A.
Anyone applying for a building and/or zoning permit in Springettsbury
Township will be required to complete a stormwater management (SWM)
questionnaire in order to determine if they are subject to the requirements
of this chapter and to what level. This questionnaire will be reviewed
by the Township staff who will then make a determination as to the
stormwater impact level and required permitting. The following levels
have been established under this chapter:
|
Table 1 - Stormwater Management Requirements
| |||||
|---|---|---|---|---|---|
|
Impact Level
|
DIA Area
(square feet)
|
Volume Control Required
|
SWM Plan Requirements
|
Rate Control Requirements
| |
|
Level 1
|
Less than 1,000
(or 5% of lot area)
|
Yes
|
None
|
None
| |
|
Level 2
|
between 1,000 and 5,000
|
Yes
|
Simplified
|
None
| |
|
Level 3
|
greater than 5,000
|
Yes
|
Full Plan
|
Applicable
| |
B.
For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 281-11.
C.
SWM site plans approved by Springettsbury Township, in accordance with § 281-22, shall be on site throughout the duration of the regulated activity.
D.
Springettsbury Township may, after consultation with DEP, approve
measures for meeting the state water quality requirements other than
those in this chapter, provided that they meet the minimum requirements
of, and do not conflict with, state law including, but not limited
to, the Clean Streams Law.[1] The Township shall maintain a record of consultations
with DEP pursuant to this subsection.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
E.
For all regulated earth disturbance activities, erosion and sediment
control BMPs shall be designed, implemented, operated, and maintained
during the regulated earth disturbance activities, i.e., during construction,
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law. Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual) 2, No. 363-2134-008 (April 15, 2000), as amended and updated.
G.
Impervious areas.
H.
Stormwater flows from and onto adjacent property shall not be created,
increased, decreased, relocated, or otherwise altered without written
notification of the adjacent property owner(s). Such stormwater flows
shall be subject to the requirements of this chapter.
I.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter, as stated in § 281-3, Purpose, by implementing measures to:
(a)
Minimize disturbance to floodplains, wetlands, wooded areas,
and existing vegetation.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(f)
Minimize soil disturbance and compaction. Topsoil, if removed,
shall be replaced to a minimum depth equal to its depth prior to removal
or four inches, whichever is greater. (Additional topsoil may be needed
for vegetation other than sod.)
(3)
To the maximum extent practicable, incorporate the techniques for
low-impact development practices described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual).
J.
The design of all facilities in areas of carbonate geology or karst
topography shall include an evaluation of measures to minimize adverse
effects, including hydrogeologic studies if required by the municipality.
K.
Infiltration BMPs shall be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter. In addition,
infiltration BMPs shall include pretreatment BMPs where appropriate.
L.
Normally dry, open-top storage facilities, designed as such, shall
completely drain both the volume control and rate control capacities
over a period of time not less than 24 hours and not more than 72
hours from the end of the design storm. However, any designed infiltration
at such facilities is exempt from the minimum twenty-four-hour standard,
i.e., may infiltrate in a shorter period of time, so long as none
of the stormwater flowing into the infiltration facility is discharged
directly into the surface waters of the commonwealth. (Inordinately
rapid infiltration rates may indicate the presence of large fractures
or other conditions for which an additional soil buffer may be required.)
M.
The design storm volumes and precipitation intensities to be used
in the analysis of discharge or runoff shall be obtained from the
Precipitation-Frequency Atlas of the United States, Atlas 14, Volume
2, Version 3.0, U.S. Department of Commerce, National Oceanic and
Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological
Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 14 can
be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
N.
O.
Various BMPs and their design standards are listed in the DEP Stormwater
Best Management Practices Manual (latest edition).
Any regulated activity that meets the following exemption criteria
is exempt from the part(s) of this chapter as specified herein. However,
the requirements of the ordinance shall otherwise remain in effect.
The criteria for exemption in this section apply to the total development
proposed, including instances in which the development is proposed
to take place in phases. The date of enactment of this chapter shall
be the starting point from which future development and the respective
proposed impervious surface computations shall be cumulatively considered
and regulated. Exemption shall not relieve an applicant from implementing
such measures as necessary to meet the intent of this chapter, or
compliance with any NPDES permit requirements.
A.
Regulated activities that create DIAs equal to or less than 1,000
square feet are exempt from the peak rate control and the SWM site
plan preparation requirements of this chapter.
B.
Regulated activities that create DIAs greater than 1,000 square feet
and equal to or less than 5,000 square feet are exempt only from the
peak rate control requirement of this chapter.
C.
Agricultural activity is exempt from the rate control and SWM site
plan preparation requirements of this chapter provided the activities
are performed according to the requirements of 25 Pa. Code Chapter
102, and in accordance with the following:
(1)
All agricultural operations shall be conducted in such a manner as
will minimize or eliminate erosion and sedimentation. The Township
may require that a conservation plan or an erosion and sedimentation
plan approved by the York County Conservation District be filed with
the Township and followed.
(2)
No agricultural operations shall damage or destroy any stormwater
management structures or facilities, including open channels or interfere
with the operation of such structures or facilities, whether located
on private land or within any public right-of-way.
(3)
No agricultural operations shall be conducted within the right-of-way
of any highway and the plowing and tilling of land along any highway
shall be in such matter as to prevent stormwater from being directed
onto the berm or cartway of any highway.
D.
Forest management and timber operations are exempt from the rate
control and SWM site plan preparation requirements of this chapter
provided the activities are performed according to the requirements
of 25 Pa. Code Chapter 102.
E.
Domestic gardening and landscaping are exempt from specific approval
and permitting under this chapter so long as those activities are
associated with one, and only one, dwelling unit and the activities
comply with all other applicable ordinances and statutes.
G.
The municipality may deny or revoke any exemption pursuant to this
section at any time for any project that the municipality determines
poses a threat to public health, safety, property or the environment.
The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Except as listed in § 281-11 above, water volume controls shall be implemented for all levels of stormwater impact using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. Those applicants in Springettsbury Township with Level 1 and 2 impacts will use Method B (CG-2) to calculate the required volume control (see Subsection B below). For regulated activity areas of a larger size equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant (or his agent) may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year, twenty-four-hour duration
precipitation.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of this commonwealth. Removal
options for the first one inch of runoff include reuse, evaporation,
transpiration, and infiltration.
(3)
Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed stormwater runoff shall be infiltrated.
A.
For computation of predevelopment peak discharge rates, 100% of the
existing impervious area of a project site, when present, shall be
considered meadow.
B.
Stormwater management facilities shall be provided so that the peak
rates of runoff for stormwater originating on the project site must
meet the following conditions, for all watersheds within the site:
(1)
The two-year and ten-year post-development peak flow must be less
than or equal to the calculated two-year and ten-year predevelopment
peak flow, respectively, except where the proposed activity involves
a subdivision of five or fewer single-family residential lots, providing
that no new streets are involved.
(2)
The twenty-five-, fifty-, and one-hundred-year post-development peak
flows must be less than or equal to 50% of the corresponding calculated
twenty-five-, fifty-, and one-hundred-year predevelopment peak flows.
A.
For the purposes of the Act 167 stormwater management (plan) elements,
contained within the York County Integrated Water Resources Plan,
and this chapter, design policy pertaining to stormwater management
facilities for Pennsylvania Department of Transportation (PennDOT)
and Pennsylvania Turnpike Commission (PTC) roadways and associated
facilities is provided in Section 13.7 (Antidegradation and Post-Construction
Stormwater Management Policy) of PennDOT Publication No. 13M, Design
Manual Part 2 (August 2009), as developed, updated, and amended in
consultation with the Pennsylvania Department of Environmental Resources
(DEP).
B.
As stated in DM-2.13.7.D (Act 167 and municipal ordinances), PennDOT
and PTC roadways and associated facilities shall be consistent with
Act 167 plans. Dm-2.13.7B (Policy on Antidegradation and Post-Construction
Stormwater Management) was developed as a cooperative effort between
PennDOT and DEP. DM-2.13.7.C (Project Categories) discusses the anticipated
impact on the quality, volume, and rate of stormwater runoff.
C.
Where standards in the Act 167 elements of the IWRP and this chapter
are impractical, PennDOT or the PTC may request assistance from DEP,
in consultation with the county, to develop an alternative strategy
for meeting state water quality requirements and the goals and objectives
of the Act 167 elements within the IWRP.
D.
For the purposes of the Act 167 elements in the IWRP and this chapter,
road maintenance activities are regulated under 25 Pa. Code Chapter
102.
A.
Stormwater collection system. The stormwater collection system shall
be designed and approved based upon the following criteria:
(1)
General. Peak discharge shall be computed using the Rational Formula,
Q = CIA.
|
Where:
| ||||
|
Q
|
=
|
Peak discharge in cubic feet per second.
| ||
|
C
|
=
|
Runoff factor expressed as a percent of the total water falling
on an area.
| ||
|
I
|
=
|
The rate of rainfall for the time of concentration of the drainage
areas in inches per hour for a given stormwater frequency (rainfall
intensity).
| ||
|
A
|
=
|
The drainage area expressed in acres.
| ||
|
The runoff factor "C" is a percentage factor which represents
the proportion of the total quantity of water falling on the area
that remains as runoff.
| ||||
[1]
Editor's Note: Table 1 is included at the end of this chapter.
[2]
Editor's Note: Table 2 is included at the end of this chapter.
(2)
Storm intensity-duration-frequency curves are presented on Figure
1.[3] The curves provide for variation in rainfall intensity
according to:
(a)
Storm frequency.
[2]
When a pipe or culvert is intended to convey the discharge from
a stormwater management facility, its required capacity shall be computed
by the Rational Method and compared to the peak outflow from the stormwater
management facility for the fifty-year storm. The greater flow shall
govern the design of the pipe or culvert.
[3]
When a pipe is part of a storm sewer system and crosses the
roadway, it shall be designed as a storm sewer with the same design
storm as the remainder of the drainage system.
[4]
Greater design frequencies may be justified on individual projects
and may be required by the Township Engineer.
[5]
A fifty-year storm frequency may be required for design of the
stormwater collection system to ensure that the resultant stormwater
runoff from the post development storm is directed into the management
facility.
[3]
Editor's Note: Figure 1 is included at the end of this chapter.
(3)
Inlet placement. In general, inlets shall be spaced such that, based
upon the Rational Method, time of concentration (tc) equals five-minute
and ten-year rainfall intensity; the area contributing to the inlet
shall not produce a peak runoff of greater than four cubic feet per
second (cfs). Also, inlets shall be spaced so that their efficiency
based upon efficiency curves published by the Pennsylvania Department
of Transportation is not less than 65% (Figures 2 through 6).[4] All stormwater inlets shall be placed at the edge of pavement
or along the curb line. Additional inlets shall be placed at the upper
side of street intersections to prevent stormwater from crossing an
intersection. Other devices such as high-efficiency grates or perforated
pipe may be required if conditions warrant.
[4]
Editor's Note: Figures 2 through 6 are included at the end
of this chapter.
(4)
Pipe and swale capacity.
(a)
Manning's equation shall be used for the design of all stormwater
pipes and for open-channel design:
|
V
|
=
|
(1,486/n) R2/3 S1/2
| |
|
V
|
=
|
Velocity of the water in feet per second.
| |
|
R
|
=
|
Hydraulic radius which is equal to the net effective areas (A)
divided by the wetted perimeter (WP).
| |
|
S
|
=
|
Slope of energy line (for approximation, use water surface slope
in wetted stream bed slope in dry stream).
| |
|
N
|
=
|
The roughness coefficient. Roughness coefficients are presented
in Table 3.[5]
|
[5]
Editor's Note: Table 3 is included at the end of this chapter.
(b)
The maximum permitted velocity in an unlined swale shall be
five feet per second.
(5)
Culverts. In all cases where drainage is picked up by means of a
head wall and inlet or outlet conditions control, the pipe shall be
designed as a culvert. The minimum diameter of any culvert shall be
15 inches on Township roads. Culverts on state roads shall meet PennDOT
specifications. The procedure contained in Hydraulic Engineer Circular
No. 5 and No. 13, as prepared by the U.S. Department of Transportation
titled Federal Highway Administration, Washington, D.C., shall be
used for the design of culverts.
B.
Design criteria; stormwater management facilities.
(1)
Surface detention and retention basins.
(a)
General. Peak discharge and runoff shall be computed using either
the soil-cover complex method contained in Urban Hydrology for Small
Watersheds, Technical Release No. 55, published by Engineering Division,
Soil Conservation Service, United States Department of Agriculture,
dated January 1975, except as modified herein or the Modified Rational
Method for drainage areas less than five acres.
(b)
Outflow determination. The maximum permitted stormwater discharge, in cubic feet per second, from any site shall not exceed the capacity of the receiving pipe or structure, nor the calculated peak discharge from the site at predevelopment ground cover and soil conditions for all design storms specified below. For the purpose of this article, predevelopment ground cover conditions shall be assumed to be "meadow" as defined in Urban Hydrology for Small Watersheds, Technical Release No. 55, published by Engineering Division, Soil Conservation Service, United States Department of Agriculture, dated January 1975 (or, if using the Rational Method use "C" = 0.2). The maximum permitted stormwater discharge shall be calculated using either the SCS method or the Modified Rational Method for the same intervals for twenty-four-hour rainfalls having recurrence intervals of two, five, 10, 25, 50, and 100 years. The rainfall depths for the SCS method shall be calculated in accordance with § 281-10M of this chapter.
(c)
The PennDOT intensity-duration-frequency (IDF) curves shall
be used to generate the proper rainfall intensity for design.
(d)
Existing runoff volume. Existing runoff volume, in inches, shall be determined using the SCS method at predevelopment conditions for the rainfall depth listed in Subsection B(1)(b) herein.
(e)
Future runoff volume. The future runoff volume, in inches, shall be determined using the SCS method at post-development conditions (including any future expansion) for the rainfall depths listed in Subsection B(1)(b) above. Stormwater management facilities shall be provided so that the peak rates of runoff for stormwater originating on the project site must meet the following conditions, for all watersheds within the site:
[1]
The two-year and ten-year post-development peak flow must be
less than or equal to the calculated two-year and ten-year predevelopment
peak flow respectively, except where, the proposed activity involves
a subdivision of five or fewer single-family residential lots, providing
that no new streets are involved.
[2]
The twenty-five-, fifty-, and one-hundred-year post-development
peak flows must be less than or equal to 50% of the corresponding
calculated twenty-five-, fifty-, one-hundred-year predevelopment peak
flows.
(f)
Minimum required detention storage. The minimum required detention
storage shall be determined by routing the post-development hydrograph
through the stormwater management facility, using either manual methods
or computerized routing. Routing shall be based upon the modified
PULS method, other routing methodologies using accepted engineering
practice shall be subject to the approval of the Township Engineer.
(g)
Emergency spillway. Emergency spillways or overflow structures
shall be designed to pass the peak flow resulting from a one-hundred-year
storm computed at post-development conditions. All retention basins
and detention basins shall be provided with an emergency spillway.
(h)
Minimum bottom slope. All detention basins shall have a minimum
bottom slope of 4% in unpaved areas.
(i)
Maximum depth. The permitted maximum depth for detention or
retention basins shall be six feet, measured from the bottom of the
emergency spillway to the lowest point in the basin. Retention basins
over 18 inches in depth shall be fenced in accordance with Township
ordinances.
(j)
Side slopes. The maximum permitted side slopes for detention
or retention basins shall be four horizontal to one vertical.
(k)
No stormwater management facilities shall be installed over
existing or proposed utility mains and services.
(l)
For small projects the volume of seepage pits must be a minimum
of one cubic foot of pit for each three square feet of new impervious
surface added.
C.
Infiltration basin: above ground or subsurface.
(1)
Do not infiltrate in compacted fill. Infiltration in native soil
without prior fill or disturbance is preferred but not always possible.
Areas that have experienced historic disturbance or fill are suitable
for infiltration provided sufficient time has elapsed and the soil
testing indicates the infiltration is feasible. In disturbed areas
it may be necessary to infiltrate at a depth that is beneath soils
that have previously been compacted by construction methods or long
periods of mowing, often 18 inches.
(2)
A level infiltration area (1% or less slope) is preferred. Bed bottoms
should always be graded into the existing soil mantle, with terracing
as required to construct flat structures. Sloped bottoms tend to pool
and concentrate water in small areas, reducing the overall rate of
infiltration and longevity of the BMP. Infiltration areas should be
flat, nearly so, or on contour.
(3)
The loading ratio describes the ratio of impervious drainage area
to infiltration area, or the ratio of total drainage area to infiltration
area. In general, the following loading ratio guidelines are recommended:
(a)
Maximum impervious loading ratio of 5:1 relating impervious
drainage area to infiltration area.
(b)
A maximum total loading ratio of 8:1 relating total drainage
area to infiltration area.
(c)
Maximum impervious loading ratio of 3:1 relating impervious
drainage area to infiltration area for karst areas.
(4)
The hydraulic head or depth of water should be limited. The total
effective depth of water should generally not be greater than two
feet to avoid excessive pressure and potential sealing of the bed
bottom. Typically the water depth is limited by the loading ratio
and drawdown time and is not an issue.
(5)
Drawdown time must be considered. In general, infiltration BMPs should
be designed so that they completely empty within 72 hours.
(6)
All infiltration BMPs should be designed with a positive overflow
structure.
(7)
Infiltration BMPs that are subject to soil movements and deposition
must be constructed with suitably well-draining nonwoven geotextiles
to prevent to movement of fines and sediment into the infiltration
system.
(8)
Avoid severe slopes (greater than 20%), and toes of slopes, where
possible. Specific on-site investigations by experienced personnel
need to be made to determined acceptability of each case.
