Author: Rebecca Alexandre
Recent sensational reports about potential flooding from ‘supertides’ might have temped us to point the finger of blame at carbon emissions, global warming or climate change. However, in this case we should be turning our attention to the moon.
On March 20 2015, the moon will be contributing to an increase in coastal flood risk. A coincidence of astronomical events that affect the tides is set to take place on and around that date. These include the vernal equinox, perigean spring tides and the 18.6-year lunar nodal cycle, all of which have enlivened discussions amongst scientists and the public about how significant, or insignificant, tidal impacts will be on the British coastline.
The astronomical factors that influence tidal heights are numerous and complex. For hundreds of years these had been a mystery to man until Sir Isaac Newton first explained gravity’s role in tidal movement in 1687. Simply put, oceanic tides are governed by, principally, the position of the moon and the sun, and the distance and angles between these and the Earth in their respective orbits. The moon has an elliptical path around the Earth in its monthly orbit, and the Earth has an elliptical path around the sun in its yearly orbit. When these three bodies are aligned, as is the case when we observe a new moon or full moon, there is a cumulative gravitational effect resulting in periodic tidal extremes - higher than average high tides and lower than average low tides. These are spring tides, which occur regularly, twice every lunar month, throughout the year.
However, three to four times a year, the full moon or new moon may occur when the moon is at perigee. During a perigee, when the moon’s elliptical orbit brings it closest to the Earth, the lunar gravitational pull on the oceans is even more powerful. The moon at such times is known as a supermoon and it causes unusually high and low spring tides: perigean spring tides or supertides. One such supermoon will occur on March 20.
The moon also has an 18.6-year-long nodal cycle which is associated with orbital changes and the tilt of the earth. This cycle affects the tides as well and happens to be at its peak this year. Additionally, March 20 marks the vernal equinox and during an equinox, the lunar and solar tidal forces can be enhanced.
The combination of these events has understandably caused many to wonder about the possibility of coastal flooding. In some places, similar circumstances have reportedly caused sea levels to rise to 0.5m higher than a normal spring tide.
But before the moon is denounced as the major cause of coastal flooding, it is important to recognise that astronomical high tides alone are unlikely to cause extreme flooding. Although the moon provides much of the force behind the tides, it is the interrelationship of the actual shoreline configuration, low barometric pressure, strong onshore winds and many other factors that determine the final flood risk to coastal communities. Areas with a high flood risk and those which are already saturated from heavy rainfall are the most vulnerable. The Environment Agency (EA) and the Met Office are working together to monitor local conditions, and the EA will issue flood alerts and warnings where needed.
Whether a supertide will coincide with adverse meteorological factors to cause coastal flooding in the coming days has yet to be seen. Nevertheless, it is a reminder of the awesome laws that govern the moon and its effect on our oceans.
For more information on these and other related phenomena, check out:
ADDITIONAL NOTE: 24 March 2015
The supertide that accompanied the events outlined above has been dubbed 'the tide of the century' with a coefficient of 119 out of a possible maximum of 120.