[Ord. 2008-1, 4/1/2008, § 301]
1.
Applicants proposing regulated activities in Schuylkill River Watershed which do not fall under the exemption criteria shown in § 23-142 shall submit a drainage plan consistent with the Schuylkill River Watershed Stormwater Management Plan to the Township of Douglass for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
2.
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.
3.
The drainage plan must be designed consistent with the sequencing provisions of § 23-124 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 this Part 1C.
4.
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
Part.
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 Part. 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.
6.
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 § 23-126, Subsection 1D.
7.
Any stormwater management facilities regulated by this Part that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by Pennsylvania DEP through the joint
permit application process, or, where deemed appropriate by Pennsylvania
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 Pennsylvania DEP.
8.
Any alteration that affects stormwater flow directly or indirectly
toward a PennDOT facility shall be subject to PennDOT regulations.
9.
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.
10.
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 Douglass.
11.
All stormwater runoff shall be treated for water quality.
12.
Transference of runoff to or from an EV/HQ watershed is prohibited
unless otherwise authorized by DEP, DRBC or SRBC.
[Ord. 2008-1, 4/1/2008, § 302]
Permits must be obtained for all relevant regulated activities,
in compliance with any and all applicable local, county, state and
federal regulations.
[Ord. 2008-1, 4/1/2008, § 303]
1.
No regulated earth disturbance activities within the Township of
Douglass shall commence until the Township of Douglass receives an
approval from the Berks County Conservation District of an erosion
and sediment control plan for construction activities.
2.
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).
3.
In addition, under 25 Pa. Code, Chapter 92, a DEP "NPDES construction activities" permit is required for regulated earth disturbance activities. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or Berks County Conservation District must be provided to the Township of Douglass. The issuance of an NPDES construction permit (or permit coverage under the Statewide General Permit (PAG-2) satisfies the requirements Subsection 1.
4.
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.
5.
Additional erosion and sediment control design standards and criteria
are recommended to 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 to maintain maximum infiltration
capacity.
B.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
achieved final stabilization.
[Ord. 2008-1, 4/1/2008, § 304]
1.
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.
A.
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 Township
requirements.
B.
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 that disturb one acre or more included consideration of
the following issues:
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 and B (areas conducive to infiltration), special geologic features, any existing recharge areas and any other requirements outlined in the Township Subdivision and Land Development Ordinance [Chapter 22].
C.
Prepare a draft project layout avoiding sensitive areas identified
in Subsection 2A.
D.
Identify site specific existing conditions drainage areas, discharge
points, recharge areas and hydrologic soil groups A and B.
F.
Satisfy infiltration objective (§ 23-125) 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.
H.
Determine what management district the site falls into (Appendix
23-1-D) and conduct an existing conditions runoff analysis.
I.
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.
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 (§ 23-128).
K.
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
[Ord. 2008-1, 4/1/2008, § 305]
1.
Maximizing the ground water recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing ground water 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 § 23-103, and take advantage of utilizing any existing recharge areas.
2.
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 paragraph .A.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 subparagraphs (3)(a) or (3)(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:
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(a)
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NRCS Curve Number Equation.
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The NRCS runoff equation shall be utilized to calculate infiltration
requirements (I) in inches.
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Equation 23-125
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I (Infiltration requirement, in inches) = (200 / CN) - 2
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Where:
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CN = SCS (NRCS) curve number of existing conditions contributing
to the infiltration facility.
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This equation is displayed graphically in, and the infiltration
requirement can be determined from Figure 23-125.
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It has been determined that infiltrating 0.46 inches of runoff
from the impervious areas will aid in maintaining the hydrologic regime
of the watershed. However, the rounded number 0.5 inches will be used.
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Figure 23-125. Infiltration requirement based upon NRCS
Curve Number.
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The retention (infiltration) volume (Rev) required to meet the infiltration requirement would therefore be
computed as:
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Equation 23-125
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Rev = (0.5 or I, whichever is greater)*
impervious area (square feet)/(12 in/ft) = Cubic Feet
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Where:
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I = infiltration requirements (in inches).
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(b)
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Annual Recharge — Water Budget Approach. If the goals
of subparagraph (3)(a) cannot be achieved, then 0.5 inches 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
23-125 or the curve in Figure 23-125 should be used to determine the
infiltration requirement.
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The retention (infiltration) volume (Rev) required again would therefore be computed as:
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Rev = (0.5 or I, whichever is greater)*
impervious area (square feet)/(12 in/ft) = Cubic Feet
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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 sub-grade
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 Douglass
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 Subsection 2A. If this investigation
proves infeasible, infiltration can occur on areas underlain by carbonate
rock by following the recommended procedure below in conjunction with
Figure 23-B-1 in Appendix 23-1-B. However, the applicant is not required
to use infiltration in carbonate areas even if the site falls into
the "recommended" range on Figure 23-B-1 in Appendix 23-1-B. If infiltration
is not proposed, the calculated infiltration volume (Subsection 2A)
shall be treated by an acceptable BMP.
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Infiltration BMP loading rate percentages in Figure 23-B-1 in
Appendix 23-1-B shall be calculated as follows:
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The area tributary to the infiltration BMP shall be weighted
as follows:
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Area Description
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Weighting
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All disturbed area to be made impervious
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100%
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All disturbed areas to be made pervious
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50%
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All undisturbed impervious areas
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100%
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All undisturbed pervious areas
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0%
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Soil thickness is to be measured from the bottom of any proposed
infiltration BMP. The effective soil thickness in Figure 23-B-1 in
Appendix 23-1-B is the measured soil thickness multiplied by the thickness
factor based on soil permeability, as follows:
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Permeability Range
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Thickness Factor
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6.0 to 12.0 inches/hr
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0.8
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2.0 to 6.0 inches/hr
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1.0
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1.0 to 2.0 inches/hr
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1.4
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0.75 to 1.0 inches/hr
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1.2
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0.5 to 0.75 inches/hr
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1.0
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The design of all facilities over Karst shall include an evaluation
of measures to minimize adverse effects.
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D.
Stormwater Hotspots.
(1)
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 be 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 wash-off
after construction. EPA's NPDES stormwater program requires some
industrial sites to prepare and implement a stormwater pollution prevention
plan.
(a)
Examples of Hotspots:
1)
Vehicle salvage yards and recycling facilities.
2)
Vehicle fueling stations.
3)
Vehicle service and maintenance facilities.
4)
Vehicle and equipment cleaning facilities.
5)
Fleet storage areas (bus, truck, etc.).
6)
Industrial sites (based on Standard Industrial Codes).
7)
Marinas (service and maintenance).
8)
Outdoor liquid container storage.
9)
Outdoor loading/unloading facilities.
10)
Public works storage areas.
11)
Facilities that generate or store hazardous materials.
12)
Commercial container nursery.
13)
Other land uses and activities as designated by
an appropriate review authority.
(b)
The following land uses and activities are not normally considered
hotspots:
(2)
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 Douglass or water
authority.
F.
Infiltration facilities shall be used in conjunction with other innovative
or traditional stormwater control facilities that are found within
the Pennsylvania DEP State BMP Manual.
G.
Caution shall be exercised where salt or chloride (Township 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 Department'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 Douglass.
J.
The Township of Douglass 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 the Zoning Ordinance [Chapter 27], 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.
[Ord. 2008-1, 4/1/2008, § 306]
1.
The applicant shall comply with the following water quality requirements
of this Part 1A:
A.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 23-125 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.
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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 Schuylkill River watershed:
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Equation 23-126
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WQv =[(P)(Rv)(A)]/12
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WQv = Water Quality Volume (acre-feet)
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P = 1 inch
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A = Total contributing drainage area to the water quality BMP
(acres)
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Rv = 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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This volume requirement can be accomplished by the permanent
volume of a wet basin or the detained volume from other BMPs.
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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.
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B.
For areas within defined special protection subwatersheds which include
Exceptional Value (EV) and High Quality (HQ) waters, 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 Pennsylvania
DEP 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 Areas 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 Part, the existing buffer shall be
maintained.
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 Douglass. The issuance of an NPDES construction
permit (or permit coverage under the statewide general permit) satisfies
the requirements of paragraph .A.
[Ord. 2008-1, 4/1/2008, § 307]
1.
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).
2.
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.
[Ord. 2008-1, 4/1/2008, § 308]
1.
The Schuylkill River Watershed has been divided into stormwater management
districts as shown on the Management District Map in Appendix 23-1-D.
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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 128-1. 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
128-1.
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Table 128-1 — Water Quantity Requirements
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Management District
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Proposed Condition Design Storm
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Existing Condition Design Storm
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A
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2-year
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Reduce to
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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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50-year
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50-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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Reduce to
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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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50-year
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25-year
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100-year
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50-year
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Equivalent Release Rate
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C*
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2-year
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Reduce to
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1-year
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100%
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5-year
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5-year
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100%
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10-year
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10-year
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100%
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25-year
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25-year
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100%
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50-year
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50-year
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100%
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100-year
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100-year
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100%
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Unclassified Management District**
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Proposed Condition Design Storm
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Existing Condition Design Storm
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|---|---|---|---|
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Remaining areas of Douglass Township, excluding areas within
the Swamp Creek Watershed
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2-year
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Reduce to
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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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50-year
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50-year
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100-year
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100-year
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*
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The minimum performance standard for development sites in District
C is that stormwater management controls must be provided to limit
the post-development peak rate of stormwater runoff to a rate that
is equal to or less than the existing peak rate of stormwater runoff.
This minimum performance criteria was set through a policy directive
of the Pennsylvania Department of Environmental Protection. However,
in the previous development of the Act 167 Plan, this district was
originally established as a conditional no-detention district such
that development sites which could discharge directly to the main
channel of the Schuylkill River and its major tributaries, or indirectly
to the main channel through an existing stormwater drainage system
(i.e., storm sewer or open channel) could do so without control of
the post-development peak rate of runoff.
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**
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The Unclassified Management District includes all areas of the
Township of Douglass shown as unmapped, in "white," on the Watershed
Map in Appendix 23-1-D, excluding the Swamp Creek Watershed which
is located in the extreme northeast corner of the Township, and is
covered by a separately available ordinance for the Swamp Creek Act
167 Plan [Part 2].
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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 23-1-D) and § 23-128, of this Part.
3.
District Boundaries. The boundaries of the Stormwater Management
Districts are shown on an official map that is available for inspection
at the Township office. A copy of the official map at a reduced scale
is included in the Appendix 23-1-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 § 23-128. 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.
[Ord. 2008-1, 4/1/2008, § 309]
1.
Applicants may select a combination of runoff control techniques
that are most suitable to control stormwater runoff from the site.
All controls shall be subject to approval of the Township of Douglass.
The Township Engineer may request specific information on design and/or
operating features of the proposed stormwater controls in order to
determine their suitability and adequacy in terms of the standards
of this Section.
2.
The applicant shall consider the effect of the proposed stormwater
management techniques on any special soil conditions or geological
hazards that may exist on the development site. In the event such
conditions are identified on the site, the Township Engineer may require
in-depth studies by a geotechnical engineer.
3.
Basins and Impoundments. Any BMP that is a dam, as defined in 25
Pa. Code, Chapter 105, shall be designed according to the requirements
in those regulations. Any stormwater BMP that does not constitute
a dam under 25 Pa. Code, Chapter 105, but is designed to store runoff
and requires a berm or earthen embankment (i.e., detention basin),
shall be designed to comply with the following:
A.
Basins with earthen embankments shall be designed with an emergency
spillway that has capacity and stability to accommodate the one-hundred-year
post-development hydrograph, in a manner that will not damage the
integrity of the facility and will not create a downstream hazard
emergency spillway design shall assume that all other basin outlets
are nonfunctional. Where practical, the emergency spillway shall be
constructed in undisturbed ground, and locations in fill should be
avoided. Regardless of location, appropriate stabilization measures
shall be provided.
B.
The height of the embankment shall be designed to provide a minimum
1.0 foot of freeboard above the maximum elevation computed when the
one-hundred-year peak post-development flow passes through the emergency
spillway. Embankment heights shall not exceed 15 feet, unless approved
by the Township of Douglass.
C.
The minimum top width of earthen embankments shall be equal to three-fourths
of the embankment height, but in no case shall the top width be less
than eight feet.
D.
Interior and exterior embankment side slopes shall not be greater
than three horizontal to one vertical.
E.
All pipes, conduits, etc., passing through basin embankments shall
have properly spaced concrete anti-seep collars. Minimum collar projection
beyond the pipe shall be two feet, and maximum collar spacing is 14
times the projection. The joint between collar and pipe shall be watertight.
F.
A key trench of relatively impervious material shall be provided
within all basin embankments. The key trench shall be at least two
feet deep, or extend down to stable subgrade, whichever is deeper.
Minimum bottom width for 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 three feet wide at the top, having
maximum side slopes of one horizontal to one vertical shall extend
for the full length of the embankment, and the top elevation shall
be set at the ten-year design water surface elevation.
G.
All basins shall be structurally sound and shall be constructed of
durable materials. The completed structure and the foundation of all
basins shall be stable under all probable conditions of operation.
Embankments shall be placed in maximum eight-inch lifts to a minimum
of 95% of maximum dry density, as established by ASTM D-1557. Compaction
test results shall be provided to the Township of Douglass upon request.
Embankments shall be constructed six inches above design elevations
to allow for settlement.
H.
All basins not including groundwater recharge and/or water quality
storage shall include an outlet structure to permit draining the basin
to a completely dry condition within 24 hours following the end of
the design rainfall.
I.
All discharge control devices with appurtenances (except discharge
pipes) shall be made of reinforced concrete and stainless or hot dip
galvanized steel. Bolts/fasteners and any orifice plates are to be
stainless or galvanized steel. Outlet barrels/pipes shall have a minimum
diameter of 15 inches and shall be made of reinforced concrete pipe
with watertight joints, or approved equivalent. Where installation
conditions merit, structural calculations that address the actual
design requirements will be required.
J.
All basins employing small orifices (i.e., less than six inches diameter
or equivalent area), shall be provided with an apron of concrete or
similar durable material which abuts the orifice invert and extends
a minimum of 18 inches in all directions from the orifice, to prevent
potential blockage by vegetative growth or debris, and to allow for
easy cleaning of the area approaching the orifice. Appropriate trash
racks are required for all orifice openings.
K.
Low flow channels shall be provided from each water carrying facility
to the outlet structure for all basins that do not include groundwater
recharge and/or water quality storage. Low flow channels shall be
1% minimum slope and shall be designed to enable ease of maintenance.
All basins that do include groundwater recharge and/or water quality
storage shall not be required to have a low flow channel.
L.
Minimum slope within a basin that does not include groundwater recharge
and/or water quality storage shall be 2% positive grade to the low
flow channel.
M.
Design storms for the computation of retention basin volumes shall
be based upon a twenty-four-hour storm with one-hundred-year return
period (SCS Type II storm).
N.
The effect on downstream areas if the basin embankment fails shall
be considered in the design of all basins. The basin shall be designed
to minimize the potential damage caused by such failure of the embankment.
O.
All structures (detention basins, cisterns, etc.), other than those
used for groundwater recharge volume and water quality volume, must
completely drain within 24 hours after the end of the design storm.
P.
Soils used for the construction of basins shall have low erodibility
factors ("K" factors).
Q.
Minimum floor elevations for all structures that would be affected
by a basin, other temporary impoundments, or open conveyance systems
where ponding may occur shall be two feet above the one-hundred-year
water surface. If basement or underground facilities are proposed,
detailed calculations addressing the effects of stormwater ponding
on the structure and water-proofing and/or flood-proofing design information
shall be submitted for approval.
4.
Stormwater Collection and Conveyance Systems. Gutters, inlets, pipes,
conduits, swales, ditches and any other means for collecting and/or
conveying stormwater shall be designed to comply with the following:
A.
All storm sewer pipes, culverts and bridges (excluding detention
and retention basin outfall structures), gutters and swales conveying
water originating only from within the boundaries of the development
site shall be designed for a twenty-five-year storm event. All storm
sewer pipes, culverts and bridges (excluding detention and retention
basin outfall structures) conveying water originating from off site
shall be designed for a fifty-year storm event. Facilities that convey
runoff to stormwater management facilities (e.g., detention basins,
etc.) shall be designed for the one-hundred-year storm event, unless
adequate alternate means for conveying one-hundred-year flow to the
facilities are provided. Drainage easements having widths no less
than 20 feet shall be provided to contain and convey the one-hundred-year
frequency flood throughout the development site. Easements shall begin
at the furthest upstream property line of the proposed development
site in a watershed.
B.
Water obstructions shall convey runoff from the twenty-five-year
design storm with a minimum of 2.0 feet of freeboard measured below
the lowest point along the top of the roadway, without damage to the
drainage structure or the roadway, unless more restrictive local,
state and/or federal regulations apply. A concentrated discharge of
stormwater to an adjacent property shall be within an existing watercourse
or otherwise an easement and appropriate agreements shall be required.
C.
Storm sewer pipes other than those used as roof drains, detention
basin underdrains, and street subbase underdrains, shall have a minimum
diameter of 15 inches and be either Class III reinforced concrete
pipe (RCP) or smooth-lined corrugated high-density polyethylene pipe
(HDPE). Corrugated metal pipe is not permitted. HDPE pipe joints shall
be silt-tight at a minimum (e.g., ADS N-12 ST/IB, or equal), and RCP
joints shall have pre-lubed O-ring gaskets. Watertight specification
is required in areas of carbonate geology and elsewhere as may be
specified by the Township of Douglass. Where installation conditions
merit (e.g., deep fills) structural calculations that address the
actual design requirements will be required.
D.
Storm sewer pipes and culverts shall be installed on sufficient slopes
to provide a minimum velocity of three feet per second when flowing
full.
E.
Storm sewers shall be placed within in the cartway of curbed streets
and parallel to the road shoulders of streets without curbs. At curbed
street intersections, inlets shall be placed on the tangent and not
on the curved portion of the curbing. Storm sewer crossings of streets
shall be perpendicular to the street center line to the maximum extent
practicable.
F.
Storm facilities not located within a public right-of-way shall be
contained in and centered within an easement not less than 20 feet
wide. Easements shall follow property boundaries where possible.
G.
Manning "n" values used for design of pipes and culverts shall be
in accordance with generally accepted engineering practice. Adequate
documentation shall be provided in support of the chosen values.
H.
All storm sewer pipe and culverts shall have a minimum cover of 18
inches. Embedment for all storm sewer pipe and culverts shall consist
of AASHTO No. 8 (1B) aggregate from a minimum of six inches of bedding
beneath the pipe, to a minimum of 12 inches over the pipe. Aggregate
requirements for street construction shall take precedence over the
1B specification, but in no case shall total aggregate cover over
the pipe be less than 12 inches. Compaction shall be in conformance
with manufacturer's specifications. Select backfill shall complete
the trench to pavement subgrade or topsoil layer in grassed areas.
I.
Curves, tees, elbows and wyes are not permitted in pipes other than
those used as roof drains, detention basin under drains, and street
subbase underdrains. Manholes or inlets are required to facilitate
such configurations.
J.
Manholes and inlets shall not be spaced more than 400 feet apart
for pipes with diameters of 24 inches and less, and not more than
500 feet apart for pipes of greater diameter. Ladder rungs shall be
placed in manholes and inlets with depths exceeding five feet.
K.
When there is a change in the pipe size in a manhole or inlet, the
elevations of the top of the pipes shall be the same, or the smaller
pipe(s) higher. A minimum drop of 0.1 foot shall be provided from
the inlet invert elevation(s) to the outlet invert elevation.
L.
Manholes, inlets, headwalls, endwalls and end sections proposed for
dedication or located along streets or subject to vehicular traffic,
shall conform to the requirements of PennDOT, or to specifications
as otherwise modified by the Township of Douglass. Manhole covers
shall have the word "STORM" cast clearly on the lid.
M.
Inlets along curbed streets shall be spaced to limit depth along
the curb to a maximum of three inches during a ten-year storm. Inlet
capacities and by-pass calculations shall be determined by PennDOT
design techniques.
N.
Pipe underdrains and/or pavement base drains shall be provided in
areas known or otherwise documented to have a seasonal high water
table, and as directed by the Township of Douglass Engineer.
O.
Appropriate headwalls, endwalls or end sections shall be used where
stormwater runoff enters or leaves the storm sewer horizontally from
a natural or man-made channel. Such facilities shall conform to the
requirements of PennDOT, or to specifications as otherwise modified
by the Township of Douglass.
P.
Adequate erosion and sediment control protection shall be provided
along all open channels, and at all points of discharge.
Q.
Stormwater roof drains, sump pumps, and pipes shall not directly
discharge water into a street right-of-way or discharge into a sanitary
sewer or storm sewer.
R.
All existing and natural watercourses, channels, drainage systems,
wetlands and areas of surface water concentration shall be maintained
in their existing condition unless an alteration is approved by the
Township of Douglass and any other necessary approving body.
S.
Flow velocities from any storm sewer may not result in erosion of
the receiving channel.
T.
Energy dissipaters shall be placed at the outlets of all storm sewer
pipes, culverts, and bridges where flow velocities exceed maximum
permitted channel velocities as specified in the Pennsylvania DEP
Erosion and Sediment Pollution Control Manual, as amended.
U.
The following conditions shall be met for all swales:
(1)
Capacities and velocities shall be computed using the Manning
equation Stabilization (i.e., linings, etc.) shall be as specified
in the Pennsylvania DEP Erosion and Sediment Pollution Control Manual,
as amended.
(2)
All vegetated swales shall have a minimum slope of 1% unless
approved by the Township of Douglass.
(3)
"N" factors shall be based upon accepted engineering design
practices as approved by the Township of Douglass.
(4)
All swales shall be designed to concentrate low flows to minimize
siltation and meandering.
[Ord. 2008-1, 4/1/2008, § 310]
1.
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 23-130 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 Township of Douglass may allow the use of the Rational Method
to estimate peak discharges from drainage areas that contain less
than 200 acres. The Soil Cover Complex Method shall be used for drainage
areas greater than 200 acres.
|
Table 23-130
| ||
|---|---|---|
|
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<60min), or as approved by the Township
of Douglass.
|
|
Other Methods
|
Varies
|
Other computation methodologies approved by the Township of
Douglass.
|
|
Note:
| |
|
*
|
Successors to the above methods are also acceptable. These successors
include WINNTR55 for TR55 and WINTR20 for TR20 and SWMM.
|
2.
All calculations consistent with this Part using the Soil Cover Complex
Method shall use the appropriate design rainfall depths for the various
return period storms according to NOAA's National Weather Service
Precipitation Frequency Data Server (PFDS), or NOAA Atlas 14. 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.
3.
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 Tables 23-1-B-1 or
23-1-B-2 in Appendix 23-1-B of this Part.
4.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods from NOAA's National Weather Service Precipitation
Frequency Data Server (PFDS), or NOAA Atlas 14.
5.
Times of concentration 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.
6.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table 23-1-B-1 in Appendix 23-1-B of this Part.
7.
Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table 23-1-B-2
in Appendix 23-1-B of this Part.
8.
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 23-1-B-3
in Appendix 23-1-B of the Part. Full flow shall be assumed for closed
conduits.
9.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part using any generally
accepted hydraulic analysis technique or method.
10.
The design of any stormwater detention facilities intended to meet
the performance standards of this Part 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 Douglass 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.
[Ord. 2008-1, 4/1/2008, § 311]
1.
All wet basin designs shall incorporate biologic minimization controls
consistent with the West Nile guidance found in Appendix 23-1-F.
2.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), water encroachments, and
any work involving wetlands governed by Pennsylvania DEP 25 Pa. Code,
Chapter 105, regulations (as amended or replaced from time to time
by Pennsylvania DEP), are subject to Pennsylvania DEP 25 Pa. Code,
Chapter 105, regulations.
3.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge (DEP Erosion and Erosion, Sediment
and Pollution Control Manual).
4.
The Township of Douglass reserves the right to disapprove any design
that would result in the construction in or continuation of a stormwater
problem area.
5.
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.
6.
Stormwater controls, conveyance facilities and BMPs are subject to
additional design and construction criteria as may be included in
a separate stormwater specifications document.
