Kosrae's Coast
Reducing coastal hazard risks on Kosrae
coastal inundation
The causes
Flooding of land on Kosrae most commonly occurs due to a combination of high tide levels occurring at the same time as moderate to large wave conditions. With the exception of typoon events (see typhoon page) which are rare on Kosrae, coastal flooding tends to occur:
Between the late 1970s and late 1990s we have also been in a period where El Nino events have tended to be more common (and hence sea levels have tended to be depressed).
We are now in a phase where it is possible over the next 20 or so years that La Nina conditions may be more common - resulting in higher sea-levels and as a result potentially more frequent inundation and damaging coastal events than has occurred over the last 20 to 30 years.
- Between November and February and June to August.
- During strong phases of La Nina.
- The most significant is the astronomical tide - the regular rise and fall of water level due to the influence of the moon and the sun. Tide levels on Kosrae tend to be higher between November and February and between June and August.
- The influence of El Nino and La Nina oscillations. During strong El Nino events, sea levels around Kosrae tend to be depressed. During strong La Ninas, the opposite occurs and sea levels tend to be higher. This can cause variations in sea-level of up to 0.25 m (10 inches) or more.
- The effect of continuous north-east trade winds which tend to increase tide levels between November and April.
Between the late 1970s and late 1990s we have also been in a period where El Nino events have tended to be more common (and hence sea levels have tended to be depressed).
We are now in a phase where it is possible over the next 20 or so years that La Nina conditions may be more common - resulting in higher sea-levels and as a result potentially more frequent inundation and damaging coastal events than has occurred over the last 20 to 30 years.
Predictions of high and low astronomical tides for Kosrae for 2008. Higher tides occur around November to February and June to August periods which is why coastal flooding often tends to occur around these times.
21-Feb-2000: High tides + La Nina conditions resulting in higher sea levels and inundation.
Other factors that also influence water levels over the fringing reef, including:
- The influence of local weather - winds and low atmospheric pressure can also increase water levels, known as storm surge. With the exception of typhoon conditions, the effect of storm surge on sea-levels around Kosrae tends to be quite small.
- Wave set-up - where waves breaking on the edge of the reef "pump" water on to reef causing localised increases in water level over the reef.
Nov '07 - Feb '08: High tides + La Nina conditions resulting in higher sea levels and inundation.
Low-lying areas around Kosrae that are at risk of inundation due to high sea levels and future sea-level rise.
22-Dec-1999: High tides + La Nina conditions resulting in higher sea levels and inundation.
In this section:
- Coastal inundation
Downloads
(C) Doug Ramsay & KIRMA, 2008
Tides and climate variability
Weather and waves
Where larger waves, due to deeper water depths over the reef flat, reach the shoreline, run-up the beach, and overwash the coastal berm behind the beach.
This is natural process and is an important process in building up the height of the coastal land behind the beach. The land upon which much of the housing and road is built would not be there without this process occurring. Typically wave run-up and overtopping of the coastal berm would be expected to occur about 2-4% of the time.
However, due to the way parts of the coastline of Kosrae have evolved, there are areas where the coastal berm has not built up as high as in other areas. These areas tend to be more prone to overwashing and coastal damage. The most at risk areas are:
A final factor is the potential effects of future sea-level rise due to climate change. This will increase the frequency and extend of overwashing and inundation that occurs.
This is natural process and is an important process in building up the height of the coastal land behind the beach. The land upon which much of the housing and road is built would not be there without this process occurring. Typically wave run-up and overtopping of the coastal berm would be expected to occur about 2-4% of the time.
However, due to the way parts of the coastline of Kosrae have evolved, there are areas where the coastal berm has not built up as high as in other areas. These areas tend to be more prone to overwashing and coastal damage. The most at risk areas are:
- South Malem, particularly Fulkrin, Kotfwa and Pal
- Walung coast particularly between Insiaf and Pilyuul.
A final factor is the potential effects of future sea-level rise due to climate change. This will increase the frequency and extend of overwashing and inundation that occurs.
Overwashing waves
