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HomeMy WebLinkAboutSewerDesignReport_07262021Sanitary Sewer System Design Report Annie Subdivision Phase 4 Bozeman, MT 109 E. MAIN STREET, BOZEMAN, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com ANNIE SUBDIVISION PHASE 4 Sanitary Sewer System Design Report REPORT DATE: May, 2021 AUTHOR: Mathias Hanssen, E.I. STAFF ENGINEER Gary Fox, P.E. PROJECT ENGINEER WGM Group, Inc. ANNIE SUBDIVISION PHASE 4 Sanitary Sewer System Design Report CONTENTS 1.0 PROJECT DESCRIPTION ................................................................. 1 2.0 FLOW DATA AND DESIGN ........................................................... 1-2 3.0 IMPACT ON EXISTING WASTEWATER FACILITIES .................... 2-3 APPENDICES A – VICINITY MAP 1 ANNIE SUBDIVISION PHASE 4 Sanitary Sewer System Design Report 1.0 PROJECT DESCRIPTION The development will be comprised of approximately 19 residential lots, and an open space/park lot, water and sewer extensions, and a stormwater management system. For sewer service, the development proposes to extend existing City of Bozeman mains at Rogers Way and Daffodil Street that terminate at the northern and western boundaries of this tract, respectively. Four (4) inch sewer services will connect each lot to these proposed main extensions. 2.0 FLOW DATA AND DESIGN The new wastewater generation for the project was based upon the total population to be served by the new main and services. The following assumptions and calculations were made per City of Bozeman Design Standard and Specifications Policy. 18 √ 4 √ ℎ: ℎ TABLE 1 – Peaking Factor based on Harmon’s Formula Peaking Factor Calculation # of Dwelling Units 25 DU # of People per Dwelling 2.11 people/DU* Population 53 people Peaking Factor 2.24 *Based on City of Bozeman Design Standards and Specifications Policy ! " # $ %&’( % ) !)& " 18 √ 4 √ 2 ANNIE SUBDIVISION PHASE 4 Sanitary Sewer System Design Report TABLE 2 – Wastewater Demand Calculation Based on TABLE 1 Peaking Factor The minimum eight (8) inch piping will be used with this development. Laid at a minimum slope of 0.4%, eight (8) inch piping with a Manning’s coefficient of 0.013 at 75% full, has a capacity of 0.65 cfs, or 291 gpm. Therefore, the proposed pipe size is more than adequate to convey the anticipated 8.07 gpm of peak wastewater flow rate. 3.0 IMPACT ON EXISTING WASTEWATER FACILITIES The existing eight (8) inch SDR 35 PVC sewer main in Daffodil is sloped at 1%. At 75% full, its maximum capacity is 1.03 cfs, or 463 gpm. To the North, the existing eight (8) inch SDR 35 PVC sewer main is sloped at 1.2%. With a maximum capacity of 75% full, it can accommodate 1.125 cfs, or 506 gpm. Referencing the City of Bozeman Wastewater Capacity viewer, both mains are less than 50% obligated. If the worst-case scenario were assumed and both mains are at 50% capacity, the Roger Main would have a peak flow of 303 gpm and the Daffodil Main would have a peak flow of 277 gpm. This liberally assumes that the “less than 50% obligated” is referring to the entire pipe diameter, not the 75% maximum set by the City of Bozeman Design and Specifications Policy. If the “less than 50% obligated” is referring to the 75% pipe full maximum, there would be more capacity in the pipe than what WGM Group has assumed. Wastewater Demand Calculation Lot Area 3.24 acres Assumed Infiltration Rate* 150 gallons/acre/day Infiltration Rate 486 gpd Wastewater Generation Rate* 89 gallons/day/person Average Wastewater Flow Rate 3.60 gpm Average Wastewater Flow Rate 216 gallons/hour Peak Wastewater Flow Rate 8.07 gpm Peak Wastewater Flow Rate 484 gallons/hour *Based on City of Bozeman Design Standards and Specifications Policy 4 ANNIE SUBDIVISION PHASE 4 Sanitary Sewer System Design Report APPENDIX A VICINITY MAP © 2020 Microsoft Corporation © 2020 Maxar ©CNES (2020) Distribution Airbus DS WGMGROUPOFWWW.WGMGROUP.COMSUBDIVISION OVERVIEW DURSTON RESIDENTIAL (2305 DURSTON RD) BOZMAN, MONTANADECEMBER 18, 202011NPRELIMINARYPLOTTED:SAVED:12/18/2012/18/20PROJECT SITELOT 1, ANNIESUB PH. 3CANNIE SUB. PH. 3CPLAT J-482ANNIE SUB. PH. 1PLAT J-135DURSTONMEADOWSPH. 2, PLAT J-258DURSTONMEADOWSPH. 1, PLAT J-238C.O.S.1817KABLESUB.PLATF-23-BWESTPARK MANOR4TH ADD.PLAT J-184ANNIE SUB.PH. 3APLAT J-343DURSTON RDN 25TH A V E 19TH AVEWILLOWBROOKCONDOSCOS 1855