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701 Xenia Avenue South | Suite 300 | Minneapolis, MN 55416 | (763) 541-4800 Building a legacy – your legacy. Equal Opportunity Employer | wsbeng.com Memorandum To: Sue Polka, PE, City of Arden Hills From: Matt Schulz, EIT, WSB & Associates, Inc. Jake Newhall, PE, WSB & Associates, Inc. Jim Stremel, PE, WSB & Associates, Inc. Date: March 12, 2018 Re: City of Arden Hills - 2018 Street and Utility Improvement Project (2018 PMP Project) Indian Oaks Area Downstream Drainage Review WSB Project No. 010111-000 WSB & Associates, Inc. has completed the initial drainage investigation downstream of the Indian Oaks wetland for the Arden Hills 2018 PMP project. I. PURPOSE AND H&H MODELING The City of Arden Hills is planning to reconstruct Indian Oaks Court and Indian Oaks Trail during the 2018 construction season. HydroCAD analysis completed during the project design phase shows the existing condition 100-year storm event will likely result in flooding to properties adjacent to the wetland south of Indian Oaks Court, here-in referred to as “IOC Wetland”, and will likely result in flooding to properties adjacent to the rock landscape ponding area along the IOC Wetland’s outlet between Indian Oaks Court and Colleen Avenue, here-in referred to as “Ponding Area”. See Attachment A for a location map or Figure 1. This drainage review was authorized by the City to evaluate increased peak flow rates downstream of the IOC Wetland and Ponding Area to further reduce the flooding risks to the adjacent residents. The analysis determined if increasing peak discharge rates leaving the IOC Wetland would cause any detrimental impacts downstream. Modifications to downstream drainage systems, in order to accommodate an increase in peak discharge rate, were also explored. This memorandum addresses a feasibility level study and does not include design level planning and analysis. IOC Wetland The IOC Wetland has been previously observed to overflow its banks and inundate the adjacent properties as well as Indian Oaks Court. Flooding risks are proposed to be reduced by lowering the Wetland outlet, reducing the permanent water level, and increasing available live storage area. This improvement will reduce flooding risks but when coupled with maintaining existing peak discharge rates, does not provide adequate capacity to provide any freeboard between the 100-year flood level and the low opening elevation of the home at 1416 Indian Oak Court. Ponding Area As part of the initial Arden Hills 2018 PMP project stormwater analysis, it was discovered that the Ponding Area is also susceptible to flooding. Reduction of flooding risk at this area was also evaluated as part of this drainage study. The Arden Hills 2018 PMP project currently proposes to reconstruct the storm sewer through this location and install additional inlets to reduce flooding potential. These improvements would also keep stormwater in the pipes instead of daylighting to the surface as it does today.