Geology Reference
In-Depth Information
Table 4. Results of the wave model for the simplifi ed
carbonate platform bathymetry. Refer to Table 3 for
parameter defi nitions. If h b > platform depth (5 m) + surge
(refer to Table 2 for the surge value), the wave breaks
before reaching the platform. If h t < platform depth (5 m) +
surge, the sediment on the platform top is not transported.
The values for the platform differ from the results for the
ramp (Table 3) due to the different bathymetries. Similarly,
the different values in u max within this table are due to the
different bathymetrical gradients infl uencing the waves.
Recall that u max is the maximum orbital velocity obtained
prior to breaking, not the orbital velocity produced on the
fl at platform.
(a)
14
12
10
8
6
4
2
0
(b)
8
7
5
3
2
u 10 (m s 1 )
u max (m s 1 )
H b (m)
h b (m)
h t (m)
0
0
5
10
15
20
25
30
35
40
6
0.38
1.01
1.75
0.53
12
0.93
1.57
4.11
1.03
Wind speed ( u 10 ) (ms 1 )
18
1.49
2.58
6.50
1.17
34
3.12
6.60
12.35
0.99
Fig. 7. Wave model results for the simple ramp
bathymetry at the peak frequency of the wave spectrum for
four different wind speeds: daily 6.0 m s 1 winds, the aver-
age winter cold front winds (12 m s 1 ), tropical storm winds
(18 m s 1 ), and Category 1 Hurricane strength winds (based
on the Saffi r-Simpson scale). (a) This plot illustrates the
depth at which the wave's bottom orbital velocity exceeds
the threshold velocity of sediment entrainment. At depths
shallower than these plotted values, bare sediment is being
transported. Once a wave breaks, however, the energy dis-
perses and the sediment transport is terminated (for that
wave). The depths at which the waves break are shown in (b).
If the wave begins to transport sediment at 6.5 m water
depth, but breaks in 4 m, it is only responsible for sediment
transport along a short distance of the ramp.
bottom orbital velocities and breaking limits
for a simple, unrimmed carbonate platform
(Fig. 6b). The platform confi guration does not
include shoals such as the tidal deltas of the
Northern Abacos, but understanding the poten-
tial infl uence of storm waves on a fl at platform is
useful in understanding the forces that could pos-
sibly have an impact on such a shoal system. The
results of the wave model as applied to a simpli-
fi ed platform are summarized in Table 4.
The case of the platform margin (Fig. 8)
provides insights on how a more complex
bathymetry might be impacted by storm waves.
Although sediment transport on the platform top
can be initiated from waves produced by 18 m s 1
storm winds, the bottom orbital velocities remain
below 1.2 m s 1 for even the strongest of storms
on the platform. From the results presented here
(Fig. 8; Table 4), the most infl uential storm would
be similar to the winter storm (12 m s 1 winds).
The waves produced from winds around 12 m s 1
(under the assumed duration and fetch) do not
produce bottom orbital velocities large enough
to initiate sediment transport on the platform.
Although the maximum orbital velocities are
larger than the threshold velocity, the depth of
initiation of motion is shallower than the plat-
form depth, so the sediment does not move along
the platform (the 12 m s 1 winds initiate sediment
motion at 4.1 m depth, but the platform depth
is 5.5 m with the surge set-up for this storm).
Instead, motion is initialized at the shallower-end
of the bathymetrical profi le, which could indicate
the location of a carbonate shoal body (Fig. 6b).
For winds stronger than 18 m s 1 , the waves break
studies of wave impact on exposed coasts, which
indicate that signifi cant sediment transport due to
waves will not occur below 20 m of water depth (cf.
Dietz & Fairbridge, 1968; Duane, 1976; Gordon &
Roy, 1977). A similar study on a coast with low
wave energy (Sapelo Island, Georgia; Coastal
Engineering Research Center, 1984) further limited
this depth to less than 10 m on coasts exposed to
lower wave energy (~0.25 m annually averaged
signifi cant wave height), and the model results are
also fairly consistent with these results.
These data illustrate the diffi culty of storm-
generated waves impacting broad expanses of
ramps, given the model assumptions. It appears
that in shallower areas (e.g. <5 m), such wind-
generated waves could, however, have an impact.
At these depths, bottom effects cause the waves to
disperse their energy through sediment transport
and eventual breaking.
Application to a carbonate platform
To investigate the effects of storm waves on a
platform, the model calculated the wave heights,
 
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