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Home My WebLink About20 - Design Report - Nexus Point - Sewer C. Sewer Main Design Report - Exhibit SEWER MAIN DESIGN REPORT FOR: NEXUS PO IN T DEVELOPMENT BOZEMAN, NIT •r RIS G. r y1 Madison Engineering 895 Technology Blvd Ste 203 Bozeman, MT 59718 (406) 586-0262 MARCH 2020 9 co or t4 d-1 ro 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 19"' 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, four(4) 30 plex 1 & 2-bedroom units and two (2) 24 plex 2-bedroom units along with a club house for a total of 240 units in the residential development. The improvements for Phase 1 of the development are anticipated to be constructed during the summer of 2020. 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 8" 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 Manhole's located in Lantern Lane and South 19t' 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 1 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: residents gallons \ C240 units * 2.11 * 89 ! = 45,070 gpd unit capita/day / An approximate population for the Nexus Point Multi-Unit Residential Development at full build out is calculated as: residents 240 units * 2.11 unit = 507 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 79 persons. This equivalent population can be utilized to determine a peaking factor: Qmax) 18 + P•5 _ 18 +. 507-5 QAve I 4 + P•5 4 +. 507-5 = 3.97 (P =Population/thousands) Based on a total population of 507 people, a peaking factor of 3.97 was calculated. The peak flow rate anticipated from the development based on an average daily flow of 45,070 gallons per day is: Peak Flow Rate = (45,070 gpd x 3.97) =178,928 gpd = 125 gpm = 0.28 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 181,927 gpd (0.28 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 3/4 pipe depth, which has adequate capacity for the combined flow of 0.56 cfs (0.28 cfs population peak flow rate and inflow and infiltration rate of 0.28 cfs). 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 Nexus Point Development Wastewater Design Report Page 2 of 3 system on Arnold Street will flow north to proposed manholes before turning east and connecting to an existing manhole located in South 19"' 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. 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