Geology Reference
In-Depth Information
(a)
Tropical wave
Category 1 Hurricane
Category 2 Hurricane
Category 3 Hurricane
Category 4 Hurricane
Category 5 Hurricane
(b)
Tropical wave
Category 1 Hurricane
Category 2 Hurricane
Category 3 Hurricane
Category 4 Hurricane
Category 5 Hurricane
Tropical storm
Tropical storm
30
30
100
100
90
80
70
60
50
40
90
80
70
60
50
40
Fig. 2.
Hurricane tracks and intensity plots for Hurricanes Frances (a) and Jeanne (b) from September 2004. The red arrow
indicates the location of the study area in the northern Abacos, Bahamas. Figures are modifi ed from the National Hurricane
Center/Tropical Prediction Center's website (www.nhc.noaa.gov). The colours of the storm tracks indicate the intensity of
the storm: tropical storm (white), category 1 hurricane (pink), category 2 hurricane (yellow), category 3 hurricane (orange),
category 4 hurricane (red) and category 5 hurricane (black).
The Abacos are prone to tropical cyclones
from June through to November. In 2004 alone,
two major tropical cyclones passed directly over
the study area. On 4 September 2004, Hurricane
Frances (Fig. 2a) passed through this region as
a category 3 storm on the Saffi r-Simpson scale
(sustained winds of 50-58 m s
1
), decreasing
in intensity to a category 2 (sustained winds of
43-49 m s
1
) upon departure (Beven, 2004). Less
than a month later, on 25 September, Hurricane
Jeanne (Fig. 2b) passed through the same area as
a strong category 3 storm. These storms did con-
siderable damage to the Bahamian communities
in the area.
the deltas. Many fl ood deltas include rock ridges
parallel to the island trend, and not all facies are
represented in every fl ood lobe due to the different
sizes and shapes. South of the Abaco island chain,
the surface of Little Bahama Bank consists largely
of peloid-skeletal wackestone to packstone and is
stabilized by seagrass.
Oceanographic and meteorological setting
The physical setting of the study area dictates
the daily oceanographic forces to which it is
exposed. The tidal deltas are exposed to the open
Atlantic Ocean to the north and are therefore
prone to larger swells from this direction than from
the shallower bank to the south, although the
continuous sections of the barrier reef reduce
the impact of these larger swells. Similarly, the
islands shield the bank from incoming open-
ocean swells, creating a mixed tidally dominated
system that is predominately semidiurnal. The
tidal range in the region averages around 1 m
(cf. Reeder, 2007).
The area is infl uenced by several winter cold
fronts annually. Such events have been docu-
mented as a signifi cant cause of erosion in areas
such as the northwest coast of Andros Island,
Bahamas (Shinn
et al
., 1969; Rankey & Morgan,
2002). These fronts are accompanied by stronger
than average winds from the north and northeast
that differ both in direction and in magnitude from
the otherwise dominating easterly trade winds.
Observations
An ultra-high-resolution (2.4 m
2
pixel size)
QuickBird remote sensing image acquired on 27
November 2003, prior to the passage of these two
storms, provides information on pre-storm mor-
phology. For comparison, a second QuickBird
image was obtained on 16 March 2005, after the
passage of the storms. These data, calibrated by
fi eld observations, provide an important foun-
dation for this study; however, differences in
boundary conditions for each image acquisition
(different atmospheric conditions, time frames,
wave climate, tide level, etc.) limited quantitative
comparisons between images. Bathymetry derived
from satellite data (Stumpf
et al
., 2003) is not accu-
rate enough to measure centimetre-scale changes.
Therefore, it cannot be determined if sediment
