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HomeMy WebLinkAboutENG #8 - Sanitary Sewer Memo 06-24-2020 1 Sewer This section is a response to the requirements of the City of Bozeman Unified Development Ordinance Section 38.220.060 relative to Water and Sewer. The original Sanitary Sewer Design Report provides analysis for the entire PUD. The sewer main in Fowler/Avenue serves roughly the west two-thirds of the Bozeman East Subdivision including Phase 5. The remainder of the subdivision is served by a connection just south of the Huffine Lane/College intersection. Flows from Phase 5 are designed to flow east to the existing sewer main in Fowler Avenue. Per the original design report the sanitary sewer segment for Phase 5 is termed “1W”. With the design of Lot 1 & 2, the layout has been revised so that the flows are split into “1.1W” and “1.2W” (see updated sewer main exhibit). “1.1W” will be sewer flow from Lot 2 that flow east via Technology Boulevard West. “1.2w” will be sewer flow from Lot 1 that will flow via a sewer service directly into the sewer main in Fowler Road. Appendix A “The Bozeman Gateway” Sewer Flow Spreadsheet-West Drainage Basin, indicates that the with a design flow rate of 3495 gpd/ac, that the peak flow from Phase 5 will be 85.1gpm. More broadly, the table from Appendix A indicates that the total peak flow from the west side of Bozeman Gateway is 435.3gpm. Since the report was written, the City of Bozeman Design Standards and Specifications have been revised to reflect a lower required flow rate for B-2 zoning, decreasing from 3495 gal/ac/day to 2000 gal/ac/day. Based on the Sewer Facility Plan’s revised flow of 2000 gal/ac/day, the wastewater produced by Phase 5 as well as the total flows from the west side of the subdivision is calculated as follows: Sewer Flow - Phase 5 Assumptions: - Phase 5 Area – 6.9 acres - Design Flow Rate – 2000gpd/ac - Proposed Hotel has 138 rooms - Proposed Hotel guest occupancy rate of 1.5 guests per room = 207 guests - Proposed Hotel will have approximately 15 employees Phase 5 Average Flow Rate 6.9 acres x 2000gpd/ac = 13,800gpd = 0.02cfs = 8.98gpm Population = 207 guests + 15 employees = 222 people Peaking Factor (PF) = (18+(population/1000)^0.5)/(4+(population/1000)^0.5) = 4.13 Peak Flow = 13,800gpd (avg daily flow) x 4.13 = 56,994gpd Infiltration = 150 gpd/acre x 6.9 acres = 1,035gpd 2 Total Peak Flow Phase 5 = 56,994gpd + 1,035gpd = 58,029gpd = 0.09cfs = 40.3gpm Sewer Flow – Western Side of Bozeman Gateway Assumptions: - Western Side Area - 48.7 acres - Design Flow Rate – 2000gpd/ac - Peaking Factor – 3.53 West Side Average Flow Rate = 48.7 acres x 2000gpd/ac = 97,400gpd Peak Flow = 97,400gpd (avg daily flow) x 3.53 = 343,822gpd Infiltration = 150 gpd/acre x 48.7 acres = 7,305gpd Total Peak Flow West Side = 343,822gpd + 7,305gpd = 351,127gpd = 0.54cfs = 243.9gpm Impact on Existing Wastewater Facilities The manhole located at the intersection of Fowler and Technology Blvd was dipped on 12/30/2019 and was found to have a sewage depth of 0.2’. Due to the timing of this measurement during the holiday season, it is conservatively assumed that the average existing flow depth is 0.5’. The outlet pipe has a diameter of 21”. At a depth of 0.5’ the pipe is approximately 29% full and flowing at a rate of 1.06cfs (475.8gpm), see attached Existing Pipe Flow report. Assuming a minimum slope of 0.1%, the 21” pipe has a capacity flowing full of 6.3cfs (2,827.6gpm), see attached 21in PVC Flowing Full report. The contribution of 0.09cfs from Phase 5 will increase the flow to 1.15cfs (516gpm) which equates to a pipe depth of 0.53 or 30.3%, see attached Proposed Pipe Flow Report. The existing sewer mains 21” pipe in the vicinity of the project can handle the increase in flows based on the information in the original Sanitary Sewer Design Report, the reduction in B-2 design flow rates and the existing measured flow. Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Dec 31 2019 Existing Pipe Flow Circular Diameter (ft) = 1.75 Invert Elev (ft) = 100.00 Slope (%) = 0.10 N-Value = 0.011 Calculations Compute by: Known Depth Known Depth (ft) = 0.50 Highlighted Depth (ft) = 0.50 Q (cfs) = 1.061 Area (sqft) = 0.57 Velocity (ft/s) = 1.86 Wetted Perim (ft) = 1.98 Crit Depth, Yc (ft) = 0.37 Top Width (ft) = 1.58 EGL (ft) = 0.55 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Sunday, Dec 29 2019 21in PVC Flowing Full Circular Diameter (ft) = 1.75 Invert Elev (ft) = 100.00 Slope (%) = 0.10 N-Value = 0.011 Calculations Compute by: Q vs Depth No. Increments = 10 Highlighted Depth (ft) = 1.58 Q (cfs) = 6.312 Area (sqft) = 2.28 Velocity (ft/s) = 2.77 Wetted Perim (ft) = 4.38 Crit Depth, Yc (ft) = 0.93 Top Width (ft) = 1.05 EGL (ft) = 1.69 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Thursday, May 14 2020 Proposed Pipe Flow Circular Diameter (ft) = 1.75 Invert Elev (ft) = 100.00 Slope (%) = 0.10 N-Value = 0.011 Calculations Compute by: Known Q Known Q (cfs) = 1.15 Highlighted Depth (ft) = 0.53 Q (cfs) = 1.150 Area (sqft) = 0.62 Velocity (ft/s) = 1.85 Wetted Perim (ft) = 2.05 Crit Depth, Yc (ft) = 0.39 Top Width (ft) = 1.61 EGL (ft) = 0.58 0 1 2 3 4 Elev (ft)Depth (ft)Section 99.50 -0.50 100.00 0.00 100.50 0.50 101.00 1.00 101.50 1.50 102.00 2.00 Reach (ft) OPEN SPACE 2 OPEN SPACE 3 OPEN SPACE 4 COMMON AREA 1ALOT 1A1 LOT 3A LOT 1A LOT 4 LOT 5 LOT 6 LOT 7 LOT 5 OPEN SPACE 5COMMON AREA 2A OPEN SPACE 6 LOT 14A COMMON AREA 3A LOT 16A COMMON AREA 4A LOT 17A LOT 25A LOT 22B LOT 46 LOT 45 COMMON AREA 12 LOT 48 COMMON AREA 7 COMMON AREA 6A LOT 24A LOT 23A OPEN SPACE 10 LOT 1 LOT 2 LOT 3 LOT 4 LOT 1A LOT 5 LOT 6 LOT 3 LOT 4 OPEN SPACE 1 LOT 1 LOT 6 LOT 7 COMMON AREA 5A LOT 19A LOT 2A1 LOT 4A LOT 35 LOT 36 COMMON AREA 11 LOT 37 LOT 38 COMMON AREA 10 LOT 40 LOT 41 LOT 42 COMMON AREA 14 COMMON AREA 13 LOT 44 LOT 43 OPEN SPACE 1B COMMON AREA 8 OPEN SPACE 1A LOT 2 SSSSSSSSSSSSSSSSSSSSSSWWSSWWSS SS SS SS SSW SS W W W W W W W W W W W W W W W W W WWWWWWWWW W WWWWW W W W WWWWWWWWWWWW WSSSS SS SS SS SS SS SS SS SS SSWSSW SSSSSSSSSSSSSSSSSSSSSSSSWWWWWWWWWWWWWW W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W WWW SS SSW SS SS SS SS SS SS SS SS SS SS SS SS SS SS SS SS SS SS SS SSWSSSSSS W SSSSWWWSS SS SS SS SS CONNECTION POINT EXST. MH JO516 CONNECTION POINT EXST. MH JO602 2W 1.1W 3W 5W 4W 1E 2E 3E 4E 1W ANALYZED SANITARY SEWER SEGMENT SEE APPENDIX A & B PROPOSED SANITARY SEWER MAIN PROPERTY BOUNDARY LEGEND SSWWWW W W SSSS 1.2W BOZEMAN GATEWAY SEWER LAYOUT