58 WAVES 



gales from the southwest and west are common here during the winter, 

 and the effective fetch of something like 300 miles from the English 

 to the Danish coast (depending on the precise locality) is theoretically 

 sufficient for a 30-mile wind to generate waves 330 to 340 feet long, or 

 waves 1 15 to 1*25 feet long with a 20-mile wind. But the water is less 

 than 131 feet (40 meters) deep for a distance of 60 to 70 miles out 

 from Denmark, and less than 65 feet deep for miles, so that the waves 

 would begin to "feel the bottom" when they still were 60 to 70 miles 

 out from the Danish side during a 30-mile gale blowing across from 

 the English shore. And by the time they reached the 5-fathom line, 

 the ratios between their lengths and their heights would be only about 

 0.7 as great as it had been before their deformation commenced. 



It is no wonder, then, that the waves of the eastern side of the North 

 Sea are proverbially steep and dangerous for small craft in westerly 

 gales, as are those in the western part of the English Channel also 

 whenever storm seas are heaving into it from the open Atlantic. 



But is is only while the wind is blowing strongly onshore, or fol- 

 lowing an onshore gale, that areas of shoaling bottom (even if exten- 

 sive) are plagued in this way by the development of steep seas of 

 troublesome size. THe submarine shelf that fronts the southwest 

 coast of Florida is worth citing in this connection, for while the 30 

 to 35-fathom contour lies 70 to 120 miles out from the land there, and 

 even the 10-fathom contour is 25 to 40 miles out, the wind is most com- 

 monly offshore, at all seasons of the year, or blows along the shore. 

 When it does blow onshore, it is shown on the monthly Pilot Charts 

 as seldom stronger than about 15 nautical miles per hour (No. 3 

 Beaufort) . Consequently, smooth seas prevail there the year round, 

 except on rare occasions, as when a winter norther develops. 



THE SIZES OF WAVES THAT ARE DEVELOPED IN 

 SHALLOW WATER 



The shapes and heights of the waves that are produced where strong 

 winds blow across extensive stretches of shoal water are a matter of 

 concern to the operators of small craft, both in enclosed sounds and 

 estuaries, as well as along open coasts fronted by a gently sloping 

 bottom during periods when strong winds blow parallel to the general 

 trend of the shoreline. In situations of this sort, the depth of the water 

 may directly limit the heights of waves if it is less than say 10 to 12 feet, 

 for waves begin breaking when they grow to the point where their 

 heights are approximately equal to the depth below undisturbed sea 

 level. 17 But while the development of breakers usually results in the 

 total extinction of the wave forms upon the shore, waves that grow 



17 Waves generated in shallow water in tank experiments broke where the depth was 

 filial to 0.8 times the wave height. 





