This table is produced to enable analysis of the principal risk factors of the various Storm Events over the last 50 years. It will be noticed that the data features only tidal heights and barometric pressure. Quantification of wave energy is more complex, but is related to both wave direction and duration. A wave buoy is located at 50.292166,-3.6165, which is in the centre of Start Bay at the Northern end of the Skerries Bank. It is part of the Coastal Monitoring Programme.

It is known that Storm Events nearly always arise during times of Low Barometric Pressure. If that occurs during Spring Tides then the higher level of the sea is what places the coastline of risk.

It will be noted that none of the storms listed here have a Predicted tidal height of greater than 5.8 metres, whereas heights of 6.0 metres at Devonport are occasionally predicted. Extreme predicted tidal ranges (very high HW and very low LW) occur mainly February-April and August-October.

Hence, if very low barometric pressure coincides with a strong sustained Easterly wind during a Spring HW of 6 metres then more severe damage than seen so far will be experienced.

NOTES AND DEFINITIONS

Devonport (part of Plymouth) tide predictions are used here as their calculation is more robust than those for minor ports. Unfortunately, they are still published by the Admiralty in a format of one decimal place, which gives a resolution of 10 centimetres. Proprietary and free computer software typically produce predicted heights to two decimal places, which gives a resolution of 1 centimetre. In legal cases it has become standard practice to use Admiralty predictions. Writing your own computer software will give tidal heights to 6 or 7 decimal places. Devonport is the nearest Standard Port to Torcross. It has a tide gauge, the data of which are analysed to obtain the tidal constituents. Those constituents are used in calculations to predict times and heights of the tide.

High Water at Torcross is approximately 25 minutes after High Water at Devonport.

During Low Barometric pressure actual HW times are often 10-70 minutes before predictions.

Barometric Pressure values quoted in the table are for Crediton or Exeter Airport. Hence they are not perfectly representative. Consider the values as ±5 mb.

hPa (hectoPascals) is an identical unit of pressure to mb (millibars).

In SW England it is usual for Sea Level to be raised above predicted tidal heights by 1 cm for every 1 millibar below 1013 mb (mean annual barometric pressure for Plymouth).

Wind Set Up can add additional height to sea level. It depends on a strong wind blowing towards a coastline for a long period of time. At Torcross this would mean a strong Easterly gale for more than a few hours. Wind direction can vary considerably during the passage of a Low Pressure system which makes calculation complex. It needs to be considered as it can be in excess of 0.3 m during gales. In the above table Wind Set Up increases in tidal height are NOT included in the Adjusted HW level column.

HW (High Water) level is expressed as metres above Chart Datum, as used on Admiralty Charts. Chart Datum zero is approximately 3 metres below Ordnance Survey Mean Sea Level in the area of Start Bay.

Storm names were not ascribed in Britain until about 2015. They are now used throughout NW Europe. It sometimes appears that there is a race between national weather centres to name an approaching low pressure system! They are convenient for tabloid newspaper reporting, but the most relevant information is the actual barometric pressure, and its rate of change.

The Coastal Monitoring Programme Wave Buoy location in Start Bay has slightly different Latitude and Longitude from time to time. It was established on 2007 but has been set adrift several times with corresponding down time. Several web sites publish live data and the Coastal Monitoring Programme use it to raise alerts if wave heights exceed about 3 metres.

CONCLUSIONS

Storms causing damage to the A379 road are becoming more frequent.
Damage to the road is becoming more severe.
Hard structures, considered sea defences, are failing.