Geology Reference
In-Depth Information
Sand waves can migrate ~4 m in a month under
fair-weather conditions in a tidal channel
(Gonzalez & Eberli, 1997). The observed sand
wave migration rate of 20 m yr
1
at the tidal
deltas is lower than this documented rate, but
these rates are highly dependent on the tidal
velocity. On the tidal deltas, the current velocit-
ies range between 0.24 and 0.30 m s
1
, so the
rates of sand wave migration may be much
smaller here.
The lobes themselves remained generally sta-
tionary, and modifi cations occurred mainly in
sand wave crests. Overall, on the tidal deltas,
there was no major reorganization of the shoal
geometries (e.g. no new 'spillover lobes'). Only
minor changes are apparent, such as removal
of small patches of seagrass (Fig. 5, red boxes),
smearing of sand waves on shoal crests, and
slight expansion of the shoal by deposition of a
thin layer of sand ~60 m outward in some areas
(Fig. 5, orange and green boxes). In the context
of the entire carbonate platform, these are small
perturbations, and, although it is possible that
such small perturbations may be further ampli-
fi ed by daily processes, this does not appear to
occur here. Observations (in summer 2005) of
sand waves reforming and seagrass re-establish-
ing suggest that the system was returning to its
pre-storm state.
Field observations led to further questions about
the infl uence of storms on the platform. No major
discernable sediment erosion or deposition was
observed in the fi eld.
Many of the areas of 'shoal widening' on the
remote sensing images proved to be a thin layer of
sediment that was transported off the shoal crest
onto adjacent grassy areas rather than a large addi-
tion to the lobe, and these areas were beginning to
stabilize by seagrass in summer 2005. These
observations indicate that the daily tides, winds
and waves of the region are restoring the system
from the minor perturbations brought about by
storms. Although storms may modify the system
in the short term, it appears unlikely that such
perturbations will be preserved.
(a)
Pre-Storm
2003
N
N
500 m
(b)
Post-Storm
2005
N
500 m
(c)
2003
2005
N
500 m
Fig. 4.
Extent of the geomorphological changes that
occurred over the 1.5-year period between remote sens-
ing image acquisitions, during which the two storms
(Hurricane Frances and Hurricane Jeanne) passed. The
majority of the study area experienced few to no modi-
fi cations, but the crests of some tidal deltas did move.
This fi gure highlights key pre-storm crest locations
(a, and green lines in c) and post-storm crest locations
(b and yellow lines in c), selected to illustrate mor-
phological adjustments between the remote sensing
images. The modifi cations are compared in c. Most
of the crests on both fl ood and ebb deltas shifted to
the southwest, but this is not the case for all the crests
highlighted. See text for more details. Images copyright
DigitalGlobe.
MODELLING STORM WAVES: WHAT
MIGHT THEY DO?
The fi eld and remote sensing observations
suggest relatively little impact of the storms on
this carbonate system. In the context of the often
assumed geological importance of storms, this
