Coastal erosion on the Grand Atlantic site, South Shore, and risks facing Residential and Tourism ConstructionBy David WingateThe entire low-cost housing development sits on the Southampton geologic formation, which is distinguished by its poor cementation due to its relatively recent deposition as a dune (circa 80,000 years before present). Thus, instead of being like regular Bermuda stone when excavated for building foundations, it is more like the similar aged S.A.L. sand quarry in Southampton.The development is located south of South Shore Road and two of the units are only 50 feet (16 metres) from the lip of a 60-foot (17-metre) high coastal cliff face in that Southampton formation.Near the base of that cliff the Southampton formation is underlain by a protosol (a poorly developed soil formed in the approximate 30,000 year period before the Southampton formation dune was deposited on top of it), and this in turn is underlain by the Belmont formation, a very hard marine limestone shelf just a little above present day sea level.The base of that cliff backs on to an ephemeral beach. When the beach is present it can build up to one metre high on top of the broad level Belmont marine shelf and extend 10 20 metres seaward from the base of the cliff to tideline, but when hurricanes or winter storms wash it away, which happens frequently, the Belmont marine shelf is exposed at or just above tideline and wave wash can extend all the way to the cliff base.The protosol is even more weakly cemented than the aeolianite above it, and because of its location, just 3-7 feet (1-2 metres) above sea level, it is exposed to wave erosion in storms and hurricanes when storm surge can result in a temporarily higher sea level (in extreme hurricanes up to 6-13 feet (2 4 metres) higher than normal !). In other words the un-cemented protosol and part of the poorly cemented cliff above it actually lies at or below sea level in a major hurricane and is additionally subject to hurricane wave battering above that level. This unique combination of geological features and exposure results in extremely rapid erosion during the relatively brief duration of hurricane events. Indeed, I know of no other location on Bermuda that is more prone to rapid erosion than this particular site.The fact that nearly all of the erosion along the South Shore occurs in rare but catastrophic hurricane events was first recognised by the famous American naturalist Addison E. Verrill, who commented in his comprehensive natural history of the Bermuda islands that the great hurricane of 1899 caused more erosion in one terrible night than had occurred within the memory of the oldest living inhabitants!There have been six significant hurricane events in the 25-year period, 1985-2010, viz: Emily (1987); Dean (1989); Felix (1995); Gert (1999); Fabian (2003); and Igor (2010). I have calculated, based on that record and examination of photos of man-made structures that had been built into or below that cliff face during the previous 25-year era of peak tourism all now destroyed that the average lateral erosion of that cliff in each hurricane event was approximately 0.5 metres and that the cumulative lateral erosion has averaged ten feet (three metres), but varying greatly depending on the specific conditions at each site. In a few places only three feet (one metre) of erosion may have occurred but in others the cliff has been cut back by 13 -16 feet (four to five metres)! Much of the erosion has been due to abrasive wind and wave action on the soft rock face but some has also been caused by undercutting and cliff collapse. Photographs taken as recently as the early 1980s show an extensive talus slope in front of the cliff which was densely vegetated with native coastal plants and casuarinas trees that helped to protect the cliff face behind from wave erosion. All of that talus and vegetation has now been washed away along with the man-made beach facilities so that the cliff itself is now exposed to the full force of the waves.Assuming the same rate of erosion into the future, it is possible to predict that the face of the cliff could be eroded back to the location of the nearest buildings presently under construction in about 75 years.However, four factors associated with the currently accepted predictions of global warming make it virtually certain that the rate of cliff erosion here will not remain the same but, rather, will increase at an accelerating rate. These factors are:a. Sea level is rising at an accelerating rate.b. Hurricanes are increasing in intensity.c. The higher the hurricane category, the higher the storm surge.d. The destructive power of a hurricane increases by the cube of its wind speed. (Whereas a category 5 hurricane has only twice the wind speed of a category 1, its destructive force is nearly ten times as great.)Confirmation that the rate of cliff erosion is already on the increase as a result of global warming is readily obtained by looking backwards in time from 1985. In the 25 years from 1960 to 1985, none of the tourism-related structures built on or below that cliff were significantly damaged by any storm event, but in the 25 years since 1985, they have all been totally destroyed!Some people who live in that area of Warwick and who have had the advantage of observing the rate of change there during regular beach walks over several decades are even more emphatic regarding the increased rate of cliff erosion. They believe the new condominiums may be threatened in as little as ten years.Of course the actual rate of erosion that will occur over the next 25 years cannot be accurately predicted because it will depend on how many hurricanes come close enough to Bermuda to cause catastrophic damage and this is beyond the realm of predictability, but it is possible to envision a worst-case scenario of a direct hit category 5 hurricane that could erode that cliff nearly back to the buildings in one single catastrophic event! Given current trends I think it is reasonable to predict that cliff erosion will pose a serious threat to those buildings within the next 25-50 years, which is well below the expected lifetime of a Bermuda building.The foregoing conclusions are predicated on the assumption that nothing can, or will be done, engineering-wise, to protect that cliff against further hurricane erosion. Although the developers have Planning permission to build an anti-erosional “toe wall” or seawall at the base of the cliff and have since stated that they intend to harden the entire cliff face by coating it with gunnited concrete, I would argue that the scale of engineering works required to withstand a major hurricane would have to be so great that it would cost as much or more than the cost of building the housing units! Anything less will be bound to fail as confirmed by the wreckage of numerous previously built seawalls and other anti-erosion barriers all along Bermuda's South Shore coastline.I am not saying that it can't be done. These days almost anything is possible engineering-wise. The real issue here is not whether it can be done but whether the developers and the Government have factored in the immense cost of doing so in a way that could genuinely prevent any further hurricane erosion. It is appropriate to enquire also whether the Bermudian public and our tourist visitors would appreciate having our beautiful South Shore coastline modified into the monolithic concrete seawall that would be required to do so.