Agenda - Public Works Committee - 04/18/2000

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PW CASE UPDATE ON 149TM AVENUE STORM SEWER PROJECT (IMPROVEMENT PROJECT #00-23) By: Steve Jankowski, City Engineer Background: Attached is the CIP sheet describing the storm sewer project that is planned to be constructed in conjunction with the T.H. #47 STP project this year. The storm pipe extending across T.H. #47 will be constructed and funded as a portion of the highway project. The remaining work is anticipated to be funded from the storm water utility. At the present time there are two drainage issues. First, the small pond on the northeast comer of 149th Avenue and T.H. #47 experiences a very rapid rise in water level during a storm event causing the adjacent residence at 5260 149th Lane to flood. A second issue is associated with the larger wetland system that lies along the border of Amber Ridge and Gorham Sandy Acres. This wetland system outlets only when water levels are high, and only then via an 18-inch pipe. The result is that high water levels resulting from precipitation events decline slowly. Some of the neighboring properties in Gorham Sandy Acres have lawns that are at wetland elevation, and have complained in the past about soggy lawns after rain events. Attached to this case is a figure illustrating the existing drainage system, and two additional options analysis by SPY Inc. The storm drainage system for this wetland complex was modeled. The model anticipated storm water contributions from the wetland on the west side of T.H. #47 and it's entire drainage area. The residence at 5260 149th Lane currently floods, primarily because store, water does not leave via the swale. Option A proposes installing an outlet pipe from the pond to the wetland at a cost of $36,250 total project cost. This will eliminate the flooding of the residence and provide 1.5 feet of face board for the 100 year storm event. Option A will not significantly change the fact that the wetland will remain at an elevated water level following storm events. The attached hydrograph shows the high water for option A would remain for more than five days. Option B provides the same outlet from the pond to the wetland, but also includes a new storm pipe in 149th Avenue, which becomes the main outlet to remove the water from the wetland complex. This option has a total project cost of $115,000. The chief benefit of this alternative over Option A is that it reduces the period of elevated water level in the wetland from 5.25 days to 1.75 days. The purpose of this case is to familiarize the committee with the goals of this project and what various options will cost. It is important to note also what this project will not accomplish. It will not eliminate the large fluctuations in water elevation on the storm water pond. It will also not eliminate soggy lawns on the parcels adjacent to the wetland, but will significantly reduce the duration of high water in wetland complex.