HomeMy WebLinkAboutAppendix W.1 - Sanitary Sewer Design Report 03-24-2021 Design Report - Page 1 of 4
INTRODUCTION
The Blackwood Groves Subdivision is a proposed 120-acre development located at the south side
of the City of Bozeman in Gallatin County, Montana. The proposed project is zoned as Residential
Emphasis Mixed Use (REMU) and will feature a variety of commercial, residential, and park/open
space developments. As part of the project, public improvements including water, sanitary sewer,
stormwater, and City road infrastructure are proposed.
SANITARY SEWER SYSTEM LAYOUT
The project is located within the City’s Cattail Creek Sanitary Sewer Drainage Basin, and the City
has required that all sewer flow from the proposed project remain within this drainage basin.
Therefore, in order to connect to the Cattail Creek Drainage basin, an 18” sanitary sewer main
extension will be installed in the S. 19th Avenue right of way from this project north to the existing
manhole in the S. 19th Avenue / Graf Street intersection.
This 18” sewer main extension is part of the City’s master wastewater facility plan, and also in the
Capital Improvements Plan for this portion of the City. As part of the wastewater facility plan, a
10” sewer main will need to be extended to a manhole in S. 19th Avenue at the southern limit of
work for the project to allow for continued southward extension in the future. Since the majority of
the property on the west side of S. 19th Avenue along the project frontage has already been
developed and provided access to City sewer mains, the proposed sewer design for this project
extends the 10” sewer main through the project’s interior streets to the project’s southern boundary
on South 19th Avenue. In addition to the 18” and 10” sewer mains, the City’s wastewater facility
plan also requires the extension of a 12” sanitary sewer main to the southeast portion of the project
for future extension to the S. 3rd Avenue / Goldenstein Lane intersection.
Once the 18” sewer main enters the site, the main will split into a 10” and 12” at the Avenue A and
Cambridge Drive intersection. The 10” sewer main will run south down Avenue A to Street E, then
west on Street E to South 19th Avenue. A 10” sewer main stub will be installed at the edge of the
South 19th Avenue improvements. The 12” sewer main will extend east down Cambridge Drive to
South 15th Avenue. Then head south down South 15th Avenue to the intersection with Street B. The
Design Report - Page 2 of 4
main will then extend east on Street B to South 11th Avenue. Once on South 11th Avenue the main
will extend south to the project limits. A 12” sewer main stub will be installed at the southern
project limits on South 11th for future development.
All of the proposed single-family residential lots will be served by City standard 4” sewer service
stubs. The larger lots (planned for multi-family and commercial developments) will be served by a
combination of 8” sewer main stubs for future extension within the lots, and individual sewer
service stubs, where practical. The size and location of these proposed stubs and services will be
finalized with the infrastructure design of the individual phases of the development.
DESIGN ASSUMPTIONS
The projected sanitary sewer flows for the proposed project were calculated using the design
values in the City of Bozeman Design Standards and Specifications. Since all of the lots proposed
for multi-family development have a preliminary concept design, anticipated dwelling units could
be estimated for each of the multi-family lots. The flows from the residential lots were estimated
using a design population of 2.17 people per dwelling unit, and residential flow rate of 65 gpcd, per
city standards. A residential peaking factor was calculated using the Harmon Formula for the
projected residential population for the entire project. This was then used to determine the
projected peak hour flows.
Although the project is zoned as REMU, several of the lots are expected to have a commercial use.
Since the design of these lots is not yet finalized, the flow rate of 3,000 gal/acre/day for B-3 zoning
designations from Table V-1 “Wastewater Flow Rate for Zoned Undeveloped Areas” in the City
Design Standards was conservatively used to estimate the projected daily flow from these lots. The
maximum peaking factor of 4.5 was also assigned to these commercial lots to estimate the peak
hour flow rate.
Inflow and Infiltration for the proposed development was calculated using the city standard
estimate of 150 gal/acre/day of infiltration. This infiltration rate was applied to the entire
development, with the exception of the two eastern parks that are not adjacent to sanitary sewer
Design Report - Page 3 of 4
main extensions, and several sections of the streets where sewer mains are not proposed. This flow
rate was also applied to the portions of the S. 19th Avenue right of way where the 18” sewer main
extension is proposed.
DESIGN LOAD
The projected flows from the development were calculated on a block-by-block basis, and also
categorized per individual phases of the development. Tables containing these calculations can be
found in Appendix B.
Below are the projected design loads for the proposed development based on the calculations in
Appendix B:
Residential Population Estimate: 1,668 people
Average Daily Flow Rate: 157,710 gal/day
Daily Infiltration Rate: 16,621 gal/day
Peak Hour Flow Rate: 440.3 gal/minute
The capacity of 8”, 10”, 12”, and 18” sewer mains at their minimum allowable slopes were
checked using Manning’s Equation: Q = (1.486/n)AR2/3S1/2
8‐inch PVC Pipe @ 75% Capacity
Pipe Diameter =8in
Manning's n= 0.013 (for PVC)
Min. Slope =0.004ft/ft
Area =0.3491ft2
Perimeter =2.0944ft
Hydraulic Radius =0.1667ft
Qfull =0.7643 cfs
Q0.75 =0.5732cfs
257.3 gpm
10‐inch PVC Pipe @ 75% Capacity
Pipe Diameter =10in
Manning's n= 0.013 (for PVC)
Min. Slope =0.0028ft/ft
Area =0.5454ft2
Perimeter =2.6180ft
Hydraulic Radius =0.2083ft
Qfull =1.1594 cfs
Q0.75 =0.8695cfs
390.3 gpm
Design Report - Page 4 of 4
CONCLUSION
Based on these calculations, the proposed 18” and 12” sewer main extensions are sufficiently sized
to convey the proposed sanitary sewer flows from the entire subdivision. The proposed flows are
to be conveyed by multiple 8” sewer main extensions that will connect to the 10”, 12” and 18”
mains at several locations, so at no point will the entire development be concentrated into a single
section of 8” or 10” sewer mains.
The proposed sanitary sewer main extensions within the development will be sized and finalized
with infrastructure design of the individual phases of the development.
G:\C&H\19\190390\Design Reports\Sewer\Preliminary Plat\Sewer Design Report - Body.Docx
12‐inch PVC Pipe @ 75% Capacity
Pipe Diameter =12in
Manning's n= 0.013 (for PVC)
Min. Slope =0.0022ft/ft
Area =0.7854ft2
Perimeter =3.1416ft
Hydraulic Radius =0.2500ft
Qfull =1.6711 cfs
Q0.75 =1.2533cfs
562.5 gpm
18‐inch PVC Pipe @ 75% Capacity
Pipe Diameter =18in
Manning's n= 0.013 (for PVC)
Min. Slope =0.0012ft/ft
Area =1.7671ft2
Perimeter =4.7124ft
Hydraulic Radius =0.3750ft
Qfull =3.6388 cfs
Q0.75 =2.7291cfs
1,224.9 gpm
APPENDIX A
Sanitary Sewer Exhibit
APPENDIX B
Sanitary Sewer Flow Analysis
Block Use DU's
Lot Area
(sf)
Lot Area
(acres)
Total Area
(sf)
Total Area
(acres)Population Avg. Flow Rate
(gpd)
Infiltration
Rate (gpd)
Peak Hr. Flow
Rate (gpm)
1 Commercial N/A 205,948 4.73 256,908 5.90 N/A 14,190 885 45.0
2 Residential 6 36,497 0.84 40,297 0.93 13 845 139 2.2
3 Residential 6 36,000 0.83 40,500 0.93 13 845 140 2.2
4 Residential 6 52,434 1.2 52,434 1.20 13 845 181 2.3
5 Residential 6 63,000 1.45 63,000 1.45 13 845 217 2.3
6 Residential 12 57,600 1.32 62,400 1.43 26 1,690 215 4.4
7 Residential 12 57,280 1.31 57,280 1.32 26 1,690 197 4.4
8 Residential 20 104,870 2.41 139,983 3.21 43 2,795 482 7.4
9 Residential 49 134,021 3.08 237,220 5.45 106 6,890 817 18.0
10 Residential 32 87,475 2.01 108,030 2.48 69 4,485 372 11.6
11 Residential 32 86,994 2.00 99,166 2.28 69 4,485 342 11.6
12 Residential 40 104,146 2.39 104,146 2.39 87 5,655 359 14.6
13 Commercial N/A 130,612 3.00 220,927 5.07 N/A 9,000 761 28.7
14 Residential 140 182,825 4.2 220,710 5.07 304 19,760 760 50.6
15 Residential 90 109,458 2.51 109,458 2.51 195 12,675 377 32.4
16 Commercial N/A 70,480 1.62 88,005 2.02 N/A 4,860 303 15.4
17 Commercial N/A 176,868 4.06 176,868 4.06 N/A 12,180 609 38.5
18 Commercial N/A 131,497 3.02 131,497 3.02 N/A 9,060 453 28.6
19 Residential 52 183,260 4.21 219,586 5.04 113 7,345 756 19.1
20 Residential 21 57,963 1.33 82,173 1.89 46 2,990 283 7.8
21 Residential 24 86,739 1.99 104,956 2.41 52 3,380 361 8.8
22 Residential 21 57,659 1.32 112,075 2.57 46 2,990 386 7.8
23 Residential 14 74,124 1.7 74,124 1.70 30 1,950 255 5.1
24 Residential 122 168,111 3.86 168,111 3.86 265 17,225 579 44.1
25 Residential 34 82,401 1.89 157,709 3.62 74 4,810 543 12.6
26 Residential 14 85,959 1.97 90,898 2.09 30 1,950 313 5.2
27 Residential 10 66,949 1.54 66,949 1.54 22 1,430 231 3.8
28 Residential 6 83,216 1.91 83,216 1.91 13 845 287 2.3
R.O.W. N/A N/A N/A N/A 1,457,165 33.45 N/A N/A 5,018 3.5
Totals 769 63.70 110.79 1,668 157,710 16,621 440.3
Notes:
persons per dwelling unit 2.17 1.
Daily WW Generation Rate (gpcd) 65
Commercial WW Generation Rate (gal/ac/day) 3,000 2.
Infiltration Rate (gal/ac/day) 150
Residential Peaking Factor (harmon Formula) 3.65 3.
Commercial Peaking Factor 4.50
4.
Conservative Peaking Factor of 4.5 applied to commercial
lots
Residential Peaking Factor from input table is applied to
all residential lots
The Right of Way (R.O.W.) area calculation includes the
portion of the 19th Avenue R.O.W. that will have sanitary
sewer main installed in it, but excludes several portions
of streets where no sewer main is expected to be
installed
March 24, 2021
Table 1 ‐ Sanitary Sewer Flow Analysis ‐ By Block
Blackwood Groves Subdivision
Table Input
Commercial B‐3 ww generation rate of 3,000 gpd
assumed for commercial lots
Phase Block Use DU's
Lot Area
(sf)
Lot Area
(acres)Total Area (sf)Total Area
(acres)Population Avg. Flow
Rate (gpd)
Infiltration
Rate (gpd)
Peak Hr. Flow
Rate (gpm)
2 Residential 6 36,497 0.84 40,297 0.93 13 845 139 2.2
3 Residential 6 36,000 0.83 40,500 0.93 13 845 140 2.2
4 Residential 6 52,434 1.20 52,434 1.20 13 845 181 2.3
5 Residential 6 63,000 1.45 63,000 1.45 13 845 217 2.3
6 Residential 12 57,600 1.32 62,400 1.43 26 1,690 215 4.4
7 Residential 12 57,280 1.31 57,280 1.32 26 1,690 197 4.4
8 Residential 20 104,870 2.41 139,983 3.21 43 2,795 482 7.4
10 Residential 32 87,475 2.01 108,030 2.48 69 4,485 372 11.6
11 Residential 32 86,994 2.00 99,166 2.28 69 4,485 342 11.6
R.O.W.N/A N/A N/A N/A 495,956 11.39 N/A N/A 1,708 1.2
Total 132 13.37 26.61 285 18,525 3,993 49.6
14 Residential 140 182,825 4.20 220,710 5.07 304 19,760 760 50.6
15 Residential 90 109,458 2.51 109,458 2.51 195 12,675 377 32.4
16 Commercial N/A 70,480 1.62 88,005 2.02 N/A 4,860 303 15.4
R.O.W.N/A N/A N/A N/A 228,538 5.25 N/A N/A 787 0.6
Total 230 8.33 14.85 499 37,295 2,227 99.0
9 Residential 49 134,021 3.08 237,220 5.45 106 6,890 817 18.0
12 Residential 40 104,146 2.39 104,146 2.39 87 5,655 359 14.6
R.O.W.N/A N/A N/A N/A 248,235 5.70 N/A N/A 855 0.6
Total 89 5.47 13.54 193 12,545 2,031 33.2
19 Residential 52 183,260 4.21 219,586 5.04 113 7,345 756 19.1
20 Residential 21 57,963 1.33 82,173 1.89 46 2,990 283 7.8
21 Residential 24 86,739 1.99 104,956 2.41 52 3,380 361 8.8
R.O.W.N/A N/A N/A N/A 157,639 3.62 N/A N/A 543 0.4
Total 97 7.53 12.96 211 13,715 1,943 36.1
22 Residential 21 57,659 1.32 112,075 2.57 46 2,990 386 7.8
23 Residential 14 74,124 1.70 74,124 1.70 30 1,950 255 5.1
R.O.W.N/A N/A N/A N/A 73,015 1.68 N/A N/A 251 0.2
Total 35 3.02 5.95 76 4,940 892 13.1
17 Commercial N/A 176,868 4.06 176,868 4.06 N/A 12,180 609 38.5
18 Commercial N/A 131,497 3.02 131,497 3.02 N/A 9,060 453 28.6
R.O.W.N/A N/A N/A N/A 31,127 0.72 N/A N/A 107 0.1
Total 0 7.08 7.79 0 21,240 1,169 67.2
24 Residential 122 168,111 3.86 168,111 3.86 265 17,225 579 44.1
25 Residential 34 82,401 1.89 157,709 3.62 74 4,810 543 12.6
R.O.W.N/A N/A N/A N/A 127,975 2.94 N/A N/A 441 0.3
Total 156 5.75 10.42 339 22,035 1,563 57.0
26 Residential 14 85,959 1.97 90,898 2.09 30 1,950 313 5.2
27 Residential 10 66,949 1.54 66,949 1.54 22 1,430 231 3.8
28 Residential 6 83,216 1.91 83,216 1.91 13 845 287 2.3
R.O.W.N/A N/A N/A N/A 94,680 2.17 N/A N/A 326 0.2
Total 30 5.42 7.71 65 4,225 1,157 11.5
1 Commercial N/A 205,948 4.73 256,908 5.90 N/A 14,190 885 45.0
13 Commercial N/A 130,612 3.00 220,927 5.07 N/A 9,000 761 28.7
R.O.W.N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
Total 0 7.73 10.97 0 23,190 1,646 73.7
769 63.70 110.79 1,668 157,710 16,621 440.3
7
6
8
9
Totals:
2
4
5
1
3
Blackwood Groves Subdivision
Table 2 ‐ Sanitary Sewer Flow Analysis ‐ By Phase
March 24, 2021