IV. Retreating Shorelines
185
When groins were built at Lagos to counter harbor silting, sand accumulated west
of Lighthouse Beach Groin, and the strand grew about 400 m (1320 ft)seawards,
while on the east strand the shoreline retreated 1,250 m (4063 ft) in about 60
years. Wave energy has increased in spots, as a result of greater depths that
developed near groins. Along this segment of the Nigerian coast, beach retreat
has averaged from 5 to 9 m (16 to 30 ft) per year since 1970; however, the
problem is not confined to beaches in Lagos State.
A natural breakwater may build, and result in beach stabilization by reducing
wave power, where accumulation of removed material takes place at relatively
shallow depth. Erosion varies in time and space : along the American Atlantic
shore it is winter-intense and during the summer accretion may instead take
place 4. Quantification must take into account volume of sediment moved at tidal,
seasonal and long term scale and transport capacity of waves, currents, winds 5.
Gently sloping sandy beaches are far more vulnerable than gravel beaches, and
rocky coasts resist better to the waves' onslaught. Seasonal changes are marked
and the damage caused by storm waves in the winter may be compensated during
the calm summers when no other factors intervene. This is due to long-period
waves in the summer. However, too often the coastal dune barrier, is tampered
with, breached or even more frequently, in industrialized countries,
unhesitatingly flattened by bulldozer !
And the recent popularizing of dune buggies and other vehicles has increased the
damage (Mahoney, 1980; Primack, 1982, op cit.). Pathways for storms are thus
created and the dune no longer carries out its beach protective role.
Storm waves riding on an abnormally high sea level of a storm surge can cause
erosion, sedimentation, and short-term flooding. Storm waves erode foredunes
and bluffs and carry sediment offshore and alongshore to beachways. High surges
from large coastal storms transport sediments from the beach and foredune onto
the back of the barrier, resulting in upward and inland gain in these areas as the
ocean front is eroded. Damage to and danger for buildings is considerable.
Dunes play the role of stabilizers sometimes "feeding the beach" sometimes
receiving beach sand. Overwash may actually start new dune building.
During quiet periods sand is returned onshore to the beaches, but not to the
dunes ; storms cause landward progress of dune erosion. In Rhode Island this can
be 7 m (22.5 ft) during a single major storm.
Counteractions include the building of sea-walls, and the like but these are very
expensive and interfere with the sand supply of adjacent areas; another measure
calls for replenishment, by new sand, of the beach; and even more drastic
measures require the setting back of all buildings from the foredune crest 6.
185
When groins were built at Lagos to counter harbor silting, sand accumulated west
of Lighthouse Beach Groin, and the strand grew about 400 m (1320 ft)seawards,
while on the east strand the shoreline retreated 1,250 m (4063 ft) in about 60
years. Wave energy has increased in spots, as a result of greater depths that
developed near groins. Along this segment of the Nigerian coast, beach retreat
has averaged from 5 to 9 m (16 to 30 ft) per year since 1970; however, the
problem is not confined to beaches in Lagos State.
A natural breakwater may build, and result in beach stabilization by reducing
wave power, where accumulation of removed material takes place at relatively
shallow depth. Erosion varies in time and space : along the American Atlantic
shore it is winter-intense and during the summer accretion may instead take
place 4. Quantification must take into account volume of sediment moved at tidal,
seasonal and long term scale and transport capacity of waves, currents, winds 5.
Gently sloping sandy beaches are far more vulnerable than gravel beaches, and
rocky coasts resist better to the waves' onslaught. Seasonal changes are marked
and the damage caused by storm waves in the winter may be compensated during
the calm summers when no other factors intervene. This is due to long-period
waves in the summer. However, too often the coastal dune barrier, is tampered
with, breached or even more frequently, in industrialized countries,
unhesitatingly flattened by bulldozer !
And the recent popularizing of dune buggies and other vehicles has increased the
damage (Mahoney, 1980; Primack, 1982, op cit.). Pathways for storms are thus
created and the dune no longer carries out its beach protective role.
Storm waves riding on an abnormally high sea level of a storm surge can cause
erosion, sedimentation, and short-term flooding. Storm waves erode foredunes
and bluffs and carry sediment offshore and alongshore to beachways. High surges
from large coastal storms transport sediments from the beach and foredune onto
the back of the barrier, resulting in upward and inland gain in these areas as the
ocean front is eroded. Damage to and danger for buildings is considerable.
Dunes play the role of stabilizers sometimes "feeding the beach" sometimes
receiving beach sand. Overwash may actually start new dune building.
During quiet periods sand is returned onshore to the beaches, but not to the
dunes ; storms cause landward progress of dune erosion. In Rhode Island this can
be 7 m (22.5 ft) during a single major storm.
Counteractions include the building of sea-walls, and the like but these are very
expensive and interfere with the sand supply of adjacent areas; another measure
calls for replenishment, by new sand, of the beach; and even more drastic
measures require the setting back of all buildings from the foredune crest 6.