Consider Stormwater Improvements at 149th Lane - A Continuation of Discussion Related to 2011 Flooding Concerns

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 a larger stormwater improvement plan set. Staff has finalized plans for these items, incorporated additional items that were approved at the Public Works Committee meeting in March, and now has these items out for bid.  Depending on direction received tonight, for potential improvements to alleviate some of the outstanding concerns, additional plans will need to be prepared.

The homeowner referenced on the petition as the contact person for this item has been notified of the meeting and has received a copy of the agenda.

Staff prepared a brief RFP in the fall of 2011, to investigate the areas of concern, and distributed it to members of the City's consultant pool in the fall of 2011. The goal of this project was to have an independent third party evaluate the areas, provide options for corrective actions, and associated estimates to implement the work. Hakanson Anderson was awarded the project to undertake these investigations, and they have since completed their analysis and compiled a report that references potential solutions for each area identified. Attached to this case is the final report.

This item was introduced in general terms at the February Public Works Committee meeting, with the direction being that staff would come back before the committee over the next several months to discuss the merits of each potential improvement in more detail.  This case will focus on section two of the report - 149th Lane.

This area is west of TH 47 and east of 149th Lane.  The area in question is adjacent to the large DNR wetland which abuts TH 47.  The petition was initiated by the resident at Lot 3, Block 1, Ramsey Meadows 4th Addition (5410 149th Lane NW).  The homeowner contacted the City about water standing in the backyard.  Review of the grading plan indicates this area was part of the wetland mitigation plan prepared for the plat and is controlled by the Wetland Conservation Act rules.  There is a draintile that connects the backyard to the DNR wetland.  Stormwater from the wetland flowed into the back yard which is lower than the wetland.  A drainage and utility easement is recorded over the low area of the property.  The DNR lists the normal water level of the wetland as 860.0.  There is a retaining wall with steps in the backyard, with an elevation at the lowest step at approximately 860.1   The backyard slopes down from the steps so there will be water present when the wetland is at normal elevation.

There is a pipe under TH47, which is the outlet for the wetland, and there is an outlet structure on the pipe that controls the water elevation.  The lowest point on the structure was approximately 2 1/2 feet above the normal water elevation at the time the elevated water was occurring last summer.  The structure was quickly modified to lower the outflow elevation in an attempt to alleviate the high water concern.  The overflow is now 0.8 feet above the normal water elevation, but the structure can still be modified slightly to lower the invert to the approximate normal water level of the wetland (860).

The funding source for this improvement would be the Stormwater Utility Fund.  It is estimated that this structure modification would cost between $2,000 - $5,000, but staff would like to meet with local contractors to discuss all options prior to making a final determination.

Staff recommends modifying the outlet structure to lower the wetland outfall elevation, placement of a flared end section, and removal of vegetation around the outlet pipe.

Motion to recommend to the City Council that staff seek quotes to modify the outlet structure to lower the outlet elevation of the wetland.