There is a well-recorded and photographed history of coastal flooding in and around the harbour at West Bay and up the Brit Valley towards Bridport. The list of years when extensive flooding and storm damage occurred goes back on record to 1744, the year after the first piers were completed. Recently severe damage or flooding has occurred in 1974, 1978, 1988,1990,1991,1995 and 1996.

Both West Dorset District Council and the Environment Agency both have flood defence and coast protection responsibilities. They have long sought a solution to the problem. The West Bay Coastal Defence and Harbour Improvements Scheme of 2002/3 is the latest of many attempts to protect West Bay. The cost of repairing the damaged historic piers and sea defences had reached a point where a major new scheme was required, and reports were commissioned to analyse the existing defences and assess the risk of future flooding. This information was part of the case put forward by West Dorset District Council for getting grant aid to build the new West Bay Coastal Defence and Harbour Improvements Scheme.

Coastal flooding can be caused by structural damage to coastal defences allowing seawater to flow through a breach or flooding may arise from overtopping of existing defences. There are several factors to consider here.

A. Likelihood of extreme wave conditions
B. Areas at risk from flooding
C. The condition and estimated lifespan of flood and coastal defences
D. Effect of climatic change and predicted sea level rises

A. Likelihood of extreme wave conditions

Fetch lengths at West Bay vary according to the direction of the wind, but with over 5,000 kilometres from the nearest landmass, the prevailing south westerly winds are able to generate waves and swell over a very long distance.

A look at the wind/wave roses which researchers have drawn up from data collected at West Bay show that the most frequently occurring winds come from the SW (220N). See figure 15.

The highest waves also came from this direction. This drives the sea onshore with ferocious waves able to erode and transport material away and attack defences. The orientation of the two original piers also allows the wind to funnel in and create a wave swell with turbulent conditions for boats entering the harbour. Photographs like the one at figure 16 of huge waves crashing over the piers, give some idea of the battering that this coast is exposed to.

Note that wave direction is not necessarily the same as wind direction, as nearshore waves are refracted and create a different wave environment.

West Bay's exposure to the Atlantic Ocean means that both swell and wind waves may cause damage and overtopping. Engineers describe this as a bi-modal sea. Flume studies have shown that a swell sea with the same wave height as a wind sea caused an increase in overtopping by a factor of between 2 and 3 times. Currently knowledge of impacts of swell waves is limited, but it is believed that they are potentially very damaging.

For the purpose of designing engineering solutions to West Bay flood and coastal defences, hydraulic studies were carried out at HR Wallingford, bringing together actual data on wave heights etc with sophisticated modelling both physical and computer generated. The highest extreme waves modelled at Wallingford were over 7 metres high with a period of between 8-9 seconds, predicted to occur as a 1 in 200 year event. With the present regime this would cause massive flooding and damage.

Assessment of likelihood of extreme water levels and wave heights occurring together based on HR Wallingford results.

At West Bay, flooding and damage depends on both wave and water level conditions. A large wave storm may not cause problems if tidal water levels are low. Equally a storm surge and high tide levels may not cause extensive damage or flooding if wave conditions are mild. It is the combination of the two that create trouble. In order to establish the flood risk, the engineers need to combine the probabilities of both occurring together.

Extreme water levels were worked out using carefully adjusted estimates based on POL statistics from Devonport and Weymouth. Predictions for swell waves are more difficult as most models use only wind waves. Meteorological surges are also generated remotely and further complicate the picture. This information was used in design.

B. Areas at risk from flooding before the 2002 West Bay Coastal Defence and Harbour Improvements Scheme

Analysis of the map at figure 17 shows the various areas at risk from flooding assuming a 3.3m Above Ordnance Datum (AOD) flood.

The map is based on a Digital Terrain Model. Flood routes were also identified and compared with photos of previous flooding events. Some houses are cut off in high floods; others may be inundated in certain conditions. Heavy storms are likely to bring additional damage from shingle within storm waves overtopping defences.

The area most at risk from coastal flooding is to the east of the harbour entrance, which is only protected by the shingle ridge of East Beach. Once over this, seawater has easy access to West Bay Road and the valley beyond.

In recent years there has been a major loss of material from East Beach with a consequent increase in flood risk. Serious flooding occurred in 1990, but this was less severe than the 1974 flood shown in the photo of West Bay Road at figure 18.

Periodic beach overtopping has enforced the need to build up the shingle ridge and replenish it to create a stronger flood barrier, especially for the cottages on the beach itself. Unlike the West Beach, there has been no hard engineering approach to the East Beach, as it's an SSSI. However serious consideration of further options is being undertaken.

Economic considerations

In 2001 the value of properties at risk from flooding was assessed in order to make the case for spending so much money to protect them i.e. a cost benefit analysis. Also costed in were properties, which would be lost as a result of sea wall or pier failure followed by erosion of the land behind the defences.

C. The state of flood and coastal defences before the 2002 West Bay Coastal Defence and Harbour Improvements Scheme

A summary of coastal defence problems is shown at figure 19.

• The sea walls to the west of the harbour were being undermined by erosion.
  Failure of these walls would result in:
  
  • widespread and permanent tidal flooding
  • loss of properties through coastal erosion
    
    
• The two ancient piers were in urgent need of major strengthening.
  Failure of the piers would lead to:
  • widespread and permanent tidal flooding
  • loss of properties
  
  
• The highly mobile East Beach sea defence bank is at risk of being breached during storms. If the terminal support of East Pier were to be lost then the beach would be severely reduced in size and very likely to be breached.
  Failure of the East bank will lead to:
  • widespread and permanent tidal flooding.

Estimating lifespan of existing defences

A residual life summary is shown at figure 20.

The problems of the West Bay coastal defences were assessed in 2001 by structural engineers and WDDC. They examined 8 "defence lengths" and noted their method of construction and condition. The residual lives (length of time before failure) were assessed.

It was predicted that there was a 50% chance that the sea walls and piers would be subject to a major failure within the next 5 years.

D. Effect of climate change at West Bay

Rising sea levels

Coastal engineers are now advised to take into account expected rises in sea level due to global warming and isostatic readjustment when designing coastal defences. Mean global sea level rise of 1/2mm/year over the last century is now widely accepted. At West Bay sea level rise is assumed by DEFRA to be 5mm/yr. Within the present harbour, sea defences are much lower than the coastal defences and are therefore vulnerable to rising sea levels.

Increased storm frequency

There is some evidence that the return periods of extreme weather events are shorter than before, with implications for flood risk. An increase in storm frequency and intensity might be expected with climate change but research remains to be done. Impacts will depend on the local beach morphology and the effectiveness of any flood defence structures.

Changes in winds and waves

Higher sea levels and changes to storm tracks would impact on wave climates and affect the height of the waves that break onto the shoreline and against coastal defences. Offshore wave heights in the North Atlantic have increased by 1-2% since 1960. Between 1960 and 1986 observed mean annual wave heights off Cornwall increased 22mm/yr (CIRIA 1996). At West Bay a trend of rising wave heights of only 4mm/yr has been observed probably due to climate change.

Changes to the river

The risk of river flooding is likely to increase with greater winter rainfall and more intense storms. Changes in land use with further loss of flood plain due to housing development will also affect the capacity of the drainage basin to deal with water. If the result of global warming is an increase in fluvial flow then flooding is more likely and any flood prevention scheme will need to take this into account.

The River Brit flood defences may be breached if the sluice gates are left open during an extreme water level event in the harbour and the water levels in the river's catchment are also high. A high sea surge accompanied by high river levels would allow fluvial flow to back up and weir over the embankments into the surrounding area. The map at figure 21 shows the various areas predicted to be affected by a 4 metre flood.

Summary of implications of climate change for West Bay

• Increase in frequency and magnitude of overtopping along all the sea front and harbour
  
  
• Exposure to larger waves may cause damage and possible breaching of sea defences
  
  
• An increase in flood risk from the harbour and River Brit flood defences due to sea level rises and fluvial flow back up
  
  
• A change in the pattern of long shore drift.