HomeMy WebLinkAbout23O - 3 Storm Calcs - HotelProject:23O - Atwell Suites Hotel - Bozeman MT
Subject:Storm Drainage Calculations
Date:4/22/2025
Engineer:Kevin May, P.E.
Drainage Basin Information
Land Use Commercial
Drainage Area (ac)2.685 (Includes Hotel Property & Shared Parking Area)
Slope (%)2
Runoff Coeff.0.720 (See Below)
Frequency Adjust. Factor (Cf)1.1 (25-yr Rainfall Frequency)
Hotel Drainage Basin: C-Factor Calculation
Item Area (sf)C-Factor
Imperv. Surfacing 90569.6 0.9
Landscaping Area 26369.9 0.1
Total Area 116939.5
Composite C Factor 0.720
Contributing Area (sf)116939.5
Contributing Area (ac)2.685
Time of Concentration Calculations - Developed Site
Flow Length (ft)125 (Ridge to Inlet B-3)
Slope (%)2.0
Time of Concentration (min)5.0 (Per Figure I-1)
Flow Length (ft) =450 (Inlet B-3 to Ret. Pond)
Slope (%) =0.005
n =0.013
R =0.08
V =1.54 ft/sec 12" Area 0.131 ft2
Time of Concentration (min)4.88 12" WP 1.571 ft (50% full)
R = A/WP
Total Tc =9.88 min
Peak Flow Calculations
Design Storm 25-year (Per CoB Design Stds II.A.3.e)
Intensity @ Tc (figure I-2)2.47 in/hr
Peak Runoff Rate (cfs)5.25 (with C-factor adjustment of 1.1)
Retention Pond Sizing Calculations
Given: Q=CiA & V = 7200Q
C-Factor 0.720
I (in/hr)0.41
A (ac)2.685
Q (cfs) =0.792
V (cf) =5703
Overland Flow
Channelized/Pipe Flow
𝑉=1.486
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