Fuzzy Formative Scenario Analysis for woody material transport related risks in mountain torrents

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Abstract

Extreme torrent events in alpine regions have clearly shown a variety of process patterns involving morphological changes due to increased local erosion and deposition phenomena, and clogging of critical flow sections due to woody material accumulations. Simulation models and design procedures currently used in hazard and risk assessment are only partially able to explain these hydrological cause–effect relationships because the selection of appropriate and reliable scenarios still remains unsolved. Here we propose a scenario development technique, based on a system loading level and a system response level. By Formative Scenario Analysis we derived well-defined sets of assumptions about possible system dynamics at selected critical stream configurations that allowed us to reconstruct in a systematic manner the underlying loading mechanisms and the induced system responses. The derived system scenarios are a fundamental prerequisite to assure quality throughout the hazard assessment process and to provide a coherent problem setting for risk assessment. The proposed scenario development technique has proven to be a powerful modelling framework for the necessary qualitative and quantitative knowledge integration, and for coping with the underlying uncertainties, which are considered to be a key element in natural hazards risk assessment.

Introduction

Particularly since the 1990s, considerable damage occurred in the European Alps due to torrent processes (1999, 2002, 2005, and 2008) and inundation (2002, 2005, 2006). This development has been attributed to both risk-influencing factors, changes in the intensity and magnitude of processes (e.g., Houghton et al., 2001, Solomon et al., 2007) and an increase in values at risk exposed (Fuchs et al., 2005, Keiler et al., 2006, Fuchs and Keiler, 2008). As a result, society increasingly realised – also on the political level – that despite of the considerable amounts of public money spent for conventional technical mitigation and hazard mapping, a comprehensive protection of settlements and infrastructure against any loss resulting from hazard processes is not affordable and economically justifiable (Weck-Hannemann, 2006, Fuchs et al., 2007a). People and political decision makers are increasingly aware of this situation. Thus, in some Alpine countries a paradigm shift took place from hazard reduction to a risk culture (PLANAT, 2004), while dealing with natural hazard risk in other countries still remains conservative until now (Stötter and Fuchs, 2006, Fuchs et al., 2008, Holub and Fuchs, 2009).

The analysis of natural hazard risk is embedded in the circle of integral risk management, including a risk assessment from the point of view of social sciences and economics, and strategies to cope with the adverse effects of hazards. The underlying objective for risk management is the planning and implementation of protective measures in an economically efficient and societal agreeable manner. Thus, risk assessment includes both risk analysis and risk valuation within a defined system at the intersection between different disciplines (Renn, 2008a, Renn, 2008b, Fuchs, 2009). For this reason, the scales of valuation (temporal, spatial, degree of detail) have to be well defined for a sustainable risk minimisation. To be able to compare different types of hazards and their related risks, and to design and implement adequate risk reduction measures, a consistent and systematic approach has to be established. While a hazard analysis focuses on natural processes such as debris flows and floods with related woody material transport, the method of risk analysis additionally includes the qualitative or quantitative valuation of elements exposed to these hazards, i.e. their individual values and the associated vulnerability (Fuchs et al., 2007b).

The event documentation of recent alpine river floods and torrent processes, such as debris flows and excessive bed load transport in gravel bed streams, highlighted considerable shortcomings in the current procedures used for natural hazard and risk assessment (Berger et al., 2007, Autonome Provinz Bozen-Südtirol, 2008). In particular, the effects of changing channel morphology and cross-sectional clogging imputable to woody material transport phenomena were found to amplify process intensities significantly (e.g., Diehl, 1997, Lyn et al., 2007, Mazzorana et al., 2010). Furthermore, existing hazard maps turned out to be not as reliable as expected (e.g., Bezzola and Hegg, 2007, Holub and Fuchs, 2009). In order to improve risk analyses and to support decision making, underlying scenarios have to be re-built based on such issues (Girod and Mieg, 2008), in particular with respect to sources of uncertainty that affect the predictability of the hazard process paths (e.g., Paté-Cornell, 1996, Merz et al., 2008).

To apply the risk equation and redesign the underlying scenarios we propose a nested scenario approach composed of different levels (Fig. 1). According to the parameters of the risk equation, this nested approach is composed of (1) natural hazard scenarios; (2) exposure scenarios; (3) vulnerability scenarios; (4) analyses of values at risk; resulting in (5) risk scenarios. According to the conceptualisation of risk, these nested components have multiple functional dependencies among each other, resulting in compound intersections (Fig. 1).

In this paper we focus on the hazard part of the risk equation, i.e. the investigation of woody material transport related hazard scenarios in mountain torrents. Acknowledging the fact that the definition of robust woody material transport related risk scenarios is necessarily based on an accurate deduction of consistent and reliable hazard scenarios, the case study presented here addresses the following issues: (1) identification of an adequate natural hazard scenario level structure, hereafter denominated as system loading scenario level; and (2) identification of an appropriate scenario level for the description of possible system responses taking place at critical stream configurations (e.g. bridges), hereafter denominated as system response scenario level. In Fig. 2 possible hazard and risk scenarios along a stream configuration are shown. The importance of a robust definition of either consistent system loading scenarios (e.g. flood with high woody material transport rates) or system response scenarios (e.g. system changes such as possible bridge clogging) is indicated to reliably infer the main consequences for the exposed objects (e.g. roads and buildings) in terms of risk.

With respect to the determination of hazard scenarios for debris flows and flood processes characterised by woody material transport, a series of uncertainties have to be considered, namely:

  • (1)

    uncertainties about the possible range of rheological behaviour and the concentration of solids in the liquid–solid mixture of debris flows;

  • (2)

    uncertainties in system loading assumptions (e.g., duration-intensity related uncertainties, uncertainties related to sediment transport rates, uncertainties emerging from woody material transport);

  • (3)

    uncertainties in system response mechanisms (e.g., localised obstructions that divert the flow patterns, influence of small-scale topological features);

  • (4)

    uncertainties concerning the protection system functionality and mitigation effectiveness (e.g., failure propensities of key components within the protection system, sediment dosing behaviour of retention basins, dike failures); and

  • (5)

    uncertainties concerning morphological changes inducing hazard processes (e.g., large erosion phenomena on alluvial fans, flow path changes in steep mountain rivers).

These uncertainties cannot yet be precisely mirrored by common 2D-hydrodynamics simulation models. We postulate here, on the basis of the comprehensive analysis of event documentations, that uncertainties regarding the statistical extrapolations of peak discharges for long return period flood events increase if the floods were accompanied by considerable sediment transport. This trend was found to be even more accentuated if woody material transport takes place (for an overview, see Montgomery and Piégay, 2003). It is a fact that the accuracy, precision and reliability of extrapolations for discharge time series with longer return periods significantly depends on the robustness of the underlying measured discharge time series. Such robust measurement series are comparatively scarce for sediment transport rates in alpine catchments and practically unavailable for woody material transport rates. Moreover, compared to liquid discharge, the currently used investigation methods and calculation procedures are less accurate if sediment dynamics and woody material transport characterise the hazard process.

In order to overcome these shortcomings related to measured data and uncertainties, we propose a concept to support a balanced strategy of investigation based on the integration of available and retrievable qualitative and quantitative knowledge of uncertainties. The approach aims at an identification of relevant impact factors and an exploration of their systemic role by determining possible system loading conditions and system response mechanisms at hydraulic weak points along mountain streams during extreme events. Hence, a comprehensive assessment of the process-response system is feasible and affordable. Therefore we extended and tested a Formative Scenario Analysis approach originally proposed by Scholz and Tietje (2002). Formative Scenario Analysis is based on qualitatively assessed impact factors and the expert-rated quantitative relations between these factors, such as impact and consistency analysis. Within this framework, “formative” indicates a generic mathematical structure behind the scenarios that is combined with quantitative and qualitative expert assessments (Tietje, 2005). Apart from the hazard assessment sensu stricto, all subsequently linked products, such as risk maps, intervention plans, and mitigation concepts benefit from this coherent derivation procedure for hydrological hazards involving woody material transport.

The requirement of a modelling framework that enables rational integration of qualitative and quantitative knowledge in order to analyse complex and often unstructured problems becomes essential if the elements of uncertainty are considerable on both, the system loading and the system response side (Funtowicz and Ravetz, 1994, Kolkman et al., 2005, Refsgaard et al., 2007).

Similar arguments are valid from a system response perspective. If flooding processes were not characterised by considerable sediment load and woody debris transport, currently used hydraulic simulation tools would provide reliable results. However, if sediment loads and woody material transport phenomena occur, complex system responses can be expected, particularly with respect to critical stream configurations such as constrictions at bridge cross-sections. Transported woody material might be entrapped at bridge piers leading to debris accumulation at individual piers. Moreover, if the distance between the bridge piers is smaller that the design log length of woody material (Diehl, 1997), a spanning blockage debris accumulation might occur. Such spanning blockage accumulations, occluding relevant parts of the cross-section, considerably reduce the flow discharge capacity. As a consequence, a change in the flow pattern from open channel flow conditions to orifice flow conditions is detectable. Additionally, considerable scour depths will develop at the pier toes and abutments, destabilising the entire structure of the bridge. On the upstream side of the construction, lateral overflow becomes increasingly probable as a consequence of backwater effects.

Argumentations outlined above had shown that either from the system loading, or from the system response perspective, a practical and effective solution has to be developed in order to close the existing gaps and to increase the reliability and robustness of natural hazard risk management. Therefore, within a scenario development framework (Mahmoud et al., 2009), we applied a level-based scenario approach for woody material transport in torrents and related mountain rivers. The major focus was on the explorative analysis of consequences emerging from hazards induced by woody material transport during extreme flood events at critical channel cross-sections. Therefore, we used Formative Scenario Analysis in combination with Fuzzy set theory to enhance knowledge representation. By applying Rough Set Data Analysis we validated the accuracy prediction of the selected set of consistent scenarios generated by Formative Scenario Analysis.

Section snippets

Risk concept

Risk has been a focal topic of many scientific and professional disciplines as well as practical actions. Consequently, a broad range of conceptualisations of the term exist that nevertheless show as a general basic principle, the combination of the likelihood that an undesirable state of reality may occur as a result of natural events or human activities (e.g., Fell et al., 2008). Originating from technical risk analyses, the concept of risk with respect to natural hazards is defined as a

Model implementation

In this section, the model being set up is implemented based on the case study on selected woody material transport induced hazard scenarios at hydraulic weak points. In order to implement this model, ten individuals were selected from different stakeholder groups, all of which have at least ten years professional experience in applied natural hazard management. Three of these experts were related to the category of academic university research, three to the category of administrative bodies in

Results

By the application of the Formative Scenario Analysis procedure, it was both possible to assess and validate the expert knowledge contained in the mental system map (Fig. 4), as well as to specify and weight the relevant system components in the system loading and system response level. Based on a reasonable identification and robust selection of the relevant key variables, followed by an accurate characterisation of each key variable in terms of activity and passivity ratings, the multiple

Conclusion

Current methods of risk analyses for natural hazards are, from an engineering point of view, based on quantitative methods of impact assessment to a given environmental setting, and require the assessment of processes as well as values exposed. With respect to torrent processes, these quantitative methods usually include process-based numerical analysis, which necessitates precise data on input parameters. Therefore, some limitations occur by applying such approaches. Above all, complex flow

Acknowledgements

The authors would like to express their sincere thanks to two anonymous referees for their insightful comments on an earlier draft of this paper.

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