The Moon and its influence on coastal flooding

  • Full Moon - Photo by Nick Ansell/PA Wire/Press Association Images

    Stormy weather accompanied by high tide - Photo by Niall Carson/PA Wire/Press Association Images

    A person struggles in high winds - Photo by Danny Lawson/PA Archive/Press Association Images

  • The Moon and its influence on coastal flooding
    15.12.2012 15:00

    So farre, so fast the eygre drave,
    The heart had hardly time to beat
    Before a shallow seething wave
    Sobb’d in the grasses at oure feet:
    The feet had hardly time to flee
    Before it brake against the knee,
    And all the world was in the sea.
    So said Jean Ingelow in her 19th Century poem ‘The High Tide on the Coast of Lincolnshire’

    A number of this week’s news stories have focused on the recent flood events in parts of the UK. Several of these give a brief mention to the fact that these have been exacerbated by a combination of rain falling on already saturated surfaces and high tides, yet very little explanation is given as to what causes high tides.

    What causes tides?

    The term ‘tide’ refers to the periodic ebb and flow of the oceans in relation to the Earth. Most locations have semi-diurnal tides which mean there are two high tides each day, during which time beaches appear to shrink as the seawater progresses further and further up the shore.

    Tides are caused by the gravitational pull between the Earth, the Moon and the Sun. The strength of this gravitational pull varies based on the position of the Moon and the Sun in relation to the Earth and is strongest when the Sun and Moon are aligned. Therefore, our tides are at their most extreme during a full moon and during a new moon and are known as ‘spring tides’. Conversely, our weakest tides occur when the sun and the moon are at right angles in relations to the Earth – these are known as ‘neap tides’.

    Tides are amplified close to the shore and in estuaries, in part due to the increased surf as waves reach shallow waters. The Bristol Channel has the second highest tidal range in the world, with the Bay of Fundy in Canada having the highest at approximately 17 metres.

    What made the recent weather event particularly hazardous?

    The recent unsettled weather in the UK posed a particularly significant risk of coastal flooding because the spring tide was accompanied by high winds and low atmospheric pressure. Both of these factors help to intensify the already powerful effect of a spring tide. The new moon occurred on Thursday 13th at 08:42, however, its effect on the tides lasts for a few days either side. Then on Thursday evening, a low pressure system and its associated fronts brought 33mm of rainfall to Plymouth over the course of 24 hours. During this time, winds gusts peaked at up to 50mph at various locations in the south-west.

    Forecast for the coming few days…

    There is a slightly drier prospect over the next few days with sunny spells and scattered showers; however some of these may be locally heavy in the south and west. It will turn unsettled again in the middle part of next week with further Atlantic systems bringing strong winds and spells of rain to all areas. However, this will coincide with a weaker ‘neap tide’, suggesting that the tides will not play such a significant role in any flood risk.

    By: John Lee