A.
For all regulated activities, unless preparation of a SWM site plan is specifically exempted in § 358-12, Exemptions:
B.
SWM site plan approved by the municipality, in accordance with § 358-22, Authorization to construction and term of validity, shall be on site throughout the duration of the regulated activity.
C.
The municipality 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]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, erosion and sediment
control BMPs shall be designed, implemented, operated, and maintained
during the regulated earth disturbance activities (e.g., 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.[2] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Manual (E&S Manual)
No. 363-2134-008 (April 15, 2000), as amended and updated.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
E.
For all regulated activities, implementation of the volume controls in § 358-13, Volume controls, is required.
F.
Impervious areas:
G.
Stormwater flow onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without a written agreement
from the adjacent property owner(s). Such stormwater flows shall be
subject to the requirements of this chapter.
H.
All regulated activities shall include such measures, to the maximum
extent practicable, to:
(1)
Protect health, safety, and property and the public welfare.
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Maintain natural drainage patterns.
(d)
Avoid erosive flow conditions in natural flow pathways.
(e)
Minimize the creation of impervious surfaces.
(f)
Minimize thermal impacts to waters of this commonwealth.
(g)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
Incorporate the techniques for low impact development practices.
I.
Discharge roof drains and sump pumps to infiltration or vegetative
BMPs or satisfy the criteria for DIAs.
J.
Infiltration BMPs should 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.
K.
Stormwater storage facilities should 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.
L.
Twenty-four-hour values.
(1)
The design storm volumes to be used in the analysis of peak rates
of discharge will be the twenty-four-hour values as shown in the following
table:
|
Storm Frequency
|
24-Hour depth in inches
|
|---|---|
|
2-year
|
2.45
|
|
10-year
|
3.44
|
|
25-year
|
4.07
|
|
100-year
|
5.13
|
(2)
Or the twenty-four-hour values 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/
M.
N.
Various BMPs and their design standards are listed in the BMP manual.
O.
Off-site areas which 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.
P.
Any stormwater management facility designed to store stormwater runoff
and not eligible for a waiver from permit requirements pursuant to
25 Pa. Code § 105.12 requiring a berm or earthen embankment
shall be designed to provide an emergency spillway to handle flow
up to and including the 100-year post-development conditions. The
height of embankment must be a minimum one foot above the maximum
pool elevation computed when the facility functions for the 100-year
post-development inflow.
Q.
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands pursuant to 25 Pa. Code Chapter 105 regulations, as amended or replaced, shall be designed, permitted, and constructed in accordance with 25 Pa. Code Chapter 105.
R.
Any drainage conveyance facility and/or channel that does not fall under Pa. Code Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm. Conveyance facilities discharging to or exiting from stormwater management facilities shall be designed to convey the design flow to or from that structure.
S.
Storm sewers must be able to convey post-development runoff from
a ten-year-storm event without surcharging inlets, where appropriate.
T.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
U.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices and shall not result in
the creation or continuation of adverse hydrologic or adverse hydraulic
conditions within the watershed.
A.
Regulated activities that create impervious areas smaller than 2,500 square feet are exempt from the peak rate control and stormwater management site plan requirements of this chapter unless the municipality disqualifies the project for an exemption pursuant to § 358-12G.
B.
Regulated activities that create impervious areas of 2,500 square feet to 5,000 square feet require a small project application to be submitted to Springfield Township and volume controls must be installed to meet the requirements of the BMP manual, or meet the requirements for the disconnected impervious area (DIA) credits in § 358-16, Small project application.
C.
Agricultural activity is exempt from this 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.
D.
Forest management and timber operations are exempt from the rate
control and the SWM site plan preparation requirements of this chapter,
provided the activities are performed according to the requirements
of 25 Pa. Code Chapter 102.
E.
Oil and gas 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.
G.
Springfield Township may deny or revoke any exemption pursuant to
this section at any time for any project that Springfield Township
believes may pose a threat to public health, safety, property or the
environment.
The low impact development practices, like those provided in
the BMP manual shall be utilized for all regulated activities to the
maximum extent practicable. Water volume controls shall be implemented
using the Design Storm Method in Subsection A or the Simplified Method
in Subsection B. For regulated activity areas 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 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) 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 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 runoff should be infiltrated.
Post-development peak discharge rates shall not exceed the predevelopment
peak discharge rates for the, two-, ten-, twenty-five-, and 100-year-storm
events.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method (stormwater conveyance systems only)
or a soil cover complex methodology as follows:
A.
Major and minor conveyance systems up to 20 acres may be designed
using the Rational Method. The Rational Method may also be used in
sizing the minor conveyance systems for larger sites. Any stormwater
runoff calculations involving drainage areas greater than 20 acres,
including on-site and off-site areas, shall use a generally accepted
runoff hydrograph technique that is based on the NRCS Soil Cover Complex
Method. Runoff hydrograph methods must be used for major drainage
system designs for all systems with greater than 20 acres of drainage
area and for design of all stormwater storage facilities.
B.
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms presented § 358-11, General requirements, Subsection L. If a hydrologic computer model is used for stormwater runoff calculations, the duration of rainfall shall be 24 hours. The SCS Rainfall Type II curve shall be used for the rainfall distribution.
C.
For the purposes of existing predevelopment runoff rate determination,
undeveloped nonforested pervious areas, including disturbed areas,
must be considered meadow in good condition, or its equivalent in
good condition. Those areas that have existing impervious areas within
the planned development area may be included in the determination
of the predevelopment flow rate.
D.
All calculations using the Rational Method shall use rainfall intensities
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/.
E.
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. Travel time for channel and pipe flow shall
be computed using velocities determined using Manning's equation.
F.
Runoff curve numbers (CN) for both existing (predevelopment) and
proposed (post-development) conditions to be used in the Soil Cover
Complex Method shall be obtained from Chapter 2 of Urban Hydrology
for Small Watersheds, NRCS, TR-55.
G.
Runoff coefficients (C) for both existing (predevelopment) and proposed
(post-development) conditions for use in the Rational Method shall
be:
|
Runoff Factors for the Rational Equation
| ||
|---|---|---|
|
Runoff Factor "C"
| ||
|
Type of Drainage Area or Surface
|
Minimum
|
Maximum
|
|
Pavement, concrete or bituminous concrete
|
0.75
|
0.95
|
|
Pavement, bituminous macadam or surface-treated gravel
|
0.65
|
0.80
|
|
Pavement, gravel, macadam, etc.
|
0.25
|
0.60
|
|
Sandy soil, cultivated or light growth
|
0.15
|
0.30
|
|
Sandy soil, woods or heavy brush
|
0.15
|
0.30
|
|
Gravel, bare or light growth
|
0.20
|
0.40
|
|
Gravel, woods or heavy brush
|
0.15
|
0.35
|
|
Clay soil, bare or light growth
|
0.35
|
0.75
|
|
Clay soil, woods or heavy growth
|
0.25
|
0.60
|
|
City business sections
|
0.60
|
0.80
|
|
Dense residential sections
|
0.50
|
0.70
|
|
Suburban, normal residential areas
|
0.35
|
0.60
|
|
Rural areas, parks, golf courses
|
0.15
|
0.30
|
|
Runoff Coefficient for Rural Watersheds
| ||||
|---|---|---|---|---|
|
Slope
| ||||
|
Extreme
|
High
|
Normal
|
Low
| |
|
Relief (Cr)
|
0.28-0.35 steep, rugged terrain with average slopes above 30%
|
0.20-0.28 hilly, with average slopes of 10-30%
|
0.14-0.20 rolling, with average slopes of 5-10%
|
0.08-0.14 relatively flat land, with average slopes of 0-5%
|
|
Soil Infiltration (Ci)
|
0.12-0.16 no effective soil cover, either rock or thin soil
mangle of negligible infiltration capacity
|
0.08-0.12 slow to take up water, clay or shallow loam soils
of low infiltration capacity or poorly drained
|
0.06-0.08 normal; well drained light or medium textured soils,
sandy loams
|
0.04-0.06 deep sand or other soil that takes up water readily,
very light well drained soils
|
|
Vegetative Cover (Cv)
|
0.12-0.16 no effective plant cover, bare or very sparse cover
|
0.08-0.12 poor to fair; clean cultivation, crops or poor natural
cover, less than 20% of drainage area over good cover
|
0.06-0.08 fair to good; about 50% of area in good grassland
or woodland, not more than 50% of area in cultivated crops
|
0.04-0.06 good to excellent; about 90% of drainage area in good
grassland, woodland, or equivalent cover
|
|
Surface (Cs)
|
0.10-0.12 negligible; surface depression few and shallow; drainageways
steep and small; no marshes
|
0.08-0.10 well defined system of small drainageways; no ponds
or marshes
|
0.06-0.08 normal; considerable surface depression storage lakes
and ponds and marshes
|
0.04-0.06 much surface storage, drainage system not sharply
defined; large floodplain storage or large number of ponds or marshes
|
|
The total runoff coefficient based on the four runoff components
is:
C = Cr + Ci + Cv + Cs
|
|
as obtained from Pub. 584, PennDOT Drainage Manual, Sections
7.5.c thru 7.5.g. Where uniform flow is anticipated, the Manning equation
shall be used for hydraulic computations such as the capacity of open
channels, pipes, and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Pub. 584, PennDOT Drainage
Manual Section 7.3.
|
H.
Outlet structures for stormwater storage facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method. Orifices smaller
than three inches diameter are not recommended; however, if the qualified
professional can provide proof that the smaller orifices are protected
from clogging by use of anticlogging devices, then smaller orifices
may be permitted.
I.
The design of any stormwater storage facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities.
