Figure 3. Barrier islands with a drumstick shape
receive much of their sand supply from large
ebb-tidal deltas formed by outgoing tides
within the adjacent inlet.
(Image modified from
Hayes, 1979)
that leads to accretion rather than having net erosion, and
wave and tidal processes distribute the available sands
to modify the island’s geomorphology.
When a wave breaks, its stored energy is released, providing
the force to transport sand. As a wave washes up onto the
beach (called the swash), sand is carried with it. Gravity then
causes the water to flow down the beach’s slope toward the
sea. Sand grains are carried with this backwash (Figure 2).
Since waves most often approach a shoreline at an angle, a
sand grain travels at an angle up the beach during the swash,
and down the slope during the backwash. The grain does not
return to its point of origin, but instead is transported a short
distance along the length of the beach. The next wave picks up
the sand grain and again carries it up the beach at an angle,
and the backwash moves it downslope.
With the passage of hundreds of angled breaking waves, a
sand grain zig-zags its way down the length of the shoreline,
a process known as longshore transport (also referred to as
longshore drift). The direction of longshore transport is often
referred to as the downdrift direction—similar to a river’s
downstream direction. You may have experienced the related
longshore current while swimming or floating on a raft
near to shore, and the current has dragged you far down the
shoreline from your beach towel! Longshore transport and
longshore current are continual processes as long as waves are
approaching the shoreline at an angle. If wave direction and
angle change, the longshore transport is affected.
Larger waves release more energy, and the longshore
transport energy is greater. Along the South Carolina
coast, the stronger and larger sediment-transporting waves
occur during storms. Since most storms that approach and
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attack our beaches arrive from the northeast, the overall
longshore transport direction for most of the S. C. coast is
generally north-to-south. Later in this article you’ll see how
extremely influential this longshore transport direction is
in determining the overall geomorphology of many of our
barrier islands, including Kiawah.
The role of tides along a barrier coast is most important in
the breaks between the islands referred to as inlets. Inlets are
the channels that allow tidal waters to flow landward during
the rising, or flood tide, and seaward during the falling, or
ebb tide. Along the SC coast, two high-low-high tidal cycles
occur daily, and at Kiawah, the vertical difference in the
water’s height between high and low tides, referred to as the
tidal range, varies between 5.5 and 7.0 feet. The result of this
moderately large tidal range is that twice a day a large volume
of water moves very swiftly through inlets in each direction!
Because inlets are narrow and constrict the water flow
(similar to placing your thumb over the end of a garden
hose nozzle), inlet tidal currents can be dangerously strong,
and have the ability to transport a tremendous amount of
sand. During an ebb tide, sand is carried seaward. These
tidal currents run into the landward-flowing waves and,
similar to a head-on collision, the tidal water’s flow is slowed
significantly. With the reduced energy from the colliding
tidal and wave energies, sands that were being transported
settle to the seafloor. Over time, a lobe of sand is constructed
seaward of the inlet. This submerged lobe, called an ebb-tidal
delta, is an enormous body of sand that can supply longshore
transported sand to the adjacent downdrift barrier island.
Stono Inlet on Kiawah’s northeast end provides a tremendous
natural source of sand to Kiawah Island.
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