HomeMy WebLinkAbout6-1 Revised Sewer Narrative 04-05-2021 1
Sewer
This section is a response to the requirements of the City of Bozeman Unified Development
Ordinance Section 38.220.060 relative to Water and Sewer. The original Sanitary Sewer Design
Report provides analysis for the entire PUD including Phase 4. Flows from Phase 4 are designed
to flow west to the existing sewer main in Fowler Avenue (see updated sewer main exhibit).
Since the report was written, the City of Bozeman Design Standards and Specifications have
been revised to reflect a lower required flow rate for B-2 zoning, decreasing from 3495
gal/ac/day to 2000 gal/ac/day. The following calculations are done to illustrate that the existing
system will meet the required sewer design criteria for Phase 4 of the Bozeman Gateway.
Residential (Lot 35)
Since the first submittal of the Preliminary Plat Phase 4, the number of total units for the four
apartment buildings in Lot 35 has been determined to be 231 units. The capacity requirement for
the Lot 1 sewer service is as follows:
231 apartment units (Buildings 1-4) x 2.17 people per dwelling unit (COB Wastewater
Collection Facilities Plan Update Final, Jun 2015) = 501.27 people
501.27 people x 64.4 gallons per capita per day (COB Wastewater Collection Facilities
Plan Update Final, Jun 2015) = 32,282 gallons per day (gpd)
The wastewater produced by Lot 35 is 32,282gpd or 0.05cfs (22.4gpm).
Remainder of Phase 4
The remainder of Phase 4 less Lot 35 would have an area of 6.84 acres. Based on the Sewer
Facility Plan’s 2000 gal/ac/day for 6.84 acres, the wastewater produced by the remainder of
Phase 4 is 13,680gpd or 0.02cfs (9.5gpm).
Phase 4 Sewer Impact
The manhole located at the intersection of Fowler and Technology Blvd was dipped on
12/30/2019 and was found to have a sewage depth of 0.2’. Due to the timing of this measurement
during the holiday season, it is conservatively assumed that the average existing flow depth is
0.5’. The outlet pipe has a diameter of 21”. Assuming a minimum slope of 0.1%, a 21” pipe has a
capacity flowing full of 6.3cfs (2,827.6gpm). At a depth of 0.5’ the pipe is approximately 29%
full and flowing at a rate of 1.06cfs (475.8gpm).
The contribution of Phase 4 increases the flow by 0.07cfs to 1.13cfs (507gpm) which equates to
a pipe depth of 0.52 or 30%. The 21” pipe has more than enough capacity to accommodate the
development.
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Sunday, Dec 29 2019
21in PVC Flowing Full
Circular
Diameter (ft) = 1.75
Invert Elev (ft) = 100.00
Slope (%) = 0.10
N-Value = 0.011
Calculations
Compute by: Q vs Depth
No. Increments = 10
Highlighted
Depth (ft) = 1.58
Q (cfs) = 6.312
Area (sqft) = 2.28
Velocity (ft/s) = 2.77
Wetted Perim (ft) = 4.38
Crit Depth, Yc (ft) = 0.93
Top Width (ft) = 1.05
EGL (ft) = 1.69
0 1 2 3
Elev (ft)Section
99.50
100.00
100.50
101.00
101.50
102.00
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Dec 31 2019
Existing Pipe Flow
Circular
Diameter (ft) = 1.75
Invert Elev (ft) = 100.00
Slope (%) = 0.10
N-Value = 0.011
Calculations
Compute by: Known Depth
Known Depth (ft) = 0.50
Highlighted
Depth (ft) = 0.50
Q (cfs) = 1.061
Area (sqft) = 0.57
Velocity (ft/s) = 1.86
Wetted Perim (ft) = 1.98
Crit Depth, Yc (ft) = 0.37
Top Width (ft) = 1.58
EGL (ft) = 0.55
0 1 2 3
Elev (ft)Section
99.50
100.00
100.50
101.00
101.50
102.00
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Monday, Apr 5 2021
Proposed Pipe Flow
Circular
Diameter (ft) = 1.75
Invert Elev (ft) = 100.00
Slope (%) = 0.10
N-Value = 0.011
Calculations
Compute by: Known Q
Known Q (cfs) = 1.13
Highlighted
Depth (ft) = 0.52
Q (cfs) = 1.130
Area (sqft) = 0.60
Velocity (ft/s) = 1.88
Wetted Perim (ft) = 2.02
Crit Depth, Yc (ft) = 0.38
Top Width (ft) = 1.60
EGL (ft) = 0.57
0 1 2 3
Elev (ft)Section
99.50
100.00
100.50
101.00
101.50
102.00
Reach (ft)