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