INTRODUCTION 



In order to forecast sea and swell from 

 weather data, it Is necessary to know the char- 

 acter of the waves produced by a given wind 

 blowing for a known length of time over a 

 known stretch of water called the fetch. Prior 

 to 1942 such knowledge was based on empirical 

 relationships, many of which were inconsistent 

 among themselves. 



In the fall of 1942 a need for sea and swell 

 forecasts arose in connection with the planned 

 invasion of North Africa. The preliminary 

 conclusions used in the African and Mediter- 

 ranean operations were helpful, but consistent 

 results were not achieved until the summer of 

 1943. By that time all oceanographic work had 

 been transferred to the United States Navy 

 where, under contracts let by the Hydrographic 

 Office and the Bureau of Ships, studies of sea, 

 swell, and surf were carried on at an acceler- 

 ated pace. 



The present manual is a revision of H. 0. 

 Misc. 11,275, Wind Waves and Swell, Prin- 

 ciples IN Forecasting. As a whole, the 

 changes are few in view of the intensity with 

 which wave research has been carried out since 

 1943. The basic numerical relationships be- 

 tween wave height and period as functions of 

 wind speed, fetch, duration, and distance of 

 decay differ somewhat from those proposed in 

 1943 and used subsequently with considerable 

 success in sea and swell forecasting during the 

 war. However, empirical data available in 

 1943 were too incomplete to be shown as rela- 

 tions between nondimensional parameters and 

 could not be used for comparison between 

 theory and observation. But through the ef- 

 forts of agencies partaking in wave research, 

 new and more comprehensive material has now 

 made it possible to check the nondimensional 

 relationships against observations. 



Three problems are involved in forecasting: 

 (1) Forecasting the length and height of the 

 waves in the open sea, (2) forecasting the 

 swell reaching exposed or partially exposed 

 anchorages, and (3) forecasting the height of 



breakers and the amount of surf on any given 

 beach. The first problem involves two steps: 

 {a) Determination of height and period of the 

 waves which emerge from any given wind area 

 and which may arrive as swell on a distant 

 coast, and (&) determination of the travel time 

 and the decrease of the height of the waves as 

 they proceed from the wind area. For the 

 second and third problems an additional factor 

 is involved, namely, the determination of the 

 transformation of the waves as they enter into 

 shallow water and wash the beach. 



This manual deals with the generation of 

 waves by wind and with the travel of waves in 

 deep water after they have left the regions of 

 strong winds. Methods are described for de- 

 termining the characteristics of wind waves by 

 means of data from adequate, consecutive syn- 

 optic weather maps and for forecasting swell 

 off coasts. 



Relationships between the waves and the 

 three important variables, wind at the sea sur- 

 face, fetch (the stretch of water over which 

 the wind blows), and duration (the length of 

 time that the wind has blown) are discussed. 

 These relationships have been developed on the 

 basis of theoretical considerations and empiri- 

 cal laws developed by various observers of 

 waves. They are presented in graphical form 

 for use in forecasting wind waves and swell. 



In order to use the graphs most effectively, 

 it is necessary to understand clearly their phys- 

 ical significance and limitations. Forecasts, 

 therefore, should not be attempted until the 

 forecaster has studied the first part of this 

 paper which describes the processes leading to 

 the growth and decay of waves. 



Tests of the method made to date indicate 

 that swell forecasts can be made with more 

 certainty than that of most meteorological fore- 

 casts. Prognostic charts are not important for 

 the forecasting of swell because considerable 

 time elapses between the generation of waves 

 in distant storm areas and their arrival at the 



IV 



