed.
V. 
Impervious areas:
(1) 
The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.
(2) 
For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.
(3) 
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls established in this chapter and the peak rate controls established in this chapter do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
The following permit requirements may apply to certain regulated earth disturbance activities and must be met prior to commencement of regulated earth disturbance activities, as applicable:
A. 
All regulated earth disturbance activities subject to permit requirements by DEP under regulations at 25 Pennsylvania Code Chapter 102, as amended.
B. 
Work within natural drainageways subject to permit by DEP under 25 Pennsylvania Code Chapter 105, as amended.
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 Pennsylvania Code Chapter 105, as amended.
D. 
Any stormwater management facility that would be located on a commonwealth highway right-of-way or require access from a commonwealth highway shall be subject to approval by PennDOT.
E. 
Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by DEP under 25 Pennsylvania Code Chapter 105, as amended.
A. 
No regulated earth disturbance activities within the Borough shall commence until the Borough receives an approval from the Conservation District of an erosion and sediment control plan for construction activities.
B. 
DEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pa. Code § 102.4(b).
C. 
In addition, under 25 Pa. Code Chapter 92a, a DEP "NPDES Construction Activities" permit is required for regulated earth disturbance activities.
D. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the Borough. The issuance of an NPDES Construction Permit [or permit coverage under the statewide General Permit (PAG-2)] satisfies the requirements of Subsection A of this section.
E. 
A copy of the erosion and sediment control plan and any required permit, as required by DEP regulations, shall be available on the project site at all times.
F. 
Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.
A. 
The design of all regulated activities shall include the following to minimize stormwater impacts:
(1) 
The applicant shall find practicable alternatives to the surface discharge of stormwater, such as those listed in Appendix F,[1] Table F-5, the creation of impervious surfaces, and the degradation of waters of the commonwealth and must maintain as much as possible the natural hydrologic regime of the site.
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
(2) 
An alternative is practicable if it is available and capable of implementation after taking into consideration existing technology and logistics in light of overall project purposes and other municipal requirements.
(3) 
All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.
B. 
The applicant shall demonstrate that the regulated activities were designed in the following sequence. The goal of the sequence is to minimize the increases in stormwater runoff and impacts to water quality resulting from the proposed regulated activity:
(1) 
Prepare an existing resource and site analysis map (ERSAM) showing environmentally sensitive areas, including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, stream buffers, and hydrologic soil groups. Land development, any existing recharge areas, and other requirements outlined in the municipal SALDO[2] shall also be included.
[2]
Editor's Note: See Ch. 450, Subdivision and Land Development.
(2) 
Establish a riparian buffer according to this chapter's requirements.
(3) 
Prepare a draft project layout avoiding sensitive areas as identified under this chapter.
(4) 
Identify site-specific existing conditions drainage areas, discharge points, recharge areas, and hydrologic soil Groups A and B (areas conducive to infiltration).
(5) 
Evaluate nonstructural stormwater management alternatives to minimize earth disturbance, minimize impervious surfaces, and break up large impervious surfaces.
(6) 
Satisfy the groundwater recharge (infiltration) objectives of this chapter and provide for stormwater pretreatment prior to infiltration.
(7) 
Provide for water quality protection in accordance with the water quality requirements of this chapter.
(8) 
Provide stream bank erosion protection in accordance with the stream bank erosion requirements of this chapter.
(9) 
Prepare final project design to maintain existing conditions drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and, to the maximum extent possible, to ensure that the remaining site development has no surface or point discharge.
(10) 
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements of this chapter.
(11) 
Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge, or other structural control.
The applicant shall comply with the following water quality requirements of this chapter:
A. 
No regulated earth disturbance activities within the Borough shall commence until approval by the Borough of a plan which demonstrates compliance with postconstruction commonwealth water quality requirements.
B. 
The BMPs shall be designed, implemented, and maintained to meet commonwealth water quality requirements and any other more stringent requirements as determined by the Borough.
C. 
To control postconstruction stormwater impacts from regulated earth disturbance activities, commonwealth water quality requirements can be met by BMPs, including site design, which provide for replication of preconstruction stormwater infiltration and runoff conditions so that postconstruction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. As described in the DEP Comprehensive Stormwater Management Policy (No. 392-0300-002, September 28, 2002, and as thereafter amended), this may be achieved by the following:
(1) 
Infiltration. Replication of preconstruction stormwater infiltration conditions;
(2) 
Treatment. Use of water quality treatment BMPs to ensure filtering out of the chemical and physical pollutants from the stormwater runoff; and
(3) 
Stream bank and streambed protection. Management of volume and rate of postconstruction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).
D. 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The retention volume computed under this chapter may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the retention volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site.
(1) 
To achieve this goal, the following calculation formula is to be used to determine the water quality storage volume (WQv) in acre-feet of storage required by this chapter:
WQv = [(P)(Rv)(A)] ÷ 12
Where:
WQv
=
Water quality volume (acre-feet)
P
=
1 inch
A
=
Area of the project contributing to the water quality BMP (acres)
Rv
=
0.05 + 0.009(I) where I is the percent of the area that is impervious surface ((impervious area/A)* 100)
(2) 
This volume requirement can be accomplished by the permanent volume of a wet basin or the detained volume from other BMPs. Where appropriate, wet basins shall be utilized for water quality control and shall follow the guidelines of the BMP Manual.
(3) 
Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall provide for protection from clogging and unwanted sedimentation.
(a) 
For areas within defined special protection subwatersheds that include EV and HQ waters, the temperature and quality of water and streams shall be maintained through the use of temperature-sensitive BMPs and stormwater conveyance systems.
(b) 
To accomplish the above, the applicant shall submit original and innovative designs to the Borough Engineer for review and approval. Such designs may achieve the water quality objectives through a combination of different BMPs.
(c) 
If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be maintained with and encouraged to use appropriate native vegetation (refer to Appendix H of the Pennsylvania Handbook of Best Management Practices for Developing Areas for plant lists). If the applicable rear or side yard setback is less than 50 feet or a stream traverses the site, the buffer width may be reduced to 25% of the setback and/or to a minimum of 10 feet. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained. This does not include lakes or wetlands.
(d) 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Borough. The issuance of an NPDES Construction Permit [or permit coverage under the statewide General Permit (PAG-2)] satisfies the requirements of Subsection A of this section.
A. 
In addition to the control of water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement is to design a BMP to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility).
B. 
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake.
C. 
In Conditional Direct Discharge Districts, the objective is not to attenuate the storms greater than the two-year recurrence interval. This can be accomplished by configuring the outlet structure not to control the larger storms or by a bypass channel that diverts only the two-year stormwater runoff into the basin or, conversely, diverts flows in excess of the two-year storm away from the basin.
A. 
Naberth Borough is located within the Darby and Cobbs Creeks Watershed, the Valley Creek Watershed, and the Gulley Run Watershed.
(1) 
In addition to the requirements specified in Tables 441-18.1 and 441-18.2 below, the erosion and sedimentation controls, the nonstructural project design, the groundwater recharge, the water quality, and the stream bank erosion requirements of this chapter shall be implemented.
(2) 
Standards for managing runoff from within the applicable watersheds for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms are shown in Tables 441-18.1 and 441-18.2. Development sites within District A must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Tables 441-18.1 and 441-18.2.
Table 441-18.1
Peak Rate Control Standards for Stormwater Management District A for the Darby- and Cobbs Creeks Watershed
District
Proposed Condition Design Storm
Existing Condition Design Storm
A
2-year
1-year
5-year
5-year
10-year
10-year
25-year
25-year
100-year
100-year
B. 
General. Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms.
C. 
District boundaries. The boundaries of the stormwater management districts are shown on an official map that is available for inspection at the Borough and County Planning offices. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the SWM site plan.
D. 
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located. 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 discharge site. In this case, peak discharge in any direction may follow Management District A criteria, provided that the overall site discharge meets the management district criteria for which the discharge is located.
E. 
Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
F. 
Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the management district criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management district criteria.
G. 
No harm option.
(1) 
The applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the stormwater management plan. The no harm option is used when an applicant can prove that the proposed conditions hydrographs can match existing conditions hydrographs and if it can be proved that the proposed conditions will not cause increases in peaks at all points downstream.
(2) 
Proof of no harm must be shown based upon the following downstream impact evaluation which shall include a downstream hydraulic capacity analysis consistent with this chapter to determine if adequate hydraulic capacity exists. The applicant shall submit to the Borough this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(a) 
The hydrologic regime of the site must be maintained.
(b) 
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 Borough.
(c) 
The evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.
(d) 
The peak flow values to be used for downstream areas for the design return period storms (two-, five-, ten-, twenty-five-, fifty-, and 100-year) shall be the values from the calibrated model for the respective watershed. These flow values can be obtained from the original Act 167 watershed stormwater management plans.
(e) 
Applicant-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 this chapter.
(f) 
Financial distress shall not constitute grounds for the Borough to approve the use of the no harm option.
(g) 
Capacity improvements to conveyance facilities or obstructions may be provided as necessary to implement the no harm option as long as it can be demonstrated through the downstream hydraulic capacity analysis that the improvements would not create any harm downstream.
(h) 
Any no harm justifications shall be submitted by the applicant as part of the SWM site plan submission requirements.
(3) 
Downstream hydraulic capacity analysis. Any downstream hydraulic capacity analysis conducted in accordance with this chapter 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. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual.
(b) 
Natural or man-made channels or swales must be able to convey 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 need to 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.
(4) 
Hardship option. The standards and criteria outlined in this chapter are designed to maintain existing peak flows and volumes throughout the Borough's watersheds as they become developed. If the standards and criteria established are too restrictive for a particular landowner or applicant, the applicant shall request a waiver from Borough Council for land developments, and otherwise obtained from the Borough Engineer, and must demonstrate a hardship and demonstrate that the existing drainage network in some areas may be capable of safely transporting slight increases in flows without causing a problem or increasing flows elsewhere. Any landowners pleading the hardship option and granted a waiver will assume all liabilities that may arise due to exercising this option and will, in writing, indemnify the Borough and the Borough Engineer for any damages that may arise related to flooding or stormwater on a form drafted by the Borough Solicitor. Cost or financial burden shall not be considered as factors contributing to a hardship. The applicant may consider, for Borough Council's consideration, off-site management controls or contributing to the Municipal Stormwater Control and BMP Operation and Maintenance Fund as long as the stormwater management controls are within the same watershed and Borough Council or the Borough Engineer, as applicable, approves of such option.
(5) 
Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as recommended by the Borough Engineer and, if a land development, approved by Borough Council:
(a) 
Meet the full requirements specified by Tables 441-18.1 and 441-18.2 and this chapter; or
(b) 
Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.
H. 
Other watersheds. Within other watersheds, the criteria for peak runoff control are designed to reduce the postdevelopment peak flow to existing conditions. Development sites must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 441-18.2.
Table 441-18.2
Peak Rate Control Standards in Other Watershed Areas
Proposed Condition Design Storm
Reduce to Existing Conditions Design Storm
2-year
2-year
5-year
2-year
10-year
10-year
25-year
25-year
50-year
50-year
100-year
100-year
(1) 
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.
(2) 
Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(3) 
Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the peak rate control standards noted above. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the peak rate control standards.
(4) 
No harm option.
(a) 
Any proposed development site not located in an area which can discharge directly to the main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary), the applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove to the satisfaction of Borough Council that no harm would be caused by discharging at a higher runoff rate than that specified by the stormwater management plan. The no harm option is used when an applicant can prove to the satisfaction of Borough Council that the proposed conditions hydrographs can match existing conditions hydrographs, and if it can be proved to the satisfaction of Borough Council that the proposed conditions will not cause increases in peaks at all points downstream.
(b) 
Proof of no harm must be shown based upon the following downstream impact evaluation which shall include a downstream hydraulic capacity analysis consistent with this chapter to determine if adequate hydraulic capacity exists. The applicant shall submit to the Borough this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
[1] 
The hydrologic regime of the site must be maintained.
[2] 
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 Borough.
[3] 
The evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.
[4] 
Applicant-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 this chapter.
[5] 
Financial distress shall not constitute grounds for the Borough to approve the use at the no harm option.
[6] 
Capacity improvements may be provided 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 applicant as part of the SWM site plan submission.
(5) 
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:
(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. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual.
(b) 
Natural or man-made channels or swales must be able to convey 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.
(6) 
Hardship option. The standards and criteria outlined in section are designed to maintain existing peak flows and volumes throughout the Borough's watershed basins as they become developed. There may be certain instances however where the standards and criteria established are too restrictive for a particular landowner or applicant. If the hardship option is pursued, the applicant shall request a waiver from Borough Council for land developments, otherwise the waiver shall be obtained from the Borough, and must demonstrate a hardship and demonstrate that the existing drainage network in some areas may be capable of safely transporting slight increases in flows without causing a problem or increasing flows elsewhere. Any landowners pleading the hardship option and granted a waiver will assume all liabilities that may arise due to exercising this option and will, in writing, indemnify the Borough and the Borough Engineer for any damages that may arise related to flooding or stormwater on a form drafted by the Borough Solicitor. Cost or financial burden shall not be considered as factors contributing to a hardship. The applicant may propose, for Borough Council's consideration, off-site management controls and/or contributing to the Borough's Stormwater Control and BMP Operation and Maintenance Fund, as long as the stormwater management controls are within the same watershed and Borough Council, or the Borough Engineer, if not a land development, approves of such option.
(7) 
Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions, if recommended by the Borough Engineer and, if a land development, approved by Borough Council:
(a) 
Meet the full requirements specified by Table 441-18.2 and Subsections H(1) through (7) above; or
(b) 
Reduce the total impervious surface on the site by at least 20%, based upon a comparison of existing impervious surface to proposed impervious surface.
A. 
Stormwater runoff from all development sites with a drainage area of greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 441-19.1 summarizes acceptable computation methods, and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The Borough may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Soil Cover Complex Method shall be used for drainage areas greater than 200 acres.
Table 441-19.1
Acceptable Computation Methodologies for Stormwater Management Plans
Method
Developed By
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans where limitations described in TR-55
HEC-1/HEC-HMS
US Army Corps of Engineers
Applicable where use of a full hydrologic computer is desirable or necessary
PSRM
Penn State University
Applicable where use of a hydrologic model is desirable or necessary; simpler than TR-20 or HEC-1
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites less than 200 acres, or as approved by the municipality and/or Municipal Engineer
Other methods
Varies
Other computation methodologies approved by the municipality and/or Municipal Engineer
B. 
The following criteria shall be used for runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
(a) 
Wooded sites shall use a ground cover of woods in good condition. Portions of a site having more than one viable tree of a DBH of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.
(b) 
The undeveloped portion of the site, including agriculture, bare earth, and fallow ground shall be considered as meadow in good condition, unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Tables F-2 or F-3 in Appendix F[1] of this chapter.
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
(2) 
For development and redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.
C. 
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.
D. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table F-2 in Appendix F[2] of this chapter.
[2]
Editor's Note: Appendix F is included as an attachment to this chapter.
E. 
Runoff coefficients(c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table F-3 in Appendix F[3] of this chapter.
[3]
Editor's Note: Appendix F is included as an attachment to this chapter.
F. 
Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table F-4 in Appendix F[4] of this chapter.
[4]
Editor's Note: Appendix F is included as an attachment to this chapter.
G. 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.
H. 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Borough 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.
A. 
Any stormwater facility located on commonwealth highway rights-of-way shall be subject to approval by PennDOT.
B. 
All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H.[1]
[1]
Editor's Note: Appendix H is included as an attachment to this chapter.
C. 
Any stormwater management facility (i.e., detention basin) required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to handle flow up to and including the 100-year proposed conditions. The height of embankment must provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year proposed conditions inflow. Should any stormwater management facility require a dam safety permit under DEP Chapter 105, the facility shall be designed in accordance with Chapter 105[2] and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may be required to pass storms larger than the 100-year event.
[2]
Editor's Note: See 25 Pa. Code Chapter 105.
D. 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands governed by DEP Chapter 105 regulations (as amended or replaced from time to time by DEP) shall be designed in accordance with Chapter 105[3] and will require a permit from DEP.
[3]
Editor's Note: See 25 Pa. Code Chapter 105.
E. 
Any other drainage conveyance facility that does not fall under Chapter 105[4] regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
[4]
Editor's Note: See 25 Pa. Code Chapter 105.
F. 
Any drainage conveyance facility and/or channel not governed by Chapter 105[5] regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-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. Roadway crossings located within designated floodplain areas must be able to convey runoff from a 100-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
[5]
Editor's Note: See 25 Pa. Code Chapter 105.
G. 
Storm sewers must be able to convey proposed conditions runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.
H. 
Adequate erosion protection shall be provided along all open channels and at all points of discharge.
I. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Borough reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.
J. 
Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.
K. 
Normally dry, open top, storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.
L. 
Unless otherwise specified under this chapter, the design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland.
The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors.
A. 
The Design Storm Method (CG-1 in the BMP Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions:
(1) 
Do not increase the postdevelopment total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation.
(2) 
For modeling purposes:
(a) 
Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.
(b) 
Twenty percent of existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions.
B. 
The Simplified Method (CG-2 in the BMP Manual) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require design of stormwater storage facilities. For new impervious surfaces:
(1) 
Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces.
(2) 
At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of the commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.
(3) 
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 0.5 inch of the permanently removed runoff should be infiltrated.
(4) 
This method is exempt from the requirements related to peak rate controls.
A. 
In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer.
B. 
Except as required by 25 Pa. Code Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 35 feet from the top of the stream bank (on each side).
C. 
Minimum management requirements for riparian buffers.
(1) 
Existing native vegetation shall be protected and maintained within the riparian buffer easement.
(2) 
Whenever practicable invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.
D. 
The riparian buffer easement shall be enforceable by the Borough and shall be recorded in the Office of the Montgomery County Recorder of Deeds, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area as required by the Borough's Zoning Code,[1] unless otherwise specified.
[1]
Editor's Note: See Ch. 500, Zoning.
E. 
Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
F. 
The following conditions shall apply when public and/or private recreation trails are permitted within riparian buffers:
(1) 
Trails shall be for nonmotorized use only.
(2) 
Trails shall be designed to have the least impact on native plant species and other sensitive environmental features.
G. 
Septic drainfields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.