Geology Reference
In-Depth Information
possible that the daily forces may be enough
to readjust the system back to its previous mor-
phological state.
Tropical cyclones are powerful enough to
produce large waves and storm surges, both of
which can generate strong currents. The models
(Figs 7 and 8) illustrate that although not all
storms (from winter storms to strong cyclones)
produce waves strong enough to have an impact
on the bottom in all areas, they appear to have
important roles in two regions. The fi rst area is
at the shoreline. In all cases, energy is expended
on the shoreline due to breaking; this energy
would be greater than that due to the daily (non-
storm) waves. In these areas, more pronounced
changes are expected. In the Abacos study area,
many of the islands are rocky, and do not have
sandy beaches, which may explain the lack of
change at the shoreline. The second important
area is the 'outer shelf' between the reef and the
shore. Some, but not all, storms produce waves
large enough to be impacted by the bottom in this
area. These deeper areas (between 5 and 12 m
deep) are beyond the penetration depths of daily
waves (Fig. 8a). Tide- and wind-generated cur-
rents, which are weaker in these less-restricted
regions, become much stronger in the presence
of high winds and storm surges. Thus, in these
areas, some sediment may have been suspended
and transported.
Another possible interpretation for the lack of
signifi cant change is that the geomorphological
elements formed in response to storms earlier in
the geological history of the region, and are now
in 'equilibrium' with such high-energy condi-
tions. Many tropical cyclones and storm fronts
pass through this area (Landsea
et al
., 2003)
and the tidal delta region of the Abacos must
have been subjected to a very large quantity of
storms ranging from tropical cyclones to winter
cold fronts throughout its history. However, this
hypothesis is unlikely for several reasons. Visual
observations at the study site revealed active sed-
iment transport with the daily tidal cycles. These
observations suggest that the forms are not relict,
being modifi ed only by high energy storm events,
but they are changing daily. Also, if they were to
be formed by storms, as 'spillover lobes', it would
be diffi cult to obtain such well-defi ned lobes on
both sides of the islands. The storm patterns as
discussed previously would most likely lead
to larger 'fl ood' lobes than 'ebb' lobes, whereas
the observations illustrate the opposite relation-
ship. Therefore, it is unlikely that these geomor-
phological bodies were formed by storms, and
it is more likely that they are the result of daily
forces.
DISCUSSION
Why only subtle changes?
Strong storms are destructive. Many lives have
been lost to tropical cyclones (e.g. the Galveston
storm of 1900), and they can cause extensive
property damage (e.g. Hurricane Andrew, 1992;
Hurricane Katrina, 2005). In regions containing
sandy barrier islands and beaches, a single storm
can cause considerable damage along the shore
due to storm surges and waves (Hill
et al
., 2004;
Keen
et al
., 2004; Stone & Orford, 2004; Stone
et al
., 2004). However, impacts on tidally infl u-
enced subtidal seascapes such as the oolitic tidal
deltas and shelf margin of the Northern Abacos
appear much less pronounced.
The remote sensing images clearly demonstrate
that although there were no major lasting geomor-
phological changes (e.g. new 'spillover lobes',
or extensive movement of sands) as a result of
passage of these tropical cyclones, there are indeed
some geographically limited changes to this sys-
tem. In the study area, between the 2003 and 2005
image acquisitions, crests of sand waves appear
to have moved, and sand was transported off the
shoal crest over adjacent grassy areas. If these
changes were not due to day-to-day processes
(the observations do not permit this possibility
to be eliminated), these small apparent changes
imply either of the following.
(a)
Changes were simply too small to observe on
the ultra-high-resolution remote sensing data,
or there simply were no changes. This could
be due to the presence of a storm surge, which
diminishes the impact of the storm waves on
the platform by increasing the water depth.
This result indicates that, although storms
may be powerful, they are not the most impor-
tant forces shaping geomorphology in this and
analogous depositional systems.
There were alterations due to storms, but
(b)
they returned to their original state after the
storms and before acquisition of the second
image and fi eld observations. If this situation
is valid, it would emphasize the role of daily
processes in shaping the overall morphology
of the shelf.
