HomeMy WebLinkAboutDRAINAGE CALCS - 19-00723 - Mountain America Credit Union Office Call Center
Prepared For:
Mountain America Credit Union
9800 South Monroe Street
Sandy, Utah
MACU
Rexburg
Call Center
2nd East Street & Main Street
Rexburg, Idaho
Anderson, Wahlen & Associates, Inc.
2010 North Redwood Road
Salt Lake City, Utah 84116
Phone: (801) 521-8529
Fax: (801) 521-9551
Date: July 29, 2019
Project #: 19-003
Drainage Analysis
11/22/19
Table of Contents
Introduction ........................................................................................................................................................... 1
Methodology .......................................................................................................................................................... 1
Analysis ................................................................................................................................................................... 1
Conclusions ............................................................................................................................................................. 1
Appendices
Vicinity Map
25 Year 24 Hour Rational Calculation
100 Year 24 Hour Rational Calculation
Intensity, Duration, and Frequency Relationship
1
Introduction
The subject property is located in the Southwest quarter of Section 20, Township 6 South, Range 40 East
of the Boise Meridian. The property is located at 2nd East Street and Main Street, Rexburg, Idaho.
The project will consist of a vacant lot with a surface covering of asphalt and undeveloped soil. The
property area is 1.4 acres and will consist of constructing a new building, parking lot and new
landscaping.
Storm water runoff generated at the site will be required to be retained onsite. This will be
accomplished through capturing the storm water in 6 inlet boxes and 1 catch basin, and routing the
storm water through a Junction box with a snout and then retaining the storm water in an ADS
underground storage chamber system.
Methodology
The modified rational method was used to determine the required storage volume for this site. The City
of Rexburg runoff coefficients are found within the State of Idaho Catalog of Storm Water Best
Management Practices, Appendix D. The IDF curve for Zone G is used to determine the 25-year and 100-
year return period intensities. The 25-year return period is used to size the onsite stormwater storage.
The 100-year return period is used to find the peak flow rate in the system.
A percolation rate of 2 inches per hour was used in these calculations. This rate, along with the area of
the underground storage chambers was used to calculate the volumetric rate that the water will
percolate. This new rate was then used to calculate the total volume required to hold the 25-year storm.
The rational method requires a composite runoff coefficient based on land type. To determine the
composite runoff coefficient the site was divided into three land uses (landscape, new roof, and new
hard surfaces). Individual runoff coefficients of 0.15, 0.85, and 0.9 were used for landscape, new roof,
and new hard surfaces respectively. The weighted average value for this site is 0.73 and is used in the
calculations.
Analysis
From the percolation rate of 2 inches per hour the detention volume required for the site is 4,217 cubic
feet (see appendix for rational calculations). The underground storage chambers will provide a total
volume of 4,494 cubic feet. This larger volume is due to the underground storage chambers coming in
set sizes, this size being the best fit to accommodate both the required volume and the percolation rate.
The extra volume will also provide a surplus of storage in the event of a larger storm.
2
Conclusions
The 100-year 24-hour storm event with a 2 inches per hour percolation rate resulted in a required
detention volume of 4,217 cubic feet based with the provided storage being 4,494 cubic feet.
Underground storage chambers will be used to detain all storm water generated at the site.
3
Vicinity Map
MACU – Rexburg Call Center: 2nd East Street & Main Street
Rexburg, Idaho
4
25 Year 24 Hour Rational Calculation
5
100 Year 24 Hour Rational Calculation
Design Manual Hydraulics Appendix B
Page 36 of 112
Figure B-8
Sheet 7 of 9