RINGSTEAD

Rock armoured groynes and beach replenishment at Ringstead has protected the properties but obscured the geological interest.

'Hold the Line'

Ringstead is a small hamlet of 18 houses and a caravan site lying some 2 miles to the east of Osmington. The village faces into Weymouth Bay and is protected, to some extent, by low reefs of Jurassic Osmington Oolite limestone and a shingle beach.

Two storm events in November 1989 and January 1990 stripped the beach, transporting the shingle off shore, far enough for it to become trapped and essentially lost. As a result, the beach level was reduced, allowing larger waves to reach the low cliffs of Kimmeridge Clay accelerating the rate of erosion.

The preferred scheme put forward by West Dorset District Council address a number of problems;

· Beach stability
· Back shore stability
· Environmental effects of the proposal
· Drainage to the cliffs and hinterland

The scheme consists of beach replenishment and the inclusion of a rock armoured groyne/strong point. The backshore area was regarding and the foreshore reinstated with imported beach material to an appearance similar to that pre 1989. The cliff faces and surrounding areas were regarded and landscaped. Drainage was installed to control water levels.

Environmental considerations

· The coast lies within the Dorset Heritage Coast and is designated as a Site of Special Scientific Interest, particularly for its geology.

· The scheme has slowed erosion rates
· It has restored the beach
· Access to the water has been provided via a slipway

· The scheme has greatly reduced access to the basal Kimmeridge Clay in the foreshore and low cliffs. This is a key location at an International level.
· Erosion has increased immediately to the east of the current defences, although the cause is uncertain.

Information from West Dorset District Council

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The following is an extract from West Dorset District Council's Engineering Division on the coast defence situation at Osmington Mills:

The geology at Osmington Mills is complex. Upper Jurassic clays, the Kimmeridge Clay, is faulted below the Osmington Oolite. As the cliffs retreat, the dip of the strata increase the proportion of oolitic limestone forming the cliff, which in turn, will increase the likelihood of landslippage.

However, coast defence options at this locality are limited and likely to be costly. The cost benefit analysis is not favourable for implementing a coast defence at this time. A tricky situation!


An improvised defence structure consisting of stone gabions and geo-textile was installed just west of the car park in 1998 by a private landowner. This scheme was unsuccessful as it added weight to the 'head' of the landslide, making movement more likely. Within a year, the structure had slipped to the beach and was completely destroyed.

In 1993, Consulting Engineers, Peter Brett Associates, were commissioned by the Council to investigate the mechanisms for failure and to make recommendations for remedial works. A report on the investigation was published in December 1993. The results of the investigation indicated that joints are present within the oolitic sequence and that, as the backscar increases in height, the possibility of a major failure of the crag increases. This it was considered could be imminent. However, a cost benefit analysis indicated that a scheme to stabilise the coastal slope and the oolitic crag would not qualify for grant aid funding from MAFF.

Whilst a scheme is not economically viable, the Council is continuing to monitor the situation.

The geology here, viewed simplistically, is that of a Corallian Crag of oolitic limestone which has a coastal slope fronting it of Kimmeridge Clay. There are three types of failure mode that are occurring:

a) Mudslide - Near surface mudslide failure of the Kimmeridge Clay, as the toe is eroded by the sea. As this slumps seaward, increasing backscar face of oolitic limestone is exposed.

b) Spalling - Spalling of the exposed face of the oolitic limestone due to weathering of closely spaced joints and bedding planes. This results in the toppling of limestone blocks from the exposed face which are than transported to beach level by mudslides. Failure is likely to be relatively small scale, but persistent.

c) Translational Slides - Investigation indicates that the oolitic limestone is faulted and jointed. It is considered that larger scale joints/faults within the oolitic sequence act as release surfaces allowing the generation of deeper seated slides. There is evidence that a large scale failure, of a 10m deep block, occurred in 1989. The mechanism is such that there will be quiescent periods interrupted by large scale failures.

Information from West Dorset District Council