Agenda - Council - 02/12/2008 - Laserfiche WebLink

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• CASE #� UPDATE ON ERMINE BOULEVARD STORM SEWER PROJECT By: Steven Jankowski, City Engineer Background: Recall that on October 24, 2006 the City Council authorized preparation of plans and specifications for the replacement of the existing 72 inch culvert located on Ermine Boulevard 200 feet west of Eaton Street. This culvert carries flow from a tributary of Trott Brook and currently has open joints which are causing the loss of material from beneath the roadway. It is also the intent of the project to pass higher flows through. the new culvert and thereby avoid an overflow of Ermine Boulevard during the 100 year event. This will lower the floodplain upstream of the Ermine Boulevard crossing and effectively remove the house at 9421 Ermine Boulevard from the floodplain which is an additional goal of this project. The location of this project is illustrated on the attached figure. Because of the need to modify the floodplain, our consultant was required to apply the floodplain model utilizing the new culvert and stream approach. In doing this a flaw was discovered in the floodplain model. Because of some work that was being performed by another consultant on an adjacent project in neighboring Sherburne County, the 100 year flood through this stream segment appears to have a discharge of 578 cfs versus 979 cfs • in the Federal Emergency Management Agency (FEMA) model, a reduction of 59 percent. Although this flow reduction will reduce the size of the culvert required, it is not alone sufficient to remove this property from the floodplain. Originally the project had anticipated replacing the existing 72 inch round culvert with two 12 ft by 6 ft box culverts at an estimated construction cost of $153,000. At the reduced flow a single culvert 14 ft by 6 ft culvert would reduce the cost by $30,000 to a cost of $123,000. A change in the floodplain for whatever reason needs to be documented through a Letter of Map Revision (LOMR) by FEMA. There is a cost of $4,400 for each LOMR review that FEMA performs. At the present time Minnesota DNR is scheduled to begin an update mapping and floodplain analysis for the section of Trott Brook ruiu-iing through Ramsey in late 2009 or possible 2010. It would be possible to eliminate the regulatory agency documentation cost associated with this project by waiting for DNR to perform this work. Below are several options for the City to consider in proceeding with this project along with their relative advantages and disadvantages. Option 1— The City would (1) direct its consultant to proceed with the remodeling of the floodplain using the lower flood flow; (2) submit this work for a CLOMR (Conditional Letter of Map Amendment) to FEMA for review and concurrence; (3) construct the project in the Summer of 2008; and (4) obtain a LOMR following • construction of the project. 183