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Table 1 Precipitation Event Frequency Percent Historic Updated Return FrequencyProbability Precipitation Precipitation 12 Depth (inches)Depth (inches) 1-year 100% 2.4 2.5 2-year 50%2.8 2.8 5-year 20%3.6 3.5 10-year10%4.2 4.2 25-year4% 4.8 5.3 50-year2% 5.3 6.3 100-year 1% 5.9 7.4 Sources: Technical Publication No. 40 (1) U.S. Weather Bureaus (Hershfield, 1961) (2) NOAA Atlas 14, Volume 8 (2013) In recent years, there has been more debate and discussion around the topic of rainfall depths and the frequency of larger storm events. As described in an issue paper on this topic prepared during the development of the Minnesota Stormwater Manu Minnesota has been rising since the 1930s. This increase is attributed to an increas frequency of heavy to extreme precipitation events (Karl and Kni Climatologist suggests that the amount of precipitation occurrin increasing in recent decades, and that about 100 years ago that fraction was similar higher than what it is today. The trends are changing and will likely continue to change. From perspective, these changes in precipitation may require larger pipes and ponds to capture, convey and treat the runoff from more intense events. The City w outcomes of the changes in design guidance as well as review its standards for design of extreme event overflow areas for new and redevelopment projects. 2.2Topography and Drainage More than 10,000 years ago, Roseville evolved into a series of bluffs and upland hills that defined the adjacent lowlands, a network of drainage ways, lakes and marshes. The topography of northern Ramsey County (and Roseville) is irregulaelevations varying approximately 160 feet: from 1,000 feet above Mean Sea Level (MSL) near County Road B2 and Western Avenue to a low of 840 feet above MSL south Because of its topographic characteristics, the City of Roseville lies at the headwater of three subwatersheds: The western part of the City, which falls under the jurisdiction of Rice Creek Watershed District, drains northerly toward Rice Creek that drains into the Mississippi River. Flows leave the City and are transferred downstream through Ramsey County Ditch 4 and 5. The southeasterly part of the City drains to the south and east into the Trout Brook interceptor and then to the Mississippi River, a part of the Capitol Region Wa. The east-central and northeast areas drain northeasterly into Lake Owasso which overflows into Grass Lake in Shoreview. This area is a part of the Ramsey-Washington- Metro Watershed District. In order to better understand how the surface-water system works, the CSWMP divides the city into sub-watershed areas based on surface drainage features and the storm ComprehensiveSurface Water Management PlanROSEV120222 City of RosevillePage 4