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How is H2% calculated from significant wave height data?

I'm processing a multi-year dataset from a wave buoy that records significant wave height (Hs) spectra. For my coastal engineering design, I need the extreme value statistic H2%. I understand it's a percentile, but do I use the raw sea surface elevation time series, individual wave heights derived from zero-crossing analysis, or the Hs values themselves? The literature seems to assume this knowledge.

 

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By Sumitra R Answered 2 years ago

A common point of confusion. H2% is an extreme wave height statistic, so you cannot calculate it directly from the time-averaged Hs values. You need a time series of individual wave heights. First, perform a zero-crossing analysis on your raw sea surface elevation data to get a series of individual wave heights (H_i). Then, rank these H_i from largest to smallest. H2% is approximately the height at the rank position (0.02 * N), where N is the total number of waves. For a more robust estimate, I typically fit the ranked data to a theoretical distribution like Rayleigh or Weibull and read the 98th percentile from the fitted curve, which smooths out sampling variability.

 

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