Dynamics of Bald Head Island Beach Topography

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Dynamics of Bald Head Island Beach Topography
Helena Mitasova Department of
Marine, Earth and Atmospheric Sciences, NCSU,
Raleigh, T. Drake, MEAS NCSU, R. Harmon, US Army
Research Office, D. Bernstein , NCSU,
Geodynamics Research supported by National
Research Council, Army Research Office and Bald
Head Island Conservancy
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Research objectives - explore the possibilities
to gain new insights into short-term 3D coastal
topography evolution by combining modern mapping
technologies with Open source GRASS GIS -
develop methodology for cost effective
short-term monitoring and analysis of topographic
change to support sustainable coastal
management - quantify and compare the impact of
hurricanes and anthropogenic changes on evolution
of beach morphology at Bald Head Island, NC
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Modern Mapping Technologies
Bathymetry multibeam and conventional
sonar (Geodynamics llt., J.McNinch, H.Miller)
Beach topography RTKGPS (D.Bernstein NCSU,Geodyna
mics)
Coastal topography LIDAR USGS/NOAA/NASA
ATM-II, EAARL

• Challenges massive data sets, oversampling,
  noise
• complex surfaces with subtle features (dunes,
  berms)
• heterogeneous coverage

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Surfaces from LIDAR and GPS data
Sampling points Grey LIDAR Black RTK-GPS
LIDAR surface
RTKGPS surface
Cape Fear 1997, 2002
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RTK GPS improved survey design
isotropic RST
anisotropic RST
cros-shore profiles shoreline survey pattern
250m
cros-shore longshore profiles survey pattern
RST with anisotropic tension
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RTK GPS survey design accuracy
Binned LIDAR data were sampled by RTK-GPS survey
points. DEM was then interpolated and compared
with the LIDAR data 4643 grid points for 5m,
13108 grid points for 3m resolution.
survey pattern / RST gridding no. of
points MAE m RMSE csh profiles / isotropic
179 / 55 0.73
0.78 csh profiles / anis. optim.
179 / 55 0.43
0.36 lsh profiles / anis., optim.
990 / 757 0.27 0.12 cshlsh
profiles / anis., optim. 1169 / 789
0.21 0.08 same at 3m resolution
1169 / 988 0.19
0.07 cshlsh profiles / anis., opt.
subset 0.16 0.05
DEM approximated from cross-shore profiles is the
least accurate. Long-shore profiles, anisotropy
and optimized approximation parameters can
significantly improve the accuracy of DEM.
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Bald Head Island
Human and natural impacts on evolution of
topography and bathymetry 1996
nourishment 1998, 1999 hurricanes Bonnie,
Floyd 2001 - channel deepening and re-alignment,
beach nourishment 2003 - Isabel
ATM LIDAR 1997-2000, EAARL LIDAR
2003 USGS/NOAA/NASA RTKGPS 2001-03 supported
by BHIC
old channel
elevation m
Single and multi beam sonar 2000, 2001, 2002
10m resolution bathy-topo model from multiple
sources
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South Beach evolution 1997-2000
Overlayed 1997 and 2000 LIDAR surfaces central
section is relatively stable, rest erodes while
changing its shape and moving landwards
Annual sand loss rate 3500 m3/ha Shoreline
erosion rate up to 30ft/year 15 of sand was
deposited behind the foredune landward movement
West Center East
convex -gt concave
2000 zgt0m zlt0m
stable pivot area
stable
concave -gt convex
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Slope and curvature change
Severely eroding area approximated and analyzed
by RST
Homes
1998
Slope
Profile curvature
concave convex
2000
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Change after nourishment
Dec. 2001 1 million m3 of sand added
May 2002
Dec 2002
Sep 2002
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Change after nourishment
March, May, August 2003
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Change after nourishment profiles
LIDAR 2000 RTKS Dec. 2001 May 2002
September 2002
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Volume and change
time period loss
gain loss rate
m3
m3 m3/ha.year 1 y 1997
- 1998 160000 42000 4400
2 y 1998 - 2000
254000 48000 3500
3 y 1997 - 2000 376000
65000 3500
2001
1,000,000 5 mo Dec01 - May02 220000
2000 13200 4 mo
May02 - Sep02 162000 80000 12100
4 mo Sep02 - Dec02
108000 107000 8100 Dec02
Mar03 Mar03 - May03
May03 - Sep03
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Dynamic model of BHI
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Dynamic model of BHI
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Dynamic model of BHI
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Conclusions I
Combination of modern mapping techniques with
Open source GIS provides unique insight into 3D
coastal topography evolution at high spatial and
temporal resolution. GIS based analysis and
visualization allows us to quantify the observed
changes (elevation, shoreline, volume, slope and
shape) and evaluate effectiveness of
stabilization measures. The developed methodology
is being further enhanced and applied to other
areas.
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Conclusions II
Bald Head Island Analysis based on LIDAR and RTK
GPS data shows systematic, spatially variable
erosion of the beach accompanied with beach shape
change. After renourishment the rates increased
in the west section and the beach became more
stable in the east. Rotation behavior is similar
to the one observed at other small islands west
and east end erode at different rates causing
rotation that tends to reverse (latest reversal
in late 90ies?)
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Future
Analysis of the entire area as a single system -
bathymetry (fate of eroded sand back to channel,
CF shoal, sandbars ?) - new LIDAR survey -
modeling (SBEACH, DELFT3D)
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Cape Fear elevation change
LIDAR 1997-2000(grey) RTK GPS Dec. 2001
Dec. 2002
1997-2000 2000-Dec. 2001
Dec. 2001-Dec. 2002
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Shoreline change where we are now
1997-2003 -250ft 1914-1962 1140 ft 1962-2003
- 780ft
2003
1914
1962
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South Beach change 1997-2000
Elevation change 1997-2000
Volume change loss 376,000 m3 gain 30,000 m3
m
loss gain
detail draped over 1997 surface
detail draped over 1997 surface
acceleration
Second order change 1997-1998-2000
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Moved volumes after nourishment
Sep. 2002 - Dec. 2001
RTK GPS
Aug. 2000 - Fall 1997 LIDAR
Sep. 2002-Aug. 2000 RTK GPS - LIDAR
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