4,432 Diffuse Retum Flow. Wind- and wave-induced water drift, pressure gradients at the bottom due to setup, density différences due to
suspended sédiment and température, and other mechanisms produce patterns
of motion in the surf zone that vary from highly organized rip currents
to broad diffuse flows that require continued observation to detect. Diffuse retum flows may be visible in aerial photos as fronts of turbid
water moving seaward from the surf zone. Such flows may be seen in the
photos reproduced in Sonu (1972, p. 3239).
4.433 Rip Currents. Most noticeable of the exchange mechanisms between
offshore and surf zone are rip currents. (See Figure 4-14, and Figure
A-7, Appendix A.) Rip currents are concentrated jets that carry water
seaward through the breaker zone. They appear most noticeable when long,
high waves produce wave setup on the beach. In addition to the classical rip currents, there are other localized currents directed seaward
from the shore. Some are due to concentrated flows down gui lies in the
beach face, and others can be attributed to interacting waves and edge
wave phenomena.
(Inman, Tait, and Nordstrom, 1971, p. 3493.) The origin
of rip currents is discussed by Arthur (1962), and Sonu (1972).
Three-dimensional circulation in the surf is documented by Shepard and
Inman (1950), and this complex flow needs to be considered, especially in
evaluating the resuit s of laboratory tests for Coastal engineering purposes
However, at présent, there is no proven way to predict the conditions that
produce rip currents or the spacing between rips. In addition, data are
lacking that would indicate quantitatively how important rip currents are
as sédiment transporting agents.
4.44 LONGSHORE CURRENTS
4.441 Velocity and Flow Rate. Lo^gsb°^_^rthetzone°ofPbreaking waves
shoreline, and are restricted mainly e
generated by the longand the shoreline. Most longshore
approach the shoreline.
shore component of motion in waves tha
Longshore currents typically hâve mean
longshore current velocor less. Figure 4-15 shows a histogram °
'
1968. DeSpite frequent
ities measured at 36 sites in California
.
most data agréé with
reports of exceptional longshore curren
fpet/ner second are unusual.
Figure 4-15 in showing that speeds ab°v®. prvations, most of which appear
A compilation of 352 longshore current o
speed, showed that the maxito be biased toward conditions producing
6and that the highest “^servaobserved speed was 5.5 feet per
(Galvin and Nelson, 1967.)
tions were reported to hâve been w^d'^
speeds, they are in]P°rta"5
Although longshore currents general y
lonp the shore for extende pe
in littoral processes because they
o
motion by the breaking waves,
ods of time, transporting sédiment se
velocThe most important variable in
shoreline. H°wever>
,
ity is the angle between the wave cres
iongshore transport rate epe
volume rate of flow of the current and the longsn
4-45
suspended sédiment and température, and other mechanisms produce patterns
of motion in the surf zone that vary from highly organized rip currents
to broad diffuse flows that require continued observation to detect. Diffuse retum flows may be visible in aerial photos as fronts of turbid
water moving seaward from the surf zone. Such flows may be seen in the
photos reproduced in Sonu (1972, p. 3239).
4.433 Rip Currents. Most noticeable of the exchange mechanisms between
offshore and surf zone are rip currents. (See Figure 4-14, and Figure
A-7, Appendix A.) Rip currents are concentrated jets that carry water
seaward through the breaker zone. They appear most noticeable when long,
high waves produce wave setup on the beach. In addition to the classical rip currents, there are other localized currents directed seaward
from the shore. Some are due to concentrated flows down gui lies in the
beach face, and others can be attributed to interacting waves and edge
wave phenomena.
(Inman, Tait, and Nordstrom, 1971, p. 3493.) The origin
of rip currents is discussed by Arthur (1962), and Sonu (1972).
Three-dimensional circulation in the surf is documented by Shepard and
Inman (1950), and this complex flow needs to be considered, especially in
evaluating the resuit s of laboratory tests for Coastal engineering purposes
However, at présent, there is no proven way to predict the conditions that
produce rip currents or the spacing between rips. In addition, data are
lacking that would indicate quantitatively how important rip currents are
as sédiment transporting agents.
4.44 LONGSHORE CURRENTS
4.441 Velocity and Flow Rate. Lo^gsb°^_^rthetzone°ofPbreaking waves
shoreline, and are restricted mainly e
generated by the longand the shoreline. Most longshore
approach the shoreline.
shore component of motion in waves tha
Longshore currents typically hâve mean
longshore current velocor less. Figure 4-15 shows a histogram °
'
1968. DeSpite frequent
ities measured at 36 sites in California
.
most data agréé with
reports of exceptional longshore curren
fpet/ner second are unusual.
Figure 4-15 in showing that speeds ab°v®. prvations, most of which appear
A compilation of 352 longshore current o
speed, showed that the maxito be biased toward conditions producing
6and that the highest “^servaobserved speed was 5.5 feet per
(Galvin and Nelson, 1967.)
tions were reported to hâve been w^d'^
speeds, they are in]P°rta"5
Although longshore currents general y
lonp the shore for extende pe
in littoral processes because they
o
motion by the breaking waves,
ods of time, transporting sédiment se
velocThe most important variable in
shoreline. H°wever>
,
ity is the angle between the wave cres
iongshore transport rate epe
volume rate of flow of the current and the longsn
4-45