SPECIAL PUBLICATION NO. 43
Table 4. Moment Wave Height Statistical Relationships (after Balsime and Carter, 1984a, 1984b).
Portion of Wave Record Spectral Relationships Shore-Breaking Relationships
Considered
All Waves A rage Wave Average Breaker Hei4ht = H Height = Hb
All Waves H= 0. 885 H H = 0.98 H,
Highest 30% H= 0. 625 H Hb = 0. 813 H,,
Highest 10% H= 0.493 /140 H = 0, 73 /Ho
Highest 1% H= 0.375 -4 H = 0. 637 /4,
Definitions:
Average Wave Root-MeaN-Square Wave
N N H,= significant Height = H= H Height = H.= -N I/ wave heght
NOTE: Formulas apply to both H and Hb; H,, H10, and H1 are calculated using the form of the equation as for the average wave height.
represents a conceptual plane not located Therefore, by the process of and/or not calculated such that it is not elimination MSL is defined as the surf base necessarily comparable to NGVD). The (it is also the tide base, not to be confused remaining tidal datum is, then, MSL. Other with the concept of the wave base). Upon than its identification by elimination of other inspection of Figures 1, 2, and 3, it is readily datums, there are strong motivating reasons apparent that MSL, like the other datums, why MSL is the proper tidal datum reference has variability. Why, then, would we select to use when dealing with coastal processes it as a convention for reference? Water (i.e., force and response elements). As we levels are not globally coincident in the have already learned, principal force vertical sense for very real reasons. elements include astronomical tides, storm However, MSL is a measure representative tides, and waves. Astronomical tides are, by of the entire distribution of the metonic definition, already accounted for when using astronomical tide, and is the only one of the MSL, and storm tides are extreme events tidal datums that has statistical continuity though accounted for as described in the and comparability of results from place to preceding section. Waves, however, place. Noting that for open coastal waters constitute an ubiquitous phenomenon near MSL is equivalent to MTL (Swanson, 1974, constant in nearshore coastal waters (except p.4), then the MTL measure remains to for coasts with a substantial zero wave represent the central tendency of the tide energy component), distribution since metonic measures of highs and lows are used in its determination. The
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