wave forecasts or in using historical weather charts for compiling 

 statistical data on waves that probably resulted from storms in the past 

 is beyond the scope of this paper For the techniques of the procedure 

 the reader is referred to Hydrographic Office Publication NOo 11, 275 

 mentioned above For a complete description of the theory involved re- 

 ference should be made to U, S. Navy Hydrographic Publication 601 , "Wind 

 Sea and Swells Theory of Relations for Forecasting" , by Sverdrup and 

 Munk, March 1947 o The accuracy of wave predictions which are made from 

 weather charts is of interest to engineers who rely on such information „ 

 To provide such data forecasting of sea and swell for comparison with ob- 

 servations was initiated shortly after the installation of the wave record- 

 ers at Point Sur and Heceta Head„ Several months of wave records have per- 

 mitted a statistical analysis to be made of the accuracy of the forecast- 

 ing methodo The comparison at Point Sur covered about 270 forecasts from 

 April to December 194-7 ° At Heceta Head comparisons covered the period May 

 to December 1947 and involved about 200 forecasts „ The actual comparison 

 of recorded to forecast values was, in general, good,, A statistical 

 analysis shows that 97 per cent of the recorded significant increases in 

 wave height were forecasts Sixty-nine of the arrival times were pre- 

 dicted within six hours The arrival times usually were predicted earlier 

 than those actually occurring,, Fifty-three per. cent of the forecast wave 

 heights were within one foot of the recorded heights, and eighty per cent 

 were within two feet Sixty- three per cent of the forecast periods were 

 within two seconds of the recorded periods The forecast periods usually 

 were lower than the recorded periods „ In general, it can be stated that 

 the forecasting technique results in a high degree of reliability for fore- 

 casting the arrival of significant increases in wave height, and for pro- 

 gnosticating the heights of the waves These two factors are the important 

 items controlling marine and shore activities „ A complete discussion of 

 the accuracy of forecasting appears in the article "A Comparison Between 

 Recorded and Forecast Waves on the Pacific Coast" by Jo Do Isaacs and 

 Thorndike Saville, Jr», which was presented before the New York Academy 

 of Sciences, March 1948 o 



Refraction of Waves 



As mentioned above, when waves move shoreward and approach a shore 

 line at an angle, refraction occurs and important changes to the height 

 and direction of travel of the waves results The magnitude of these 

 changes is best considered to be a map which shows the wave crests at a 

 given time, or the successive positions of a particular wave as it moves 

 shoreward „ Figure 5 shows a typical diagram which was prepared for the 

 area in the vicinity of Moss Landing in Monterey Bay, California Note 

 that on this diagram there are a series of lines, orthogoaals, drawn 

 perpendicular to the wave crests These orthogonal lines are used in 

 estimating variations in wave height due to refraction Such estimates 

 are made by assuming that the wave energy between any two orthogonals 

 remains constant and, for steady state conditions, the same energy 

 should flow past all positions between the orthogonals A consideration 

 of the fundamental equation for the power of a wave shows that the ratio 



