IV. Retreating Shorelines
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187
In tropical climates, mangroves may colonize inlets or part of estuaries. They
retain mud particles, although it is not known to what extent, and are thus an
accretion agent.
Mahin mud Beach in Nigeria retreated 200 to 600 m (600 to 1800 tt) between
1964 and 1981. Profiling between 1984 and 1987 revealed continuing climate,
sediment characteristics, shore and offshore topography, land use practices and
lack of sediment starving. [A..C. 1be et al, 1989, Erosion management strategies
for the Mahin Mud Beach, Ondo Sate, Nigeria : Coastal zone '89, 1, 821-835 ].
Superimposed are regional subsidence and global eustatic rise. Area lack of
construction material require a search for erosion control alternative strategies
such as re-afforestation program and installation of a variety of non concrete
offshore breakwaters. The Mahin Beach, 75 km (46.5 mi) long between Lagos
and the Benin River, has one of the fastest erosion rates anywhere in Nigeria,
though accretion occurs at the western end. Homes had to be moved inland at
least twice in the last 5 years.
Erosion here endangers coastal settlements, accelerates salt water intrusion,
modifies the hydraulic regime, and threatens oil wells and facilities. Settlers
contributed to the problem by clearing the adjacent mangrove, decimating the
mangrove forest along shoreline to erect buildings, making fish traps, boat tyingups, cooking, construction drying racks, etc. Yet mangroves" network of aerial
roots is particularly effective in stabilizing unconsolidated sediment, dampening
tidal currents and trapping sediment. Wherever the mangrove has been cleared,
the coastline inflects. The custom of digging small canals to the sea creates
artificial inlets which enhance erosion, while livestock, particularly pigs,
destabilizes the soil and accelerates erosion.
Oil and gas extraction, onshore and offshore, accelerates the natural subsidence
of the coastal geosyncline, fluid extraction reduces fluid pressure in the reservoir,
leading to an increase of '~ffective" stress (or grain to grain stress) in the
systems, compaction results, hence the basis subsides. The rise in sea level and
subsidence compound the problem.
Remedial options include reafforestation, public enlightenment program to
preserve the mangrove, damping wave impact with non concrete floating
breakwaters that also would trap and retain sediment. Such breakwaters could be
made of used tires, disused oil filled hoses linked together to form an underwater
'~forest" and artificial sea weeds, all of which can be assembled off the site and
then transported and 'tlumped" Artificial seaweed appears to be the most
disputable solution.
An erosion study involves taking profiles '~ross-island" or up to a point beyond
the dune barrier, whichever the case may be, nearshore profiles and sediment
samples.
......
187
In tropical climates, mangroves may colonize inlets or part of estuaries. They
retain mud particles, although it is not known to what extent, and are thus an
accretion agent.
Mahin mud Beach in Nigeria retreated 200 to 600 m (600 to 1800 tt) between
1964 and 1981. Profiling between 1984 and 1987 revealed continuing climate,
sediment characteristics, shore and offshore topography, land use practices and
lack of sediment starving. [A..C. 1be et al, 1989, Erosion management strategies
for the Mahin Mud Beach, Ondo Sate, Nigeria : Coastal zone '89, 1, 821-835 ].
Superimposed are regional subsidence and global eustatic rise. Area lack of
construction material require a search for erosion control alternative strategies
such as re-afforestation program and installation of a variety of non concrete
offshore breakwaters. The Mahin Beach, 75 km (46.5 mi) long between Lagos
and the Benin River, has one of the fastest erosion rates anywhere in Nigeria,
though accretion occurs at the western end. Homes had to be moved inland at
least twice in the last 5 years.
Erosion here endangers coastal settlements, accelerates salt water intrusion,
modifies the hydraulic regime, and threatens oil wells and facilities. Settlers
contributed to the problem by clearing the adjacent mangrove, decimating the
mangrove forest along shoreline to erect buildings, making fish traps, boat tyingups, cooking, construction drying racks, etc. Yet mangroves" network of aerial
roots is particularly effective in stabilizing unconsolidated sediment, dampening
tidal currents and trapping sediment. Wherever the mangrove has been cleared,
the coastline inflects. The custom of digging small canals to the sea creates
artificial inlets which enhance erosion, while livestock, particularly pigs,
destabilizes the soil and accelerates erosion.
Oil and gas extraction, onshore and offshore, accelerates the natural subsidence
of the coastal geosyncline, fluid extraction reduces fluid pressure in the reservoir,
leading to an increase of '~ffective" stress (or grain to grain stress) in the
systems, compaction results, hence the basis subsides. The rise in sea level and
subsidence compound the problem.
Remedial options include reafforestation, public enlightenment program to
preserve the mangrove, damping wave impact with non concrete floating
breakwaters that also would trap and retain sediment. Such breakwaters could be
made of used tires, disused oil filled hoses linked together to form an underwater
'~forest" and artificial sea weeds, all of which can be assembled off the site and
then transported and 'tlumped" Artificial seaweed appears to be the most
disputable solution.
An erosion study involves taking profiles '~ross-island" or up to a point beyond
the dune barrier, whichever the case may be, nearshore profiles and sediment
samples.