Agenda - Public Works Committee - 02/21/2012

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Public Works Committee 5.2. Meeting Date: 02/21/2012 By: Tim Himmer, Engineering/Public Works Title: Discuss Investigations Related to 2011 Flooding Concerns Background: Last summer the City experienced several significant rainfall events that lead to many localized flooding concerns, and resident complaints. The large volumes of precipitation that occurred over a short period of time appears to have elevated the groundwater within areas of the City, and prohibited the generous rate of infiltration that typically takes place in the Anoka sandplain. Throughout the summer and fall staff worked hard at registering and responding to the calls, and evaluating the situations on an individual basis to determine whether quick fixes could be implemented (culvert obstructions, re -ditching, etc.) to alleviate the immediate concerns. This item was discussed at the Public Works Committee on August 15, 2011, and at that time staff summarized the areas of concern that were being investigated based upon citizen complaints received. The attached summary was presented at the meeting; which outlines the concern identified, actions steps to evaluate the concern, additional investigations that would be necessary to fully understand the situation, and recommendations for specific projects that could be implemented rather quickly and inexpensively. At that time we classified the issues into 3 categories: 1. Those that required no further action. They were evaluated and corrected, or did not need correcting because the water was fully contained within a dedicated drainage & utility easement (functioning as designed). 2. Those that required additional investigation and evaluation before deciding on a long term solution, and implementing corrective actions. 3. Those that had an identified recommendation for immediate action. At that meeting the Committee briefly discussed the areas of concern, and directed staff to prepare plans and specifications for the items identified in category 3. Once this direction was ratified by the City Council on September 13, 2011, and the plans completed, it was too late in the season to secure bids and complete the improvements in 2011. It was then decided to delay construction until 2012, where some of the improvements could be completed by inclusion in the street maintenance program. On August 23, 2011 the City Council accepted a petition, from residents in the neighborhood near 149th Lane NW and TH 47, requesting that the City review the drainage problems associated with the wetland complex in that location. The area is largely comprised of the RAMSEY MEADOWS subdivision (5 separate subdivisions), which was built around a large wetland complex and included significant easements encompassing the 100 year flood elevation. The petition was initiated by the resident at 5410 149th Lane NW because they had a considerable amount of standing water in their backyard. Staff conducted a site visit and reviewed plat documents for the property in question, and determined that the backyard of all the homes along the east side of 149th Lane had rather large platted drainage and utility easement at the rear of their properties that corresponded to the 100 year flood elevation of the surrounding water body. At that time staff also reviewed the wetland outfall and downstream receiving bodies (pipes, ponds, culverts, and ditches) to determine whether there was an obstruction that could be holding back the water. While no obstructions were found it was discovered that the outlet pipe from the wetland, that was installed as part of the TH 47 reconstruction project, may have been installed at an incorrect elevation. Staff immediately modified that structure, and was able to provide some relief of surface water levels. It was also determined that the property owner had constructed some improvements in their backyard (retaining wall construction and/or grading revisions) that may have reduced some of the storage capacity in the area and compounded the flooding situation on their parcel. Council direction from that meeting was to analyze the sub -watershed for this area in an effort to determine whether improvements could be made that would correct the concern identified and provide additional relief from long term