For various values of h, the force, F, is tabulated below: 



h, meters 0.5 1.0 1^5 2_ 1 2.5 



F, newtons per meter 73,100 138,300 203,900 270,800 339,500 



NOTE: --Calculations will show that C^ > 1 at the maximum surge height 

 (where the rate of change of surge height -»■ 0) . This indicates that 

 the calculated value is conservative for design purposes. It can be 

 seen that the hydrostatic pressure component of the force is a rela- 

 tively small part of the total force. 



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As indicated by example problem 23 and shown in Figure 66, there is 

 a gradual rise in water level at the front of the surge, although this 

 change in water level appears to occur rapidly with respect to time 

 because of the forward velocity of the surge. A surge on a dry bed has 

 a much flatter front than a bore approaching a shoreline. This is seen 

 in laboratory tests. The buoyant force of the leading edge of the surge 

 tends to lift objects into the surging water, and the force of the surge 

 will then carry these objects forward. 



Wilson and T0rum (1968) report on the case at Seward, Alaska, of a 

 tsunami surge overtaking a pickup truck being driven from the shoreline. 

 The truck was swept up by the surge and carried forward like a surfboard 

 into nearby woods . 



The water velocity near the leading edge of a surge is relatively 

 high, and the height of the leading edge is relatively low (i.e., the 

 buoyant force is low). Therefore, it is possible that the surge force 

 may destroy a structure before the buoyant force lifts it into the flow. 



c. Drag Forces . The velocity of the water in the surge produced 

 by the tsunami runup creates a drag force which tends to move a structure 

 in the direction of the surge. If the velocity is assumed to remain 

 relatively constant under the surge, i.e., acceleration is negligible 

 and its effects can be ignored, then it can be assumed that the inertia 

 or mass coefficient, C„, approaches zero so that the drag force in 

 newtons is • 



V D -9-C D Kf (336) 



where 



p = the density of seawater = 1.026 grams per cubic centimeter 

 = 1,026 kilograms per cubic meter 



C n = a coefficient of drag, depending on the body (Table 6) 



A = the projected area of the body normal to the direction of 

 flow in square meters 



u = the velocity of the water in meters per second 



177 



