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

The purpose of this paper is to describe and evaluate the nature of th e European historical archives which are suitable for the assessment o f the temporal occurrence and forecasting within landslides studies, u sing the British south coast as an example. The paper is based upon th e 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 a s possible. Thus, the analysis of archives is, potentially, an essenti al preliminary to the study of the temporal occurrence of landslide ev ents. The range of such data sources extends from isolated, fortuitous ly dated sites from the Quaternary assemblage, through inferred event impacts using dendrochronology or lichenometric time series to histori cal 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 considera ble value in establishing the characteristics of geomorphological proc esses. These include narrative in literature, prints and other artwork , terrestrial and aerial photographs, remote sensing series, newspaper s, incidental statements and scientific journals and reports. There ar e numerous difficulties in accessing, extracting, organising, databasi ng and analysing such data because they are not usually collated for s cientific use. Problems involve such incalculable errors as: the exper ience, training and conscientiousness of the observer; the editing and recording process; judging the validity of the data used and the haph azard nature of recorded events in time and space. Despite these diffi culties, such data do yield a record which adds to the representative temporal sample at 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 Ven tnor complex on the Isle of Wight initially established in 1991 by Geo morphological Services Limited (GSL), now of Rendel Geotechnics, and s upplemented by the collections of the first author. The record display s an increase in landslide events over the present century, due probab ly to increasing technology and awareness of hazard and the developmen t of process geomorphology. However, the landslide record was subseque ntly correlated with the Ventnor precipitation series. This indicated that wet year sequences usually gave rise to significant landslide eve nts. The increasing variability and number of rainfall events predicte d by various climatic units, e.g. the Hadley Centre, may therefore ind icate a fundamental increase in landslide events in the future.