of water across the ground, five types of forces may result from the 

 surging water: 



(a) Buoyant forces caused by partial or total submergence 

 in the surging water. When water or water pressure intrudes 

 under a structure, the buoyant force tends to lift the structure 

 from its foundations. Vehicles and other large items may also 

 be lifted up into the surging water. 



(b) Surge forces caused by the leading edge of the surge 

 impinging on a structure. This leading edge has the appearance 

 of an elongated wedge, and the force of the surge on a structure 

 gradually increases as a function of the increase in surge height. 

 The buoyant force also increases as a function of surge height, 



so that a structure may be carried forward by the leading edge 

 of the surge, or may be destroyed in place if the surge force is 

 high enough and the buoyant force is not sufficient to lift the 

 structure from its foundations. 



(c) Drag forces caused by the high velocity of the surging 

 water, where the water level is relatively constant. These 

 forces will displace buildings or other items in the direction 

 of the current, and the high velocity flow may cause severe 

 erosion of the ground and damage waterfront structures by scour- 

 ing material at the base of the structure. 



(d) Impact forces caused by buildings, boats, or other 

 material carried forward by the surging water. These forces 

 may either destroy other structures on impact or create momentum 

 which, when added to other forces, will move a structure in the 

 direction of the current. 



(e) Hydrostatic forces caused by partial or total submer- 

 gence of structures by the tsunami. This can result in cracking 

 or collapse of a structure or wall. 



a. Buoyant Forces . Buoyant forces are defined by the weight of the 

 displaced water when objects are partially or totally submerged. For 

 saltwater, taking the density p = 1.026 grams per cubic centimeter (1.99 

 pound-seconds squared per foot 4 ) , the buoyant force is 



F s = pg V (324) 



where V is the displaced volume of water*. This assumes water intrudes 

 under the structure. 



************** EXAMPLE PROBLEM 21 ************* i 



GIVEN : A house occupies a floor area of 225 square meters (2,422 square 

 feet) . Calculations to predict tsunami runup have indicated a probable 

 surge depth of 2 meters at that location. It is assumed that the flow 

 of water will be at a constant depth around the house. 



172 



