High (King) tides

When the tropic bird (sik) speaks while flying the tide will be coming in. (Kosraean saying)

Flooding of low-lying land on Kosrae most commonly occurs due to high tide levels. Very high tide levels (often known as king tides) and the risk of flooding vary through the year and between year to year.

High tide flooding has caused damage a number of times in the recent past notably:

The highest tides, most significant flooding and any resulting damage are often due to a combination of things influencing sea levels or where larger than normal wave conditions coincide with high tide levels.

The slides below explain more about the high tides that Kosrae experiences (hover over slide to pause).

Very high tides are a common cause of coastal flooding on Kosrae. In December 1999 very high tides caused flooding on Lelu Island.
The high tides also caused flooding and damage to property located on the shoreline at Malem...........
........and almost resulted in the restaurant at Sandy Beach Hotel being washed away.
A few weeks later, around the 21 February 2000, high tides again caused flooding, wave overwash and damage, for example at Malem.
In February 2008, high tides again caused damage particularly along the Malem coast. Photo: KIRMA
Between December 2013 and February 2014, high tides again resulted in flooding, for example at Mosral.
So why do these high tides occur and why do they occur at some times of the year and only on some years and not on others.......lets explore Kosrae's tides a little further.
What causes tides and why does Kosrae have one large tide and one small tide each day?
Tides are the rise and fall of sea levels caused by the gravitational forces exerted by the Moon and the Sun and the rotation of the earth. This is known as the astronomical tide.
Kosrae has mixed semi-diurnal tides, that is there are two tides with one larger tide and one smaller tide each day. The difference in tide each day is due to Kosrae's tide being a combination of factors that cause two tides a day and a weaker factor that only causes one tide per day. The plot above shows high and low tide levels over 2 days between 8-10 January 2005.
What are Spring and Neap tides?
Every 14 days, just after the time of the new and full moon, the range in tide levels are at their highest. These are called spring tides. Seven days later the tide range is much lower, called neap tides.
On Kosrae the moon has the largest influence on the tide, with the sun's influence slightly less than half that of the moon. When the effect of the Moon and Sun combine, just after new and full moons, spring tides occur. A week later, the effect of the Moon and Sun partially cancel each other out, resulting in the lower neap tides.
Why are the tides higher between December and February?
This plot shows all the tide levels for 2005. It shows high spring tides in January / February and again in late December reaching over 7 ft. The highest tides typically occur on Kosrae during these three months. This is because the diurnal (once daily) tide component has its largest influence around the summer solstice (in December), and to a lesser extent during the winter solstice (June).
What other influences of the moon and sun (astronomical) are there on tides on Kosrae?
1. Due to the shape of the orbit of the moon around the earth every 27.5 days there are times that the moon is closer to the earth. When it is at its closest point, this is called the lunar perigree and results in higher tides.
2. Tides on Kosrae are also higher when the moon is at its maximum angle to earth's equator twice during a cycle of 27.32 days. This is known as the lunar declinational cycle.
3. A 8.85 year cycle (cycle of lunar perigee) also influences high tides on a 4.4 year cycle. The above plot shows the influence of this on the highest tide each year between 2000 and 2030.
When some or all of these influences coincide with a normal high Spring tide, higher high tide levels or king tides can occur. When all these factors coincide – the highest astronomical tide occurs.
What about the effect of El Nino and La Nina on tide levels?

The 2 to 5 year cycle of El Nino Southern Oscillation can have a significant impact on sea levels on Kosrae. The effects of El Nino or La Nina can occur for months to a year or more.
During El Nino phases, sea levels and hence all tide levels on Kosrae are pushed down and the possibility of high tide flooding is reduced.
However, during La Nina phases sea levels on Kosrae are pushed up and hence all tide levels are higher than normal.
Many of the times when high tide flooding has occurred (such as the December to February periods in 1999-2000, 2007-2008 and 2013-2014) this has been due to high Spring tides occurring with La Nina conditions.
How do we measure tides and sea levels on Kosrae?
Since 2011 sea levels have been measured at the Marine Resources wharf in Lelu Harbour. This measures sea levels every 10 minutes. The sea level data is then sent over the mobile phone network to the internet. At the moment staff in KIRMA have access to the sea level data almost as soon as it is measured. The data is backed up in New Zealand at NIWA.
A temporary sea level recorder was also installed at Okat dock. The intention was to record one year of sea levels here and then move it to Walung for a year and then to Utwe. This would allow us to understand any variability in the heights and timing of tides around Kosrae. However the recorder has malfunctioned a number of times so we are still measuring sea levels at Okat.
Why do we need to measure tides when we have tide tables available?
Once we had recorded about one year of sea levels in Lelu Harbour the data was analysed to enable us to provide more accurate predictions of tide levels than was previously available. This analysis has now been done and is used to produce the tide tables available from this website or from the KIRMA office in Tofol.
Whilst we could produce monthly tide tables for a whole year ahead or more, on Kosrae we also incorporate predictions of whether sea levels are going to be higher or lower due to things like El Nino or La Nina. The latter predictions are only available one to three months ahead, hence the reason that tide tables are only available for up to three months in the future. Kosrae is the first place in the Pacific where the tide tables include both the astronomical tide prediction and the prediction of sea level fluctuation.
However, whether high tide levels cause significant damage or not really depends on the wave conditions that coincide with high tide. General forecasts of wave conditions are only available up to about 6 days ahead.
If wave conditions are only slightly higher than normal at high tide, wave overwashing and overtopping can occur and flooding and damage can be much more significant, particularly along the Malem coast.

The astronomical tide on Kosrae can now be predicted for any day in the past or in the future. Of course the actual high tide level on each day will also depend on other factors, such as whether we are in an El Nino or La Nina phase (which can only be predicted a few months in advance), or the particular weather conditions, which can only be predicted a few days in advance.

Did you know?

The highest astronomical tide between the years 2000 and 2030 occurred on Tuesday 11 January 2005 with a high tide level of 7.27 ft.
In 2015 the highest astronomical tide of the year occurred on Wednesday 21 January, with a high tide level of 7.2 ft.
The flooding events that happened in December 2013 to February 2014 occurred in part due to the third highest astronomical tide between 2000 and 2030.

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