HomeMy WebLinkAboutSmart Foods Stormwater Study_070121
PRELIMINARY
Prepared For: Smart Foods
101 South 200 East, Suite 200
Salt Lake City, Utah 84111
(801) 521-4781
Smart Foods -
Bozeman
1601 West Main Street
Bozeman, Montana
Anderson, Wahlen & Associates, Inc.
2010 North Redwood Road
Salt Lake City, Utah 84116
Phone: (801) 521-8529
Fax:(801) 521-9551
Date: May 24, 2021
Job #: 21-212
Table of Contents
Introduction .................................................................................................................................................. 1
Location ........................................................................................................................................................ 1
Methodology ................................................................................................................................................ 1
Analysis ......................................................................................................................................................... 2
Stormwater Treatment ................................................................................................................................ 4
Low Impact Development ............................................................................................................................ 4
Conclusion .................................................................................................................................................... 5
Appendices
Project Location
Lag Time Calculations
Runoff Coefficients for Modified Rational Method
Modified Rational Calculations
Bozeman Design Specification
Bozeman Precipitation Depths
INTRODUCTION
Smart Foods grocery store is proposing to construct a grocery store in Bozeman, MT on the southwest
corner of North 15th Avenue and West Beall St. The address is 1601 West Main Street. The store will be
approximately 22,000 square feet. Site construction activities will include but is not limited to asphalt
paving, curb and gutter, and grading.
The purpose of this report is to examine the following:
· Determine the overall allowable storm water release rate to the public storm drain system.
· Determine the required detention using the 10-year, 2-hour storm event.
· Determine the required retention based on Bozemen City LID requirement of 0.5 inches/acre.
· Determine conveyance capacity of structures for a 25-year storm event.
· Explain how storm water will be treated prior to being released into the public system.
· Explain how the Low Impact Development (LID) best management practices are implemented.
LOCATION
The commercial development is located on an existing lot with various retail stores. The location of the
proposed Smart Foods grocery store will be directly east of the Harbor Freight store. See Appendix for
project map.
METHODOLOGY
The 10- year, 2-hour storm depth was provided in the Bozeman Storm Water Facilities Plan. This value is
0.73 inches. Additionally, a LID retention requirement of 0.5 inches/acre was provided by the City of
Bozeman. It is required that the post development runoff does not exceed that of the existing conditions.
The modified rational method is one of Bozeman City’s approved methods for calculating stormwater
runoff and is used throughout this report. The lag time and corresponding rainfall intensity were also
calculated and can be found in the appendix.
ANALYSIS
Contributing Areas
Even though the existing condition is currently asphalt paving, for the purpose of this study the existing
condition will be assumed to be native soils, with Hydrologic soil group D. This will result in a
conservative approach to designing the required storm drain system.
The development of this area will result in a single drainage basin of 2.21 acres that will drain to an on-
site detention basin. The proposed development will consist of approximately 70% impervious area with
the remaining 30% being landscaping. In the absence of hydrologic soil data for this area, it is assumed
to be soil class D for existing and developed conditions.
Allowable Release Rate
As previously mentioned, the release rate cannot exceed that of the existing conditions. To determine
the allowable release rate, the modified rational method was used. The lag time for the condition was
calculated to be 5.27 minutes (see Appendix).
The figure below shows the intensity calculations approved by the City of Boseman.
The rainfall intensity was calculated to be 0.82 inches per hour for the existing condition. This value was
then used to calculate the runoff.
The runoff was calculated using the Modified Rational Method shown below.
For the existing conditions, Q=(0.24) * 0.82 * 2.21
Q=0.4
Therefore, the allowable release rate for the developed condition is 0.4 cfs.
Storage Requirements
An above ground detention basin will be constructed with the capacity to detain the 10-year, 2-hour
storm. This results in a required detention volume of 5,856 c.f. The basin is designed to not exceed the
allowable release rate. This is accomplished with the use of a wier that will restrict flows.
To meet the LID requirement of 0.5 inch per acre will be retained and infiltrated back to the surrounding
groundwater table.
The onsite basin will be constructed to provide a minimum of 5,856 c.f. of detention storage. To meet
the LID requirement a portion of this 5,856 c.f. will be treated and infiltrated back to the groundwater
table. The total volume to be infiltrated is 4,011 c.f., which is 0.5 inches per acre.
Storm Drain Pipes
The stormwater will be conveyed from the site to the basin via a trench drain that runs along the site from
north to south. The water within the basin will sheet flow to the trench drain. Per city requirements, on-
site storm drain design must be sufficient to convey runoff from the 25-year storm. The depth of the 25-
Year, 2-Hour storm is 0.88 inches (see Appendix). This will require storm drain pipes to convey 0.4 cfs in
order to meet the city’s requirement.
STORM WATER TREATMENT
Per the City of Bozeman, it is required to infiltrate, evapotranspire, and/or capture for reuse the post-
construction runoff generated from 0.5 inches/acre. To meet this requirement, the onsite storage basin
will have an outfall structure with an invert elevation above the bottom of the storage basin. Considering
that the onsite basin will provide stormwater treatment for an area of 2.21 acres, a volume of 4,011 c.f.
will be treated by infiltration. This will be accomplished by installing a layer of sand on the bottom of the
basin. This sand will be effective in filtering out suspended solids before entering back to the groundwater
table.
LOW IMPACT DEVELOPMENT
The purpose of Low Impact Development (LID) requirements is to minimize the impact of the proposed
development on the natural environment, as it pertains to stormwater runoff. One approach to LID is to
allow a portion of the stormwater runoff from the site to infiltrate back into the ground to recharge local
aquifers. To meet local LID requirements, 0.5 inches/acre of stormwater runoff must be treated before
entering back to the water table. The method being used will effectively filter out debris and will improve
the effect this area has on downstream stormwater systems. Previously, there was no treatment.
CONCLUSION
The above ground stormwater storage system designed for Smart Foods will be able to detain stormwater
from the 10-Year, 2-Hour Storm.
The allowable release rate based on historical flows was determined to be 0.4 cfs.
The designed release rate will not exceed 0.4 cfs with use of a weir.
The storage system will be designed to treat 4,011 c.f. by sand infiltration and meet LID requirements.
The on-site structures are designed to convey runoff from the 25-year storm event.
Installation of this storm drain system will result in a positive impact on the public storm drain facilities
located downstream. By implementing LID methods, the stormwater entering the public facilities will have
less debris. Additionally, this development will not contribute anymore flows to the public storm drain
system than it did in the existing condition.
Final tc
Design: CN K Area (ac)Area (mi2)Length (ft) Slope (%)ti (min)Overland Flow Only? Length (ft) Slope (%) Velocity (fps)tt (min)Total Length (ft)tc=(L/180)+10 (min)min Lag Time (hr) Remarks
Existing Site 94 0.851 2.21 0.00345 300 4.0 4.92 NO 85 4.0 4.040 0.35 385 12.14 5.27 0.053
Developed Site 87 0.758 2.21 0.00345 300 4.0 6.74 NO 182 4.0 4.040 0.75 482 12.68 7.49 0.075
Lag Time Calcs - Smart Food Bozeman
Company: Anderson Wahlen & Associates
By: Grayson Smith Description: Drainage Basins Lag Time Calcs
Date: 2021/05/20 File Name: Lag Time Calcs
Drainage Basin Data Initial/Overland Time (ti) Travel Time (tt)tc Check (Urbanized Basins)
Smart Foods Bozeman
Existing Condition - Meadow - (Soil Class D)
Area Type Area (ft^2)C
Meadow (Soil D) 96,461.0 0.24
Subtotal=96,461
Total= 2.21 acres
C average=0.24
Frequency: 10-Year
Duration: 2-Hour
Tc= 5.27 (See Lag Time Calculations)
Intensity(in/hr)= A*(Tc/60)^-0.6 0.82
Q(cfs)=CiA= 0.4
Storm Water Calculations - Existing Conditions