[Ord. 878, 5/12/2014]
1.
Applicants proposing regulated activities in the municipality which are not exempt under § 19-106 shall submit an SWM site plan to the municipality for review and approval in accordance with Parts 3 and 4. SWM site plans approved by the municipality shall be on site throughout the duration of the regulated activity.
2.
The stormwater management and runoff control criteria and standards
in this chapter shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity, even if the development is to take place
in stages.
3.
No regulated activity within the municipality shall commence until:
A.
The municipality issues approval of an SWM site plan, which demonstrates
compliance with the requirements of this chapter; and
B.
The applicant has received a letter of adequacy or approval for the
erosion and sediment control plan review by the municipality and the
Conservation District (if required) and has received all other local,
state and federal permit approvals required for the project involving
the regulated activity.
4.
Neither submission of an SWM site plan under the provisions herein
nor compliance with the provisions of this chapter shall relieve any
person from responsibility for damage to any person or property otherwise
imposed by law.
5.
The applicant shall design the site to minimize disturbances to land, site hydrology and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 19-304 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
6.
To the maximum extent practicable, post-construction stormwater shall
be discharged within the drainage area of the same stream or water
body receiving the runoff prior to construction of the proposed regulated
activity.
7.
For regulated activities with one acre or more of proposed earth
disturbance, existing drainage peak rate discharges up to and including
the one-hundred-year storm onto or through adjacent property(ies)
or downgradient property(ies), including diffuse drainage discharge,
shall not be altered in any manner without written permission from,
and, where applicable as determined by the municipality, an easement
and agreement with, the affected landowner(s) for conveyance of discharges
onto or through their property(ies). Such discharge shall be subject
to any applicable discharge criteria specified in this chapter.
A.
For regulated activities with one acre or less of proposed earth
disturbance, the applicant shall provide written notification to the
affected landowner(s) describing the proposed regulated activity and
proposed discharge(s), unless otherwise required by the municipality.
8.
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in §§ 19-305, 19-306, 19-307 and 19-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
9.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-year storm event
peak runoff rate reduction standards for new development required
by this chapter, the applicant shall propose alternate methods to
mitigate the bypass of the BMPs, subject to the approval of the Municipal
Engineer. In no case shall resulting peak rate be greater than the
predevelopment peak rate for the equivalent design storm.
10.
For all regulated activities, erosion and sediment control BMPs shall
be designed, implemented, operated, and maintained during the regulated
activities (i.e., during construction) as required to meet the purposes
and requirements of this chapter, to meet the erosion and sediment
control requirements of the municipality, if applicable, and to meet
all requirements under Title 25 of the Pa. Code and the Clean Streams
Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
11.
For all regulated activities, permanent BMPs and conveyances shall
be designed, implemented, operated, and maintained to meet the purposes
and requirements of this chapter and to meet all requirements under
Title 25 of the Pa. Code, the Clean Streams Law, and Act 167.
12.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the municipality. The municipality
reserves the right to disapprove any design that would result in construction
in an area affected by existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
13.
Existing wetlands, either on the site or on an adjacent property,
shall not be used to meet the minimum design requirements for stormwater
management or stormwater runoff quality treatment. Stormwater discharges
to existing wetlands shall not degrade the quality or hydrologic integrity
of the wetland.
14.
Hotspot Runoff Controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Municipal
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
A.
Vehicle salvage yards and recycling facilities;
B.
Vehicle fueling stations;
C.
Vehicle service and maintenance facilities;
D.
Vehicle and equipment cleaning facilities;
E.
Fleet storage areas (bus, truck, etc.);
F.
Industrial sites based on Standard Industrial Classification codes;
G.
Marinas (service and maintenance areas);
H.
Outdoor liquid container storage;
I.
Outdoor loading/unloading facilities;
J.
Public works storage areas;
K.
Facilities that generate or store hazardous materials;
L.
Commercial container nursery;
M.
Contaminated sites/brownfields;
N.
Other land uses and activities as designated by the municipality.
15.
Contaminated and Brownfield Sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Municipal Engineer's discretion,
the minimum infiltration requirement may be reduced or eliminated
commensurate with the contaminated area and the required water quality,
and runoff control measures may be increased to mitigate the reduced
infiltration requirement for the contaminated area.
16.
Additional Water Quality Requirements. The municipality may require
additional stormwater control measures for stormwater discharges to
special management areas, including, but not limited to:
A.
Any water bodies listed as "impaired" by PADEP.
B.
Any water body or watershed with an approved total maximum daily
load (TMDL).
C.
Any areas of known existing flooding problems.
D.
Any critical areas with sensitive resources (e.g., state-designated
special-protection waters, cold-water fisheries, carbonate geology
or other groundwater recharge areas that may be highly vulnerable
to contamination, drainage areas to water supply reservoirs, etc.).
17.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the Municipal Engineer, for testing and design standards for BMPs;
and where there is a conflict with the provisions of this chapter,
the most restrictive applies.
18.
For areas underlain by karst or carbonate geology that may be susceptible
to the formation of sinkholes and other karst features, the location,
type, and design of infiltration BMPs shall be based on a site evaluation
conducted by a qualified licensed professional and based on the PA
BMP Manual or other design guidance acceptable to the Municipal Engineer.
19.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with any applicable floodplain management provisions enacted by the Borough of Oxford, including, but not limited to, Part 17, Floodplain Management, § 27-1701 et seq., as amended, of Chapter 27, Zoning, of the Code of the Borough of Oxford, and shall be designed to maintain the flood-carrying capacity of the floodway such that the base flood elevations are not increased, either upstream or downstream. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.
20.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of the top-of-bank
of all perennial and intermittent waterways, water bodies (lakes,
ponds, etc.) and wetlands, except for activities otherwise approved
by state or local agencies (e.g., stream restoration projects, road
crossings, subsurface utility projects, etc.). At the Municipal Engineer's
discretion, and with Conservation District and PADEP approval where
necessary, the nondisturbance buffer may be reduced because of setback
or other site constraints, but never be less than 10 feet.
[Ord. 878, 5/12/2014]
1.
The following permit or other regulatory requirements may apply to
certain regulated activities and shall be met prior to (or as a condition
of) final approval by the municipality of the SWM site plan and prior
to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the municipality.
B.
Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under 25 Pa. Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
[Ord. 878, 5/12/2014]
1.
No regulated activity within the municipality shall commence until:
A.
The municipality receives documentation that the applicant has received:
(1)
A "letter of adequacy" from the Conservation District or other
approval from PADEP in compliance with 25 Pa. Code Chapter 102 for
an erosion and sediment control plan for construction activities,
if applicable;
(2)
A PADEP NPDES construction activities permit as required under
25 Pa. Code Chapter 92, if applicable; and
(3)
Evidence of any other permit(s) or approvals required for the
regulated activities.
B.
An erosion and sediment control plan has been approved by the municipality,
if required.
2.
A copy of the erosion and sediment control plan and any required
permit(s), as required by PADEP regulations, shall be available on
the site at all times.
[Ord. 878, 5/12/2014]
1.
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall demonstrate in its SWM site plan (as required in § 19-402, Subsection 1C) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
B.
Second, provide a prioritized listing of these resources and features
to identify:
(1)
Those to be incorporated into the site design in a manner that
provides protection from any disturbance or impact from the proposed
regulated activity;
(2)
Those to be protected from further disturbance or impact but
for which the proposed regulated activity will provide improvement
to existing conditions;
(3)
Those that can be incorporated into and utilized as components
of the overall site design in a manner that protects or improves their
existing conditions while utilizing their hydrologic function within
the limits of their available capacity (e.g., for infiltration, evapotranspiration,
or reducing pollutant loads, runoff volume or peak discharge rates,
etc.) to reduce the need for or size of constructed BMPs; and
(4)
Those that may be considered for alteration, disturbance or
removal.
C.
Third, develop the site design to achieve the following:
(2)
Minimize earth disturbance (both surface and subsurface);
(3)
Maximize protection of or improvement to natural resources and
special management areas;
(4)
Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(6)
Maximize infiltration and the use of natural site infiltration
features, patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading
patterns or preserve existing topography in order to evenly distribute
runoff and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open-channel conveyance
techniques that provide infiltration and water quality benefits, and
landscape-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of § 19-304, Subsection 1B and C. Appendix A presents additional discussion of natural hydrology site design and sources of information for conservation design, low-impact design, and sustainable design.[1]
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
[Ord. 878, 5/12/2014]
1.
To control post-construction stormwater impacts from regulated activities
and meet state water quality requirements, BMPs shall be provided
in the site design that replicate predevelopment stormwater infiltration
and runoff conditions, such that post-construction stormwater discharges
do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The applicant shall comply with the following
water quality and runoff volume requirements for all regulated activities,
including all new development and redevelopment activities:
A.
The post-construction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation (design storm). The water
quality and runoff volume to be managed shall consist of any runoff
volume generated by the proposed regulated activity over and above
the predevelopment total runoff volume and shall be captured and permanently
retained or infiltrated on the site. Permanent retention options may
include, but are not limited to, reuse, evaporation, transpiration,
and infiltration.
C.
The design of the facility outlet shall provide for protection from
clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 19-306. The infiltration volume required under § 19-306 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
H.
When the regulated activity contains or is divided by multiple drainage
areas, the water quality and runoff volume shall be separately addressed
for each drainage area.
I.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
J.
Areas located outside of the site (i.e., areas outside of the regulated
activity) may be excluded from the calculation of the water quality
and runoff volume requirements.
[Ord. 878, 5/12/2014]
1.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 19-305.
B.
For regulated activities involving new development, the volume of
a minimum of one inch of runoff from all proposed impervious surfaces
shall be infiltrated.
D.
If the requirements of § 19-306, Subsection 1B or C, cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however, in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.
E.
Only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site shall a waiver from § 19-306 be considered by the municipality.
F.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
G.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
H.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
I.
A detailed soils evaluation of the site shall be conducted by a qualified
professional and, at a minimum, shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Municipal Engineer) and in general shall:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(2)
Provide field tests, such as double-ring infiltrometer or other
hydraulic conductivity tests (at the elevation of the proposed infiltration
surface), to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention
(infiltration) volume based on field-determined infiltration capacity
(and apply safety factor as per applicable design guidelines) at the
elevation of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall
be demonstrated to the Municipal Engineer that the soils are conducive
to infiltration on the identified lots.
J.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention
(infiltration) volume within three days 72 hours) from the end of
the design storm.
K.
All infiltration practices shall:
(2)
Be set back at least 25 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
unless otherwise approved by the Municipal Engineer;
(3)
For any infiltration practice that collects runoff from shared
or multiple features and that is located within 50 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
L.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a hotspot is located in the area draining to a proposed
infiltration facility, an evaluation of the potential of groundwater
contamination from the proposed infiltration facility shall be performed,
including a hydrogeologic investigation (if necessary) by a qualified
licensed professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a "wellhead protection area" of a public
water supply well, infiltration practices shall be in conformance
with the applicable approved source water protection assessment or
source water protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
M.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Municipal
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
N.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation is provided to the municipality demonstrating that the
potential or likelihood of subsidence or sinkholes is minimal. Evaluation
of site conditions and infiltration design shall rely on guidance
in the PA BMP Manual (as amended) or other guidance acceptable to
the Municipal Engineer.
O.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
P.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
Q.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include, but are not limited to, leaf traps, gutter guards and cleanouts.
R.
All infiltration practices shall have appropriate positive overflow
controls.
S.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
T.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or
like facilities shall be covered with drainage fabric. Fabric shall
be nonwoven fabric acceptable to the Municipal Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage
bed/trench or like facility, and provisions for the collection of
debris shall be provided in all facilities.
[Ord. 878, 5/12/2014]
1.
For regulated activities involving new development with one or more
acres of earth disturbance, the applicant shall comply with the following
stream channel protection requirements to minimize stream channel
erosion and associated water quality impacts to the receiving waters:
A.
The peak flow rate of the post-construction, two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour duration precipitation, using the
SCS Type II distribution.
B.
To the maximum extent practicable, and unless otherwise approved
by the Municipal Engineer, the post-construction, one-year, twenty-four-hour
storm flow shall be detained for a minimum of 24 hours and a maximum
not to exceed 72 hours from a point in time when the maximum volume
of water from the one-year, twenty-four-hour storm is stored in a
proposed BMP (i.e., when 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 orifice is at the invert of the proposed
BMP).
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter, unless otherwise approved by the Municipal
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the 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.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates, subject
to the Municipal Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
[Ord. 878, 5/12/2014]
1.
The applicant shall comply with the following peak flow rate control
requirements for all regulated activities, including those that involve
new development and redevelopment.
A.
Post-construction peak flow rates from any regulated activity shall
not exceed the Predevelopment peak flow rates as shown for each of
the design storms specified in Table 308.1.
|
Table 308.1
| |||
|---|---|---|---|
|
Peak Rate Control Standards
| |||
|
(Peak Flow Rate of the Post-Construction Design Storm
Shall be Reduced to the Peak Flow Rate of the Corresponding Predevelopment
Design Storm Shown in the Table)
| |||
|
Post-Construction Design Storm Frequency
(24-hour duration)
|
Predevelopment Design Storm
| ||
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| ||
|
2-year
|
1-year
|
2-year
| |
|
5-year
|
5-year
|
5-year
| |
|
10-year
|
10-year
|
10-year
| |
|
25-year
|
25-year
|
25-year
| |
|
50-year
|
50-year
|
50-year
| |
|
100-year
|
100-year
|
100-year
| |
C.
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total proposed impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 308.1, using the redevelopment ground cover assumptions presented in § 19-309, Subsection 4.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
[Ord. 878, 5/12/2014]
1.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 309.1 summarizes acceptable computation methods.
The method selected for use shall be based on the individual limitations
and suitability of each method for a particular site. The use of the
Rational Method to estimate peak discharges for drainage areas greater
than five acres shall be permitted only upon approval by the Municipal
Engineer.
|
Table 309.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for SWM Site Plan
| |||
|
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 where limitations described
in TR-55 are met
| |
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
| |
|
Rational Method
(or commercial computer package based on Rational Method)
|
Emil Kuichling (1889)
|
For sites up to 5 acres, or as approved by the municipality
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
| |
2.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return-period storms
consistent with this chapter. Rainfall depths used shall be obtained
from NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or infiltration, water quality and runoff volume functions, the duration
of rainfall shall be 24 hours.
3.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates, or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times-of-concentration shall
be calculated based on the methodology recommended in the respective
model used. Times-of-concentration for channel and pipe flow shall
be computed using Manning's Equation.
4.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
A.
For regulated activities involving new development, the following
ground cover assumptions shall be used:
B.
For regulated activities involving redevelopment, the following ground
cover assumptions shall be used:
(1)
For areas that are woods (as defined in Part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(2)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(3)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as "meadow" ground cover.
C.
The applicant shall determine which stormwater standards apply to
the proposed regulated activity as follows:
(1)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(2)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(3)
At the discretion of the Municipal Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
(a)
Apply the stormwater standards (redevelopment or new development)
that are associated with the activity that involves the greatest amount
of land area; or
(b)
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
5.
Runoff CN for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table B-1 in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
6.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
B-2 in Appendix B of this chapter.[2]
[2]
Editor's Note: Appendix B is included as an attachment to this chapter.
7.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
8.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended).
Hydraulic computations to determine the capacity of open channels
shall be consistent with methods and computations contained in the
Federal Highway Administration Hydraulic Engineering Circular Number
15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with
Table B-3 in Appendix B of this chapter.[3]
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
[Ord. 878, 5/12/2014]
1.
Any BMP intended to hold standing water for four days or longer shall
be designed to incorporate biologic controls consistent with the West
Nile Guidance found in Appendix C, PADEP Document 363-0300-001 "Design
Criteria - Wetlands Replacement/Monitoring" (as amended),[1] or contact the Pennsylvania State Cooperative Wetland
Center or the Penn State Cooperative Extension Office for design information.
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
2.
Any stormwater basin required or regulated by this chapter designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to safely convey flow up
to and including the one-hundred-year proposed conditions. The height
of embankment shall provide a minimum one foot of freeboard above
the maximum pool elevation computed when the facility functions for
the one-hundred-year proposed conditions inflow. Should any BMP require
a dam safety permit under 25 Pa. Code Chapter 105 regulations, the
facility shall be designed in accordance with and meet the regulations
of 25 Pa. Code Chapter 105 concerning dam safety. In addition, 25
Pa. Code Chapter 105 may require the safe conveyance of storms larger
than the one-hundred-year event.
3.
Any drainage conveyance facility and/or channel not governed by 25
Pa. Code Chapter 105 regulations shall be designed to convey, without
damage to the drainage facility or roadway, runoff from the twenty-five-year
storm event. Larger storm events (fifty-year and one-hundred-year
storms) shall also be safely conveyed in the direction of natural
flow without creating additional damage to any drainage facilities,
nearby structures, or roadways.
4.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
5.
Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a one-hundred-year design
storm consistent with Federal Emergency Management Agency National
Flood Insurance Program floodplain management requirements.
6.
Any facility located within a PennDOT right-of-way shall comply with
PennDOT minimum design standards and permit submission and approval
requirements.
7.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Municipal Engineer.
[Ord. 878, 5/12/2014]
1.
Need for Basins, Storm Sewers, Culverts, Bridges and Other Stormwater
Facilities.
A.
Basins, storm sewers, culverts, bridges and other structural installations
shall be provided to safely accommodate stormwater, where natural
nonstructural practices are not feasible and where stormwater flows
otherwise would have adverse impact on the environment and the general
welfare of the Borough of Oxford and its citizens, in order to:
(1)
Maintain natural hydrologic regimes of streams and watercourses.
Such flows may be redirected as required, subject to the approval
of PADEP.
(2)
Promote drainage of all low points along the line of streets.
Overflow swales shall be designed to convey the full one-hundred-year
storm flows away from all street low points. These swales shall be
located to prevent flooding of the downslope lots.
(3)
Intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained, and to prevent
substantial flow of water across intersections or flooding of intersections
during the design storm stipulated elsewhere in this chapter.
(4)
Ensure adequate and unimpeded flow of stormwater under driveways
in, near, or across natural watercourses or drainage swales. Suitable
pipes or other waterway openings shall be provided as necessary.
2.
Design of Basins, Storm Sewers, Culverts, Bridges and Other Stormwater
Facilities.
A.
Stormwater basins shall be installed prior to any earthmoving or
land disturbances that they will serve, unless otherwise approved
by the Borough Engineer. The phasing of their construction shall be
noted on the erosion and sediment control plan. Permanent vegetation
shall be established prior to denuding any other land, unless the
basin functions as an erosion and sediment control measure.
B.
Stormwater management facilities shall be designed to provide for
stormwater infiltration wherever feasible. For any facilities proposed
to be underlain by karst or carbonate geology, a detailed geologic
evaluation of the project site shall be performed to determine the
suitability for infiltration, including both the potential for groundwater
contamination and potential for sinkhole formation. The evaluation
shall be performed by a qualified geologist and/or soil professional
and, at a minimum, address soil permeability, depth to bedrock, susceptibility
to sinkhole formation, and subgrade stability. Soils used for the
construction of basins shall have low-erodibility factors (K factors).
The Borough may require an impermeable liner to be installed up to
the one-hundred-year design water surface elevation for any proposed
stormwater management facilities that may be susceptible to causing
sinkhole formation and/or groundwater contamination.
C.
Energy dissipaters and/or level spreaders shall be installed at points
where pipes or drainageways discharge from stormwater management facilities.
Multiple outlet structures and multiple outlet piping from the facility
may be required by the Borough to reduce the impact of point discharges.
D.
The following slope restrictions shall apply to stormwater basins:
(1)
Exterior slopes of compacted soil shall not exceed one-foot
vertical for three feet horizontal and may be further reduced if the
soil has unstable characteristics.
(2)
Interior slopes of the basin shall not exceed one-foot vertical
in three feet horizontal except with approval of the Borough and:
(a)
Where maximum water depth will not exceed three feet; or
(b)
When a two-inch rainfall in one hour will not exceed the capacity
of the basin in one hour; or
(c)
Where concrete, stone or brick walls are used with side slopes
proposed to be steeper than one-foot vertical in three feet horizontal,
in which case the basin shall be fenced by a permanent fence 42 inches
in height and a ramp of durable, nonslip materials for maintenance
vehicles shall be provided for access into the basin.
(3)
The minimum bottom slope shall be 3% for grass and 0.75% for
concrete paving. One percent may be used for grass if an underdrain
system is provided. A concrete low-flow channel may be required for
basins where the distance from the inlet pipe to the outlet structure
exceeds 100 feet, depending upon site conditions. The minimum channel
width shall be four feet. The channel shall be constructed of four-inch-thick
concrete (3,300 psi, twenty-eight-day strength) over four inches of
PennDOT No. 2A stone.
E.
Stormwater basins shall also be designed to meet the following requirements:
(1)
The maximum berm height shall be 15 feet, unless otherwise approved
by the Borough and the PADEP.
(2)
The minimum top-of-berm width shall be eight feet.
(3)
Outlet pipes shall have a minimum diameter of 12 inches, except
for pipe lengths exceeding 100 feet, in which case the minimum diameter
shall be 15 inches.
(4)
Properly spaced antiseep collars (six inches' minimum thickness)
shall be installed on all basin outlet pipes. Design calculations
shall be provided.
(5)
All berms shall be constructed with a compacted, relatively
impervious (Unified Soil Classification CL-ML or CL) key trench and
core. The key trench shall extend at least two feet into undisturbed
subsoil (below topsoil layer). The minimum bottom width of the trench
shall be four feet, and the minimum top width of the core shall be
eight feet. The side slopes of the compacted core and trench shall
not exceed one horizontal to one vertical, and the top elevation of
the core shall be set at or above the twenty-five-year design water
elevation.
(6)
The top of berm shall be constructed at least six inches above
the design elevations to allow for settlement of the embankment.
F.
Stormwater basin outlet structures and emergency spillways:
(1)
Outlet structures within basins which will control peak discharge
flows and distribute the flows by pipes to discharge areas shall be
constructed primarily of concrete or masonry material and shall have
childproof, nonclogging trash racks over all design openings, except
those openings designed to carry perennial stream flows. Trash rack
material should be epoxy-coated galvanized or stainless steel. Other
materials are subject to the approval of the Borough.
(2)
Six inches of freeboard shall be provided between the crest
of the primary outlet structure and the invert of the emergency spillway.
(3)
Emergency spillways shall be constructed in undisturbed earth
wherever possible. When constructed in fill, sod, precast concrete
paving blocks, concrete or permanent erosion-control matting shall
be used. Design calculations shall be submitted indicating that the
specified material can withstand velocities based on the one-hundred-year
design storm event. When using sod, it shall be applied along the
inside slope above the twenty-five-year water surface elevation, along
the face and sides of the spillway and down the outside slope to existing
grade. Emergency spillways shall be designed to safely convey the
one-hundred-year basin inflow hydrograph through the basin assuming
the principal outlet is completely blocked and the basin water surface
elevation is equal to the spillway invert elevation.
G.
Stormwater basin inlet and outlet structures shall be located at
maximum distances possible from one another. The Borough may require
a rock filter berm or rock-filled gabions between inlet and outlet
areas when the distance is deemed insufficient for sediment trappings.
H.
Permanent grasses or stabilization measures shall be established
on the sides of all earthen basins by hydroseeding within five days
of initial construction (or conversion from sediment basin or sediment
trap). The Borough may require jute or erosion-control matting to
be installed inside the basin or on the basin embankment.
I.
Stormwater runoff shall discharge to a suitable natural drainagecourse
or storm sewer system. Where not possible or not permitted, level
spreading devices or other suitable facilities shall be designed with
sufficient capacity to convey the one-hundred-year storm event without
creating any safety flooding or property hazard. Securing of necessary
stormwater management easements for this purpose shall be the sole
responsibility of the applicant.
J.
The Borough may require soil samples from the site to be analyzed
to determine if these soils are suitable for berm embankment construction.
If the soils are found to be unsuitable, the applicant shall import
suitable soils for constructing any proposed berm embankments.
K.
Grass swales not specifically designed as BMP devices shall have
a minimum bottom slope of 2%.
L.
Storm sewer design:
(1)
Design for conveyance systems shall be based upon a twenty-five-year
storm event (minimum).
(2)
Storm sewer lines within street rights-of-way shall be placed
immediately in front of the curb when parallel to the right-of-way.
Locating storm sewers under curbs, in curves or at street intersections
shall be avoided.
(3)
Storm sewers shall have a minimum diameter of 15 inches and
only smooth-lined, high-density polyethylene pipe or reinforced cement
concrete pipe shall be used, unless otherwise approved by the Borough
Engineer. The minimum grade of the pipe shall be 0.50%.
(4)
In karst or carbonate areas, watertight pipe connections are
required, and appropriate specifications shall be included on the
plans.
(5)
Headwalls, endwalls, or end sections shall be required on all
open pipes.
(6)
All storm sewer components (pipes, inlets, manholes, etc.) shall
be constructed per current PennDOT standards and criteria as outlined
in Publication 408 (Specifications), Publication 13M (Design Manual
Part 2, Highway Design) and Publication 72M (Roadway Construction
Standards), latest versions.
(7)
All storm sewer inlets in paved areas shall be fitted with bicycle-safe
grates.
(8)
When there is a change in pipe size through an inlet, the top
inside elevation of the outlet pipe shall be at or below the top inside
elevations of all incoming pipes.
(9)
Storm sewer design shall be based upon PennDOT design methods.
Inlet efficiency and bypass flow shall be determined for all inlets,
and the gutter flow spread shall not exceed 1/2 the travel lane width.
Where parking is permitted, the maximum gutter flow spread shall not
exceed eight feet. Gutter spread calculations shall be based upon
a ten-year storm event (minimum). The Borough may require that a hydraulic
grade-line analysis be performed on storm sewer systems.
(10)
Culverts shall be evaluated for inlet and outlet control restrictions.
(11)
Subsurface drainage systems shall have manholes, inlets or junction
boxes spaced at intervals not exceeding 300 feet and located wherever
there is a change in alignment or grade.
(12)
For drainage lines of 48 inches' diameter or greater manholes
may be spaced at intervals greater than 300 feet with approval of
the Borough Engineer.
(13)
PennDOT Type "C" concrete inlet top units are to be provided
with a five-inch by twenty-four-inch cast-iron "Dump No Waste - Drains
To Waterways" (with one-half-inch raised lettering) plate with trout
logo as manufactured by E. Jordan Iron Works, or approved equal. PennDOT
Type "M" concrete inlet top units are to be provided with a three-inch
by twenty-four-inch cast-iron "Dump No Waste - Drains To Waterways"
(with one-half-inch raised lettering) plate with trout logo as manufactured
by E. Jordan Iron Works, or approved equal. Bottoms of covers are
to be clearly marked with grade of iron (ASTM A48, Class 35B), product
number and date of manufacture.
(14)
The words "Dump No Waste - Drains To Waterways" in one-and-one-fourth-inch
raised letters with bass logo shall be cast or stamped into the storm
sewer manhole covers as manufactured by E. Jordan Iron Works, or approved
equal. Bottoms of covers are to be clearly marked with grade of iron
(ASTM A48, Class 35B), product number and date of manufacture.
(15)
Erosion shall be minimized at storm sewer discharge points through
the use of proper ground cover or riprap.
M.
Design of Bridges and Culverts.
(1)
Bridge and/or culvert construction shall be in accordance with
the PennDOT specifications and shall meet the requirements of the
PADEP. The appropriate permits and approvals must be acquired by the
applicant.
(2)
Culverts shall be provided with wing walls and constructed for
the full width of the right-of-way. If the character of the road is
expected to change for future planning, the cartway of the bridge
shall be made to anticipate this condition. On each side of the bridge
cartway, the bridge railing must be set back from the edge of the
final cartway, and this area may be used to place sidewalks, present
or future.
3.
Graded slopes shall not be steeper than three horizontal units to
one vertical unit, unless otherwise approved by the Borough Engineer.
4.
A minimum of six inches of topsoil shall be provided on all disturbed
areas prior to final seeding and mulching.
5.
Mature healthy trees of at least six inches' diameter at breast
height (DBH) and other significant existing vegetation within the
limits of earth disturbance shall be located in the field on the SWM
site plan and shall be retained and protected, unless otherwise approved
by the Borough Engineer. The filling of soil over the roots of trees
to be preserved is prohibited (roots are presumed to extend out from
the tree as far as the tree's branches extend outward).
