HomeMy WebLinkAbout02 - Design Report - Valley West Ph 1 - Sewer DESIGN REPORT
FOR THE
SANITARY SEWER
IN
VALLEY WEST SUBDIVISION - PHASE #1
BOZEMAN, MONTANA
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Prepared By: Morrison-Maierle, Inc.
901 Technology Blvd
PO Box 1113
Bozeman, Montana 59771-1113
Date: January 29, 2002
File: 3384.004 043 0310
Valley West Subdivision — Phase #1
Sanitary Sewer Design Report
Introduction
This report provides a basis for design of the Valley West Subdivision — Phase
#1 sanitary sewer collection system. A comprehensive design report evaluating
the capacity of the existing sewer facilities serving the entire Valley West
Subdivision Planned Unit Development (PUD) is provided in the Preliminary Plan
submittal for the PUD. The PUD report does not identify any capacity problems
for the existing collection piping to serve the development.
The sewer system serving Valley West Subdivision — Phase #1 will be designed
and installed in accordance with WQB Circular No. 2; Montana Public Works
Standard Specifications (MPWSS); City of Bozeman modifications to the
MPWSS; the Uniform Plumbing Code; and the City of Bozeman Wastewater
Facility Plan.
In accordance with the City of Bozeman Design Standards and Specifications
Policy this report is being submitted for approval by the City of Bozeman prior to
plan and specification submittal. The applicant intends to utilize the self-
certification process provided for in these design standards.
Valley West Sanitary Sewer Page 1 of 13
Deviation Requests
Utility Design Criteria
B. Sanitary Sewer System Design Criteria
3. A deviation from the requirement that new sewer lines be sized
to flow at no more than 50% of full capacity at peak hour
conditions upon full build out of the development is requested.
Justification: While the Valley West Subdivision — Phase #1
sewer improvements described in this report are designed using
this criterion; we feel that this policy may result in oversizing of
sewer lines. Oversizing the pipes can result in lower flow
velocities and reduce the pipe's ability to flush itself. This will
result in increased maintenance in the future. We suggest that
the pipes be designed to convey peak flows at a 75% flow
depth.
4. A deviation from the requirement to use the zoning densities
identified in Table V-1 of the City of Bozeman Design Policy is
requested.
Justification: The underlying zoning for the property is R-3.
Using Table V-1 in the City of Bozeman Design Standards
yields an allowable density of 12 dwelling units per acre for the
property. The Valley West Subdivision Planned Unit
Development identifies development densities for the entire
subdivision area. These planned densities are considerably
less than the maximum allowable densities allowed by zoning.
Designing for the densities identified in Table V-1 will result in
over conservative pipe design for the subdivision. We request
that the densities identified in the Valley West Planned Unit
Development Preliminary Plan submittal be used for design.
City of Bozeman Standard Pipe Locations
A deviation is requested from the standard pipe locations specified in the City of
Bozeman Modifications to the Montana Public Works Standard Specifications
Standard Drawing No. 02713-4 — Water & Sewer Main and Services Location
Standards. Our request is to place the sanitary sewer main serving the lots
Babcock Street in alternate locations to those identified in the standard drawing.
Valley West Sanitary Sewer Page 2 of 13
Our specific request is to locate the sewer main between Vaughn Drive and
Stafford Ave. in the alley. The alley right-of-way is 30-feet wide, and the pipe will
be located 10-feet north of the south right-of-way line. The sewer main serving
lots 54 and 55 will be located in a 30-foot wide utility and access easement
between the lots.
Justification: Using the existing sewer main in Babcock Street to serve the lots
will require a street cut to connect each service. The easement provided in the
front of the homes will be occupied by storm drain and water utilities and does
not provide adequate width to meet separation requirements for the sewer utility.
Valley West Sanitary Sewer Page 3 of 13
Average Daily Sewage Generation
City of Bozeman Design Standards require that sanitary sewer lines serving
residential areas be designed to accommodate an average daily flow rate of 72
gallons per capita per day. An infiltration rate of 150-gallons per acre per day
shall be added to all flow calculations for new sewers. All new sewer lines
installed in association with Valley West Subdivision — Phase #1 wily serve
residential areas.
Residential zoning densities for new developments are also identified in the
design standards. The underlying zoning for Valley West Subdivision — Phase
#1 is R-3. Table V-1 of the design standards identifies an allowable zoning
density of 12 dwelling units per acre in the R-3 zone to be used of designing new
sewers.
The Valley West Subdivision — Phase #1 encompasses portions of Wastewater
Drainage Area 1 and Wastewater Drainage Area 2 as identified in the Valley
West Sewer Main Extension Design Report included in the Valley West
Subdivision Planned Unit Development Preliminary Plan submittal. Drainage
Area 1 drains into an existing 8" main in Durston Road. Drainage Area 2 drains
into an existing main located in Cottonwood Road consisting of pipes that are 12"
diameter and larger. Both the Durston Road and Cottonwood Road sewer mains
were sized and constructed in 1998 to serve the Valley West Subdivision.
The Valley West Subdivision Planned Unit Development Preliminary Plan
identifies a range of 144 — 195 dwelling units to be constructed within the 31.21
acre Phase 1. It needs to be noted that 6.53 acres of Phase 1 consist of parks
and open space, leaving 24.68 acres for residential development. Using the
zoning densities identified in the City of Bozeman Design Standards, an
allowable zoning density of 296 dwelling units is calculated (12 x 24.68 = 296).
This is considerably higher than the range identified in the Preliminary Plan. To
be consistent with the design standards, pipe capacities will consider both the
allowable zoning density and the maximum density range identified in the
Preliminary Plan.
The total Drainage Area 1 encompasses approximately 50 acres. The PUD
Preliminary Plan identifies this area to be occupied by up to 414 residential
dwelling units, with the portion of Valley West Subdivision — Phase #1
contributing to Drainage Area 1 consisting of up to 108 dwelling units. Using the
zoning densities identified in the City of Bozeman Design Standards, an
allowable zoning density of 600 (12 x 50 = 600) dwelling units is calculated for
the entire Drainage Area 1, with the portion of Valley West Subdivision — Phase
#1 contributing to the area consisting of up to 145 dwelling units (12 x 12.1 =
145).
Valley West Sanitary Sewer Page 4 of 13
Drainage Area 2 encompasses approximately 30 acres. The PUD Preliminary
Plan identifies this area to be occupied by up to 173 residential dwelling units,
with the portion of Valley West Subdivision — Phase #1 contributing to Drainage
Area 2 to will consist of up to 87 dwelling units. Using the zoning densities
identified in the City of Bozeman Design Standards, an allowable zoning density
of 360 (12 x 30 = 360) dwelling units is calculated for the entire Drainage Area 1,
with the portion of Valley West Subdivision — Phase #1 contributing to the area
consisting of up to 151 dwelling units (12 x 12.6 = 151).
The following tables provide design average daily flows from each drainage area
and the contributing areas from the Valley West Subdivision — Phase #1. Table 1
provides flows estimated from the allowable zoning densities established in the
City of Bozeman Design Standards. Table 2 provides flows estimated from the
densities identified in the Valley West Subdivision Planned Unit Development
Preliminary Plan.
Table 1: Average Daily Flows — City of Bozeman Design
Standard Allowable Zoning Densities
Average Total
Drainage Residential Daily Area Infiltration Average
Basin Units Population Sewage (acres) (gpd) Daily Flow
Density Flow (gpd)
(gpd)
1 600 1,380 99,360 50 7,500 106,860
Phase 1 145 334 24,102 12.1 1,815 25,920
2 360 828 59,616 30 4,500 64,120
Phase 1 151 347 25,006 12.6 1,890 26,900
Table 2: Average Daily Flows — Valley West Subdivision
Planned Unit Development Densities
Average Total
Drainage Residential Daily Area Infiltration Average
Basin Units Population Sewage (acres) (gpd) Daily Flow
Flow
(gpd) (gpd)
1 total 414 952 68,558 50 7,500 76,060
1 phase 1) 108 248 17,885 12.1 1,815 19,700
2 total 173 398 28,649 30 4,500 33,150
2 phase 1 87 200 14,407 12.6 1,890 16,300
Valley West Sanitary Sewer Page 5 of 13
In summary, the total design average daily sewage flow, based on the City of
Bozeman Design Standard allowable zoning densities, from Wastewater
Drainage Area 1 into the 8" sanitary sewer trunk main in Durston Road, is
106,860 gpd (74.2 gpm) with 25,920 gpd (18.0 gpm) generated by the Valley
West Subdivision — Phase #1. The total flow from Wastewater Drainage Area 2
into the 12" sanitary sewer trunk main in Cottonwood Road is 64,120 gpd (44.5
gpm) with 26,900 gpd (18-.7 gpm) generated by the Valley West Subdivision —
Phase #1.
The total design average daily sewage flow, based on densities identified in the
Valley West Subdivision Planned Unit Development Preliminary Plan, from
Wastewater Drainage Area 1 into the 8" sanitary sewer trunk main in Durston
Road is 76,060 gpd (52.8 gpm) with 19,700 gpd (13.7 gpm) generated by the
Valley West Subdivision — Phase #1. The total flow from Wastewater Drainage
Area 2 into the 12" sanitary sewer trunk main in Cottonwood Road is 33,150 gpd
(23.0 gpm) with 16,300 (11.3 gpm) generated by the Valley West Subdivision —
Phase #1.
Valley West Sanitary Sewer Page 6 of 13
Peak Flows
Peak hour flows are calculated using the peaking formula provided in the City of
Bozeman Design Standards and Specification Policy.
PF = 18 + P1/2
4 + P1/2 (P = Population in thousands)
Tables 3 and 4 present peak flows based on the average daily flow calculated
previously in this report.
Table 3 Peak Hour Flows — City of Bozeman Design Standard Allowable
Zoning Densities
Drainage Basin Ave. Daily Flow (qpm) Peaking Factor Peak Flow (qpm)
Area 1 (total) 74.2 3.8 282
Area 1 (phasel) 18.0 4.1 73.8
Area 2 (total) 44.5 4.0 178
Area 2 (phase 1) 18.7 4.1 76.7
Table 4 Peak Hour Flows —Valley West Subdivision Planned Unit
Development
Drainage Basin Ave. Daily Flow (qpm) Peaking Factor Peak Flow (qpm)
Area 1 (total) 52.8 3.8 200.6
Area 1 (phasel) 13.7 4.1 56.1
Area 2 (total) 23.0 4.0 92
Area 2 (phase 1) 11.3 4.1 46.3
As noted in the deviation request section, we propose to utilize the design flows
listed in Table 4, above, for the Valley West Subdivision. The following pipe
design section provides calculations based on the peak flows presented in Table
4.
Valley West Sanitary Sewer Page 7 of 13
Sanitary Sewer Pipe Design
Wastewater Drainage Area 1
Using the approved densities presented in the Valley West Planned Unit
Development and the peak flows provided in Table 4, the sewer pipe transporting
wastewater from the entire Wastewater Drainage Area 1 for the Valley West
Subdivision Planned Unit Development will need to convey a peak flow of 200.6
gpm.
The capacity of an 8" diameter sewer pipe installed at a slope of 0.53% can be
calculated as follows:
Manning's Equation Q = (1.49/n) AR2/3S1/2
A Manning's friction factor of 0.013 will be used as required by the City of
Bozeman Design Standards. Per the City of Bozeman Design Standards, new
sewers will be sized to flow at 50% of full capacity during peak hour conditions
upon full build-out of the development.
Using Appendix A in "Open Channel Hydraulics" by Chow, the maximum
capacity of an 8" diameter PVC sewer pipe will occur under the following
conditions:
y/do = 0.5;
Using Appendix A the corresponding AR2 = 0.1558
dos/3
Calculating AR2/3 = (0.1558)(0.678/3) = 0.0536
Qpeak = (1.49)(0.0536)(0.0053)1/2 = 0.447 cfs or 200.6 gpm
0.013
An 8" diameter PVC pipe installed at a minimum slope of 0.53% is
sufficient to serve the entire Wastewater Drainage Area 1 for the
Valley West Subdivision Planned Unit Development.
Valley West Subdivision — Phase #1 Contributing to Wastewater Drainage
Area 1
Using the approved densities presented in the Valley West Planned Unit
Development and the peak flows provided in Table 4, the sewer pipes
transporting wastewater from the Valley West Subdivision — Phase #1 will need
to convey a peak flow of 56.1 gpm.
Valley West Sanitary Sewer Page 8 of 13
The capacity of an 8" diameter sewer pipe installed at a slope of 0.4% can be
calculated as follows:
Manning's Equation Q = (1.49/n) AR2r3Sv2
A Manning's friction factor of 0.013 will be used as required by the City of
Bozeman Design Standards. Per the City of Bozeman Design Standards, new
sewers will be sized to flow at 50% of full capacity during peak hour conditions
upon full build-out of the development.
Using Appendix A in "Open Channel Hydraulics" by Chow, the maximum
capacity of an 8" diameter PVC sewer pipe will occur under the following
conditions:
y/do = 0.5;
Using Appendix A the corresponding AFe 3 = 0.1558
doai3
Calculating AR2/3 = (0.1558)(0.67813) = 0.0536
Qpeak = (1.49)(0.0536)(0.004)12 = 0.389 cfs or 174.4 gpm
0.013
An 8" diameter PVC pipe installed at the minimum recommended
slope of 0.4% is sufficient to serve the entire Valley West Subdivision
— Phase #1.
Wastewater Drainage Area 2
Using the approved densities presented in the Valley West Planned Unit
Development and the peak flows provided in Table 4, the sewer pipe transporting
wastewater from the entire Wastewater Drainage Area 2 for the Valley West
Subdivision Planned Unit Development will need to convey a peak flow of 92.0
gpm.
As calculated above an 8" diameter sewer pipe installed at the minimum
recommended slope of 0.4% will convey 174.4 gpm at a 50% flow depth.
An 8" diameter PVC pipe installed at the recommended minimum
slope of 0.4% is sufficient to serve the entire Wastewater Drainage
Area 2 for the Valley West Subdivision Planned Unit Development.
Valley West Sanitary Sewer Page 9 of 13
Valley West Subdivision — Phase #1 Contributing to Wastewater Drainage
Area 2
Using the approved densities presented in the Valley West Planned Unit
Development and the peak flows provided in Table 4, the sewer pipes
transporting wastewater from the Valley West Subdivision — Phase #1 will need
to convey a peak flow of 46.3 gpm.
As calculated above, the capacity of an 8" diameter sewer pipe installed at the
minimum recommended slope of 0.4% is 174.4 gpm at a 50% flow depth.
An 8" diameter PVC pipe installed at the minimum recommended
slope of 0.4% is sufficient to serve the entire Valley West Subdivision
— Phase #1.
Valley West Sanitary Sewer Page 10 of 13
Impacts to Existing Facilities
Durston Road Trunk:
The Valley West Subdivision Planned Unit Development Wastewater Drainage
Area 1 will drain to an existing 8" diameter sanitary sewer trunk line installed
specifically to serve this area. The controlling pipe on this trunk line has a slope
of 0.45%. The capacity of the pipe is calculated as follows:
Manning's Equation Q = (1.49/n) AR213S1i2
A Manning's friction factor of 0.013 will be used as required by the City of
Bozeman Design Standards.
Using Appendix A in "Open Channel Hydraulics" by Chow, the maximum
capacity of an 8" diameter PVC sewer pipe will occur under the following
conditions:
y/do = 1.0;
Using Appendix A the corresponding AR2/3 = 0.3117
dosi3
Calculating AR2/3 = (0.3117)(0.67813) = 0.1071
Qpeak = (1.49)(0.1071)(0.0045)1/2 = 0.823 cfs or 370 gpm
0.013
Using the approved densities presented in the Valley West Planned Unit
Development and Table 4, the sewer pipe transporting wastewater from the
entire Wastewater Drainage Area 1 for the Valley West Subdivision Planned Unit
Development will need to convey a peak flow of 200.6 gpm. This is
approximately 54% of the capacity of the pipe.
The existing 8" diameter PVC pipe in Durston Road is sufficient to serve
the entire Valley West Planned Unit Development Wastewater Drainage
Area 1 including the contributing portion of the Valley West Subdivision —
Phase #1.
Valley West Sanitary Sewer Page 11 of 13
Cottonwood Road Trunk:
The Valley West Subdivision Planned Unit Development Wastewater Drainage
Area 2 will drain to an existing 12" diameter sanitary sewer trunk line installed
specifically to serve this area. The controlling pipe on this trunk line has a slope
of 0.63%. The capacity of the pipe is calculated as follows:
Manning's Equation Q = (1.49/n) AR2i3Sv2
A Manning's friction factor of 0.013 will be used as required by the City of
Bozeman Design Standards.
Using Appendix A in "Open Channel Hydraulics" by Chow, the maximum
capacity of an 12" diameter PVC sewer pipe will occur under the following
conditions:
y/do = 1.0;
Using Appendix A the corresponding ARC = 0.3117
dosr3
Calculating AR2/3 = (0.3117)(1.08/3) = 0.3117
Qpeak = (1.49)(0.3117)(0.0063)1/2 = 2.84 cfs or 1273 gpm
0.013
Using the approved densities presented in the Valley West Planned Unit
Development and Table 4, the sewer pipe transporting wastewater from the
entire Wastewater Drainage Area 1 for the Valley West Subdivision Planned Unit
Development will need to convey a peak flow of 92.0 gpm. This is approximately
7.2% of the capacity of the pipe.
The existing 12" diameter PVC pipe in Cottonwood Road is sufficient
to serve the entire Valley West Planned Unit Development
Wastewater Drainage Area 2 including the contributing portion of the
Valley West Subdivision — Phase #1.
Valley West Sanitary Sewer Page 12 of 13
Design Specifics:
All manholes will be 48-inch diameter with eccentric cone top section. If drop
manholes are necessary, the drop will be constructed inside the manhole and a
60-inch diameter manhole will be used. Manholes will have full-depth channels.
Manhole spacing will be at maximum intervals of 400'.
All sewer mains will be 8-inch diameter PVC pipe.
Sewer services shall be 4-inch diameter connected to the main with in-line wyes.
Service line stubs will be extended to the property line with a maximum slope of
Y2-inch per foot and a minimum slope of '/-inch per foot. Service lines will be
located 15-feet from the downstream lot line.
A 12' wide gravel access road will be constructed to provide access to sanitary
sewer manholes not located within a paved roadway or alley. Easements will be
provided where sewer mains are located outside the area to be platted.
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Valley West Sanitary Sewer Page 13 of 13
EXISTING SEWER
TRUNK MAIN
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EXISTING SANITARY SEWER SYSTEM JIMORRISON
PORTION OF VALLEY WEST PUD (CONSTRUCTED 1998) VALLEY WEST SUB.
DRAINAGE AREA 1 PHASE 1 SEWER iw
' PROPOSED SANITARY SEWER SYSTEM MMERLE,INC. EXISTING&PROPOSED
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SEWER FACILITIES
PORTION OF VALLEY WEST PUD % COLLECTOR TIE-IN LOCATIONS _- PROPOSED PHASE 1 50 1002 0 CLIENT.ASPEN
DRAINAGE AREA 2 RELD WORK-- DATE:01/21/02 PLOTTED DATE: Jan/29/2002
SANITARY SEWER SYSTEM SCALE:V=100 DRAWING NAME:
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340 IF OF 8" 400 IF OF 8"
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115 IF 12"
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