This article reports on a seminar held at the Institution of Civil Engineers in May 2004. This, and a previous meeting in 2002, stemmed from Government-funded research into soft cliff erosion. While the previous meeting dealt with the theory, this one dealt with more practical issues by way of four papers. Dr Neil Dixon described work carried out on landsliding of unprotected London Clay cliffs at Warden Point on the Isle of Sheppey. Dr Alan Clark described practical examples of engineering schemes at Lyme Regis and at Castlehaven, Niton and the Military Road on the Isle of Wight. The final two papers described aspects of work on the Norfolk coast: Dr Mike Walkden presented a process-based regional model of shoreline recession while Peter Frew of North Norfolk District Council described the problems faced by his authority in managing a retreating coastline where property is under threat.
1. INTRODUCTION
The Coastal Engineering Advisory Panel of the Maritime Board hosted a half-day seminar at Great George Street on 27 May 2004. Entitled ‘Soft Coastal Cliff: Understanding and Managing Retreat’, it attracted some 60 participants. The four speakers had been assembled by Dr Beatrice Baudet of University College London and Dr Mike Walkden of Bristol University, who himself led the second session.
This report was prepared following the meeting but remained unpublished. Given the present general acknowledgement that changing climate will have a significant effect on cliff erosion, particularly that of soft cliffs, it is considered that the papers are still relevant despite the time since they were presented.
2. THE FIRST SESSION
Mr Hugh Payne, chairman of the first session, commented that the meeting was a follow-up to a previous meeting in April 2002. The feedback from that meeting was that there was a concentration on the theory of the subject but little on the practical side. This meeting was intended to redress the situation. He pointed out that both meetings stemmed from research into cliff erosion and that the results of the 1995–1999 work funded by the Ministry for Agriculture, Fisheries and Food (MAFF; now the Department for Environment, Food and Rural Affairs (DEFRA)) was the subject of a publication.1
Dr Neil Dixon, the first speaker, a senior lecturer in the Geotechnical Department at Loughborough University, spoke on ‘Landsliding in London Clay Coastal Cliffs’. This work, in conjunction with Professor Edward Bromhead of Kingston University, was essentially on the causes of landslides. In particular, factors controlling rotational landsliding in the unprotected London Clay coastal cliffs at Warden Point, Isle of Sheppey, in the Thames estuary were described.
The cliffs vary in height between 37 m and 45 m and the clay thickness is of the order of 127 m. Observations have been made over 30 years, supported by an extensive subsurface investigation including 56 piezometers. Dr Dixon commented that the site was ideal for the study of landslide mechanics since the geology is uncomplicated, the strata are relatively uniform and homogeneous and the geotechnical properties of London Clay are well known.
The studies resulted in an understanding of the pore water pressure in the cliffs, including the role of undrained unloading resulting from slope formation, and pore pressures within the landslide mass ‘carried down’ the slope.
Most engineering experience of coastal landslides in UK is of reactivation of pre-existing landslides, not of first-time failures. In contrast, the November 1971 failure at Warden Point (site K39) was a first-time event. A feature of the failure was its speed in comparison with reactivated slides. Dr Dixon also referred to the toppling failure on the nearby Herne Bay cliffs in May 1988 (site K38).
The authors had prepared a model to allow calculation of pore water pressures associated with the degrading slide mass. This allows back-analysis to be carried out which extends the residual shear strength envelope for London Clay to high effective normal stress levels, and explains the rate of reactivated slide movements.
The conclusion from the Warden Point study is that depressed pore water pressure in the natural slopes subject to unloading is a significant effect. The zone of such pressure moves inland with the slope as the coastline retreats. It is local to the slope, but may extend up to 50 m inland.
The second paper in the first session was entitled ‘Sea Coastal Cliffs–Managing Coastal Retreat’ and had been prepared by Stephen Fort. His absence in Vietnam meant that Dr Alan Clark, also of High-Point Rendel, presented the paper.
The paper described practical examples of engineering schemes relating to the management of unstable cliffs categorised as
unstable protected
unstable unprotected
unstable unprotected where protection was not an option.
The case studies for the first two categories were Lyme Regis in Dorset and Castlehaven, Niton on the Isle of Wight. The third category was represented by two examples on the south coast of the Isle of Wight, involving the A3055 Military Road.
Despite its thirteenth century history, Lyme Regis is sited on pre-existing landslides protected by coastal defences. Reactivation of these coastal landslides by loss or collapse of the defences, especially in view of greater storminess, would result in considerable property and infrastructure damage. As it was, a number of properties had been lost in the past.
Instability at Lyme Regis, along slip surfaces controlled by limestone marker horizons within the Lias geology, has been affected by groundwater levels, inappropriate excavation and construction, and both loss of defences and foreshore lowering.
Dr Clark described the remedial measures adopted–a slope stabilisation scheme and substantial piled foreshore works. It was hoped the works would start in 2005 at a cost of some £15–20 million.
Turning to the Isle of Wight, the second case study was Castlehaven, on the Niton Undercliff, part of the much larger undercliff which is a post-glacial landslide complex that is approximately 12 km long and 500 m wide.
The fishing village is backed by steep, actively eroding cliffs some 15 m high, which have retreated up to 40 m since 1862. As elsewhere, coastal erosion of soft cliffs and coastal retreat can reactivate older landslides, such that movement may be experienced significantly further back from the coast. The geology comprises Upper Greensand overlying Gault Clay, but the failure mechanism is complex.
The site has a number of environmental designations and any remedial scheme had to recognise these, particularly the ecological impact on foreshore reefs. However, precisely because of the further impact of uncontrolled erosion, a scheme could be designed–and was currently under construction–with a robust benefit–cost ratio.
The two examples of unstable unprotected cliff where protection of the toe was not an option were also on the Isle of Wight and concerned the Military Road on the south-west coast. Here the cliffs, which are up to 70 m high, are eroding and in places the highway is only 10 m from the cliff edge.
With two issues of environmental designations preventing cliff protection and realignment of the highway, but also a requirement to retain the route as long as possible, particularly for tourism, the first scheme was devised whereby all works were carried out from the highway. These comprised deep piles connected by a capping beam and separate drainage, the provision of inclinometers, and inclined tension piles. The works were completed in September 2003, at a cost of £2·5 million, and envisaged the eventual ‘suspension’ of the road.
At the second site, where the eroding slope is of Wealden shales and marls and the road was likely to be in danger within 10 years, a realignment was possible.
The presenter's conclusion was that although environmental constraints may be onerous they could form a key driver for an innovative solution, which was economically and technically sound, but the scheme promoter had to have the enthusiasm to persuade others of its viability.
The two papers provoked a lively discussion.
3. THE SECOND SESSION
Mr Brian Waters introduced the second session.
Dr Mike Walkden from the University of Bristol, the first presenter, has been developing a process-based regional model of shoreline recession. He explained that a process-based model was better for predicting future recession than one based on past recession rates as drivers of recession are anticipated to alter with predicted changing climate. The model therefore uses information on drivers such as water levels, surges, waves and currents together with physical properties of the materials to consider erosion within the whole of the shoreline system (the beach, the shore platform underlying and fronting the beach, and the cliffs themselves) as well as interactions along the coast. The model can simulate the local effects of coastal defence structures. He had found that erosion of the shore platform had a significant effect on recession of the cliffs and its inclusion was critical to the success of the model.
He demonstrated the model with reference to work undertaken on the north Norfolk coast. It was successfully validated against recession derived from four OS surveys between 1885 and 2002, giving three different recession periods. So far, it has been used to predict 19 different scenarios over the next 50 years using future climate predictions from the Tyndall Centre. Among the scenarios considered were holding the line of the present coast where it was being defended at present and abandoning all defences. Between those more extreme cases, the effects of defending and not defending certain lengths of coast were examined. Where the coast has been defended historically, the coast is forward of its natural position. It was found that there were large initial increases in recession in those areas when defences were removed with, as expected, a corresponding reduction in recession in the immediate downdrift area until the coastline assumed its equilibrium state when more even rates were resumed. Work is still ongoing in collaboration with the Tyndall Centre to consider scenarios over the next 100 years.
In response to questions, Dr Walkden confirmed that interactions were governed more by sediment transport than by the strength of the materials being eroded, and that a relatively simple relationship for material strength was adequate. The model can differentiate between defence using structures and beach nourishment as a form of defence, but not between different types of defence structure. The effect of beach recharge has been investigated and it was found that, while recharge boosted the beach and led to an initial drop in erosion, rapid erosion followed as the beach platform in front of the recharge lowered. Dr Walkden was able to reassure those interested in shoreline management planning that the model could be used to assess the effects of shoreline management policies at the different epochs for which the latest guidelines required consideration.
Peter Frew of North Norfolk District Council has the unenviable task of managing the coastline on which Dr Walkden demonstrated his model. It has a length of 34 km of which 22 km is defended in some manner. An added complication is that 21 km has some form of Nature Conservation designation, including candidate special area of conservation (cSAC) and site of special scientific interest (SSSI), which restricts the operations that can be undertaken. Management policies set down in the first generation of shoreline management plans required 45% of this length of coastline to be held on its present line, with managed retreat over another 38% and nothing done on the remaining 17%. It was acknowledged that these policies were not realistic and that, in the second generation of shoreline management plans there would be ‘no active intervention’ at undefended lengths and an increase in managed realignment where the line was currently being held.
Mr Frew then illustrated the problems faced with examples from Overstrand, Trimmingham and Happisburgh. These are all small communities close to the cliff top that are currently defended, but with defences that are now threatened with collapse. Under the present system of scheme appraisal and prioritisation, it is unlikely that new defences could be justified at these locations or be eligible to receive government grant aid. At Happisburgh, 800 m of the original defences have been lost and 25 properties have had to be abandoned since 1986. If retreat is implemented there will be a loss of homes, businesses and farmland together with considerable public opposition. On the other hand there will be benefits in terms of habitat creation and increased sediment availability.
In the short term Mr Frew considered there was a need to enforce planning policy to prevent new development in areas expected to be lost to the sea. Other possible approaches were the purchase of threatened land and property, compensation for losses or relocation of entire settlements inland. Short-term defence to slow erosion may be useful to allow more time for managers and property owners to come to terms with the situation. The public needs to be educated to reduce the present expectation that they will always be defended. Many settlements have been lost to the sea in the historical past. Relocation of settlements in a timely manner needs more consideration and coordination between DEFRA and the Office of the Deputy Prime Minister (now the Department for Communities and Local Government), which is responsible for land-use planning. Compensation will need the approval of the Treasury.
Mr Waters, the chairman of the session, summed up by referring to the essentially practical nature of the papers presented, which at the same time indicated coastal management options for perhaps up to 100 years. One point that had been well made was that the provision of weight at the toe of a cliff by recharge and/or armouring could be negated by lowering of the foreshore platform beyond it.
He felt that some emotive aspects of retreat and loss of property could be met by state-sponsored compensation and that a better calculation of benefit–cost ratios might result from including a value for sediment put into a system from erosion. Similarly it should be possible to give notional value to environmental habitat formed by retreat.
A vote of thanks to the speakers, the organisers (Beatrice Baudet and Mike Walkden) and to Marta Dziedzicki of the Institution of Civil Engineers was carried by acclamation.
