with reclamations and seawalls (both Government and private). Causeway construction between reef islands can
have negative physical and biological, as well as social,
effects. The position, length, construction method, width
of the reefs, and the pattern of water exchange and current
strengths mean that causeways can have an influence on
both water and sediment transport.
Seawalls pose particular problems. Many are temporary structures built to protect land or expensive assets
from erosion. On Tarawa and in the Marshall Islands, they
are generally built of coral boulders and beachrock
on the margin of land and often extend out onto or across
the active beach. Where vertical walls are constructed,
these tend to reflect wave energy and can cause scouring
at their base and undercutting (Figure 9). Alternatively,
wave overtopping causes infiltration of water leading to
structural collapse.
Reclamation and backfilling with sand have provided
many people with extra space on small reef islands. Reclamations act like groynes blocking sediment transport
along active beaches, with sediment accumulating on the
updrift side of structures, while the downdrift side experiences coastal erosion as has occurred on Majuro Atoll
(Xue, 2001). Other disturbances to sediment transport
pathways can also have severe implications, for example,
boat channels, blasted or excavated across the reef flat to
provide access for boats at all stages of the tide, also provide conduits for beach sand to be lost over the reef edge.
Atolls, Figure 8 The atoll of Pingelap, Micronesia, showing a cross-section of Deke Island, and the elevation of the island and the
surface elevation of the freshwater lens (after Ayers and Vacher, 1986).
ATOLLS
65
have negative physical and biological, as well as social,
effects. The position, length, construction method, width
of the reefs, and the pattern of water exchange and current
strengths mean that causeways can have an influence on
both water and sediment transport.
Seawalls pose particular problems. Many are temporary structures built to protect land or expensive assets
from erosion. On Tarawa and in the Marshall Islands, they
are generally built of coral boulders and beachrock
on the margin of land and often extend out onto or across
the active beach. Where vertical walls are constructed,
these tend to reflect wave energy and can cause scouring
at their base and undercutting (Figure 9). Alternatively,
wave overtopping causes infiltration of water leading to
structural collapse.
Reclamation and backfilling with sand have provided
many people with extra space on small reef islands. Reclamations act like groynes blocking sediment transport
along active beaches, with sediment accumulating on the
updrift side of structures, while the downdrift side experiences coastal erosion as has occurred on Majuro Atoll
(Xue, 2001). Other disturbances to sediment transport
pathways can also have severe implications, for example,
boat channels, blasted or excavated across the reef flat to
provide access for boats at all stages of the tide, also provide conduits for beach sand to be lost over the reef edge.
Atolls, Figure 8 The atoll of Pingelap, Micronesia, showing a cross-section of Deke Island, and the elevation of the island and the
surface elevation of the freshwater lens (after Ayers and Vacher, 1986).
ATOLLS
65