04-04-2016 Council Packet - Laserfiche WebLink

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feasibility study was completed in January 2016 and during review by City staff, an additional outlet alternative was requested to be included in the study. The original preferred alternative included a 72-inch pipe outlet to Peltier Lake from 20th Avenue, a new culvert crossing under I-35E, storm sewer along the proposed Otter Lake Trail extension, and regional ponding facilities to detain peak storm flow and prevent increasing the flood levels on Peltier Lake. The requested alternative includes the new culvert crossing under I-35E, storm sewer and regional ponding facilities; however it replaces the 72-inch pipe outlet with an open channel greenway between Peltier Lake Drive and 20th Avenue. The conceptual alignment and typical cross-section have been developed with the intent to provide live storage for flood events within the channel; be in agreement with the City’s AUAR and greenspace requirements; and avoid wetland impacts as much as possible. The greenway option also provides additional water quality treatment opportunities for the study area, above and beyond the City’s and RCWD’s development requirements. At the January Environmental Board meeting, additional data on the water quality calculations for the pipeline option was requested. RCWD provided guidance on the calculation of water quality impacts for this conceptual analysis using a simple land use analysis. This analysis is based on a 1.1-inch rainfall depth, which corresponds to RCWD volume reduction requirements, and uses total phosphorus concentrations provided by the District. Environmental Board member Heiskary provided different total phosphorus concentrations which change the calculations. Heiskary’s calculations result in the need for additional water quality treatment BMP’s. WSB staff is reviewing the basis for these revised numbers and will adjust the design accordingly. It is expected that that greenway option would be able to accommodate additional treatment requirements with minimal additional cost. Finally, the Environmental Board requested information on how existing drainage rights would be maintained at the January board meeting. The proposed project will not adversely impact existing drainage rights. No property will lose drainage rights due to the installation of the new outlet; the design of the system includes all upstream flows and will continue to pass ditch flows downstream, as under existing conditions. With the new outfall, it is likely that upstream property owners will experience increased capacity and less maintenance of their system as water can move through the system more efficiently. As development in the study area occurs, these individual developments, along with the property owner, will have to petition the Rice Creek Watershed District for re-alignment or abandonment of the drain tile on their property. This process is dictated by the Minnesota Statutes Chapter 103E and includes public hearings to ensure no adverse impacts to benefitted landowners will occur. The following is a tentative schedule to finalize the feasibility: April 2016 – Hold informational meeting with property owners and other stakeholders April 2016 – Present the final feasibility study to the Environmental Board for