Denys Brunsden

A critical assessment of the sensitivity concept in geomorphology

Abstract The landscape sensitivity concept concerns the likelihood that a given change in the controls of a system or the forces applied to the system will produce a sensible, recognisable, and persistent response. The idea is an essential element of the fundamental proposition of landscape stability. This is described as a function of the spatial and temporal distributions of the resisting and disturbing forces and is known as the factor of safety or the stability index. The resistance of a system is defined by the system specifications: its structure, strength properties, transmission linkages, coupling efficiency, shock absorption capacity, complexity and resilience. The disturbing forces include the steady application of energy from the specified tectonic, climatic, biotic, marine and human environmental controls. Change takes place through time and space as a normal process–response function to these specifications and involves material transport, morphological evolution and structural rearrangement. These, in turn, progressively change the system specifications, which alters the performance through time. To make progress with these issues, the nature of waves of aggression, temporal adjustments to disturbing forces, spatial interactions with structure, divergent pathways of change propagation, evolution of ‘barriers to change,’ effects of inheritance, decoupling, and the effects of change on system specifications all need to be understood at all temporal and spatial scales.

Large scale geomorphological mapping and Highway engineering design

Summary Geomorphological mapping in site investigations for highway engineering is proving to be a rapid means of obtaining much relevant information about ground conditions. Such mapping requires the recognition of both the origin of surface features and the geological processes that still influence them. In addition information about materials is recorded and inferences made about their extent. The paper identifies eight aims of a geomorphological survey for highway engineering and discusses these in the context of small-scale maps and large-scale plans. The latter are illustrated by case studies from Nepal and South Wales. The review identifies the established applications of geomorphological mapping to site investigation and concludes that further work can usefully be directed towards a broadening of the geomorphological content of site investigation.

The nature, use and problems of historical archives for the temporal occurrence of landslides, with specific reference to the south coast of Britain, Ventnor, Isle of Wight

Abstract The purpose of this paper is to describe and evaluate the nature of the European historical archives which are suitable for the assessment of the temporal occurrence and forecasting within landslides studies, using the British south coast as an example. The paper is based upon the British contribution to the Environment programme EPOCH, 1991–1993. A primary requirement of a research programme on process occurrence is to determine the event frequencies on as many time and space scales as possible. Thus, the analysis of archives is, potentially, an essential preliminary to the study of the temporal occurrence of landslide events. The range of such data sources extends from isolated, fortuitously dated sites from the Quaternary assemblage, through inferred event impacts using dendrochronology or lichenometric time series to historical records of causal factors such as rainfall data and more recently, deliberately recorded packages of cumulative or continuous data. Most countries have extensive historical sources which may be of considerable value in establishing the characteristics of geomorphological processes. These include narrative in literature, prints and other artwork, terrestrial and aerial photographs, remote sensing series, newspapers, incidental statements and scientific journals and reports. These are numerous difficulties in accessing, extracting, organising, databasing and analysing such data because they are not usually collated for scientific use. Problems involve such incalculable errors as: the experience, training and conscientiousness of the observer; the editing and recording process; judging the validity of the data used and the haphazard nature of recorded events in time and space. Despite these difficulties, such data do yield a record which adds to the representative temporal sample as a level above some threshold reporting position. It therefore has potential for specific statistical analysis. An example of a reasonable temporal landslide record is the data base of the Ventnor complex on the Isle of Wight initially established in 1991 by Geomorphological Services Limited (GSL), now of Rendel Geotechnics, and supplemented by the collections of the first author. The record displays an increase in landslide events over the present century, due probably to increasing technology and awareness of hazard and the development of process geomorphology. However, the landslide record was subsequently correlated with the Ventnor precipitation series. This indicated that wet year sequences usually gave rise to significant landslide events. The increasing variability and number of rainfall events predicted by various climatic units, e.g. the Hadley Centre, may therefore indicate a fundamental increase in landslide events in the future.

Geomorphological roulette for engineers and planners: some insights into an old game

Engineers have always appreciated that a civil engineering project should be based on a clear understanding of the geomorphology of the site of the proposed works. It is an old ‘game’! However, although it is good practice to take full account of the morphology and processes of the landscape during an investigation there is a tendency to restrict the work to basic mapping and identification of features rather than a true understanding of the site in space and time. Particular criticisms are that not enough attention is paid to the effects of landform change, the identification of inherited material conditions, the frequency and magnitude of processes and the nature of risk. Ways of improving the service are identified. It is suggested that future work should be based on a full understanding of the conceptual basis for modern geomorphology. In the past thirty years there has been a revolution in the theoretical basis of the subject as well as a remarkable improvement in the technical capability. A major statement is therefore made on the nature of geomorphology and this is then summarized as a possible conceptual basis for application to engineering and planning. The theme of the paper will be illustrated by a description of the Lyme Regis Environmental Improvements Scheme in West Dorset which is in the design stage and has been based on a full understanding of the geomorphology and geology of the area and the employment of a Geo-Team at all stages of the project.

The morphology of degraded landslide slopes in South West Dorset

Summary Geomorphological techniques, including morphological mapping and profiling have been used to investigate the distribution of old landslides on the southern slopes of the Char valley in West Dorset. It is shown that these field techniques provide data which can be usefully employed to precede and supplement geotechnical investigations. The spatial pattern of landslide morphology is described and a tentative chronology suggested. The survey reveals a similar, although more extensive and complete, pattern of landslide degradation to that reported for slides on London Clay and the Lias Clay of the East Midlands.

Rapid geomorphological assessments for engineering

Summary Geomorphological mapping is being used increasingly in engineering projects (especially at the reconnaissance and site-investigation stages) as a rapid, highly cost-effective means of assessing both potential hazards and resources. This is normally achieved through a field mapping programme which is dependent on the availability of aerial photographs, and which involves the correct interpretation of landforms as to their origins, material composition, and associated present-day geomorphological processes. The results of such a survey need to be presented in a form which is of direct use to an engineering geologist and/or engineer. This is best achieved, in most cases, by the production of problem or site-orientated maps derived from the primary geomorphological survey. The case studies, drawn from several investigations in arid land environments, were made by a small team of (3-5) geomorphologists and geologists, working for short but concentrated periods of time in the field. A search for fine aggregates in Bahrain was closely associated with the identification and mapping of contributing drainage networks and source areas for alluvial fan deposits. Thelocation of trial pits and boreholes at a site in Dubai was rationalised in terms of a reconnaissance map of both landforms and their constituent materials; while at Suez, extrapolation between existing trial pit and borehole sites was made possible from the geomorphological mapping of landform boundaries. Hazards to engineering from flooding (at Suez) and dune migration (in Dubai) were identified, assessed, and mapped on the basis of a knowledge of their place in the overall geomorphological system.

A preliminary geomorphological assessment of part of the Karakoram Highway

Summary The Karakoram Highway has been constructed through extremely difficult high arid mountainous terrain characterized by long, steep slopes, highly fractured and heavily weathered rocks, extensive areas of debris accumulation and locally high rates of geomorphological activity. The problems for road construction imposed by difficulties of access and remoteness have been compounded by rockslope instability, debris flows, river and meltwater surges, and fluctuating glacier snouts. As a consequence, the employment of sophisticated engineering techniques was abandoned in favour of widespread blasting and an extended period of labour-intensive maintenance. This paper is concerned with an 147 km stretch of the Highway and essentially subdivides into two parts. The first part describes the terrain conditions and main hazards posed to road construction, and includes a geomorphological assessment of the chosen alignment. The second section presents the results of a detailed survey of 128.8 km of the Highway in terms of the patterns of disruption experienced during the first three years of its life, and the distribution of a variety of hazards which will continue to endanger the road. This survey clearly reveals the importance of a good appreciation of geomorphological processes to highway engineering in arid mountains, as well as indicating the potential for geomorphological inputs to highway design.

Geomorphological Investigations for the Channel Tunnel Terminal and Portal

Extensive geomorphological investigations of the Channel Tunnel UK portal and terminal site were commissioned by TML during 1986/87, prior to the commencement of the detailed ground investigations. These studies were primarily concerned with identifying and delimiting the spatial extent of landforms, superficial materials and in particular the evidence for contemporary or relict slope instability. The investigations took the form of large-scale (1:500) geomorphological mapping of the proposed development area. This paper describes the results of the mapping and how the information was incorporated into the site investigation and subsequent construction programme. In addition, some conclusions are drawn about the geomorphological evolution of the landslides that had developed off the chalk escarpment in the study area.

Engineering geomorphology on the coast : lessons from west Dorset

The central aim of this paper is to describe the general context in which an applied geomorphological investigation for a management project on a Heritage coast will be set. We attempt to show how the decisions may be affected by historical legacies and public or administrative attitudes. Modern attitudes to the coast in Great Britain are summarized in the light of recent studies by the Department of the Environment and the Ministry of Agriculture, Fisheries and Food. The Dorset coast in southwest England is used to illustrate the main points. The paper describes the coastal features, explains the historical legacy of use, and examines problems of contemporary coastal management. The paper concludes with a consideration of the natural geomorphological principles of landscape design which might be employed as part of the guiding concepts.

The reactivation of a landslide during the construction of the Ok Ma tailings dam, Papua New Guinea

The Ok Ma dam (Ok is a local word meaning river) was to form part of the permanent tailings disposal system designed for the Ok Tedi gold and copper opencast mine located at Mount Fubilan in the Star Mountains of Papua New Guinea. During the construction works a landslide involving approximately 35 million cubic metres of soil and rock moved downslope to partially fill the foundation excavations of the dam. The failure of this landslide was the start of an insurance litigation that finally reached the Supreme Court of Papua New Guinea in November 1989. The geotechnical, geomorphological and engineering geological data that were available at the time of the failure are presented together with an evaluation of the key deductions from the case study.