HomeMy WebLinkAbout05_2.0_Stormwater Report_04282021
Stormwater Report:
for
N 3rd Duplexes
Gallatin County, Montana
April, 2021
Prepared For:
302 Partners, LLC
Prepared By:
Hyalite Engineers, PLLC
Brett Megaard, PE
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N 3RD DUPLEXES
ENGINEER’S STORM DRAINAGE DESIGN REPORT
Owner: 302 Partners, LLC
2304 North 7th Ave.
Bozeman, MT 59715
Introduction
This report provides a design basis and hydrologic calculations for sizing stormwater
infrastructure for North 3rd Duplexes. The project site is located in the north east corner of
the intersection of West Beall Street and North 3rd Avenue. This project in total is 0.26 acres
of residential development constructed in one proposed phase. The owner would like to
construct two duplex units as well as a detached garage and storage building on the lot. The
existing site has a single residential structure with topography sloping at approximately -2.5%
to the north east. There is currently no existing stormwater infrastructure on the lot. All
precipitation is currently either contained on site through absorption and infiltration or runs
offsite to the north east.
The City of Bozeman Design Standards and Specifications (March 2004) was used as the
primary guideline for this stormwater drainage design.
General Design
The proposed development of the site will be 62% impervious area containing buildings and
concrete driveways/walkways and 38% pervious area of landscaping. Two test pits were dug
on site in March 2021 to a depth of 8’. Sandy Lean Clay was found from surface to a depth of
approximately 4’, a layer of Gravel with Clayey Sand was found from approximately 4’ to 6’,
and Gravel with Some Cobbles with Sand was found from 6’ to 8’. No ground water was
encountered in the excavation of the test pits. The absence of groundwater and the presence
of gravels make infiltration an appropriate solution to post development stormwater runoff.
The proposed duplexes will have roof drains that will capture rainwater which will then be
routed to an underground retention chamber. The site is graded so that runoff from the
courtyard area will runoff into a proposed yard inlet and them routed to the same
underground retention chamber.
Hydrologic Methodology
The modified rational method was used to calculate retention volumes. The modified rational
method is used to compute runoff for storm durations equal to or greater than time of
concentration. Total runoff volume is computed by multiplying the duration of the storm by
the runoff rate. Retention volumes were calculated using the following formulas:
Q = CIA
V = DQ
Where:
C = Weighted C Factor
I = Storm Intensity (in/hr)
A = Area (acres)
Q = Runoff (cfs)
D= Duration (s)
V = Volume (cf)
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Retention volumes were analyzed for the 10-year 2-hour and 100-year 2-hour and 24-hour
storms using Figure I-3 from The City of Bozeman design Standards (March 2004).
Runoff coefficients and watershed flow calculations are included in Appendix A.
Infiltration Chamber Sizing
The infiltration chamber is sized to have enough capacity for the 10-year, 2-hour storm
without taking into account infiltration and also sized to retain and infiltrate the runoff from the
100-year, 2-hour and 24-hour storms with infiltration. The infiltration chamber will be installed
at a depth so that the bottom of the chamber is in native gravels with cobbles where the
infiltration rate is 2.6 in/hr (DEQ 8, Table 2 of Appendix C). Using the infiltration rate of 2.6
in/hr the infiltration chamber is sized to drain in less than 48 hours.
Flooding
Excessive runoff from a large storm event will be routed such that runoff does not inundate
home sites, drainfields or over top the roadway. Finished floor elevations of buildings will be
set higher than the surrounding ground with finished ground sloping away from them. In the
event that the ponds overtop, stormwater will spill into the alley to the north east and
eventually be routed to city storm sewers through existing curb and gutters.
Appendix A – Stormwater Retention Pond Calculations