Bathymetry and Bottom Characteristics in the Surf Zone During
SandyDuck
Dickson, P.J., Gallagher, E. L., and Drake, T.G., 1999, Bathymetry
and bottom characteristics in the surf zone during SandyDuck:
EOS Trans. AGU, 80 (17), Spring Meeting Suppl., S194. (Special
session: Bedform Patterns and Processes)
Digital sidescan sonar imagery and detailed profiles of bathymetry
obtained using an array of acoustic altimeters were collected
daily for over one month in the nearshore during the SandyDuck
97 field experiment at the Army Field Research Facility (FRF)
at Duck, North Carolina. Seven 1 MHz sonic altimeters mounted
on the FRF's amphibious CRAB sampled seafloor elevations at 48
Hz. After altimeter positions are computed from measurements of
CRAB attitude and RTK-GPS, the vertical resolution of the altimeters
is about 5 cm. A dual frequency (100 MHz and 500MHz) digital sidescan
sonar having about 10 cm horizontal resolution was mounted on
the CRAB a fixed distance of about 1 m above the seafloor. Data
were collected along 20 cross-shore profile lines about 25 m apart
extending from the beach to 6 m water depth. Sidescan images having
nearly 100% overlap were obtained seaward of a sand bar at 1 m
depth under low-energy conditions; at relatively higher energy
conditions sonar performance degraded significantly due to bubbles
and suspended sediment in the water column. Integration of sonar
and altimetric observations provides a unique estimate of bottom
characteristics, including geometric roughness and gross sedimentology,
over large, contiguous areas of the shallow-water environment.
Such integration also helps resolve ambiguities that typically
arise during interpretation of side-scan imagery. For example,
areas of high acoustic backscatter in sidescan images may correspond
to bedforms or to differences in bed material.
While wave ripples and megaripples were observed on the seaward
side of the sand bar in sidescan sonar images and altimeters,
correlation between imaged ripples and altimeter data are often
low. Possible causes of the low correlation could be suspended
sediment masking the sidescan image, ripples too small for the
altimeters to resolve, or changes in the bed between sidescan
images and altimeter passes (typically 15 to 20 minutes). Sonar
image degradation can be difficult to detect; on the other hand,
excess suspended sediment or bubbles are typically much more obvious
in the altimeter data. Correlation between sonar imagery and altimeter
data is sometimes good. For example, during a period when significant
wave height was greater than 1 m and wave period less than 10
s, a pattern of long-crested, low-amplitude ripples is observed
in both datasets. Mean ripple spacing is 5 m and maximum ripple
height is a several cm, and ripple crests were oriented transverse
to wave propagation.Examples of bedforms in the nearshore will
be examined for conditions resulting in high correlation between
the two data sets, and possible causes of low correlation between
the two measurement types will be investigated.
Supported by the Coastal Dynamics Program of the
Office of Naval Research.