In addition to the performance standards and design criteria
requirements of this article, the applicant shall comply with the
following water quality requirements of this article:
A. Adequate storage and treatment facilities will be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under §
173-14 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P).
To achieve this goal, the following criterion is established:
The following calculation formula is to be used to determine
the water quality storage volume, (WQv), in acre-feet of storage for
the Delaware River South Watershed:
WQv = [(P)(Rv)(A)]/12
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Where:
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WQv = Water Quality Volume (acre-feet).
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P = Rainfall amount equal to 90% of events producing
this rainfall (in).
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A = Area of the project contributing to the water
quality BMP (acres).
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Rv = 0.05 + 0.009(I) where I is the percent of the
area that is impervious surface (impervious area/A*100).
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The P value for the five PennDOT rainfall regions is
shown in Figure B-2 in Appendix B of this chapter and as shown in
Appendix Table B-5 of this chapter. Since the Delaware River South
is in PennDOT Region 5, the P value to be utilized to meet this requirement
is 2.04 inches.
B. Design
of BMPs used for water quality control shall be in accordance with
design specifications outlined in the Pennsylvania Handbook of Best
Management Practices for Developing Areas or other applicable manuals.
The following factors shall be considered when evaluating the suitability
of BMPs used to control water quality at a given development site:
(1) Total
contributing drainage area.
(2) Permeability
and infiltration rate of the site soils.
(3) Slope
and depth to bedrock.
(4) Seasonal
high water table.
(5) Proximity
to building foundations and well heads.
(7) Land
availability and configuration of the topography.
(8) Peak
discharge and required volume control.
(10) Efficiency of the BMPs to mitigate potential water quality problems.
(11) The volume of runoff that will be effectively treated.
(12) The nature of the pollutant being removed.
(13) Maintenance requirements.
(14) Creation/protection of aquatic and wildlife habitat.
(16) Enhancement of aesthetic and property value.
C. To accomplish
the above, the applicant shall submit original and innovative designs
to the municipality for review and approval. Such designs may achieve
the water quality objectives through a combination of BMPs (best management
practices).
D. The BMPs
must be designed, implemented and maintained to meet state water quality
requirements, and any other more stringent requirements as determined
by the municipality.
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 shall use a generally accepted calculation technique
that is based on the NRCS soil cover complex method. Table 173-19-1
summarizes acceptable computation methods. It is assumed that all
methods will be selected by the applicant based on the individual
limitations and suitability of each method for a particular site.
B. The municipality
may allow the use of the rational method to estimate peak discharges
from drainage areas that contain less than 200 acres. The rational
method is recommended for drainage areas under 100 acres.
C. All calculations
consistent with this chapter using the soil cover complex method shall
use the appropriate design rainfall depths for the various return
period storms according to the region for which they are located as
presented in Table B-1 in Appendix B of this chapter. If a hydrologic
computer model such as HEC-1 or HEC-HMS is used for stormwater runoff
calculations, then the duration of rainfall shall be 24 hours. The
SCS curve shown in Figure B-1, Appendix B of this chapter shall be
used for the rainfall distribution.
D. For the
purposes of predevelopment flow rate determination, undeveloped land
shall be considered as "meadow" in good condition, unless the natural
ground cover generates a lower curve number or rational "C" value
(i.e., forest), as listed in Table B-2 or B-3 in Appendix B of this
chapter. For areas of prior mining disturbance (i.e., strip mining,
mine spoil areas, etc.), the designer must first identify in which
mining affected area the site is located, using the Management District
Map in Appendix D of this chapter. The appropriate curve number or
rational "C" value from Table B-2 or Table B-3 in Appendix B of this
chapter should then be used.
E. 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 the Pennsylvania Department
of Transportation (PennDOT) Design Rainfall Curves (1986) (Figures
B-2 to B-3). 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.
F. 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 chapter.
G. 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 chapter.
H. Where
uniform flow is anticipated, Manning’s 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
this chapter.
I. Outlet
structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
J. The design
of any stormwater detention facilities intended to meet the performance
standards of this chapter shall be verified by routing the design
storm hydrograph through these facilities using the storage-indication
method. For drainage areas greater than 200 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 that
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
Table 173-19-1. Acceptable Computation Methodologies For Stormwater
Management Plans
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Method
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Method Developed By
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Applicability
|
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TR-20 (or commercial computer package based on TR-20)
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USDA NRCS
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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
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HEC-1, HEC-HMS
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US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
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PSRM
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Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
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Rational method (or commercial computer package based on rational
method)
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Emil Kuichling (1889)
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For sites less than 200 acres, or as approved by the municipality
and/or Municipal Engineer
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Other methods
|
Varies
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Other computation methodologies approved by the municipality
and/or Municipal Engineer
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Development on lands adjacent to the Delaware Canal which propose
to discharge stormwater into the canal must obtain special right-of-way
approval from the Department of Conservation and Natural Resources
(DCNR). The DCNR has established a separate policy for granting right-of-way
for stormwater drainage into the Delaware Canal, which shall be used
in conjunction with this chapter.