The Tobyhanna Creek Watershed has been divided
into three stormwater management districts as follows:
A. District A.
|
Subareas
|
Postdevelopment
(years)
|
Predevelopment
(years)
|
---|
|
1-4, 7, 8
|
2.33
|
2.33
|
|
21-40, 43
|
10
|
10
|
|
45-67, 73
|
50
|
50
|
B. District B.
|
Subareas
|
Postdevelopment
(years)
|
Predevelopment
(years)
|
---|
|
5, 6
|
10
|
2.33
|
|
9-20
|
50
|
10
|
C. District C. Development sites which can discharge
directly to a stream or watercourse, may do so without control of
postdevelopment peak rate of runoff. If the postdevelopment runoff
is intended to be conveyed to a stream or watercourse, assurance must
be provided that such system has adequate capacity to convey the increased
peak flows. When adequate capacity of the downstream system does not
exist and will not be provided through improvements, the postdevelopment
peak rate of runoff must be controlled to the predevelopment peak
rate as required in District A provisions (postdevelopment flows to
predevelopment flows for the two-and-thirty-three-hundreths-, ten-
and fifty-year storms). The subwatershed areas which are included
in this district are: 41, 42, 44, 68-72, and 74-77.
The following are the minimum water quality
criteria established to meet the objective of the plan. Because the
standard for water quality may result in a fairly small outlet orifice
in detention facilities, the Municipal Engineer shall require a minimum
standard such as a three-inch diameter orifice.
A. In addition to the performance standards and design
criteria requirements of Article XXI of this article, the land developer
shall comply with the following water quality requirements of this
article unless otherwise exempted by the provisions of this article.
In addition to the water quantity requirements of this article, the
land developer shall control for water quality as follows:
(1) Residential areas.
(a)
Stormwater management facilities shall be provided
to detain the one-year, twenty-four-hour design storm using the SCS
Type II distribution or applicable methedology. Provisions shall be
made so that the one-year storm takes a minimum of 24 hours to drain
from the facility from a point where the maximum volume of water captured
from the facility for the one-year storm is achieved (i.e., the maximum
water surface elevation achieved in the facility). On a smaller site
such as where the time-of-concentration is 15 minutes or less, the
criteria shall apply to the appropriate calculation methodology, i.e.,
the modified rational method.
(b)
Release of this water can begin at the start
of the storm (i.e., the invert of the water quality orifice is at
the invert of the facility). The design of the facility shall consider
and minimize the chances of clogging and sedimentation potential.
(2) Commercial/industrial areas. Commercial and industrial
sites shall detain the first 1/2 inch of runoff for a twenty-four-hour
period.
(3) As an alternative to the requirements of Subsection
A(1) and
(2), the water quality objectives may be achieved through a combination of BMP's including, but not limited to, infiltration structures, detention/retention basins, vegetated filter strips and buffers or any combination of BMP's listed in Appendix 4 of the Tobyhanna Creek Watershed Stormwater Management Plan, Volume II. The combination of BMP's shall be designed according to the requirements listed under §
124-80D, BMP selection criteria, and in consultation with the Municipal Engineer.
B. To accomplish Subsection
A above, the land developer may submit original and innovative designs to the Municipal Engineer for review and approval.
C. In selecting the appropriate BMPs or combinations
thereof, the land developer shall consider the following at a given
development site.
(1) Total contributing drainage area.
(2) Permeability and infiltration rate of the site soils
(hydrologic soil group).
(3) Slope and depth to bedrock (soils).
(4) Seasonal high-water table.
(5) Proximity to building foundations and well heads.
(7) Land availability and configuration of the topography.
D. The following additional factors SHOULD be considered
when evaluating the suitability of the EMP's used to control water
quality at a given development site:
(1) Peak discharge and required volume control.
(3) Efficiency of the BMP's to mitigate potential water
quality problems.
(4) The volume of runoff that will be effectively treated.
(5) The nature of the pollutant being removed.
(6) Maintenance requirements.
(7) Creation/protection of aquatic and wildlife habitat.
(9) Enhancement of aesthetic and property value.
Stormwater runoff from all development sites
shall be calculated using either the rational method or a soil-cover-complex
methodology.
A. Any stormwater runoff calculations involving drainage
areas greater than 200 acres, including on- and off-site areas, shall
use a generally accepted calculation technique that is based on the
NRCS soil-cover-complex-method. Table 1 summarizes acceptable computation methods. It is assumed
that all methods will be selected by the design professional based
on the individual limitations and suitability of each method for a
particular development site. The municipality may approve the use
of the rational method to estimate peak discharges from drainage areas
that contain less than 200 acres.
B. All calculations consistent with the requirements
of this article using the soil-cover-complex method shall use the
appropriate design rainfall depths for the various return period storms
presented in Table B-1 in Appendix B of this article. If a hydrologic computer model such as PSRM or HEC-1 is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The NRCS S-curve shown in Figure B-1, Appendix
B of this article shall be used for the rainfall distribution.
C. For the purposes of predevelopment flow rate determination,
undeveloped land shall be considered as meadow conditions, unless
the natural ground cover generates a lower curve number or rational
C value (i.e., forest).
D. All calculations using the rational method shall use
rainfall intensities consistent with appropriate times of concentration
for overland flow and return periods from the design storm curves
from PA Department of Transportation Design Rainfall Curves (1986)
(Figure B-2), Appendix B. Times-of-concentration for overland flow
shall be calculated using the methodology presented in Chapter 3 of
Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced
from time to time by NRCS). Times-of-concentration for channel and
pipe flow shall be computed using Manning's equation.
E. Runoff curve numbers (CN) for both existing and proposed
conditions to be used in the soil-cover-complex method shall be obtained
from Table B-2 in Appendix B of this article.
F. Runoff coefficients (c) for both existing and proposed
conditions for use in the rational method shall be obtained from Table
B-3 in Appendix B of this article.
G. Where uniform flow is anticipated, the Manning equation
shall be used for hydraulic computations and to determine the capacity
of open channels, pipes, and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Table B-4 in Appendix B of
the article. Outlet structures for stormwater management facilities shall be designed to meet the performance standards in §
124-78 of this article using any generally accepted hydraulic analysis technique or method.
H. The design of any stormwater detention facilities intended to meet the performance standards of this article in §
124-78 of this article shall be verified by routing the design storm hydrograph through these facilities using the storage-indication method. For drainage areas greater than 20 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality may approve the use of any generally accepted full hydrograph approximation technique which shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
I. The municipality has the authority to require that
computed existing runoff rates be reconciled with field observations
and conditions. If the designer can substantiate through actual physical
calibration that more appropriate runoff and time-of-concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and recommendations of the Municipal Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question. Calibration should not be misrepresented
to mean a best guess as to what might happen based on experience,
as is oftentimes the case.
|
TABLE 1
Acceptable Computation Methodologies for Stormwater Management
Plans
|
---|
|
Method
|
Method Developed By
|
Applicability
|
---|
|
TR-20 (or commercial package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer
model is desirable or necessary
|
|
TR-55 (or commercial computer package based
on TR-55)
|
USDA NRCS
|
Applicable for land development plans within
limitations described in TR-55
|
|
HEC-1
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer
model is desirable or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer
model is desirable or necessary; simpler than TR-20 or HEC-1
|
|
Rational method (or commercial computer package
based on rational method)
|
Emil Kuichling (1889)
|
For sites less than 20 acres, or as approved
by the municipality and Municipal Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by
the municipality and Municipal Engineer
|
The ability to retain and maximize the groundwater
recharge capacity of the area being developed is encouraged. Design
of the stormwater management facilities shall give consideration to
providing groundwater recharge to compensate for the reduction in
the percolation that occurs when the ground surface is paved and roofed
over. A detailed geologic evaluation of the development site shall
be performed to determine the suitability of recharge facilities.
The evaluation shall be performed by a qualified geologist and/or
soil scientist, and at minimum, address soil permeability, depth to
bedrock, susceptibility to sinkhole formation,and subgrade stability.
Where pervious pavement is permitted for parking lots, recreational
facilities, nondedicated streets, or other areas, pavement construction
specifications shall be noted on the plan.