ERDC/CHL CETN II-43
June 2000
Concentration Profile
4
3.5
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
Concentration, g/L
Figure 3. Example concentration profile from October
1997 STORM experiment
Temporal variation of sand concentration and orbital velocity occurs on short time scales, at a
small fraction of a wave period. Sand concentrations peak with the passage of each wave cycle,
sometimes changing from near zero to several hundred mg/L and back to near zero during a half
wave cycle. Current direction and magnitude also change continually through a wave cycle,
reversing direction and reaching a maximum every half cycle. To resolve the transport of
suspended sand by orbital wave motion, both sand concentration and orbital velocity must be
sampled at high frequency, typically at greater than 2 Hz.
Without the deployment of multiple instrument arrays, measuring across the nearshore requires
several hours to collect data at each station and to move from one station to the next. Fortunately,
wave and current conditions during a storm are generally quasi-steady over the period of an hour
or more. However, tidal currents and elevations at midtide may be significant at the time scale of
an hour. To minimize tidal variations, data collection across the surf zone was limited to the
periods of high and low tide, when tidal currents and elevations change minimally.
EQUIPMENT: The instruments described in this section were used during the STORM
experiments. Other instruments are available to measure the same quantities. The primary
sensors for measuring sediment flux are a vertical array of OBS (Downing, Sternberg, and Lister
1981) or other instrument to measure sediment concentration and electromagnetic current meters
(EMCM) or other instrument to measure flow velocities. An example of a vertical array used in
the October 1997 STORM experiments is shown in Figure 3. This array consisted of 10 OBS and
four EMCMs attached updrift of a support frame. The OBS are vertically distributed so that most
of the sensors are positioned within 1.0 m of the ocean bottom. The four EMCMs are located
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