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City of Arden Hills - Indian Oaks Area Downstream Drainage Review March 12, 2018 Page 6 Building a legacy – your legacy. Equal Opportunity Employer | wsbeng.com - 24” HDPE pipe from Indian Oaks Court to Colleen Avenue - 24” RCP along Colleen Avenue - Install Additional Inlet at Colleen Avenue Sag for revised sewer configuration and improve roadway ponding. - Directional drill 18” DR-17 HDPE parallel to existing 15” RCP. Directional drilling new storm sewer will minimize disturbance to adjacent properties and avoid excavation conflicts with utilities and timber retaining wall. See Figure 4. - Re-establish the emergency overflow (EOF) between the Ponding Area and Colleen Avenue and EOF north of the Colleen Avenue to 1’ below the adjacent house low opening. - Provide additional riprap at storm sewer outfall to Colleen Wetland. - Raise bituminous trail grade adjacent to the Colleen Wetland to 933.10 at EOF (Approximately 200ft of trail improvements). - Outlet control structure with 27” discharge pipe for Colleen Wetland rate control and 24” culvert - Pavement removal/replacement and other associated work. The 21”/24” discharge pipe option results in a 100-year high water level elevation in the Wetland and Ponding Area of 961.52 and 957.80, respectively. Phased Plan We propose that these drainage improvements should be installed as part of a phased plan. Phase 1 will be constructed with the Arden Hill 2018 PMP Project and includes all improvements between the IOC Wetland and the existing manhole on Colleen Avenue. Phase 2 construction will include the other drainage improvements and have the option to be included as part of a future City project with additional improvements. Orifice plates located in the storm sewer structures may be necessary to maintain peak discharge rates leaving the Wetland and Ponding Area to at or below existing conditions until the downstream improvements are complete Increased Inundation The HydroCAD analysis indicates the existing condition 100-year highwater level in the Colleen Wetland encroaches onto the property of 1457 Colleen Avenue, past the easement. Increasing pipe sizes and mitigating the increased peak flow rates at the trail crossing to will raise 100-year highwater levels and cause additional encroach onto private properties. Figure 5 shows the increase in highwater level for the 18” option and 21”/24” option in the Colleen Wetland. The 21”/24” option results in an estimated 4’ additional encroachment on the 1457 Colleen Avenue property and 25’ of encroachment on the 1449 Colleen Avenue property. The 18” options results in slightly less encroachment than the 21”/24” option. The highwater bounce area of additional inundation is located along a steep wooded slope on the low side of a retaining wall. The amount of inundation will be confirmed based on final design, refined analysis, and design-level survey.