A.
Applicants proposing regulated activities in Schuylkill River Watershed which do not fall under the exemption criteria shown in § 320-29 shall submit a drainage plan consistent with the Schuylkill River Watershed Stormwater Management Plan to the Township of Exeter 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. Appendix E, Low-Impact
Development Practices,[1] provides several alternatives.
[1]
Editor's Note: Appendix E is included at the end of this chapter.
C.
The drainage plan must be designed consistent with the sequencing provisions of § 320-21 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 Article IV of this Part 2.
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
Part 2.
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 Part 2. If diffused
drainage discharge is proposed to be concentrated and discharged onto
adjacent property, the applicant must document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge,
or otherwise prove that no erosion, sedimentation, flooding or other
impacts will result from the concentrated discharge.
G.
Any stormwater management facilities regulated by this Part 2 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 Exeter.
L.
Transference of runoff to or from an exceptional value/high quality
(EV/HQ) watershed is prohibited unless otherwise authorized by PADEP,
Delaware River Basin Commission (DRBC) or Susquehanna River Basin
Commission (SRBC).
Permits must comply with any and all applicable local, county,
state and federal regulations.
A.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control."
B.
No regulated earth disturbance activities within the Township of
Exeter shall commence until the Township of Exeter receives an approval
from the Conservation District of an erosion and sediment control
plan for construction activities.
C.
PADEP 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).
D.
In addition, under 25 Pa. Code Chapter 92, a PADEP NPDES construction
activities permit is required for regulated earth disturbance activities.
E.
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 Exeter. The issuance of an NPDES construction permit or permit coverage under the statewide general permit (PAG-2) satisfies the requirements of Subsection A.
F.
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.
G.
Additional erosion and sediment control design standards and criteria
shall 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.
H.
Sediment and erosion pollution control facilities shall be designed
according to applicable governmental standards, specifically the following:
(1)
PADEP sediment and erosion control plan requirements in Chapter 102,
Title 25 Pa. Code.
(2)
Berks County Conservation District standards, which has been delegated
by the Pennsylvania Department of Environmental Protection to enforce
the erosion and sediment pollution control plan.
(3)
It shall be the responsibility of the applicant to submit the application
and other necessary material to the Berks County Conservation District.
Comments shall be received and compliance therewith accomplished prior
to final plan approval.
I.
Erosion and sediment pollution control devices shall be installed
concurrently with earthmoving activities and whenever a situation
is created which would contribute to increased erosion.
J.
No earthmoving or stripping of vegetation will be conducted in areas
of greater than 33% slope unless specific approval is obtained from
the Township.
K.
Earthmoving and the addition of fill shall be maintained where possible
and practicable to preserve desirable natural features and the topography
of the site. Changes in grade and topography and other earthmoving
shall be in accordance with the approved erosion and sediment pollution
control plan.
L.
Stripping of vegetation, regrading or other development shall be
done in such a way that will minimize erosion.
M.
All clearing and grading operations shall comply with the requirements
for tree preservation included in the Exeter Township Zoning Ordinance.[1]
(1)
Land disturbance shall be limited to the actual construction site
and an access strip. The amount of disturbed area and the duration
of exposure shall be kept to a practical minimum.
(2)
To the maximum extent practicable, mature, healthy trees of at least
six inches trunk width measured three feet above the average surrounding
ground level shall be retained and protected. Such trees shall not
be removed except as provided on the approved subdivision or land
development plan in accordance with the applicable provisions of the
Exeter Township Zoning Ordinance. The filling of soil over the roots
of trees to be preserved is prohibited. (The roots are presumed to
extend out from the tree as far as the tree's branches extend outward.)
N.
Procedures for protecting soils or geologic structures with water
supply potential from contamination by surface water or other disruption
by construction activity shall be established in consultation with
the Township and such areas shall include, at minimum, those underlain
by carbonate limestone formations. The Township may require pollution
control facilities to be provided on existing and proposed stormwater
management systems within or adjacent to the project site.
O.
Provisions for protecting existing wells or other water supplies
shall be established.
P.
Unless otherwise permitted, graded slopes shall not be steeper than
three horizontal units to one vertical unit.
Q.
A minimum of four inches of topsoil shall be provided on all disturbed
areas prior to final seeding and mulching.
R.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge (PADEP Erosion, Sediment and Pollution
Control Manual).
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. See
Appendix E, Low-Impact Development Practices,[1] for several alternatives.
(1)
An alternative is practicable if it is available and capable of being
done after taking into consideration 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.
[1]
Editor's Note: Appendix E is included at the end of this chapter.
B.
For projects disturbing one acre or more, the applicant shall:
(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 the municipal
Subdivision and Land Development Ordinance.[2]
(2)
Establish appropriate buffers for each of the delineated environmentally sensitive areas per all applicable Township ordinances. (See § 320-23D for stream buffers and § 320-22B(11) 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 objective (§ 320-22) 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.
(8)
Determine what management district the site falls into (Appendix
D[3]) and conduct an existing conditions runoff analysis.
[3]
Editor's Note: Appendix D is included at the end of this chapter.
(9)
Prepare a 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 (§ 320-25).
(11)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
A.
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. These measures are required to be consistent with § 320-12, and take advantage of utilizing any existing recharge areas.
B.
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 the applicant seeks relief due to site-specific limitations, the Township has the right to hire an independent professional, at the applicant's expense, to assess the applicant's claim. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from Subsection B(1)(c) depending on demonstrated site conditions and shall be the greater of the two volumes.
(1)
Infiltration BMPs shall meet the following minimum requirements:
(a)
Infiltration requirements:
[1]
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 Subsection B(1)(c)[1] or [2], depending upon demonstrated site conditions.
(b)
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:
[1]
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
[2]
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.
[3]
The infiltration facility shall be capable of completely infiltrating
the required retention (infiltration) volume within four days (96
hours).
[4]
Pretreatment shall be provided prior to infiltration.
(c)
The size of the infiltration facility shall be based upon the
following volume criteria:
[1]
NRCS curve number equation.
[a]
The NRCS runoff equation shall be utilized to calculate
infiltration requirements (I) in inches.
|
Equation 320-22.1
| |
|---|---|
|
I (Infiltration requirement, in inches) = (200/CN) - 2
|
|
Where:
| ||||
|---|---|---|---|---|
|
CN
|
=
|
SCS (NRCS) curve number of existing conditions contributing
to the infiltration facility.
| ||
|
This equation is displayed graphically in, and the infiltration
requirement can be determined from Figure 320-22.1.
| ||||
[b]
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 320-22.1. Infiltration requirement based upon NRCS
Curve Number.
|
[c]
The retention (infiltration) volume (Rev) required to meet the infiltration requirement would
therefore be computed as:
|
Equation 320-22.2
| ||
|---|---|---|
|
Rev =
|
(0.5 or I, whichever is greater)* impervious area (square feet)/(12
in/ft) = cubic feet
| |
|
Where:
| ||||
|---|---|---|---|---|
|
I
|
=
|
infiltration requirements (in inches)
| ||
[2]
Annual recharge: water budget approach.
[a]
If the goals of Subsection B(1)(c)[1] 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 320-22.1 or the curve in Figure 320-20.1 should be used to determine the infiltration requirement.
[b]
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
|
(2)
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 Township Engineer shall be notified a minimum
of two working days prior to the site evaluation so that the evaluation
may be witnessed by the Township Engineer or its designee. The general
process for designing the infiltration BMP shall be:
(a)
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.
(b)
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.
(c)
Design the infiltration structure for the required retention
(Rev) volume based on field determined capacity
at the level of the proposed infiltration surface.
(d)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the Township of Exeter
that the soils are conducive to infiltrate on the lots identified.
(3)
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 B(1). If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure 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 B-1 in Appendix B. If infiltration is not proposed, the calculated infiltration volume [Subsection B(1)] shall be treated by a facility which prevents the required volume of runoff from being discharged off site. 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.
(a)
Infiltration BMP loading rate percentages in Figure B-1 in Appendix
B shall be calculated as follows:
 |
(b)
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%
|
(c)
Soil thickness is to be measured from the bottom of any proposed
infiltration BMP. The effective soil thickness in Figure B-1 in Appendix
B is the measured soil thickness multiplied by the thickness factor
based on soil permeability, as follows:
|
Permeability Range
(inches/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
|
(d)
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 at the end of this chapter.
(4)
Stormwater hotspots.
(a)
Following is a list of examples of designated stormwater hotspots.
If a site is designated as a stormwater 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 (i.e., 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. It shall be noted that EPA's
NPDES stormwater program requires some industrial sites to prepare
and implement a stormwater pollution prevention plan. Examples of
stormwater 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
stormwater hotspots:
(c)
While large highways [i.e., those roads with 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.
(5)
Caution shall be exercised where infiltration is proposed in source
water protection areas as defined by the Township of Exeter or Water
Authority.
(6)
Infiltration facilities shall be used in conjunction with other innovative
or traditional stormwater control facilities that are found within
the PADEP State BMP Manual.
(7)
Caution shall be exercised where salt or chloride (e.g., 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.
(8)
The infiltration requirement in high-quality or exceptional-value
waters shall be subject to the Department's Chapter 93 Antidegradation
Regulations.
(9)
Dependent upon certain land use or stormwater hotspots, an impermeable
liner will be required in detention basins where the possibility of
groundwater contamination exists. Applicant may be required to submit
a detailed hydrogeologic investigation for review and approval by
the Township of Exeter.
(10)
The Township of Exeter 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.
(11)
For projects that disturb one acre or more, unless a stricter
setback is otherwise specified in the Zoning Ordinance,[2] the following minimum setback requirements for infiltration
facilities shall apply:
(a)
From water supply wells: 100 feet.
(b)
From building foundations: 10 feet downslope or 100 feet upslope.
(c)
From septic system drainfields: 50 feet.
(d)
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: 50 feet.
(e)
From the property line unless documentation is provided to show
all setbacks from wells, foundations and drainfields on the neighboring
property will be met: 100 feet.
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 § 320-22 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 Watershed:
|
Equation 320-23.1
| |
|---|---|
|
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 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. (Refer 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 required setback, to a minimum of 10 feet, at the discretion
of the Township after consideration of the water quality rating of
the stream in question and the need to protect it. If an existing
buffer is legally prescribed (i.e., deed, covenant, easement, etc.)
and it exceeds the requirements of this Part 2, the existing buffer
shall be maintained. The Township of Exeter may permit a smaller or
larger buffer width if desired, but the buffer width may not be less
than 10 feet. This does not include lakes, wetlands, or permitted
stream crossing.
E.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office must be provided to the Township of Exeter. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of § 320-23A.
A.
In addition to control of the water quality volume, in order to minimize
the impact of stormwater runoff on downstream streambank 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 condition
one-year storm takes a minimum of 24 hours to drain from the facility,
measured 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 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.
Division; standards for managing runoff.
(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 rates from each subarea in the Schuylkill River Watershed for the two-year through one-hundred-year design storms are shown in Table 320-25.1. Development sites located in each of the districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 320-25.1, unless a more stringent standard is imposed by Subsection A(2) below.
|
Table 320-25.1 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]
|
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%
| |||
[2]
Note: The Owatin Creek Watershed No. 515 (designated as C)
shall be added to the B District.
[1]
Editor's Note: Appendix D is included at the end of this chapter.
(2)
For
any site which proposes to discharge into an established stormwater
drainage conveyance system (i.e., a system of swales, ditches, channels,
pipes, conduits, culverts and storm sewers), the site may only discharge
at a rate that does not exceed the site's proportional share of the
flow capacity of the most restrictive element (e.g., a culvert) of
the downstream conveyance system. The site's proportional share of
the flow capacity is equivalent to the ratio of the proposed development
site acreage to the total watershed acreage served by the conveyance
system upstream of the most restrictive element. Therefore, if the
proposed development site is 10% of the total watershed area, the
discharge from the proposed development shall not exceed 10% of the
available downstream capacity at peak flow with travel time considered.
The fifty-year storm event shall be the design storm for this provision
unless a greater capacity is warranted due to consideration of public
safety.
B.
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)[3]and this section of this Part 2.
[3]
Editor's Note: Appendix D is included at the end of this chapter.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on the Schuylkill River Watershed Act 167 Stormwater
Management Plan that is available for inspection at the municipal
office. A copy of the Schuylkill River Watershed Act 167 Stormwater
Management Plan at a reduced scale is included in Appendix D.[4] 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]
Editor's Note: Appendix D is included at the end of this chapter.
D.
Sites located in more than one district. For a proposed development
site located within two or more stormwater management district category
subareas, the peak discharge rate from any subarea shall meet the
management district criteria for which the discharge is located, as
indicated in this section. 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.
E.
Off-site areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
F.
Site areas. Where the site area to be impacted by a proposed development
activity is less than 50% of the total site area, only the proposed
impact area shall be subject to the management district criteria.
In other words, unimpacted areas bypassing the stormwater management
facilities would not be subject to the management district criteria.
G.
Developed sites which are expanding or are being partially redeveloped
and currently have stormwater storage facilities which were approved
by the Township. The new impervious areas and redeveloped areas shall
comply with the provisions of this Part 2 while the existing impervious
area may continue to be controlled by the existing stormwater management
facilities providing that the existing stormwater management facilities
are functioning properly and no drainage problem exists. The design
engineer shall contact the Township Engineer prior to performing the
stormwater management calculation to determine the best approach to
demonstrating compliance with this Part 2.
H.
Developed sites which are expanding or are being partially redeveloped
and currently do not have stormwater storage facilities that were
approved by the Township. The new impervious areas and redeveloped
portions of the site shall comply with the provisions of this Part
2 while the existing impervious area may continue function without
the benefit of stormwater management facilities providing that no
drainage problems exist. If drainage problems do exist on the site
or at the point of stormwater discharge from the site, the correction
of the drainage problem to the satisfaction of the Township shall
be incorporated into the design for the new impervious area or separate
facilities shall be provided. The design engineer shall contact the
Township Engineer prior to performing the stormwater management calculation
to determine the best approach to demonstrating compliance with this
Part 2.
I.
Redevelopment of sites. Sites that are being redeveloped, as defined
in this Part 2, shall be in conformance with the provisions of this
Part 2.
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 320-26.1 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 less than 200 acres. The soil-complex
method shall be used for drainage areas greater than 200 acres.
|
Table 320-26.1
Acceptable Computation Methodologies For Stormwater Management
Plans
| |||
|---|---|---|---|
|
Method*
|
Method Developed By
|
Applicability
| |
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
| |
|
Rational method (or commercial computer package based on rational
method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres and with time of concentration
less than 60 minutes (tc less than 60 minutes), or as approved by
the Township of Exeter
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township of
Exeter
| |
|
*
|
Note: Successors to the above methods are also acceptable. These
successors include WinTR-55 for TR-55 and WinTR-20 for TR-20 and SWMM.
|
B.
All calculations consistent with this Part 2 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 B-1 in Appendix B of this Part
2.[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 at the end of this chapter.
C.
For the purposes of existing conditions flow rate determination,
undeveloped land and redeveloped land shall be considered as "meadow"
in good condition, unless the natural ground cover has a curve number
or rational "C" value (i.e., forest) lower than that for meadow, as
listed in Table B-1 or B-2 in Appendix B of this Part 2.
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 as acceptable method for Tc in undeveloped
areas.
E.
Time of concentration shall be based on the following design parameters:
(1)
Sheet flow. The maximum length for each reach of sheet or overland
flow before shallow concentrated or open channel flow develops is
150 feet. Flow lengths greater than 100 feet shall be justified based
on the actual conditions at each development site. The maximum construction
phase and postdevelopment sheet flow length for unpaved surfaces shall
be 100 feet for most situations (150 feet for areas which will remain
undisturbed).
(3)
Open channel flows. At points where sheet and shallow concentrated
flows concentrate in field depressions, swales, gutters, curbs, or
pipe collection systems, the travel times and downstream end of the
development site between these design points shall be based upon Manning's
equation and/or acceptable engineering design standards as determined
by the Township Engineer.
F.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the soil-cover complex method shall be obtained from
Table B-1 in Appendix B of this Part 2.[3]
[3]
Editor's Note: Appendix B is included at the end of this chapter.
G.
Runoff coefficients (c) for both existing and proposed conditions
for use in the rational method shall be obtained from Table B-2 in
Appendix B of this Part 2.[4]
[4]
Editor's Note: Appendix B is included at the end of this chapter.
H.
Where uniform flow is anticipated, the Manning's 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 B-3 in Appendix B of
this Part 2.[5] Full flow shall be assumed for closed conduits.
[5]
Editor's Note: Appendix B is included at the end of this chapter.
I.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part 2 using any generally
accepted hydraulic analysis technique or method.
J.
The design of any stormwater detention facilities intended to meet
the performance standards of this Part 2 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 Exeter 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.
K.
During the construction progression of a project, the discharge of
stormwater runoff from the site may follow distinct patterns that
will be altered during the course of the construction. In such an
event, the comparison of the construction phase discharge rate to
the predevelopment rate shall be performed for each separate and distinct
pattern. At no time throughout the construction phase of a project
shall the peak discharge rate discharging from any location along
the boundary of a site exceed the predevelopment peak discharge rate
at that location.
L.
Where applicable, construction phase and postdevelopment time of
concentration to a storage facility shall be the time of concentration
in the storm sewer to its point of discharge into the storage facility.
Overland flow from the discharge point to the outlet structure shall
not be included in the time of concentration.
M.
When designing regional storage facilities for office parks, industrial
parks, etc., the following design criteria shall be followed:
(2)
A six-minute time of concentration shall be used to calculate the
postdevelopment basin inflow hydrographs (five minutes for the rational
method). Longer times of concentration shall be allowed for the portions
of the drainage area that will remain undeveloped or where it can
be demonstrated that longer times will occur in the developed area
after full potential development occurs. The hydrographs for the developed
and undeveloped portions of the drainage area shall be combined prior
to routing through the stormwater facility.
(3)
For lots which will have on lot detention or retention basins, the
predevelopment time of concentration for the lot shall be the same
as the predevelopment time of concentration for the entire predevelopment
drainage area in which it is located.
A.
General regulations.
(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 Exeter.
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, which must be promptly submitted by the applicant.
(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)
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.
(4)
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).
(5)
The Township of Exeter reserves the right to deny or reject any design
that would result in the construction of or continuation of a stormwater
problem area.
B.
Detention and retention basins. 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:
(1)
All wet basin designs shall incorporate biologic minimization controls
consistent with the West Nile Guidance found in Appendix F.[1]
[1]
Editor's Note: Appendix F is included at the end of this chapter.
(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 shall 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 homeowner's
association.
(4)
Basins with earthen embankments shall be designed with an emergency
spillway that has capacity and stability to accommodate the one-hundred-year
postdevelopment 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 and the use of
a concrete spillway maybe required in areas where it is constructed
in fill.
(5)
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 postdevelopment flow passes through the emergency
spillway. Embankment heights shall not exceed 15 feet, unless approved
by the Township of Exeter.
(6)
The minimum top width of earthen embankments shall be equal to 3/4
of the embankment height, but in no case shall the top width be less
than eight feet.
(7)
Side slopes shall be kept as close to the natural land contours as
practical. Interior and exterior embankment side slopes shall not
be greater than three horizontal to one vertical. Depending upon the
location and the intended use of the detention facilities during nonfunctioning
times, a side slope of less than three horizontal to one vertical
may be required by the Board of Supervisors. When a detention basin
is to be maintained by a future residential lot owner, the maximum
side slope shall be four horizontal to one vertical.
(8)
All pipes, conduits, etc., passing through basin embankments shall
have properly spaced concrete antiseep collars. Minimum collar projection
beyond the pipe shall be two feet, maximum collar spacing is 14 times
the projection, and the minimum thickness shall be 12 inches. The
joint between collar and pipe shall be watertight.
(9)
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 shall be calculated and considered when performing
the detention facility routing calculations. The tailwater elevation
shall be based on the frequency of the storm being routed.
(10)
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) shall be provided
within all basin embankments. The 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 four 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 twenty-five-year
design year water surface elevation.
(11)
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 Exeter upon request.
Embankments shall be constructed a minimum of six inches above design
elevations to allow for settlement.
(12)
All basins not including groundwater recharge and/or water quality
volume 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.
(13)
All discharge control devices (outlet structures) 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.
(14)
All basins employing small orifices (i.e., orifices 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.
(15)
All outlet structures in above ground detention/retention basin
shall have the top of grate set at least three inches lower than the
invert of the emergency spillway to allow discharge through the outlet
structure and associated outlet barrel/pipes prior to discharge through
the emergency spillway.
(16)
Outlet structures for underground detention facilities shall
be provided within a reinforced concrete junction box or inlet box.
Flows through the outlet structure 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 postdevelopment
inflow without surcharging the junction box.
(17)
Underground detention facilities that propose pipe/culvert storage
as a means of detention shall be constructed of reinforced concrete
or smooth-lined corrugated high-density polyethylene pipe (HDPE),
with watertight joints meeting ASTM D-3212. Where installation conditions
merit, structural calculations that address the actual design requirements
will be required.
(18)
Underground detention and infiltration facilities utilizing
pipe, box culverts or chambers as the means of storage and located
in areas receiving vehicular traffic shall be designed to carry earth
dead and AASHTO HS20 live load conditions. Under warranted conditions,
AASHTO HS25 shall be the design live load.
(19)
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 one-percent 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.
(20)
The minimum slope within a basin that does not include groundwater
recharge and/or water quality storage shall be two-percent positive
grade to the low-flow channel.
(21)
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.
(22)
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.
(23)
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 Exeter may require the installation
of an impermeable liner in detention basins.
(24)
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 waterproofing and/or floodproofing design information
shall be submitted for approval.
(25)
In addition to the above, wet ponds shall be designed in accordance
with the design parameters as found in the Pennsylvania Department
of Environmental Protection, Pennsylvania 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
Exeter.
(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.
C.
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:
(1)
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.
(2)
When designing storm sewers that discharge into a detention/retention
facility, a tailwater condition at the storm sewer discharge point
shall be considered where applicable. The tailwater elevation shall
be equal to the detention facility water surface elevation resulting
from the design storm that was used to design the storm sewer.
(3)
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.
(4)
Storm sewer pipes, to be constructed within the right-of-way of existing
streets or streets offered for dedication to the Township of Exeter,
other than those used as roof drains, detention basin underdrains,
and street subbase underdrains, shall have a minimum diameter of 15
inches and be Class III reinforced concrete pipe (RCP). 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. Where installation conditions merit (e.g., deep fills),
structural calculations that address the actual design requirements
will be required.
(5)
Storm sewer pipes, to be constructed on private property or streets
not offered for dedication to the Township of Exeter, other than those
used as roof drains, detention basin underdrains, and street subbase
underdrains, shall have a minimum diameter of 15 inches and be 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. Where installation
conditions merit (e.g., deep fills), structural calculations that
address the actual design requirements will be required.
(6)
Storm sewer pipes and culverts shall be installed on sufficient slopes
to provide a minimum velocity of three feet per second when flowing
full; however, a minimum slope of not less than 0.005 feet/feet shall
be provided.
(7)
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.
(8)
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.
(9)
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.
(10)
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. PA 2A stone placed in eight inches
lifts and compacted to 95% of maximum dry density, as per PennDOT
Publication 408, latest edition, shall complete the trench to pavement
subgrade. Compaction test results shall be provided to the Township
of Exeter upon request. Select backfill shall complete the trench
to topsoil layer in grassed areas.
(11)
Curves, tees, elbows and wyes are not permitted in pipes other
than those used as roof drains, detention basin underdrains, and street
subbase underdrains. Manholes or inlets are required to facilitate
such configurations.
(12)
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.
(13)
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 one-tenth foot shall be provided
from the inlet invert elevation(s) to the outlet invert elevation.
(14)
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 Exeter. Manhole covers shall
have the word "STORM" cast clearly on the lid.
(15)
Inlets along curbed streets shall be spaced to limit gutter
spread to maximum of 1/2 of the width of the travel lane during a
ten-year storm. Inlet capacities and bypass calculations shall be
determined by PennDOT design techniques.
(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)
Inlets shall be located at curb tangents on the uphill side of street intersections and at other locations as may be required by the Township 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.
(18)
Storm sewer inlets proposed to be sumped shall be a minimum
of six feet long.
(19)
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.
(20)
Inlets shall be designed and located to prevent hazards to vehicles,
bicycles and pedestrians.
(21)
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 Engineer.
(22)
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 Exeter.
(23)
Adequate erosion and sediment control protection shall be provided
along all open channels, and at all points of discharge.
(24)
Stormwater roof drains, sump pumps, and pipes installed after
the effective date of this Part 2 shall not directly discharge water
into a street right-of-way or discharge into a sanitary sewer or storm
sewer unless approval is given by the Township.
(25)
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 Exeter and any other necessary approving body.
(26)
Flow velocities from any storm sewer may not result in erosion
of the receiving channel.
(27)
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 PADEP Erosion and
Sediment Pollution Control Manual, as amended.
(28)
Drainage facilities, including detention ponds, shall not convey
water onto any existing or proposed public road right-of-way.
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, 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 to detention/retention facilities
and from emergency spillways and areas where damage to property would
result.
(3)
Open channels located adjacent to streets in residential subdivision
and/or developments 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,
a 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 ft/ft
nor greater than 0.100 ft/ft.
E.
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 § 320-25.
(1)
Infiltration testing shall be conducted in accordance with § 320-22B(2).
(2)
A minimum depth of 36 inches shall be provided between the bottom
of the infiltration facility and the limiting zone.
(3)
The infiltration and/or percolation rate shall be sufficient to accept
the additional stormwater load and drain completely as determined
by field tests conducted by the applicant's design professional.
(4)
The infiltration facility shall be capable of completely infiltrating
the required retention/detention volume within 24 hours.
(5)
Pretreatment shall be provided prior to infiltration.
(6)
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 § 320-25. This is in consideration of loss of function during freezing weather.
(7)
When runoff is piped underground to the infiltration facility; 50% of the detention volume required to satisfy § 320-25 may be infiltrated, providing the following conditions are also met:
(a)
There exists at least 18 inches between the top of the infiltration
facility and the ground surface.
(b)
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.
(c)
The point of discharge of the emergency overflow shall be in
accordance with the provisions of this Part 2. The preferred point
of discharge is into a detention or retention facility.
F.
Any infiltration and water quality facilities proposed to meet the requirements of §§ 320-22 and 320-23 shall 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 shall be in accordance with the guidelines provided in the Pennsylvania Stormwater Best Management Practices Manual, latest edition.
G.
Construction of streets.
(1)
All streets shall be designed 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.
