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I" y'~it~ ,.,..d., . ____._._.. ~. ". t t DNR requirement for a design that preserves the natural character of a protected water and their shoreland, see DNR rules Section 6115.0210 Subp 1 A. 2.3 Structural The design of this lakeshore protection was based on the successful application of this technique on lakes in the north metropolitan area of Minneapolis and St. Paul. This design requires the use of a large boulder benn with a gravel backfill. 'A 4-foot high berm was selected based on the hydrology of the lake. The top of the boulder benn is at elevation 887 feet which is 1 foot higher than the 100-year lake level of 886 feet. The normal pool elevation in the lake is 884.2 feet. Calculations showed' that the height of waves generated in this lake is approximately 3/4 feet, using the SCS Minnesota Technical Release 2 "Slope Protection for Dams and Lakeshore, April 1988". The top of boulder berm is high enough to protect against nonnal wave action in the lake and the 100-year flood level. The boulder berm was also designed to resist normal ic~ acuon in the lake. From field observations in Spring 1997, the ice damage due to runup on the bank was approximately 1 1/2 fe~t higher than the normal lake level. This runup height was determined by observing the depressions left in the bank from the ice action. A small vertical bank along the water edge gave some indication as to the top of ice in the lake during periods of significant ice thrust on the lake. Calculations were made for ice action against the boulders. As ice pushed against the boulders, passive earth pressures will be mobilized in the soil/gravel behind the boulders. The total resisting force due to passive earth pressures plus sliding friction due to the weight of the boulders was calculated to be approximately 3,400 lbs per foot width. By analyzing the action of ice against the boulders, we observed that the boulders were structurally capable of withstanding the following: . Ice thrust for ice thickness of 1.5 feet in the lake without solar effect and for rate of temperature rise of 5 degrees F per hour. The situation of not having solar effect is when there is snow cover over the ice. Greatest thrusts from ice are observed during time of temperature rise, and the temperature rise value used was deemed an average situation for this region. . Ice thrust for ice thickness of I feet in the lake with solar effect and for rate of temperature rise of 5 degrees F per hour. The two ice thrusts appear to be reasonable for this lake. 2