By Dr Jason McIlvenny and Dr Philip Gillibrand, Environmental Research Institute, North Highland College UHI
The tides around the coast of the United Kingdom fluctuate between “spring” tides and “neap” tides according to the moon’s phase, full moons and new moons giving rise to the largest range tides, known as “spring” tides. That means that both the highest high tides and the lowest low tides (i.e. the largest range) occur at “springs”, while at neap tides the tidal range is smallest. The word “neap” is thought to have originated from the Middle English word ‘neep’ meaning small. The word spring refers to the tide springing up and not the season of spring.
In simple terms, spring tides occur when the gravitational effects of the moon and the sun are aligned, giving the greatest net effect. At neap tides, the gravitational pull of the sun and the moon act in perpendicular directions, so the net effect is smaller.
There are other, more subtle, effects on the tides, due to slow variations in the relative orbits of the earth, moon and sun. For example, due to the tilt of the earth and orbital changes, there is an astronomical tidal cycle with an 18.6 year period (known as the ‘nodal period’). During 2015 we will reach the peak of this cycle. Therefore at springs, the large tidal range will be even larger with higher than normal high tides, leading to the so-called supertides. The largest tides will occur at the spring and autumnal equinoxes:
- 20-21 February 2015
- 29-30 September 2015
Tides at the equinoxes are generally the largest of the year, so this year will be the largest seen for a number of years.
The high astronomical tides will not cause coastal flooding alone around the Scottish coastline unless combined with the effects of storm conditions. Storm systems can cause an increase in water level known as a storm surge which can elevate the normal tidal height as much as 1.5 meters.
In contrast to this, the upcoming astronomical tides will increase the average spring tide height up to 30 cm around Scotland (most places however will be increased only 10 to 20 centimetres). The difference in tides between locations depends on local conditions such as the shoreline topography and contour of the ocean floor.
At Scrabster, for example, an average spring tide consists of a high water of approximately just over 5 meters and a low water of approximately 0.5 m. The super tide in January increased this tidal ‘range’, with a high water level of 5.3 m and a low of 0.2 m. It might not sound much (increase of ~25 cm on normal tidal levels); however, if combined with a storm surge there is an increased risk of coastal flooding.
In Inverness the super-tide for January caused a predicted high tide of 5.0 m and a low tide of 0.7 m, whereas an average spring tide (for example, on 22 December 2014) the high water mark was 4.7m and the low tide was 0.9m).
Some tidal statistics from around Scotland are given in the table below:
|Location||Area||HighestTide 2012||HighestTide 2013||HighestTide 2014||HighestTide 2015|
|Millport||South West||2.14||2.15||2.19||2 Feb 2014||2.15||23 Jan 2015|
|Ullapool||North West||2.95||2.98||3.10||1 Feb 2014||3.01||22 Jan 2015|
|Wick||Far North East||2.14||2.16||2.25||11 Sep 2014||2.29||30 Sep 2015|
|Moray Firth||Moray Firth||2.60||2.60||2.74||11 Sep 2014||2.78||30 Sep 2015|
|Aberdeen||North East||2.46||2.43||2.58||11 Sep 2014||2.63||30 Sep 2015|
|Leith||Firth of Forth||3.14||3.11||3.3||11 Sep 2014||3.34||30 Sep 2015|
Areas of low-lying topography are most at risk of flooding. Outside Scotland in French Guiana, for example, the super tides will cause a 6 cm increase in the average spring tide height, which will lead to a 90 m shoreline retreat (http://www.nature.com/ngeo/journal/v1/n3/full/ngeo127.html). So the threat posed by super-tides depends strongly on the nature of the local coastal area.
Storm surges occur due to the winds and low pressure associated with a large depression or storm. Low atmospheric pressure allows the local sea surface to rise, and strong winds can pile up water against a coastline. Combined, these two effects can elevate sea level by several meters (the storm surge during Hurricane Katrina in the Bay of Mexico in 2005 was about 9 m). Closer to home, in January 2005 a large storm (see figure) created a storm surge which elevated the normal level of the tides in the north coast of Scotland by 1.5 meters. The storm led to significant flooding in communities along the north coast of Scotland.
12 January 2005 storm
View over the river from Janet Street, Thurso, 12 Jan 2005
Scrabster Harbour office front door, 12 Jan 2005
Thurso Harbour 12 Jan 2005
For more information, contact:
Dr Jason McIlvenny, Post-Doctoral Research Associate, Environmental Research Institute, North Highland College UHI. firstname.lastname@example.org
Dr Philip Gillibrand, Senior Research Fellow, Environmental Research Institute, North Highland College UHI. email@example.com Tel: 01847 889686