| So they build seawalls... What
do we have to lose? By JEFF SELINGO, Wilmington Morning
Star
Published: 08/25/96
How man-made barriers affect beach dynamics...
Jetties: Reshaping Islands
Photo courtesy / ORRIN PILKEY
Jetties interrupt the longshore current's flow of suspended sand particles. The
interruption of this flow robs downcurrent beaches of sand, which is retained by the
jetty. This inlet, at Tavira in southern Portugal, shows a downcurrent island starved by
an upcurrent jetty. |
The Newjerseyization of the Emerald
Coast  BEACHES OR
BEDROOMS?
North Myrtle Beach, The World's
Widest Beach?
Barrier Islands: Always changing
Renourish beaches or lose them...
ORRIN H. PILKEY
LISTEN TO MR PILKEY ON NPR
"SHIFTING SANDS" |
Photo courtesy / ORRIN PILKEY
The presence of groins can drastically alter the shape of an island. These
groins at Cape May, N.J., have created pockets of sand along the beach. By holding the
sand that would have gone to the downdrift beach in the distance, the groins have caused
its deep, U-shaped profile, a setback of nearly 3/4 mile. |
Groins: Trapping sand
|
Private Seawalls: A Chain Reaction
|
1. Waves stir up sand. Part of the sand is moved offshore, and part is carried
by longshore currents. Sand carried by longshore currents is a major source of
replenishment for the beach. |
| 2. This give and take results in a net loss of sand. Still, if allowed to
advance naturally, the shoreline is always bordered by a sandy strip of beach. An
unimpeded beach stays about the same size, advancing landward. Buildings in the path of
this advance are threatened. |
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3. Seawall is built in front of one house: - Sand accumulates
on upcurrent side of seawall.
- Beach is scoured away in front of seawall.
- Longshore currents can't deposit sand downcurrent.
- Beach in front of neighbor's house is deprived of sand. |
| 4. Neighbor builds a seawall. Sand depletion and scouring of beach continue. |
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| The moving shore... 
|
When the Cape Hatteras lighthouse was built, in 1870, it was nearly half a mile
from shore. Its location on one of the most exposed, wave-intensive shorelines in the
state has put in the path of a rapidly retreating shoreline. Without the seawall at its
base, a temporary measure, the lighthouse would have already been claimed by the sea. The
National Park Service wants to move the lighthouse. |
| The lighthouse at Morris Island, S.C., south (and downcurrent) of Charleston,
stood on the shore during WWII. Left behind by the rapid retreat of Morris Island, the
lighthouse stands alone 1,300 ft. from shore. The island's retreat was hastened when
jetties built in the Charleston area retained the sand that would have fed the downcurrent
island. |
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| Sources: William Cleary, Geology Dept., UNCW; 'The Corps and
the Shore' by Orrin Pilkey Jr. and Katherine Dixon; 'From Currituck to Calabash: Living
With North Carolina's Barrier Islands' by Orrin Pilkey Jr., William Neal, Orrin Pilkey Sr.
and Stanley Riggs |
