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P. Wang et al. / Coastal Engineering 46 (2002) 175211
also reported from field studies (e.g., Beach and
The magnitude of the temporal variations did not vary
Sternberg, 1992; Miller, 1998, 1999; Puelo et al.,
significantly with sensor elevation throughout the
2000). Two examples, one each from the spilling
measured water column. The standard deviations at
(upper panel) and plunging (lower panel) breaker
other cross-shore locations in the surf zone were of
zones at 10.1 m from shoreline, are illustrated in
similar magnitudes (Table 3). The two landwardmost
gages are not included in Table 3 due to the spikes
are shown. Sediment suspension under breaking
probably caused by exposure of the sensors and
waves was largely episodic (Fig. 10). Intermittent
intensive air-bubble entrainment (Hamilton et al.,
events of sediment suspension dominated the tempo-
2001). Similar ranges of variation were obtained after
ral variation patterns. Sediment concentration varied
removal of the spikes.
over a greater range under the plunging breakers than
For the plunging case examples, one standard
under the spilling breakers with greater sediment
deviation was about 58% of the mean, indicating
concentration measured high in the water column
slightly more relative variation than the spilling case
due to the much more active sediment suspension
caused by the plunging breakers. Temporal variations
case, the magnitude of the variations was not signifi-
of suspended sediment concentration in the mid-surf
cantly influenced by vertical elevation across the mid-
zone for both cases showed similar patterns as
surf zone. Considerably greater variations were meas-
observed at the spilling breaker line.
ured near the plunging breaker line (Table 3). This
Close examination of the time series of sediment
could be influenced by the intense turbulence and also
concentration at the different elevations indicates that
reduced sensor performance due to the substantial air
the variations were not exactly in phase although
entrainment at the plunging breaker line. Reversal of
qualitatively correlated suspension events at different
longshore-current direction occurred only occasion-
levels were evident at times. However, large concen-
ally (Fig. 9).
tration measured at one level sometimes did not have
directly corresponding high values at other levels. An
4.2. Temporal variations and periodicities of sediment
example is shown in Fig. 10 (upper panel). The large
concentration
concentration at 3 cm above bed measured between
208 and 214 s did not have corresponding concen-
Great temporal variations of suspended sediment
tration jumps at other elevations. At the plunging
concentration, ranging over several orders of magni-
breaker line, greater or similar sediment concentration
tude, were measured. Similarly large variations were
was occasionally measured at a higher elevation when
compared to data from a lower elevation. An example
Table 3
can be found in the lower panel of Fig. 10 between
Temporal variation, as indicated by one standard deviation, of
292 and 295 s, when concentration measured at 21 cm
longshore current for the spilling and plunging cases
above bed was similar or greater than that measured at
Spilling
Plunging
7 cm. A constant phase shift among concentrations at
breaker case
breaker case
different elevations could not be identified from visual
Percentage of the mean value
inspection.
49.4 F 3.9a
51.8 F 6.9b
ADV3
(4.1 m from shoreline)
Visual observations during the experiments indi-
ADV4
(5.7 m from shoreline)
54.3 F 6.4
61.0 F 5.4
cated that sediment suspension events were closely
ADV5
(7.1 m from shoreline)
44.2 F 2.8
59.5 F 7.7
related to the detailed spatial patterns of wave break-
ADV6
(8.5 m from shoreline)
42.4 F 3.2
61.3 F 8.1
ing. This was particularly apparent for plunging
ADV7
(10.1 m from shoreline)
48.6 F 3.7
92.8 F 17.3
ADV8
(11.6 m from shoreline)
54.1 F 11.7
95.9 F 14.7
breakers. A cloud of sediment was suspended into
Measurements were conducted at 1/3 still-water depth from the
the water column at the plunge point and then
bottom.
dispersed across shore. Horizontal advection seemed
a
Values represent the average of nine measurements at different
to play a significant role in dispersing the sediment
longshore locations, and the alongshore variation.
suspended at the plunging point. The locations of the
b
Values represent the average of four measurements at different
breaking point for irregular waves were influenced by
longshore locations, and the alongshore variation.
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