SEAWAY 



33 



0^2 



o 

 5 



200 400 GOO 800 1000 

 _l I \ I I I 



40 60 80 



Fetch (Mautical Miles) 



100 



Fig. 31 Graph of t against fetch (from Darbyshire, 1952) 



y = Hr/r 



_ {T/V - 0.24) - 

 0.0027 



(62) 



The resultant, curve is shown l\v the solid lines in Fig. 30. 

 This relationship and the plots of Mg. 30 were (obtained 

 by considering wa\-e components of the same period un- 

 der the action of winds of different speeds. The maxi- 

 mum value of Hr/T is 0.44 and it occurs at T/V = 0.24. 

 At T/V = 0.33 the value of Hj. is \'ery small, only one 

 fourteenth of the maxinuun \alue of //j-. 



The eciuivalent wave height H is then found \>y inte- 



Table 6 



Hmaia Menu wave height (.avcnige of all 1,1) 



waves ) 

 H E(|uivalent wave height (height of 1.2//n,ean 



simple sine wave having same 



energy content as cumplieatcd wave 



pattern ) 

 Hs Significant wave height (average of I A)H„„,„„ 



one-third highest waves) 

 Hn.aK Ma.ximiim wave height (higliest in iOO 2.4//meaii 



waves) or 1 .5Hs 



grating Hj- (IT from to co and taking the scjuare root of 

 the result, which gi\-es 



H = 0.027 V- (63) 



where // is in feet, V in knots. 



P'or a fetch shorter than 100 miles the correction of the 

 foregoing is obtained from oli.servatioiis made on a lake 

 and on the Iri.sh Sea, yielding 



H = 0.027 1''"= (1 



-o-iuV 



(64) 



where x is the fetch in nautical miles. In addition a 

 somewhat more complicated if more accurate expression 

 is given. The variation of wave period with fetch is 

 shown in Fig. 31. 



On the basis of the previous work of Barber (1950), 

 the relative values of H and of other forms of sea defini- 

 tion are given in Table 6. 



According to Barber, approximately 13 per cent of the 

 waves ha\'e heights greater than the significant height. 

 Examination of wave records has shown that the signifi- 



14 _ 



« _ 



2 4 G 8 10 



(a) Computed '•Has/e Period (sec.) 



5 10 



(b) Computed Height Ctt.) 



Fig. 32 (<?) Comparison of computed and observed wave periods (Sverdrup 



and Munk's data), (h) Comparison of computed and observed wave heights 



(Sverdrup and Munk's data) 



