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This page saved from http://www.ndbc.noaa.gov/wavecalc.shtml, summer 2001.
Significant wave height, Hs, is approximately equal to the average of
the highest one-third of the waves. Hs is calculated using:
where m0 is the variance of the wave displacement time
series acquired during the wave acquisition period. However, since wave
displacement time series are not returned from NDBC's wave measurement
systems, variance is calculated using the nondirectional wave spectrum
according to the following relationship:
where the summation of spectral density, S(f), is over all frequency
bands, f, of the nondirectional wave
spectrum and df is the bandwidth of each band. NDBC wave analysis
systems typically sum over the range from 0.03 to 0.40 Hz with frequency
bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with
bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high
frequencies. Dominant, or peak, wave period, Tp, is the period corresponding to the
frequency band with the maximum value of spectral density in the
nondirectional wave spectrum. It is the reciprocal of the peak frequency,
fp:
Dominant period is representative of the higher waves encountered
during the wave sampling period. Greater detail on the processing of NDBC
wave data can be found here. NDBC also provides estimates of the height and period of wind-seas and swell on
each station page. Values for these quantities are calculated by applying
the above process to the respective wind-sea and swell portions of the
wave spectrum. The algorithm used to estimate wave steepness is taken from work done
by William Buckley, discussed in a paper that appeared in the Journal of
Naval Engineers, September, 1988, titled " Extreme and Climatic Wave
Spectra for Use in the Structural Design of Ships". The algorithm involves
the relationship between significant wave height (Hs) and dominant wave
period, or more precisely, its inverse, peak wave frequency (fp). The algorithm follows: where Hswell and Hwindwv are the respective significant heights of the
swell and wind waves which are quantities that are also computed.
How are significant wave height, dominant period, and wave steepness
calculated?
val = exp(-3.3 * ln(fp))
if Hs > (1/250 * val)
steepness = 'very steep';
elseif Hs > (1/500 * val)
steepness = 'steep';
elseif Hs > (1/1000 * val)
steepness = 'average';
else
if Hswell >= Hwindwv
steepness = 'swell';
else
steepness = 'average';
This page was last modified on Thursday, 21-Dec-00 16:01:02 CST