Abstract

Hurricane Bonnie struck the North Carolina coast on August 26, 1998.  A category 3 storm 
packing winds of 115 mph (185 kph), Bonnie came ashore near Cape Fear, NC, close to 
the tracks of Hurricanes Bertha and Fran in the summer of 1996.  One week after the 
storm, NASA, USGS, and NOAA surveyed the area of impact using the Airborne 
Topographic Mapper (ATM), a scanning airborne laser altimeter.  These  data were 
compared to ATM data collected one year prior to Bonniešs landfall.  Beach erosion and 
accretion create distinctive patterns in plots showing elevation changes between the two 
data sets. On relatively low areas of Topsail Island, NC, dunes were breached resulting in 
both sheet and channelized overwash.  Overwash deposits 0.5-1.0 meters thick extended as 
much as 150 meters landward of the shoreline, in some areas extending under oceanfront 
houses and covering roads.  On higher parts of Topsail Island, few significant impacts 
were observed.  Although prior to landfall, Hurricane Bonnie was approximately  the same 
strength as Hurricane Fran, Bonnie caused far fewer impacts to the barrier island 
environment.  Using the ATM to acquire pre- and post-storm topography over hundreds of 
kilometers of coast greatly improves quantification of storm impacts.

North End

At the north end of Topsail Island a broad area of erosion is observed.  On the change plot, 
this area shows as an area of bright reds and yellows on the beach adjacent to the remains 
of the dune line.  The area eroded during Hurricane Bonnie is clearly seen as a 
topographically high spot on the beach in the 1997 elevation data.  Overwash can be seen 
landward of the area of erosion as a thin blue line.  In profile comparison 1, 2.5 meters of 
vertical erosion on the beach face is evident.  This profile also shows that a significant 
portion of the foredune was also removed.  In profile comparison 2, the dunes appear to 
have been completely eroded away.  Profile comparison 3 shows as much as 3.0 meters of 
vertical beach erosion and complete destruction of the foredune.


 Middle

The low dunes at Topsail Beach, which had been rebuilt after Hurricane Fran in 1996, 
were again overwashed by Hurricane Bonnie.  This is evident in the change plot where 
dark blue (accretion) lies directly landward of reds and yellows (erosion).  In the pre-storm 
elevation data, a faint, straight continuous pale green line can be seen directly seaward of 
the front row of beach houses.  This pale green line represents the post-Fran reconstructed 
dune.  In the 1998 elevation data the green line is broken in areas and "smeared" inland.  
The change plots show the extent of sheet overwash (modified by crews moving sand off 
the roads before the ATM data was flown).  Houses appear as gray (non-change) areas in 
the blue band of overwash.  In profile comparison1, sand was removed from the beach and 
transported inland resulting in 2.5 m of vertical accretion.  Similarly, profile comparison 2 
shows extensive erosion and overwash deposition amounting to 0.5-0.9 meters of vertical 
accretion.  Note the accretion commonly occurs landward of houses indicating that flow 
during the storm went underneath the houses, most of which were built on stilts.  In profile 
comparison 3, the proximity of the house to the dune obscures the fate of the dune, 
however, the photograph shows there is overwash under and behind the house.  Note the 
"new" house 200 meters landward of the beach.  This house was build after the 1997 
survey.

South end

Old Topsail Inlet is located just off the south end of Topsail Island.  The change plot shows 
that during Hurricane Bonnie this inlet was filled and overwashed.  To the south of the inlet 
the yellows and reds indicate beach erosion.  Directly landward of that erosion, overwash 
fans (blue) can be seen.  Profile comparisons 1 and 2 show extensive erosion of beach and 
dunes with overwash deposits further landward.  Overwash deposits were as much as 1.0 
meter thick.  Profile comparison 3 shows a broad area of sheet overwash filling the channel 
of Old Topsail Inlet.  This profile cuts across beach that was eroded during the storm.  The 
1997 elevation data also shows the location of the channel before the storm.