P. Wang et al. / Coastal Engineering 46 (2002) 175211
201
Fig. 21. Vertical profiles of rms velocity at various cross-shore locations for both spilling and plunging cases. Numbers in the parentheses
indicate the still-water depth.
using a formulation similar to Eq. (8). It is worth
column, where the sediment concentration is the
emphasizing that Ca_water_column provides one estimate
greatest, could cause an underestimation of average
over a rapidly upward-decreasing trend that spans up
values.
to 4 orders of magnitude. Caution should be exercised
Although the suspended sediment concentrations at
when using the depth-averaged sediment concentra-
the plunging breaker line above 3 cm from the bed
tions. Nevertheless, averaged values over the meas-
were more than one order of magnitude greater than
ured part of the water column are used in the
that in other parts of the surf zone (Fig. 17), the
following to demonstrate a general cross-shore varia-
Ca_water_column was only three to five times greater
tion of sediment concentration in the surf zone.
due to the similar near-bed concentrations. The
Although the bottom sensor was deployed directly
Ca_water_column across most of the surf zone remained
on the bed, it functioned mainly as an elevation
relatively uniform, ranging from 0.5 to 1.2 g/l for both
reference for the rest of the sensors, instead of
spilling and plunging cases (Fig. 22). A similar gen-
measuring sediment concentration at the bed. The
eral magnitude of surf-zone suspended sediment con-
concentration obtained from the bottom sensor was
centration, 0.5 to 1.0 g/l, was obtained by Kana (1979)
questionable and was not included in the depth
and Zampol and Inman (1989) in the field using
averaging. The Ca_water_column included positions from
instantaneous bottle samplers. The Ca_water_column at
1 cm above the bed and spanned over the range of
the spilling breaker line were approximately 1.5 g/l; at
measurements. Near the water surface above the wave
the plunging breaker line, the Ca_water_column were
trough, the measurements have greater uncertainty
approximately 2.5 g/l (Fig. 22). Across most of the
due to the varying water depth and frequent exposure
mid-surf zone, the Ca_water_column was slightly greater
of the sensors. No extrapolation toward the water
under spilling breakers than under the plunging break-
surface was conducted. The Ca_water_column discussed
ers. The reason for this is not clear. Sediment concen-
trations measured at an elevation approximately equal
in the following did not include the top 20% to 35%
to 20% of the water depth from the bed matched the
of the water depth. Given the rapidly upward-decreas-
Ca_water_column values (excluding the bottom 1 cm and
ing sediment concentration, the exclusion of the upper
20% to 35% water column could result in consider-
the top 20% to 35% water depth) reasonably well
able overestimation of average values. On the other
hand, the exclusion of the bottom 1 cm of water
still-water depth, where representative current meas-
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