A.
All regulated activities in the municipality shall be subject to
the stormwater management requirements of this chapter.
B.
Storm drainage systems shall be designed to preserve natural watercourses
except as modified by stormwater detention facilities, recharge facilities,
water quality facilities, pipe systems or open channels consistent
with this chapter.
C.
The existing locations of concentrated drainage discharge onto adjacent
property shall not be altered without written approval of the affected
property owner(s).
D.
Areas of existing diffused drainage discharge onto adjacent property
shall be managed such that, at minimum, the peak diffused flow does
not increase in the general direction of discharge, except as otherwise
provided in this chapter. If diffused flow is proposed to be concentrated
and discharged onto adjacent property, the developer must obtain the
written approval of the affected property owner(s). Areas of existing
diffused drainage discharge shall be subject to any applicable release
rate criteria in the general direction of existing discharge whether
they are proposed to be concentrated or maintained as diffused drainage
areas.
E.
F.
Post-construction BMPs shall be designed, installed, operated and
maintained to meet the requirements of the Clean Streams Law[3] and implementing regulations, including the established
practices in 25 Pa. Code Chapter 102 and the specifications of this
chapter as to prevent accelerated erosion in watercourse channels
and at all points of discharge.
[3]
Editor's Note: See 35 P.S. § 691.1 et seq.
G.
No earth-disturbance activities associated with any regulated activities
shall commence until approval by the municipality of a plan which
demonstrates compliance with the requirements of this chapter.
H.
Techniques described in Appendix E (Low Impact Development)[4] of this chapter are encouraged because they reduce the
costs of complying with the requirements of this chapter and the state
water quality requirements. The applicant is required to evaluate
practicable alternatives to the surface discharge of stormwater, the
creation of impervious surfaces and the degradation of waters of the
commonwealth, and must maintain as much as possible the natural hydrologic
regime.
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
I.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc. are encouraged, where soil
conditions permit, to reduce the size or eliminate the need for detention
facilities or other structural BMPs.
J.
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered in any manner which could cause property
damage without permission of the affected property owner(s) and shall
be subject to any applicable discharge criteria specified in this
chapter.
K.
All stormwater runoff shall be treated for water quality.
L.
Transference of runoff to or from an EV/HQ watershed is prohibited
unless otherwise authorized by DEP, DRBC or SRBC.
M.
The Board may require that a landowner or developer provide reasonable
corrective measures to alleviate an existing off-site storm drainage
problem areas that may be affected by the proposed subdivision and/or
land development. It shall be the responsibility of the landowner
or developer to obtain all drainage easements on, over, or through
other properties, and the Township, its agents, workmen, servants
and employees shall be indemnified and held harmless from any liability.
The following permit requirements apply to certain regulated
and earth-disturbance activities and must be met prior to commencement
of regulated and earth-disturbance activities, as applicable:
A.
All regulated and earth-disturbance activities, subject to permit
requirements by DEP under regulations at 25 Pa. Code Chapter 102.
B.
Work within natural drainageways, subject to permit by DEP under
25 Pa. Code Chapter 102 and Chapter 105.
C.
Any stormwater management facility that would be located in or adjacent
to surface waters of the commonwealth, including wetlands, subject
to permit by DEP under 25 Pa. Code Chapter 105.
D.
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 DEP under 25 Pa. Code Chapter
105.
A.
No regulated earth-disturbance activities within the municipality
shall commence until approval by the municipality of an erosion and
sediment control plan for construction activities. Written approval
by DEP or a delegated County Conservation District shall satisfy this
requirement.
B.
A written erosion and sediment control plan is required by DEP regulations
for any earth-disturbance activity of 5,000 square feet or more under
Pa. Code § 102.4(b).
C.
A DEP NPDES Stormwater Discharges Associated with Construction Activities
Permit is required for regulated earth-disturbance activities of one
acre or greater under Pa. Code Chapter 102.
D.
Evidence of any necessary permit(s) for regulated earth-disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the municipality before the commencement
of an earth-disturbance activity.
E.
A copy of the erosion and sediment control plan and any permit, as
required by DEP regulations, shall be available at the project site
at all times.
A.
No regulated earth-disturbance activities within the municipality
shall commence until approval by the municipality of a drainage plan
which demonstrates compliance with this chapter.
B.
The water quality volume (WQv) shall be captured and treated with evapotranspiration and/or direct recharge BMPs. The WQv shall be calculated as the difference in runoff volume from predevelopment to post-development for the twenty-four-hour, two-year return period storm. This may be calculated using either the Soil-Cover-Complex Method using the two-year rainfall depth as noted in § 415-17H. The effect of closed depressions on the site shall be considered in this calculation. The WQv shall be captured and treated in a manner consistent with the standards outlined in § 415-13. For sites less than five acres, or as approved by the Municipal Engineer, the Rational Method may be used to calculate the WQv.
C.
The WQv shall be calculated for each post-development drainage direction
on a site for sizing BMPs. Site areas having no impervious cover and
no proposed disturbance during development may be excluded from the
WQv calculations and do not require treatment.
D.
For sites within watersheds with high quality (HQ) or exceptional
value (EV) waters as designated by DEP, if an applicant is proposing
to use a BMP that ponds water on the land surface and may receive
direct sunlight, the discharge from that BMP must be treated by infiltration,
a vegetated buffer, filter strip, bioretention, vegetated swale or
other BMP that provides a thermal benefit to protect the designated
waters from thermal impacts.
E.
Sites where applicants intend to use infiltration BMPs must meet
the following criteria:
(1)
Depth to bedrock below the invert of the BMP greater than or equal
to two feet.
(2)
Depth to seasonal high-water table below the invert of the BMP greater
than or equal to two feet; except for infiltration of residential
roof runoff where the seasonal high-water table must be below the
invert of the BMP.
(3)
Soil permeability (as measured using the standards listed in Appendix
C of the Pennsylvania Stormwater Best Practices Manual) greater than
or equal to 0.1 inch/hour and less than or equal to 10 inches per
hour.
F.
Site areas proposed for infiltration shall be protected from disturbance
and compaction except as necessary for construction of infiltration
BMPs.
G.
If infiltration of the entire WQv is not proposed, the remainder
of the WQv shall be treated by acceptable BMPs for each discharge
location. Acceptable BMPs are listed in Appendix F.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
H.
Stormwater runoff from hot-spot land uses shall be pretreated. Guidance
regarding acceptable methods of pretreatment is located in Appendix
F.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
I.
The use of infiltration BMPs is prohibited on hot-spot land use areas
unless the applicant can demonstrate that existing and proposed site
conditions, including any proposed runoff pretreatment, create conditions
suitable for runoff infiltration under this chapter.
J.
Applicants shall request, in writing, public water suppliers to provide the Zone I Wellhead Protection radius, as calculated by the method outlined in the Pennsylvania Department of Environmental Protection Wellhead Protection regulations, for any public water supply well within 400 feet of the site. In addition to the setback distances specified in § 415-13E, infiltration is prohibited in the Zone I radius as defined and substantiated by the public water supplier in writing. If the applicant does not receive a response from the public water supplier, the Zone I radius is assumed to be 100 feet.
K.
The municipality may, after consultation with DEP, approve alternative
methods 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.[3]
[3]
Editor's Note: See 35 P.S. § 691.1 et seq.
L.
Soils testing. If on-lot infiltration is proposed as part of a project,
the applicant's design professional must demonstrate to the satisfaction
of the Township that the soils are conducive to infiltration at the
proposed location of infiltration facilities. Soils testing shall
be completed as follows:
(1)
Soils testing must be conducted by a qualified design professional
and at a minimum shall address depth to limiting zone, soil permeability,
and subgrade stability. Soils testing must be observed by a representative
of the Township.
(2)
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.
(3)
Conduct field testing including: test pits to determine soil horizons
and depth to limiting zone and permeability tests, at the elevation
of the proposed infiltration facility surface, to determine the appropriate
hydraulic conductivity rate. Double ring infiltrometer or hydraulic
conductivity tests should be used to determine soil permeability (percolation
tests are not recommended for design purposes). Site evaluation and
soils testing should be conducted in accordance with Appendix C of
the Pennsylvania Stormwater Best Management Practices Manual.
(4)
The proposed infiltration facilities shall be designed for the required
recharge (Rev) volume based on the field determined
capacity at the surface elevation of the proposed infiltration facility.
M.
Infiltration design criteria.
(1)
All infiltration systems shall have appropriate positive overflow
controls to prevent storage within one foot of the finished surface
or grade.
(2)
Surface inflows shall be treated to prevent the direct discharge
of sediment and pollutants into the infiltration system; accumulated
sediment reduces stormwater storage capacity and ultimately clogs
the infiltration mechanism.
(3)
No sand or other particulate matter may be applied to a porous paving
surface for winter ice conditions.
(4)
During site construction, all recharge system components shall be
protected from compaction due to heavy equipment operation or storage
of fill or construction material. Recharge areas shall be protected
from sedimentation. All areas designated for recharge shall not receive
runoff until the contributory drainage area has achieved final stabilization.
(5)
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(a)
Excavation for the infiltration facility shall be performed
with equipment which will not compact the bottom of the seepage bed/trench,
or like facility.
(b)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(c)
Only clean aggregate, free of fines, shall be allowed.
(d)
The top and sides of all seepage beds, trenches, or like facilities
shall be covered with drainage filtration fabric. Fabric shall meet
the specifications of PennDOT Publication 408, Section 735, Construction
Class 1.
(e)
Perforated distribution pipes connected to centralized catch
basins and/or manholes with provision for the collection of debris
shall be provided in all facilities. The perforated pipes shall distribute
stormwater throughout the entire seepage bed/trench, or like facility.
(6)
All infiltration facilities which service more than one lot and are
considered a common facility shall have an easement provided to the
Township for future access if necessary.
This section is applicable only to the Perkiomen Creek Headwaters
Watershed District.
A.
The entire WQv as calculated in § 415-13B shall be captured and treated by either direct recharge or evapotranspiration BMPs.
B.
Lawn area up to a maximum of 33% of the entire site area may be allowed
to bypass water quality BMPs. As much proposed impervious area as
practical shall be directed to water quality BMPs.
C.
Existing impervious area that is not proposed to be treated by direct
recharge BMPs should be excluded from all water balance calculations.
D.
A maximum of 30% of the total annual rainfall for a site may be directly
recharged to groundwater using direct recharge BMPs, for runoff from
impervious areas.
(1)
For development sites with greater than thirty-three-percent proposed
impervious cover:
(a)
If all impervious cover is directed to evapotranspiration BMPs
to capture the entire two-year, twenty-four-hour event, the direct
recharge standard is met.
(b)
Up to 33% of the site as impervious cover may be directed to
direct recharge BMPs designed to capture the entire two-year, twenty-four-hour
event. All remaining impervious cover shall be directed to evapotranspiration
BMPs designed to capture the remainder of the WQv.
(c)
For evapotranspiration and/or direct recharge BMPs designed
for runoff from impervious areas designed to capture less than entire
two-year, twenty-four-hour event, Appendix C shall be used to assure
that the maximum direct recharge standard is met.
(2)
For development sites with less than thirty-three-percent proposed
impervious cover, all proposed impervious and the entire WQv may be
directed to direct recharge BMPs.
(3)
The maximum thirty-percent direct recharge standard applies on an
overall site basis, rather than in each drainage direction.
A.
Within Upper Hanover Township, there are two stormwater management
districts established as follows:
(1)
Stormwater Management District I is made up of the portion of Upper
Hanover Township that is located within the Perkiomen Creek Headwaters
Watershed. To implement the provisions of the Perkiomen Creek Headwaters
Watershed Stormwater Management Plan, the municipality is hereby divided
into stormwater management districts consistent with the Perkiomen
Creek Headwaters Release Rate Map presented in the Plan Update. The
boundaries of the stormwater management districts are shown on the
official map included in Appendix A.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
Stormwater Management District II is made up of the portion of Upper
Hanover Township outside of the Perkiomen Creek Headwaters Watershed.
The boundaries of the stormwater management districts are shown on
the official map included in Appendix A.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
Description of stormwater management districts. The two-, ten-, twenty-five-,
fifty, and one-hundred-year post-development peak runoff must be controlled
to the stated percentage of the predevelopment peak for each district
as shown in Tables 2 and 3 below.
(1)
District I: Perkiomen Creek Headwaters Watershed.
|
Table 2, Water Quantity Requirements
Perkiomen Creek Headwaters Watershed
Stormwater Management District I
| |||
|---|---|---|---|
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
2-year
|
Reduce to
|
2-year
| |
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
(2)
District II: areas outside of the Perkiomen Creek Headwaters Watershed.
|
Table 3, Water Quantity Requirements
Areas Outside of the Perkiomen Creek Headwater Watershed
Stormwater Management District II
| |||
|---|---|---|---|
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
2-year
|
Reduce to
|
2-year
| |
|
10-year
|
2-year
| ||
|
50-year
|
10-year
| ||
|
100-year
|
100-year
| ||
A.
The exact location of the stormwater management district boundaries
as they apply to a given development site shall be determined by mapping
the boundaries using the two-foot topographic contours provided as
part of the drainage plan. The district boundaries as originally drawn
coincide with topographic divides or, in certain instances, are drawn
from the intersection of the watercourse and a physical feature such
as the confluence with another watercourse or a potential flow obstruction
(e.g., road, culvert, bridge, etc.). The physical feature is the downstream
limit of the subarea and the subarea boundary is drawn from that point
up slope to each topographic divide along the path perpendicular to
the contour lines.
B.
For sites straddling major watershed divides (i.e., Perkiomen Creek
Headwaters and the remainder of the Township), runoff volumes shall
be managed to prevent diversion of runoff between watersheds, as practicable.
A.
Stormwater runoff from all development sites shall be calculated
using the Soil-Cover-Complex methodology. For sites of less than five
acres, or as approved by the Municipal Engineer, the Rational Method
may be used.
B.
The design of any detention basin intended to meet the requirements
of this chapter shall be verified by routing the design storm hydrograph
through the proposed basin using the storage Indication Method or
other methodology demonstrated to be more appropriate. For basins
designed using the Rational Method technique, the design hydrograph
for routing shall be either the Universal Rational Hydrograph or another
Rational Hydrograph that closely approximates the volume of the Universal
Rational Hydrograph.
C.
BMPs designed to store or infiltrate runoff and discharge to surface
runoff or pipe flow shall be routed using the Storage Indication Method.
D.
BMPs designed to store or infiltrate runoff and discharge to surface
runoff or pipe flow shall provide storage volume for the full WQv
below the lowest outlet invert.
E.
Wet detention ponds designed to have a permanent pool for the WQv
shall assume that the permanent pool volume below the primary outlet
is full at the beginning of design event routing for the purposes
of evaluating peak outflows.
F.
All wet basin designs shall incorporate biologic minimization controls
consistent with the West Nile Guidance found in Appendix G.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
G.
Runoff calculations using the Soil-Cover-Complex Method shall use
the Natural Resources Conservation Service Type II twenty-four-hour
rainfall distribution. The twenty-four-hour rainfall depths for the
various return periods to be used consistent with this chapter may
be taken from NOAA Atlas 14, Volume 2 Version 2.1, 2004, or the PennDOT
Intensity - Duration - Frequency Field Manual (PDT-IDF) (May 1986)
for Region 4.
(1)
The following values are taken from the PDT-IDF Field Manual:
|
Return Period
|
24-Hour Rainfall Depth
| |
|---|---|---|
|
1-year
|
2.40 inches
| |
|
2-year
|
3.00 inches
| |
|
5-year
|
3.60 inches
| |
|
10-year
|
4.56 inches
| |
|
25-year
|
5.52 inches
| |
|
50-year
|
6.48 inches
| |
|
100-year
|
7.44 inches
|
(2)
A graphical and tabular presentation of the Type II twenty-four-hour
distribution is included in Appendix C.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
H.
Runoff calculations using the Rational Method shall use rainfall
intensities consistent with appropriate times of concentration and
return periods and NOAA Atlas 14, Volume 2 Version 2.1, 2004, or the
Intensity-Duration-Frequency Curves as presented in Appendix C.
I.
Runoff curve numbers (CNs) to be used in the Soil-Cover-Complex Method
shall be based upon the table presented in Appendix C.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
J.
Runoff coefficients for use in the Rational Method shall be based
upon the table presented in Appendix C.
K.
All time of concentration calculations shall use a segmental approach
which may include one or all of the flow types below:
(1)
Sheet flow (overland flow) calculations shall use either the NRCS
average velocity chart (Figure 3-1, Technical Release-55, 1975) or
the modified kinematic wave travel time equation (Equation 3-3, NRCS
TR-55, June 1986). If using the modified kinematic wave travel time
equation, the sheet flow length shall be limited to 50 feet for designs
using the Rational Method and limited to 150 feet for designs using
the Soil-Cover-Complex Method.
(2)
Shallow concentrated flow travel times shall be determined from the
watercourse slope, type of surface and the velocity from Figure 3-1
of TR-55, June 1986.
(3)
Open channel flow travel times shall be determined from velocities
calculated by the Manning Equation. Bankfull flows shall be used for
determining velocities. Manning 'n' values shall be based on the table
presented in Appendix C.
(4)
Pipe flow travel times shall be determined from velocities calculated
using the Manning Equation assuming full flow and the Manning 'n'
values from Appendix C.[4]
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
L.
If using the Rational Method, all predevelopment calculations for
a given discharge direction shall be based on a common time of concentration
considering both on-site and any off-site drainage areas. If using
the Rational Method, all post-development calculations for a given
discharge direction shall be based on a common time of concentration
considering both on-site and any off-site drainage areas.
M.
When conditions exist such that a proposed detention facility may
experience a tailwater effect, the basin shall be analyzed without
any tailwater effect for all storm events for comparison against the
required release rates. An additional routing of the one-hundred-year
storm with the full tailwater effect shall be performed to check that
the basin has sufficient storage to contain the one-hundred-year tributary
flow with a tailwater.
N.
The Manning Equation shall be used to calculate the capacity of watercourses.
Manning 'n' values used in the calculations shall be consistent with
the table presented in Appendix C or other appropriate standard engineering
'n' value resources. Pipe capacities shall be determined by methods
acceptable to the municipality.
O.
The Pennsylvania DEP, Chapter 105, Rules and Regulations, apply to
the construction, modification, operation or maintenance of both existing
and proposed dams, water obstructions and encroachments throughout
the watershed. Criteria for design and construction of stormwater
management facilities according to this chapter may differ from the
criteria that are used in the permitting of dams under the Dam Safety
Program.
P.
Any stormwater management facility (i.e., detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year proposed
conditions and may be subject to PaDEP Chapter 105 regulations.
Q.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), water encroachments, and
any work involving wetlands governed by PaDEP Chapter 105 regulations
(as amended or replaced from time to time by PaDEP), are subject to
PaDEP Chapter 105 regulations.
R.
Any proposed roadway drainage facilities shall be designed according
to PennDOT Design Manual Part II.
S.
Storm sewers must be able to convey proposed conditions runoff from
a fifty-year design storm without flooding inlets, where appropriate.
Roadway crossings must be able to convey runoff from a one-hundred-year
design storm.
T.
In addition to the criteria in § 415-19, on-site conveyance systems designed to carry runoff to a detention basin must be able to transport the basin's one-hundred-year tributary flow either in-system, in-gutter, or overland.
U.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge (DEP erosion and Erosion, Sediment
and Pollution Control Manual).
V.
The Township reserves the right to disapprove any design that would
result in the construction in or continuation of a storm drainage
problem area.
W.
Applicants shall provide a comparative pre- and post-construction
stormwater management hydrograph analysis for each direction of discharge
and for the site overall to demonstrate compliance with the provisions
of this chapter.
X.
Within a release rate category area, for a proposed development site which has areas which drain to a closed depression(s), the design release from the site will be the lesser of a) the applicable release rate flow assuming no closed depression(s) or b) the existing peak flow actually leaving the site. In cases where b) would result in an unreasonably small design release, the design discharge of less than or equal to the release rate will be determined by the available downstream conveyance capacity to the main channel calculated using § 415-19 and the minimum orifice criteria.
Y.
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 using the capacity criteria in § 415-19 and the detention criteria in § 415-17.
Z.
For development sites proposed to take place in phases, all detention
ponds shall be designed to meet the applicable release rate(s) applied
to all site areas tributary to the proposed pond discharge direction.
All site tributary areas will be assumed as developed, regardless
of whether all site tributary areas are proposed for development at
that time. An exception shall be sites with multiple detention ponds
in series where only the downstream pond must be designed to the stated
release rate.
AA.
Where the site area to be impacted by a proposed development activity
differs significantly from the total site area, only the proposed
impact area shall be subject to the release rate criteria. The impact
area includes any proposed cover or grading changes.
BB.
Development proposals which, through groundwater recharge or other
means, do not increase either the rate or volume of runoff discharged
from the site compared to predevelopment are not subject to the release
rate provisions of this chapter.
CC.
Regional detention alternatives. For certain areas within the study
area, it may be more cost-effective to provide one control facility
for more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective developers. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined based on
the required release rate at the point of discharge.
A.
The following criteria shall be used for the design of detention
and wet basins. Any reference to detention basin shall also include
wet basins.
(1)
When detention basins are provided, they shall be designed to utilize
the natural contours of the land wherever possible. When such design
is impracticable, the construction of the basin shall utilize slopes
as flat as possible to blend the structure into the terrain. All basins
shall have maximum side slope of three horizontal to one vertical
(3:1).
(2)
A routed hydrograph and calculations shall be furnished for each
storm through the detention basin.
(3)
Emergency overflow facilities (i.e., emergency spillway) shall be
provided for all detention facilities which shall pass a one-hundred-year
storm under orifice block conditions.
(4)
No stormwater detention facility shall be placed within 50 feet of
a special geologic feature.
B.
In the design of detention basins, the following items listed below
shall be submitted to the Township for review. Any reference to detention
basin shall also include retention basin.
(1)
Design computations for the sizing of the outlet device.
(2)
A stage-storage curve for said detention/retention basin.
(3)
Flood routing and/or storage requirement calculations.
(4)
A plan showing the berm embankment and outlet structure. The plan
shall indicate top of berm elevations, width of the top of the berm,
side slopes, emergency spillway elevation, and elevations of the outlet
structure, including riser, dimensions and spacing of anti-seep collars.
(5)
A cross section through the outlet structure, emergency spillway
and berm embankment.
(6)
A detailed plan of the trash rack and anti-vortex device (if required).
(7)
The maximum side slope of earthen detention embankments shall be
three horizontal to one vertical (3:1).
(8)
The top and/or toe (whichever is closer to a property line) of any
slope shall be located a minimum of 25 feet from any property line.
(9)
Whenever possible, the side slopes and basin shape shall conform
to the natural topography.
(10)
The minimum top width of the detention basin berm shall be 10
feet.
(11)
A cutoff trench (key-way) of impervious material shall be provided
under all embankments that require fill material. The cutoff trench
shall:
(12)
In order to insure proper drainage on the floor of the basin,
a minimum grade of 2% shall be maintained for areas of sheet flow.
For channel flow, a minimum grade of 1% shall be maintained. Under
certain circumstances, such as continuous seasonal flow, the Township
may require a low-flow channel to be constructed. Wet basins may be
designed with a flat bottom upon approval by the Township Engineer.
(13)
All detention basin embankments shall be placed in a maximum
of eight-inch lifts to a minimum of 95% of maximum dry density as
established by ASTM D-1557. Prior to proceeding to the next lift,
the compaction shall be checked by the Township Engineer. The developer's
contractor shall obtain the services of a qualified laboratory technician
to conduct compaction tests on the leading and the trailing edge of
the berm along with the top of the berm. All tests shall be furnished
to the Township for review.
(14)
Whenever possible, the emergency spillway for detention basins
shall be constructed on undisturbed ground. Emergency spillways shall
be constructed of permanent material approved by the Township Engineer.
All emergency spillways shall be constructed so that the detention
basin berm is protected against erosion. The minimum capacity of all
emergency spillways shall be the peak flow rate from the one-hundred-year
design storm after development. The construction material of the emergency
spillway shall extend along the upstream and downstream berm embankment
slopes. Construction material on the upstream slope of the emergency
spillway shall extend to a minimum of two feet below the spillway
crest elevation. The downstream slope of the spillway shall as a minimum
extend to the toe of the berm embankment. The emergency spillway shall
not discharge over earthen fill and/or easily erodible material.
(15)
A minimum freeboard of six inches shall be provided between
the one-hundred-year design stormwater elevation and the invert of
the emergency spillway, and between the design flow through the emergency
spillway and the top of the berm. Bioretention BMPs with a ponded
depth less than or equal to 1.5 feet are exempt from emergency spillway
and freeboard requirements unless otherwise deemed necessary by the
Township Engineer.
(16)
Anti-seep collars shall be installed around the pipe barrel
within the normal saturation zone of the detention basin berms. The
anti-seep collars and their connections to the pipe barrel shall be
watertight. The anti-seep collars shall extend a minimum of two feet
beyond the outside of the principal pipe barrel. The maximum spacing
between collars shall be 14 times the minimum projection of the collar
measured perpendicular to the pipe. A minimum of two anti-seep collars
shall be installed on each pipe outlet.
(17)
All outlet pipes through the basin berm shall be reinforced
concrete pipe having O-ring joints. All outlet structures shall be
concrete.
(18)
Energy dissipating devices shall be placed at all basin outlets.
(19)
Easements shall be provided for all basins.
(20)
Permanent detention basins outlet structures shall be designed
to incorporate multiple stage outlet release devices.
(21)
The minimum circular orifice diameter for controlling discharge
rates from detention facilities shall be three inches. Designs where
a lesser-size orifice would be required to fully meet release rates
shall be acceptable with a three-inch orifice, provided that as much
of the site runoff as practical is directed to the detention facilities.
(22)
A cross section through the basin from the proposed pipe termination
into the basin to the termination of the outlet pipe shall be provided.
(23)
Permanent plantings for wet ponds shall be designed by a qualified
professional and shall have a mixture of plants that thrive in wet
areas.
C.
The following criteria shall be used for the design of underground
detention basins:
(1)
All below-ground stormwater detention and infiltration facilities
shall have an additional 10% of the one-hundred-year storage volume
available within the storage medium, as well as a minimum of 0.5 foot
of freeboard.
(2)
The freeboard shall be measured from the maximum water surface elevation
to the lowest overflow elevation for below-ground facilities.
A.
Storm sewers must be able to convey proposed conditions runoff from
a fifty-year design storm without flooding inlets, where appropriate.
Roadway crossings must be able to convey runoff from a one-hundred-year
design storm.
B.
On-site conveyance systems designed to carry runoff to a detention
basin must be able to transport the basin's one-hundred-year tributary
flow either in-system, in-gutter, or overland.
C.
Storm sewers, as required, shall be placed in the cartway of curbed
streets and parallel to the roadway shoulders of streets without curbs.
D.
Storm drainage pipes. The minimum diameter of all storm drainage
pipes shall be 15 inches or an equivalent thereto. All storm drainage
piping shall be laid in a straight line. Storm drainage piping shall
not be permitted under buildings or structures. The minimum grade
of piping shall be 0.5%. All pipe shall be reinforced concrete, or
smooth bore high-density polyethylene pipe. All structures exposed
to the surface shall be reinforced concrete.
E.
When proposed, manholes and inlets (catch basins) shall not be spaced
more than 400 feet apart for pipes of less than or equal to twenty-four-inch
diameter and 500 feet apart for pipes of greater than twenty-four-inch
diameter. Additional, manholes or inlets shall be placed at all changes
in alignment, grade or pipe size, and at all points of convergence
of two or more influent storm sewer lines. In addition, the following
standards shall apply:
(1)
Manholes and inlets must conform to the standards established by
PennDOT and must be supplied by a PennDOT Bulletin 15 approved supplier.
Such requirement shall be listed on the plan.
(2)
At street intersections, inlets shall be placed to prevent the flow
of water across intersections.
(3)
Inlets shall be spaced to limit the gutter spread to not more than
eight feet during the ten-year storm.
(4)
Inlets shall be placed such that flow to any single inlet shall not
exceed four cubic feet per second (cfs) for four-foot inlets and five
cfs for six-foot inlets in non-ponding areas.
(5)
Inlets with a depth greater than five feet must be provided with
ladder rungs and noted on the plan as such.
(6)
A minimum drop of two inches shall be provided between the inlet
pipe invert elevation and the outlet pipe elevation.
(7)
Inlets shall contain a marker which discourages the discharge of
anything other than stormwater into the inlet.
(8)
Inlets in paved areas shall be equipped with bicycle safe grates.
Inlets in non-paved areas shall be equipped with standard grates.
(9)
Manhole covers shall have the word "STORM" cast on the top of the
cover.
F.
Properly designed, graded (minimum two-percent bottom slope) and
lined drainage swales may be permitted in lieu of storm sewers. Swale
lining must meet the Conservation District design standards. All drainage
channels shall have a maximum side slope grade of three horizontal
to one vertical (3:1). All drainage swales shall be provided with
a minimum six-inch freeboard, measured from the top of the design
storm twenty-five-year flow to the top of the swale.
G.
Pipe underdrains and/or pavement base drains shall be provided in
areas delineated as having a "seasonal high-water table" and in areas
deemed necessary by the Township Engineer during the construction
phase of the project. Pipe underdrains and pavement base drains shall
be constructed in accordance with PennDOT Pub. 408, Section 610, as
amended.
H.
Pipe end sections and/or headwalls shall be utilized at all terminated
pipe segments.
I.
All drainage structures located within a state highway right-of-way
shall be reviewed and approved by the PennDOT. A letter from PennDOT
indicating such approval shall be submitted to the Township prior
to Township approval.
J.
Energy dissipaters shall be provided at all pipe end treatments.
K.
At the discretion of the Township, in situations in which the design
standards and requirements do not apply to the site conditions, the
Township Engineer shall suggest or provide additional and/or alternative
design methods to meet the objectives of this chapter.
L.
No stormwater conveyance facility shall be constructed within 50
feet of a special geologic feature, unless it is constructed of durable
pipe utilizing watertight joints.
M.
Drainage easements shall be provided to accommodate all storm drainage
requirements and shall be a minimum of 20 feet in width. Easements
shall be provided for all storm drainage piping and designed swales
that are not located within street rights-of-way.
A.
Where lots in a subdivision or land development are proposed to be
less than one acre, or where multifamily, or other dense housing is
proposed, the subdivider, developer or builder shall provide a drainage
system for the collection and conveyance of sump pump discharges that
will emanate from the basements of the proposed housing.
B.
The drainage system may run between rows of houses, to collect sump
pump drainage from adjacent properties when deemed appropriate by
the Township Engineer.
C.
The sump pump discharge collection system shall consist of smooth
bore plastic pipe (Sc. 40 or SD 35), and have a minimum diameter of
six inches.
D.
The collection system and sump pump discharge piping shall be a minimum
of three feet below grade.
E.
The collection system shall be designed to take discharged sump pump
water from adjacent houses and convey it by gravity to a positive
outlet at another drainage facility, such as a catch basin, where
it will be conveyed away from the residences without creating a drainage
problem. Connection to other drainage facilities may not interfere
with the receiving drainage facility function.
F.
The sump pump drainage system may also be used to convey roof water,
provided that the piping is sized appropriately.
G.
Sump pump drainage systems shall be provided with cleanouts at two-hundred-foot
intervals to permit unblocking the system if necessary.
H.
Connection of sump pumps to the collection system shall be via a
wye, tee, or saddle connection. Break-in connections shall not be
permitted.
A.
Roof drainage shall be conveyed by downspouts and other drainage
facilities to a stormwater collection facility and/or detention facility
to minimize the effects of increased runoff in multifamily or nonresidential
areas.
B.
Roof drains will not be permitted to discharge across or onto parking
areas, driveways or drive aisles in high-density residential, commercial,
shopping center or industrial districts.
C.
No roof drains shall discharge directly to streets, storm sewers,
or roadside swales without approval of the Township Engineer. Only
where topography conditions prohibit, should roof drains discharge
directly to these features.
D.
Roof drains shall discharge to infiltration areas or vegetative BMPs
to the maximum extent practicable.
E.
Roof drains may be discharged to lawns and subsequently to drainage
swales in low-density residential areas.
