Service Command (1970) provides summaries of shipboard wave observations,
Summary of Synoptic Meteorological Observations (SSMO) for the hatched
areas indicated in Figure 3-17, Cumulative distribution functions for
wave heights as determined by both hindcasting techniques and the shipboard observations are given in Figure 3-18. The average of the two
forecasting methods agréés reasonably well with the shipboard observations.
3.54 ESTIMATING WAVE DECAY IN DEEP WATER
Figures 3-19 and 3-20 are used to
the waves hâve left the fetch area but
With Figure 3-19, and given Hy, Ty, F^
possible to compute the ratios
TT
decayed wave height _ nD
fetch wave height
Hp
estimate wave characteristics after
are still travelling in deep water.
and D (the decay distance), it is
rp
decayed wave period _
fetch wave period
Tp
With Figure 3-20, it is possible to compute wave travel time between
a fetch and a coast, knowing the decayed wave period Tp and the decay
distance D.
This travel time tp is determined by dividing the decay distance
by the deepwater group velocity for waves having a period equal to the
decayed period Tp. These values enable the estimation of arrivai times
for waves at the end of the decay distance.
Waves, after leaving a generating area, will generally follow a greatcircle path toward a coast. However, sufficient accuracy is usually
obtained by assuming wave travel in a straight line on the synoptic chart.
Decay distance is found by measuring the straight line distance between
the front of a fetch and the point for which the forecast is being made.
If a forecast is being made for a Coastal area, the effects of shoaling,
refraction, bottom friction and percolation will hâve to be considered in
translating the deepwater forecast to the shore.
3.6 WAVE FORECASTING FOR SHALLOW WATER
3.61 FORECASTING CURVES
hâter depth affects wave génération. For a given set of wind and
fetch conditions, wave heights will be smaller and wave periods shorter if
génération takes place in transitional or shallow water rather than in deep
water. Several forecasting approaches hâve been made; the method given by
Bretschneider as modified using the results of Ijima and Tang (1966) is
presented here. Bretschneider and Reid (1953) consider bottom friction
and percolation in the permeable sea bottom.
Ibere is no single theoretical development for determining the actual
grout of waves generated by winds blowing over relatively shallow water.
e numerical method presented here is based on successive approximations
3-42
Summary of Synoptic Meteorological Observations (SSMO) for the hatched
areas indicated in Figure 3-17, Cumulative distribution functions for
wave heights as determined by both hindcasting techniques and the shipboard observations are given in Figure 3-18. The average of the two
forecasting methods agréés reasonably well with the shipboard observations.
3.54 ESTIMATING WAVE DECAY IN DEEP WATER
Figures 3-19 and 3-20 are used to
the waves hâve left the fetch area but
With Figure 3-19, and given Hy, Ty, F^
possible to compute the ratios
TT
decayed wave height _ nD
fetch wave height
Hp
estimate wave characteristics after
are still travelling in deep water.
and D (the decay distance), it is
rp
decayed wave period _
fetch wave period
Tp
With Figure 3-20, it is possible to compute wave travel time between
a fetch and a coast, knowing the decayed wave period Tp and the decay
distance D.
This travel time tp is determined by dividing the decay distance
by the deepwater group velocity for waves having a period equal to the
decayed period Tp. These values enable the estimation of arrivai times
for waves at the end of the decay distance.
Waves, after leaving a generating area, will generally follow a greatcircle path toward a coast. However, sufficient accuracy is usually
obtained by assuming wave travel in a straight line on the synoptic chart.
Decay distance is found by measuring the straight line distance between
the front of a fetch and the point for which the forecast is being made.
If a forecast is being made for a Coastal area, the effects of shoaling,
refraction, bottom friction and percolation will hâve to be considered in
translating the deepwater forecast to the shore.
3.6 WAVE FORECASTING FOR SHALLOW WATER
3.61 FORECASTING CURVES
hâter depth affects wave génération. For a given set of wind and
fetch conditions, wave heights will be smaller and wave periods shorter if
génération takes place in transitional or shallow water rather than in deep
water. Several forecasting approaches hâve been made; the method given by
Bretschneider as modified using the results of Ijima and Tang (1966) is
presented here. Bretschneider and Reid (1953) consider bottom friction
and percolation in the permeable sea bottom.
Ibere is no single theoretical development for determining the actual
grout of waves generated by winds blowing over relatively shallow water.
e numerical method presented here is based on successive approximations
3-42