IV. Retreating Shorelines
Chapter II : Coastal Protection
1
Introduction
1.1
Beach Dynamic Processes
179
Unconsolidated beaches result from a building process linked to accumulation
processes and appear as sandy beaches (less than one third of world beaches) or
muddy beaches (either colonized by mangroves or in sheltered districts). The
principal types are dunal systems, estuarine environments, littoral spits, lagoons
and ponds.
Beaches are built of gravel, shingle, shingle upper/sand lower, mixed
shingle/sand or sand, silicious, carbonaceous or volcanic.
A shingle beach can be re-orientated and drawn down rapidly by storm waves,
but very little of the material is drawn out beyond the breaker line, and smaller
and less steep waves tend to restore the profile. The slope of a shingle beach in
the inter-tidal zone is typically about 1 : 8 which may be drawn down to about 1 :
12 in extreme storm conditions.
A sand beach, on the other hand, continues to flatten under prolonged storm
conditions with consequent continuous removal of material from the beach head.
Material woves offshore as well as alongshore, and the beach requires much
longer to return to a 'hormal" profile. It is possible for extreme storms to draw
the beach sand into a water depth where milder waves do not influence the sea
bed and from whence the material cannot be returned to shore. If this situation is
combined with lack of material carried alongshore, artificial recharging of the
beach may be required. For a comprehensive assessment of groin performance, it
is necessary to consider the envelope of possible beach profile variation as well as
the "normal" profile.
Today the majority of beaches, due to natural and/or anthropic action, are
thinning or retreating. On the French Atlantic coast some 850 km (527 mi) of
littoral lose about a meter (3.28 ft) each year and world-wide sea-level increases
on the average from 12 to 15 cm (4.8 to 6 om) a year ~.
Chapter II : Coastal Protection
1
Introduction
1.1
Beach Dynamic Processes
179
Unconsolidated beaches result from a building process linked to accumulation
processes and appear as sandy beaches (less than one third of world beaches) or
muddy beaches (either colonized by mangroves or in sheltered districts). The
principal types are dunal systems, estuarine environments, littoral spits, lagoons
and ponds.
Beaches are built of gravel, shingle, shingle upper/sand lower, mixed
shingle/sand or sand, silicious, carbonaceous or volcanic.
A shingle beach can be re-orientated and drawn down rapidly by storm waves,
but very little of the material is drawn out beyond the breaker line, and smaller
and less steep waves tend to restore the profile. The slope of a shingle beach in
the inter-tidal zone is typically about 1 : 8 which may be drawn down to about 1 :
12 in extreme storm conditions.
A sand beach, on the other hand, continues to flatten under prolonged storm
conditions with consequent continuous removal of material from the beach head.
Material woves offshore as well as alongshore, and the beach requires much
longer to return to a 'hormal" profile. It is possible for extreme storms to draw
the beach sand into a water depth where milder waves do not influence the sea
bed and from whence the material cannot be returned to shore. If this situation is
combined with lack of material carried alongshore, artificial recharging of the
beach may be required. For a comprehensive assessment of groin performance, it
is necessary to consider the envelope of possible beach profile variation as well as
the "normal" profile.
Today the majority of beaches, due to natural and/or anthropic action, are
thinning or retreating. On the French Atlantic coast some 850 km (527 mi) of
littoral lose about a meter (3.28 ft) each year and world-wide sea-level increases
on the average from 12 to 15 cm (4.8 to 6 om) a year ~.