Agenda - Public Works Committee - 06/17/2014

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Public Works Committee 5. 2. Meeting Date: 06/17/2014 By: Bruce Westby, Engineering/Public Works Title: Consider Recommending City Council Approval of Modifications to Stormwater Management Permits, Plans, and Ordinances Purpose/Background: Purpose: The purpose of this case is to request a recommendation from the Public Works Committee to the City Council to approve the attached modifications to the City of Ramsey's stormwater management permits, plans, and ordinances. The proposed modifications account for new stormwater management requirements recently adopted by the Lower Rum River Watershed Management Organization including the incorporation of Atlas 14 precipitation data and stormwater infiltration requirement revisions. Background: The City of Ramsey and the Lower Rum River Watershed Management Organization (LRRWMO) previously applied precipitation data values from Technical Paper 40 (1961) and Hydro-35 (1977) to design and model stormwater infrastructure including such things as drainage ditches, ponds, and storm sewers. However, in recent years it was observed that this data was no longer representative of the actual precipitation being experienced in Minnesota and other states. Therefore, the National Oceanic and Atmospheric Administration (NOAA) worked with the Federal Highway Administration (FHWA) and 10 pooled fund states in the Midwest to develop updated rainfall data called Atlas 14, Volume 8, which was recently adopted by the LRRWMO for its precipitation standard. As such, the City of Ramsey must also adopt the Atlas 14 precipitation data for our drainage design standards located in the City's stormwater management permits, plans, and ordinances. In summary, the precipitation data developed through Atlas 14 incorporates a much greater sampling size generated over a 50 year period. While precipitation data from Atlas 14 is greater in some areas, it is also less in others. In one example, the precipitation amount referenced in TP-40 for the 24 hour, 100 year event in Ramsey is about 6 inches, while the 24 hour, 100 year event generates roughly 7 inches of precipitation in Ramsey according to Atlas 14. Additional information on Atlas 14 is available at NOAA's web site at http://hdsc.nws.noaa.gov/hdsc/pfds/. Regarding the proposed infiltration language modifications, the current language in the City of Ramsey's stormwater management permits, plans, and ordinances discourages infiltration within all Drinking Water Supply Management Areas (DWSMA). Staff proposes to modify that language to discourage infiltration only within delineated 10 year capture zones. Attached is an exhibit that shows the existing DWSMA boundary in the area of The COR and all associated 10 year captures zone boundaries surrounding public water supply wells. The exhibit also identifies the locations of ponds, both lined and unlined, infiltration raingardens, underground infiltration practices, and wetlands. As can be seen, numerous infiltration practices exist within 10 year capture zones including an unlined pond, 2 underground infiltration basins, and 2 raingardens. These will need to be addressed in the future. However, as can also be seen there are numerous other infiltration practices that exist in areas outside the 10 year capture zones but inside the DWSMA boundary that currently violate our ordinances and SWMP but would be allowed under the proposed language modifications. It is important to note that DWSMA boundaries are subjective and are created by drawing lines along geographical features such as rivers, section lines, streets, etc. to encompass all of the delineated 10 year capture zone boundaries. The 10 year capture zone boundaries depict areas in which stormwater that falls on the ground has the ability to infiltrate through the ground to reach the aquifer that the wellhead is located in within 10 years or less.