may be determined by first analyzing meteorological data to find deepwater conditions. Then, by analyzing refraction (Section 2.32, General -
Refraction by Bathymetry.), the changes in wave characteristics as the
wave moves through shallow water to the shore may be found. In other
areas, in particular along the North. Atlantic coast, wfiere the bathymetry
is complex, refraction procedure results are frequently difficult to interpret, and the conversion of deepwater wave data to shallow-water and nearshore data becomes laborîous and sometimes inaccurate.
Along the Gulf coast and in many inland lakes, génération of waves by
wind is appreciably affected by water depth. In addition, the nature and
extent of transitional and shallow-water régions complicate ordinary refraction analysis by introducing a bottom-friction factor and associated
wave energy dissipation.
3.4 WIND INFORMATION NEEDED FOR WAVE PREDICTION
Wave prédiction from first principles, as described above, requires
very detailed spécification of the wind field near the water surface. This
is generally developed in two steps: (1) Estimation of the mean free air
wind speed and direction, (This step may be omitted for réservoirs and small
lakes if surface wind observations are available.), and (2) Estimation of
the mean surface wind speed and direction.
When the full wave génération process is considered, a large capacity
computer must be used for the calculations, and fairly complex procedures
may be used for determining the wind field. Engineers who require wave
hindcasts for only a few locations, and perhaps for only a few dates must
employ simpler techniques. A brief discussion of the processes involved
in determining the surface wind and techniques suitable for use in determining the characteristics of the wind field needed for the simplified wave
prédiction model described in Section 3.5, Simplified Wave Prédiction Models,
are given in this section. These procedures will be accurate (within 20
percent) about two-thirds of the time. The following discussion provides
guidance for recognizing cases in which the simplified procedures are not
appropriate. Errors resuiting from disregarding the exceptional situations
tend to be random. Thus climatological summaries, based on hindcast data,
may be much more accurate than the individual values that go into them.
Wind reports from ships at sea are generally estimâtes based on the
appearance of the waves, the drifting of smoke, or the flapping of flagsalthough some are anemometer measurements. Actually, even if ail ships
were equipped with several aneometers, the wind field over the sea would
still not be known in sufficient detail or précision to permit full
exploitation of modem théories for wave génération.
Fortunately, estimâtes of the surface wind field that are usefully
accurate most of the time can be based on the isobaric pattern of synoptic
weather charts.
r
3-20
Refraction by Bathymetry.), the changes in wave characteristics as the
wave moves through shallow water to the shore may be found. In other
areas, in particular along the North. Atlantic coast, wfiere the bathymetry
is complex, refraction procedure results are frequently difficult to interpret, and the conversion of deepwater wave data to shallow-water and nearshore data becomes laborîous and sometimes inaccurate.
Along the Gulf coast and in many inland lakes, génération of waves by
wind is appreciably affected by water depth. In addition, the nature and
extent of transitional and shallow-water régions complicate ordinary refraction analysis by introducing a bottom-friction factor and associated
wave energy dissipation.
3.4 WIND INFORMATION NEEDED FOR WAVE PREDICTION
Wave prédiction from first principles, as described above, requires
very detailed spécification of the wind field near the water surface. This
is generally developed in two steps: (1) Estimation of the mean free air
wind speed and direction, (This step may be omitted for réservoirs and small
lakes if surface wind observations are available.), and (2) Estimation of
the mean surface wind speed and direction.
When the full wave génération process is considered, a large capacity
computer must be used for the calculations, and fairly complex procedures
may be used for determining the wind field. Engineers who require wave
hindcasts for only a few locations, and perhaps for only a few dates must
employ simpler techniques. A brief discussion of the processes involved
in determining the surface wind and techniques suitable for use in determining the characteristics of the wind field needed for the simplified wave
prédiction model described in Section 3.5, Simplified Wave Prédiction Models,
are given in this section. These procedures will be accurate (within 20
percent) about two-thirds of the time. The following discussion provides
guidance for recognizing cases in which the simplified procedures are not
appropriate. Errors resuiting from disregarding the exceptional situations
tend to be random. Thus climatological summaries, based on hindcast data,
may be much more accurate than the individual values that go into them.
Wind reports from ships at sea are generally estimâtes based on the
appearance of the waves, the drifting of smoke, or the flapping of flagsalthough some are anemometer measurements. Actually, even if ail ships
were equipped with several aneometers, the wind field over the sea would
still not be known in sufficient detail or précision to permit full
exploitation of modem théories for wave génération.
Fortunately, estimâtes of the surface wind field that are usefully
accurate most of the time can be based on the isobaric pattern of synoptic
weather charts.
r
3-20