diagram overlay template to correspond to the hydrographic chart being 

 used. In constructing this overlay, first determine how long each of its 

 radius-wavelength units must be. As noted previously, one radius -wave length 

 unit on the overlay must be identical to one wavelength on the hydrographic 

 chart. The next step is to construct and sketch all overlay rays and arcs 

 on clear plastic or translucent paper. This allows penciling in of the 

 scaled lines of equal K for each angle of wave approach that may be 

 considered pertinent to the problem. Thus, after studying the wave field 

 for one angle of wave approach, K lines may be erased for a subsequent 

 analysis of a different angle of wave approach. 



The diffraction diagrams in Figures 2-28 through 2-39 show the break- 

 water extending to the right as seen looking toward the area of wave dif- 

 fraction; however, for some problems the structure may extend to the left. 

 All diffraction diagrams presented may be reversed by simply turning the 

 transparency over to the opposite side. 



Figure 2-40 illustrates the use of a template overlay. Also indicated 

 is the angle of wave approach which is measured counterclockwise from the 

 breakwater. This angle would be measured clockwise from the breakwater if 

 the diagram were turned over. Figure 2-40 also shows a rectangular coordi- 

 nate system with distance expressed in units of wavelength. Positive 

 x direction is measured from the structure's tip along the breakwater and 

 positive y direction is measured into the diffracted area. 



Template Overlay 



Angle of Wave Approach 



Breakwater 



f'/'/'/'r /=z= 



I. O.I. I. '././. 1. 1. 1. '.I. '.I. J. '7^ 



-•-X 



Wave Crests 



Figure 2-40. Diffraction for a Single Breakwater Normal Incidence 



The following problem illustrates determination of a single wave 

 height in the diffraction area. 

 ************** EXAMPLE PROBLEM ************** 



GIVEN : Waves with a period of T = 8 seconds and height of H = 10 feet 

 impinge upon a breakwater at an angle of 135 degrees. The water depth 

 at the tip of the breakwater toe is dg = 15 feet. Assume that one inch 

 on the hydrographic chart being used is equivalent to 133 feet. 



2-95 



