38c Refr action Fan Diagrejns. - It is often convenient, especially where 

 a coastal area is shielded hj laiiil features from waves approaclTing in certain 

 directions, to construct refraction diagrams from shallow toward deep water, 

 111 such cases, a sheaf or fan of orthogonals may b-e projected seaward in 

 direction soire 5 or 10 degrees apart* (See Figure l^a) . VJith the deep water 

 directions deteririned by the individual orthogonals, companion orthogonals 

 mscf be projected shoreward on either side of the seaward projected ones in 

 order to deternine the refraction coefficd.ent for the various directions of 

 wave approach, (.See Figure l5b)» 



39* Refractio n Diagraji Lim itations, - In majny cases refraction dia- 

 grajis provide a reasonably accurate measure of tte changes waves undergo 

 on approaclilng a coasts '^lite often they provide the only riieasure of these 

 changes available » However _, the data deteruiined frora refraction diagrams 

 are only as valid as. the theory of their construction is accurate. The 

 orthogonal direction change equation (9) is derived for the siiiplest case 

 of s trai.ght parallel contours, and although little error is introdu.ced by 

 bringing orthogonf?J.ls over z'elativelj'- sirr^jle hj^drogi-aph^r, it is difficult 

 to carrj" an orthogonal accurately into shore over complex bottom features, 

 Moreover, tlie equation is derived for small waves moving over relatively 

 flat slope s,B Although no strict limits have been set, strict accuracy can- 

 not be expected where bottom slopes are steeper than 1 on 10, A third 

 limu-tation is inherent in tte assurrption that no energy travels laterally 

 along a wave crests No strict limits have been set, but the accuracy of 

 wave heights derived from orthogonals which bend sharply, is questionable,, 



iiOa Significant and Higher Waves . - It was noted in paragraph 9 that 

 the wave height determined from forecasting or liindcasting procedures is 

 the so-called significant wave height, the average of the one-third higher 

 heights of a given wave groups If this wave height is used as H in 

 determin3.ng the inshore wave height H through refraction analysis, H vjill 

 also be the significant wave height in the transitional and shallow xirater 

 zones e 



Itl, It has been found that a linear relationslxLp exists betT-jeen the 

 significant wave height and the mean wave height of a groups This relation- 

 ship is 



H = 0.62lt H - 0,015 (10) 



m s 



xifhere H = the significant wave height of a wave grouo (in feet) and 

 s 



H = the m^ean vrave height of the group (in feet) 



It was further found that this mean wave height could be related statistically 

 to any other height which may occur more or less frequently, Tliis relation- 

 ship is represented graphically on Figure 16 • The graph can be used to find 

 a wave height m.ore nearly the maximum of a group of waves, say one wliich 

 id.ll not be exceeded by 9^ percent of the waves* 



25 



