SUMMARY

The best forecast of sea level rise for Weymouth is about 250 mm by 2050 and 650 mm by 2100. However, some forecasts give higher figures nearer 350 mm by 2050 and 850 mm by 2100.

By 2050, tidal heights of three metres will become 'three-times-a-decade' events. The top of the new harbour wall is about three metres above chart datum so that this will be over-topped and the 'Park District' and the western half of the Town Centre will be prone to direct flooding from the sea pouring over the present harbour wall.

By 2100, three metre tides will become monthly occurrences. 

Clearly, remedial work and new harbour defences will need to be installed before the second half of the 21st century arrives.

By 2100, the harbour wall must be at least 500 mm higher than the recently built wall and a more extensive system of pumping will be needed to prevent backflow through the town's drainage system.

INTRODUCTION

Sea levels are rising all over the globe. This is widely believed to be due to global warming. There are sceptics who believe that:

This is not the place to argue these ideas but interested readers should go to the 'links' page for websites and articles. 

What concerns us here is that sea level is rising and that large areas of Weymouth lie within a couple of metres of mean low tide level in Weymouth Bay. The 'Park District' has been flooded many times in the 20th century and two bad floods in 1908 and 1955 are described in [1] and there are several photographs of residents wading through water outside their homes in [2]. Such flooding can only get more frequent as sea levels continue to rise unless drastic flood protection actions are taken during this century.

The reason that the 'Park District' has flooded so often in the century and a half since it was built has been not predominantly due to sea water pouring into the low lying estate. Rather, it has been because rain water pouring down the River Wey and into Radipole Lake has been unable to get out to sea because high tides have forced the Westham Bridge sluice gates closed.

Thus, flooding had ultimately always been due to high sea levels combined with high rainfall in the Wey River catchment area.

The sea level in Weymouth Bay is a combination of the following factors:

All of these factors will be discussed below leading to forecasts of the risks of extreme sea levels being experienced in Weymouth Bay which threaten Weymouth's Town Centre and 'Park District'.

Long-term Sea Level Rise

The threat of sea level rise was not widely recognised until relatively recently and appropriate tidal gauges were not adequately deployed to measure sea level rise rates until about the 1980s. Unfortunately, this is too short a timescale to measure accurate sea level rises and results around the UK coastline can be rather uncertain.

A major complication is the tilting of the United Kingdom as a response to the loss of the ice burden that covered much of the UK in the last Ice Age. The UK is still very slowly responding and, as a result, the relative levels of the sea and coastline are changing in quite different rates around the coastline.

Plotted from data at [3]

Figure 1

The above graph shows actual measurements of sea level rise rates relative to the coastline around the UK. The value for Weymouth (5 mm/year) is indicated by the larger filled symbol.

It can be seen that there is a trend indicating that the southern United Kingdom is sinking relative to the north. However, the short timescale of the measurements has resulted in large error bands on many measurement sets.

The global mean sea level is currently rising by about 2 mm/year [4]. However, measurements are very difficult to make and prone to errors.

Predictions of future levels are fraught with uncertainty. In part this is because it is not known what levels of greenhouse gases will be emitted over the 21st century. The Intergovernmental Panel on Climate Change has produced over two hundred possible economical, technological and political scenarios [5].

Figure 2

Figure 2, derived from data in [5], shows the upper and lower bound of the predicted Global Carbon Emissions for 256 scenarios. Clearly, no useful predictions can be made unless the scenarios are reduced to a few plausible options!

Figure 3

Figure 3 shows the upper and lower bounds for the predicted global sea level in black. The green curves are four of the most plausible scenarios for global responses to the threat of global warming. The red dotted curve is based on a Defra prediction that sea level will rise by 250 mm in Weymouth Bay by the year 2050. This figure is consistent with the measured 5 mm/year plotted on figure 1. This figure extrapolates to a sea level rise of 650 mm by 2100 taking a mean rate of acceleration for a variety of scenarios.

However, it has been stated [6]: 

"Until the last decade, the rate of sea level rise along the Dorset coast was taken to be about 3 mm per year and then was revised upwards to 5 mm per year by MAFF in the mid-1990s. DEFRA has advised (2001) that 6 mm per year should be used for future coast protection and flood defence proposals."

As a compromise, the red curve on figure 3 is used here to predict sea level rise throughout the 21st century in Weymouth Bay.

Flooding is clearly a greater risk at around times of high tide so we will assess the risk of flooding in Weymouth at around the times of high tide.

Heights of Astronomical High Tides

I have computed the probability of a high tide exceeding any given value for Weymouth Harbour in 2005, 2050 and in 2100 taking into account astronomical tide levels and the effects of increased storminess and sea level rise both predicted to be caused by climate change.

For details of how I achieved this please click here.

Figure 4

Figure 4 shows the probability of observing a high tide exceeding the value shown on the horizontal axis.  

This is a key graph as it shows the frequency of high tides reaching any given level in Weymouth Harbour.

Although the rise in sea level may seem modest, its greatest effect will be to increase the probability of the sea reaching a given level in the future.

For example, the sea currently reaches a height of 2.8 metres about once every decade. By 2050 this height may be achieved about once every month according to Figure 4.

We can read [7]

"...the rise in sea level will also change the frequency of occurrence of higher than normal water levels. The current 1-in-100-year high-water level might be expected to be exceeded every 20 years on average by 2050. There may also be secondary impacts such as changes in wave heights and wave 'reach' due to increased water depths."

However, my measurements and calculations suggest that extreme high tides will occur very much more frequently that estimated in [7] and this may be because the Dorset Coast Forum have not included the full effects of an accelerating sea level rise as well as the effects of the deepening barometric pressures forecast by the middle of the 21st century.

Flooding Risks

The actual risk of flooding is not predicted here. What is given here is an estimate of the frequency that a tide of a stated height will be experienced during the course of the 21st century and the areas of Weymouth that will be below those levels of the sea in Weymouth Harbour. 

This does not mean that those areas will be flooded. The occurrence of flooding will depend on many factors such as the effectiveness of sea defences that are already in place and which will have to be put in place before 2100 as well as the effectiveness of the pumping of surplus water out of low lying areas, such as the 'Park District', before it causes problems to residents.

The 'Park District' has been flooded frequently and two detailed and dramatic accounts of floods in 1908 and 1955 can be read at [2].

Some flood protection schemes have already been implemented.

Figure 5

Figure 5 shows part of the flood protection wall built around Weymouth Harbour in 2002 [8]. 

This is the lowest part of Weymouth and, before the wall was built, it was common to see cars parked at this spot, despite the flood warning signs, submerged in sea water on Spring high tides. With the aid of a weighted string and a tape measure, I determined that at this point by the Town Bridge, the top of the wall lies 3.1 metres above chart datum and the road is 1.9 metres above chart datum.

Spring high tides in Weymouth Harbour are typically about 2.0 - 2.3 metres so that this section of roadway is below sea level at some time in most months. Figure 4 shows that there is no chance of the new sea wall being over-topped at present. However, by 2050 the sea defence will be over-topped about once every decade on average and several times every year by 2100.

When this area floods the sea water will extend well into the Town Centre. Even in the 1990s, before the sea wall was built, sea water flooded Commercial Road as far as the entrance to the Multi-Story car-park - a distance of 300 metres from the Town Bridge.

Sea water over-topping the new harbour defence wall would extensively flood the Town Centre street adjacent to Weymouth Harbour.

The adjacent figure 6 shows approximate land height contours in Weymouth Town Centre. These are plotted relative to the chart datum upon which tidal levels are based and not on the Ordnance Survey Datum used on land maps.

The red dots enclose land less than about two metres above tidal datum and the blue contours enclose land below about three metres above chart datum.

For a larger scale map (220 kB) please click here.

The land enclosed in red dots - the central area of the 'Park District' and Radipole Park Drive - are below sea level at some time in most months. 

However, this does not mean that they are flooded because they have no direct access to the sea. Past floods have occurred when heavy rainfall sweeping into Radipole Lake, has been unable to run to sea because of a high Spring tide and pumps have failed to hold back the flow of water back up the drains.

Obviously, the pumps work adequately on the overwhelming number of Spring tides otherwise flooding would be a frequent event every year instead of once every few decades.

When the River Wey is not in flood then the sluice gates on Westham Bridge (slightly below centre of the picture) stop the sea flooding back into Radipole Lake and threatening the 'Park District' and Radipole Park Drive.

However, by 2050, tidal heights of three metres will be 'three-times-a-decade' events. The top of the new harbour wall is about three metres above chart datum so that this will be over-topped and land within the blue dotted area will be prone to direct flooding.

By 2100, three metre tides will become monthly occurrences see figure 4. 

The above image is copyright Dorset County Council 2000 and is reproduced here with permission.

This map is very similar to the Environment Agency's 'Flood Risk' map, see [9]

The whole of the area enclosed in blue dots will be below sea level at some time every month. This can be seen from figure 6 to be the entire 'Park District' and the western half of the Town Centre.

Clearly, remedial work and new harbour defences will need to be installed before the second half of the 21st century dawns.

By 2100, a new harbour wall must be built at least 500 mm higher than the recently built wall and a vastly more extensive system of pumping will be needed to prevent backflow through the town's drainage system.

References

1.    http://www.weymouth.gov.uk/main.asp?svid=137&svaid=268&svapid=2451

2.    Attwool, M. and West, J., (1995) Weymouth: An illustrated history. Blandford, Dorset, The Dovecote Press, ISBN 0 946159 14 9

3.    http://www.pol.ac.uk/psmsl/datainfo/rlr.trends

4.    http://www.agu.org/revgeophys/dougla01/node3.html#SECTION00030000000000000000

5.    IPCC report (2001) Climate Change 2001: The Scientific Basis. Contributions of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.

6.  Dorset Coast Digital Archive http://www.dcda.org.uk/1-3coastalform/3detailed.html

7.   Dorset Coast Forum http://www.dorsetcoast.com/index.jsp?articleid=331646

8.   Weymouth Harbour flood defence http://archive.thisisdorset.net/2002/01/28/92594.html

9.   Environment Agency (2001) Flood Forecasting Map  http://maps.environment-agency.gov.uk/wiyby/mapController