HomeMy WebLinkAbout18 - Design Report - Creekstone Condos - Stormwater (Addendum to Laurel Glen Subdivision Ph 2 Lot 1 Block 34 Report rc
Engineering and Surveying Inc.
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February 26,2018
City of Bozeman
Attn: Shawn Kohtz
Engineering Department
20 E. Olive Street
Bozeman,MT 59718
RE: Addendum to 2007 Stormwater Design Report,Drainage Area 3-Creekstone Condos
Site Plan Application#16254
Dear Shawn,
This letter is intended to serve as an addendum to the original stormwater design report for the
above referenced project that was produced in 2007. More specifically,there were no stormwater
facilities shown for Drainage Area 3 and this has been rectified in the revised site plan included
herein.
As shown on the enclosed stormwater calculations, Drainage Area 3 will produce 862 cubic feet
of stormwater. Each proposed dry well is designed to retain 432 cubic feet of stormwater. Since
they are spaced at the halfway point and the endpoint of the swale, each drywell will capture half
of the stormwater volume of 431 cubic feet.
A drainage swale is now proposed to be installed along the eastern boundary of the development
near Phases 3 through 8, as shown on the C1.0 drawing. This swale will capture stormwater from
Drainage Area 3 and direct it northward and into two proposed dry wells to capture and infiltrate
the runoff.The drainage swale is intended to function as a stormwater conveyance facility,but will
also support stormwater percolation and treatment. This further ensures that there is adequate
stormwater capacity. The drainage swale is designed to convey a 25-year design flow. The
proposed swale will have maximum 4H:1V embankment slopes and will be 6" deep to provide
adequate stormwater conveyance.
Reviewed By,
Matt Hausauer,P.E.
Enclosed:
1) Dry Well and Swale Design Calculations
2) Original(2007) Creekstone Stormwater Design Report
Civil/Structural Engineering and Surveying
DRAINAGE AREA #3
REQUIRED SWALE CAPACITY
Contributing Area C Area (ft ) C *Area
Hardscape 0.95 10306 9791
Landscape 0.2 14673 2935
Total 24979 12725
A =Area (acres) 0.57
C= Weighted C Factor 0.51
2. Calculate Tc(Time to Concentration)
Tc Overland Flow
Tc= 1.87(1.1-CC)D1/2/Sv3
-----------------------------------
;Storm
S =Slope of Basin (%) 1% lReturn (yrs) Cf
C = Rational Method Runoff Coefficient 0.51 12 to 10 1
Cf= Frequency Adjustment Factor 1.1 111 to 25 1.1
D = Length of Basin (ft) 550 1126 to 50 1.2
51 to 100 1.25
To Overland Flow(minutes) 113.35
Tc Total= 113.35
3. Calculate Flow(Rational Formula)
Q=CIA
C =Weighted C Factor 0.51 (calculated above)
=0.78 Tc'-'4(in/hr) 0.52 (25-yr storm)
A=area (acres) 0.57 (calculated above)
Q=REQUIRED SWALE CAPACITY(cfs) 0.15 (assuming no carry flow)
PROVIDED SWALE CAPACITY
1.Drainage Swale Capacity
Q = (1.486/n)AR"Sv2
n = Mannings Coefficient 0.035 (Unlined Channel)
b =Base Width (ft) 0.50
z=Channel Side Slope (ft/ft) 4 (Max 4:1 slope)
d =depth (ft) 0.50
A=Area (ft) 1.25
P =Wetted perimeter(ft) 4.62
R= Hydraulic Radius A/P (ft) 0.27
S =slope(%) 0.91%
Q= PROVIDED SWALE CAPACITY(cfs) 2.121
DRYWELL #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2) C "Area
Hardscape 0.95 5153 4895
Landscape 0.2 7337 1467
Total 12490 6363
A =Area (acres) 0.29
C= Weighted C Factor 0.51
2. Calculate Required Volume
Q=CIA
V=7200Q
C=Weighted C Factor 0.51
1 = Intensity(in/hr) 0.41 (10 yr,2hr storm)
A=Area (acres) 0.29
Q= Runoff(cfs) 0.06
V=REQUIRED VOL(ft) 431
3. Calculate Drywell Volume
Existing Soil Condition Fine Sand
Percolation Rate (min/in) 10 (see Circular DEQ 4,
Percolation Rate (ft/hr) 0.50 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft) 4.5
Drywell Gravel Area (ft) 150.2
Infilitration Volume (ft) 150.22
Required Drywell Depth (ft) 6.23
Proposed Drywell Depth (ft) 6.25 (round up to nearest 3")
Provided Volume Inc. Perc. ft3 432
DRYWELL #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2) C "Area
Hardscape 0.95 5153 4895
Landscape 0.2 7337 1467
Total 12490 6363
A =Area (acres) 0.29
C= Weighted C Factor 0.51
2. Calculate Required Volume
Q=CIA
V=7200Q
C=Weighted C Factor 0.51
1 = Intensity(in/hr) 0.41 (10 yr, 2hr storm)
A=Area (acres) 0.29
Q = Runoff(cfs) 0.06
V= REQUIRED VOL(ft) 431
3. Calculate Drywell Volume
Existing Soil Condition Fine Sand
Percolation Rate (min/in) 10 (see Circular DEQ 4,
Percolation Rate (fUhr) 0.50 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft) 4.5
Drywell Gravel Area(ft) 150.2
Infilitration Volume (ft) 150.22
Required Drywell Depth (ft) 6.23
Proposed Drywell Depth (ft) 6.25 (round up to nearest 3")
Provided Volume Inc. Perc. ft3 432