Figure 4-22, from Inman (1957) and including data from two earlier
studies, shows the velocity needed to start motion in a sédiment bed of
a given grain size. These results are in general agreement with other
studies relating critical velocity to grain size. Also shown in this
figure are maximum velocities above which ripples tend to be smoothed
off, in qualitative agreement with conditions for bed forms in unidirectional flows. (Southard, 1972.)
From Figure 4-22, it appears that maximum wave-induced bottom velocities between 0.4 and 1.0 foot per second are sufficient to initiate sand
motion under waves.
In field studies, Inman (1957) found that ripples are
always présent whenever computed maximum velocities exceed 0.33 foot per
second, and Cook and Gorsline (1972) report ripples above a velocity range
of 0.5 to 0.6 foot per second. Equation 4-19 can be used to détermine the
combination of wave conditions and depth that produces any given critical
value of U/?7(2a; at the bottom. Figure 4-23 shows the relation between depth
and wave height, for given wave péri ods, for a critical velocity, Umax =
0,5 foot per second.
4.523 Seaward Limit of Significant Transport» Figures -
moye bot_
and a knowledge of offshore wave climate ^uggest ta
hs Qf 10Q tQ 400
tom sédiments over most of the c°n^e* |
Géologie studies indicate
feet or more) during some time of the ye .
al sediments over
that fine material has been winnowed fro
question is,
-».».,t.
«bat is the maximum depth to which the
aiscusses field data that
tant in Coastal engineering? This section
supply partial answers to this question.
a. Bathymetry. Dietz (1963) and others ^^r/o^h^-^
nearshore sands, smoothing out irregular
nuasi-eouilibrium surface
offshore transport. This smoothing pro uces
contours, nearly
in the nearshore zone which forms relatively straight contours,
parallel to the shoreline.
Most bathymétrie charts with closely spaced contours as illu
by Figures 4-24 and 4-25 show that isobaths near the shore r
p
to the shoreline; further offshore, the contours may indicate Une
shoals (Duane, et al., 1972), or other irregular submarine eatures.
Following the idea of Dietz (1963), the depth below which shore-^^
Parallel contours give way to irregular contours is assum
transition between the nearshore zone where sands are moved by the
in significant quantifies and the offshore zone where sand is moved
‘fesser quantifies. Possible exceptions to this shore-P*”“!^f°n^d
e are the contours around river deltas and in ets, or
&es occur in the nearshore zone.
4-65
studies, shows the velocity needed to start motion in a sédiment bed of
a given grain size. These results are in general agreement with other
studies relating critical velocity to grain size. Also shown in this
figure are maximum velocities above which ripples tend to be smoothed
off, in qualitative agreement with conditions for bed forms in unidirectional flows. (Southard, 1972.)
From Figure 4-22, it appears that maximum wave-induced bottom velocities between 0.4 and 1.0 foot per second are sufficient to initiate sand
motion under waves.
In field studies, Inman (1957) found that ripples are
always présent whenever computed maximum velocities exceed 0.33 foot per
second, and Cook and Gorsline (1972) report ripples above a velocity range
of 0.5 to 0.6 foot per second. Equation 4-19 can be used to détermine the
combination of wave conditions and depth that produces any given critical
value of U/?7(2a; at the bottom. Figure 4-23 shows the relation between depth
and wave height, for given wave péri ods, for a critical velocity, Umax =
0,5 foot per second.
4.523 Seaward Limit of Significant Transport» Figures -
moye bot_
and a knowledge of offshore wave climate ^uggest ta
hs Qf 10Q tQ 400
tom sédiments over most of the c°n^e* |
Géologie studies indicate
feet or more) during some time of the ye .
al sediments over
that fine material has been winnowed fro
question is,
-».».,t.
«bat is the maximum depth to which the
aiscusses field data that
tant in Coastal engineering? This section
supply partial answers to this question.
a. Bathymetry. Dietz (1963) and others ^^r/o^h^-^
nearshore sands, smoothing out irregular
nuasi-eouilibrium surface
offshore transport. This smoothing pro uces
contours, nearly
in the nearshore zone which forms relatively straight contours,
parallel to the shoreline.
Most bathymétrie charts with closely spaced contours as illu
by Figures 4-24 and 4-25 show that isobaths near the shore r
p
to the shoreline; further offshore, the contours may indicate Une
shoals (Duane, et al., 1972), or other irregular submarine eatures.
Following the idea of Dietz (1963), the depth below which shore-^^
Parallel contours give way to irregular contours is assum
transition between the nearshore zone where sands are moved by the
in significant quantifies and the offshore zone where sand is moved
‘fesser quantifies. Possible exceptions to this shore-P*”“!^f°n^d
e are the contours around river deltas and in ets, or
&es occur in the nearshore zone.
4-65