8.3 SR 02-06-2023 - Laserfiche WebLink

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1. Prioritize gravity flow over pressurized flow; 2. Maintain a minimum cover depth of 7.5-feet; 3. Minimize gravity sewer depth installation deeper than 30-feet; 4. Design gravity trunk sewermains to flow 85-percent full under peak hourly flow conditions; 5. Install gravity sewer pipe per the minimum Ten-States Standards' recommended slopes; 6. Design forcemains to maintain flow velocities between 2.0 and 5.0 fps; 7. Design forcemains to maintain headloss gradients below 10 ft per 1,000 ft; 8. Recommend lift stations firm capacities that exceed by 25-percent the projected peak hourly flows; 9. Lift station design shall be submersible type with a wet well and a valve vault; and 10. Limit wet well depths to 35-feet when possible. A summary of the recommended sanitary sewer infrastructure for the northeast study areas is listed below: Trunk Sewermain (gravity flow): It is recommended to install trunk sewermains ranging from 10- inch to 18-inch diameter along Cleveland Street NW and Twin Lakes Road NW to minimize the need for lift stations. Eight-inch sewermains can be extended beyond the trunk mains to serve the remaining portions of the study area. Lift Stations (pressurized flow): Although the ground elevations throughout the study area are conducive to maximizing gravity flow, trunk lift stations are still required to pump wastewater. At least three trunk lift stations will be needed to serve the study area. Additional smaller lift stations orsmall low pressure systems may be needed to serve minor portions ofthe study area as indicated in Figure A20. The need for these lift stations will depend on the final grading within particular developments near the wetland areas. A summary of the recommended trunk lift stations is shown in Table 4.9. Each lift station's wet well depth is the minimum recommended depth to serve the sewersheds identified in Figure A20. Deeper wet wells can be installed if a more conservative design is desired. Table 4.9 — Preliminary Northeast Lift Station Design�'� Ultimate Initial Firm Ultimate Firm No. Min. Wet Forcemain Peak Pumping Pumping Pumps Forcemain Name Well Diameter Hourly Flow Capacity Capac(ty (Initial/ Depth (ft) Quantity (inch) (gpm) (gpm) (gpm) Ultimate) Regional 1,100 500 — 750 1,500 2/3 41 Dual 8& 12 West 140 185 185 2 35 Single 6 North 170 225 225 2 20 Single 6 (1) Ultfmate firm pumpfng desfgn capacftfes and forcemafn dfameters shall be revfsed durfng each Ifft statfon's desfgn phase based on final development densities. (2) Initially, two (2) 750 gpm pumps can be installed at the Regional Lift Station. As development increases, a third 750-gpm pump can be added to increase the lift station's firm pumping capacity to 1,500 gpm. Wastewater pumped by the northeast Regional Lift Station will be conveyed via dual forcemain directly into the City's VWVfP. Because of this, it is likely that a new receiving structure will be needed at the facility. Initially, the 8-inch forcemain alone can be used when the lift station's pumping capacity is lower and both forcemains can be used as the lift station's pumping capacity increases over time. It is recommended to install both forcemains at the same time to reduce installation costs. Air release valve manholes should be installed at the high points of the forcemains as shown in Figure A20. 4.6 System Evaluation and Capacity Analysis for Northwest Areas A portion of the wastewater generated in the northwest areas will be pumped by the existing Windsor Lift Station. As shown in Table 4.2, the Windsor Lift Station has an existing residual capacity of 196 gpm (or 82-percent). Peak hourly flow in this lift station's sewershed is projected to increase to approximately 300 gpm for ultimate development conditions. Given that the existing lift station capacity is only 240 gpm, the pumping capacity of this lift station may require upsizing in the future. Ultimate peak hourly flows for this sewershed may never reach 300 gpm. Since the Windsor Lift Station has sufficient residual capacity for the Feasibility Report Northeast & Northwest Urban Service Area Expansion Study City of Elk River, MN WSB Project No. 020010-000 Page 19