A Summary of Relevant Sea Level Reports by the

Proudman Oceanographic Laboratory.

The following two reports are available on the POL web site, www.pol.ac.uk. POL is one of the foremost oceanographic laboratories in the country. It is their data that is used to compile tide tables such as those published by the Laver Publishing Co.

POL Internal Document No. 140

Integrated effects of climate change on coastal extreme sea levels.

R Laver, et al, July 2001.

The effects considered are:

1. Global changes in mean sea level, in the case of Liverpool these go back to about 1760.

2. Regional land movements, based on archeological evidence going back 16,000 years.

3. Tidal changes due to changes in sea level, based on analysis of the harmonic motion of the sea in tidal basins.

4. Changes in storminess due to increasing atmospheric CO 2, based on POL data produced for the STOWASUS-2100 international study.

The results are simply added together to give the resultant changes in mean sea level. They agree well with corresponding MAFF (2000) data for the East Coast, give lower values for the South Coast and give higher values for the West Coast, particularly the Lancashire coast where values of about 8 mm/year are shown on a contour style map.

The Environment Agency appear to use the MAFF figure of 4 mm/year for the Fylde Coast.

There are large margins of error on all the data, probably +/- 2 mm or more, as there are with the MAFF data.

POL Internal Document No. 64

Extreme sea-levels at the UK A-class sites. Dixon & Tawn, March 1994

This is a comprehensive mathematical analysis. I have not attempted to follow the maths. but just looked at the results. They have studied only selected A-Class sites where hourly sea levels are available over adequate periods and analysed this data statistically. The sites nearest to the Fylde are Holyhead and Heysham, and at Heysham this spans 21 years between 1964 - 90. Note: Liverpool and Fleetwood are not included. The baseline for data is 1990

and the maximum astronomical tide (MAT) for Liverpool is 5.7 mAOD

The results are described in terms of:

Ÿ Mean sea level , after removal of waves, (Chart datum figures are used, which equal Ordnance Datum + 4.9 m, which is the same at Liverpool)

Ÿ Extreme sea levels are based on astronomical tides with any additional surge, but they do not separate the effects of the various causes of surges.

Ÿ They use the terms Return Levels and Return Periods, the sea levels that could give flooding and the expected frequency of such events.

Ÿ They also use the term “design level” of defences, where height of defences to cope with a 1 in 50 year level equates to about a 1 in 113 year water level.

Ÿ They assume the sea level remains constant and that there is a trend in surge level with time due to global warming, etc.

They evaluated various statistical techniques for estimating levels and conclude that for Heysham the “Revised Joint Probabilities Method” (RJPM) was most satisfactory. This technique allowed longer term annual maxima to be used to improve the estimates from the shorter term hourly measurements, it can also take trends of sea (surge) level into account and tide/surge interaction.

There is a great deal of scatter in the data comparing sea levels and surge height.

At Heysham surge level seems to be independent of tide height so extreme water levels are reported from high tide and high surge, (so add surge to tide?) The observed maxima were:-

Tide:- 5.65 mAOD Surge:- 2.71 m Sea level:- 6.66 mAOD

i.e. the highest surges were not seen at the highest tides (Is that due to shortage of data? Only about 5% of tides are above 5 mAOD but 65% are less than 4 mAOD and only one year was included when the MAT was possible, 1979. Nevertheless, it suggests that it is not good enough just to add the typical high values of surge to typical high values of astronomical tide with shorter term data).

The extreme levels for Heysham are calculated as 11.31 m (6.4 mAOD) with a standard error of 0.075 m. The highest sea level observed had a return of 110 years (an estimated 0.5 m short of the maximum possible).

Sea level change for Heysham is given as 8.4 mm/year.

Larger surges at Heysham over the 21 years were concentrated between days 0-100 and 238 - 365 in the year ( = 8 months), i.e. away from summer and but covering times of high tides.

They don’t plot storm duration against frequency but recorded data for storms in excess of 72 h and that there are slightly higher sea level maxima for storms of over 50 h duration (11.9 m versus 11.8 m).