HomeMy WebLinkAbout007 Nexus Point Sewer Design Report
SEWER MAIN
DESIGN REPORT
FOR:
NEXUS POINT MULTI-UNIT
RESIDENTIAL DEVELOPMENT
BOZEMAN, MT
Prepared By:
Madison Engineering
895 Technology Drive, Suite 203
Bozeman, MT 59718
(406) 586-0262
MARCH 2023
Nexus Point Development
Wastewater Design Report
Page 1 of 3
NEXUS POINT DEVELOPMENT
SEWER SYSTEM DESIGN REPORT
A. General Project Description
This report will give an overview of the proposed wastewater collection system for the
development of the Nexus Point development on Lot 1 of minor sub 235 in S23, T02S, R05E
P.M.M, located West of south 19th Avenue and North of Lantern Lane in Bozeman, MT. The
zoning for the 19.99-acre area is listed as R-O, residential office district. The proposed buildings
are designed as two (2) multi-floor office buildings, six (6) 12 plex 2-bedroom units, two (2) 24
plex 2-bedroom units, three (3) 36 plex 1 & 2-bedroom units, one (1) 34-plex 1 & 2 bedroom
units that includes a café, along with a club house for a total of 262 units in the residential
development. The improvements for Phase 1 of the development were constructed during the
summer of 2020. The improvements for Phase II of the development are anticipated to be
constructed in 2023 and 2024.
Included in this report is a description of the proposed wastewater collection system and design
parameters. Once the wastewater system is constructed and approved, the system will become a
part of the City of Bozeman (C.O.B.) system. The following references were used in the
wastewater facilities design and capacity analysis:
a. COB Design Standards and Specifications Policy, 2017. Addendum #6
b. COB Modification to Montana Public Works Standard Specifications (MPWSS)
c. City adopted MPWSS and adopted Addenda
d. COB Wastewater Facility Plan, 2017
e. WQB Circular No. 2
f. City of Bozeman Zoning Map
The procedures and parameters outlined in the COB Design Standards and Specifications Guide
were used in the design of the sewer infrastructure for the proposed development.
B. Capacity Analysis
The proposed wastewater collection system consists of 3,234 linear feet (0.61 miles) of new 8β
sewer main and will provide service to the multi-unit residential & office development. The
effluent generated from the proposed buildings will be collected by the wastewater collection
system and ultimately flow west and north to existing COB Manholeβs located in Lantern Lane
and South 19th Avenue.
The wastewater demands from the proposed subdivision were estimated based on a flow per capita
basis, which provided a more conservative flow than a per zoned acre basis. (COB Table V-1
specifies an average daily flow rate of 980 gal/ac-day for R-O zoning, resulting in 19,590gpd =
19.99 ac x 980 gal/ac-day).
The proposed sewer mains have been sized to flow at no more than 75-percent of full capacity at
peak hour conditions of full build out, and a Manning coefficient of 0.013 was used for PVC pipe.
Nexus Point Development
Wastewater Design Report
Page 2 of 3
The average daily flow is based on the number of units, 2.11 residents per unit, and 89 gallons per
capita per day, the latter two values based on COB design standards:
(πππ units β2.11 residents
unit β89 gallons
capita/day )=ππ,πππgpd
The 2 future office buildings consist of 11,300 sf and 9,900 sf at full build out. Based on a
conservative basis the office buildings will use 0.273 gpd/sf. per water supply planning book by
David W. Prasifka. Taking these factors into consideration the office buildings will contribute:
(π.ππππππ₯π₯π₯π₯π₯ π₯ππ₯ πππ±
π₯π₯π₯ππ₯π ππππ₯β(ππ,πππ+π,πππ)π₯π₯π₯ππ₯π ππππ₯ )= ππ,πππ ππ₯π
One building located in phase 2, is set to include a café/restaurant. Based on a conservative basis
the café will use 55 gpd/seat per water supply planning book by David W Prasifka. Taking this
into consideration the café will contribute:
(ππππ₯π
π₯πππ₯βππ π₯πππ₯π₯/π₯πππ₯π₯ππ₯π₯π₯)=π,πππ ππ₯π
Based on these parameters, the development at full build out (262 units + 2 office buildings + café)
will have an estimated daily flow of 66,136 gpd
An approximate population for the Nexus Point Multi-Unit Residential Development at full build
out is calculated as:
πππ units +ππ π₯ππ₯π’πππ₯π₯π₯ (π₯ππ± ππππ π₯πππ₯π₯ππ₯π₯π₯)+πππ π₯ππ₯π’πππ₯π₯π₯ (ππ,πππ ππ
πππππ
ππ πππ π ππ
)
β2.11 residents/unit =πππ persons
This is a more realistic population value compared to utilizing the 5.2 units per acre (Table V-1
from COB design standards) applied to 19.99 acres, which only results in 104 persons.
This equivalent population can be utilized to determine a peaking factor:
(ππππ₯
ππ΄π£π
)=18 +π.5
4 +π.5 =18 +.πππ.5
4 +.πππ.5 =π.ππ
(P = Population/thousands)
Nexus Point Development
Wastewater Design Report
Page 3 of 3
Based on a total population of 923 people, a peaking factor of 3.82 was calculated. The peak flow
rate anticipated from the development based on an average daily flow of 66,136 gallons per day
is:
Peak Flow Rate = (66,136 gpd x 3.82) = 252,639.52 gpd
= 175 gpm
= 0.39 cfs
Additionally, an infiltration flow of 2,999 gpd (150 gal/acre/day x 19.99 acres) has been added to
the flow, resulting in a total flow of 255,638.52 gpd (0.40 cfs).
The sewer main size for the development will be an 8-inch, based on minimum required pipe size
and designed at a 0.40% slope. An 8-inch PVC sewer main at 0.40% slope has a capacity of 0.64
cfs at ¾ pipe depth, which has adequate capacity for the proposed peak flow of 0.28 cfs effluent
generated from the site. Calculations are included in the end of this report.
C. Proposed Collection System
PVC pipe meeting COB specifications will be used for the sewer mains. The effluent from the
Nexus Point multi-unit residential development will drain into a proposed 8-inch diameter sanitary
sewer main via 4-inch & 6-inch sewer services. The effluent in the 8-inch wastewater collection
system on Arnold Street will flow north to proposed manholes before turning east and connecting
to an existing manhole located in South 19th Avenue, the wastewater collection system on Lantern
Lane will flow west to an existing manhole. The maximum manhole spacing will be 400 feet for
the 8-inch main. Manholes will be constructed according to COB specifications. Sewer services
will be 4-inch to 6-inch PVC installed at 2% minimum grade. Sewer services will connect into the
main with in-line gasketed wyes. The sewer main will maintain a minimum separation of 10-feet
horizontally and 18-inches vertically from any proposed or existing public water mains.
D. Downstream Capacity Analysis
The sanitary sewer mains were designed in conformance with the City of Bozeman Wastewater
Improvement Plan. It is anticipated that the City of Bozeman sanitary sewer system has enough
capacity to accommodate the main extension as this was included in the onsite design of the
downstream collection system. A capacity letter from a City Engineer will be required prior to
DEQ approval of the system.
E. Conclusion
As outlined above, a proposed 8-inch sanitary sewer collection system serving the residential
development has the capacity for the effluent generated from the subdivision. The wastewater
sewer collection system was designed on a conservative premise for future development. This
project will require the approval of the City of Bozeman Engineer and MDEQ.