increased turbulence generated by the jet may be swept back through 

 the bottom constriction as the flow pattern reverses with the passing 

 waves . 



Because of this, development of an accurate mathematical description 

 of the lift force phenomena for nonlinear waves that would cover the 

 complete transformation of the lift force record with increasing bottom 

 clearance, and yet be flexible enough to allow application of any higher 

 order theory, would be a formidable, if not impossible, task. Since the 

 lift force model developed for linear theory seems to fit the experi- 

 mental data reasonably well, even for waves that were obviously nonlinear, 

 it should provide a useful tool for engineering calculations, even though 

 it may not be flexible enough and theoretically correct to allow the use 

 of higher order wave theories. The value of the maximum horizontal 

 velocity, u,^^^, can be calculated under the wave crest using any higher 

 order wave theory; this value can then be used in the linear lift force 

 model, possibly giving a better approximation of the lift forces induced 

 by highly nonlinear waves. 



II. EXPERINENTAL INVESTIGATION 



1 . Experimental Equipment . 



Model experiments were performed in three different wave tanks. The 

 two-dimensional tests were done in a 1-foot-wide wave channel in the 

 Hydraulic Engineering Laboratory (HEL) at the University of California, 

 Berkeley. The three-dimensional tests were started in the 8-foot-wide 

 Naval Architecture (NA) tow tank, and then continued in the 8-foot-wide 

 HEL wave tank where the majority of the experiments were conducted, 

 both located at the Richmond Field Station of the University of California. 

 The 1-foot wave channel is 100 feet (30.48 meters) long; the 8-foot HEL 

 wave tank and NA tow tank are 180 and 200 feet (54.86 and 60.96 meters) 

 long, respectively. All tests were conducted at approximately the middle 

 of the tanks. A Stillwater depth of 2 feet (60.96 centimeters) was used 

 in the two dimensional tests, and a 3- foot (91.44 centimeters) water 

 depth was used in the three-dimensional experiments. 



A flapper-type generator is located at one end of each of the HEL 

 wave tanks; the NA tow tank has a piston- type wave generator. The wave 

 period is controlled by varying the speed of the electric motors which 

 drive the wave generators. A cam mechanism with a variable stroke length 

 is connected between the drive motor and the flapper, and the wave height 

 is varied by changing the stroke length. A wave filter, consisting of a 

 series of vertical screens, was placed in front of the wave generator in 

 the 1-foot-wide wave channel to smooth out any irregularities in the 

 generated waves due to reflections from the flapper. A permeable beach 

 was installed at the opposite end of each of the tanks to absorb the 

 wave energy and minimize the wave reflections from that end of the wave 

 tank. 



36 



