Consider Stormwater Improvements at 148 th 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 homeowners adjacent to his ponding area have been notified of the meeting, and have 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 one of the report - 148th Lane

The site is east of Nowthen Boulevard (CSAH 5), southeast of Ramsey Elementary School.   There is a low area across three (3) lots south of 148th Lane, that was created as a stormwater pond for the area and wetland mitigation for the development.  There is a storm sewer inlet to this area along the west edge, which drains the subdivision to the north.  There is also an outlet from this area on the east end that connects to a pipe that runs easterly and parallel to 148th Lane.  The invert of this outlet pipe is approximately 2.5 feet above the low point in the basin.  The outlet pipe also connects to a structure in the boulevard of 148th Lane that takes storm water runoff from the roadway and directs it easterly as described above.  The inverts on both these pipes are at approximately the same elevation, and therefore water may be entering this basin that is not intended to flow there from the stormwater system.  It was also observed that the basin has several undulations in the ground surface that do not allow the water to evenly spread out across the entire drainage and utility easement; basically there appears to be three separate, somewhat isolated basins.

Hakanson Anderson explored several options for installing another outlet and determined that each option would not significantly affect the high water elevation in the basin, and would justify the costs associated with performing them.  Additional easements would need to be acquired, and disruption of neighboring properties would occur.  Staff is currently researching check valves that could be placed in the outlet pipe at  the manhole to block drainage from the street and only allow water to exit the basin through this structure, thereby eliminating additional flow to the area.

The funding source for these improvements is the storm water utility fund.  It is estimated that regrading the basin and installing a backflow valve on the outlet pipe would cost approximately $7,500 to $12,500.

Staff recommends regrading the backyards of the three properties to better distribute water in the basin, and re-establish the original pond contours.  It is further recommended to install a backflow valve on the outlet pipe to direct storm water runoff from 148th Lane to the intended downstream receiving body.

Motion to recommend to the City Council that staff seek quotes to regrade the backyards of the three properties to better distribute water in the basin and re-establish the original pond contours, and to install a backflow valve on the outlet pipe to direct storm water runoff from 148th Lane to the intended downstream receiving body.