values of the mhp to whp ratio for wave generators in use at different 

 laboratories (using values of k in equation 6-24 of 1.5 and 2.0 for 

 plunger and flap generators, respectively) are presented in Table 6-1. 



(2) Tsunami Generators . Tsunamis are seismic sea waves caused 

 by submarine earthquakes and volcanic eruptions of the ocean floor. The 

 explosion of an atomic bomb underwater can also generate a tsunami. An 

 active seismic belt extends around a major part of the Pacific Ocean with 

 all bordering coastal zones subject to the attack of tsunamis. These dis- 

 turbances travel in a series of waves of long lengths and periods, at the 

 speed of 



V = (gd)l/2 ^ (6-26) 



Tsunamis in the open ocean have lengths as much as 100 miles or more, and 

 can travel at speeds in excess of 600 miles per hour, depending on the 

 depth of water; their heights in the open ocean are unknown, but probably 

 do not exceed 1 or 2 feet. As tsunamis approach shore, shoaling effects 

 cause a decrease in speed and wavelength and an increase in height. The 

 heights of tsunamis can be greatly increased in bays if the bay's config- 

 uration and bottom contours are such that resonance is obtained between 

 the period of the incident waves and the natural period, or one of the 

 harmonics of the water mass in the bay. Tsunami wave periods from 5 to 

 60 minutes are common, and wave heights on the Pacific coast were as high 

 as about 20 feet at Crescent City, California, in 1964 (Keulegan, Harrison, 

 and Mathews, 1969), and about 35 feet at Hilo Harbor, Hawaii, in 1960 

 (Eaton, Richter, and Ault, 1961). Heights of nearly 100 feet have been 

 reported in Japan (Ichiye, 1958). When tsunamis approach shore, in the 

 water depths where coastal structures are located, two types of phenomena 

 may occur, depending on the steepness of the wave front. If the steepness 

 is large the front advances in the form of a wall of water, or bore, and 

 breaks in the manner of surf where the water depth is approximately equal 

 to the bore height. For smaller steepnesses the tsunami appears as a rise 

 in sea level, and will simply overflow the land area if the rise above 

 mean sea level is of sufficient height relative to the elevation of the 

 coastal areas. Thus, coastal structures should be designed to withstand 

 the frontal attack of bores with steep fronts and the overtopping of these 

 structures due to either type of tsunamis where the height is sufficient 

 to cause overtopping. 



Theoretical and experimental investigations of three types of bore 

 generators (piston, gate, and pneumatic) were made by Shen (1965); a 

 theoretical study of pneumatic wave generators was made by Keulegan 

 (1966). Based on these studies, the pneumatic generator is considered 

 the most practical for use in wave flumes to determine the stability of 

 coastal structures that will be subjected to the attack of tsunamis. 

 This type of generator consists of a pneumatic tank similar to that 

 shown in Figure 6-10. The upper part of the tank has a constant cross 

 section, and the lower part transforms gradually into a horizontal nozzle. 

 The angle of expansion of the nozzle is small enough to prevent separation 

 of flow. Water is drawn into the tank with a vacuum pump to the height 



35 



