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Estimates from the National Weather Service (NWS) TP -40 publication for the precipitation depth of a 24 -hour duration event for various return frequencies are presented in Table 1. The NWS is currently in the process of producing updated rainfall frequency estimates using a longer period of rainfall observations and state -of -the -art statistical methods. Updated estimates for Minnesota are anticipated in the spring of 2013. Table 1 Precipitation Event Frequency Return Frequency Percent Probability Precipitation Depth inches 1 -year 100% 2.4 2-year 50% 2.8 5 -year 20% 3.6 10-year 10% 4.2 25 -year 4% 4.8 50- ear 2% 5.3 100 -year 1% 5.9 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 Manual, precipitation in Minnesota has been rising since the 1930s. This increase is attributed to an increased frequency of heavy to extreme precipitation events (Karl and Knight 1998). The State Climatologist suggests that the amount of precipitation occurring as large events has been increasing in recent decades, and that about 100 years ago that fraction was similar to or even higher than what it is today. The trends are changing and will likely continue to change. From a stormwater management perspective, these changes in precipitation may require larger pipes and ponds to capture, convey and treat the runoff from more intense events. The City will continue to monitor the 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. 2.2 Topography 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 irregular with ground elevations 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 of Lake McCarrons. Because of its topographic characteristics, the City of Roseville lies at the headwater of three subwatersheds: • The western part of the City drains northerly toward Rice Creek that drains into the Mississippi River, which falls under the jurisdiction of Rice Creek Watershed District; • 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 Watershed; and • 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. Comprehensive Surface Water Management Plan ROSEV120222 City of Roseville Page 5