fluenced by the secondary wind and the forecast 

 should be modified accordingly. 



If later maps showed that waves from this 

 fetch were no longer affecting Brest, the swell 

 originating on 13 March 0630Z (fetch A)i 

 should be diminished by cutting back in the 

 fetch as indicated on page 21. This gives the 

 following results with [7 = 37 knots, F — 650 

 nautical miles, D = 1,300 nautical miles, and 

 td = 26 hours : 



(1) 

 = 275 nautical miles 

 = 1,675 nautical miles 

 = 26 feet 

 = 9.2 seconds 



.22 

 = 16.4 seconds 

 = 67 hours 

 = 5.7 feet 



(2) 



= 150 nautical miles 

 = 1,800 nautical miles 

 = 23 feet 

 = 7.9 seconds 



The complete forecast then is : 



■Pmir 

 Hd/Hip = 



to 



F 

 D 



Tjr 



Ho/Hp = .15 



= 16.1 seconds 

 = 73 hours 

 rr: 3.5 feet 



(3) 



= 75 nautical miles 



= 1,875 nautical miles 



= 18 feet 



= 6.3 seconds 

 Hd/Hp ^ .08 

 To =15.8 seconds 

 tjy = 79 hours 



Hd =1.4 feet 



to 

 Hn 



F 

 D 



H^ 

 Tp 



Therefore, the forecast would be for waves 

 3-5 feet high on the morning of the 15th of 

 March with occasional waves near 8 feet^, in- 

 creasing by afternoon to 7-9 feet with occa- 

 sional waves near 11 feet-, decreasing after 

 midnight to 5-7 feet, and to less than 2 feet 

 by mid-afternoon of the 16th of March. The 

 actual forecast heights are sometimes plotted 

 on a graph against the time of arrival to give 

 more precise determinations, but it is doubtful 

 if the method of forecast itself is sufficiently 

 accurate to make this refinement worthwhile. 



It must be emphasized that this forecast rep- 

 resents deep-water conditions. The height and 

 period of breakers observed on any particular 

 beach would depend on the underwater beach 

 topography for that particular area. Pro- 

 cedures for forecasting changes which occur 

 when swell moves into shallow water are set 

 forth in separate manuals (H.O. Pub. Nos. 234 

 and 605) . 



1 Waves from fetch B are always negligible and may 

 be neglected. 



^ These figures result from adding the swells from 

 each fetch and multiplying by 1.29 to give the average 

 of the highest 10 percent. (See p. 4.) 



REFERENCES 



The scientific background for this paper will 

 be found in the following references: 

 Arthur, R. S. "Variability in Direction of Wave 



Travel," New York Academy of Science, Annals, 



vol. 51, pp. 511-521, 1949. 

 Baeber, N. F. and Ursell, F. "The Generation and 



Propagation of Ocean Waves and Swell, Part I, 



Wave Periods and Velocities," Royal Society of 



London, Philosophical Transactions, Ser. A, vol. 240, 



pp. 527-560, 1948. 



"Study of Ocean Swell," Nature, vol. 159, pp. 



205, 1947. 



Basquin, E. a. The Diminution of Ocean Swell. Un- 

 published manuscript. 1951. 



Bates, C. C. "A Procedure for Rapidly Forecasting 

 Ocean Swell," American Meterological Society, Bul- 

 letin, vol. 28, pp. 228-232, 1947. 



Cornish, V. Ocean Waves and Kindred Geophysical 

 Phenomena. Cambridge: University Press. 1934. 



30 



