A. 
All regulated activities in the Borough of Westmont which do not fall under the exemptions criteria shown in § 191-20 shall submit a drainage plan consistent with this chapter to the municipality for review. These criteria shall apply to the total proposed development even if development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the waiver criteria.
B. 
Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.
C. 
The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the adjacent property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.
D. 
Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer 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 the concentrated discharge.
E. 
Where a development site is traversed by watercourses drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Also, maintenance, including mowing of vegetation within the easement shall be required, except as approved by the appropriate governing authority.
F. 
When it can be shown that, due to the topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, through the general permit process.
G. 
Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the joint permit application process, or, where deemed appropriate by PADEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the developer or his agent to show that the land in question cannot be classified as wetlands, otherwise approval to work in the area must be obtained from PADEP.
H. 
Any stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PADOT).
I. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc. are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities.
J. 
Roof drains must not be connected to street, sanitary or storm sewers or roadside ditches to promote overload flow and infiltration/percolation of stormwater where advantageous to do so. When it is more advantageous to connect directly to street or storm sewers, then it shall be permitted on a case-by-case basis by the municipality.
K. 
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 the proposed development and any existing stormwater currently conveyed to subject channel or swale. 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 the 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 a minimum, pass the increased twenty-five-year-return-period runoff.
A. 
The following general standards shall be applied to all development within the Borough of Westmont to control stormwater volume and promote flow attenuation.
(1) 
All site development in the Borough of Westmont that does not meet the exemption criteria shall submit a drainage plan consistent with the provisions of this chapter to the municipality for review and approval. This requirement shall apply to the total proposed development even if the development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway area and any new street or sidewalk. Any area initially designated to be gravel or crushed stone shall be assumed to be impervious.
(2) 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities. Roof drains must not be connected to street, sanitary or storm sewers or roadside ditches.
(3) 
Runoff from the site shall not be concentrated or increased runoff discharged onto adjacent property without the written consent of the adjacent landowners in the form of a drainage easement.
B. 
All developments which create impervious surface shall provide capacity for and treatment of the surface water runoff in accordance with the applicable governmental regulations in effect at the time of development.
A. 
General criteria.
(1) 
Applicants may select runoff control techniques, or a combination of techniques, which are most suitable to control stormwater runoff from the development site. All controls shall be subject to review by the Municipal Engineer. The Municipal Engineer shall make a recommendation to the Council of the Borough of Westmont, and final approval shall be made by a majority vote of said Council in accordance with current applicable governmental regulations for subdivision and land development. The Municipal Engineer 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 section.
(2) 
The applicant should consider the effect of the proposed stormwater management techniques on any special soil conditions or geological hazards which may exist on the development site. In the event such conditions are identified on the site, the Municipal Engineer may require in-depth studies by a competent geotechnical engineer. Not all stormwater control methods may be advisable or allowable at a particular development site.
(a) 
In developing a stormwater management plan for a particular site, stormwater controls shall be selected according to the following order of preference:
[1] 
Infiltration of runoff on-site.
[2] 
Flow attenuation by use of open vegetated swales and natural depressions.
[3] 
Stormwater detention/retention structures.
(b) 
Infiltration practices shall be used to the extent practicable to reduce volume increases and promote groundwater recharge. A combination of successive practices may be used to achieve the applicable minimum control requirement. Justification shall be provided by the applicant for rejecting each of the preferred practices based on actual site conditions.
(c) 
Open detention/retention facilities shall not be permitted within residential areas as part of an infill project.
[1] 
The applicant may request a waiver from the requirement. All such requests for waiver shall be submitted in writing.
[2] 
It shall be the responsibility of the applicant for a waiver of this part to show that the modification will not create a safety risk and that the modification is consistent with the best management practices and current engineering design standards.
B. 
Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm of earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year postdevelopment conditions. The height of embankment must be set as to provide a minimum 1.0 foot to freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year postdevelopment inflow. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than the one-hundred-year event.
C. 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed in PADEP Chapter 105 regulations (as amended or replaced from time to time by PADEP), shall be designed in accordance with Chapter 105 and will require a permit from PADEP. Any other drainage conveyance facility that doesn't fall under Chapter 105 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 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility that constitutes a dam as defined in PADEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PADOT right-of-way must meet PADOT minimum design standards and permit submission requirements.
D. 
Any drainage conveyance facility and/or channel that does not fall under Chapter 105 regulations must be capable of conveying, without damage to the drainage structure or roadway, runoff from the proposed development. 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 one-hundred-year design storm. Any facility located within a PADOT right-of-way must meet PADOT minimum design standards and permit submission requirements.
E. 
Storm sewers must be capable of conveying postdevelopment runoff from the proposed development without surcharging inlets.
F. 
Adequate erosion protection shall be provided along all open channels and at all points of discharge.
G. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the occupancy or continuation of an adverse hydrologic or hydraulic condition within the watershed.
Stormwater runoff from all development sites shall be calculated using either the rational method or a soil-cover-complex methodology.
A. 
Any stormwater runoff calculation involving drainage areas greater than five acres, including on- and off-site areas, shall use a generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 304.1 summarizes acceptable computation methods. 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 Municipal Engineer may approve the use of the Rational Method to estimate peak discharges from drainage areas that contain less than five acres.
B. 
All calculations consistent with this chapter using the soil-cover-complex method shall use the appropriate design rainfall depths for the various return period storms presented in § 191-17B of this chapter. If a hydrologic computer model such as PSRM or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The NRCS curve numbers, shown in Attachment B[1] of this chapter, shall be used for the rainfall distribution.
[1]
Editor's Note: Attachment B is on file in the Borough offices and may be examined there during regular business hours.
C. 
For the purposes of predevelopment flow rate determination, undeveloped land shall be considered as "meadow" good condition, unless the natural ground cover generates a lower curve number or Rational `C' value (i.e., forest).
D. 
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, current edition. 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). Time of concentration for channel and pipe flow shall be computed using Manning's equation.
E. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Attachment D[2] of this chapter.
[2]
Editor's Note: Attachment D is on file in the Borough offices and may be examined there during regular business hours.
F. 
Runoff coefficient (c) for both existing and proposed conditions for use in the rational method shall be obtained from Attachment B of this chapter.
G. 
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 currently accepted design practices. 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 the designed stormwater management system.
I. 
The municipality has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the designer can substantiate through actual physical calibrations that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall date for the particular site in question.
Table 304.1 Acceptable Computation Methodologies for
Stormwater Management Plans
Method
Method Developed by
Applicability
TR-20 or commercial package based on TR-20
USDA – NRCS
When use of full model is desirable or necessary
TR-55 or commercial package based on TR-55
USDA – NRCS
Applicable for plans within the models limitations
HEC-1
U.S. Army Corps of Engineers
When full model is desirable or necessary
PSRM
Penn State University
When full model is desirable or necessary
Rational Method or commercial package based on Rational Method
Emil Kuiching (1889)
For sites less than 5 acres
Other methods
Various
As approved by the Municipal Engineer
A. 
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Cambria County Conservation District and the standards and specifications of the appropriate municipal government.
B. 
Additional erosion and sedimentation control design standards and criteria that must be or are recommended to be applied where infiltration BMP's are proposed and include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization.
A. 
The ability to retain and maximize the groundwater recharge capacity of the area being developed is encouraged. Design of the 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. A detailed geologic evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified geologist and/or soil scientist, and at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. Where previous pavement is permitted for parking lots, recreational facilities, nondedicated street, or other areas, pavement construction specifications shall be noted on the plan.
B. 
Site underlain by limestone.
(1) 
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 groundwater contamination and, where necessary, sinkhole formation. Soils used for the construction of basins shall have low-erodibility factors ("K" factors). The municipality may require the installation of an impermeable liner in detention basins.
(2) 
It shall be the developer's responsibility to verify if the site is underlain by limestone.
A. 
General requirements. In addition to the performance and standards and design criteria requirements of Articles IV and V of this chapter, the land developer shall comply with the following water quality requirements of this article unless otherwise exempted by provisions of this chapter:
(1) 
No discharge materials, toxic or otherwise, shall be permitted into any stormwater management system except as may be permitted by applicable laws of the Commonwealth of Pennsylvania or United States. Where required by federal and state regulation, the landowner or developer shall be responsible for obtaining an NPDES permit for stormwater discharges.
(2) 
In addition to the quantity requirements of this section, the land developer shall:
(a) 
Achieve the water quality objectives using a combination of best management practice (BMPs) including, but not limited to, infiltration structures, detention/retention basins, vegetation filter strips and buffers.
(b) 
The combination of BMPs shall be designed in accordance with BMP selection criteria listed in Subsection B and in consultation with the municipal engineer.
B. 
BMP selection criteria.
(1) 
The following factors must be considered in BMP combination selection:
(a) 
Total contributing area.
(b) 
Permeability and infiltration rate of the site soils.
(c) 
Slope and depth to bedrock.
(d) 
Seasonal high-water table.
(e) 
Proximity to building foundations and well heads.
(f) 
Erodibility of soils.
(g) 
Land availability and configuration of the topography.
(2) 
The following additional factors should be considered when evaluating the suitability of the BMPs used to control water quality at a given development site.
(a) 
Peak discharge and required volume control.
(b) 
Streambank erosion.
(c) 
Efficiency of the BMPs to mitigate potential water quality problems.
(d) 
The volume of runoff that will be effectively treated.
(e) 
The nature of the pollutants being removed.
(f) 
Maintenance requirements.
(g) 
Recreation value.
(h) 
Enhancement of aesthetics and property values.