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Fig. 13. Comparison of different images of the Garin alluvial fan: after the 1977 event - as the image shows, the debris flow affected a part of the fan without causing any damage to the house destroyed in 2008. In 1998 - the image shows the house destroyed in 2008 as it was until the day before the 2008 event. Risk Management for the Future - Theory and Cases

Figure 13 Comparison of different images of the Garin alluvial fan: after the 1977 event - as the image shows, the debris flow affected a part of the fan without causing any damage to the house destroyed in 2008. In 1998 - the image shows the house destroyed in 2008 as it was until the day before the 2008 event. Risk Management for the Future - Theory and Cases

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Historically the early FRM strategies focused on the implementation of flood control countermeasures, and are generally addressed to as “defence strategies” (Stalenberg and Vrijling, 2006). These flood control practices are widely documented during the Renaissance. For example, Coccapani’s opera entitled “Trattato del modo di ridurre il fiume Arno in un canale” (Tractate on how to transform the Arno river into a channel, Coccapani, 1610) focused on structural controls of the river, either for reducing the inundation risk in the surrounding agricultural areas or for navigation purposes. In that opera, Coccapani harshly criticized the intervention already made, stating that “fluvial accidents [in Arno river] are not due to natural defects, but mainly to improper interventions, made by inexperienced architects”. Also Galileo Galilei (Acanfora, 1990) confirmed that the structural measures to canalise Arno river have been extensively used for river “corrections”, for navigation purposes (see Figure 1).
Historically the early FRM strategies focused on the implementation of flood control countermeasures, and are generally addressed to as “defence strategies” (Stalenberg and Vrijling, 2006). These flood control practices are widely documented during the Renaissance. For example, Coccapani’s opera entitled “Trattato del modo di ridurre il fiume Arno in un canale” (Tractate on how to transform the Arno river into a channel, Coccapani, 1610) focused on structural controls of the river, either for reducing the inundation risk in the surrounding agricultural areas or for navigation purposes. In that opera, Coccapani harshly criticized the intervention already made, stating that “fluvial accidents [in Arno river] are not due to natural defects, but mainly to improper interventions, made by inexperienced architects”. Also Galileo Galilei (Acanfora, 1990) confirmed that the structural measures to canalise Arno river have been extensively used for river “corrections”, for navigation purposes (see Figure 1).
Fig. 2. Elements composing the risk, defined as the superposition of hazard, exposure anc vulnerability. The arrows indicate the effect on risk due to an increase in hazard, vulnerability, exposure (WMO, 2009).
Fig. 2. Elements composing the risk, defined as the superposition of hazard, exposure anc vulnerability. The arrows indicate the effect on risk due to an increase in hazard, vulnerability, exposure (WMO, 2009).
Fig. 3. Aerial pictures of Arno river (Italy) near Montelupo Fiorentino in the period 1954 (left)-1993 (right). Urbanisation that took place especially during the fifties and the sixties, substantially increased the number of receptors along rivers and altered the processes of formation and propagation of floods along watercourses (Autorita di Bacino del fiume Arno, 1997). Flood Risk Management in Rivers and Torrents
Fig. 3. Aerial pictures of Arno river (Italy) near Montelupo Fiorentino in the period 1954 (left)-1993 (right). Urbanisation that took place especially during the fifties and the sixties, substantially increased the number of receptors along rivers and altered the processes of formation and propagation of floods along watercourses (Autorita di Bacino del fiume Arno, 1997). Flood Risk Management in Rivers and Torrents
When the modelling uncertainties do not allow us to obtain results within reasonable tolerances (Figure 4), generally more heuristic approaches are followed for example with the help of empirical evidences. In particular, risk assessment in alluvial fan flooding is affected by stronger uncertainties (see Section 4). In these cases a detailed back-analysis is a basic step to estimate (even roughly) the order of magnitude of the hazards, by means of the reports recorded in newspapers, geomorphological or statistical approaches (Franzi and Bianco, 2000).
When the modelling uncertainties do not allow us to obtain results within reasonable tolerances (Figure 4), generally more heuristic approaches are followed for example with the help of empirical evidences. In particular, risk assessment in alluvial fan flooding is affected by stronger uncertainties (see Section 4). In these cases a detailed back-analysis is a basic step to estimate (even roughly) the order of magnitude of the hazards, by means of the reports recorded in newspapers, geomorphological or statistical approaches (Franzi and Bianco, 2000).
Fig. 5. Florence flooding in 1966, November 34-4th(left) and the so-called “mud angels” (right), that are students and volunteers that helped to move the artistic works of art and to recover books during and after flooding. Risk Management for the Future - Theory and Cases
Fig. 5. Florence flooding in 1966, November 34-4th(left) and the so-called “mud angels” (right), that are students and volunteers that helped to move the artistic works of art and to recover books during and after flooding. Risk Management for the Future - Theory and Cases
Risk Management for the Future - Theory and Cases In order to allow a comprehensive assessment of risks, risk evaluation should elaborate risks scenarios, that is individuate and represent the more significant situations for every level of risks. In this frame, the capacity to simulate different situations, and the availability of flexible analysis instruments can strongly influence the capacity to let a_ better understanding of the consequences of flooding.
Risk Management for the Future - Theory and Cases In order to allow a comprehensive assessment of risks, risk evaluation should elaborate risks scenarios, that is individuate and represent the more significant situations for every level of risks. In this frame, the capacity to simulate different situations, and the availability of flexible analysis instruments can strongly influence the capacity to let a_ better understanding of the consequences of flooding.
Table 4. Strategies in risk treatment. At present, the necessity to take structural countermeasures in flood management, is often a consequence of urbanization and of the flood management strategies followed in the past. Historically the enhancement of protection against floods favored the occupation of floodplains, increasing the total number of receptors potentially at risk. At the present state, the necessity to maintain the economical activities on floodplains as well as the need to protect urban areas, force local authorities to implement, and even expand and extend the present systems of structural protection countermeasures. Risk Management for the Future — Theory and Cases
Table 4. Strategies in risk treatment. At present, the necessity to take structural countermeasures in flood management, is often a consequence of urbanization and of the flood management strategies followed in the past. Historically the enhancement of protection against floods favored the occupation of floodplains, increasing the total number of receptors potentially at risk. At the present state, the necessity to maintain the economical activities on floodplains as well as the need to protect urban areas, force local authorities to implement, and even expand and extend the present systems of structural protection countermeasures. Risk Management for the Future — Theory and Cases
The results of flood risk management should be monitored by authorities, in order to better calibrate the planned interventions. Collected data should improve the informative systems and databases, increasing the total number of recordings of past events (Bovo et a. 2007). Monitoring should also include the after flood debriefing, in order to indicate to managers the lesson learned, and suggest what enhancements should be implemented. In this frame flood protection plans result to be an ongoing process, as they change in time, enhancing the territorial understanding of the flood risks and allowing a constant adaptation to changing circumstances and changing societal requirements. A good example of Italian experience in the mitigation-preparadness-recovery cyclic process can be found in Bovo et al. 2007, as far as Regione Piemonte (Italy) is concerned. By referring to the FRM plan developed by the Po Basin District Authority (indicated with PAI), in the next section, the effectiveness of the FRM strategies is discussed, by referring to either to floods in main rivers or to flash floods in torrents. In particular the role played by the uncertainties in the risk assessment are discussed and put into evidence.
The results of flood risk management should be monitored by authorities, in order to better calibrate the planned interventions. Collected data should improve the informative systems and databases, increasing the total number of recordings of past events (Bovo et a. 2007). Monitoring should also include the after flood debriefing, in order to indicate to managers the lesson learned, and suggest what enhancements should be implemented. In this frame flood protection plans result to be an ongoing process, as they change in time, enhancing the territorial understanding of the flood risks and allowing a constant adaptation to changing circumstances and changing societal requirements. A good example of Italian experience in the mitigation-preparadness-recovery cyclic process can be found in Bovo et al. 2007, as far as Regione Piemonte (Italy) is concerned. By referring to the FRM plan developed by the Po Basin District Authority (indicated with PAI), in the next section, the effectiveness of the FRM strategies is discussed, by referring to either to floods in main rivers or to flash floods in torrents. In particular the role played by the uncertainties in the risk assessment are discussed and put into evidence.
Table 6. Technical definition of the fluvial corridors approach. Risk vulnerability and exposure components are estimated by means of qualitative scales (Table 3). Risk classes are ranked by intersecting the different exposure/vulnerability classes with hazards, obtaining four risk classes. At the present implementation state of PAI, risks are not mapped on topographic layers but refer to administration units. In other words, for each municipality, a risk class has been assigned, without mapping the risks on floodplains. As to the receptors at risk, they are indirectly pointed out in the implementation norms (PBDA, 2001b), which refer to the most relevant categories of receptors which are inside each flooding corridor, discriminating the actions needed, the regulation of activities, prescriptions and prohibitions (Table 7).
Table 6. Technical definition of the fluvial corridors approach. Risk vulnerability and exposure components are estimated by means of qualitative scales (Table 3). Risk classes are ranked by intersecting the different exposure/vulnerability classes with hazards, obtaining four risk classes. At the present implementation state of PAI, risks are not mapped on topographic layers but refer to administration units. In other words, for each municipality, a risk class has been assigned, without mapping the risks on floodplains. As to the receptors at risk, they are indirectly pointed out in the implementation norms (PBDA, 2001b), which refer to the most relevant categories of receptors which are inside each flooding corridor, discriminating the actions needed, the regulation of activities, prescriptions and prohibitions (Table 7).
One of the limitation of the followed approach is that the hazard is defined by referring to the inundation of the design discharges, without considering that locally the real hazard conditions can be very different. For example, some urbanized areas are in the same
One of the limitation of the followed approach is that the hazard is defined by referring to the inundation of the design discharges, without considering that locally the real hazard conditions can be very different. For example, some urbanized areas are in the same
Fig. 8. Flooded areas near Ivrea, with the superposition of the fluvial corridors (before the event, black colour) and after revision of the PAI (in blue colour). Risk Management for the Future - Theory and Cases
Fig. 8. Flooded areas near Ivrea, with the superposition of the fluvial corridors (before the event, black colour) and after revision of the PAI (in blue colour). Risk Management for the Future - Theory and Cases
Fig. 9. Zoning of the areas near Ivrea (PBDA, 2008). Each colour corresponds to a different protection level. In yellow and orange areas there is a deficit in the protection level; in blu areas there is a surplus.
Fig. 9. Zoning of the areas near Ivrea (PBDA, 2008). Each colour corresponds to a different protection level. In yellow and orange areas there is a deficit in the protection level; in blu areas there is a surplus.
Fig. 10. Aerial view of Garin, affected by the debris flow of Cassarot creek occurred on 29 May 2008. 1 - destroyed house; 2 - more destroyed building; 3 - location where one of the cars hit by the flow was found (the fourth victim); and 4 - location where the ambulance and the tractor, hit by the flow, were found. However the bulletin was based on a rainfall forecasting at a regional scale and it could not give more precise indications on the exact location where the highest rainfall intensities might have occurred. Therefore the issue of the determination of the possible receptors at risk was just “downscaled” from a total 2400 fan areas, to about 100 fan areas of the Pellice river basin (293 km2). What is more impressive is that some hours before the event, there were no rainfall evidences about a possible flooding on Villar Pellice. Risk assessment in real time conditions showed to have strong limitations.
Fig. 10. Aerial view of Garin, affected by the debris flow of Cassarot creek occurred on 29 May 2008. 1 - destroyed house; 2 - more destroyed building; 3 - location where one of the cars hit by the flow was found (the fourth victim); and 4 - location where the ambulance and the tractor, hit by the flow, were found. However the bulletin was based on a rainfall forecasting at a regional scale and it could not give more precise indications on the exact location where the highest rainfall intensities might have occurred. Therefore the issue of the determination of the possible receptors at risk was just “downscaled” from a total 2400 fan areas, to about 100 fan areas of the Pellice river basin (293 km2). What is more impressive is that some hours before the event, there were no rainfall evidences about a possible flooding on Villar Pellice. Risk assessment in real time conditions showed to have strong limitations.
Flood Risk Management in Rivers and Torrents
Flood Risk Management in Rivers and Torrents
Fig. 11. Location of the raingauges near Villar Pellice with the intensities and cumulated rainfalls on 29-30 May 2009 (the start time for all of the graphs was midnight on 29 May). The red arrow shows the debris flow time occurrence. the imminence of the event. Rainfalls intensities in the hours before the flooding showed a strong spatial variability but anyhow they cannot considered as exceptional. At Bobbio Pellice rain-gauge station (the nearest to the flooding area) the rainfalls intensities maintained for several hours less than 10 mm/h (an intensity which is not exceptional for alpine areas), and increased suddenly from about 12 mm/h to more than 45mm/h, when probably the debris flood was triggered (Figure 11).
Fig. 11. Location of the raingauges near Villar Pellice with the intensities and cumulated rainfalls on 29-30 May 2009 (the start time for all of the graphs was midnight on 29 May). The red arrow shows the debris flow time occurrence. the imminence of the event. Rainfalls intensities in the hours before the flooding showed a strong spatial variability but anyhow they cannot considered as exceptional. At Bobbio Pellice rain-gauge station (the nearest to the flooding area) the rainfalls intensities maintained for several hours less than 10 mm/h (an intensity which is not exceptional for alpine areas), and increased suddenly from about 12 mm/h to more than 45mm/h, when probably the debris flood was triggered (Figure 11).
Fig. 12. Hazard mapping on the Villar Pellice fan. In this picture, the areas that are shown are classified by areas at risk: CP - area characterised by high debris flow hazard and partially protected; Cn - area characterised by medium debris flow hazard; Ee - area characterised by very high water flood hazard. The red line shows the limit of debris flow deposits. The experience gained in the alluvial flooding allowed the regional authorities to make more steps towards real time assessment of hazards and risks. In particular it was observed (Table 11) that the assessment of the risks could be improved by increasing the risk perception of people living in endangered areas, and helping majors of small municipalities to take decisions in real time by experts (Booker et a., 2009). Moreover a collaboration among weather forecasting centres and experts is also necessary, so that the real time estimations of local rainfalls obtained by radars can allow to take decisions more rapidly and to improve the performance of civil protection strategies and protocols (Borga et al., 2006).
Fig. 12. Hazard mapping on the Villar Pellice fan. In this picture, the areas that are shown are classified by areas at risk: CP - area characterised by high debris flow hazard and partially protected; Cn - area characterised by medium debris flow hazard; Ee - area characterised by very high water flood hazard. The red line shows the limit of debris flow deposits. The experience gained in the alluvial flooding allowed the regional authorities to make more steps towards real time assessment of hazards and risks. In particular it was observed (Table 11) that the assessment of the risks could be improved by increasing the risk perception of people living in endangered areas, and helping majors of small municipalities to take decisions in real time by experts (Booker et a., 2009). Moreover a collaboration among weather forecasting centres and experts is also necessary, so that the real time estimations of local rainfalls obtained by radars can allow to take decisions more rapidly and to improve the performance of civil protection strategies and protocols (Borga et al., 2006).
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