For a structure completely immersed in water, i.e., flow passes 

 around all sides o£ the structure (including both the top and the bottom), 

 the forces act against the projected surface area of the structure 

 normal to the direction of flow. Coefficients of drag, C^;, can be 

 obtained from the Table for known structure dimensions. For a structure 

 resting on the ground, where there is neither underflow nor overflow, 

 the structure can be treated as an infinitely long (i.e., infinitely 

 high) structure to determine the appropriate coefficient from the Table . 



When a structure rests on the ground and overtopping water creates 

 overflow where there is no underflow, or when a structure is supported 

 or floating above the ground so that there is underflow but no overflow, 

 the following procedure may be used to determine the coefficient of drag. 

 The drag acts against the submerged part of the structure. Create a 

 "mirror" of the submerged part at the surface where no flow occurs. This 

 gives a completely immersed structure with a height equal to twice the 

 submerged height of the actual structure. The coefficient of drag for 

 the actual structure can then be obtained from the Table, using the width 

 of the actual structure and a height equal to twice the submerged height 

 of the actual structure. This is illustrated further in the example 

 problem. 



The velocity of the structure, u^,, as a function of time, is 



u-u = u - — 7 T- (3) 



^ aut + 1 



and the force, F, accelerating the building at any instant in time is 



F'= pVa (u - u^)' 



(4) 



where p is the mass density of water (seawater - 2 slugs per cubic foot 

 (1,026 kilograms per cubic meter) and freshwater 1.94 slugs per cubic 

 foot) . 



************** EXAMPLE PROBLEM ************** 



GIVEN: A tsunami is 12 feet (3.66 meters) high at the shoreline, and 

 moves on to the shoreline as a steep-fronted surge. A building is 

 swept forward, and impacts with another building after being carried 

 through a distance of 20 feet (6.1 meters). The building is rectan- 

 gular, 40 feet (12.2 meters) wide and 14.4 feet (4.4 meters) deep in 

 the direction of flow, and is submerged to a depth of 10.5 feet (3.2 

 meters) as it is carried forward (Fig. 3). The velocity of the surge 

 is approximated as u = 36 feet (11 meters) per second. 



FIND: 



(a) The time required for the building to impact with the other 

 building; 



