[Ord. 266, 7/11/2005, § 301]
1.
All regulated activities in East Branch Perkiomen Creek watershed that do not fall under the exemption criteria shown in § 25B-402 shall submit a drainage plan consistent with the East Branch Perkiomen Creek Watershed Stormwater Management Plan to the municipality for review. This criterion 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 exemption criteria.
2.
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 25B.
3.
The drainage plan must be designed consistent with the sequencing provisions of § 25B-302 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Part 4 of this Chapter 25B.
4.
The 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 25B.
5.
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 25B. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the applicant must document that adequate downstream conveyance facilities transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding, or other harm will result from the concentrated discharge.
6.
Whenever a watercourse is located within a development site, it shall
remain open in its natural state and location and should not be piped,
impeded, or altered (except for road crossings). It is the responsibility
of the developer to stabilize existing eroded stream/channel banks.
7.
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.
8.
When it can be shown that, due to 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 the municipality and the DEP through
the joint permit application process, or, where deemed appropriate
by DEP, through the general permit process.
9.
Any stormwater management facilities regulated by this Chapter 25B that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by DEP through the joint permit application process, or, where deemed appropriate by DEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the applicant 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 DEP.
10.
Any stormwater management facilities regulated by this Chapter 25B that would be located on or discharge into state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
11.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., are required to reduce
the size or eliminate the need for detention facilities.
12.
Roof drains must not be discharged to streets or roadside ditches
or connected to sanitary or storm sewers. Overland flow and infiltration/percolation
of stormwater shall be promoted where site conditions allow. If a
developer wishes to connect directly to streets or storm sewers, it
shall be permitted on a case-by-case basis only after review and approval
by the municipality.
13.
Special requirements for watersheds draining to high quality (HQ)
and exceptional value (EV) waters: The temperature and quality of
water and streams that have been declared as exceptional value and
high quality are to be maintained as defined in Chapter 93, Water
Quality Standards, 25 Pa. Code, of Pennsylvania Department of Environmental
Protection rules and regulations. Temperature sensitive BMPs and stormwater
conveyance systems are to be used and designed with-storage pool areas
and supply outflow channels and should be shaded with trees. This
will require modification of berms for permanent ponds and the relaxation
of restrictions on planting vegetation within the facilities, provided
that capacity for volumes and rate control is maintained. At a minimum,
the southern half on pond shorelines shall be planted with shade or
canopy trees within 10 feet of the pond shoreline. In conjunction
with this requirement, the maximum slope allowed on the berm area
to be planted is 10 to one. This will lessen the destabilization of
berm soils due to root growth. A long-term maintenance schedule and
management plan for the thermal control BMPs is to be established
and recorded for all development sites.
[Ord. 266, 7/11/2005, § 302]
1.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts.
A.
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 of the site.
B.
An alternative is practicable if it is available and capable of being
done after taking into consideration cost, existing technology and
logistics in light of overall project purposes, and other municipal
requirements.
C.
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.
2.
The applicant shall demonstrate that they designed the regulated
activities in the following sequence to minimize the increases in
stormwater runoff and impacts to water quality:
A.
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, floodplains, stream buffer zones, hydrologic soil groups A, B, C, and D, any existing recharge areas and any other requirements outlined in the municipal Subdivision and Land Development Ordinance [Chapter 22]. Establish stream buffer according to recommended criteria or applicable ordinances.
B.
Prepare a draft project layout avoiding sensitive areas identified
in Subsection 2A and minimizing total site earth disturbance as much
as possible. The ratio of disturbed area to the entire site area and
measures taken to minimize earth disturbance shall be included in
the ERSAM.
C.
Identify site specific existing conditions drainage areas, discharge
points, recharge areas, and hydrologic soil groups A and B.
F.
Satisfy groundwater recharge (infiltration) objective (§ 25B-304) and provide for stormwater treatment prior to infiltration.
H.
Determine what management district the site falls into (Appendix
25B-D) and conduct a predevelopment runoff analysis.
I.
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance and impervious
surfaces, and to reduce runoff to the maximum extent possible, the
use of surface or point discharges.
J.
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 25B-306).
K.
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
[Ord. 266, 7/11/2005, § 303]
1.
In addition to the performance standards and design criteria requirements
of this Part, the applicant shall comply with the following water
quality requirements of this Part.
A.
Adequate storage and treatment facilities will be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under § 25B-304 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is
the storage capacity needed to capture and to treat a portion of stormwater
runoff from the developed areas of the site produced from 90% of the
average annual rainfall (P).
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To achieve this goal, the following criterion is established:
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The following calculation formula is to be used to determine
the water quality storage volume, (WQv), in
acre-feet of storage for the East Branch Perkiomen Creek watershed:
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WQv = [(P)(Rv)(A)]
12
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Equation: 25B-303.1
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WQv
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=
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Water quality volume (acre-feet)
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P
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=
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Rainfall amount equal to 90% of events producing this rainfall
(in)
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A
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=
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Area of the project contributing to the water quality BMP (acres)
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Rv
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=
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0.05 + 0.009(I) where I is the percent of the area that is impervious
surface (impervious area/A* 100)
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The P value for the five PennDOT rainfall regions is shown in Figure B-2 in Appendix 25B-B of this Chapter 25B within this plan and as shown in Appendix Table 25B-B-5. Since the East Branch Perkiomen Creek is in PennDOT Region 4, the P value to be utilized to meet this requirement is 1.95 inches.
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B.
Design of BMPs used for water quality control shall be in accordance
with design specifications outlined in the Pennsylvania Handbook of
Best Management Practices for Developing Areas or other applicable
manuals. The following factors shall be considered when evaluating
the suitability of BMPs used to control water quality at a given development
site:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and well heads.
(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.
(16)
Enhancement of aesthetic and property value.
C.
To accomplish the above, the applicant shall submit original and
innovative designs to the municipality for review and approval. Such
designs may achieve the water quality objectives through a combination
of BMPs (best management practices).
[Ord. 266, 7/11/2005, § 304]
1.
Infiltration BMPs shall meet the following minimum requirements:
A.
Regulated activities will be required to recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from Subsection 1A(2)(a) or (b) depending upon demonstrated site conditions.
(1)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
(b)
An infiltration and/or percolation rate sufficient to accept
the additional stormwater load and drain completely as determined
by field tests conducted by the applicant's design professional.
(c)
The recharge facility shall be capable of completely infiltrating
the recharge volume within four days (96 hours).
(d)
Pretreatment shall be provided prior to infiltration.
(e)
The requirements for recharge are applied to all disturbed areas,
even if they are ultimately to be an undeveloped land use such as
grass, since studies have found that compaction of the soils during
disturbance reduces their infiltrative capacity.
(2)
The recharge volume (Re) shall be computed by first obtaining the infiltration requirement using methods in either Subsection 1A(2)(a) or (b) then multiplying by the total proposed impervious area. The overall required recharge volume for a site is computed by multiplying total impervious area by the infiltration requirement.
(a)
NRCS Curve Number Equation. The following criteria shall apply.
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The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
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For zero runoff:
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P = I (Infiltration) =
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(200
CN)
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—2 Equation: 25B-304.1
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Where:
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P=I
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=
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Infiltration requirement (inches)
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CN
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=
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SCS(NRCS) curve number of the existing conditions contributing
to the recharge facility
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This equation can be displayed graphically in, and the infiltration
requirement can also be determined from Figure 25B-304.1.
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The recharge volume (Rev) required would
therefore be computed as:
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Rev=
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I* impervious area (SF)
12
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= Cubic Feet (CF) Equation:
25B-304.2
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(b)
Annual Recharge Water Budget Approach. It has been determined that infiltrating 0.6 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of Subsection 1A(2)(a) cannot be achieved, then 0.6 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 77. Above a curve number of 77, Equation 25B-304.1 or the curve in Figure 25B-304.1 should be used to determine the Infiltration requirement.
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Where:
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T
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=
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0.g inches
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The recharge volume (Rev) required would
therefore be computed as:
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Rev=
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I* percent impervious area (SF)
12
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= CF
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The recharge values derived from these methods are the minimum
volumes the applicant must control through an infiltration/recharge
BMP facility. However, if a site has areas of soils where additional
volume of infiltration can be achieved, the applicant is encouraged
to recharge as much of the stormwater runoff from the site as possible.
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Figure 25B-304.1. Infiltration Requirement Based upon NRCS Curve
Number.
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 Required Infiltration (I) in inches by NRCS CN
SCS Curve Number (CN)
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2.
Process.
A.
The general process for designing the infiltration BMP shall be:
A detailed soils evaluation of the project site shall be required
to determine the suitability of recharge facilities. The evaluation
shall be performed by a qualified applicant and, at a minimum, address
soil permeability, depth to bedrock, and subgrade stability.
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration practices.
(2)
Provide field tests, such as double ring infiltration tests
at the level of the proposed infiltration surface to determine the
appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required storm volume
based on field determined capacity at the level of the proposed infiltration
surface.
(4)
Where the recharge volume requirement cannot be physically accomplished
due to the results of the field soils testing, supporting documentation
and justification shall be supplied to the municipality with the drainage
plan.
(5)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
B.
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. Extreme caution shall be exercised
where infiltration is proposed in source water protection areas. The
qualified design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration/recharge facility and
perform a hydrogeologic justification study if necessary. The infiltration
requirement in High Quality/Exceptional Value waters shall be subject
to the DEP's 25 Pa. Code, Chapter 93, Antidegradation Regulations.
The municipality may require the installation of an impermeable liner
in BMP and/or detention basins where the possibility of groundwater
contamination exists. A detailed hydrogeologic investigation may be
required by the municipality.
C.
The municipality shall require the applicant to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination, should there be a mishap or spill.
D.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
[Ord. 266, 7/11/2005, § 305]
1.
In addition to the water quality volume, to minimize the impact-of
stormwater runoff on downstream streambank erosion, the 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).
2.
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 consider and minimize the chances of
clogging and sedimentation. 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. Trash racks are required
for any primary orifice.
3.
In "no detention" areas (District C) only, the objective is not to
attenuate the larger storms. This can be accomplished by configuration
of the outlet structure not to control the larger storms, or by a
bypass or channel to divert only the two-year flood into the basin
or divert flows in excess of the two-year storm away from the basin.
[Ord. 266, 7/11/2005, § 306]
1.
The East Branch Perkiomen Creek watershed has been divided into stormwater
management districts as shown on the watershed map in Appendix 25B-D.
3.
Standards for managing runoff from each subarea in the East Branch
Perkiomen Creek watershed is shown below. Development sites located
in each of the A, B, or C Districts must control proposed conditions
runoff rates to existing conditions runoff rates for the design storms
as follows:
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District
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Design Storm Proposed Conditions
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Design Storm Existing Conditions
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A
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2-year
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1-year
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5-year
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5-year
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10-year
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10-year
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25-year
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25-year
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100-year
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100-year
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B
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2-year
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1-year
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5-year
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2-year
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10-year
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5-year
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25-year
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10-year
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100-year
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50-year
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C-1
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2-year
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1-year
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5-year
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2-year
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10-year
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10-year
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25-year
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25-year
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C-2*
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2-year
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1-year
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5-year
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2-year
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*
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Explanation of District C-2: Development sites which can discharge directly to the East Branch Perkiomen Creek main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) may do so without control of proposed conditions peak rate of runoff greater than the five-year storm. Sites in District C will still have to comply with the water quality criteria (§ 25B-303), the groundwater recharge criteria (§ 25B-304), and streambank erosion criteria (§ 25B-305). If the proposed conditions runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the increased peak flows or will be provided with improvements to furnish the required capacity. If storms greater than the two-year storm cannot be conveyed to the stream or watercourse is a safe manner, the proposed conditions peak rate of runoff must be controlled to the existing conditions peak rate as required in District C-I provisions (i.e., twenty-five-year proposed conditions flows to twenty-five-year existing conditions flows) for the specified design storms.
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[Ord. 266, 7/11/2005, § 307]
2.
District Boundaries. The boundaries of the stormwater management
districts are shown on an official stormwater district map that is
available for inspections at the municipal office. A copy of the map
at a reduced scale is included in Appendix 25B-D. 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 drainage plan.
3.
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 be the existing conditions peak discharge for that subarea as indicated in § 25B-306. 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.
4.
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.
5.
Site Areas. Where the area of a site being impacted by a proposed
development activity differs significantly from the total site area,
only the proposed disturbed area utilizing stormwater management measures
shall be subject to the management district criteria, unimpacted or
undisturbed areas that do flow into or are bypassing the stormwater
management facilities would not be subject to the management district
criteria.
6.
"No Harm" Option. For any applicant whose application exceeds the exemption criteria delineated in § 25B-402 of this Chapter 25B, 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 plan. The "no harm" option is used when an applicant can prove that the proposed conditions hydrographs can match existing conditions hydrographs, or if it can be proved that the proposed 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 Subsection 7 to determine if adequate hydraulic capacity exists. The applicant shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
A.
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 municipality.
B.
The evaluation shall continue downstream until the increase in flow
diminishes due to additional flow from tributaries and/or stream attenuation.
C.
The peak flow values to be used for downstream areas for the design
return period storms (two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year) shall be the values from the calibrated model for
the East Branch Perkiomen Creek watershed. These flow values can be
obtained from the watershed plan.
D.
Applicant 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 § 25B-303, Subsection 1C.
E.
Financial considerations shall not constitute grounds for granting
a no-harm exemption.
F.
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.
G.
Any "no harm" justifications shall be submitted by the applicant
as part of the drainage plan submission per Part 4.
7.
Downstream Hydraulic Capacity Analysis. Any downstream capacity hydraulic analysis conducted in accordance with this Chapter 25B 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 Department of Environmental Protection's 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 the Department of Environmental Protection, 25 Pa. Code, Chapter
105, regulations (if applicable) and, at minimum, pass the increased
twenty-five-year return period runoff.
8.
Regional Stormwater Management Facilities Alternatives. For certain
areas within the study area, it may be more cost-effective to provide
one control facility for more than one development site than to provide
an individual control facility for each development site. The initiative
and funding for any regional runoff control alternatives are the responsibility
of prospective applicants. The design of any regional control facility
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional control facility would be determined
on a case-by-case basis using the hydrologic model of the watershed
consistent with protection of the downstream watershed areas. "Hydrologic
model" refers to the calibrated model as developed for the stormwater
management plan. It is a requirement that, even if regional facilities
are proposed for the water quantity control, that the water quality,
streambank erosion, and recharge criteria be accomplished on-site,
or as close to the source of the runoff as possible.
[Ord. 266, 7/11/2005, § 308]
1.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
2.
Any stormwater management facility (i.e., detention basin BMP) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Chapter 25B shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must be set as to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions peak inflow: Should any stormwater management facility require a dam safety permit under 25 Pa. Code, Environmental Protection, Chapter 105, Dam Safety and Waterway Management, 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 one-hundred-year event.
3.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), and any work involving wetlands
as directed in DEP Chapter 105 regulations (as amended or replaced
from time to time by DEP), shall be designed in accordance with Chapter
105 and will require a permit from DEP. Any other drainage conveyance
facility that does not 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 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.
Any drainage/conveyance facility and/or channel that does not fall
under 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. 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 PennDOT right-of-way must
meet PennDOT minimum design standards and permit submission requirements.
5.
Storm sewers must be able to convey proposed conditions runoff from
a twenty-five-year design storm without surcharging inlets, where
appropriate.
6.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
7.
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
creation of, exacerbation or continuation of an adverse hydrologic
or hydraulic condition within the watershed.
[Ord. 266, 7/11/2005, § 309]
1.
Stormwater runoff from all development sites shall be calculated
using either the rational method or a soil cover complex methodology.
A.
Any stormwater runoff calculations shall use generally accepted calculation
technique that is based on the NRCS soil cover complex method. Table
25B-309-1 summarizes acceptable computation methods. It is assumed
that all methods will be selected by the applicant based on the individual
limitations and suitability of each method for a particular site.
The municipality may allow the use of the rational method to
estimate peak discharges from drainage areas that contain less than
200 acres. The rational method is recommended for drainage areas under
100 acres.
B.
All calculations consistent with this Chapter 25B using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms according to the region for which they are located as presented in Table 25B-B-1 in Appendix 25B-B of this Chapter 25B. If a hydrologic computer model such as HEC-1 or HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The SCS "S" curve shown in Figure 25B-B-1, Appendix 25B-B of this Chapter 25B shall be used for the rainfall distribution.
C.
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table 25B-B-2 in Appendix 25B-B of this Chapter 25B. For the purposes of existing conditions flow rate determination, undeveloped 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 Table 25B-B-2 or 25B-B-3 in Appendix 25B-B of this Chapter 25B. For areas of prior mining disturbance (i.e., strip mining, mine spoil areas, etc.), the designer must first locate in which of mining affect area the site is located, using the Management District Map in Appendix 25B-D. The appropriate curve number or Rational "C" value from Table 25B-B-2 or Table 25B-B-3 should then be used.
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 (1986) (Figures 25B-B-2 to 25B-B-3). 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.
E.
The designer shall consider that the runoff from proposed sites graded
to the subsoil will not have the same runoff conditions as the site
under existing conditions even if topsoiled and seeded. The designer
may increase their proposed condition CN or C value to reflect proposed
soil conditions.
F.
Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table 25B-B-3 in Appendix 25B-B of this Chapter 25B.
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 Table 25B-B-4 in Appendix 25B-B of this Chapter 25B.
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Chapter 25B 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 25B shall be verified by routing the design storm hydrograph through these facilities using the storage-indication method. 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 municipality 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.
|
Table 25B-309-1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|
Method
|
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 within limitations described
in TR-55
| |
|
HEC-1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer 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
| |
[Ord. 266, 7/11/2005, § 310]
1.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with 25 Pa. Code, Chapter 102, Rules and Regulations, Part 1, 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 Bucks County Conservation District.
2.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
shall include the following:
A.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity.
B.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
received final stabilization.
