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Variability in beach topography and forcing along Oak Island, North Carolina

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

Abstract: This study analyzed variability in beach topography along Oak Island, North Carolina as well as local wave, water level, and wind conditions over a two year period to identify patterns of variability in shoreline change and potential processes forcing these patterns. Empirical Orthogonal Function (EOF) analysis was used to identify dominant modes of variability in beach topography along the western 17 km of Oak Island between June 2004 and June 2006. This analysis allows identification of separate patterns of variability in time series data, each with a specific spatial and temporal signature, and each explaining a specific percent of variance in the data. Kinematic GPS was used to collect beach topography data bimonthly at 12 shore-perpendicular beach profile transects (dune to MLW) spaced at 0.5 - 2.0 km intervals along the beach. The EOF analysis was then performed on a vectorized time series of beach surface elevations for the entire study reach measured during the 13 separate surveys to identify both crossshore and along-shore variability. Wave, water level and wind data were compiled from a number of local stations to identify potential processes forcing shoreline variability. A number of averaging windows were applied to these parameters ranging from one day before each survey to the entire period between surveys. These parameters were then correlated against each mode to identify significant relationships and determine whether these processes were more important over shorter or longer time periods before each survey. Results of the EOF analysis include two dominant patterns of variability in beach topography which combine to explain 63% of all variability in the data. The first mode, explaining 44% of variance in the time series, showed large scale shoreline retreat of the entire study reach between August and December 2005. This mode was determined to be forced by storm surge and wave activity during Hurricane Ophelia and two successive extratropical storms during that period. The second mode, explaining 19% of variance, reflected seasonal cross shore variability of the beach profile with accretion during the summer and fall and erosion during the winter. A cross-shore pivot point between seasonal profiles described by this mode was identified between MSL and MLW for a majority of the transects. Transect 1 fluctuated out of phase with the rest of the transects in this mode indicating potential seasonal flux in along-shore sediment transport which could be related to slight shifts in wave direction or inlet dynamics. This mode strongly correlated to seasonal variability in wave height and energy measured over one to four week averaging windows before each survey. This suggests seasonal forcing of the beach profile by seasonal changes in wave climate. Strong negative correlations were identified in the second mode between increased water levels the day before each survey and landward transport of sediment along transects 2-12. This relationship seems counterintuitive but the strength of correlation indicates significance. Finally, transects 1 and 2 in the vicinity of Lockwood’s Folley Inlet exhibited the highest vertical and crossshore variability within the study area, supporting expansion of the currently defined Inlet Hazard Area (IHA) to the newly proposed IHA which would include transect 2.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Keywords
Coast changes--North Carolina--Oak Island, Environmental monitoring--North Carolina--Oak Island, Shorelines--North Carolina--Oak Island, Topographical surveying--North Carolina--Oak Island
Subjects
Topographical surveying -- North Carolina -- Oak Island
Environmental monitoring -- North Carolina -- Oak Island
Shorelines -- North Carolina -- Oak Island
Coast changes -- North Carolina -- Oak Island