07/07/87 - Laserfiche WebLink

Home

-~/ Department~',- epar,¢.,- of Natural Resources Division of Waters Volume 1, Issue 3 Spring, 1987 I ICE POWER ! By Gerald L. Paul In the Spanish language, exclamation marks are used at both cads of a sentence or declaration. The tide of this article, "; Ice Power ?, warrants this same added emphasis to draw your attention to the awesome force which ice un- leashes upon Minnesota's shoreland almost every year. Ice is a marvelous form of our most precious resource. No hockey game, cocktail, winter fishing or Minnesota winter would be complete without it. But there is a dark side to ice] It has to do with a property of ice that scientists and engineers casually identify as the coefficient of thermal ewpansion. In numerical expression this property is repre- sented as 0.000051 inches per degree centigrade. It is de£med as the ratio of the change in length per degree centigrade to the length at 0 degrees centigrade. In layman's terms, this simply means that ice gets larger as it's temper- ature rises. '¢,rhen conditions are right, this latent force 'Mil unleash havoc on shoreland and any structure man has un- wisely constructed in its path. For example, for a lake that is one mile across, when the ice's temperature rises from 14 to 32 degrees fahrenheit (equivalent to 20 degrees celsius), the ice sheet will expand laterally a total of approximately 32 inches, almost 3 feet! Tiffs can occur in a matter of hours when there is no snow cover on the ice sheet. "...for a lake that is one mile across, when the ice's temperature rises from 24 to 32 degrees ... the ice sheet will expand laterally a total of ajg- proximately 32 inches, almost 3 feetL.." Unfortunately, the reverse is not true. The ice sheet ~qll not contract to its original size when the temperature goes do~a again. This paradox is based upon another of the inherent properties of ice. Namely, that ice is much stronger in compression than it is in tension. To the lakeshore owner, this spells trouble! There would be little impact if a lake's ice would expand a little when the temper- ature rises ...if only the ice sheet would shrink when the temperature goes down again. But it doesn't happen this The west shore of Big Cormorant Lake, Backer County suffered from 'ice power' this winter. Taken on March 5, 1987, this photo demonstrates that aesthetics Is not the only reason structures need proper setback. (Photo #1) way ...at least not enough to matter. What actually happens is that the ice sheet will expand a small amount during a temperature rise, with the resultant force of its compressive strength directed to its point of contact with the shoreline. But as the temperature drops, trouble starts. The ice sheet then tries to shrink back to its original shape - like a piece of steel that expands when heated and contracts when cooled. The ice tries to pull it's massive weight inward in a tensile force struggle within itself. The pulling force is relentless. But since the ice's ten- sile strength is weak, and the ice literally ruptures itself, breaking open into slxrinkage cracks. Some cracks can be wide, others no more than "hairline cracks", but collectively their widths equal that the ice sheet had contracted. Continued on page 2 · .. IN THIS ISSUE ... Title Page lee Power ......................................................................................... DOW' To Speed Up Permit Process .............................................. 9 Platting the Destiny of Our Shoreland ......................................... 13 The Financial Cost of Negligent FP Management ...................... 5 Cottonwood River Gets a Facelif'L ................................................ 6 Minnesota' Water Bank Program .................................................. 8