the surf zone and wave reflection. Use of video equipment allowed each wave 

 to be examined during frame -by- frame playback as waves shoaled up to the break 

 point. The point where the wave front became vertical was selected for use as 

 the break-point definition because it was a unique point that could be readily 

 observed on the video monitor and measured by reference to the grid placed on 

 the side glass. 



132. The horizontal location of the break point was a critical param- 

 eter because the water depth over the bar became increasingly shallow with 

 small horizontal increments , which made h b and thus breaker depth index 

 sensitive to location. Plunge distance was also measured from the break 

 point; therefore, a change in break-point location results in a different 

 plunge distance. Although the remote sensing system was of high quality, 

 determination of the exact location where the wave front became vertical was 

 subject to judgment. The importance of precisely locating the break point 

 required iteration of frame advancements and retreats during videotape anal- 

 ysis, which became tedious, but once the break point was defined, the breaker 

 height and depth could be obtained with consistency. For some waves the break 

 point occurred between videotape frames. In these cases, the height and depth 

 at breaking were measured from the previous frame. Breaker height and depth 

 were measured from the video monitor using the grid placed on the tank wall as 

 a reference. The 2-in. grid spacing could be scaled from the monitor to 



1/4 -in. increments. 



133. The breaker depth was measured under the crest of the wave from 

 SWL, which was well defined on the grid. The SWL is the water level that 

 would exist if waves ceased but tide and storm surge remained (in the case of 

 the field) , whereas MWL is the time -averaged water surface elevation including 

 the presence of waves. Since tide and storm surge did not exist in the tank, 

 SWL was simply the quiescent water level in the tank. The MWL can easily be 

 obtained from the wave gages; however, breaker depth was not always located at 

 a gage; thus, it was convenient to use SWL as the datum for analyzing the 

 videotapes . Water depth was also measured under the horizontal face of the 

 wave from SWL for comparison with depth under the wave crest. For spilling 

 and plunging waves, the break point was located near the crest, resulting in 

 little difference in 7 b . However for collapsing waves, the depth at the 

 break point could be up to one -half the depth under the crest, which doubled 

 the value of 7 b . 



85 



