FLORIDA GEOLOGICAL SURVEY
specific direction of propagation. Where tide might be considered to be maintained, water depths are such that waves remain say, for 1/2 to 1 hour. Doubling this value, relatively stable, the wave record (such as since two highs occur in each tidal day for that measured by a wave gauge) will the semidurnal tide, then MHW is actually represent all wave trains (i.e., multiple trains) maintained for about 4 to 8 per cent of the passing the gauge. Multiple wave train time (e.g., 14 and 28 days a year). height and period measurements are termed Superimposed upon MHW is the significant the special wave record or wave field,. wave height which, by definition, neglects Shot -breaking waves, however, do not 70 per cent of the wave record (assuming conform to spectral wave statistics. This that Hs adequately includes any significant occurs because in nearshore waters, waves zero wave energy component; Balsillie, are ultimately limited by water depth 1993b). Clearly, such an application would according to db = 1.28 Hb (McCowan, be inappropriate for one applying such force 1894; Balsillie, 1983a; Balsillie, 1999b; elements to annual or long-term conditions. Balsillie and Tanner, 1999) where Hb is the Unfortunately, however, such misapplicawave crest height at shore-breaking and db tions, of which this is just one example, are is the water depth where the wave breaks. commonplace. On the other hand, such an Hence, shore-breaking waves engender application might have more viable moment wave statistics for sn wave application if it included a storm surge (i.e., trains since a wave train with larger waves peak combined storm tide minus the will break further offshore than one with astronomical tide) to represent the peak smaller waves, combined storm tide and attendant wave activity which occurred coincident with the
It follows, then, that moment wave peak astronomical tide. This latter case, statistics vary depending upon whether they however, has application only to identify a represent the spectral wave record or single conservative design elevation for a structure shore-breaking wave trains. The most (e.g., perhaps a pier) which is a monergistic commonly applied nearshore wave height tidal datums application, but certainly not to statistics are the average wave height H, profile response which constitutes a root-mean-square wave height Hrms, synergistic application. significant wave height Hs (average of the
highest 30 per cent waves of record), H10 Previously discussed use/application (average of the highest 10 per cent waves examples have already led to the elimination ofrecord), and H1 (average of the highest 1 of extreme datum planes (i.e., MHHW, per cent waves). Each of these moment MWH, MLW, MLLW) as has the preceding measures is applied in the design of coastal example, and MSL and NGVD remain for engineering solutions by defined prescription, consideration. The NGVD reference is not, Relating moment measures for spectral and of course, a tidal datum. It is rather, for all shore-breaking wave cases are listed in Table practical purposes a geodetic datum for 4 to illustrate the variability of relating computational reference, that although for coefficients. open-coast gauges has a departure generally less than 0.5 of a foot from MSL for Florida,
Let us look at an example of tide the long-term primary departure of MSL and conditions to which we might superimpose NGVD is subject to influences of sea level certain wave conditions. Figure 8 illustrates rise or fall (shorter-term natural deviations 6 days of an astronomical tide record. have been discussed above). Hence, it Suppose one inspects the case where MHW should not be utilized as a datum, and Hs are, for whatever reason(s), selected particularly where global data are involved for use. From the plots, each peak of the (i.e., where the non-tidal vertical reference
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