The Gabion Revetment at Hengistbury Head
Hardly a thing of beauty, but effective none the less.
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.
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The tell-tale rusty-red Irondoggers incorporated into Christchurch castle either by the Normans in the 11th century or by the Parlimentarians in the 1640's. |
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 was already a factor people were aware of in the late 18th century.
But what happened next very nearly spelt the end for Hengistbury Head. In the 1850's the Hengistbury Mining Company removed many thousands of tons of Ironstone Doggers from the beach and from the waters around Hengistbury Head. The result was catastrophic erosion. It was not until almost the middle of the 20th century before measures were taken to arrest the decline, but by then half of the promontory had been lost.
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.
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Taken in 2001. A pile of talus from a recent collapse waits to be washed away A small number of Ironstone doggers begin their new job as protectors of the cliff. |
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 put the risk at 3% or less.
Gabion Revetment
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
The Gabion Revetment at Hengistbury Head
Hardly a thing of beauty, but effective none the less.
Beach Replenishment
One technique 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
Reef
One new and revolutionary idea has recently come forward which is to build an artificial reef off the head. While the main driving force for this idea to provide a tourist attraction for surfers, it could also, possibly replace the original marine iron stone reef/environment removed in the 1850's by the Hengistbury Mining Co. Currently a reef is planned for further West near the old Boscombe Pier but there is still quite some pressure for further investigation into the possibiliy of a dual purpose reef (catering for both beach defence and surfers) nearer Hengistbury Head
Natures defence - Ironstone Doggers
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.