The causes, effects and mitigation strategies relating to ..., Lecture notes of Geology

Download The causes, effects and mitigation strategies relating to ... and more Lecture notes Geology in PDF only on Docsity! SXG 390 M.Hinton. R0416915 The causes, effects and mitigation strategies relating to coastal landslides at Highcliffe and Naish Farm on the Dorset – Hampshire border. A Report submitted as the examined component of the Project Module SXG 390. Michael Trevor Hinton R0416915 September 2007 SXG 390 M. Hinton. R0416915 2 Abstract Coastal landsliding on the Hampshire – Dorset border is examined, particularly with respect to its causes, potential effects and mitigation strategies. These landslides occur on a cliff face that forms part of the Highcliffe to Milford SSSI and are significant in that the geological interest is compromised by stabilisation works. Degradation processes in this particular geological situation have been closely studied and are reasonably well understood. Also described is the package of engineering solutions dealing with the causes of landsliding which has been applied to the western (Highcliffe) sector which is currently stable while the eastern (Naish Farm) sector has continued to degrade providing geologists, stratigraphers and palaeontologists with fresh exposures to study. The effects of continued cliff degradation on the Highcliffe sector could have been disastrous for cliff top residents but the treatment applied to the coastal slope here has concealed the geology. The non-treatment, of the Naish Farm site has resulted in coast protection problems for Christchurch Borough Council, the authority administering the Highcliffe sector, where a terminal problem manifested itself during the 1980s. Because the Highcliffe sector was substantially stabilised by 1985 the research base predates this date. Currently some of the relevant material has been collected together and is included in a website originating from the School of Ocean and Earth Sciences of Southampton University. To place this study in a more modern, broader context information gleaned from The DEFRA/EA Soft Cliffs, Prediction of Recession Rates and Erosion Control Techniques publication is considered and the particular issues involving marine erosion, weathering and the passage of ground water are referred to. Conflicting outlooks relating to stabilisation and potential solutions applicable to the Naish Farm segment are examined in conclusion. 281 words SXG 390 M. Hinton. R0416915 5 1. Introduction 1.1 Scope of work In a geological study of the Barton Beds Burton (1933) mentions ‘sudden landslips’ which cause a loss of clifftop land of ‘at least a foot a year’. Although landslides on the part of the coast in question cannot be said to measure up to the dramatic slips to be seen in mountainous parts of the world they nevertheless are a ‘geohazard’ in their own right. Although there is no known recorded loss of life or physical injury resulting from landslides here baring fossil hunters becoming stuck in the clay, landowners have been greatly concerned by the insidious erosion of their property. The development of Highcliffe village as a retirement area saw extensive development of land, in some cases precariously and some would say unwisely close to the cliff top. 1.2 Objectives The objectives of this report are listed below:- • Describe the geology of the coastal cliff at Highcliffe, Dorset and Naish Farm, Hampshire. • Determine the probable physical processes that have caused landsliding at the two sites. • Examine and describe the effects of landsliding at the two sites. • Describe and evaluate the measures taken to mitigate the effects of landsliding at Highgcliffe. • Describe the concept of the ‘compromise solution’ as a mitigation measure which may be suitable for application at Naish Farm. Any attempts at mitigation of this hazard must be based on understanding the causes and whilst coastal landslides mainly result from erosion of the cliff toe by the sea other, sometimes complex, factors have a major part to play. This report firstly examines the geology of the coastal cliff. An understanding of this is essential as the ground conditions underpin the causes of landslides. The site is an SSSI because of the geological interest and this has led, in the past, to differences of opinion as to how and why, or even if, mitigation measures should be applied. SXG 390 M. Hinton. R0416915 6 1.3 Methodology Much of the literature studied for this report has been described as ‘parochial’, which is inevitable due to the localised nature of the geohazard. There are general principles which can be applied to the study and prevention of coastal landslides but inevitably local research is required before any attempts are made to understand the causes of landslides at a specific site. Mitigation inevitably is closely linked to the causes meaning that engineering schemes are also site specific and localised. As a result of this fact many of the references used in this report are not peer reviewed studies but are commentaries on a local situation by local authority engineering staff or their consultants. Because many of the references were extracted from local authority records they are unlikely to be readily available to the casual enquirer, there being no search system in place to cope with public enquiries. The writer has taken advantage of his employment situation to search records, and acknowledges Christchurch Borough Council Engineering Services Section, specifically Peter Barker, Head of Operations, now retired, for his permission and encouragement to use information relevant to SXG390. (502 words) 2. Literature review Because the subject of this study is provincial and localised the literature also tends to be provincial and localised. Of the 17 references 8 were gleaned from the Engineering Services archives of Christchurch Borough Council and of these, 7, i.e. Barton (1973), Burden (1991), Burton (1933), Halcrow Maritime (1999), Mockridge (1982), Tyhurst (Undated) and Tyhurst (1993) are somewhat dated technical reports or peer reviewed papers relating to geology, landslide research or stabilisation projects on the coastal cliff at Highcliffe on Sea at the extreme eastern boundary of this local authorities area of jurisdiction. Bristow et al. (1991), Hooke (1998) and Melville and Freshney (1982) are textbooks, either specific geological publications, or in the case of Hooke (1998), a textbook, a substantial amount of which relates to a case study involving conservation issues raised by the Highcliffe – Naish Farm coastal degradation situation. McInnes (2003) is a publication by the Standing Committee On Problems Associated with the Coastline, a group of interested agencies covering the central south coast. There are no specific references in this publication to the Highcliffe – Naish Farm site although coastal landsliding issues on SXG 390 M. Hinton. R0416915 7 the Isle of Wight figure significantly. The publication is only referred to here for figures given for sea level rise for the central south coast as a result of climate change. Internet searching revealed 5 sites with either a specific or generalised relevance to this report. Barton and Coles (1984) and Barton et al. (2006) deal with further research into Barton Clay cliff degradation. Lee (2002) is a fairly recent DEFRA/EA publication on the management of soft cliffs and the Maine Geological Survey, Coastal Landslide Hazards (2005) is an interesting highly illustrated website devoted to soft cliff landsliding in Maine, USA, used here for its high standard of presentation and educational value. There is no attempt made here to compare or align the information given to the Highcliffe – Naish Farm Site, it is used simply for its generalisations and visual treatment of soft cliff landsliding. West (ongoing), is primarily devoted to cataloguing local coastal geology and is constantly updated, many studies are listed including some used in this report and the history of coastal landsliding in Christchurch Bay is documented. It is intended for general interest use although primarily focussed on students at all levels up to degree standard. Coastal landslides are not new but have occurred ever since the sea lapped a raised shoreline. To classify as a geohazard a coastal landslide must be perceived as a threat to human interests and in the case of the Highcliffe – Naish Farm site the threat is to residential property, at least as far as the Highcliffe sector is concerned. At Naish Farm the geohazard is low value land loss combined with ‘knock on’ effects further along the coastline. To mitigate the effects of landsliding the causes must be removed and whilst the causes of coastal landslides are broadly similar to those demonstrated in Maine Geological Survey, Coastal Landslide Hazards (2005) the situation at Highcliffe – Naish Farm is modified by the geological situation peculiar to the site. Although the main theme of this reference is the likely acceleration in the rate of landsliding as a result of sea level rise caused by global warming on the Maine (USA) coast the actual mechanics of cliff landsliding are clearly represented. The processes of toe erosion, rotational slumping, the cyclic nature of landslides and the tell tale or detective signs of subtle ground movements are clearly illustrated. Lee (2002) explains that each individual site is unique because the causes are entirely dependent on local conditions and that whilst marine erosion of the cliff toe promotes slope instability other factors such as site geology and ground water behaviour play a significant part in landsliding characteristics. SXG 390 M. Hinton. R0416915 10 Deposits related to an ancient river system that once occupied the Solent area during periods of low sea level. Amongst pioneering commentators the work of Burton (1933) stands out as important here as it was he who assigned the lettering system to the individual horizons of the Barton Beds and his system is followed by all subsequent commentators. Burtons interest was in the superbly preserved fossils to be found in the matrix of the sandy clay and sand sediments and consequently his ‘faunal horizons’ mark changes in the fossil record in preference to lithological changes. Burton listed horizons A1, A2, A3, B, C, D, E, F, G, H, I, J, K, and L with horizon A0 being added by a later commentator (Figure 1) . Our interest only concerns horizons up to about F as the remainder lie further to the east outside the sites in question. When Burton carried out his study the whole of the cliff face was available to him as there were no attempts at stabilisation at that time. Interestingly Burton mentions ‘sudden landslips’ which he estimates to result in a cliff top recession rate of ‘certainly more than a foot during a year’. More recent commentators in Hooke (1998) recognised a series of marine transgressions and regressions in the sequence. Daley quotes Hooker (1976) in describing the Barton sediments as representing four transgressive – regressive cycles. The lower beds in the Highcliffe – Naish Farm section are described as Cycle 1 (Beds A0 to A3), commencing with a pebble bed representing a transgressive lag. Bed A3 with its abraded fossils represents reworking due to regressive phase shallowing. Cycle 2 (Beds B – I) commences with a sharp burrowed junction with rolled shells probably sourced from Bed A3. The glauconitic silty clays at the base of this cycle represent a transgressive phase. 3.2 SSSI status The whole of the cliff face fronting Christchurch Bay which includes the 3 km combined Highcliffe and Naish Farm sites is a Site of Special Scientific Interest first notified in 1953 (Halcrow Maritime, 1999), Under Section 28 of the Wildlife and Countryside Act (1981) the Site was notified in March 1991. The cliff face exposes the only complete late - middle to early - late Eocene sequence in the world and is the Type Section or International Stratotype for The Bartonian stage of the Eocene, (Hooke, 1998), the age of which ranges from 41.3 Ma to 37.0 Ma. Consequently the natural, gently sloping to the east, rock exposures revealed on this stretch of SXG 390 M. Hinton. R0416915 11 coastline are highly valued by geologists, stratigraphers and palaeontologists. Additionally the cliff east of Chewton Bunny forms fairly easily accessible, excellent, barely vegetated examples of degrading soil slopes of interest to engineering geologists. Burden (1991) mentions their value as a teaching aid visited by generations of students :- ‘The site has been visited by student undergraduate parties for many years and has played a vital role in the training of geologists, on whom the economy depends to find coal, oil, gas, water and building materials’. Also,’……… regularly used by established academics and research students from both Britain and overseas’. It is against this geological background that the causes, effects and mitigation strategies relating to coastal landslides at Highcliffe and Naish Farm are considered. Differing outlooks, especially in recent years, have resulted in clashes of interest and a ‘Property vs. fossils’ debate has ensued which resulted in a public enquiry (Burden, 1991), called to examine Christchurch Borough Councils proposal to extend its coast protection works into the Naish Farm sector. (762 words) 4. The causes of landsliding at the two sites 4.1 General overview of causes Before a boundary change in 1974 split this part of the coast between Hampshire and Dorset both the adjacent Highcliffe and Naish Farm sites were in the county of Hampshire. Also, it should be pointed out, that at this time only preliminary stabilisation works had been carried out at the eastern end of the Highcliffe site (Mockridge, 1982). Bartons (1973) pioneering study therefore examined almost the complete combined lengths of the two adjacent sites, almost 3 km, which is made up of the gently easterly dipping Barton strata from horizon A0, at beach level in the west to horizon F, at cliff top level in the east. About 5 m of Pleistocene gravels, the Plateau Gravels, overlie the clay strata combining to give a cliff top height of 30 – 35 m. The factors involved in coastal landsliding are listed as (Lee, 1982) :- SXG 390 M. Hinton. R0416915 12 External factors:- • Undermining of the cliff by wave action • Oversteepening of the cliff by wave action • Unloading of the cliff by removal of debris from the toe • Lowering of beach levels Internal factors:- • Weathering • Stress relief and swelling • Strain softening • Groundwater level changes • Shrinkage 4.2 Analysis of landsliding processes at the two sites Barton concluded that it was the geological make up of the site which caused the cliff to assume a terraced profile, (Barton, 1973), and calculated that the overall gradient of the cliff would be stable at 14 degrees (Barton and Coles, 1984). Because of the rapid rate of degradation and minimal vegetation cover he concluded that this was an ideal site on which to study soil slope failure mechanisms. The processes which resulted in landsliding along the complete combined length of both sites were examined and five were listed as combining together to cause the transport of cliff top and mid cliff material from the top of the Plateau Gravel progressively down to beach level, a maximum descent of 30 – 35 m. The primary cause of land sliding is listed as marine erosion of the cliff toe, the process resulting mainly from winter storm conditions which overwash the base of the cliff. Weathering and the percolation of groundwater which issues from the gravel – clay interface also contributes substantially to cliff sliding (Lee, 2002). Partly attributed to weathering and partly to the saturation of the clays by groundwater are the erosive effects of streams and mudflows. Weathering is the main cause of scarp spalling, the collapse of the often near vertical gravel face at the cliff top resulting from the saturation of the gravels during storm conditions when they are subject to driving rain SXG 390 M. Hinton. R0416915 15 5.2 Engineering solutions Soft cliff stabilisation is dependant on the implementation of measures to overcome the following five factors (Lee, 2002) • Reducing pore water pressure in slopes through surface and sub surface drainage. • Reducing de-stabilising forces by re profiling cliff or landslides. • Increasing stabilising forces by adding weight to the toe. • Supporting unstable areas by construction of retaining structures. • Preventing the erosion of exposed slopes and cliffs. Implementing these stabilisation measures contrasts quite well with Bartons five degradation processes (Barton, 1973) with cliff toe protection and surface and sub surface drainage measures emerging as the principal factors in achieving cliff stability. The detail of the mitigation measures applied to the Highcliffe cliff is set out in Mockridge (1982) and Tyhurst (1993). By 1971 a stout timber revetment and 11 sheet piled groynes stretched the length of the cliff between Highcliffe Castle and Chewton Bunny. The purpose of these structures was to protect the cliff toe and to encourage a pebble beach to accumulate behind the revetment. Following a Halcrow design (Mockridge, 1982) a diaphragm wall was sunk into the clay substrate at the eastern end of the cliff some tens of metres behind the cliff top, the purpose of which was to prevent ground water oozing from the gravel–clay interface. This accumulated water was piped via a series of drains directly down to beach level and was harmlessly absorbed into the foreshore. A horizontal drain ran along the cliff about half way down the slope with the purpose of piping any accumulated ground water from the slope face directly into the drainage system. The lower slopes which could be drenched with sea spray during storms were drained by means of regularly spaced rock filled ditches. Attention given to vegetation on the slopes and terraces meant that salt tolerant grass mixes had to be developed. The obvious benefits resulting from maintaining a vegetation cover being that root systems hold together potentially unstable soils with plants taking up a certain amount of moisture during their biological processes. SXG 390 M. Hinton. R0416915 16 Additionally, dense evergreen foliage on the upper slopes was encouraged to provide an umbrella cover to disperse the concentrated effects of wind and rain during winter storm conditions. Tyhursts (1993) description with accompanying simplified diagram (Figure 3) clearly sets out the mitigation strategy of engineering works that by 1985 had been applied to the complete Highcliffe frontage and his undated study, Tyhurst (undated) describes the attention given to the vegetation at the site with salt tolerant grasses sown on the lower slopes and selective use of small trees and shrubbery to protect, drain and assist in stabilising the upper slopes. Subsequently the 11 sheet piled groynes were progressively converted to substantial random rubble moles to deflect direct wave action and the high maintenance timber revetment was buried beneath many thousands of tonnes of imported gravel leaving a mixed sand and gravel beach which formed a pleasant summer recreational amenity. By the early 1990s a total of £2.5 M had been invested in the Highcliffe frontage (Hooke, 1998), probably amounting to around £4 M at late 1990s prices. This amounts to about £3 k per m run. As a measure of the effectiveness of the stabilisation techniques, apart from some minor soil creep on steep slopes there has been no major movement on this cliff section for at least 10 years and during summer months the stabilised coastal slope forms a pleasant vegetated south facing recreational amenity with a view of the approaches to the Solent, The Isle of Wight and The Needles. Figure 4 is reproduced from Halcrow Maritime (1999) and indicates the forecast cliff recession at Naish Farm by 2049 based on a broad estimate of a 1 m/yr recession rate. The mitigation proposal from Halcrow, if mitigation is appropriate, is to stabilise both ends of the site allowing the large central section to reach an equilibrium situation. It is unlikely that similar stabilisation techniques to those applied at Highcliffe will ever be applied to the Naish Farm section of the cliff due to conservation issues combined with negative cost benefit analysis results. However, both Lee (2002) and Barton in Hooke (1998) discuss ‘ compromise solutions’ which may be suitable for application to Earth Science Conservation Sites where it might be appropriate to slow rather than prevent cliff degradation. The construction of offshore reefs or nourishment of beaches are two techniques that are mentioned as possible solutions as they do not directly interfere with exposed geology but give a degree of protection to the cliff toe. (892 words) SXG 390 M. Hinton. R0416915 17 6. Conclusions Hopefully this report has given the reader a basic understanding of the causes, effects and mitigation strategies that apply to landslides on this stretch of the coast of Christchurch Bay spanning the Dorset – Hampshire border. The large chapter devoted to the geology of the site is felt to be prerequisite to an investigation of causes and this is stressed by all sources. Because of the early geological interest in the coastal cliff there was a substantial literature base prior to more recent engineering investigations which preceded mitigation schemes. Whether or not coastal landslides require mitigation depends on ones point of view, whether or not the landslide is viewed as a ‘geohazard’ and whether or not the expense of mitigation can be justified. The techniques used at the Highcliffe segment of the site would be eminently suitable for use at Naish Farm because of the geological similarity but the expense combined with environmental considerations preclude this at present. Compromise solutions such as offshore reefs or beach nourishment, or allowing erosion and degradation to occur between strong points may be a way forward for mitigation at Naish Farm. In any case this report has attempted to show that at the location in question causes and processes are well understood, that the perception of effects is somewhat related to ones point of view and that successful mitigation strategies are available should they be needed and the expense justifiable. It is commonly understood that sea level rise resulting from climate change will accelerate coastal landsliding worldwide and sooner or later policy makers will need to make difficult decisions as to whether to simply allow this to occur or make partial efforts to slow down the process rather than halt it. (322 words) Word total :- 4696 SXG 390 M. Hinton. R0416915 20 7. References Barton M E 1973. The Degradation of the Barton Clay cliffs of Hampshire. Quarterly Journal of Engineering Geology. Vol 6 pages 423 – 440. Barton M E and Coles B J 1984. The Characteristics and Rates of the Various Slope Degradation Processes in the Barton Clay cliffs of Hampshire. Quarterly Journal of Engineering Geology, London, 17 117 – 136. (Abstract in West [constantly updated]) Barton M E, Hillier S and Watson G V R 2006. The Slip Surface in the D zone of the Barton Clay. Quarterly Journal of Engineering Geology and Hydrogeology. 39:44, 357 – 370, Geological Society. (website) http://cat.inist.fr/?aModele=afficheN&cpsidt=18276293 (Accessed April 2007) Bristow C R, Freshney E C and Penn I E 1991. Geology of the country around Bournemouth. Memoir for geological sheet 329. British Geological survey. Burden P R , (inspector). 1991. Report on public inquiry into a scheme of coast protection works at Chewton Bunny. Submitted to the Minister for Agriculture Fisheries and Food for approval under section 5 of the Coast Protection Act 1949. 24 pages. Burton E St John 1933. Faunal Horizons of the Barton Beds in Hampshire. Proceedings of the Geologists Association. Vol 44 131-167 Halcrow Maritime 1999. Poole and Christchurch Bays Shoreline Management Plan. Volume 2 Physical and Natural Environment. Hooke J (editor) 1998. Coastal Defence and Earth Science Conservation. The Geological Society. Lee E M 2002. Soft Cliffs: Prediction of Recession Rates and Erosion Control Techniques. DEFRA/EA (website) http://www.defra.gov.uk/environ/fcd/CliffsRepFinal2.pdf (Accessed April 2007) SXG 390 M. Hinton. R0416915 21 Maine Geological Survey, Coastal Landslide Hazards 2005. (website) http://www.maine.gov/doc/nrimc/mgs/explore/hazards/landslide/facts landslide.htm (Accessed April 2007) McInnes R G 2003. Coastal defence, a non technical guide. SCOPAC. Cross Publishing. Melville R V and Freshney E C 1982. British Regional Geology. The Hampshire Basin and adjoining areas. Natural Environmental Research Council. Mockridge R G 1982. Highcliffe cliffs – the maintenance of coastal slopes. Proceedings of a conference organised by the Institution of Civil Engineers. University of Southampton. Pages 235 – 242. Tyhurst M F Undated. Coastal vegetation work at Highcliffe on Sea. Christchurch Borough Council. Technical Services Division. Tyhurst M F 1993. Cliff Stabilization Techniques at Highcliffe. A students guide. Christchurch Borough Council. Technical Services Division. Tyhurst M F 2000 Global warming. The Effect of Sea level Rise on Christchurch’s Coast. Christchurch Borough Council. Technical Services Division. West I (constantly updated) Geology of the Wessex Coast. (website) http://www.soton.ac.uk/~imw/ (Accessed April 2007)