V. SIMPLIFIED METHODS FOR ESTIMATING WAVE CONDITIONS 



When estimates of wave heights, periods, and directions are needed, the 



most accurate procedures are the numerical methods discussed in Chapter 3, 



Section III. However, there are often cases where neither the time available 



nor the cost justifies using complex numerical methods. In these cases, a 



simplified method may be justified. Chapter 3, Section V,3 presents a series 



of equations and nomograms that give significant wave height by H and 



m 



period of the spectral peak, T for a given windspeed and fetch or 



duration. Estimating surface winds is treated in Chapter 3, Section IV. 



Estimating fetch length is treated in Chapter 3, Section V,l. 



The spectrally based significant wave height H is four times the 



m 



square root of the variance of the sea surface elevation. In deep water H 



is approximately equal to the significant wave height H , which is based on 

 counting and measuring individual waves (see Chapter 3, Section 11,5). In 



shallow water, H becomes less than H . In both deep and shallow 

 m ^ 



water, H is based on the wave energy; this is not true for H„ . 

 m s 



o 



The following assumptions pertain to these methods. The methods will be 

 used for cases where fetches are short (80 to 120 kilometers (50 to 75 miles) 

 or less) and the wind can be assumed uniform and constant over the fetch. 

 Cases where the wind field varies rapidly in time or with distance over the 

 fetch or where swell from distant sources propagates into the area are best 

 treated numerically. Since these conditions are rarely met and wind fields 

 are not usually estimated accurately, do not assume the results are more 

 accurate than warranted by the accuracy of the input or the simplicity of the 

 method. Good, unbiased estimates of all parameters for input to the wave 

 equations should be sought and the results interpreted conservatively. 

 Individual input parameters should not each be estimated conservatively, since 

 to do so may bias the results. 



1 . Delineating a Fetch . 



A fetch has been defined subjectively as a region in which the windspeed 

 and direction are reasonably constant. Confidence in the computed results 

 begins to deteriorate when wind direction variations exceed 15° ; confidence 

 deteriorates significantly when direction deviations exceed 45° . The computed 

 results are sensitive to changes in windspeed as small as 1 knot (0.5 meter 

 per second), but it is not possible to estimate the windspeed over any sizable 

 region with this precision. For practical wave predictions it is usually 

 satisfactory to regard the windspeed as reasonably constant if variations do 

 not exceed 5 knots (2.5 meters per second) from the mean. A coastline upwind 

 from the point of interest always limits a fetch. An upwind limit to the 

 fetch may also be provided by curvature or spreading of the isobars as 

 indicated in Figure 3-20 (Shields and Burdwell, 1970) or by a definite shift 

 in wind direction. Frequently the discontinuity at a weather front will limit 

 a fetch, although this is not always so. 



3-39 



