Compare Tide and Surge

5 December 2013 and Feb 2015

February 20 and 21 2015 see some of the highest tides of the year each at 10.3 metres in the Liverpool Bay Tide Table, the highest tide of the year being 29 September at 10.4 metres.  It is not only tide height that creates flood risk it is when high tide in combination with onshore wind and low pressure create surge conditions and these combinations are difficult to predict. The photos below compare high tide sea levels on 20 and 21 February 2015 with the high tide of 5 December 2013 when conditions similar to the Towyn Floods of 1990 were experienced.

The images serve to benchmark events. The calm sea tide of mid day 20 February was about the same height as the slightly later tide of 21 Feb when a brisk west wind raised waves with the effect that can be seen in the photos. Also from the photos it looks like the surge created on 5 Dec 2013 on a tide of 9.9m was about 1 metre higher than the exceptional high tides of 20 / 21 Feb when the tide was 10.3m with implications for coastal defences.

Note: Approximately – Tide tables are based on GMT times and a standard high barometric pressure of 1013.25 millibars. Flood risk is greater when pressure is lower and water rises 1 cm for each 10 millibars so at a low pressure of 930 millibars a tide table height of 10 metres would be an actual 10.7 metres.

Incidentally the prom can be seen to be partly filled with sand so reducing the overflow capacity and from the 21 Feb the Foryd harbour “Blue” bridge was closed to all traffic at the start of a long maintenance programme and this has implications for an emergency in that evacuation across the bridge could not take place and emergency services (RNLI, Fire) may not be able to get into Kinmel Bay across the bridge from Rhyl.

While high tide can be predicted years in advance the other conditions of flood risk are less predictable long in advance for example low pressure can be forecast a few days in some cases and surge conditions are monitored but wind direction and force can switch quickly and in our area with the river Clwyd combines coastal and river conditions.

High Tide