the subject of Section 4.4, include longshore currents and mass-transport
currents in the onshore-offshore direction. It is possible to hâve ripple
forms moving shoreward while residual currents above the ripples carry
suspended sédiment clouds in a net offshore direction. Information on the
transport of sédiment above ripples is given in Bijker (1970), Kennedy and
Locher (1972), and Mogridge and Kamphuis (1972).
b. Surf Zone. The stress of the water on the bottom due to turbulence and wave-induced velocity gradients moves sédiment in the surf zone
with each passing breaker crest. This sédiment motion is both bedload
and suspended-load transport. Sédiment in motion oscillâtes back and
forth with each passing wave, and moves alongshore with the longshore
current. On the beach face, the landward terminât ion of the surf zone,
the broken wave advances up the slope as a bore of graduaily decreasing
height, and then drains seaward in a graduaily thinning sheet of water.
Frequently, the draining return flows in guilies and carries sédiment to
the base of the beach face.
In the surf zone, ripples cause significant sédiment suspension, but
here there are additional eddies caused by the breaking wave. These eddies
hâve more energy and are larger than the ripple eddies. The greater energy
suspends more sand in the surf zone than offshore. The greater eddy size
mixes the suspended sand over a larger vertical distance. Since the size
is about equal to the local depth, significant quantifies of sand are suspended over most of the depth in the surf zone.
Since breaking waves suspend the sédiment, the amount suspended is
partly determined by breaker type. Data from Fairchild (1972, Figure 5),
show that spilling breakers usually produce noticeably lower suspended
sédiment concentrations than do plunging breakers. See Fairchild (1972)
and Watts (1953) for field data; Fairchild (1956 and 1959) for lab data.
Typical suspended concentrations of fine sand range between 20 parts per
million and 2 parts per thousand by weight in the surf zone, and are about
the same near the ripple crests in the offshore zone.
Studies of suspended sédiment concentrations in the surf zone by
atts (1953) and Fairchild (1972) indicate that sédiment in suspension in
the surf zone may form a significant portion of the material in longshore
ransport. However, présent understanding of sédiment suspension, and
e practical difficulty of obtaining and Processing sufficient suspended
sédiment samples hâve limited this approach to predicting longshore trans4.513 Profiles. Profiles are tw ahow élévation varies with die* W° 1JJensional vertical sections showing
4-18) are usually «asJJed
?aStal
(See Figs. 4-1 and
shelf profiles, nearshore profiles^o/h t0, the shoreli"e> and may be
shore and beach profiles a™ in* *
beach profiles. Changes on nearthe interprétation of littoral
ated’
are highly important in
of combined beach and nearshorpP C^feS‘. The measurement and analysis
lng studies of littoral processes™1 1 leS is a maj°r Part of most engineer4-54
currents in the onshore-offshore direction. It is possible to hâve ripple
forms moving shoreward while residual currents above the ripples carry
suspended sédiment clouds in a net offshore direction. Information on the
transport of sédiment above ripples is given in Bijker (1970), Kennedy and
Locher (1972), and Mogridge and Kamphuis (1972).
b. Surf Zone. The stress of the water on the bottom due to turbulence and wave-induced velocity gradients moves sédiment in the surf zone
with each passing breaker crest. This sédiment motion is both bedload
and suspended-load transport. Sédiment in motion oscillâtes back and
forth with each passing wave, and moves alongshore with the longshore
current. On the beach face, the landward terminât ion of the surf zone,
the broken wave advances up the slope as a bore of graduaily decreasing
height, and then drains seaward in a graduaily thinning sheet of water.
Frequently, the draining return flows in guilies and carries sédiment to
the base of the beach face.
In the surf zone, ripples cause significant sédiment suspension, but
here there are additional eddies caused by the breaking wave. These eddies
hâve more energy and are larger than the ripple eddies. The greater energy
suspends more sand in the surf zone than offshore. The greater eddy size
mixes the suspended sand over a larger vertical distance. Since the size
is about equal to the local depth, significant quantifies of sand are suspended over most of the depth in the surf zone.
Since breaking waves suspend the sédiment, the amount suspended is
partly determined by breaker type. Data from Fairchild (1972, Figure 5),
show that spilling breakers usually produce noticeably lower suspended
sédiment concentrations than do plunging breakers. See Fairchild (1972)
and Watts (1953) for field data; Fairchild (1956 and 1959) for lab data.
Typical suspended concentrations of fine sand range between 20 parts per
million and 2 parts per thousand by weight in the surf zone, and are about
the same near the ripple crests in the offshore zone.
Studies of suspended sédiment concentrations in the surf zone by
atts (1953) and Fairchild (1972) indicate that sédiment in suspension in
the surf zone may form a significant portion of the material in longshore
ransport. However, présent understanding of sédiment suspension, and
e practical difficulty of obtaining and Processing sufficient suspended
sédiment samples hâve limited this approach to predicting longshore trans4.513 Profiles. Profiles are tw ahow élévation varies with die* W° 1JJensional vertical sections showing
4-18) are usually «asJJed
?aStal
(See Figs. 4-1 and
shelf profiles, nearshore profiles^o/h t0, the shoreli"e> and may be
shore and beach profiles a™ in* *
beach profiles. Changes on nearthe interprétation of littoral
ated’
are highly important in
of combined beach and nearshorpP C^feS‘. The measurement and analysis
lng studies of littoral processes™1 1 leS is a maj°r Part of most engineer4-54