[Ord. 621, 2/22/1993, § 601]
The following provisions shall be considered the overriding
performance standards against which all proposed stormwater control
measures shall be evaluated:
A.
Any landowner and any person engaged in the alteration or development
of land which may affect stormwater runoff characteristics shall implement
such measures as are reasonably necessary to prevent injury to health,
safety or other property. Such measures shall include actions as are
required:
B.
The stormwater management plan for the development site must consider
all the stormwater runoff flowing over the site.
C.
Where existing storm sewers are reasonably accessible, proposed developments
may be required to connect with the storm sewer system unless insufficient
capacity or other reasons can be demonstrated to prevent the connection.
D.
No discharge of toxic materials into any stormwater management system
shall be permitted.
E.
For any development that is to be constructed in stages, the applicant
must demonstrate that stormwater facilities will be installed to manage
stormwater runoff safely during each stage of development.
[Ord. 621, 2/22/1993, § 602]
The stormwater performance standards contained in this section
are intended to be used as a guide in applying and interpreting all
specific ordinance requirements. Unless otherwise specified, the following
provisions shall apply.
[Ord. 621, 2/22/1993, § 603; as amended by Ord.
685, 5/11/1998]
1.
Stormwater management facilities on all development sites shall control
the peak stormwater discharge for the two-, ten-, twenty-five- and
one-hundred-year design storms. The SCS 24 hour, Type II Distribution
rainfall depths which shall be used for analyzing stormwater runoff
with the TR-55 methodology are as follows:
|
Design Storm Return Period
|
24-Hour Rainfall Depth in Inches
| |
|---|---|---|
|
2-year
|
2.6
| |
|
10-year
|
3.9
| |
|
25-year
|
4.5
| |
|
100-year
|
5.2
|
2.
The Rational Method shall be used for analyzing the stormwater runoff
for small watersheds as specified in this Part. The rainfall intensities
for the design storms are as specified:
A.
(For additional information or data on other return periods, consult
the "Rainfall Duration Frequency Tables for Pennsylvania," produced
by PA DEP Office of Resource Management, Bureau of Dams and Waterways
Management, Division of Stormwater Management, Harrisburg, February
1983, or in its most recent update.)
|
Design Storm Return Period
|
5-Minute Duration Rainfall in Hour
| |
|---|---|---|
|
2
|
4.1
| |
|
10
|
5.2
| |
|
25
|
5.6
| |
|
100
|
6.5
|
[Ord. 621, 2/22/1993, § 604]
1.
Development Sites. For the purposes of computing peak discharges and runoff hydrographs from development sites and drainage areas larger than three acres, calculations shall be performed using the methodologies presented in SCS Publication, Technical Release 55 (TR 55) or an approved method for analyzing these types of watersheds. For development sites less than three acres, the Rational Method may be utilized using the design storm criteria shown in § 26-153 or an approved method for analyzing these types of watersheds. The Borough Engineer may approve the use of simulation computer programs for the stormwater analysis and design. The appropriate calculations and worksheets or acceptable computer printouts as approved by the Borough Engineer must be submitted regardless of the methodology used for these calculations. An approved simulation of the modified puls (Hastead's Method Pond 2, HEC 1) methodology shall be used in analysis for routing the design storm hydrographs through the detention/retention facility. The proper stage/storage charts and outflow hydrographs must be submitted for approval. All graphs and/or charts used must be submitted.
2.
Stormwater Collection/Conveyance Facilities. For the purpose of designing storm sewers, open swales and other stormwater runoff collection and conveyance facilities, the Rational Method shall be applied. Rainfall intensities for a twenty-five year design storm with a minimum five minute duration must be used and should be obtained from the Pennsylvania Department of Transportation's Part II Manual, Chapter 10, Rainfall Intensity Charts. However, if the exiting Tc is larger than five minutes, it should be used as the rainfall duration.
3.
Detention/Retention Pond. The inflow hydrograph shall be routed through
the detention/retention facility by using the modified puls method.
When using the rational method for developing the inflow hydrograph,
the detention time must be a minimum of 30 minutes for a five minute
storm intensity as specified previously. The routing charts and outflow
hydrographs for each design storm must be submitted.
[Ord. 621, 2/22/1993, § 605; as amended by Ord.
685, 5/11/1998]
1.
General Design Guidelines.
A.
Applicants may select runoff control techniques, or combinations
of techniques, which are most suitable to the level of stormwater
runoff control required, the type of development, and the natural
features of the site. Cost of maintenance shall be one of the considerations
in the designs selected. All controls are subject to the approval
of the Borough Engineer. The Engineer may request specific information
on design and/or operating features of the proposed stormwater controls
in order to determine their suitability and adequacy in terms of the
standards of this Part.
B.
In selecting and designing stormwater management systems and controls,
applicants may be guided by the following references:
(1)
"Urban Hydrology for Small Watersheds," Technical Release No.
55, USDA, Soil Conservation Service, June 1986 (or most recent edition).
(2)
"Part II Design Manual," the Pennsylvania Department of Transportation,
1990 edition (or most recent update).
(3)
"HEC 1 and HEC 2," U.S. Army Corps of Engineers, most recent
edition.
(4)
"Soil Erosion and Sedimentation Control Manual," Pennsylvania
Department of Environmental Protection, March 1982.
(5)
"Standards and Specifications for Soil Erosion and Sediment
Control," Maryland Water Resources Administration, 1983.
(6)
"Urban Stormwater Management," Special Report No. 49, American
Public Works Administration, 1981.
(7)
"Water Resources Protection Measures in Land Development - A
Handbook," University of Delaware Water Resources Center, April 1974.
(8)
"Design and Construction of Sanitary and Storm Sewers," WPCF
Manual of Practice No. 9, Water Pollution Control Federation, 1970.
C.
The applicant should consider the effect on the proposed stormwater
management techniques of any special soil conditions or geological
hazards which may exist on the development site. In the event such
conditions are identified on the development site, the Borough may
require in-depth studies by a competent geotechnical engineer.
2.
Criteria for Stormwater Detention Facilities.
A.
If detention facility(ies) are utilized for the development site,
the facility(ies) shall be designed so that the post development peak
runoff rates from the site are controlled to those rates calculated
for the predevelopment peak runoff rate for the aforementioned design
storms as well.
B.
All detention facility(ies) shall be designed and equipped to safely
pass the post-development one-hundred-year storm runoff flows without
damaging (i.e., impairing the continued function of) the facilities.
The facility(ies) must have an emergency outlet which is able to discharge
the post development one-hundred-year peak flow.
C.
Shared storage facilities, which provide detention of runoff for
more than one development site, may be considered within a single
subarea. Such facilities shall meet the design criteria contained
in this section. In addition, runoff from the development sites involved
shall be conveyed to the facility in a manner so as to avoid adverse
impacts, such as flooding or erosion, to channels and properties located
between the development site and the shared storage facilities.
D.
Where detention facilities will be utilized, multiple-use facilities,
such as lakes, ballfields or similar recreational uses, shall be considered
first as storage facilities wherever feasible, subject to the approval
of the Borough.
E.
Other considerations which should be incorporated into the design
of the detention facilities include:
(1)
Inflow and outflow structures should be designed and installed
to prevent erosion and bottoms of impoundment type structures must
be protected from soil erosion. All outlet velocities must be calculated
and submitted. If riprap is used, the sizing calculations must be
submitted.
(2)
Control and removal of debris both in the storage structure
and in all inlet or outlet devices must be a design consideration.
(3)
Inflow and outflow structures, pumping stations, and other structures
should be protected and designed to minimize safety hazards.
(4)
The interior slope of a detention/retention pond shall be 3:1.
Restriction of access (fences, walls, etc.) may be necessary depending
on location of the facility.
(5)
Detention/retention ponds with a water depth greater than 10
feet shall require a supporting report from a geotechnical engineer
and shall be constructed under the supervision of the geotechnical
engineer.
(6)
Landscaping must be provided for the facility which harmonizes
with the surrounding area.
(7)
Facility must be accessible for the maintenance purposes, considering
the frequency and type of equipment that will be required. A maintenance
schedule must be submitted and approved by the Borough.
(8)
Details of the facility must be shown on the plan.
(9)
A riprap channel with underdrain for low flows must be sized
properly and provided within the pond.
(10)
The minimum required freeboard for a detention/retention pond
is one foot and for a detention tank or sump is 1/2 of a foot above
the one-hundred year water surface elevation.
(11)
If an underground facility (i.e., tank, sump) is proposed, a
cleanout manhole with steps must be provided.
(12)
An emergency spillway must be provided for all detention/retention
facilities. For basins, the spillway must be sized to pass the entire
one-hundred-year post development storm with one foot of freeboard.
Underground systems must have an emergency outlet set above the 0.5
foot of required freeboard capable of passing the one-hundred-year
post-development storm.
(13)
Outlet control structures shall be constructed of reinforced
concrete (cast-in-place, precast, or block) and provided with debris
grates approved by the Borough Engineer.
(14)
All impoundment areas shall be adequately underdrained to prevent
long term ponding of water.
(15)
All detention facilities shall be provided with an access road
(with a legal easement) for maintenance purposes. Such roads shall
be a minimum of 10 feet wide, have a maximum grade of 15%, and be
of a stone or impervious surface. Control and removal of debris both
in the storage facility and in all inlet or outlet devices shall be
a design consideration. Inflow and outflow structures, pumping stations
and other structures shall be protected and designed to minimize safety
hazards. Landscaping shall be provided for the facility which harmonizes
with the surrounding area.
(16)
An as-built drawing shall be required for each stormwater detention
facility constructed. The drawing shall represent an engineering certification
of the volume of the facility and its depth vs. storage relationship.
This relationship shall be shown on the drawing in table form. The
drawing shall be stamped by a registered professional engineer and
submitted to the Borough within 60 days of the completion of the facility.
No facility will be accepted until this requirement has been fulfilled.
3.
Criteria for Collection/Conveyance Facilities.
A.
All stormwater runoff collection or conveyance facilities, whether
storm sewer or other open or closed channels, shall be designed in
accordance with the following basic standards:
(1)
All sites shall be graded to provide drainage away from and
around the structure in order to prevent any potential flooding damage.
(2)
Lots located on the high side of streets shall extend roof and
French drains to the storm sewer in the street or to the gutter line
of the street if no sewer exists. Lowside lots shall extend roof and
French drains to a stormwater collection/conveyance system or natural
watercourse in accordance with the approved stormwater management
plan for the development site.
(3)
Collection/conveyance facilities should not be installed parallel
and close to the top or bottom of a major embankment to avoid the
possibility of causing the embankment to fail.
(4)
All collection/conveyance facilities shall be designed to convey
the twenty-five-year storm peak flow rate from the contributing drainage
area and to carry it to the nearest suitable outlet or natural watercourse.
Off-site conveyance shall be provided with easements to an existing
storm sewer system or natural watercourse.
(5)
Where drainage swales or open channels are used, they shall
be suitably lined to prevent erosion and designed to avoid excessive
velocities. Calculations must be presented for the velocities and
it must be shown that the proposed surface for the swale is adequate.
(6)
Wherever storm sewers are proposed to be utilized, they must
comply with the following criteria:
(a)
Designed to traverse under seeded and planted areas. If constructed
within 10 feet of the road paving, walks or other surfaced areas,
the drains shall have a narrow trench, and maximum compaction of backfill
to prevent settlement of the superimposed surface or development.
(b)
Installed after excavating and filling in the area to be traversed
is completed, unless the drain is installed in the original ground
with a minimum of three feet cover and/or adequate protection during
the fill construction.
(c)
Designed: (1) with cradle when traversing fill areas of indeterminate
stability, (2) designed with anchors when gradient exceeds 20%, and
(3) designed with encasement or special backfill requirements when
traversing under a paved area.
(d)
Designed to handle adequately the anticipated stormwater flow
and to be constructed and maintained economically. The minimum pipe
size shall be 15 inches in diameter.
(e)
Drain pipe, trenching, bedding and backfilling requirements
shall conform to the requirements of the Borough and/or applicable
requirements of the Pennsylvania Department of Highways Specifications,
current Form 408.
(f)
All corrugated metal pipe shall be polymer coated, with bonding
and paved inverts where prone to erode. Pipe under a Borough cartway
shall be reinforced concrete pipe with a minimum diameter of 15 inches.
(g)
Storm inlets and structures shall be designed to be adequate,
safe, self-cleaning and unobtrusive and shall be consistent with the
standards of the Borough.
(h)
Appropriate grates shall be designed for all catch basins, stormwater
inlets and other entrance appurtenances in accordance with Borough
specifications.
(i)
Manholes shall be designed so that the top shall be at finished
grade and sloped to conform to slope of finished grade. Top castings
of structures located in roads or parking areas shall be machined
or installed to preclude "rattling."
(j)
Where a proposed storm sewer connects with an existing storm
sewer system, the applicant shall demonstrate that sufficient capacity
exists in the downstream system to handle the additional stormwater
flow.
(k)
Storm sewer outfalls shall be equipped with energy dissipators
to prevent erosion and conform with applicable requirements of the
PA DEP for stream encroachments (Chapter 105 of the Department's
rules and regulations).
[Ord. 621, 2/22/1993, § 606]
Proposed erosion/sedimentation measures should be submitted with the stormwater management plan. These plans shall follow the guidelines set forth in the Borough's Erosion and Sediment Pollution Control Ordinance [Chapter 9].
