The velocity is assumed to be equal over the 2.44-meter depth so that 

 the resultant drag force acts 1.22 meters (4 feet) above ground level. 

 The moment is then 



M = 3.36 x 10 It (1.22) 



= 4.1 x lO 1 * newton-meters (2.93 x 10 4 foot-pounds) 



on each of the columns. 



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As indicated in example problem 25 drag forces and surge forces can 

 act in conjunction with buoyant forces. The buoyant forces can lift 

 buildings from their foundations, and the surge or drag forces can slam 

 them into such things as trees or other structures. Buildings that are 

 firmly attached to their foundations to resist the buoyant forces must 

 also have sufficient structural strength to withstand the drag forces 

 acting against them. The drag forces can be lessened by constructing a 

 building on an elevated platform some distance above the ground. In some 

 instances, the first floor of a building may be designed to be carried 

 away by the tsunami, thereby reducing the forces on the building and 

 protecting the higher floors. However, this may be an expensive solution 

 and has the undesirable feature of adding debris to the water. 



The high velocity of a tsunami surge can also damage structures by 

 scouring material near the structures' foundations. Shepard, MacDonald, 

 and Cox (1950) noted numerous instances of severe erosion caused by the 

 1946 tsunami in Hawaii. At Haena Bay, a sand beach eroded and sand was 

 deposited 1.2 meters deep across a highway. A section of shoreline at 

 Moloaa, was cut back about 21 meters (70 feet). At Kalaupapa, the back- 

 wash from the tsunami undermined a road. Other instances of erosion 

 were also noted. Erosion and deposition of surface material are quite 

 common when severe tsunamis occur. Imamura (1942) gives an example from 

 1707, when a tsunami washed away layered sediments which had covered an 

 old ricefield. Conversely, the Earthquake Research Institute (1954) 

 reported instances of ricefields being covered with sand by the 1933 

 Sanriku tsunami. Instances of deposition of sand are also indicated by 

 Shepard, MacDonald, and Cox (1950) in Hawaii, and by Reid and Taber (1919) 

 in Puerto Rico. Shepard, MacDonald, and Cox noted that dense stands of 

 grass prevented or greatly diminished ground erosion during the 1946 

 tsunami in Hawaii. 



d. Impact Forces . The high velocity of a tsunami surge will sweep 

 large quantities of material forward with the surge. This material may 

 include automobiles, trees, petroleum tanks, buildings, debris from build- 

 ings, or other materials in the path of the surge. A large boulder moved 

 by the 1960 tsunami at Hawaii is shown in Figure 68. In higher latitudes, 

 when tsunamis occur during the winter, the material may include large 

 quantities of broken ice. 



183 



