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Home My WebLink About015 Sewer Report WATER & SEWER MAIN DESIGN REPORTS FOR: 2131 GRAF MULTI-UNIT RESIDENTIAL DEVELOPMENT BOZEMAN, MT Prepared By: Madison Engineering 895 Technology Drive, Suite 203 Bozeman, MT 59718 (406) 586-0262 AUGUST 2020 2131 GRAF MULTI-UNIT RESIDENTIAL 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 Icon Apartment Homes at 2131 Graf multi-unit residential development Lot 2 of minor sub 235 located in S23, T20S, R05E P.M.M, located south of Stucky Road and north of Graf Street in Bozeman, MT. The zoning for the 25.99 acre area is listed as R-4, residential high density. The proposed apartment buildings are designed as three (3)-3 plex (Type A), nine (9)-32 plex (Type B), and five (5)-32 plex (Type C) for a total of 457 units in the residential development. The improvements for Phase 1 of the development are anticipated to be constructed during the Fall 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 #7 b. COB Modification to Montana Public Works Standard Specifications (MPWSS) c. City adopted MPWSS and adopted Addenda d. COB Wastewater Facility Plan, 2017 e. MDEQ 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 2,714 linear feet (0.51 miles) of new 8” sewer main and will provide service to the multi-unit residential development. The effluent generated from the proposed apartment buildings will be collected by the wastewater collection system and ultimately flow north to Lantern Lane and west to an existing COB SSMH. 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 1,950 gal/ac-day for R-4 zoning, resulting in 49,901 gpd = 25.59ac x 1,950 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. 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: ൬457 units ∗ 2.11 residents unit ∗ 89 gallons capita/day ൰ = 85,820 gpd An approximate population for the project at full build out is calculated as: 457 units ∗ 2.11 residents unit = 964 persons This is a more realistic population value compared to utilizing the 10.4 units per acre (Table V-1 from COB design standards) applied to 25.99 acres, which only results in 570 persons. This equivalent population can be utilized to determine a peaking factor: ൬𝑄௠௔௫ 𝑄஺௩௘ ൰ =18 +𝑃.ହ 4 +𝑃.ହ =18 +.964 .ହ 4 +.964 .ହ = 3.81 (P = Population/thousands) Based on a total population of 964 people, a peaking factor of 3.81 was calculated. The peak flow rate anticipated from the development based on an average daily flow of 85,820 gallons per day is: Peak Flow Rate = (85,820 gpd x 3.81) = 326,974 gpd = 227 gpm = 0.51 cfs Additionally, an infiltration flow of 3,839 gpd (150 gal/acre/day x 25.59 acres) has been added to the flow, resulting in a total flow of 330,813 gpd (0.51 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 0.52 cfs effluent generated from the site. Calculations are included in the end of this report. In addition, two of the proposed buildings will utilize the existing 8” sewer main in Graf St. The total number of units using this sewer is 64 units. The resultant flow rate is 12,019 gpd, or 0.02 cfs. The peak flow rate is 45,792 gpd, or 0.07 cfs. C. Proposed Collection System PVC pipe meeting COB specifications will be used for the sewer mains. The effluent from the development will drain into a proposed 8-inch diameter sanitary sewer main via 4-inch or 6-inch sewer services. The effluent in the 8-inch wastewater collection system will then flow north to Lantern Drive and west to an existing manhole and existing 8” sewer main in Lantern Drive. 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. The sewer effluent from the proposed development will flow into the existing Meadow Creek Trunk Sewer which was designed for this property. 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.