in the area affected by diffraction to the incident wave height K, in the 

 area unaffected by diffraction. Thus, H and H^^ are determined by H = 



The diffraction diagrams shown in Figures 2-28 to 2-39 are constructed in 

 polar coordinate form with arcs and rays centered at the structure's tip. The 

 arcs are spaced one radius-wavelength, unit apart and rays 15° apart. In 

 application, a given diagram must be scaled up or down so that the particular 

 wavelength corresponds to the scale of the hydrographic chart being used. 

 Rays and arcs on the diffraction diagrams provide a coordinate system that 

 makes it relatively easy to transfer lines of constant K' on the scaled 

 diagrams. 



When applying the diffraction diagrams to actual problems , the wavelength 

 must first be determined based on the water depth at the toe of the structure. 

 The wavelength L in water depth dg , may be found by computing dg/L^ = 

 d /5.12T^ and using Appendix C, Table C-1, to find the corresponding value of 

 dg/L. Dividing dg by dg/L will give the shallow-water wavelength L. It 

 is then useful to construct a scaled diffraction diagram overlay template to 

 correspond to the hydrographic chart being used. In constructing this over- 

 lay, first determine how long each of its radius-wavelength units must be. As 

 noted previously, one radius-wavelength unit on the overlay must be identical 

 to one wavelength on the hydrographic chart. The next step is to sketch all 

 overlay rays and arcs on clear plastic or translucent paper. This allows the 

 scaled lines of equal K' to be penciled in for each angle of wave approach 

 that may be considered pertinent to the problem. After studying the wave 

 field for one angle of wave approach, K' lines may be erased for a sub- 

 sequent analysis of a different angle of wave approach. 



The diffraction diagrams in Figures 2-28 to 2-39 show the breakwater 

 extending to the right as seen looking toward the area of wave diffraction; 

 however, for some problems the structure may extend to the left. All diffrac- 

 tion 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 and also indicates 

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

 water. This angle would be measured clockwise from the breakwater if the dia- 

 gram were turned over. The figure also shows a rectangular coordinate 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. 



The following problem illustrates determination of a single wave height in 

 the diffraction area. 



*************** EXAMPLE PROBLEM 9*************** 



GIVEN ; Waves with a period T = 8 seconds and height H = 3 meters (9.84 feet) 

 impinge upon a breakwater at an angle of 135" . The water depth at the tip 

 of the breakwater toe is d = 5 meters (16.40 feet). Assume that the hydro- 

 graphic chart being used has a scale of 1:1600 (1 centimeter = 16 meters). 



2-90 



