steepness (height-to-length ratio) decreases. (Galvin, 1967.) Of the
four breaker types, spilling breakers most closely resemble the solitary
waves whose speed is described by Equation 4-13. (Galvin, 1972.) Spilling breakers differ little in fluid motion from unbroken waves (Divoky,
LeMehaute, and Lin, 1970), and thus tend to be less effective in transporting sédiment than plunging or collapsing breakers.
The most intense local fluid motions are produced by plunging breakers. As the wave moves into shallower depths, the front face begins to
steepen. When the wave reaches a mean depth about equal to its height,
it breaks by curling over at the crest. The crest of the wave acts as a
free-falling jet that scours a trough into the bottom. At the same time,
just seaward of the trough, the longshore bar is formed, in part by sédiment scoured from the trough and in part by sédiment transported in ripples moving from the offshore.
The effect of the tide on nearshore currents is not discussed, but
tide-generated currents may be superimposed on wave-generated nearshore
currents, especially near estuaries. In addition, the changing élévation
of the water level as the tide rises and falls may change the area and
the shape of the profile through the surf zone, and thus al ter the nearshore currents.
4.43 ONSHORE-OFFSHORE CURRENTS
4.431 Onshore-Offshore Exchange. Field and laboratory data indicate
that water in the nearshore zone is divided by the breaker line into
two distinct water masses between which there is only a limited exchange
of water.
The mechanisms for the exchange are: mass transport velocity in
shoaling waves, wind-induced surface drift, wave-induced setup, currents
induced by irregularities on the bottom, rip currents, and density currents. The resulting flows are significantly influenced by, an act on,
the hydrography of the surf and nearshore zones. Figure 4-13 shows t e
nearshore current System measured for particular wave con itions on
e
Southern California coast.
to be extensive exchange of
At first observation, there appears
However, the breaking
water between the nearshore and the sur
’ been withdrawn from the
wave itself is formed largely of water
water then reenters the
surf zone after breaking. (Galvin,
•
that only a limited amount
surf zone as part of the new breaking wave, This inference is supported
of water is actually transferred oftsn
.
6788) which show that
by the calculations of Longuet-Higgins
veiocity distributions,
little mixing is needed to account or
with speeds much slower
Most of the exchange mechanisms ^n^cate. taken as an estimate of the
than the breaking-wave speed, which ™a^toral zone indicated by Equation
maximum water particle speed in the i
4.13.
4-43
four breaker types, spilling breakers most closely resemble the solitary
waves whose speed is described by Equation 4-13. (Galvin, 1972.) Spilling breakers differ little in fluid motion from unbroken waves (Divoky,
LeMehaute, and Lin, 1970), and thus tend to be less effective in transporting sédiment than plunging or collapsing breakers.
The most intense local fluid motions are produced by plunging breakers. As the wave moves into shallower depths, the front face begins to
steepen. When the wave reaches a mean depth about equal to its height,
it breaks by curling over at the crest. The crest of the wave acts as a
free-falling jet that scours a trough into the bottom. At the same time,
just seaward of the trough, the longshore bar is formed, in part by sédiment scoured from the trough and in part by sédiment transported in ripples moving from the offshore.
The effect of the tide on nearshore currents is not discussed, but
tide-generated currents may be superimposed on wave-generated nearshore
currents, especially near estuaries. In addition, the changing élévation
of the water level as the tide rises and falls may change the area and
the shape of the profile through the surf zone, and thus al ter the nearshore currents.
4.43 ONSHORE-OFFSHORE CURRENTS
4.431 Onshore-Offshore Exchange. Field and laboratory data indicate
that water in the nearshore zone is divided by the breaker line into
two distinct water masses between which there is only a limited exchange
of water.
The mechanisms for the exchange are: mass transport velocity in
shoaling waves, wind-induced surface drift, wave-induced setup, currents
induced by irregularities on the bottom, rip currents, and density currents. The resulting flows are significantly influenced by, an act on,
the hydrography of the surf and nearshore zones. Figure 4-13 shows t e
nearshore current System measured for particular wave con itions on
e
Southern California coast.
to be extensive exchange of
At first observation, there appears
However, the breaking
water between the nearshore and the sur
’ been withdrawn from the
wave itself is formed largely of water
water then reenters the
surf zone after breaking. (Galvin,
•
that only a limited amount
surf zone as part of the new breaking wave, This inference is supported
of water is actually transferred oftsn
.
6788) which show that
by the calculations of Longuet-Higgins
veiocity distributions,
little mixing is needed to account or
with speeds much slower
Most of the exchange mechanisms ^n^cate. taken as an estimate of the
than the breaking-wave speed, which ™a^toral zone indicated by Equation
maximum water particle speed in the i
4.13.
4-43