A.
Applicants proposing regulated activities in Township of Muhlenberg which do not fall under the exemption criteria shown in § 297-22 shall submit a drainage plan consistent with this chapter to the Township of Muhlenberg for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
B.
The applicant
is required to evaluate 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 drainage plan must be designed consistent with the sequencing provisions of § 297-14 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and to protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article III of this chapter.
D.
Existing points
of concentrated drainage that discharge onto adjacent property shall
not be altered in any manner which could cause property damage without
permission of the affected property owner(s) and shall be subject
to any applicable discharge criteria specified in this chapter.
E.
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 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.
F.
Where a development site is traversed by existing watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall conform to the stream buffer requirements contained in § 297-16D of this chapter.
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 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 PADEP.
H.
Any alteration
that affects stormwater flow directly or indirectly toward a PennDOT
facility shall be subject to PennDOT regulations.
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
or other structural BMPs.
J.
Roof drains shall
not be connected to impervious surfaces in order to promote overland
flow and infiltration/percolation of stormwater where advantageous
to do so. When site conditions preclude infiltration/percolation,
then it shall be permitted on a case-by-case basis by the Township
of Muhlenberg.
K.
All stormwater
runoff shall be treated for water quality.
L.
Transference
of runoff to or from an EV/HQ watershed is prohibited unless otherwise
authorized by DEP, DRBC or SRBC.
Permits must comply with any and all applicable
local, county, state and federal regulations.
A.
No regulated
earth disturbance activities within the Township of Muhlenberg shall
commence until the Township of Muhlenberg 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.
[Amended at time of adoption of Code (see Ch. 1, General
Provisions, Art. I)]
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 Township of Muhlenberg. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of Subsection A.
E.
A copy of the
erosion and sediment control plan and any required permit, as required
by DEP regulations, shall be available at 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 and 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
contributory drainage area to the infiltration BMP has achieved final
stabilization.
A.
For projects
disturbing one acre or more, the design of all regulated activities
shall include evaluation of 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.
(1)
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.
(2)
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 he/she designed the regulated activities that
disturb one acre or more include consideration of the following issues:
(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, floodplains, stream buffer zones, hydrologic soil groups A and B (areas conducive to infiltration), special geologic features, any existing recharge areas and any other requirements outlined in Chapter 310, Subdivision and Land Development, of this Code.
(2)
Establish appropriate buffers for each of the delineated environmentally sensitive areas per Chapter 355, Zoning, of this Code. [See § 297-16D for stream buffers and § 297-20B(2) for special geologic feature buffers.]
(4)
Identify
site-specific existing conditions drainage areas, discharge points,
recharge areas and hydrologic soil groups A and B.
(6)
Satisfy infiltration objectives (§ 297-15) and provide for stormwater pretreatment prior to infiltration. Pretreatment may not be necessary for rooftop runoff which enters the infiltration facility directly from a roof leader.
(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 the remaining site
development has no surface or point discharge.
(10)
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 (§ 297-18).
(11)
Manage
any remaining runoff through treatment prior to discharge, as part
of detention, bioretention, direct discharge or other structural control.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration 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 disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs, which can be over-designed to compensate for the infiltration losses due to parking areas. It is recommended that roof runoff be directed to infiltration BMPs, which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 297-3 and take advantage of utilizing any existing recharge areas. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, due to seasonal high water table, soil permeability rate, soil depth or setback distances from special geologic features, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 297-15A(3), depending on demonstrated site conditions and shall be the greater of the two volumes.
A.
Infiltration
BMPs shall meet the following minimum requirements:
(1)
Infiltration requirements. Regulated activities will be required to infiltrate, where site conditions permit, 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 infiltrated shall be determined from § 297-15A(3)(a) or (b), depending upon demonstrated site conditions.
(2)
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 infiltration
facility shall be capable of completely infiltrating the required
retention (infiltration) volume within four days (96 hours).
(d)
Pretreatment
shall be provided prior to infiltration.
(3)
The size
of the infiltration facility shall be based upon the following volume
criteria:
(a)
NRCS curve
number equation.
[1]
The
NRCS runoff equation shall be utilized to calculate infiltration requirements
(I) in inches.
|
Equation 297-15.1
|
|
I (Infiltration requirement, in inches) = (200/CN) –
2
|
|
Where:
| |||
|---|---|---|---|
|
CN = SCS (NRCS) curve number of existing conditions
contributing to the infiltration facility.
|
[2]
This
equation is displayed graphically in, and the infiltration requirement
can be determined from Figure 297-15.1.
[3]
It
has been determined that infiltrating 0.46 inch of runoff from the
impervious areas will aid in maintaining the hydrologic regime of
the watershed. However, the rounded number 0.5 inch will be used.
 Figure 297-15.1. Infiltration Requirement
Based Upon NRCS Curve Number
|
[4]
The
retention (infiltration) volume (Rev) required
to meet the infiltration requirement would therefore be computed as:
|
Equation 297-15.2
| |
|---|---|
|
Rev =
|
(0.5 or I, whichever is greater) * impervious
area (square feet) / (12 inch/feet) = cubic feet
|
|
Where:
| |||
|---|---|---|---|
|
I = infiltration requirements (in inches).
|
(b)
Annual
recharge = water budget approach.
[1]
If the goals of § 297-15A(3)(a) cannot be achieved, then 0.5 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 297-15.1 or the curve in Figure 297-15.1 should be used to determine the infiltration requirement.
[2]
The
retention (infiltration) volume (Rev) required
again would therefore be computed as:
|
Rev =
|
(0.5 or I, whichever is greater) * impervious
area (square feet) / (12 inch/feet) = cubic feet
|
B.
Soils. A detailed
soils evaluation of the project site shall be required where practicable
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified design professional and, at a minimum,
address soil permeability, depth to bedrock and subgrade stability.
The general process for designing the infiltration BMP shall be:
(1)
Analyze hydrologic
soil groups as well as natural and man-made features within the site
to determine general areas of suitability for infiltration practices.
In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
is not permitted to be ruled out without conducting these tests.
(2)
Provide field
tests such as double-ring infiltrometer or hydraulic conductivity
tests (at the level of the proposed infiltration surface) to determine
the appropriate hydraulic conductivity rate. Percolation tests are
not recommended for design purposes.
(3)
Design the
infiltration structure for the required retention (Rev) volume based on field-determined capacity at the level of the proposed
infiltration surface.
(4)
If on-lot
infiltration structures are proposed by the applicant's design professional,
it must be demonstrated to the Township of Muhlenberg that the soils
are conducive to infiltrate on the lots identified.
C.
Carbonate areas. The applicant is required to investigate the ability of all areas on the site which are not underlain by carbonate rock to meet the infiltration requirements of § 297-15A. If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure 124-B-1 in Appendix B.[1] However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "Recommended" range on Figure 124-B-1 in Appendix B. If infiltration is not proposed, the calculated infiltration volume (§ 297-15A) shall be treated by an acceptable BMP. Acceptable methods for treating this volume are storage and reuse, bioretention, wet ponds and soil composting. Other methods may be acceptable, and all methods should generally follow the design guidelines outlined in the Pennsylvania Stormwater Best Management Practice Manual, latest edition.
(1)
Infiltration
BMP loading rate percentages in Figure 124-B-1 in Appendix B shall
be calculated as follows:
 |
(2)
The area
tributary to the infiltration BMP shall be weighted as follows:
|
Area Description
|
Weighting
|
|---|---|
|
All disturbed area to be made impervious
|
100%
|
|
All disturbed areas to be made pervious
|
50%
|
|
All undisturbed impervious areas
|
100%
|
|
All undisturbed pervious areas
|
0%
|
(3)
Soil thickness
is to be measured from the bottom of any proposed infiltration BMP.
The effective soil thickness in Figure 124-B-1 in Appendix B is the
measured soil thickness multiplied by the thickness factor based on
soil permeability, as follows:
|
Permeability Range
(inches per hour)
|
Thickness Factor
|
|---|---|
|
6.0 to 12.0
|
0.8
|
|
2.0 to 6.0
|
1.0
|
|
1.0 to 2.0
|
1.4
|
|
0.75 to 1.0
|
1.2
|
|
0.5 to 0.75
|
1.0
|
(4)
The design
of all facilities over karst shall include an evaluation of measures
to minimize adverse effects.
[1]
Editor's Note: Appendix B is included as an
attachment to this chapter.
D.
Stormwater hotspots.
Following is a list of examples of designated hotspots. If a site
is designated as a hotspot, it has important implications for how
stormwater is managed. First and foremost, untreated stormwater runoff
from hotspots shall not be allowed to recharge into groundwater where
it may contaminate water supplies. Therefore, the Rev requirement shall not applied to development sites that fit into
the hotspot category. (The entire WQv must still be treated.) Second,
a greater level of stormwater treatment shall be considered at hotspot
sites to prevent pollutant washoff after construction. The EPA's NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan.
(1)
Examples
of hotspots:
(a)
Vehicle
salvage yards and recycling facilities.
(b)
Vehicle
fueling stations.
(c)
Vehicle
service and maintenance facilities.
(d)
Vehicle
and equipment cleaning facilities.
(e)
Fleet
storage areas (bus, truck, etc.).
(f)
Industrial
sites (based on Standard Industrial Codes).
(g)
Marinas
(service and maintenance).
(h)
Outdoor
liquid container storage.
(i)
Outdoor
loading/unloading facilities.
(j)
Public
works storage areas.
(k)
Facilities
that generate or store hazardous materials.
(l)
Commercial
container nursery.
(m)
Other
land uses and activities as designated by an appropriate review authority.
(2)
The following
land uses and activities are not normally considered hotspots:
(3)
While large
highways [average daily traffic volume (ADT) greater than 30,000]
are not designated as a stormwater hotspot, it is important to ensure
that highway stormwater management plans adequately protect groundwater.
E.
Caution shall
be exercised where infiltration is proposed in source water protection
areas, as defined by the Township of Muhlenberg, a water authority,
or water company.
F.
Infiltration
facilities shall be used in conjunction with other innovative or traditional
stormwater control facilities that are found within the PADEP BMP
Manual.
G.
Caution shall
be exercised where salt or chloride (municipal salt storage) would
be a pollutant since soils do little to filter this pollutant and
it may contaminate the groundwater. The qualified design professional
shall evaluate the possibility of groundwater contamination from the
proposed infiltration facility and perform a hydrogeologic justification
study if necessary.
H.
The infiltration
requirement in high quality or exceptional value waters shall be subject
to the DEP's Chapter 93 antidegradation regulations.
I.
Dependant upon
certain land use or hotspots, an impermeable liner will be required
in detention basins where the possibility of groundwater contamination
exists. A detailed hydrogeologic investigation may be required by
the Township of Muhlenberg.
J.
The Township
of Muhlenberg shall require the applicant to provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
K.
For projects that disturb one acre or more, unless otherwise specified in Chapter 355, Zoning, of this Code, the following setbacks for infiltration facilities shall apply:
(1)
One hundred
feet from water supply wells.
(2)
Ten feet
downslope or 100 feet upslope from building foundations.
(3)
Fifty feet
from septic system drainfields.
(4)
Fifty feet
from a geologic contact with carbonate bedrock, unless a preliminary
site investigation is done in the carbonate bedrock to show the absence
of special geologic features within 50 feet of the proposed infiltration
area.
(5)
One hundred
feet from the property line unless documentation is provided to show
all setbacks from wells, foundations and drainfields on the neighboring
property will be met.
The applicant shall comply with the following
water quality requirements of this article.
A.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 297-15 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the infiltration 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. To achieve this goal, the following criterion is established:
(1)
The following
calculation formula is to be used to determine the water quality storage
volume (WQv) in acre-feet of storage for the Schuylkill River and
Maiden Creek watersheds:
|
Equation 297-16.1
|
|---|
|
WQv = [(P)(Rv)(A)]/12
|
|
Where:
| |||
|
WQv = Water quality volume
(acre-feet).
| |||
|
P = 1 inch.
| |||
|
A = Total contributing drainage area 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.
(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.
B.
For areas within
defined special protection subwatersheds which include exceptional
value (EV) and high quality (HQ) waters and cold water fishery (CWF),
the temperature and quality of water and streams shall be maintained.
C.
To accomplish
the above, the applicant shall use innovative or traditional stormwater
control facilities that are found within the PADEP State BMP Manual.
D.
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 appropriate native vegetation (reference to Appendix H of Pennsylvania
Handbook of Best Management Practices for Developing Area for plant
lists). If the applicable rear- or side-yard setback is less than
50 feet, the buffer width may be reduced to 25% of the setback 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.
E.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Township of Muhlenberg. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of § 297-16A.
A.
In addition to
control of the 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
peak 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 a three-inch-diameter
orifice, and a trash rack shall be installed to prevent clogging.
On sites with small contributing drainage areas 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.
A.
Schuylkill River
Watershed.
(1)
The Schuylkill
River Watershed has been divided into stormwater management districts
as shown on the Management District Map in Appendix D.[1]
(b)
Standards
for managing runoff from each subarea in the Schuylkill River Watershed
for the two-year through one-hundred-year design storms are shown
in Table 297-18.1. Development sites located in each of the districts
must control proposed conditions runoff rates to existing conditions
runoff rates for the design storms in accord with Table 297-18.1.
|
Table 297-18.1
| ||||
|---|---|---|---|---|
|
Water Quantity Requirements
| ||||
|
Management District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
|
Equivalent Release Rate
| |
|
A
|
2-year
|
Reduce To
|
1-year
|
—
|
|
5-year
|
5-year
|
100%
| ||
|
10-year
|
10-year
|
100%
| ||
|
25-year
|
25-year
|
100%
| ||
|
50-year
|
50-year
|
100%
| ||
|
100-year
|
100-year
|
100%
| ||
|
B
|
2-year
|
Reduce To
|
1-year
|
—
|
|
5-year
|
2-year
|
30%
| ||
|
10-year
|
5-year
|
75%
| ||
|
25-year
|
10-year
|
75%
| ||
|
50-year
|
25-year
|
75%
| ||
|
100-year
|
50-year
|
75%
| ||
|
C
|
2-year
|
Reduce To
|
1-year
|
100%
|
|
5-year
|
5-year
|
100%
| ||
|
10-year
|
10-year
|
100%
| ||
|
25-year
|
25-year
|
100%
| ||
|
50-year
|
50-year
|
100%
| ||
|
100-year
|
100-year
|
100%
| ||
[1]
Editor's Note: Appendix D is included as an
attachment to this chapter.
(2)
General.
Proposed condition rates of runoff from any regulated activity shall
not exceed the peak release rates of runoff prior to development for
the design storms specified on the Stormwater Management District
Watershed Map (Appendix D)[2] and § 297-18 of this chapter.
[2]
Editor's Note: Appendix D is included as an
attachment to this chapter.
(3)
District
boundaries. The boundaries of the stormwater management districts
are shown on an official map that is available for inspection at the
municipal office. A copy of the official map at a reduced scale is
included in the Ordinance Appendix 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.
(4)
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, as indicated in § 297-18. 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 shall follow Management District A criteria, provided that the overall site discharge meets the management district criteria for which the discharge is located.
(5)
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.
(6)
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.
B.
Maiden Creek
Watershed.
(1)
The Maiden
Creek Watershed has been divided into stormwater management districts
as shown on the Management District Map in Appendix D.[3] Corresponding release rate percentages associated with
each stormwater management district is provided as well.
(b)
Standards
for managing runoff from each subarea in the Maiden Creek Watershed
for the two and thirty-three-hundredths-, ten- and fifty-year design
storms are shown in Table 297-18.2. Development sites located in each
of the districts must control proposed condition runoff rates to existing
conditions runoff rates for the design storms in accordance with Table
297-18.2.
|
Table 297-18.2
| ||||
|---|---|---|---|---|
|
Water Quantity Requirements
| ||||
|
Management District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
|
Equivalent Release Rate
| |
|
A
|
2-year
|
Reduce To
|
1-year
|
-
|
|
5-year
|
5-year
|
100%
| ||
|
10-year
|
10-year
|
100%
| ||
|
25-year
|
25-year
|
100%
| ||
|
100-year
|
100-year
|
100%
| ||
|
B-1
|
2-year
|
Reduce To
|
1-year
|
-
|
|
10-year
|
5-year
|
75%
| ||
|
25-year
|
10-year
|
75%
| ||
|
50-year
|
25-year
|
75%
| ||
|
100-year
|
100-year
|
100%
| ||
|
B-2
|
2-year
|
Reduce To
|
1-year
|
-
|
|
5-year
|
2-year
|
30%
| ||
|
25-year
|
5-year
|
50%
| ||
|
50-year
|
10-year
|
50%
| ||
|
100-year
|
100-year
|
100%
| ||
|
B-3
|
2-year
|
Reduce To
|
1-year
|
-
|
|
5-year
|
2-year
|
30%
| ||
|
10-year
|
5-year
|
75%
| ||
|
50-year
|
25-year
|
75%
| ||
|
100-year
|
100-year
|
100%
| ||
|
C
|
2-year
|
Reduce To
|
1-year
|
-
|
|
5-year
|
5-year
|
100%
| ||
|
10-year
|
10-year
|
100%
| ||
|
25-year
|
25-year
|
100%
| ||
|
50-year
|
50-year
|
100%
| ||
|
100-year
|
100-year
|
100%
| ||
[3]
Editor's Note: Appendix D is included as an
attachment to this chapter.
(2)
General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff prior to development for the design storms specified on the Maiden Creek Stormwater Management District Watershed Map (Appendix D) and § 297-18 of this chapter.
(3)
District
boundaries. The boundaries of the stormwater management districts
are shown on an official map that is available for inspection at the
municipal office. A copy of the official map at a reduced scale is
included in the Ordinance Appendix D-1. The exact location of the
Maiden Creek 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.
(4)
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, as indicated in § 297-18. 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 shall follow Management District A criteria, provided that the overall site discharge meets the management district criteria for which the discharge is located.
(5)
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.
(6)
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.
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 297-19-1
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 Rational
Method shall be used to estimate peak discharges from drainage areas
that contain fewer than 200 acres. The Soil- Cover-Complex Method
shall be used for drainage areas greater than 200 acres.
|
Table 297-19-1
| ||
|---|---|---|
|
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
|
US 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 and with time
of concentration less than 60 minutes (tc is less than 60 minutes),
or as approved by the Township of Muhlenberg and/or Township Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by
the Township of Muhlenberg and/or Township Engineer
|
|
* Note: Successors to the above methods are
also acceptable. These successors include WINNTR55 for TR55 and WINTR20
for TR20 and SWMM.
|
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 according to the region in which they are located as
presented in Table 124-B-1 in Appendix B of this chapter.[1] If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS
is used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference to NOAA
Atlas 14.
[1]
Editor's Note: Appendix B is included as an
attachment to this chapter.
C.
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 124-B-1 or B-2 in Appendix B of this chapter.
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 NOAA Atlas 14 Precipitation-Frequency Atlas of the
United States (2004, revised 2006). 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.
NRCS lag equation divided by 0.6 is an acceptable method for Tc in
undeveloped areas.
F.
Runoff coefficients
(c) for both existing and proposed conditions for use in the Rational
Method shall be obtained from Table 124-B-2 in Appendix 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 Table 124-B-3 in Appendix B of the chapter. Full
flow shall be assumed for closed conduits.
H.
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.
I.
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. 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 (i.e., TR-20, TR-55, HEC-1, PSRM). The
Township of Muhlenberg 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.
General regulations.
(1)
Any facilities
that constitute water obstructions (e.g., culverts, bridges, outfalls,
or stream enclosures), water encroachments and any work involving
wetlands governed by PADEP Chapter 105 regulations (as amended or
replaced from time to time by PADEP) are subject to PADEP Chapter
105 regulations.
(2)
Adequate
erosion protection shall be provided along all open channels and at
all points of discharge (DEP Erosion and Sediment Pollution Control
Manual).
(3)
The Township
of Muhlenberg reserves the right to disapprove any design that would
result in the construction in or continuation of a stormwater problem
area.
B.
Detention and
retention basins.
(2)
No stormwater
detention facility shall be placed within 50 feet of a special geologic
feature. No stormwater conveyance facility shall be constructed within
50 feet of a special geologic feature, unless it is constructed of
durable pipe utilizing watertight joints.
(3)
Privately
owned detention facilities and BMPs should be situated such that lot
lines do not divide the facility. Maintenance of an individual facility
shall be the responsibility of only one lot owner unless the facility
is located in a common area maintained by a homeowners' association.
(4)
Any stormwater
management facility (i.e., detention basin) designed to store runoff
and requiring a berm or 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 proposed conditions
and may be subject to PADEP Chapter 105 regulations. The emergency
spillway shall either be placed in undisturbed earth or constructed
of concrete.
(5)
Minimum grades
for turf areas inside detention basins shall be 1%, and maximum side
slopes of retention/detention basins shall be 33% (three units horizontally
to one unit vertically). Depending upon the location and the intended
use of the detention facilities during nonfunctioning times, a side
slope of less than 33% for one or both slopes may be required by the
Board of Commissioners. Side slopes shall be kept as close to the
natural land contours as practical, but when a detention basin is
to be maintained by a future residential lot owner, the maximum side
slope shall be 25% (four units horizontally to one unit vertically).
(6)
Where the
outfall point of a proposed detention facility is located at a point
subject to tailwater conditions, a tailwater elevation at the outfall
point will need to be assumed when performing the detention facility
routing calculations. The tailwater elevation assumed shall be based
on the frequency of the storm being routed. For example, if a ten-year
storm is being routed through the facility, the ten-year water surface
elevation needs to be determined at the discharge point and assumed
as the downstream tailwater condition.
(7)
Outlet structures
for underground detention facilities shall be provided within a reinforced
concrete junction box. Flows through the junction box shall be attenuated
with a concrete weir wall with adequately sized orifices. The dimensions
of the weir wall should be designed to allow passage of the one-hundred-year
post-development inflow without surcharging the junction box.
(8)
Underground
detention facilities which propose pipe storage as a means of detention
shall be constructed of reinforced concrete pipe (RCP) or smooth-lined
corrugated plastic pipe (SLCPP).
(9)
Wet ponds
shall be designed in accordance with the design parameters as found
in the Pennsylvania Department of Environmental Protection Stormwater
Best Management Practices Manual, latest edition, and the following:
(a)
Water
surface area shall not exceed 1/10 of the tributary drainage area.
(b)
Shoreline
protection shall be provided to prevent erosion from wave action.
(c)
Minimum
normal water depth shall be four feet. If fish are to be used to keep
the pond clean, a minimum of 1/4 of the pond area shall be a minimum
of 10 feet deep.
(d)
Facilities
shall be provided to allow the pond level to be lowered by gravity
flow for cleaning purposes and shoreline maintenance.
(e)
Aeration
facilities as may be required to prevent pond stagnation shall be
provided. Design calculations to substantiate the effectiveness of
these aeration facilities shall be submitted with final engineering
plans. Agreements for the perpetual operation and maintenance of aeration
facilities shall be prepared to the satisfaction of the Township of
Muhlenberg.
(f)
In the
event that the water surface of the pond is to be raised for the purposes
of storing water for irrigation or in anticipation of the evapotranspiration
demands of dry weather, the volume remaining for storage of excess
stormwater runoff shall still be sufficient to contain the design
year storm runoff.
(10)
Retention
and/or detention basins which are designed with earth fill dams shall
incorporate the following minimum standards:
(a)
The height
of the dam shall not exceed 15 feet, unless approved by the Township
of Muhlenberg and the Pennsylvania Department of Environmental Protection.
(b)
The minimum
top width of dams up to 15 feet in height shall be equal to 2/3 of
the dam height, but in no case shall the top width be less than eight
feet.
(c)
The side
slopes of the settled earth fill shall not be less than three horizontal
to one vertical.
(d)
A key
trench, at least two feet deep or extending down to stable subgrade,
whichever is deeper, of compacted relatively impervious material (Unified
Soil Classification CL or ML). Minimum bottom width for the key trench
shall be four feet. Maximum side slopes for the key trench shall be
one horizontal to one vertical. A compacted impervious core at least
eight feet wide at the top, having a maximum side slope of one horizontal
to one vertical, shall extend for the full length of the embankment,
and the top elevation shall be set at the design year water surface
elevation.
(e)
All pipes
and culverts through dams shall have properly spaced anti-seep collars
(minimum six inches thick).
(f)
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 Township of Muhlenberg may require the installation
of an impermeable liner in detention basins.
(11)
The following
standards shall apply to all low-volume detention basins. A low-volume
detention basin is considered any man-made surface impoundment designed
to provide no more than 0.10 acre-foot of storage between the lowest
outlet elevation to the top of berm elevation, and a maximum one-hundred-year
ponding depth no greater than three feet.
(a)
The minimum
berm width shall be five feet.
(b)
The side
slopes of both the interior and exterior embankments shall be no steeper
than two horizontal units to one vertical unit.
(c)
Low-volume
detention basins shall be designed to accommodate the one-hundred-year
post-development storm such that the maximum water surface elevation
is less than the emergency spillway elevation.
(d)
Emergency
spillways shall be designed to safely convey to one-hundred-year post-development
basin inflow with a minimum six inches of freeboard above the water
surface elevation.
(12)
The following standards shall apply to all stormwater management facilities proposing infiltration as a method to reduce the proposed condition runoff to the existing condition design storm as required by § 297-18.
(b)
A minimum
depth of 36 inches between the bottom of the infiltration facility
and the limiting zone.
(c)
The 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.
(d)
The infiltration
facility shall be capable of completely infiltrating the required
retention/detention volume within 24 hours.
(e)
Pretreatment
shall be provided prior to infiltration.
(f)
When stormwater runoff accesses the infiltration bed or trench via surface flow, the volume of runoff controlled by infiltration shall be limited to 25% of the detention volume required to satisfy § 297-18. This is in consideration of loss of function during freezing weather.
(g)
When runoff is piped underground to the infiltration facility, 50% of the detention volume required to satisfy § 297-18 may be infiltrated, providing the following conditions are also met:
[1]
There
exists at least 18 inches between the top of the infiltration facility
and the ground surface.
[2]
An
emergency overflow is provided capable of controlling the maximum
year design storm for the watershed and district in which the facility
is located when the infiltration facility is independent of the detention
facility.
[3]
The
point of discharge of the emergency overflow shall be in accordance
with the provisions of this chapter. The preferred point of discharge
is into a detention or retention facility.
C.
Storm sewers.
(1)
Any proposed
roadway drainage facilities shall be designed according to the more
restrictive of PennDOT Design Manual Part II or the Township of Muhlenberg
standards.
(2)
Storm sewers
must be able to convey proposed conditions runoff from a twenty-five-year
design storm without flooding inlets. At the Township of Muhlenberg's
discretion, drainage facilities shall be designed to convey proposed
conditions runoff from the one-hundred-year design storm without flooding
inlets in sag areas of streets.
(3)
When designing
storm sewers that discharge into a detention facility, a tailwater
condition at the storm sewer discharge point needs to be assumed where
applicable. The assumed tailwater elevation should be the equal to
the detention facility water surface elevation resulting from whatever
year design storm was used to design the storm sewer. For example,
if the storm sewer is being designed to convey a twenty-five-year
storm, the downstream tailwater elevation should be assumed as the
twenty-five-year stormwater surface elevation of the detention facility.
(4)
Subsurface
drainage systems shall have manholes spaced at intervals not exceeding
400 feet and located wherever branches are connected or sizes are
changed and wherever there is a change in alignment or grade. Inlets
may be used instead of manholes when approved by the Township Engineer.
(5)
Storm sewer
lines within street rights-of-way shall be located next to the curbline
and shall parallel the curbline as far as practical.
(6)
Subsurface
drainage systems shall have curb inlets located at curb tangents on
the uphill side of street intersections and at other locations as
may be required by the Township of Muhlenberg as necessary to intercept
runoff. Design and location of curb inlets shall be in accordance
with Pennsylvania Department of Transportation Design Manual Part
2 and be approved by the Township Engineer.
(7)
All proposed
inlets within curbed streets shall be designed such that the top-of-grate
elevations are equal to the top-of-base-course elevation to permit
proper drainage of the streets prior to wearing course placement.
(8)
Inlets shall
be designed and located to prevent hazards to vehicles, bicycles and
pedestrians.
(9)
Reinforced
concrete culvert pipe Class III or better, with rubber O-ring joints,
shall be used for all storm sewer lines to be constructed within street
rights-of-way. Minimum diameter shall be 15 inches. Smooth lined corrugated
polyethylene pipe (SLCPP) may be used, at the option of the subdivider,
for lines to be located on private property that will not be offered
for dedication to the Township of Muhlenberg.
(10)
If SLCPP is being proposed, as permitted in § 297-20C(9), in areas underlain with limestone geology, water-tight joints will be required within the storm sewer system. Only pipe joints which are capable of meeting the testing criteria outlined by ASTM D-3212 will be considered water-tight.
(11)
All drainage
facilities shall be maintained to retain their design capacity.
(12)
Drainage
facilities, including detention ponds, shall not convey water onto
any existing or proposed public road right-of-way.
(13)
Pipes and
storm sewers shall have a minimum slope not less than 0.005 foot per
foot.
(15)
The inflow
inverts of the proposed inlets shall be no less than 0.2 foot above
the outflow invert elevation. The inflow pipe(s) crown elevation(s)
shall be equal to or above the crown elevation of the outflow pipe.
In specific cases, this requirement may be waived at the discretion
of the Township Engineer.
(16)
All proposed
inlets within curbed streets shall be designed such that the top-of-grate
elevations are equal to the top-of-base-course elevation to permit
proper drainage of the streets prior to wearing course placement.
(17)
Storm sewer
inlets proposed to be sumped shall be a minimum of six feet long and
no deeper than six feet.
D.
Open channels.
(1)
Open channels
shall be designed in accordance with the channel design procedures
found in the Department of Environmental Protection Bureau of Water
Quality Protection's Erosion and Sediment Pollution Control Program
Manual.
(2)
All open
channels shall be designed to convey the twenty-five-year stormwater
flow. In addition, open channels shall be designed to convey the one-hundred-year
stormwater flow from emergency spillways and areas where damage to
property would result.
(3)
Open channels
located adjacent to residential streets shall be limited to a top
width of six feet, depth of 1.5 feet, and if trapezoidal in cross
section, a minimum bottom width of two feet.
(4)
Open channels
located along nonresidential streets, in side yards, rear yards, or
open space shall be limited to a top width of 12 feet, depth of three
feet, and if trapezoidal in cross section, a minimum bottom width
of two feet. Open channels located in side yards, rear yards, or open
space shall provide a minimum of six inches of freeboard.
(5)
The minimum
longitudinal slope shall not be less than 0.010 foot per foot nor
greater than 0.100 foot per foot.
E.
Any infiltration and water quality facilities proposed to meet the requirements of §§ 297-15 and 297-16 should be designed in a manner consistent with the requirements of the NPDES permit, if one is required. Design of the infiltration and water quality facilities should be in accordance with the guidelines provided in the Pennsylvania Stormwater Best Management Practices Manual, latest edition.
F.
Construction
of streets.
(1)
All streets
shall be designed as to provide for the discharge of surface water
from their rights-of-way.
(2)
The slope
of the crown on proposed streets shall not be less than 1/4 inch per
foot and not more than 1/2 inch per foot.
(3)
The typical
slope of the shoulder areas shall not be less than 3/4 inch per foot
and not more than one inch per foot.
(4)
Cross drainage
in intersections or in tangent sections of roadway will not be permitted.
(5)
Gutter flow
shall not exceed 1/2 of the travel lane width.
