The Defend the Dunes Trust

Revised Estimates of Return Levels and Return Periods at St. Annes and their relevance to the Blackburn Children’s Home site.

This assessment of sea level returns and return periods is based on the following background information:-

Ÿ The principal factors affecting astronomical tides levels are relatively short term so that at any particular coastal position the tidal ranges and frequency of high tides observed in any one year are a fairly good measure of the frequencies for all years. Minor influences superimposed on the major factors result in the very highest tides that return every 18 years, 1997, 2015, etc.

Ÿ Reed’s Nautical Almanac ⁽¹⁾ shows the astronomical mean high water levels are the same for Liverpool, Preston, Fleetwood and Heysham, therefore the tide levels at Liverpool also apply along the Fylde Coast.

Ÿ The Proudman Oceanographic Laboratory estimates the increase in sea level along the Lancashire Coast arising from global warming, etc. is about 8 mm/year. ⁽²⁾

Ÿ The Proudman Oceanographic Laboratory has studied the surges in sea level as a result of meteorological factors for the British Isles, including those at Heysham, the nearest available data collection point to the Fylde Coast.⁽³⁾ Nearly all large sea level surges were observed to occur in the 8 winter months. Over the 21 years of measurement the maximum surge was +2.7 m. and about 25 surges of 1.7 m or more were recorded. The most satisfactory method of estimating future flooding return levels and return periods from the available data was found to be the Revised Joint Probabilities Method (RJPM).

Ÿ A surge of about 2.3 m is reported for the Fylde Coast in 1977 with additional wave heights of from 1.5 to 3 m depending on beach characteristics.⁽⁴⁾

Ÿ The standard atmospheric pressure used to compute tide tables is 1028 mbar and under severe storm conditions the atmospheric pressure may fall to about 950 mbar, therefore, based on the American Practical Navigator, ⁽⁵⁾ the drop in atmospheric pressure alone can cause elevations of sea level of up to about 0.7 m.

On the basis that surges along the Lancashire Coast can reach 2.7 m and that low atmospheric pressure alone can result in a surge of 0.7 m a 1.7 m surge is considered to be a reasonably conservative level to assume for forecasting for the Fylde Coast under severe storm conditions. Applying this surge value to three typical higher tide heights the following annual probabilities of sea level returns were derived from the tidal predictions for Liverpool for 2002 & 2003. ^{(6 )}

Tide height Sea level Adjusted % of days in the 8

mACD mAOD* mAOD** winter months

2002 2003 Mean

9.0 4.1 6.2 43.3 45.4 44

9.5 4.6 6.7 17.9 20.4 19

9.9 5.0 7.1 7.1 12.9 10

* Chart Datum - 4.9 = Ordnance Datum in metres ⁽⁶⁾

** Adjusted for global warming to 2050 and with 1.7 m surge added.

Assuming that there are on average two significant storms per year during the 8 winter months (for example from observations carried out by the Defend the Dunes Trust there were two in the early months of 2002) and that on average one storm in ten can give a large surge (based on the fact that the prevailing winds in the area are southwesterly and this is also the direction of maximum fetch) then the probability of a tide of 9.0 m coinciding with low atmospheric pressure, a storm and a surge of 1.7 m is 0.44 x 2 x 0.1 = 0.09,which gives a return period of 11 years.

The results for the same three typical return levels are:-

Adjusted % of days in Return period

mAOD** winter months (years)

6.2 44 11

6.7 19 26

7.1 10 50

In the attached Figure these estimates are compared with POL RJPM data for Heysham (adjusted for global warming to 2050) and with the Environment Agency estimate of a flooding risk of one in 200 y with a return level of 6.2 mAOD ⁽⁷⁾ (in this case adjusted for global warming using EA estimate of trend in sea levels of 0.4 mm/y). It is clear from this that the estimates of predicted sea levels presented here compare well with those of POL but that the EA data point does not. Even applying the higher estimates for sea level rise to the EA data still gives a much lower estimate of flooding risk.

It is noteworthy that POL ⁽³⁾ emphasise the value of using longer term historical data to improve the accuracy of the results. Local records of flooding obtained from local newspaper and other reports indicates that sea flooding has occurred on several occasions along the Fylde Coast since 1821. In particular, more recent data indicate that coastal flooding has occurred at sites with comparable geography to the St. Annes dunes in 1896, 1903, 1907, 1927, and 1977, i.e. 5 times in 108 years. ⁽⁸⁾ Thus historical records of flooding are in good agreement with the POL results and with those developed here. In addition it must be remembered that these estimates of sea levels take no account of the superimposed height of waves which must be expected under storm conditions and these are reported to have been up to 1.5 m for more gently shelving beaches, as is the case at St. Annes. ⁽⁴⁾

The POL data and our own predictions make it clear that flooding is a serious risk (1 in 50 year return with a sea level of 7.1 mAOD) at the Blackburn Home site where parts of the slab level are at 6.4 mAOD. ⁽⁹⁾ As noted elsewhere ⁽⁸⁾ flooding has not been reported in the past at Blackburn Children’s Home but the nearby cutting has been made through the dunes since the last serious flood in 1977 and it has a low sill. We suggest that flooding could arise from sea water coming through that nearby cutting, over Clifton Drive North (height 6 mAOD) and onto the site, or in the case of a more persistant storm from catastrophic failure of the line dunes. It may be argued that should a high sea level be anticipated this cutting could be blocked but analysis of the last severe floods ⁽⁴⁾ makes it clear that the emergency services would be at full stretch under very severe storm conditions. Furthermore, the scale of damage inflicted on sections of the sand dunes on 1st Feb. 2002 under modest storm and tidal conditions of only short duration⁽⁸⁾ shows the scale of defences necessary.

D A Moore

29th December 2002

REFERENCES

1. Reed’s Nautical Almanac, 2003, MacMillan Press.

2. Proudman Oceanographic Laboratory Internal Document No. 140, 2001

3. Proudman Oceanographic Laboratory Internal Document No. 64, 1994

4. Notes on a MAFF Meeting of Regional Land Drainage Chairmen Report, 3rd May 1978.

5. American Handbook of Navigation, by Bowditch.

6. Liverpool Tide Table 2002 & 2003, Laver Publishing Co.

7. Environment Agency, Flood Plain Maps, January 2002.

8. An assessment of the Risk of Flooding..... Defend the Dunes Report, May 2002

9. Assessment flood risk at the Blackburn Children’s Home site, Parkman Ltd, March 2002.