Geology Reference
In-Depth Information
into the backbarrier and to the fl ood-tidal delta due to
the increased capacity of the tidal currents. At migrat-
ing inlets, new fl ood-tidal deltas are sequentially
formed as the inlet moves along the coast and encoun-
ters new open water areas in the backbarrier. An almost
continuous, mostly sub-tidal shoal system has been
produced by this process along the bayside of Katama
Spit on the southern coast of Martha's Vineyard,
Massachusetts (FitzGerald and Pendleton 2002). At
most stable inlets, however, sand comprising the fl ood
delta is mostly recirculated between a relatively fi xed
number of depocenters. The transport of sand to and
from fl ood deltas is controlled by the time-velocity
asymmetry of the tidal currents (Boothroyd and
Hubbard
1975
). During the rising tide, fl ood currents
reach their strongest velocities near high tide when the
entire fl ood-tidal delta is covered by water. Hence,
there is a net transport of sand up the fl ood ramp,
through the fl ood channels, and onto the ebb shield.
Some of the sand is moved across the ebb shield and
into the surrounding tidal channel. During the falling
tide, the strongest ebb currents occur near mid to low
water. At this time, the emerging ebb shield diverts the
currents around the delta. The ebb currents erode sand
from the landward face of the ebb shield and transport
it along the ebb spits and eventually into the inlet chan-
nel where once again it will be moved onto the fl ood
ramp thus completing the sand gyre.
3. Swash platform
-
This is a broad shallow sand
platform located on both sides of the main ebb
channel, defi ning the general extent of the ebb delta
and dominated by wave action.
4. Channel margin linear bars
-
These are bars that
border the main ebb channel and sit atop the swash
platform. These bars tend to confi ne the ebb fl ow
and are exposed at low tide.
5. Swash bars - Waves breaking over the terminal lobe
and across the swash platform form arcuate-shaped
swash bars that migrate onshore. The bars are usu-
ally 50-150 m long, 50 m wide, and 0.5-2.0 m in
height.
6. Marginal-fl ood channels. These are shallow channels
(up to 2 m deep at mean low water) located between
the channel margin linear bars and the onshore
adjacent beaches. The channels are dominated by
fl ood-tidal currents.
The general shape of an ebb-tidal delta and the
distribution of its sand bodies reveal the relative mag-
nitude of different sand transport processes operating
at a tidal inlet. Ebb-tidal deltas that have elongated
main ebb channel and channel margin linear bars that
extend far offshore are common for tide-dominated
inlets, including those having strong tidal-versus-wave
energy and/or large tidal prisms (e.g., Georgia, south-
ern South Carolina, Florida Gulf Coast). Wave-
generated sand transport plays a secondary role in
modifying delta shape at these inlets. Because most
sand movement is in the onshore-offshore direction,
the ebb-tidal delta overlaps a relatively small segment
of inlet shoreline. This affects the extent to which this
part of the coast undergoes erosional and depositional
changes caused by inlet processes.
Wave-dominated inlets tend to be small relative to
tide-dominated systems. Their ebb-tidal deltas are
driven onshore, close to the inlet mouth by the domi-
nant wave processes. Commonly, the terminal lobe and/
or swash bars form a small arc outlining the periphery
of the delta (e.g., Nauset Inlet, Massachusetts; Little
River, South Carolina; Matanza Inlet, Florida) In many
cases the ebb-tidal delta of these inlets is entirely sub-
tidal (e.g., Blind Pass, Florida). In other instances, sand
bodies clog the entrance to the inlet, leading to the for-
mation of several major and minor tidal channels.
At mixed-energy tidal inlets, the shape of the delta
is the result of tidal and wave processes. These deltas
have a well-formed main ebb channel, which is a
product of dominant ebb-tidal currents. Their swash
12.2.1.2 Ebb-Tidal Deltas
An ebb-tidal delta is a complex of shoals and channels
on the seaward side of the inlet. It consists of sand that
is intercepted from the longshore transport system and
is carried seaward and deposited by ebb-tidal currents,
where it is subsequently modifi ed by incident waves
and ambient tidal currents. Ebb deltas exhibit a variety
of forms dependent on the relative magnitude of wave
and tidal energy of the region as well as geological
controls. Despite this variability, most ebb-tidal deltas
contain the same general features including:
1. Main ebb channel
-
This is a seaward shallowing
channel that is scoured in the ebb-tidal delta sand
and is dominated by ebb-tidal currents.
2. Terminal lobe
-
Sediment transported out the main
ebb channel is deposited in a lobe of sand forming
the terminal lobe. The deposit slopes relatively
steeply on its seaward side. The outline of the termi-
nal lobe is well defi ned by breaking waves during
storms or periods of large wave swell at low tide.
