Impact forces from material carried forward by the current are not 

 as easily analyzed as other forces. The drag force will initially 

 accelerate material which is swept up into the current. The velocity 

 of forward motion of such material depends on the distance the material 

 has moved, the ratio of the drag force to the actual mass of the object 

 in motion, and the resistance created by the object dragging against 

 the ground or impacting and grinding against other objects. 



Analyzing the effects of a structure impacting with another structure 

 also requires knowledge of the rigidity of the structures and the angle 

 of impact. If the flat side of one structure impacts with the flat side 

 of a second structure, the impact force is spread over a wide area. How- 

 ever, if a corner of the first structure impacts with the flat side of 

 the second structure, the force is concentrated and there will be a 

 greater tendency to crush the impacting structures. It should be remem- 

 bered that if a structure is partially flooded, the water within the 

 structure becomes a part of the mass of the structure. 



Considering an object being swept forward from a stationary position 

 by a moving fluid such as a tsunami surge, the velocity of the fluid, 

 u, with respect to the ground is assumed to be constant, and the velocity 

 of the object, u-u, with respect to the ground varies as the object is 

 accelerated. The velocity, u'^, of the object approaches the velocity, 

 u, of the fluid after the object has moved some distance (i.e., the 

 velocity of the object approaches some terminal velocity). The force 

 accelerating the body is a combination of drag forces and inertia forces, 

 and is given by the equation 



(u - u.) 2 d(u - u,) 

 F - C n pA P- ♦ C u PV - (337) 



J £> K 2 M r dt 



where 



C = the coefficient of drag 



p = the density of water 



A = the cross-sectional area of the object transverse to 

 the direction of motion 



(u - u, ) = the velocity of the water with respect to the object 



^b 



at any instant in time 



C = the inertia or mass coefficient 

 M 



V = the volume of water displaced by the object 



= time 



185 



