A.
For purposes of stormwater management, the municipality of Baldwin
Borough is divided into stormwater management release rate zones.
These zones are designated in the Monongahela River Watershed Plan,
and a map of these zones may be found in Appendix D[1] of this chapter.
(1)
The following release rate zones are located within Baldwin Borough:
|
Zone 2
|
Zone 21
| |
|
Zone 4
|
Zone 22
| |
|
Zone 6
|
Zone 24
| |
|
Zone 9
|
Zone 32
| |
|
Zone 12
|
Zone 34
| |
|
Zone 15
|
Zone 35
| |
|
Zone 43
|
(2)
One or more of these districts may be further subdivided into subareas
which have similar hydrological characteristics and drain to a common
point.
[1]
Editor's Note: Appendix D is included at the end of this chapter.
B.
The location and boundaries of the watershed(s) and subareas are
shown on the Municipal Stormwater Management District Zone Map which
is hereby adopted as part of this section. A copy of this District
Map is available through the Borough Office.
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 required
and shall be evaluated according to the following standards:
(1)
To assure that the maximum rate of stormwater runoff is no greater
after development than prior to development activities; or
(2)
To manage the quantity, velocity and direction of resulting stormwater
runoff in a manner which otherwise adequately protects health and
property from possible injury.
B.
The stormwater management plan for the development site must consider
all the stormwater runoff flowing over the site.
C.
No discharge of toxic materials into any stormwater management system
shall be permitted. Where required by federal and state regulation,
the landowner or developer shall be responsible for obtaining a NPDES
permit for stormwater discharges.
D.
For any development that is to be constructed in stages, the applicant
must demonstrate that stormwater facilities shall be installed to
manage stormwater runoff safely during each stage of development.
E.
For the analysis of redevelopment projects 20% of the existing impervious
surface shall be considered meadow in good condition.
F.
Where existing storm sewers are reasonably accessible, proposed developments
shall be required to connect with the storm sewer system unless insufficient
capacity or other reasons can be demonstrated to prevent the connection.
The Borough Engineer shall determine what is reasonably accessible.
The stormwater performance standards contained in this article
are intended to implement the standards and criteria contained in
the Monongahela River Stormwater Management plan, adopted and approved
in accordance with the Pennsylvania Storm Water Management Act.[1] If there is any discrepancy between the provisions of
this section and the standards and criteria of the plan, or if the
watershed plan is subsequently amended, then the standards/criteria
of the current watershed plan shall govern.
[1]
Editor's Note: See 32 P.S. § 680.1 et seq.
A.
Stormwater management facilities on all development sites shall control
the peak stormwater discharge for the one-, two-, ten-, twenty-five-,
fifty-, and one-hundred-year storm frequencies. The SCS twenty-four-hour
Type II Rainfall Distribution shall be used for analyzing stormwater
runoff for both pre- and post-development conditions. The twenty-four-hour
total rainfall for these storm frequencies in the watershed are:
|
Design Storm Return Period
|
24-Hour Rainfall Depth in Inches
| |
|---|---|---|
|
1-year
|
1.97
| |
|
2-year
|
2.50
| |
|
10-year
|
3.61
| |
|
25-year
|
4.31
| |
|
50-year
|
4.40
| |
|
100-year
|
5.71
|
B.
The Modified Rational Method shall be used for analyzing the stormwater
runoff for small watersheds as specified in this article. The rainfall
intensities for the design storms are as specified. (For additional
information or data on other return periods, consult the Rainfall
Duration Frequency Tables for Pennsylvania, produced by PaDEP, Office
of Resource Management, Division of Stormwater Management, Harrisburg,
February 1983, or in its most recent update.)
A.
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 TR 20, HEC 1, Penn State Runoff Method (PSRM), Modified Rational Method, 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 § 142-63 or an approved method for analyzing these types of watersheds. The Borough 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 must be submitted regardless of the methodology used for these calculations. An approved simulation of the modified puls (Haested Method Pond 2, HEC 1) routing methodology shall be used in analysis for routing the design storm hydrographs through the detention/retention facility. The proper stage/storage and stage/discharge and routed outflow hydrographs must be submitted for approval.
B.
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 existing Tc is larger than five
minutes, it should be used as the rainfall duration.
C.
Detention/retention basin. The inflow hydrograph shall be routed
through the detention/retention facility by using the Modified Puls
method or other recognized routing method subject to the approval
of the Borough and the County. The routing and outflow hydrographs
for each design storm must be submitted.
D.
Pre-development conditions. Pre-development conditions shall be assumed
to be those which exist on any site at the time of adoption of the
Monongahela River Stormwater Management Plan. Hydrologic conditions
for all areas with pervious cover (i.e., fields, woods, lawn areas,
pastures, cropland, etc.) shall be assumed to be in good condition,
and the lowest recommended SCS runoff curve number (CN) shall be applied
for all pervious land uses within the respective range for each land
use and hydrologic soil group.
A.
Definition. The release rate percentage defines the percentage of
the pre-development peak rate of runoff that can be discharged from
an outfall on the site after development. It applies uniformly to
all land development or alterations within a subarea.
B.
Procedure for use:
(1)
Identify the specific subarea in which the development site is located
from the watershed map located in Appendix D of this Part 2[1] and obtain the subarea release rate percentage from the
following corresponding zone release rate percentages:
|
Assigned Release Rate Area
|
Release Rate Percentage
| |
|---|---|---|
|
Zone 2
|
100
| |
|
Zone 4
|
60
| |
|
Zone 6
|
60
| |
|
Zone 9
|
90
| |
|
Zone 12
|
50
| |
|
Zone 15
|
70
| |
|
Zone 21
|
50
| |
|
Zone 22
|
60
| |
|
Zone 24
|
70
| |
|
Zone 32
|
70
| |
|
Zone 34
|
50
| |
|
Zone 35
|
70
| |
|
Zone 43
|
100
|
|
All other areas not located within an assigned zone by the Monongahela
River Watershed Plan are assigned the release rate of 100%.
|
[1]
Editor's Note: Appendix D is included at the end of this chapter.
(2)
Compute the pre- and post-development runoff hydrographs for each
stormwater outfall on the development site using an acceptable calculation
method for the one-, two-, ten-, twenty-five-, fifty- and one-hundred-year
storms. If the post-development peak runoff rate and the runoff volume
is less than or equal to the pre-development peak runoff rate and
volume, then additional stormwater control shall not be required at
the outfall. If the post-development peak runoff rate and volume are
greater than the pre-development peak runoff rate and volume, then
stormwater detention shall be required. The maximum allowable release
rate form the detention facility shall be calculated by multiplying
the subarea release rate percentage by the pre-development rate of
runoff from the development site for each of the four design storms.
A.
An applicant may seek to exceed the otherwise applicable subarea
release rate percentage by performing the "no-harm evaluation." This
evaluation requires an independent engineering analysis to demonstrate
that other reasonable options exist to prevent the occurrence of increased
stormwater runoff discharge rates and/or velocities, or that measures
can be provided to prevent increased stormwater discharge rates and/or
velocities from increasing flood elevations and accelerating erosion
at all downstream points in the watershed.
B.
A no-harm evaluation will be considered only in the following instances:
(1)
In the Monongahela River Watershed: only in instances where the discharge
to a stream channel from the development site occurs directly to the
Monongahela River, to an adequately sized storm or combined sewer
which discharges directly into the Monongahela River, or through a
properly sized and designed regional stormwater detention facility.
C.
The analysis for the no-harm evaluation shall be submitted to the
Municipal Engineer and the Allegheny County Planning Department for
review and approval.
D.
The no-harm evaluation shall be prepared by a registered engineer
who is experienced in hydrology and hydraulics. The no-harm evaluation
analysis shall be completed using the following procedure.
E.
The analysis shall be completed using an approved calculation methodology
in the following manner:
(1)
Develop the runoff hydrograph(s) for the design storms of the site
and areas tributary to it using the approved methodology for pre-development
conditions using the land use characterizations contained in the Monongahela
River Watershed Stormwater Management Plan.
(2)
Develop the post-development discharge hydrograph from the proposed
site using the approved methodology.
(3)
Subtract the runoff hydrograph ordinates under pre-development conditions
(Step 1) from the discharge hydrograph ordinates (Step 2), maintaining
the time scales of both hydrographs for one-to-one correspondence.
(4)
Obtain the hydraulic model for the existing conditions for the Monongahela
River Watershed from the county.
(5)
Locate the subbasin(s) in which the proposed development is located
and into which the discharge hydrograph enters. If more than one subbasin
receives this incremental flow, divide the flow accordingly.
(6)
Add the incremental increase computed in Step 3 to the runoff hydrograph
of the subbasin(s) identified in Step 5.
(7)
Route the adjusted runoff hydrograph through the Monongahela River
Watershed Model and note any increase in peak flows which would occur
in downstream subbasins. If no increase is noted, then the no-harm
has been demonstrated. If no increase is observed in peak flows, the
increased potential for erosion and/or sedimentation in downstream
channels resulting from any change in the flood hydrograph predicted
by the model shall be evaluated. If no increased potential can be
demonstrated by appropriated technical means, then the "No-Harm" exemption
may be requested.
(8)
If an increase in peak flow is observed in any of the downstream
subbasins or increased potential for erosion and/or sedimentation
is indicated, the no-harm exemption shall not be granted.
A.
General design guidelines.
(1)
Applicants may select runoff control techniques, or combinations
of techniques, as provided in this Part 2, Stormwater Management Ordinance,
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 2.
(2)
In selecting and designing stormwater management systems and controls,
applicants may be guided by the following references:
(a)
Urban Hydrology for Small Watersheds, Technical Release No.
55, USDA, Soil Conservation Service, June 1986 (or most recent edition).
(b)
Part II Design Manual, The Pennsylvania Department of Transportation,
1990 Edition (or most recent update).
(c)
HEC 1 and HEC 2, U. S. Army Corps of Engineers, most recent
edition.
(d)
Soil Erosion and Sedimentation Control Manual, Pennsylvania
Department of Environmental Resources, March 1982.
(e)
Standards and Specifications for Soil Erosion and Sediment Control,
Maryland Water Resources Administration, 1983.
(f)
Urban Stormwater Management, Special Report No. 49, American
Public Works Administration, 1981.
(g)
Water Resources Protection Measures in Land Development - A
Handbook, University of Delaware Water Resources Center, April 1974.
(h)
Design and Construction of Sanitary and Storm Sewers, WPCF Manual
of Practice No. 9, Water Pollution Control Federation, 1970.
(3)
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.
(5)
Infiltration practices shall be used to the extent practicable to
reduce peak runoff and promote groundwater recharge. A combination
of successive practices may be used to achieve the applicable minimum
control requirements. Justification shall be provided by the applicant
for rejecting each of the preferred practices based on actual site
conditions.
B.
Criteria for stormwater detention facilities.
(1)
If detention facility(ies) are utilized for the development site,
the facility(ies) shall be designed such that post-development peak
runoff rates from the developed site are controlled to those rates
defined by the subarea release rate percentage for the one-, two-,
ten-, twenty-five-, fifty-, and one-hundred-year storm frequencies.
(2)
All detention facilities shall be equipped with outlet structures to provide discharge control for the four designated storm frequencies. Provisions shall also be made to safely pass the post-development one-hundred-year storm runoff in the event of an outlet structure failure without damaging or impairing the continued function of the facilities (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. Should any stormwater management facilities qualify as a dam under PaDEP Chapter 105 criteria, the facility shall be designed in accordance with those regulations and meet the regulations concerning dam safety.
(3)
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.
(4)
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 and Pennsylvania Department of Environmental Protection Chapter 105 Regulations.
(5)
Other considerations which should be incorporated into the design
of the detention facilities include the following:
(a)
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.
(b)
Control and removal of debris both in the storage structure
and in all inlet or outlet devices must be a design consideration.
Trash racks on the outlet structures are required to minimize debris
that enters the structure.
(c)
Inflow and outflow structures, pumping stations and other structures
should be protected and designed to minimize safety hazards.
(d)
The interior slope of a detention/retention basin shall be 3:1
(horizontal to vertical). The water depth at the perimeter of a storage
pond should be limited to that which is safe for children. This is
especially necessary if bank slopes are steep or if ponds are full
and recirculating in dry periods. Restriction of access (fences, walls,
etc.) may be necessary depending on the location of the facility.
(e)
Detention/retention basins shall require a supporting report
from a geotechnical engineer including design recommendations for
embankment construction, interior and exterior slopes, drainage swales
and infiltration areas, and shall be constructed under the supervision
of the geotechnical engineer.
(f)
Landscaping must be provided for the facility which harmonizes
with the surrounding area. Landscaping of the pond embankment shall
not be permitted.
(g)
Facility must be accessible for maintenance purposes, considering
the frequency and type of equipment that will be required. A maintenance
schedule must be submitted and approved by the Borough.
(h)
Details of the facility must be shown on the plan.
(i)
A riprap channel with underdrain for low flows must be sized
properly and provided within the pond.
(j)
If an underground facility (i.e., tank, sump) is proposed, a
cleanout manhole with steps must be provided.
(k)
An emergency spillway must be provided for all detention/retention
facilities. For aboveground basins, the spills must be sized to pass
the entire one-hundred-year post-development storm with one foot of
freeboard.
(l)
Outlet control structures shall be constructed of reinforced
concrete (cast-in-place, precast or block) and provided with trash
racks approved by the Borough.
(m)
All impoundment areas shall be adequately underdrained to prevent
long-term ponding of water.
(n)
All detention facilities shall be provided with an access road
(with a legal easement) for maintenance purposes. Such roads shall
be minimum of 10 feet wide, have a maximum grade of 15% and be of
a stone or impervious surface.
(o)
All pond outlet structures shall have suitable gaskets to prevent
leakage and piping of water through the pond embankment. All storm
pipe installed through the pond embankment must be constructed of
reinforced concrete pipe.
(p)
An as-built drawing shall be required for each stormwater detention
facility constructed. (The drawing shall represent the volume of the
facility and the stage-storage relationship.) 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.
(q)
The exterior slope of a detention/retention pond shall not exceed
three horizontal to one vertical.
(r)
Normally dry, open-top, storage BMPs should drain in 24 hours
to 72 hours.
(s)
Infiltration BMPs should be spread out and shallow as much as
practicable.
C.
Criteria for collection/conveyance facilities. All stormwater runoff
collection or conveyance facilities, whether storm sewers 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. Low side 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 with cradle when traversing fill areas of indeterminate
stability; designed with anchors when gradient exceeds 20%, and designed
with encasement or special backfill requirements when traversing under
a paved area.
(d)
Designed to adequately handle 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 Borough and/or applicable requirements of the
Pennsylvania Department of Transportation Specifications, Publication
408, latest edition.
(f)
All pipe shall be made of either PVC, corrugated plastic pipe,
corrugated plastic pipe with smooth interior walls, corrugated metal/steel
or reinforced concrete. 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.
Proposed erosion and sedimentation pollution control measures should be submitted with the stormwater management plan. These plans shall follow the guidelines set forth in the Borough's current Erosion and Sediment Pollution Control Ordinance under a separate cover. In addition, the plan shall be prepared in accordance with the Pennsylvania Erosion/Sedimentation Regulations (25 Pa. Code, Chapter 102) and the standards and guidelines of the county conservation district.
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.
B.
DEP has regulations that require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more
under 25 Pa. Code § 102.4(b).
C.
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. The issuance of an NPDES construction permit (or permit coverage under the statewide General Permit (PAG-2) satisfies the requirements Subsection C.
E.
A copy of the erosion and sediment control plan and any required,
permit, as required by DEP regulations, shall be available at the
project site at all times.
Prior to releasing the final bond moneys, the Borough shall
receive an as-built drawing, signed and scaled by a registered surveyor,
and these plans shall include the following:
A.
The location of any roads or driveways constructed for the land development
or single-family home.
B.
The location, tops and flow lines of all storm sewers constructed
within the land development or single-family home.
C.
The location, depth and contours of the detention or retention facilities
constructed for the land development or single-family home.
D.
A detail of the outlet structure for the detention or retention facility
as constructed.
E.
The location and details of all post-construction stormwater management
BMPs including water quality devices.
F.
A revised stormwater management report reflecting the final contours
for a detention pond as constructed.
