Erosion has been a continuous factor at Hengistbury Head since the sea finally reached its base a few thousand years ago. However, Hengistbury Head was provided with a natural defence in the form of heavy Ironstone Doggers that fell from crumbling cliffs to the beach below and built protective barriers, both on the beach and off shore. Consequently erosion at Hengistbury Head was a slow if not stationary process. The area had been stable for around 2000 years.
However the Ironstone Doggers gained their name from the fact that they could provide good quality iron ore. Some of the Doggers have been found to contain 30% by weight Iron Ore. Their use and exploitation dates back to pre-history but every age since the Iron-Age has exploited the rusty red coloured stones.
In the 11th century some were used in the construction of Christchurch Castle, later in the 17th century many tons were used to build Clarendons Jetty (also called the Long Rocks). Longshore drift closed Yarrantons cut through Mudeford sandspit in the early years of the 18th century so by then it would appear that erosion was taking a hold. Later in the 18th century Smeaton recorded that Hengistbury Head was suffering major erosion, confirming that erosion has already a factor people were aware of in the late 18th century.
The major erosion mechanism effecting Hengistbury Head is driven by wave action and is called Longshore Drift. Before the current sea defences were installed, and after the Ironstone Doggers had been removed, the sea had direct access to the base of Hengistbury Head. In storm conditions wave action on the soft rocks of Hengistbury Head undermined the cliff face and caused sections of the cliff face to collapse. The prevailing wind in this region is from the South West. The waves normally break on the shore at an angle of about 45 degrees, driving in from the South-West. These waves reflect off the beach and retreat at 90 degrees to the incoming wave. After crashing on the beach the waves pick up a quantity of the sand and retreats from the beach carrying the sand away. Of course, the waves can also carry sand into the beach. Beach building or beach erosion depends on whether or not the waves drop more sand than they carry away. With Hengistbury Head more sand is carried away than is dropped so there is erosion. As the sand grains are removed they are replaced by the soft rock (talus) from the fallen cliff face. This soft rock is soon pummelled into fine sand and replaces the sand that has just been washed away. The process at Hengistbury Head would have continued until Hengistbury Head was completely washed away unless remedial action had been taken.
The original natural defence against Long Shore Drift has been provided by the large Ironstone doggers that had fallen to the beach from the eroded Upper Hengistbury Beds. They were usually far too heavy to be shifted by the sea and so formed a defensive barrier on the beach and also, they formed a reef like structure off-shore. This reef absorbed a great deal of the energy of the attacking waves and also contained the sand washed off the beach. Because of these natural defences, high energy waves reached the base of the cliff infrequently and Long Shore Drift was at least partially arrested. The beach and cliffs remained stable.
Regrettibly erosion is a problem within the whole area. Bournemouth cliffs and the nearby coastline and cliffs at Highcliffe and Barton also having suffered large scale land loss.
Bournemouth cliffs have been stabilised by the construction of a concrete promenade with long groynes. The consequent containment of loss of material from Bournemouth cliffs had essentially increased the erosion at Hengistbury Head as the arrival of material from the South-west had decreased . The building of groynes at Hengistbury has helped to contain this loss. Highcliffe had a very large concave beach defence erected in the 1960's below Steamer Point and considerable work has also been done at nearby Barton. Interestingly the beaches at Mudeford and Highcliffe have built up considerably in the last 30 years while the trailing sandbank off Mudeford Spit new seems less prominent. The end of Mudeford Sandspit has also been reinforced with a rock groyne
While the sea defences erected at Bournemouth and at Hengistbury Head have significantly slowed the erosion, regrettably it is difficult to see how full equilibrium can now be restored within this area, without the utilisation of new techniques. There has been concern of a breach across barn Field which would turn Hengistbury Head into an Island. There are mixed opinions regarding this possibility with some considering such a disaster almost inevitable within the next 50 years while others (arguably the more expert opinion) put the risk at 3% or less.
One defence scheme used at Hengistbury Head is to regularly replenish the beach with shingle. Shingle is less prone to long shore drift and reduces the overall loss of sand, however it does not completely solve the problem and has to be repeated approximately every ten years. It is also wildly unpopular with those who enjoy the sandy nature of the Hengistbury beaches. Major beach replenishment last took place at Hengistbury Head in 2005/6, focussing mainly on the vulnerable area to the south of Barn field In 2010 beach replenishment was conducted along Fishermans Walk and Southbourne Beaches. Here though the majority of the replenishment was sand.
The method used during this beach replenishment involved building a temporary 50 cms (approx) diameter pipeline from the beach to a dredger, which moored about 300 meters off the shore. The dredger pumped sand as a slurry through the pipe to the beach. The crucial part of this operation is the assembly of the pipe which is in 20 meter (approx) sections of rigid steel tubing. This is built on the beach and floated out to the dedger. In order to ensure that there is not too much lateral force on the steel pipe a tug is positioned half way from dredger to beach and prevents the tide buckling the pipe.
Along with a set of new groynes either side of the original 1938 long groyne a Gabion Revetment has been built to protect the weakest point in the eastern end of Hengistbury Head, on the South beach just before Hengistbury Head rises up to Warren Hill. While this thing is hardly a picture of Beauty it does provide an effective defence and will hopefully prevent Hengistbury Head becoming Hengistbury Island. One major problem with the gabion revetments is their life span. Inevitably, as they exist in a very harsh environment, their useful life is limited.
One new and revolutionary idea that was mooted for the coastal defence of Hengistbury Head would have involved building an artificial reef just off the coast to effectively replace the ironstone doggers that were trawled up in the 1850's. However the reef ended up being built further west and was by all accounts an unmitigated failure. It would seem unlikely that the council would have the appetite for trying the idea again anytime soon.
While all this human activity and hard work continues, Hengistbury Head is also lending a hand in its own defence. As the cliff has receded, the Ironstone Doggers have been slowly slipping down onto the beach and rebuilding the defences stolen by the Hengistbury Mining Company and others. It can be seen, on the beach at Hengistbury Head, that where Ironstone Doggers are most prevalent, the cliff has regressed less.