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Department Approval l�'�, Jy REQUEST FOR COUNCIL ACTION Date: 8/10/09 Item No.: 7.i Acting City Manager Approval � � � Item Description: Approve project and authorize advertisement for bids for Walsh Lake Subwatershed Rain Garden Project BACKGROUND On March 24, 2008, the City Council authorized a study of the storm water hydrology of the Walsh Lake subwatershed located in the neighborhood southeast of Midland Hills Golf Course ("Rosewood Neighborhood") due to neighborhood concerns about localized street flooding and damage to property. This area includes the following streets: Midland Hills Road, Draper Avenue, Rosedale Drive, Westwood � Circle, Hythe Street, Skillman Avenue, North Rosewood Lane, and South Rosewood Lane. � WSS and Associates, Inc., completed the modeling for this area, as well as the final design for the drainage � improvements to mitigate the flooding in the neighborhood. WSS's analysis showed that the flooding �� concerns in the neighborhood were separated into two distinct watersheds. The final design includes two � improvements to alleviate the flooding concerns in the neighborhood. �: The first improvement consists of the construction of 22 rain gardens throughout the neighborhood. This �. will decrease the runoff volume of water and change the rate at which water reaches the bottleneck in the �� storm sewer system at Draper Avenue and Midland Hills Road. In order to reduce the volume of water in �� the amount needed to provide flood protection, an underground storage chamber will also need to be � constructed. Past storms have caused flooding up to 2 feet higher than the lowest floor elevation at 2241 � � Rosewood Lane South. A backflow preventor will be installed in the existing storm sewer inlet and a berm �� will be constructed at 2241 Rosewood, which will prevent the street flooding from entering the yard and z� the home. The improvements will provide one foot of freeboard to the home, or the high water level will z be one foot lower than the low building elevation. Staff held several neighborhood meetings with the z; property owners to provide educational information about rain gardens and seek interested participants for z: this project. Each rain garden will be constructed in the right of way and/or on properties where the z. property owners were interested and willing to plant and maintain the rain gardens. A maintenance z� agreement will be signed by each property owner and recorded against the property. z� z The second improvement consists of the expansion of "Rosewood Pond", the wetland located between z� 2235 and 2211 Rosewood Lane North. This pond is prone to flooding during large rain events, and in the z� past, has inundated two nearby homes. The drainage study determined that a majority of the runoff to this 3� pond comes from the golf course. After reviewing several alternatives, it was determined that the most cost 3 effective solution is to increase storage in this pond. The proposed project to provide flood protection to 3; these homes is to expand and deepen the pond. Existing conditions result in up to one foot of flooding 3: above the low building elevations. The wetland expansion will provide one foot of freeboard, or the high � water level will be one foot lower than the low building elevations. The expansion would occur onto the Midland Hills Country Club (MHCC) property. City staff has worked with the MHCC to ensure that the Page 1 of 2