IV. Retreating Shorelines
157
Beach mining for sand or gravel or cobbles results evidently in the removal of a
volume of material but, additionally, and as a consequence sand mining
contributes to beach erosion. This involves off-shore as well as onshore sand
removal and has been illustrated by a long list of cases ranging from Lebanon
and Israel to Morocco and the United States. On the Oregon Coast (Siletz Spit)
72,000 m 3 (2.543 million cu.fl) of sand were removed from Schoolhouse Creek
beach : reduction of sand volume reduces its buffer effectiveness for the land.
Prior to the mining operations the beach was stable. The mining permit has not
been renewed since 1972. The demand for construction and road paving material
has encouraged off-shore dredging. Beach exploitation is common. The
environmental consequences have been recently reviewed by Charlier (1990).
However, destruction of coral reefs to supply ornamental material is equally
damaging, both ecologically and dynamically, as it enhances beach erosion by
removing "natural breakwaters".
The chapters that follow stress the natural factors of coastal erosion, an
inexorable phenomenon, and leave aside man's impact, except where directly
pertinent. It will consider causes and processes of erosion, beach response and
approaches to alleviate the problem.
1.1
The Beach
Beach materials originate far inland, carried by rivers and streams to their resting
place, from coastal formations erosion, and sometimes by onshore movement of
deeper water sediment. Beach sediments vary in sizes from fine sands to cobbles,
they can be arenaceous, volcanic or biogenic (shell-, coral reef fragments), rarely
silt or mud. Sand particles sizes and distribution, sand composition, berm type,
slopes of foreshore and inshore zone, off-shore bars are as many beach
characteristics. Geomorphological traits include lagoons, inlets and barrier
beaches. Dunes built by winds blowing over foreshore and berm carry landward
beach sand, and constitute a natural levee sometimes anchored by grass, bushes,
even trees. They provide a reservoir of beach sand and thereby a protective
barrier against wave attack
A beach must constantly adjust to wave energy and water levels. It must sacrifice
some beach, occasionally dune, to provide material for the building of an offshore bar. The beach berm, built by waves to the highest elevation that can be
reached by normal storm waves, is another protective barrier, whose value is
reduced when its sand is carried off-shore.
Overtopping of a dune results in landward transport of beach and dune sand,
thence lost to the dynamic beach system.
157
Beach mining for sand or gravel or cobbles results evidently in the removal of a
volume of material but, additionally, and as a consequence sand mining
contributes to beach erosion. This involves off-shore as well as onshore sand
removal and has been illustrated by a long list of cases ranging from Lebanon
and Israel to Morocco and the United States. On the Oregon Coast (Siletz Spit)
72,000 m 3 (2.543 million cu.fl) of sand were removed from Schoolhouse Creek
beach : reduction of sand volume reduces its buffer effectiveness for the land.
Prior to the mining operations the beach was stable. The mining permit has not
been renewed since 1972. The demand for construction and road paving material
has encouraged off-shore dredging. Beach exploitation is common. The
environmental consequences have been recently reviewed by Charlier (1990).
However, destruction of coral reefs to supply ornamental material is equally
damaging, both ecologically and dynamically, as it enhances beach erosion by
removing "natural breakwaters".
The chapters that follow stress the natural factors of coastal erosion, an
inexorable phenomenon, and leave aside man's impact, except where directly
pertinent. It will consider causes and processes of erosion, beach response and
approaches to alleviate the problem.
1.1
The Beach
Beach materials originate far inland, carried by rivers and streams to their resting
place, from coastal formations erosion, and sometimes by onshore movement of
deeper water sediment. Beach sediments vary in sizes from fine sands to cobbles,
they can be arenaceous, volcanic or biogenic (shell-, coral reef fragments), rarely
silt or mud. Sand particles sizes and distribution, sand composition, berm type,
slopes of foreshore and inshore zone, off-shore bars are as many beach
characteristics. Geomorphological traits include lagoons, inlets and barrier
beaches. Dunes built by winds blowing over foreshore and berm carry landward
beach sand, and constitute a natural levee sometimes anchored by grass, bushes,
even trees. They provide a reservoir of beach sand and thereby a protective
barrier against wave attack
A beach must constantly adjust to wave energy and water levels. It must sacrifice
some beach, occasionally dune, to provide material for the building of an offshore bar. The beach berm, built by waves to the highest elevation that can be
reached by normal storm waves, is another protective barrier, whose value is
reduced when its sand is carried off-shore.
Overtopping of a dune results in landward transport of beach and dune sand,
thence lost to the dynamic beach system.