4.32 FACTORS DETERMINING LITTORAL WAVE CLIMATE
The wave climate at a shoreline dépends on the offshore wave climate,
caused by prevailing winds and storms, and on the bottom topography that
modifies the waves as they travel shoreward.
4.321 Offshore Wave Climate. Wave climate is the distribution of wave
conditions averaged over the years. A wave condition is the particular
combination of wave heights, wave periods, and wave directions at a given
time. A spécifie wave condition offshore is the resuit of local winds
blowing at the time of the observation and the recent history of winds
in the more distant parts of the same water body. For local winds, wave
conditions offshore dépend on the wind velocity, duration, and fetch.
For waves reaching an observation point from distant parts of the sea,
a decay factor is added which preferentially filters out the higher,
shorter period waves with increasing distances.
(Chapter 3 discusses
wave génération and decay.)
4.322 Effect of Bottom Topography. As waves travel from deep water, they
change height and direction because of refraction, shoaling, bottom friction, and percolation. Laboratory experiments indicate that height and
apparent period are also changed by nonlinear deformation of the waves in
shallow water.
Refraction is the bending of wave crests due to the slowing down of
that part of the wave crest which is in shallower water.
(See Section
2.32.) As a resuit, refraction tends to decrease the angle between the
wave crest and the bottom contour. Thus, for most coasts, refraction reduces the breaker angle and spreads the wave energy over a longer crest
length.
Shoaling is the change in wave height due to conservation of energy
flux. (See Section 2.32). As a wave moves into shallow water the wave
height first decreases slightly, and then increases continuously to the
breaker position, assuming friction and refraction effects are negligible.
Bottom friction is important in reducing wave height where waves
nust travel long distances in shallow water. (Bretschneider, 1954.)
choi There ^as been only limited field study of nonlinear deformation in
in io/
er waves (B/1716, 1969), but because such deformation is common
iominn «2°^ exPeriments (Galvin, 1972), it is expected that such pheis to «iniit
?onuno11 ln the field* An effect of nonlinear deformation
both the tpq
mcoming wave crest into two or more crests affecting
both the resulting wave height and the apparent period.
shelter^narts nf
’ shoals» 311(1 other variations in hydrography also
greatest influenr e S °re’ In gei)eral» bottom hydrography has the
Because of the eff °? Wa^e^ traveling long distances in shallow water.
hâve different rhaeC f °- ?ttom hydrography, nearshore waves generally
different characteristics than they had in deep water offshore.
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The wave climate at a shoreline dépends on the offshore wave climate,
caused by prevailing winds and storms, and on the bottom topography that
modifies the waves as they travel shoreward.
4.321 Offshore Wave Climate. Wave climate is the distribution of wave
conditions averaged over the years. A wave condition is the particular
combination of wave heights, wave periods, and wave directions at a given
time. A spécifie wave condition offshore is the resuit of local winds
blowing at the time of the observation and the recent history of winds
in the more distant parts of the same water body. For local winds, wave
conditions offshore dépend on the wind velocity, duration, and fetch.
For waves reaching an observation point from distant parts of the sea,
a decay factor is added which preferentially filters out the higher,
shorter period waves with increasing distances.
(Chapter 3 discusses
wave génération and decay.)
4.322 Effect of Bottom Topography. As waves travel from deep water, they
change height and direction because of refraction, shoaling, bottom friction, and percolation. Laboratory experiments indicate that height and
apparent period are also changed by nonlinear deformation of the waves in
shallow water.
Refraction is the bending of wave crests due to the slowing down of
that part of the wave crest which is in shallower water.
(See Section
2.32.) As a resuit, refraction tends to decrease the angle between the
wave crest and the bottom contour. Thus, for most coasts, refraction reduces the breaker angle and spreads the wave energy over a longer crest
length.
Shoaling is the change in wave height due to conservation of energy
flux. (See Section 2.32). As a wave moves into shallow water the wave
height first decreases slightly, and then increases continuously to the
breaker position, assuming friction and refraction effects are negligible.
Bottom friction is important in reducing wave height where waves
nust travel long distances in shallow water. (Bretschneider, 1954.)
choi There ^as been only limited field study of nonlinear deformation in
in io/
er waves (B/1716, 1969), but because such deformation is common
iominn «2°^ exPeriments (Galvin, 1972), it is expected that such pheis to «iniit
?onuno11 ln the field* An effect of nonlinear deformation
both the tpq
mcoming wave crest into two or more crests affecting
both the resulting wave height and the apparent period.
shelter^narts nf
’ shoals» 311(1 other variations in hydrography also
greatest influenr e S °re’ In gei)eral» bottom hydrography has the
Because of the eff °? Wa^e^ traveling long distances in shallow water.
hâve different rhaeC f °- ?ttom hydrography, nearshore waves generally
different characteristics than they had in deep water offshore.
4-28