for vertical-wall breakwaters and nonbreaking waves were conducted by 

 Danel (1952). However, for model studies of some structures where the 

 maximum wave that can attack has not been determined and estimates of 

 the maximum waves based on depth limitation (H^^^ = 0.78d) are not ade- 

 quate for use as design waves, special tests must be performed to deter- 

 mine the maximum waves that can attack the test structures. Such waves 

 should be determined for both high and low design water levels, using 

 selected wave periods within the range of periods that occur in the pro- 

 totype wave environment. Tests by Danel (1952) and Jackson (1968) were 

 conducted using simple, monochromatic wave trains of constant height. 

 Model tests of proposed structures to determine the maximum wave that 

 can reach the structure, and stability tests to determine the optimum 

 design of such structures using the maximum breaking waves, have also 

 been conducted using simple wave trains of regular height and period. 

 Tests of this type using irregular wave trains would be difficult to 

 conduct in such a way as to ensure that the selected test-wave spectra 

 included the wave height -wave period combination that corresponds to the 

 maximum wave that could attack the structure, unless the maximum wave 

 had been determined previously using the simple type of wave train. 



b. Generation of Test Waves. 



(1) Short-Period Wave Generators . In stability models of coastal 

 structures where the major forces are due to short -period, wind-generated 

 waves, the test waves are generated by the periodic displacement of water 

 in a wave flume. Water is displaced by a vertical plunger, a horizontal 

 bulkhead motion, or a rigid plate hinged at the bottom and driven by a 

 crank and rod attached near the top of the plate. The latter is referred 

 to as a flap generator. Most other types of mechanical generators incor- 

 porate the characteristics of one or more of the above- listed generators. 

 The pneumatic-type wave generator, in which the periodic displacement 

 of water is accomplished by the variation of pressure above a body of 

 water in a confined pneumatic chamber, has also been used to generate 

 short-period waves; however, this type is probably better suited for the 

 generation of long waves, such as seiches, surges, tides, and tsunamis. 

 Wave generators may be used to produce simple wave trains of nearly con- 

 stant height and period, or they can be programed to generate irregular 

 waves of variable height and frequency. Irregular waves can also be gen- 

 erated in a wind-tunnel type of wave flume by the action of wind blowing 

 over the water surface, or by the combined action of wind and a mechan- 

 ical or pneumatic generator. 



Equations for determining the wave heights generated by the various 

 wave machines are useful in designing new generators, in selecting linear 

 scales for proposed models using existing generators, and in reducing the 

 work required to calibrate existing generators for use in model tests. 

 Equations based on the complete linear wave generator theory have been 

 derived for the piston and flap generators (Ross and Bowers, 1953), but 

 these equations do not apply to the plunger type and are not strictly 

 applicable for waves of large steepness (experiments by Ursell, Dean, and 

 Yu (1958) showed that the measured wave heights generated by a piston 



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