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Sundance Springs Comm Lot 2
Sewer Design Report
April 23, 2025
Project Background:
•2-buildings
•1.31 acres
•Commercial Use (mixed office, food service, retail, health gym)
•Zoning designation is B-1 (neighborhood commercial)
Design Flow:
•From City of Bozeman Design and Construction Standards (COBDCS) 5.2.4.B – Design Average
Flow: “New sanitary sewer infrastructure to serve non-residential areas shall be designed to
accommodate an average daily flow rate for the development being constructed. Typical
wastewater flow values per unit from Circular DEQ-4 shall be used.”
•From DEQ-4 Table 3.1-1&2:
o East Building:
▪Health Gym – 39 occupants – 10 gpd/occupant = 390 gpd
▪Office 1 & 2 – 13 occupants – 13 gpd/occupant = 169 gpd
o West Building
▪Restaurant – 70 occupants (175 meals) – 3 gpd/meal = 525 gpd
▪Retail Store – 4 employees – 10 gpd/employee = 40 gpd
▪Office 1 & 2 – 8 occupants – 13 gpd/occupant = 104 gpd
o Total Daily Demand
▪390 + 169 + 525 + 40 + 104 = 1,228 gpd
▪Rounded to 1400 gpd
•From COBDCS 5.2.4.C: “A base infiltration rate of 150 gallons/acre/day shall be added to
domestic flow calculations when designing all new sewers. The base infiltration rate shall be
applied to the gross area of the development.”
o Infiltration rate: 150 gallons/acre/day * 1.31 acres = 196.5 gallons/day
•Deign average flow = 1,400 + 196.5 = ~1,600 GPD
•From COBDCS 5.2.5 – Peaking Factor
o Equivalent population: 64.4 GPD per capita then 1600 GPD/64.4 GPD = 24.84
capita ~25 persons
•Qmax = 18 + (0.025)^½ = 18.16
•Qavg = 4 + (0.025)^½ = 4.16
•Peaking Factor = Qmax/Qavg = 4.37
•Design Average Flow: 1600 Gallons per day * 1 day/24 hours * 1hour/60 minutes = 1.11
gpm average
•Peak Hour = 1.11 gpm * 4.37 = 4.85 gpm
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Sewer Main Specifications:
• 8-inch PVC pipe
• Slope 0.4% or greater
• Minimum cover is four feet
• Cleanout installed within 3 feet of foundation wall
COB Standard Drawings:
• COB 02720-4 Standard Straight Manhole
• COB 02720-6 Precast Manhole Bases
• COB 02720-7 Typical Manhole Channel Details
• MPW 02730-1 Nomograph for Air Testing
• COB 02730-2 Sanitary Sewer Service Line
• COB 02730-3 Deep Sanitary Sewer Service Line
• COB 02730-4 Sanitary Sewer Cleanout
Wastewater Collection: 8’’ sanitary sewer main, 330 feet of PVC pipe, connecting to existing
city sewer line at an existing manhole. Three additional manholes are proposed. A slope of no
less than 0.4% and minimum cover of four feet are required. The proposed pipe network has
slopes between 0.6% and 2.38%. Where cover is between 4 and 5 feet, pipe must be insulated
to protect from freezing. New sewer is sized to flow at no more than 75% full.
Manhole: Three manholes are required for this sewer main extension. One where the main
extension branches from the existing main along Little Horse Drive, one at the bend in the
proposed extension, and a third at the terminus of the proposed extension. Barrel size shall be
48’’. Manholes shall have full-depth channels. When a smaller main is being connected to a
larger main at a manhole, the manhole inverts shall be set so that the 8/10 depth of flow of
each main is equal in elevation. The minimum drop across a manhole is 0.2 feet.
Sanitary Sewer Services: Diameter is 4-inches. Services shall connect to the main with in-line
gasketed wyes. Max slope of ½ inch per foot. Minimum slope is ¼ inch per foot. Installed
perpendicular to the main via inline gasketed wye. Each building shall have a separate service
line.
Manning’s Formula:
𝑉=𝑘
𝑛𝑅ℎ
2/3𝑅1/2
V = average velocity in ft/s
n = 0.013
Rh = hydraulic radius = d/4 = 0.1997
S = slope = 0.02, k = 1.49
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Manning Formula Uniform Pipe Flow at Given Slope and Depth
Inputs:
Pipe Diameter, dₒ 7.9420 in
Manning Roughness, n 0.0130
Pressure slope (possibly equal to pipe slope), S₀ 0.6000 % slope
Percent of (or ratio to) full depth (100% or 1 if flowing full) 75.0000 %
Results:
Flow, Q 375.7161 gal/min
Velocity, v 3.0246 ft/s
Velocity head, hv 0.1422 ft
Flow Area, A 0.2768 ft^2
Wetted Perimeter, P 1.3861 ft
Hydraulic Radius 0.1997 ft
Top Width, T 0.5732 ft
Froude Number, F 0.77
Shear Stress (tractive force), τ 0.1859 psf
Version 2.0 (20 June 2017)
HawsEDC Calculators
04/23/2025