HomeMy WebLinkAboutDRAINAGE REPORT - 23-00505 - Golden West Irrigation - InteriorGolden West Irrigation
DRAINAGE REPORT
Submitted to:
CITY OF REXBURG
PLANNING & ZONING DEPARTMENT
Prepared for:
Golden West Irrigation
Roy Jenkins
2256 S. Yellowstone Hwy
Rexburg, ID 83440
In
Forsgren Associates, Inc.
1137 Summers dr
Rexburg, ID 83440
December 2023
Golden West Drainage Report
INTRODUCTION
The runoff and drainage calculations in this report pertain to the storm drainage design for the
proposed Golden West Irrigation site located on south Yellowstone Hwy in the City of Rexburg,
Idaho. The site is bounded on the east by farm ground, on the west by Yellowstone Hwy, and
the on the north by private residential, and on the south by other commercial properties. The
total site acreage is 13.5 acres and the site is generally flat.
Per the approved development agreement, the City of Rexburg requires onsite retention for this
property. Per the City of Rexburg Development Code (Chapter 12, Section 3.6), the site will be
designed to retain runoff generated by the 25-year 24-hour design storm. This document
summarizes the flow and volume computations for the project site.
DESIGN METHODS :.
Determine Rainfall Denth =
CST"G
The design storm required by the City of
Rexburg for use in sizing stormwater retention
volume is the 25-year, 24-hour storm. Figure 1
is the NOAA Atlas map for the State of Idaho
precipitation depths corresponding to the 25-
year, 24-hour design storm. The NOAA Atlas
shows a total rainfall depth of 0.9 inches
corresponding with the 25-year 24-hour design
storm for the project site. Dividing the rainfall
depth (0.9 inches) by the storm duration (24
hours), results in a rainfall intensity of 0.0375
in/hr.
Determine Runoff Coefficients
Not all precipitation results in runoff. The
City of Rexburg
Pzs = 0.9 inches
ground slope, cover, vegetation, soil type,
rainfall intensity and other abstractions affect
the percentage of precipitation that results in t
runoff. Various hydrologic methods are used
to determine runoff, each accounting for those Figure'l: NOAA Atlas 2 25 YR 24-BR Precipitation
abstractions in various ways. In the City of
Rexburg, the Rational Method for determining runoff is recommended for areas less than 100
acres. The Rational Method uses a runoff coefficient "C" that predicts the fraction of water that
falls on a particular surface that results in runoff. See Table 1 below for a list of various proposed
surfaces within the development and corresponding runoff coefficients that have been selected to
represent each anticipated surface type.
C:\Usras\aswenson\Dropbox\Aaron\Work\Roy lenkins\C'antatnere Drainage Report.doc
Golden West _ _ _ Drainage Report
Roofs
1 6#
M1
35,000
Note that asphalt, sidewalks and roofs have all been assigned the same runoff coefficient.
Determine Peak Runoff
The peak runoff (Q) is determined by multiplying the land area (A) for each land surface type by the
rainfall intensity (I) and the runoff coefficient (C) according to the following Rational Method
formula:
Q = CIA where:
• Q = peak runoff(cfs)
• C = runoff coefficient
• I = rainfall intensity (in/hr)
• A = total area of the development (ac)
The peak runoff volume is determined by taking the calculated peak runoff (Q) multiplied by the
storm duration (D). Table 2 summarizes the data and calculations utilized to determine an
anticipated total storage volume of 37,788 ft3 (0.868 ac-ft) for the project site.
TABLE
2: Rational
Method Calculations
Runoff
Rainfall
Peak
Area
Description
Coefficient
Intensity
Runoff
Runoff Volume
C
I (in/hr)
Q cfs)
V (ft3
Vac-ft)
A (ac)
Roof/As halt/Sidewalk
1.83
0.90
0.0375
_ 0.073
1 15,685
0.361 _
Landscape
11.67
0.20
0.0375
0.0
22104
0.507
Total
13.50
0.082 82
37,788
0.868
Due to the flat terrain that exists on the project site, the site has been graded to drain to several
low spots in the parking and landscape areas. At these locations, water will either be detained as
surface water and/or collected via grate inlets and conveyed to underground infiltration
chambers. In order to determine the required storage obligation for each drainage basin, the total
runoff volume has been divided by total area to determine a runoff volume per acre. That
number was then multiplied by the area of each basin to determine that basin's storage
obligation. Table 3 below summarizes the runoff volume that each lot will be obligated to retain,
either via retention pond on site.
G\Users\aswmson\Dmphox\Aaron\Work\Roy Jmkins\Cantamere Drainage Repon.doc
Golden West Drainage Report
TA_ BLE 3: Stora e Requirements per Drain a a Basin
Runoff
Retention
Chambers
Drainagel
Area
Volume
Storage Type
Volume
Proposed
Basin
Available
_ _
A ac
V ft3
Surface/Subsurface/Both.
V ft3
#
_ 1
1 0.868
37,788
Surface
40,000__
0
TOTAL
0.868
37,788 1
40,000
0
Back-up computations are attached, and proposed storage improvements are shown on the
grading plan.
C:\Usems aswenson\Drophox\Aaron\Wurk\Roy Jenkins\Canfaiume Diainage Repomdoc
FORSGREN ASSOCIATES, INC
Client: Loth Group LLC
Project: Hampton Inn and Fresco Restaurant, Rexburg
By: ES Checked By:
For: Development Drainage Analysis
Condition: Based on Post -Development Conditions
Subarea 5
Calculation of Runoff Coefficient
HYDROLOGIC ANALYSIS
Rational Method
V = CIA
V = runoff volume (ft3)
C = Runoff coefficient
i = precipitation depth (ft)
A = Drainage Area (ft2)
Surface Condition
Subbasin 1
Subbasin 2
Area (ft2) Coefficient
Area (ft2) Coefficient
Building Roof
44,250 0.92
0 0.92
Asphalt
35,000 0.9
0 0.9
Landscaped Area
508,810 0.2
0 0.2
open space
0 0.2
0 0.2
total
588,060
Average Site Coefficient 0.30
Estimated Snow Melt by Degree Day Method
Max. Average Temperature for March 46.3 deg. F
Min Average Temperature for March 23.6 deg. F
Average Temperature for 24 hour period 35.0 deg. F
Estimated potential snowmelt 0.2 inches
Estimated Total Runoff Volume for 50-year event plus snow melt &113of2-yrevent
NOAA Atlas Rainfall
Subbasin 1
Subbasin 2
50-yr
2.4 inch
0.0 inch
10-yr
1.8 Inch
0.0 inch
2-yr
1.2 Inch
1.0 inch
1/3 of 2-yr (estimated)
0.40 Inch
0.00 Inch
Total precip 50-yr (in)
2.61 inch
0.2 inch
Estimated Runoff Volume (50yr)
37788 ft3
0 ft3
Water Quality Precip
Estimated Runoff Vol. 1/3 of 2yd
5799 ft3
0 ft3
12128/2023 C:\Users\aswenson\DropboxVAaron\Work\Roy Jenkins\HydroAnalysis_goldenwest