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City of Lino Lakes SWMP | February 8, 2013 Draft | 12 Table 2 . Design Storm Events Recurrence Interval (years) 24 Hour Rainfall Amount (inches) 1 2.4 2 2.8 5 3.6 10 4.2 25 4.8 50 5.3 100 5.9 Source: U.S. Weather Bureau TP -40 (Hershfield, 1961) 3.4. Topography The City’s hydrologic system includes streams, lakes and wetlands. The City is characterized by gently rolling topography interspersed with lakes, streams, floodplains, various judicial and county ditches, and wetlands that are primarily a result of glaciation. This is due to its location in the North Central Hardwood Forest Eco -region that can be sub -divided into three geomorphic settings: the McGrath Till Plain, the Twi n Cities Formation and the Anoka Sand Plain. This system is typical of a glaciated area with outwash plains and terminal moraines. The thinly spread glacial drift is characterized by circular level -topped hills with smooth side slopes averaging between 2 -3 % grades. Ground surface elevations range from 850 to 950 feet above mean sea level (MSL). Lacking a uniform drainage pattern, many small streams end in lakes or depressions. Low -lying areas contain closed basins with numerous lakes and peat bogs. Water ta ble levels normally occur within three to ten feet of the surface. All ground water levels vary seasonally. 3.5. Geology Surficial Geology The surficial geology of Lino Lakes is composed of 2 distinct areas. The City is divided almost evenly between portions of the Anoka Sandplain and an area of loamy till typical of the Eastern Saint Croix Moraine. Organic deposits are present throughout both areas (Figure 3 ). The northwestern portion of the City is considered to be part of the Anoka Sandplain geomorphic area. The Anoka Sandplain is an outwash plain formed during the retreat of the Grantsburg Lobe. The material making up the plain is principally fine sand. Depre ssions are common in the plain and were formed when isolated blocks of ice later melted. They are now filled with peat deposits or are marshes and lakes. The landscape within the Anoka Sandplain is generally considered to be a gently undulating plain. Wate r tables are near the surface in depressions and from 2 to 10 or more feet deep in rises. The southeastern portion of the City is composed of New Ulm loamy till. This area was formed by the Grantsburg sublobe of the Des Moines lobe and is characterized b y gently rolling terrain (Figure 3 ). Bedrock Geology Three bedrock layers exist beneath the study area; Mt. Simon -Jordan, Prairie Du Chien and St. P eter - Platteville. The Mt. Simon -Jordan was formed during the Upper Cambrian and parallels the same extent as the Anoka Sandplain. The Prairie Du Chien and St. Peter -Platteville areas are from the Ordovician period and parallel the glacial till in the sou theast portion of the City (Figure 4 ).