HomeMy WebLinkAboutDRAINAGE REPORT - 03-00048 - Deseret Industries - New CommercialREXBURG DESERET INDUSTRIES
DRAINAGE REPORT
CITY OF REXBURG
DEPARTMENT OF PUBLIC WORKS
Rexburg Deseret Industries Drainage Report
INTRODUCTION
The runoff and drainage calculations in this report pertain to the storm drainage design for the
Rexburg Deseret Industries site located on the north side of Rexburg bounded on the west by the
Union Pacific railroad tracks and on the east by North 2"d East Street. The property is bounded
on the south by the Teton Lanes Bowling Alley and the northern boundary is approximately 71h
North. The total acreage is 6.22 acres (270,943 square feet) Refer to Sheet SD1.1 Site Plan in
the plan set prepared by JRW Associates.
The City of Rexburg requires on site detention for this property with an allowance to release the
detained storm water to the Rexburg storm drain system over a forty-eight hour period. The
design storm for detention is the 50 -year 24-hour event. Applying the design storm to the
project area yields a runoff volume of
approximately 44,024 cubic feet.
Calculations for emptying the
detention pond in 48 hours are not
included in this report.
DESIGN METHODS
Determine Rainfall Depth
The design storm required by the City
of Rexburg for use in sizing
stormwater detention features is the
50 -year, 24-hour storm. The NOAA
Atlas shows a total rainfall depth of
2.3 inches corresponding with the
50 -year 24-hour design storm.
Figure 1 is the NOAA Atlas map for
the State of Idaho precipitation depths
corresponding to the design storm.
Determine Runoff
Not all the water that descends as
precipitation results in runoff. The
ground slope, cover, vegetation, soil
type, rainfall intensity and other
abstractions affect the percentage of
precipitation that does not percolate
into the soil. Various hydrologic
methods are used to determine runoff,
Figure 1: NOAA Atlas, 50 -years, 24-hour precipitation
K:URN\RI,XRl1RG DI\DRAINAGE REPORT D1 STORM CALCS.DOC'
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August 2003
114 111 11
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IWC 'V�� .rtmi TRN it
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• Rexbure 2.3
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Figure 1: NOAA Atlas, 50 -years, 24-hour precipitation
K:URN\RI,XRl1RG DI\DRAINAGE REPORT D1 STORM CALCS.DOC'
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August 2003
Rexburg Deseret Industries Drainage Report
each accounts for those abstractions in various ways. The Rational Method uses a runoff
coefficient "C" that predicts the fraction of water that falls on a particular surface and results in
runoff. The method selected for this analysis is the Rational Method.
In the Rational Method, development projects typically utilize a coefficient for three general
types of surface; rooftop, pavement including sidewalks, and landscaping, which for our case is
assumed to be grass. Runoff coefficients were selected for those three surface types on this
project, and the corresponding areas for each land surface type was determined based on the final
Site Plan configuration as presented on Sheet SD1.1 of the drawing set.
The volume of runoff is determined by multiplying the land area for each land surface type by the
precipitation volume and the runoff coefficient according to the following formula:
V = CIA where:
• V = volume of runoff (ft)
• C = runoff coefficient
• I = rainfall depth for design storm (ft)
• A = total area of the development (fe)
The following table presents the data collected along with the calculations that predict a total
storage volume of 44,024 cubic feet. The storage volume may be reduced by releasing storm
water concurrent with the design storm event.
Determination of Runoff Volume - Rational Method
Ki,JRWARF.XBORG DW)RAtN:AGF; RF.POR-RDI STORM CALC3.DOC
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August 2003
Area
Runoff
Rainfall
Volume
Coefficient
Depth
[Al
IC]
[I]
IV]
Descri tion
Acres ft2
-
(inches)
(ft)
Building roof
0.9
39204
0.92
2.3
6,913
Landscape
0.49
21344.4
0.2
2.3
818
Asphalt & cement
4.83
210394.8
0.9
2.3
36,293
Total
6.22
270943.2
44,024
Ki,JRWARF.XBORG DW)RAtN:AGF; RF.POR-RDI STORM CALC3.DOC
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August 2003