Agenda - Council - 08/26/2014 - Laserfiche WebLink

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CC Regular Session 7. 4. Meeting Date: 08/26/2014 By: Bruce Westby, Engineering/Public Works Information Title: Introduce Ordinance #14-09 Amending City Code Chapter 117, Article II, Division 6 (Performance Standards), Subdivision II (Stormwater Pollution Control). Purpose/Background: Purpose: The purpose of this case is to introduce Ordinance #14-09 amending City Code Chapter 117, Article II, Division 6 (Performance Standards), Subdivision II (Stormwater Pollution Control). The proposed amendments address new stormwater management requirements recently adopted by the Lower Rum River Watershed Management Organization including the incorporation of Atlas 14 precipitation data and revised stormwater infiltration requirements, as well as revisions required by the Minnesota Pollution Control Agency per their 2013 reissued General Construction Stormwater Permit and Municipal Separate Small Stormsewer System (MS4) Permit. 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 culverts, 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 ten pooled fund states in the Midwest to develop updated precipitation data, subsequently named Atlas 14, Volume 8, which was recently adopted by the LRRWMO as its precipitation standard. Now, the City of Ramsey must also adopt Atlas 14 precipitation data for our drainage design standards as specified in the City's stormwater management codes, permits, plans, and ordinances. 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 a little more than 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 stormwater infiltration amendments, the City's current stormwater management codes, permits, plans, and ordinances discourage infiltration within Drinking Water Supply Management Areas (DWSMA). Staff proposes to amend all such language to discourage infiltration only within delineated 10 year capture zones. Attached are two exhibits that show the City's easterly and westerly DWSMA's and associated delineated 10-year capture zones surrounding all 8 public water supply wells, 2 of which are within the easterly DWSMA and 6 are within the westerly DWSMA. Per the westerly DWSMA exhibit, which encompasses most of The COR, numerous lined and unlined ponds, infiltration raingardens, underground infiltration practices, and wetlands exist in this area. 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 likely need to be addressed in the near future. However, there are also numerous other infiltration practices that exist in areas outside the 10--year capture zones but inside the DWSMA boundary that currently violate our stormwater management codes, permits, plans, and ordinances, but would be allowed to remain as is if the proposed ordinance amendments are adopted. Therefore, not only will the proposed amendments save the City money, they will also allow for additional stormwater infiltration practices within the City's two DWSMA's thereby allowing for greater recharge