1. Which of the three theories for Directional
effects shall I use? Three methods are included in the program for this subject. The effective
fetch method is obtaining an equivalent fetch of the region by a weighted average process
of the fetches, from this fetch the significant wave height and period are predicted, and
from them the wave spectrum.
Savil's method that obtains an equivalent significant wave height and period, by a
weighted average process of the predicted significant wave height and period for each
sector. From the significant wave height and period the wave spectrum is predicted.
Seymour's method. The wave spectrum is computed for each sector and the final predicted
wave spectrum is obtained by adding up the energy of each sector's spectrum.
Seymour's method seems to give more realistic results for restricted fetch areas, because
the wave spectrum is predicted at each wind sector. The wave frequencies are better
represented in the final forecasted wave spectrum. With this method double pick spectra
will be predicted in some regions as many infield measurements have shown. Of course
the experience of the user and the characteristics of the water region is the final judge
of the selected method.
2. What type of wave spectrum shall I use? The most common spectrum to use is JONSWAP spectrum.
But Pierson-Moskowitz may be used if the characteristics of the water region, and the
experience for waves in the region tells this.
3. What method for significant wave
computation is preferable? The mechanics of wave generation by winds acting over water surface is a
complex issue. The wave forecasting methods are based on semi-empirical relations (SMB
methods Sverdrup, Munk, and Bretschneider), which link the significant wave height Hs and
significant wave period to wind speed, fetch, and water depth.
In the program are implemented three basic and commonly used methods. In this methods F is
the fetch, U the wind velocity, and g the acceleration of gravity, and D is the average
depth of the region. The selection of one of these methods is based on experience. A
commonly used method is to try all of them and observe the differences in the predicted
4. I know the wind velocity in Beaufort scale,
how can I transfer it to other units? Look at the program help (wind velocity topic) for a table showing the
correspondence between Beaufort scale and wind velocity in m/s or km/h.
5. If the water region is open sea, how to
define the boundary? Use a far boundary (eg. 200 km, or 200 miles ) for this boundary.
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Σεπτεμβρίου 13, 2018.