It’s 6.45 AM as I’m writing this, listening to Radio Humberside and watching the snow fall on my garden in Barton-upon-Humber. For the past hour I’ve been following the news, updates from Twitter, and the reporting from live tidal gauges, trying to get an idea of the materialising image of this storm surge.
A storm surge occurs when low atmospheric pressure during storms causes the sea level to rise. This is because the low pressure draws up the water level. This is a basic explanation, but a 1mb reduction in atmospheric pressure will result in a 1cm increase in sea level. On top of the sea level increase caused by this, storms are usually windy and whip up significant waves, adding further level to the water.
To cause flooding, a significant surge needs to coincide with a high tide, usually a high spring tide like we are experiencing at the moment. If the surge coincides with the low tide, it results in nothing more than an unusually high low tide level but is not a risk. This page from the MetOffice provides a great description of them.
This combination was what drove the warnings for flooding along the East Coast of England this morning. Fortunately, the flooding feared did not occur. It is important for me to be very clear now that the main risk, and the flooding warnings for today, refer to the tide this evening – keep listening to the Environment Agency, do what they say, and keep an eye on their warnings here.
From the National Oceanographic Centre we can see forecasts for storm surges at tidal gauge sites. Below are the forecasts for today for the Immingham gauge – the peak surge is forecast to be over 2m. If this was to occur with the high spring tide due at Immingham this evening, over 7.4m, it could produce water levels in excess of 9.4m which would be higher even than 2013. Thankfully, the peak of the surge is forecast to occur prior to the high tide. This has happened numerous times in the past, where large surges have coincided with low tides.
This does not mean there is no risk by any measure – the water levels this evening will be higher than this morning. This forecast could turn out to have predicted the timing wrong. Surges can also have the effect of drawing in the high tide, causing it to occur slightly early as demonstrated by Horsburgh and Wilson (2007) (Paywall). The winds forecast also did not occur this morning but the could arrive during the day, adding to the waves.
Storm surges are complex, with numerous facets combining to cause the risk. This makes them difficult to forecast and can evolve and change quickly. The Environment Agency have done a superb job is warning and preparing these past few days and should be commended for this work. It is also encouraging to see the greater appreciation for tidal flooding risk from the media and the public – awareness is vital component in reducing the dangers of flooding.
Stay alert and stay safe.