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Event driven sediment mobility on the inner continental shelf of Onslow Bay, NC

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Jeffrey A. Marshall (Creator)
Institution
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Advisor
Lynn Leonard

Abstract: This study seeks to further constrain near-bottom hydrodynamic current conditions required to mobilize native sediments on a high-energy sediment starved shelf environment and link these data to changes in sidescan sonar imagery of the inner-shelf environment of Onslow Bay, NC. A bottom-mounted upward looking Acoustic Doppler Current Profiler (ADCP) deployed at the OB3M study site on the lower sand flat adjacent to a low-relief marine hardbottom recorded hourly flow velocity profiles from a depth of 17.7 m. The lower sand flat is composed of two dominant surficial lithofacies consisting of patchy, but well-defined areas of well sorted fine sand and poorly sorted coarse grained material. A dual frequency high-resolution sidescan sonar system was utilized to biannually survey a 5.5 by 3.7 km area encompassing the OB3M site between March 2002 and October 2003. Mosaic imagery obtained from these surveys were used to document seasonal changes in bottom characteristics in response to twenty-three identified sediment mobility events. Measurable contributions from semidiurnal tidal flows, mean current flows dominated by subtidal wind-generated currents, as well as wave-generated oscillatory motions in the near-bottom layer during storm and non-storm conditions have been identified for the nineteen-month period bracketing two tropical storm seasons off the North Carolina coast. Calculated critical shear velocity values due to the combined effects of waves and currents indicate that the fine-grained sand fraction was mobile more than 66% of the period, frequently as incipiently suspended load and bedload, and rarely as fully suspended load. Quantitative analysis of sidescan sonar imagery demonstrate that even though hydrodynamic conditions favor mobilization of fine sands, the gross morphology and sediment distribution at this inner-shelf site remained relatively unchanged after the occurrence of several commonplace high-energy events. Seasonal sedimentation patterns, however; were found to be substantially altered after the passage of Hurricane Isabel within 225 km of the study site. Evidence from this study reveals that over the nineteen-month study period at this discrete site, the combined effects of typical high-energy events had little effect on the net distribution of bottom sediments, yet a singular extreme event was found to actively modify seabed sedimentation processes.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina at Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Keywords
Sedimentation analysis--North Carolina--Onslow Bay, Sedimentation and deposition--North Carolina--Onslow Bay, Sediments (Geology)--North Carolina--Onslow Bay
Subjects
Sediments (Geology) -- North Carolina -- Onslow Bay
Sedimentation and deposition -- North Carolina -- Onslow Bay
Sedimentation analysis -- North Carolina -- Onslow Bay