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Title: Shoreline erosion processes, Orwell Lake, Minnesota
Authors: University of North Dakota. Department of Geology.
Reid, John R. (John Reynolds), 1933-
Keywords: Cold regions
Shoreline erosion
Beach erosion
Orwell Lake, Minnesota
Issue Date: Dec-1984
Publisher: Cold Regions Research and Engineering Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: CRREL report ; 84-32.
Description: CRREL Report
Abstract: Orwell Lake, in west-central Minnesota, is a flood-control, water-management reservoir first impounded in 1953. Subsequent erosion of the shoreline and a lack of knowledge of slope erosion processes in this region prompted this study to identify and quantify the processes there. The processes were measured at selected sites between June 1980 and June 1983. Erosion of the banks is primarily caused by three processes: rain, frost thaw, and waves. The first two processes tend to move sediment to the base of the steep slopes, forming a relatively gentle surface of accumulation. Wave action then tends to move this sediment into the lake. Analysis of the data collected over three years has confirmed that wave action is the dominant erosion process, providing almost 77% of the erosion during the 1981-82 study year. During the 1981 high pool level, 2,089 Mg of sediment, mostly colluvium, was removed from the lower slopes by wave action striking the 1.62 km of eroding shoreline. More than 4,300 Mg was eroded by waves accompanying the higher pool levels of 1982. During years in which the pool level does not exceed 325.5 m in elevation, the colluvium slope builds up at the expense of the steeper slope. But during successive years with higher pool levels, the resulting thin colluvium is quickly eroded. Erosion of the primary sediment, a compact till, then occurs, forming the typical nearly vertical banks. In winter the upland surface adjacent to the lake freezes to a depth of between 1 and 2m, depending on the surface temperature, the snow cover, and the distance from exposed banks. In late winter soil aggregates, released by the sublimation of interstitial ice within the banks, begin to accumulate at the base of the slopes, often veneering snowbanks there. Once thaw begins, slab failure of bank sediment is followed by mudflows and earthflows. Thaw failure at Orwell Lake in the winter of 1981-82 accounted for over 20% of the erosion; in the spring of 1982, 824 Mg was eroded by this process and 746 Mg the following spring. Such slope failure is most intense along north-facing banks and considerably less intense on south-facing banks, where more effective desiccation and sublimation reduce the soil moisture content. Summer rainfall is responsible for the remaining 3% of the total erosion, amounting to 102 Mg in 1981 and 208 Mg in 1982. Because the banks are steep and relatively short, rainwash is infrequent; rainsplash is the most consistent process during the summer, but the infrequent storms during which rainwash occurs cause greater total erosion. Erosion by rain has increased in each of the past three summers, largely because of increased precipitation. Infrequent massive slope failures (slumps) have occurred at the east end of the lake where a buried clay-rich unit is stratigraphically and topographically positioned to favor such failures. Drought years followed by heavy spring rains probably will result in additional slope failures of this type at the east end. Unless changes are made, the banks at Orwell Lake will continue to recede. Restriction of the pool level to less than 325.5-m elevation is the least expensive solution to the problem.
Appears in Collections:CRREL Report

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