Agenda - Public Works Committee - 10/21/2003

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P.W. CASE: POTASSIUM STREET DRAINAGE CONCERNS By: Steven J. Jankowski, City Engineer Background: Within the past several weeks concern has been expressed from two residents along Potassium Street relative to the drainage surrounding the small wetland located in the backyards of 14660 Potassium Street, Jay and Tom Mault, and 14680 Potassium, Mr. Steve Roof. The east half of this wetland is located in these two yards while the. west is located in the Village of Sunfish Lake. '.Attached~to this case are several exhibits: Exhibit A illustrates the drainage area contributing to this wetland pr/or to the Village of Sunfish Lake. Exhibit B illustrates the drainage area contributing to this wetland subsequent to the Village of Sunfish Lake. Exhibit C is a copy of the plat of Chestnut Ridge 3r~ Addition which illustrates the lots surrounding wetland and the drainage and utility easement on these lots. Prior to the Village of Sunfish Lake development a storm sewer to 146th Avenue provided an outlet for this wetland once storm water would reach an elevation of 866.1. This outlet elevation was sufficient to prevent flooding of the residential structures surrounding this depression but would allow yard flooding. The grading and drainage plan for the village of Sunfish Lake reduced the contributing drainage area to this wetland depression from 12.43 acres to 4.01 acres, although it also increased the contributing impervious area by adding half of the roof areas from seventeen additional singe family units. A review of drainage submittals show the net effect of the basin modifications by the development of Village of Sunfish Lake is a reduction in runoff from a 100 year precipitation event from 0.676 acre-feet to 0.296 acre-feet. A new outlet was also provided from this depression at an elevation of 863.0. A precipitation event which consists of back to back 100-year storms, would cause the water level in this wetland to rise to a maximum elevation of 863.6. Residents located at 14660 and t4680 Potassium Street expressed concern over the drainage change, noting that this summer the low portions of their backyard appear wetter than in the past, despite the dry spell beginning in early July. A meeting was held with the residents' on Friday, October 17, 2003 on site with the residents, the developer and his engineer,' and the city engineering department. The location of the 863.0 and 863.6 contours had been surveyed and staked as well as the drainage and utility easement in these yards. These contours were contained entirely within the drainage and utility easement on the Maults property (14660), but fell outside the easement limits of Roofs property (14680) in places up to approximately ten feet. Recommendation: Although the 1 O-year water coriditions are either wholly or substantially contained within existing easements, it is felt some cooperative arrangement should be considered to alleviate resident concerns, Staff is therefore recommending that following possible measure be considered to ach/eve this goal. '