2022 01-18 CC PACKET - Laserfiche WebLink

Home

3.2 A map of the fire flow analysis for the distribution system under a MDD scenario, with one and two watermain crossings under U.S. Highway 61, can be seen in Appendix E. Both one and two watermain crossings provide adequate fire flows, but if the empty agriculture lot to the west assigned as mixed use should develop, two watermain crossings may be needed to increase available fire flow to at least 2,000 gpm. Gem Lake — White Bear Township Water Syste Ow Using White Bear Township's watermain shapefiles, 2022 MDD demands, and water tower overflow elevations as the HGL for the system, a White Bear Township fed Gem Lake water system was modelled. White Bear Township's MDD demands were spread evenly throughout their system except for assigning their top ten users' demands to their appropriate locations, and each scenario was analyzed with no water being pumped into the system by the City's wells and with their water towers full to their overflow elevation. A map of the proposed Gem Lake — White Bear Township water system can be seen in Appendix G. 3.2.1 Gem Lake — White Bear Towel% Pr es White Bear Township's two water towers in the No em have an overflow elevation of 1,090 feet. Gem Lake system pressures projected those overflow elevations and during a peak hour MDD scenario (assumed to be 130% of the MDD in gallons per minute) can be seen in the maps in Appendix G. As can be seen in Appendix G-2 the higher elevations areas of Gem Lake are projected to have inadequate pressures, with one area seeing a pressures as low as 23 psi. As discussed earlier, a pressure below 20 psi would require a boil water advisory due to the risk of contamination entering the distribution system pipes. Like with the previously discussed White Bear Lake fed water system modelling, it would be expected that pressures would periodically be a few psi lower as the storage tank elevations vary from water usage throughout the day. , The 2006 feasibility study noted that the White Bear Township towers experience large water level drops during high water demand periods, so pressures would be expected to drop significantly during those periods. Due to the low pressures experienced with White Bear Township's HGL, a pressure booster station would likely be required. To project pressures experienced in Gem Lake with a pressure booster station, the hydraulic model was run with a booster station that boosts pressures so that the highest elevation in the Gem Lake system would maintain a pressure of 50 psi. As can be seen in Appendix G-4, a pressure booster station would provide Gem Lake with adequate pressures throughout the water system. Although some areas with a lower elevation would see pressures exceed 80 psi and approach 90 psi. pressures would still be within acceptable ranges. If Gem Lake preferred to boost pressures only for the high elevation areas on the west side of the City, a separate pressure zone, achieved by use of a local pressure booster station and pressure regulating valves in the distribution system piping, could be explored. 3.2.2 Gem Lake — White Bear Township Pipe Velocities As can be seen in Appendix G-2 and Appendix GA, the proposed watermains were sized to limit velocities to below 1.7 feet per second with or without a booster station. This is well below the maximum recommended velocity of 5 feet per second. FEASIBILITY STUDY UPDATE City of Gem Lake, MN Page 8