level fluctuation can be obtained in some areas by examining historical records. However, 

 the design maximum and minimum water levels must usually be determined by computing 

 the wind setup or depression in accordance with instructions in the Shore Protection Manual 

 (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1973). 



In regions where tornados occur, httle can be done beyond following the precautions 

 recommended for hurricane-prone areas. The destructive force of a tornado exceeds the 

 practical design safety factor used for most structures. Some damage may be prevented by 

 designing buildings for quick exchange of atmospheric pressure by the use of large sacrificial 

 doors and windows to avoid complete collapse. 



The movement of sand by the prevailing winds of a coastal area creates special problems 

 that must be considered. Windblown sand may shoal harbors and cover land areas, requiring 

 frequent dune and drift -sand removal from roads, parking lots and other areas. Evidence of 

 dune formation at a harbor site should be examined, and an estimate made of its 

 seriousness. At some sites the windblown sand problem has been solved by paving, by 

 landscape-planting the dry sandy areas windward of the harbor complex, or by stabihzing 

 with sand fences and dune grass (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1973). 



c. Ice. Sheet ice can cause damage to a small-craft harbor. The best and most popular 

 precaution against ice damage is to remove boats from the water in winter. They can either 

 be removed to dry storage or hoisted out of their slips and left suspended above the water 

 surface. 



Damage to fixed and floating slips occurs in two ways. As sheet ice forms, it expands and 

 can crush floats and cut into piles. A secondary effect of this action is that if the water level 

 rises after freezing has begun, the ice sheet hugging the pUes exerts an upward force tending 

 to jack them up and reduce penetration into the soil. Repeated freezing and thawing may 

 eventually Uft the pUes completely out of the ground. Most ice damage is caused by the 

 impact of drifting floes on structures as the ice melts in spring. 



In areas where freezing does not produce a thick ice sheet, ice formation can be 

 prevented near piles, floating slips, and boats by forced-convection currents. This system is 

 discussed in Section V. Steel or metal-cladded timber piles can be driven deep enough in 

 some soils to develop great withdrawal resistance, so that the ice will sUde along the pile as it 

 rises. Floating sUps can also be designed with tapering or rounded bottoms so that the 

 pinching effect of the ice squeezes them upward (Fig. 9). 



Natural basin perimeters and revetted slopes can be severely eroded by expanding ice 

 sheets. One method of preventing this erosion is to line the perimeter slope with smooth 

 concrete. Vertical perimeter walls may be pushed back into soil behind them in winter and 

 then spring back when the ice thaws. The extent of weakening that results will depend on 

 the type of construction. 



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