[Amended 7-14-2014 by Ord. No. 2014-04]
A. Conococheague Creek Watershed has been divided into stormwater management
districts as shown on the Watershed Map in Appendix D. The Borough of Chambersburg is located entirely within
District A.
B. In addition to the requirements specified below, the groundwater recharge (§
251-16), water quality (§
251-17), and streambank erosion (§
251-18), requirements shall be implemented.
C. Standards for managing runoff are shown below. Development sites
located in the Borough of Chambersburg must control postdevelopment
runoff rates to predevelopment runoff rates for the design storms
as follows:
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Design Storm Postdevelopment
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Design Storm Predevelopment
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Two-year
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One-year
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Five-year
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Five-year
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Ten-year
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Ten-year
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Twenty-five-year
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Twenty-five-year
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Fifty-year
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Fifty-year
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One-hundred-year
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One-hundred-year
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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 generally
accepted calculation technique that is based on the NRCS soil-cover
complex method. Table 305-1 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 site. The municipality may allow the
use of the rational method to estimate peak discharges from drainage
areas that contain less than 200 acres. The soil complex method is
recommended for drainage areas greater than 200 acres.
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-HMS or HEC-1
is used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The alternating block method to develop design
hyetographs described 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 or
rational "C" value (i.e., forest), as listed in Table B-2 or B-3 in
Appendix B of this document.
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 Pennsylvania
Department of Transportation Design Rainfall Curves (1986) (Figures
B-3 to B-4). 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. Overland flow path lengths may
not exceed 100 linear feet.
[Amended 7-14-2014 by Ord. No. 2014-04]
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 chapter.
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 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 Ordinance. 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.
H. 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 Borough
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 305-1
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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
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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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United States Army Corps of Engineers
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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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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 Borough and/or Borough Engineer
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Other methods
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Varies
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Other computation methodologies approved by
the Borough and/or Borough Engineer
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Applying the water quality criteria in §
251-17 above will also help the streambank erosion problem, detain the two-year postdevelopment storm to the one-year predevelopment storm and detaining the one-year postdevelopment storm a minimum of 24 hours.