Kosrae's Coast
Reducing coastal hazard risks on Kosrae
case studies
(C) Doug Ramsay & KIRMA, 2008
Reef flat dredging
The issue
The lessons learned
Other areas on Kosrae with similar issues?
During construction of the airport at Okat in the early 1980s, the contractor responsible for dredging the small boat channel that provided the fill for the airport reclamation was behind schedule, resulting in dredged slurry being pumped in to the basin surrounding the reclamation at a faster rate than could be retained. The resulting overflow of the dredge slurry in itself caused considerable environmental impact due to sedimentation effects on the surrounding seagrass beds and coral reef. However, the overflow also resulted in there being a shortfall of fill material for the reclamation. As a result further dredging was conducted on the reef flat fronting Tafunsak village, resulting a number of deep dredge pits.
False-colour IKONOS satellite image taken in 2001 showing the dredge pits (darker rectangular areas) on the reef flat in front of Tafunsak village.
The four dredge pits were excavated to a depth of between 1.5 to 2 below the level of the reef flat, and covered a total area of 783,000 cubic metres. By 1995 the eastern pit had filled in and depths in the other three pits had reduced to 0.2 to 0.5 m below the reef flat level. Over this period around 100,000 cubic metres of sand that was being transport in a westerly direction along the Tafunsak coast and from the beach in front of Tafunsak village was trapped in these dredge pits. This resulted in a rapid retreat of the coastline at Tafunsak.
The compensation package resulted in a rock revetment being constructed along much of the affected coastline of Tafunsak, causing a complete loss of the beach. The cost was of the order of $4 million to protect less than 1 km of coastline. Whilst the reef flat material provided a cheap source of material for the airport construction, the subsequent environmental and social costs were very much higher.
- Any open coast reef flat dredging on Kosrae will cause major coastal erosion problems.
- Great care needs to be taken with any dredging activity to ensure increased sedimentation in the water does not impact on coral reef, seagrass beds or other marine ecosystems.
Great care needs to be taken to ensure that dredging at the ship repair facility is kept to a minimum and that sedimentation from dredging activities and from run-off at the site is controlled.
References
Margos, J.E. (1993). Impact of coastal construction on coral reefs in the U.S. Affiliated Pacific Islands. Coastal Management, 21, 235-269.
Xue, C (1999). Coastal Sedimentation, erosion and management on the North coast of Kosrae, Federated States of Micronesia. Journal of Coastal research 15, 4, pp927-935.
Xue, C (1999). Coastal Sedimentation, erosion and management on the North coast of Kosrae, Federated States of Micronesia. Journal of Coastal research 15, 4, pp927-935.
In this section:
- Reef dredging
(C) Doug Ramsay & KIRMA, 2008