Nearshore Sedimentary Structures
Investigators
Thomas Drake, J.Bailey Smith
Funding Source
Geologic Analysis of Shelf Beach Interchange Work Unit,
US Army Corps of Engineers, Coastal Geology and Geotechnology Program
Objectives
The principal objective is to link sediment size-sorting mechanisms to fluid dynamical
measurements using sedimentological information preserved in cores. Sediment transported by
waves and currents is typically sorted by size and density, producing distinctive geometrical
patterns or structures that are primarily manifested by variations in grain size. Cores of
sedimentary deposits taken adjacent to measurements of near-bed fluid motion and bed elevation
will be used to study the relationship between wave climate, sediment transport processes,
bathymetric evolution and the resulting sedimentary deposits in the most active portion of the
profile envelope. Processes on two disparate length scales will be investigated. The primary
objective of the small-scale studies (mm to m scale) is to examine vertical size-segregation of
grains by two bedload transport mechanisms: sheet flow transport and megaripple migration.
The primary objective of the large-scale studies (m to 100's of m) is to determine the dominant
transport mechanism(s) during bar migration. Secondary objectives are to determine the role, if
any, of rip currents in effecting the offshore transport of coarse sediment; and to resolve
ambiguities in sediment transport direction, which may arise in the interpretation of point
measurements of bed elevation.
Approach
Cores will be taken using a number of techniques by divers and from the CRAB,
depending on conditions and required sedimentological information. Detailed grain-size analyses
and measurement of the geometry of such sedimentary structures as laminae and cross bedding
will be used to study the relationship between descriptors of the fluid motion, bed elevation, and
sedimentology.
Preliminary Results
We obtained over 60 cores in close proximity to the SIO
instrument transect, and have studied selected cores and their
relationship to the hydrodynamic and bed elevation data. We
presented the following presntation 5-8 November 1995 at he
Geological Society of America Annual Meeting in New Orleans, LA.
DO DETAILED HYDRODYNAMIC AND SEDIMENTOLOGIC DATA AID
PREDICTION OF NEARSHORE STRATIGRAPHY? EXAMPLES FROM THE DUCK94
NEARSHORE FIELD EXPERIMENT
- J. Bailey Smith, U.S. Army Corps of Engineers, Coastal
Engineering Research Center, Vicksburg, MS 39180
- Thomas G. Drake, Department of Marine, Earth, and
Atmospheric Sciences, North Carolina State University, Raleigh,
NC 27695
- Edith L. Gallagher, Center for Coastal Studies, Scripps
Institute of Oceanography, La Jolla, CA 92093
- Steve Elgar, EECS, Washington State University, Pullman, WA
99164
Abstract
Nearshore vibracores and box cores collected during DUCK94
Nearshore Field Experiment (August-October 1994) near Duck,
North Carolina, offer a unique opportunity to study sedimentary
structures and stratigraphy and their relation to detailed
bed-elevation and hydrodynamic data. Most cores were taken
within a few meters of a cross-shore transect of wave, current,
and bed-elevation sensors extending about 800 m from the
shoreline to 8 m water depth. Significant bathymetry changes
occured during the experiment. In particular, winds from a
mid-October nor'easter produced 4.5 m waves and longshore
currents exceeding 1 m/s. During the storm, a single linear
summer bar migrated 100 m offshore, eroding about 1.5 m from
its former position. Cores penetrating newly-deposited bar
sands do not display strong evidence (i.e. crossbeds) of
migrating bedforms, despite bed-elevation data and ancillary
observations indicating that megaripples were present.
Synthetic stratigraphies generated from bed elevation
time-series were used to construct histograms of strata
thickness for comparison with similar histograms from cores.
Synthetic thickness histograms show greatly increased
frequencies for depositional events creating strata less than 1
cm thick, and reduced frequencies for thicker strata, relative
to data from cores obtained both by visual inspection and
x-ray. Hypotheses for these discrepancies and implications for
statigraphic studies of nearshore barrier island deposits will
be discussed.
For additional information on this project:Dept.
of Marine, Earth, and Atmospherical Sciences at North Carolina
State University
Collaboration: Guza, Elgar, Stauble, Birkemeier, others
For a list of addresses of the investigators involved in Duck
94 click here.