Agenda - Public Works Committee - 02/16/1999

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PW CASE #5 UPDATE ON STORMWATER PLANNING By: Steve Jankowski, City Engineer Background: There is a dual purpose for this case. First, it is intended to provide a summary of the previous stormwater studies conducted within the City. Secondly, it is intended to achieve consensus on what issues need to be addressed in order to move forward with a number of stormwater projects that have been identified in the City's Capital Improvement Plan. Previous Studies The 1980 Comprehensive Storm Drainage Plan was prepared by Peter Raatikka, P.E., then affiliated with Consulting Engineers Diversified, Inc. This study addressed the area of the City south of Trott Brook. The study established drainage subbasins and identified how drainage would be transmitted from basin to basin until ultimately discharging into either the Mississippi or Rum Rivers. This study proposed utilizing existing wetlands and future regional detention ponds for stormwater storage. Water storage volumes were identified along with the release rates between basins, which could be used to design ditches and storm pipes. This study also included an inventory of storm water structures in place throughout the study area at the time. Although nearly twenty years old, this study continues to be a valuable planning document. Specifically, the drainage subbasins and their relationships continue to be valid. When a more detailed study of the southeastern quadrant of the City was conducted in 1997 utilizing more precise topographic information, there was no change from the previously defined drainage patterns. Also, although the southeastern portion of this study area has developed considerably, the inventory of existing structures is relatively up-to-date in other parts of the study area. The 1997 Southeastern Storm Water Study focused on a study area that was east of Ramsey Boulevard and south of the Boy Scout camp. The boundaries of this study was limited to the area where two-foot topographic contours were available. The best topographic information outside of this area of the City are ten-foot contour intervals available from the U.S. Geological Survey Quadrangle Series. This study modeled the ponding basins and provided 100-year floor elevations on wetlands and ponding basins within the study area. This study also provided recommendations for outlets, specifying pipe sizes, and cost estimates for the elimination of land-locked drainage basins. The information provided was utilized to determine the amount of storm water trunk charge that the City collects as a development charge. The 1998 Lower Rum River Watershed Management Organization Plan was approved by the BOWSR board on May 7, 1999, replacing an earlier watershed plan that was adopted in February 1991. This report deals primarily with water quality issues including an inventory of those resources, policies for their protection, and an implementation program. It is expected that the individual four cities comprising this WMO will utilize this document to bring their own local water resource plans in conformance. /..4