40 



length at a point at which the orthogonal lines are spaced at distance 

 b is 



P=^fPo (46a) 



^__ SboPoT 



wbia 



<i-m|) 



All quantities in this equation are loiown except h and the solution 

 is made by approximation, assuming first that the quantity in paren- 

 theses (equation 45) is imity. 



The steps in construction of wave refraction diagrams for a particu- 

 lar harbor or beach are: 



1. Field data on periods, directions, and heights are first 

 analyzed for frequency to discover the most frequent and im- 

 portant combinations of direction and period to be analyzed for 

 refraction. For each important direction, one or more periods 

 will be used. 



2. For the chosen period, construct a diagram of wave velocity 

 and length as a function of depth (figs. 1 and 2). 



3. From inspection of the hydrographic chart choose a time 

 interval such that, over most of the area, the wave advance 

 during the interval will include a change in depth not over 10 

 percent of the depth. The time interval is best chosen as an 

 integral multiple of the wave period. 



4. Construct an auxiliary diagram showing the wave advance 

 during the chosen interval as a function of depth. (Wave 

 velocity times the chosen interval.) 



5. Place a transparent overlay on the hydrographic chart and 

 insert the initial wave crest position in deep water. The wave 

 will advance parallel to itself by equal increments until the depth 

 begins to affect the velocity. Usually, the effect of the bottom 

 is not appreciable until the depth has been reduced to about 

 30 percent of the deep-water length. 



6. Using the graph constructed under 4 lay oft' distances per- 

 pendicular to the wave front corresponding to each depth along 

 the crest. A refinement in the method consists in finding the 

 approximate new depth and then laying off a perpendicular 

 distance corresponding to the mean depth in the interval. 



7. Strictly speaking the wave advances in 6 should be drawn 

 as arcs and a new wave crest drawn as the envelope of these arcs. 

 If the time interval is not too long, the pependicular distance 

 will be correct within the usual accuracy of such diagrams. 



8. Continue this procedure to the point at which the smallest 

 wave of interest in the study would break. 



