Geology Reference
In-Depth Information
(a)
(b)
N
N
0.25 km
0.25 km
Fig. 5.
QuickBird high-resolution remote sensing images of a fl ood lobe of a tidal delta near Carter's Cay, Bahamas, acquired
in November 2003 (a) and in March 2005 (b). This time interval surrounds the active 2004 hurricane season that affected
the regions, and shows one of the regions more affected by the storms, with erosion of seagrass (red box), migration of bar
crests (green box) and sedimentation (orange boxes). Satellite image courtesy: DigitalGlobe.
result is not trivial, given the apparent strength of
the storms, and the fact that two passed over the
area in a short time period. To further investigate
the potential infl uence of storms, a simple
wave model was used to explore the forces that
might be exerted by storm waves on a carbonate
platform.
The model consists of two main parts: a wave
prediction module and a wave transformation
module. The wave prediction module calculates
the deep water signifi cant wave height (the aver-
age wave height of the highest 1/3 of the measured
waves in a location) and peak wave period for a
given wind speed. As the waves approach shal-
low water, the wave transformation module of the
model calculates the adjusted signifi cant wave
heights and the bottom orbital velocities pro-
duced by these waves. Breaker criteria are used to
calculate the maximum height of a wave before it
will break and are used to determine the depth at
which the wave will break. Once the wave breaks,
the heights and orbital velocities are set to zero,
terminating the propagation of the wave as well
as any sediment transport that may have been
initiated.
The wave model was run for two different
bathymetric confi gurations (Fig. 6). The simplest
bathymetry was a ramp sloping at an angle of 0.3°
from horizontal, spanning 10 km (Fig. 6a). The
second bathymetric confi guration that was stud-
ied simulated a simplifi ed fl at-topped platform. In
this case, a 5-m-deep shelf dropped steeply (14°)
to deep water at the shelf margin, mimicking some
Bahamian platforms (Fig. 6b).
(a)
0.02m
50m
0.3
°
10km
(b)
5m
1m
s
= 0.001
50m
s
= 0.015
14°
4km
Fig. 6.
Bathymetric confi gurations for the ramp (a) and
platform (b) situations modelled with the wave model. The
platform bathymetry (b) consists of three different slopes
used to determine the breaking criteria for the waves.
Deep water wave model
Initially, the deep-water wave heights (
H
0
) are
calculated for a given wind speed measured 10 m
above sea level (
u
10
) using the deep water fetch-
limited wave equations (Komar, 1976)
(1)
where
g
is the acceleration due to gravity
(~9.81 m s
2
),
F
is the fetch, and
u
*
is the wind
stress factor. Given a known
u
10
, the wind stress
factor is
Wind waves are initially dependent on the time
the wind has been blowing (duration) as well as the
