The following general standards shall be followed to protect
health, safety and property and meet water quality, volume and rate
control goals:
A. All stormwater management plans shall be designed and certified by
a registered professional.
B. All stormwater management facilities shall employ best management
practices (BMP), as defined in this chapter. Various BMPs and their
design standards are listed in the BMP Manual.
C. Where applicable, stormwater management facilities shall comply with
the requirements of Chapter 105 (Water Obstructions and Encroachments)
of Title 25, Rules and Regulations of the PADEP.
D. Stormwater management facilities that involve a state highway shall
be subject to the approval of the PennDOT and may require a highway
occupancy permit.
E. Stormwater runoff from a development site to an adjacent property
shall flow directly into a natural watercourse, into an existing storm
sewer system, or onto adjacent properties in a manner similar to the
runoff characteristics of the predevelopment flow.
F. Stormwater runoff shall not be transferred from one watershed to
another, unless the watersheds are subwatersheds of a common watershed,
which join together within the perimeter of the property and the effect
of the transfer does not alter the peak discharge onto adjacent lands,
and drainage easements from the affected landowners are provided.
Transfer between watersheds as written in Act 167 is prohibited.
G. All stormwater runoff flowing over the project site shall be considered
in the design of the stormwater management facilities.
H. Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity. Staging of earth disturbance activities and selection of
construction equipment should consider this protection.
I. Developers have the option to propose a regional stormwater management plan or participate in a regional stormwater management plan developed by others. A regional stormwater management plan may include off-site volume and rate control, as appropriate and be supported by a detailed design approved by the Township in accordance with §
185-11C. A regional stormwater management plan must meet all of the volume and rate control standards required by this chapter for the area defined by the regional stormwater management plan, but not necessarily for each individual development site. Appropriate agreements must be established to ensure the requirements of this chapter and the requirements of the regional stormwater management plan are met.
J. Prohibited discharges.
(1) Except where specifically prohibited under the "Discharges Not Authorized
by this General Permit" section, this general permit authorizes the
discharge of stormwater to surface waters from regulated small MS4s.
In addition, the following nonstormwater discharges are authorized
by this general permit as long as such discharges do not cause or
contribute to pollution as defined in Pennsylvania's Clean Streams
Law:
Waterline flushing*
Air-conditioning condensation
Landscape irrigation
Irrigation waters
Diverted stream flows
Flows from riparian habitats and wetlands
Rising groundwaters
Uncontaminated groundwater
Lawn watering
Water from crawl space pumps
Springs
Routine external building wash down (which does not use detergents
or other compounds)
Individual residential car washing**
Discharges from firefighting activities, including training
Discharges from potable sources
Water from geothermal systems
Uncontaminated discharges from foundation drains or footings
Noncontaminated hydrostatic test water discharges*
*
|
If such discharges do not contain detectable concentrations
of total residual chlorine (TRC).
|
**
|
Where cleaning agents are not utilized.
|
(2) In the event that the Township or DEP determines that any of the discharges identified in Subsection
J significantly contribute to pollution of waters of the commonwealth, the Township or DEP will notify the responsible person to cease the discharge.
(3) Upon notice provided by the Township, the discharger will have a
reasonable time to cease the discharge consistent with the degree
of pollution caused by the discharge.
K. The following connections to the Township storm sewers or stormwater
drainage systems are prohibited:
(1) Any drain or conveyance, whether on the surface or subsurface, which
allows any nonstormwater discharge including sewage, process wastewater,
and wash water to enter the separate storm sewer system, or waters
of the commonwealth, and any connections to the storm drain system
from indoor drains and sinks.
(2) Any drain or conveyance from a commercial or industrial land use
to the separate storm sewer system which has not been documented in
plans, maps, or equivalent records, and approved by the Township.
(3) This prohibition expressly includes, without limitation, connections
made in the past, regardless of whether the connection, drain or conveyance
was previously allowed, permitted, or approved by a government agency,
or otherwise permissible under law or practices applicable or prevailing
at the time of connection.
(4) Roof drains shall not be connected to streets, sanitary or storm sewers or roadside ditches, except as provided in Subsection
K(5).
(5) When it is more advantageous to connect directly to streets or storm
sewers, connections of roof drains to streets or roadside ditches
may be permitted on a case-by-case basis by the Township. It shall
be the burden of the person seeking to make the connection to demonstrate
to the Township that such connection is more advantageous and such
connection shall not violate any state or federal statute, rule or
regulation.
(6) Roof drains and sump pumps shall discharge to infiltration areas
or vegetative BMPs to the maximum extent possible.
(7) Sump pumps shall not be discharged directly to separate storm sewer
systems or waters of the commonwealth.
L. Waste disposal prohibitions. No person shall throw, deposit, leave,
maintain, keep or permit to be thrown, deposited, left, or maintained
in or upon any public or private property, driveway, parking area,
street, alley, sidewalk, or other component of the Township's
separate storm sewer system any refuse, rubbish, garbage, litter,
or other discarded or abandoned objects, articles, and accumulations,
so that the same may cause or contribute to pollution. Wastes deposited
in streets in proper waste receptacles for the purpose of collection
are exempted from this prohibition.
The green infrastructure and low-impact development practices
provided in the BMP Manual shall be utilized for all regulated activities
wherever possible. Volume control BMPs are intended to maintain existing
hydrologic conditions for small storm events by promoting groundwater
recharge and/or evapotranspiration as described in this chapter. Runoff
volume controls shall be implemented using the Design Storm Method
described in Subsection A below, or through continuous modeling approaches
or other means as described in the BMP Manual. Small projects may
use the method described in Subsection B to design volume control
BMPs.
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) The BMP selected may not increase the post-development total runoff
volume for all storms equal to or less than the two-year twenty-four-hour
storm event.
(2) For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
When the existing project site contains impervious area, 20%
of existing impervious area to be disturbed shall be considered meadow
in good condition in the model for existing conditions.
(c)
The maximum loading ratio for volume control facilities in karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the Township
if professional justification deemed satisfactory to the Township
is provided. Hydraulic depth may be used as an alternative to an area-based
loading ratio if the design hydraulic depth is shown to be less than
the depth that could result from the maximum area loading ratio.
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 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.
(4) This method is exempt from the requirements of §
185-24, Rate control.
C. Volume control for small projects. At least the first three inches
of runoff from new impervious surfaces or an equivalent volume 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.
D. A detailed geologic evaluation of the development site shall be performed in areas of karst geology to determine the design parameters of recharge facilities. A report shall be prepared in accordance with §
185-14A(10) of this chapter.
E. Storage facilities, including normally dry, open-top facilities,
shall completely drain the volume control storage over a period of
time not less than 24 hours and not more than 72 hours from the end
of the design storm. 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, provided that none of this water will
be discharged into waters of this commonwealth.
F. Any portion of the volume control storage that meets the following
criteria may also be used as rate control storage.
(1) Volume control storage that depends on infiltration is designed according
to the infiltration standards in this chapter.
(2) The volume control storage which will be used for rate control is
that storage which is available within 24 hours from the end of the
design storm based on the stabilized infiltration rate and/or the
evapotranspiration rate.
G. Volume control storage facilities designed to infiltrate shall avoid
the least permeable hydrologic soil group(s) at the development site.
H. The developer may use stormwater credits for nonstructural BMPs in
accordance with the BMP Manual. The allowable reduction will be determined
by the Township Engineer.
Rate control for large storms, up to the 100-year event, is
essential to protect against immediate downstream erosion and flooding.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland.
A. The post-development hydrograph must match the predevelopment hydrograph. Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development 100-year hydrograph to match the predevelopment 100-year hydrograph, along all parts of the hydrograph, for the development site. To match the predevelopment hydrograph, the post-development peak rate must be less than or equal to the predevelopment peak rate, and the post-development runoff volume must be less than or equal to the predevelopment volume for the same storm event. A shift in hydrograph peak time of up to five minutes may be allowable. "Volume control" volumes as given in §
185-23 above may be used as part of this option.
B. In all other cases, the post-development peak discharge onto adjacent
property shall be 50% of the predevelopment peak rate of runoff. Runoff
calculations for the predevelopment and post-development comparison
shall be provided for the two-, ten-, twenty-five-, fifty-, and 100-year
storm events. The 50% peak rate reduction is consistent with the approved
Cocalico Creek Act 167 Plan and Conestoga River Act 167 Plan and associated
release rate maps. A twenty-four-hour SCS Type II storm or an IDF
Curve Modified Rational Method storm evaluation shall be performed
as outlined within this chapter.
C. Peak rate control is not required for off-site runoff. Off-site runoff
may be bypassed around the site, provided all other discharge requirements
are met. If off-site runoff is routed through rate control facilities,
runoff coefficients for off-site discharges used to design those rate
control facilities shall be based on actual land use assuming summer
conditions in the predevelopment and winter or poor land conditions
in the post-development.
D. Normally dry, open-top storage facilities shall completely drain
the rate control storage over a period of time less than or equal
to 24 hours from the peak 100-year water surface design elevation.
E. A variety of BMPs may be employed and tailored to suit the development
site. The following is a partial listing of BMPs which can be utilized
in stormwater management systems for rate control where appropriate:
(1) Decreased impervious surface coverage.
(3) Grassed channels and vegetated strips.
(4) Bioretention areas (rain gardens).
(5) Concrete lattice block or permeable surfaces.
(6) Seepage pits, seepage trenches or other infiltration structures.
(9) Cisterns and underground reservoirs.
(13)
Other methods as may be found in the BMP Manual.
F. Small projects are not required to provide for rate control.
The methods to calculate runoff shall be as follows:
A. The USDA SCS Soil-Cover-Complex Method.
(1) The USDA SCS Soil-Cover-Complex Method, as set forth in the latest
edition of Urban Hydrology for Small Watersheds, Technical Release
No. 55, as published by SCS, shall be used where the drainage area
is greater than 20 acres; or the time of concentration is greater
than one hour; or the calculated runoff to a point of interest requires
the combination of multiple hydrographs from multiple subwatersheds
of the study area.
(2) If the SCS Method is used, Antecedent Moisture Condition 1 is to
be used in areas of karst geology when required by the Township Engineer,
and Antecedent Moisture Condition 2 is to be used in all other areas.
A Type II distribution shall be used in all areas.
B. Rational Method.
(1) The Rational Method of Q = CIA, where "Q" is the peak discharge of
the watershed in cubic feet per second, "C" is the coefficient of
runoff, "I" is the intensity of rainfall in inches per hour, "A" is
the area of the watershed in acres.
(2) The modified Rational Method shall be used where the drainage area
is less than 20 acres; and the calculated runoff to a point of interest
does not require the combination of multiple hydrographs from multiple
subwatersheds of the study area.
(3) The modified Rational Method shall be used where the drainage area
is less than 20 acres; and the calculated runoff to a point of interest
does not require the combination of multiple hydrographs from multiple
subwatersheds of the study area.
C. If "modified" or "unit hydrograph" rational methods are used to develop
hydrographs, the ascending leg of the hydrograph shall have a length
equal to three times the time of concentration (3xTc) and the descending
leg shall have a length equal to seven times the time of concentration
(7xTc) to approximate an SCS Type II hydrograph.
D. Runoff calculations shall include a hydrologic and hydraulic analysis
indicating volume and velocities of flow and the grades, sizes and
capacities of water-carrying structures, sediment basins, retention
and detention structures, and sufficient design information to construct
such facilities. The capacities of the pipes, gutters, inlets, culverts,
outlet structures, and swales shall consider all possible hydraulic
conditions. Runoff calculations shall also indicate both predevelopment
and post-development rates for peak discharge of stormwater runoff
from all discharge points.
E. For the purpose of calculating predevelopment on-site peak discharges,
all on-site runoff coefficients shall be based on actual land use
assuming summer or good land cover conditions. Post-development runoff
coefficients for off-site discharges used to design conveyance facilities
shall be based on actual land use assuming winter or poor land conditions.
Any areas designed to initially be gravel or crushed stone shall be
assumed to be impervious.
F. Criteria and assumptions to be used in the determination of stormwater
runoff and design of management facilities are as follows:
(1) Runoff coefficients should be based on the following table. If the
land use is not listed, runoff coefficients shall be chosen from other
published documentation, and a copy of said documentation shall be
submitted with the stormwater management report. Higher coefficients
may be required by the Township Engineer due to local soil conditions.
|
Runoff Coefficients "C" for Rational Formula
|
---|
Soil Group
|
A
|
B
|
C
|
D
|
---|
Slope
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%
|
---|
Land use
|
|
|
|
|
|
|
|
|
|
|
|
|
Cultivated land
|
|
|
|
|
|
|
|
|
|
|
|
|
Winter conditions
|
0.14
|
0.23
|
0.34
|
0.21
|
0.32
|
0.41
|
0.27
|
0.37
|
0.48
|
0.34
|
0.45
|
0.56
|
Summer conditions
|
0.10
|
0.16
|
0.22
|
0.14
|
0.20
|
0.28
|
0.19
|
0.26
|
0.33
|
0.23
|
0.29
|
0.38
|
Fallowed fields
|
|
|
|
|
|
|
|
|
|
|
|
|
Poor conditions
|
0.12
|
0.19
|
0.28
|
0.17
|
0.25
|
0.34
|
0.23
|
0.33
|
0.40
|
0.27
|
0.35
|
0.45
|
Good conditions
|
0.08
|
0.13
|
0.16
|
0.11
|
0.15
|
0.21
|
0.14
|
0.19
|
0.26
|
0.18
|
0.23
|
0.31
|
Forest/woodland
|
0.08
|
0.11
|
0.14
|
0.10
|
0.14
|
0.18
|
0.12
|
0.16
|
0.20
|
0.15
|
0.20
|
0.25
|
Grass areas
|
|
|
|
|
|
|
|
|
|
|
|
|
Good conditions
|
0.10
|
0.16
|
0.20
|
0.14
|
0.19
|
0.26
|
0.18
|
0.22
|
0.30
|
0.21
|
0.25
|
0.35
|
Average conditions
|
0.12
|
0.18
|
0.22
|
0.16
|
0.21
|
0.28
|
0.20
|
0.25
|
0.34
|
0.24
|
0.29
|
0.41
|
Poor conditions
|
0.14
|
0.21
|
0.30
|
0.18
|
0.28
|
0.37
|
0.25
|
0.35
|
0.44
|
0.30
|
0.40
|
0.50
|
Impervious areas
|
0.90
|
0.91
|
0.92
|
0.91
|
0.92
|
0.93
|
0.92
|
0.93
|
0.94
|
0.93
|
0.94
|
0.95
|
Weighted residential
|
|
|
|
|
|
|
|
|
|
|
|
|
Lot size 1/8 acre
|
0.29
|
0.33
|
0.36
|
0.31
|
0.35
|
0.40
|
0.34
|
0.38
|
0.44
|
0.36
|
0.41
|
0.48
|
Lot size 1/4 acre
|
0.26
|
0.30
|
0.34
|
0.29
|
0.33
|
0.38
|
0.32
|
0.36
|
0.42
|
0.34
|
0.38
|
0.46
|
Lot size 1/3 acre
|
0.24
|
0.28
|
0.31
|
0.26
|
0.32
|
0.35
|
0.29
|
0.35
|
0.40
|
0.32
|
0.36
|
0.45
|
Lot size 1/2 acre
|
0.21
|
0.25
|
0.28
|
0.24
|
0.27
|
0.32
|
0.27
|
0.31
|
0.37
|
0.30
|
0.34
|
0.43
|
Lot size 1 acre
|
0.18
|
0.23
|
0.26
|
0.21
|
0.24
|
0.30
|
0.24
|
0.29
|
0.36
|
0.28
|
0.32
|
0.41
|
|
Runoff Curve Numbers "CN" for SCS Method
|
---|
Soil Group
|
A
|
B
|
C
|
D
|
---|
Slope
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
---|
Land use
|
|
|
|
|
|
|
|
|
|
|
|
|
Cultivated land
|
|
|
|
|
|
|
|
|
|
|
|
|
Winter conditions
|
48
|
60
|
65
|
62
|
73
|
73
|
68
|
78
|
79
|
77
|
88
|
81
|
Summer conditions
|
35
|
51
|
61
|
48
|
55
|
70
|
57
|
65
|
77
|
64
|
69
|
80
|
Fallowed fields
|
|
|
|
|
|
|
|
|
|
|
|
|
Poor conditions
|
45
|
54
|
76
|
56
|
63
|
85
|
64
|
74
|
90
|
69
|
77
|
93
|
Good conditions
|
30
|
44
|
74
|
43
|
48
|
83
|
48
|
54
|
88
|
56
|
60
|
90
|
Forest/woodland
|
30
|
40
|
30
|
42
|
46
|
55
|
45
|
50
|
70
|
50
|
56
|
77
|
Grass areas
|
|
|
|
|
|
|
|
|
|
|
|
|
Good conditions
|
35
|
51
|
39
|
48
|
54
|
61
|
56
|
59
|
74
|
62
|
63
|
80
|
Average conditions
|
45
|
53
|
49
|
52
|
55
|
69
|
60
|
63
|
79
|
65
|
69
|
84
|
Poor conditions
|
48
|
55
|
68
|
56
|
67
|
79
|
66
|
74
|
86
|
73
|
81
|
89
|
Impervious areas
|
96
|
97
|
98
|
96
|
97
|
98
|
96
|
97
|
98
|
96
|
97
|
98
|
Weighted residential
|
|
|
|
|
|
|
|
|
|
|
|
|
Lot size 1/8 acre
|
71
|
75
|
77
|
74
|
76
|
85
|
78
|
80
|
90
|
81
|
83
|
92
|
Lot size 1/4 acre
|
62
|
67
|
61
|
66
|
69
|
75
|
67
|
69
|
83
|
75
|
78
|
87
|
Lot size 1/3 acre
|
59
|
65
|
57
|
64
|
66
|
72
|
65
|
66
|
81
|
74
|
77
|
86
|
Lot size 1/2 acre
|
57
|
63
|
54
|
62
|
64
|
70
|
63
|
65
|
80
|
72
|
76
|
85
|
Lot size 1 acre
|
55
|
62
|
51
|
61
|
63
|
68
|
61
|
64
|
79
|
71
|
75
|
84
|
Conveyance Facility Design Criteria
|
---|
Location
|
Within Public Street Right-of-Way or Dedicated Land
|
Outside Public Street Right-of-Way or Within Nondedicated Areas
|
---|
Loading
|
All
|
Vehicular Loading
|
Nonvehicular Loading
|
---|
(a)
|
Pipe design
|
|
|
|
|
[1]
|
Material
|
SLHDPE, RCP
|
PVC, SLHDPE, RCP
|
PVC, SLHDPE, RCP
|
|
[2]
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
|
|
[3]
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
|
|
[4]
|
Diameter (minimum)
|
15 inches
|
15 inches
|
8 inches
|
|
[5]
|
Street crossing angle
|
75° to 90°
|
N/A
|
N/A
|
|
[6]
|
Access/maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
600 feet
|
(b)
|
Inlet design
|
|
|
|
|
[1]
|
Material
|
Concrete
|
Concrete
|
N/A
|
|
[2]
|
Grate depression
|
2 inches
|
2 inches
|
1 inch minimum
|
(c)
|
Manhole design
|
|
|
|
|
[1]
|
Material
|
Concrete
|
Concrete
|
Concrete
|
(d)
|
Swale design
|
|
|
|
|
[1]
|
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
|
|
[2]
|
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
|
|
[3]
|
Slope (minimum)
|
1%
|
N/A
|
1%
|
|
[4]
|
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
|
|
[5]
|
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
|
|
[6]
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
|
(e)
|
Outlet design
|
|
|
|
|
[1]
|
End treatment
|
Headwall/endwall
|
N/A
|
Headwall/endwall or flared end section
|
|
[2]
|
Energy dissipater
|
Required
|
N/A
|
Required
|
Abbreviations: N/A = not applicable or no criteria specified;
SLHDPE = smooth lined high density polyethylene pipe; PVC = polyvinyl
chloride; RCP = reinforced concrete pipe
|
(2) Times of concentration shall be based on the following design parameters:
(a)
Sheet flow. The maximum length for each reach of sheet, or overland
flow before shallow concentrated or open channel flow develops, is
150 feet. Sheet flow shall be determined using Worksheet 3 from the
latest edition of Urban Hydrology for Small Watersheds, Technical
Release No. 55, as published by SCS.
(b)
Shallow concentrated flow. Travel time for shallow concentrated
flow shall be determined using Figure 3-1 from the latest edition
of Urban Hydrology for Small Watersheds, Technical Release No. 55,
as published by SCS.
(c)
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
site between these design points shall be based upon Manning's
Equation and/or acceptable engineering design standards, as determined
by the Township Engineer. Supporting documentation and calculations
must be submitted for review and approval.
Aboveground storage facilities. Aboveground storage facilities
consist of all stormwater facilities which store, infiltrate evaporate/transpire,
clean or otherwise affect stormwater runoff and the top of which is
exposed to the natural environment. Aboveground storage facilities
are located above the finished ground elevation. Aboveground storage
facilities do not include stormwater management facilities designed
for conveyance or cisterns.
A. Design criteria. Aboveground storage facilities shall comply with
the design criteria in the following table:
Aboveground Storage Facility Design Criteria
|
---|
|
Facility Depth
|
---|
Less than 2 feet
|
2 feet to 8 feet
|
Greater than 8 feet
|
---|
(a)
|
Embankment geometry
|
|
|
|
|
[1]
|
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
|
|
[2]
|
Interior side slope (maximum)
|
3:1
|
3:1
|
5:1
|
|
[3]
|
Exterior side slope (maximum)
|
3:1
|
3:1
|
3:1
|
(b)
|
Embankment construction
|
|
|
|
|
[1]
|
Key trench
|
Not required
|
Required
|
Required
|
|
[2]
|
Pipe collar
|
Not required
|
Required
|
Required
|
|
[3]
|
Compaction density
|
Not required
|
Required
|
Required
|
(c)
|
Internal construction
|
|
|
|
|
[1]
|
Dewatering feature
|
N/A
|
Required
|
Required
|
|
[2]
|
Pretreatment elements
|
Not required*
|
Required
|
Required
|
(d)
|
Outlet structure
|
|
|
|
|
[1]
|
Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
|
|
[2]
|
Pipe material
|
SLHDPE, PVC, RCP
|
SLHDPE, RCP
|
RCP
|
|
[3]
|
Anti-clogging device/trash rack
|
Required
|
Required
|
Required
|
|
[4]
|
Anti-vortex design
|
Not required
|
Required
|
Required
|
|
[5]
|
Watertight joints in piping
|
Yes
|
Yes
|
Yes
|
(e)
|
Spillway requirements
|
|
|
|
|
[1]
|
Spillway freeboard (minimum)
|
Not required
|
3 inches
|
6 inches
|
|
[2]
|
Width (minimum)
|
Not required
|
10 feet
|
20 feet
|
|
[3]
|
Width (maximum)
|
Not required
|
50 feet
|
50 feet
|
|
[4]
|
Spillway channel design
|
Not required
|
Required
|
Required
|
|
[5]
|
Routing of 100-year storm
|
Permitted
|
Permitted
|
Permitted
|
Note: In certain situations 2:1 side slopes may be authorized
by the Township on embankment berms of less than two feet when appropriately
landscaped and mulched.
|
Abbreviations: *Pretreatment required for infiltration BMPs
unless shown to be unnecessary; N/A = not applicable; SLHDPE = smooth
lined high density polyethylene pipe; PVC = polyvinyl chloride; RCP
= reinforced concrete pipe.
|
B. Facility depth.
(1) For the purposes of the design criteria, the facility depth is defined
to be the depth between the bottom invert of the lowest orifice and
the invert of the spillway. If there is no spillway, the top of the
berm shall be used. For basins with no orifices or outlet structure,
the bottom elevation of the basin shall be used.
(2) Facilities with a facility depth greater than six feet shall not
be permitted in residential areas.
(3) Facilities with a facility depth greater than 15 feet require a dam
permit from DEP.
C. Embankment construction.
(1) Impervious core/key trench. An impervious core/key trench, when required,
shall consist of a cutoff trench (below existing grade) and a core
trench (above existing grade). A key trench may not be required wherever
it can be shown that another design feature, such as the use of an
impermeable liner, accomplishes the same purpose.
(a)
Materials. Materials used for the core shall conform to the
Unified Soil Classification GC, SC, CH, or CL and must have at least
30% passing the No. 200 sieve.
(b)
Dimensions.
[1]
The dimensions of the core/key trench shall provide a minimum
trench depth of two feet below existing grade, minimum width of four
feet and side slope of IH:IV or flatter.
[2]
The core should extend up both abutments to the ten-year water
surface elevation or six inches below the emergency spillway elevation,
whichever is lower.
[3]
The core shall extend four feet below any pipe penetrations
through the impervious core. The core shall be installed along or
parallel to the center line of the embankment.
(c)
Compaction.
[1]
Compaction requirements shall be the same as those for the embankment
to assure maximum density and minimum permeability.
[2]
The core shall be constructed concurrently with the outer shell
of the embankment.
[3]
The trench shall be dewatered during backfilling and compaction
operations.
(2) Pipe collars. All pipe collars, when required, shall be designed
in accordance with Chapter 7 of the E&S Manual. The material shall
consist of concrete or otherwise nondegradable material around the
outfall barrel and shall be watertight.
(3) Embankment fill material. The embankment fill material shall be taken
from an appropriate borrow area or imported fill which shall be free
of roots, stumps, wood, rubbish, stones greater than six inches, frozen
or other objectionable materials.
(4) Embankment compaction. When required, embankments shall be compacted
by sheepsfoot or pad roller. The loose lift thickness shall be nine
inches or less, depending on roller size, and the maximum particle
size is six inches or less (2/3 of the lift thickness). Five passes
of the compaction equipment over the entire surface of each lift are
required. Embankment compaction to visible nonmovement is also required.
The embankment shall be structurally sound under all probable conditions
of operation.
D. Internal construction.
(1) Bottom slope. The minimum bottom slope of facilities not designed
for infiltration shall be 1%. A flatter slope may be used if an equivalent
dewatering mechanism is provided.
(2) Dewatering features. When required, dewatering shall be provided
through the use of underdrain, surface device, or alternate approved
by the Township Engineer. If the facility is to be used for infiltration,
the dewatering device should be capable of being disconnected and
only be made operational if the basin is not dewatering within the
required time frame.
(3) Pretreatment elements. When required, pretreatment elements shall
consist of forebays, or alternate approved by the Township Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
(4) Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with §
185-14C(9) of this chapter and the BMP Manual which is designed to promote infiltration.
E. Outlet configuration.
(1) For facilities with a depth of two feet or greater, a Type D-W endwall
or riser box outlet structure shall be provided.
(2) For facilities with a depth less than two feet, no outlet structure
is required.
(3) All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel. Bolts/fasteners shall be
stainless steel.
F. Spillway.
(1) Material. The spillway shall be designed to provide a nonerosive,
stable condition when the project is completed.
(2) Nonemergency use. Use of the spillway to convey flows greater than
the fifty-year design storm is permitted.
(3) Emergency use. The spillway shall be designed to convey the 100-year
peak inflow in a manner which will not damage the integrity of the
facility or the downstream drainage areas.
(4) When required, freeboard shall be measured from the top of the water
surface elevation for emergency use.
G. Breach analysis. The Township may require a breach analysis based
on site-specific conditions and concern of threat for downstream property.
When required, the breach analysis shall be conducted in accordance
with the NRCS methodology, the United States Army Corps of Engineers
methodology (HEC-1) or other methodologies as approved by the Township.
H. Fencing. Fencing, or other acceptable devices that adequately restrict
access, shall be provided for basins with slopes greater than five
horizontal to one vertical when the water surface area is greater
than one acre, and/or more than five feet deep during a 100-year storm
event. However, the Township, based upon the type and proximity of
adjacent land use, may require the access restrictions to avoid a
hazardous condition.
I. Wet ponds. Basins which are not designed to release all stormwater
shall be specifically identified wet pond basins and designed in accordance
with the BMP Manual to create biodiversity, ensure long-term viability
and provide appropriate shoreline protection and buffering.
Subsurface storage facilities. Subsurface storage facilities
consist of all stormwater facilities which store, infiltrate/evaporate/transpire,
clean or otherwise affect stormwater runoff and the top of which is
not exposed to the natural environment. Subsurface facilities are
located below the finished ground elevation. Subsurface facilities
do not include stormwater management facilities designed for conveyance.
A. Design criteria. Subsurface storage facilities shall comply with
the design criteria in the following table:
Subsurface Storage Facility Design Criteria
|
---|
|
Facility Type
|
---|
Infiltration and Storage
|
Storage Without Infiltration
|
---|
(a)
|
Facility geometry
|
|
|
|
[1]
|
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
|
|
[2]
|
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
|
(b)
|
Distribution system requirements
|
|
|
|
[1]
|
Pipe size (minimum)
|
4 inches
|
4 inches
|
|
[2]
|
Pretreatment
|
Required
|
Required
|
|
[3]
|
Loading/balancing
|
Required
|
Not required
|
|
[4]
|
Observation/access ports
|
Required
|
Required
|
Abbreviations: *Unless otherwise determined by professional
geologic evaluation.
|
B. Distribution system requirements.
(1) Pretreatment requirements. The facility shall be designed to provide
a method to eliminate solids, sediment, and other debris from entering
the subsurface facility.
(2) Loading/balancing. The facility shall be designed to provide a means
of evenly balancing the flow across the surface of the facility to
be used for infiltration.
(3) Observation/access ports.
(a)
For facilities with the bottom less than five feet below the
average grade of the ground surface, a cleanout shall be an acceptable
observation port.
(b)
For facilities with the bottom five feet or more below the average
grade of the ground surface, a manhole or other means acceptable to
the Township shall be provided for access to and monitoring of the
facility.
(c)
The number of access points shall be sufficient to flush or
otherwise clean out the system.
C. Materials.
(1) Pipe material. Distribution system piping may be PVC, SLHDPE, or
RCP.
(2) Stone for infiltration beds. The stone used for infiltration beds
shall be clean washed, uniformly graded coarse aggregate (AASHTO No.
3 or equivalent approved by the Township). The void ratio for design
shall be assumed to be 0.4.
(3) Backfill material. Material consistency and placement depths for backfill shall be (at a minimum) per all applicable pipe manufacturer's recommendations, further providing it should be free of large (not exceeding six inches in any dimension) objectionable or detritus material. Select nonaggregate material should be indigenous to the surrounding soil material for nonvehicular areas. Backfill within vehicular areas shall comply with this section unless otherwise specified in Chapter
194, Subdivision and Land Development, or any other Township ordinance or regulation. Furthermore, if the design concept includes the migration of runoff through the backfill to reach the infiltration facility, the material shall be well drained, free of excess clay or clay-like materials and generally uniform in gradation.
(4) Lining material. Nonwoven geotextiles shall be placed on the sides
and top of subsurface infiltration facilities. No geotextiles shall
be placed on the bottom of subsurface infiltration facilities.
D. Cover.
(1) When located under pavement, the top of the subsurface facility shall
be a minimum of three inches below the bottom of pavement subbase.
(2) Where located under vegetative cover, the top of the subsurface facility
shall be a minimum of 12 inches below the surface elevation or as
required to establish vegetation.
(3) Subsurface facilities shall be isolated from property lines by a
minimum of five feet and separated from dwellings with basements,
sewer disposal facilities, wells, etc., as outlined in the BMP Manual.
(4) Subsurface facilities shall be designed to safely convey and/or bypass
flows from storms exceeding the design storm. Overflow locations shall
be identified isolated from downstream property lines to the maximum
extent possible and positioned to minimize potential for adverse downstream
conditions.
When applications are submitted in phases, and if temporary
facilities are required for construction of a phase, such facilities
shall be included in the submitted plans and secured with an improvement
guarantee. All phases of development must comply with the provisions
of this chapter. In the event temporary measures cannot adequately
handle the stormwater runoff, the additional facilities shall be included
as part of the construction of the proposed phase. An expected project
time schedule shall be outlined on the plans.
The following principles shall be applied to the design plan
and construction schedule to minimize soil erosion and sedimentation:
A. Stripping of vegetation, grading or other soil disturbance shall
be done in a manner which will minimize soil erosion.
B. Natural wooded cover and vegetation shall be retained and protected,
whenever feasible.
C. The extent of the disturbed area and the duration of its exposure
shall be kept to a minimum, within practical limits.
D. Either temporary seeding, mulching or other suitable stabilization
measures shall be used to protect exposed critical areas during construction.
E. Drainage provisions shall accommodate the stormwater runoff, both
during and after construction.
F. Soil erosion and sedimentation facilities shall be installed before
any earth-disturbing activities.
G. All earth disturbance activities shall be conducted in such a way
as to minimize accelerated erosion and resulting sedimentation. Measures
to control erosion and sedimentation shall, at a minimum, meet the
standards of the LCCD, Chapter 102 (Erosion Control) of Title 25,
Rules and Regulations of the PADEP, and the E&S Manual.
H. The erosion and sedimentation control plan must be available at all
times at the project site. When required, a permit allowing earth
disturbance activity shall be obtained by the developer before any
construction on the project site shall begin.
I. Approval of an erosion and sedimentation control plan by the Township
shall not be construed as an indication that the plan complies with
the standards of any agency of the commonwealth.
J. The erosion and sedimentation control plan shall be submitted to
the LCCD for its review and approval. Copies of all information submitted
to the LCCD, including worksheets and calculations, shall be provided
to the Township. Building permits will not be issued and construction
shall not start until an NPDES permit, where required, is received.