Geology Reference
In-Depth Information
paradox. Although larger storms produce bigger
waves offshore, a category 4 or 5 storm may lead
to less wave-infl uenced transport on the platform
than a less intense storm such as a winter cold
front. The reefs, however, will probably feel more
of an impact from these large storms. The relation-
ship between storm strength and storm impact to
the carbonate system, can therefore vary spatially
across a platform.
Another limitation to this wave model is that
it only considers the peak wave period of the
wind-wave spectrum. Although this wave may
break, there are some wave height and period
combinations that would penetrate into shallower
waters, either on the ramp or along the platform.
Although the model may indicate that the waves
have broken on the reef, it is possible that some
waves produced by a particular wind speed may
yield sediment transport beyond the point of
breaking for the peak period waves.
Furthermore, wave generation by wind on the
platform was not explored in this simple wave
model. As a strong wind traverses a carbonate plat-
form, it will produce fetch and duration-limited
wind-generated waves. These waves will not have
to pass over a steep slope, so they may be able to
produce more sediment transport on the platform
than the waves coming from the deep ocean.
Finally, the estimated values of wave heights,
bottom orbital velocities and depths of initiation
of motion or breaking from the model represent
the maximum values for several reasons. In the
case of a reef-rimmed platform such as the Abaco
tidal delta region, the majority of the waves would
break over the reef, so the conditions depicted in
the fl at-topped platform model correspond to the
highest waves and orbital velocities on the reef-
rimmed platform.
(a)
14
12
10
8
6
4
2
0
(b)
7
6
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
Wind speed (
u
10
) (m s
−
1
)
Fig. 8.
The results of the wave model for fi ve frequen-
cies under four different wind strengths (6 m s
1
, 12 m s
1
,
18 m s
1
(Category 1 Hurricane))
over a shallow, unrimmed platform. Since the platform is
fl at and bottom friction is not accounted for, if the wave
does not break on the reef, it will persist throughout
the platform. This fi gure shows the depths at which each
wave produced by the different wind conditions begins
to move sediment at the bottom (a) and the depths at
which the waves break and would therefore stop infl u-
encing the bottom. The grey box in both fi gures indicates
the depth of the platform (which varies according to the
storm surge). If the wave breaks above this grey box (b),
the wave does not infl uence the platform. Similarly, if the
depth of transport initiation is below this box (a), no sedi-
ment is transported on the platform. For model runs of
storms of the order of Category 1 and greater, the waves
break at the platform edge, so there is effectively less
infl uence on the platform.
1
(Tropical Storm), 34 m s
at the edge of the platform, so the platform sedi-
ments will not be affected by these waves. There
remains a small window of storm intensities that
can infl uence the platform interior and, hence, the
carbonate shoal bodies.
Storm forces versus day-to-day processes
Model assumptions and limitations
Storms have an effect on sediment distribution.
The modelling illustrates how tropical cyclones
can yield large enough bottom orbital velocities to
move sediment, and observations after hurricanes
Frances and Jeanne illustrated limited redistribu-
tion of sediment, with the most pronounced
changes occurring in the fl ood tidal delta lobes.
However, the question remains as to how much
effect the storms have on shaping geomorpho-
logical bodies and, eventually, the sedimentary
record. Although they may have a slight impact on
geomorphology in the short term, are they enough
to counteract the daily processes?
This simple model fails to show the post-breakage
effects. As waves enter the platform over the shelf
margin, they are modifi ed by bottom effects. The
larger swells break at the platform edge upon hit-
ting shallow water, since the maximum wave that
can occur is a fraction of the water depth. The
waves that break can reform, but due to the short
fetch and shallow depths of the region between the
reefs and the islands in the study area, these regen-
erated waves cannot grow to the size and force
of the original swells that broke at the platform
edge. These dynamics introduce an interesting
