ERDC/CHL CETN II-43
June 2000
10 percent of the concentration measurements within the window limits are less than. The 10th
percentile concentration, or the background turbidity, is then subtracted from the signal to
provide a concentration that is assumed to represent the concentration of sand in the water
column.
As with the EMCM, the OBS must be regularly calibrated to assure data quality. Calibration is
provided by measuring OBS response to a known quantity of sand well mixed into a known
quantity of water. One shortcoming of OBS is that they are calibrated to a known grain size
distribution (usually sediment collected from the location where measurements are made).
Different sizes and shapes of material will scatter light differently. This will cause drift in signals
if the grain size distribution changes during deployment. This is a problem when measuring
along a cross-shore transect during storms because the material will be sorted during the storm,
leaving more coarse grain material in the nearshore and finer material offshore. For example, it is
known that fine particles, which tend to have larger surface area to mass ratios than sands, will
exaggerate a signal calibrated to fine sand. This causes problems after the offset is removed
because the entire signal has been exaggerated, possibly resulting in overestimate of sand
concentration. Wherever possible, in situ samples should be collected during deployment to
indicate if this situation exists.
During deployment periods, the OBS signal may sometimes contain spurious signals that are not
representative of the suspension and settling of sand or have signal magnitudes inconsistent with
the forcing conditions measured. Small plumes of fine sediments from other sources, aquatic life,
or suspended debris may be explanations for anomalous signals. The OBS signals must be
further analyzed after removal of offset values to eliminate signals inconsistent with the
suspension of sandy material. Offshore, this can often be done using screening tools, but
nearshore, conditions are variable enough that analysis for these spurious signals must be
performed visually.
At elevation of approximately 1 m above the sediment bed, there will be minimal sand signal in
the OBS time series during storms. OBS exhibiting this behavior should not be included in the
longshore transport estimates. Noise remaining after processing to remove background turbidity
will dominate these signals and lead to exaggerated transport estimates in the upper water
column. In addition, wave movement will result in some upper OBS protruding above the
air/water interface during a portion of some deployments. The signals from these OBS are
evident by their non-sand nature and should not be used in transport estimates. Buried OBS are
evident by their full-scale, flat-line signal. The buried portion of the signal should be omitted
from transport estimates.
Processing EMCM Signals: Currents serve to transport suspended sediments and temporal and
spatial variations in currents must be defined to accurately quantify the transport of suspended
sediments across the surf zone. At a particular data-collection station, a vertical distribution of
current meters is used to measure velocity at various elevations above the bed to define spatial
variation of current speed in the vertical direction. A high sampling frequency (~16 Hz) is
needed to define the temporal variation in the currents. Quality control checks on the collected
data are required to ensure that sediment transport is estimated correctly.
6
Previous Page
