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 §
228-10 herein 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:
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WQv
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=
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[(P)(Rv)(A)]/12
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Where:
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WQv
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=
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Water quality volume (acre-feet)
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P
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=
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Rainfall amount equal to 90% of events producing
this rainfall (inches)
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A
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=
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Area of the project contributing to the water
quality BMP (acres)
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Rv
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=
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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. Since the Delaware River South is in PennDOT Region 5,
the P value to be utilized to meet this requirement is 2.04 inches.
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B. Design of BMPs used for water quality control shall
be in accordance with design specifications outlined in the Pennsylvania
Stormwater Best Management Practices Manual 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:
[Amended 9-20-2022 by Ord. No. 2022-04]
(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).
Stormwater runoff from all development sites
shall be calculated using the following methodology.
A. Stormwater runoff calculations for the design of stormwater
detention/retention facilities shall use a generally accepted calculation
technique that is based on the NRCS soil cover complex method. Table
228-15:1 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. 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 'S' curve shown in Figure B-1, Appendix
B of this chapter shall be used for the rainfall distribution.
C. 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 (i.e.,
forest), as listed in Table B-2 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 area
the site is located, using the Management District Map in Appendix
D herein. The appropriate curve number from Table B-2 or Table B-3
herein shall then be used.
D. 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. For stormwater detention/retention facilities, 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.
F. For storm sewer facilities, 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.
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 chapter.
H. 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.
I. 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.
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Table 228-15:1. Acceptable Computation
Methodologies For Stormwater Detention/Retention Facilities
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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
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|
TR-55 (or commercial computer package based
on TR-55)
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USDA NRCS
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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
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Applicable where use of a hydrologic computer
model is desirable or necessary; simpler than TR-20 or HEC-1
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Developments 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.