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

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.