Floods and Landslides: Integrated Risk Assessment

Even non specialists have a general idea of what constitutes a landslide. Cruden recently reminded us of the precise definition: “a movement of mass of rock, earth or debris down a slope” (Cruden 1991). It is a collection of phenomena differentiated by the type of movement process involved: a fall, either free or with rebounds, a roll, a slide, a flow, which may be either fluid or viscous. And we know that one or more of these processes can happen in different places or successively in a landslide (Dikau et al. 1995).

A flood is an overflowing of water from rivers onto land not usually submerged. Floods also occur when water levels of lakes, ponds, reservoirs, aquifers and estuaries exceed some critical value and inundate the adjacent land, or when the sea surges on coastal lands much above the average sea level. Nevertheless, floods are a natural phenomenon important to the life cycle of many biota, not the least of which is mankind. Floods became a problem as humans began establishing farms and cities in the bottom-lands of streams and rivers. In doing so, they not only exposed their lives and properties to the ravages of floods, but also exacerbated floods by paving the soil and constructing the stream channels. Over time, continued urbanisation of natural floodplains has caused great annual losses of both wealth and human life. In this way, in many countries and regions of the World, floods are the most costly hazards in terms of both loss of human lives and material damage.

The first aim and purpose of the European community research project TESLEC, “The Temporal Stability and Activity of Landslides in Europe with Respect to Climatic Change” (1994–96) (Dikau et al. 1996b; Schrott and Pasuto 1997) was to prepare the technical manual “Landslide Recognition” which presents the main characteristics of different landslide types (Dikau et al. 1996a). The manual is based on a classification of a previous European project, EPOCH, “The Temporal Occurrence and Forecasting of Landslides in the European Community” (1991–1993) (Casale et al. 1994; Soldati 1996).

Natural disasters repeatedly occur with a certain frequency in time and space, nevertheless their progressive intensification during the last decades with effects of amplification and accumulation, as well as, the predominance of geographic localisation may be considered as a consequence of anthropogenic dynamics and development policies of socio-economic systems. The analysis of correlation between disasters and climate change becomes fundamental to better understand causes and effects, in order to calibrate decisions and human interventions rather than assuming disasters as an unpredictable occurrence (Margottini 1994).

Alestalo (1971) first outlined the basic principles of dendrogeomorphology. Basically geomorphic and geologic processes affect trees in a variety of ways that can be determined and dated through tree-ring analysis. Geomorphic stress can induce growth anomalies as suppression and release (sudden growth decrease or increase) (Shroder 1978; Shroder 1980). Whereas, climatic factors can also induce ring variations similar to those of geomorphic processes (Fritts 1971), it is important to exclude the climatic influence on the anomalies found in trees, affected by a geomorphic event, by control trees living in an undisturbed zone, close to the study area.

Weather radar networks operated by national meteorological agencies are well serviced in the primary meteorological requirement of daily weather reporting and forecasting. Visual images of the spatial extent and propagation of storms provided by radar are now a familiar feature of such reporting on television. However, the need for quantitative estimates of rainfall to support applications in hydrology and water resources, especially flood forecasting, has not been so well serviced. Hydrologists and meteorologists have sought to improve the reliability of weather radar and to develop “radar hydrology” products characterised by greater resolution in space and time and improved quantitative accuracy.

The engineering assessment of the stability of natural and man-made slopes as influenced by natural or induced changes to their natural environment can be aided by the application of analytical and numerical techniques.

Several human activities are characterised by a strong interaction with meteorological phenomena, particularly complex social-production-environmental systems are linked to this topic.

A team of scientists from the Department of Physical Geography at the University of Liège have worked with A. Pissart on the present research; Damien Closson carried out the Arc-Info cartography and the studies on the protection of water catchments, on the sewage possibilities and on the fertility of soils; Camille Ek studied the karstic dangers; Michel Erpicum and Georges Mabille the climatological issues, while François Petit was concerned with the flood hazards.

In recent years, thanks among other things to increasing computational and measurement capabilities, there has been a growing awareness in various areas of applied research that it is possible to predict emergency situations with a reasonable degree of approximation, especially in the fields of meteorology and hydrology. With the arrival of new measuring systems (Meteorological Radar and Meteosat Satellites), and especially with the new possibilities offered by Limited Area Models (LAM), it has finally become apparent that it is possible to create complex forecasting systems on a series of different time scales. Nevertheless, there is the feeling that the Public Administration is still reluctant to set up an organisational structure for emergency management based on the early but uncertain forecasting provided by hydrological models or by combined meteorological-hydrological models, and for the time being it is confining itself simply to the monitoring of the events under way. It was therefore felt necessary to clarify a number of basic aspects of meteorological and hydrological measuring and forecasting, in the light, among other things, of the experience gained in the development of a series of projects funded by the Commission of the European Communities, as the precondition for a correct approach to the forecasting and management of emergencies in a series of warning and alert stages with differing degrees of space-time resolution depending on the forecasting advance time.

Advances in flood forecasting concern much more than innovation in model formulation. This paper aims to provide a perspective on developments in the UK over the last twenty years, initially by reviewing models in current operational use but progressing to discuss updating methods, the use of weather radar and lastly the introduction of integrated flood forecasting systems. Updating methods allow real-time measurements, for example of river level received via telemetry, to be used to improve model performance. Weather radar allows point measurements of rainfall, from an often sparse raingauge network, to be complemented by spatially continuous measurements of the rainfall field and thereby, provide improved input to flood forecasting models. Most recently, flood forecasting systems have been developed capable of coordinating the construction of forecasts at many points across a possibly complex region and being generic in their use of models and their configuration to any set of river networks without expensive recoding. These developments are examined with an acknowledged emphasis on procedures developed at the Institute of Hydrology and particularly those methods which are incorporated in its River Flow Forecasting System or RFFS. However, these methods are reviewed with reference to techniques developed elsewhere and which are in use operationally, thereby providing an overview of the present state-of-the-art in the UK. Prospects for future improvement are considered, focusing on the potential value of digital terrain models to formulate a new generation of distributed model appropriate for operational use in combination with radar rainfall and satel-lite-derived thematic data, for example on land-use.

In order to examine the relationships between environment and man, the first being considered from a geomorphological point of view, two main possibilities can be considered (Panizza 1992) (Fig.12.1).

The results presented in this paper correspond largely to the contribution given by the authors to a research project funded by the European Commission under the Environment Programme, titled EUROFlood. The EUROFlood project has been concerned with flood hazard assessment, modelling and management. As such it was involved in determining the nature, extent and severity of flood hazards across the European Union countries, by collecting data on these hazards and modelling their likely future patterns. In addition, however, EUROFlood was also concerned with investigating methods for better management of hazards and populations vulnerable to hazardous events, so as to reduce vulnerability and enhance the use of floodplain areas. A detailed presentation of EUROFlood results can be found in Penning-Rowsell (1996). A full description of the authors’ contribution to this research project is presented in Correia et al. (1996).

It is very often not appreciated by the academic community that those affected by floods and landslides, or by other natural perils, are not or at least primarily not interested in academic research and the results produced by it but by the consequences of such disasters for them and in preventive steps. As the money spent by research is produced by the taxpayers and as one of the main aims of research in such fields should be a pragmatic service to the public, it is felt that more attention should be paid to such issues.

The paper is based on the United Nations — International Decade for Natural Disaster Reduction (UN-IDNDR) pilot study project “Mitigation of the effect of natural hazards on urban areas particularly developing country megacities” for which the writer was the project leader.

It is assumed here that the land use planning practice is mainly an action of negotiation between land private owners and/or land public authorities. The ultimate target is to improve the localization of the land uses, taking in account the natural hazards, so as to be able to alleviate the risks. An intermediate target is to improve the civil and hydraulic structures and the water bodies maintenances undertaken. Alleviate means not only a global reduction of damages, but also a progress in social equity toward the residual risks. These lasts remain always present, even when an actual application of strong land planning measures, and/or of protection structures, is undertaken and respected.

Many researchers who deal with mass movements feel that landslide hazard is generally neglected, more emphasis being given to other types of hazards such as seismic and volcanic hazards. It has been estimated that every year about 225 000 lives are lost because of natural events in general (Burton et al. 1978), among which several mass movements causing casualties are included (for a more comprehensive review of these events see Eisbacher and Clague 1984; Hansen 1984; Brabb 1991; Gares et al. 1994). The fact that landslide hazard is usually underestimated is even more unfortunate, since slope movements are usually more easily predictable and manageable than earthquakes, volcanic eruptions or hurricanes. Actually, besides high-magnitude mass movements which occur quite seldom, there is a huge number of medium to small sized landslides which are so widespread that the related cost for human society is even higher than that of catastrophic events.

Hydrogeological disarrangement is one of the most destructive natural events which every year strikes civil populations, urban settlements and infrastructures world-wide, causing thousands of casualties and remarkable damage.

Mass movements are part of the normal landscape evolution. The two antagonistic processes:

shape the surface forms of the earth.

The large number of landslides released in the Langhe Hills (Piemonte region — Northern Italy) during the intensive rainfalls of 4–6th November 1994 manifests the degree to which water may affect the stability of natural and man-excavated slopes.

The intense storm of 5–6th November 1994 produced up to 300 mm rainfall in 36 h and produced slope instabilities and flooding across the region of Piemonte. Records and literature show that widespread storms, floods and landslides occur frequently and cyclically in this region every year.

As water levels rise, the bed load of a gravel bed river is partly mobilised by drag forces. When the stream can no longer entrain these sediments, the pebbles are deposited on the bed and create locally depositional forms. These bars then perturb the flow and play an important role during subsequent floods.

This work deals with the development of a morphological model, suitable for computing the morphological changes in mountain rivers during a flood event. In most of the morphological river bed models developed earlier, the granulometric compo-sition of the sediment transport assumed to be uniform or represented by a material of some characteristic diameter, i.e. D₅₀, The granulometric composition of the river bed and, particularly in the mountain rivers, is characterised by a variety of dimension of particles, which vary from fine sand to boulders. It is obvious that the uniform material assumption is far from reality and more detailed approaches must be made.

This paper provides a brief study of Varenna basin (Ligury) through the analysis of its instability features and mass movement proneness; this area, during the five years from 1991–1995, has gone through frequent episodes of flood damages and landslides. The quick slide of topsoil debris and its displacement into the stream has dramatically increased the over-alluvion along the riverbed reaches with a strong dissipation of energy, and has made the river more liable to an esondation; besides, in correspondence of a restricted hydraulic section it reaches, several new bank erosions, being themselves responsible for the triggering mechanism in the slopes. For mountain basins, as well as the present one, it is necessary to define geomorphologic and geotechnical characteristics of the landslides, rainfall triggering threshold and thickness critical rate of the materials involved in the mass movements, in order to set out new activities for the reduction of the flood and landslides integrated hazards. These criteria have been used in the analysis of two mass-movements which occurred in Varenna valley during the autumnal rains in 1993–1994; they are still representative of the geological characteristics of most of the examined basin: the figures derived from the study show that these typical landslides always occur in presence of serpentine-schists strongly influenced by tectonic movements and widely spread along the whole basin. The geomorphologic and geotechnical data have been compared to those derived from the survey for the assessment of the slope stability, using the “back analysis” method suggested by Jambu. The results allow to define several degrees of proneness to the instability and, as well as, the degree of geomorphologic hazards in all the above-mentioned area.