ERDC/CHL CETN II-43
June 2000
near the OBS in the same cross-shore and vertical position so waves reach both sets of
instruments at nearly the same phase. For the STORM experiments, all sensors are sampled at
16 Hz. Any experiments should include a similar high frequency for sampling due to the rapidly
changing nature of sand concentration and velocity during a storm wave period. Other sensors
should include a downward-looking sonic altimeter to provide an estimate of the bed elevation
relative to the instruments and a pressure sensor to record local water level fluctuations.
OBS: OBS operate on the principle that concentration can be related to the scattering of a beam
of light emitted into the water column. The volume over which the concentration is estimated is
on the order of 1 cm3. As will be described later, OBS measurements must be calibrated for the
sediments being measured to avoid incorrect interpretations of the light scattering. Calibration
should be performed just prior to measurements to assure reliability. Suspended sediment
samples collected during the storm can be used to verify that the composition has not changed
from the composition for which the OBS were calibrated.
EMCM: EMCMs measure water velocity by relating water flowing past a sensor to an
electromagnetic response of that sensor. This relationship relies on calibration to provide an
offset value. If this offset value drifts significantly between calibrations, data quality can be
affected. Therefore, the relationship between sensor reading and velocity must be checked
periodically. An EMCM has four nodes and measures bidirectional currents, which can be
expressed in cross-shore and longshore directions. As will be described later, EMCMs can be
affected by interference from other electronic devices and precautions must be undertaken to
assure data quality.
Sonic Altimeter: A single sonic altimeter was used during the STORM experiments to identify
the location of the sediment bed relative to each instrument. The altimeter emits a 1 MHz
acoustic signal sampled at 16 Hz and measures the bottom position from the reflected signal.
Although the altimeter or other interface-detecting instruments operate fairly well during storm
periods, the interface may not be detectable during the most high-energy periods, particularly in
the area near the swash zone. The sediment bed/water interface becomes less defined during
high-energy periods due to fluidization and sheet flow movement, which may make it difficult to
define an interface. In addition, air bubbles between the altimeter and the bed will result in false
echoes.
DATA PROCESSING: Processing, analysis, and evaluation of the collected data must be
performed to ensure correct estimates of sediment transport. Processing of OBS, EMCM, and
altimeter data are described in this section.
solids in the water column, regardless of the source of reflection (sand, fine particles, organic
material, biological fouling, aquatic organisms, etc.). Assuming that fine particles that result in
turbidity are well mixed and do not settle rapidly and that sand particles are suspended and settle
out intermittently, a defined level of background turbidity can be established and subtracted from
the indicated OBS signal (Beach and Sternberg 1988). The method for estimating the
background turbidity involves the application of a moving window for which a 10th percentile
value of OBS signal is determined. This 10th percentile represents the concentration for which
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