12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
STANDARD METHODOLOGY TO STUDY DEBRIS FLOW HAZARD AND RISK MITIGATION IN ALPINE BASINS
Raffaele Rocco1, Nathalie Bétemps2, Evelyne Navillod3, Ester Buonaiuto4, Giuliana Rezzaro5 and Paolo Ropele6
INTRODUCTION
In 2005, the public administration of “Regione Autonoma Valle d'Aosta” (Italy), according to a specific program of hydraulic and geological risk prevention and prediction, developed a strategy to evaluate the debris flow hazard mapping in 155 cone areas, in order to define new measures, in addiction to the existing ones, to reduce the potential risk of debris flow.
The starting point of the evaluation process was focused on the flood and landslide hazard maps already available before 2005, concerning the different municipalities of the entire Region and designed according to the regional urban planning law. The analysis of these maps, and in particular the urban areas exposed to high or medium levels of flood and landslide hazard, defined the list of the most dangerous fans to investigate. The list of fans to be studied includes those in which were recorded historical events of debris flow.
The 155 fans, classified by their high-medium level of hazard, have been studied from 2005 to 2010 by 34 different groups of professionals consisting of an engineer, an agronomist and a geologist. A technical scientific supervisor group coordinated the study activities, according to a specific methodology in order to get a homogeneous level of analysis in the 34 different group.
METHODOLOGY
The methodology provides a fairly standardized set of steps (such as for example surveying and geological maps, land use and damages definition). Other steps must be adapted to the peculiarities of the basin under study: it is therefore of fundamental importance the experience and sensitivity of professionals for the interpretation of available information.
The cognitive phase involves determining spatial-phenomenological framework and the possibility that a debris flow in the basin under study occurs. The proposed method, in particular, attaches great importance to estimate the volumes using two approaches:
1. Geomorphologic method (Hungr et al., 1984 & Spreafico et al., 1999);
2. Volumetric method (D’Agostino et al., 2003), starting with the hydrograph built using a hydrological modelling with a return period of 200 years and a meteoric phenomenon of short duration and high intensity.
In the assessment phase, the information collected in the previous survey phase was used for zoning:
based on surveying, the critical cross sections, those in which the peak of the debris-flow is not completely collected by the torrent, have been identified. This analysis shows the volumes collected by the torrent and the coming out volumes on the fan.
After the first critical cross section, the following checks were made only using the volume collected by the torrent while the volume spilled was distributed on the fan, according to the following criteria:
1 Dr. Raffaele Rocco. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Italy
2 Dr. Nathalie Bétemps. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Via Promis 2, 11100 Aosta, Italy (email: n.betemps@regione.vda.it)
3 Dr. Evelyne Navillod. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Italy
4 Dr. Ester Buonaiuto. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Italy
5 Dr. Giuliana Rezzaro. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Italy
6 Dr. Paolo Ropele. Regione Autonoma Valle d'Aosta Dipartimento difesa del suolo e risorse idriche, Italy
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Calculation of stopping distance (several formulations);
Calculation of the maximum lateral dispersion (several formulations);
Distribution of volume on the fan, using geomorphologic criteria according to obstacles that can cause diversion;
Checking phase of the thickness of the volume distributed on the fan.
The standard criteria to edit the hazard maps provide as high hazard level the area of the fan directly interested by the debris-flow, as medium hazard level a geometric buffer to the high hazard level area and low hazard level the remaining part of the geomorphologic fan.
CRITICAL ASPECTS AND CONCLUSIONS
Among the main problems encountered it is necessary to underline those which arose in the course of activities less standardized.
The first critical aspect was to determine the propensity of the basin to generate debris flow. Even if in the catchment area many basins have a large quantity of available material, the entire quantity of this material could not be transported by a 200 years return period flow. The group of professionals, according to a focused historical research and field surveys, had to estimate the volume of material that could be transported by the torrent flow and could reach the fan and be deposited out of the torrent bed or the normal quantity of material as normal sediment transport.
A second critical aspect concerned the definition of the rheology of the debris flow, important parameter that determines the dynamics and hence the choice of the numerical approach to be used for hydraulic analysis.
Another critical aspect was the determination of the hydrograph taken as reference, in fact, the choice of the distribution of rainfall and its duration determine hydrograph with different characteristics.
The last delicate phase was hazard mapping, in fact, it was necessary to calibrate the information provided by the proposed formulations with morphological features detected on the fan, taking into account possible interference with housing, roads, depressed areas.
After the revision of the hazard maps, a new set of measures (structural interventions) was finally proposed to reduce the potential risk of debris flow in urban area.
REFERENCES
D’Agostino V., Marchi L. (2003). Geomorphological estimation of debris flow volume in alpine basins. Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment, Rickenmann &
Chen (eds). Millpress, Rotterdam, ISBN 9077017 78X, pp. 1097-1106.
Hungr O., Morgan G.C., Kellerhals R. (1984). Quantitative analysis of debris torrent hazards for design of remedial measures, Canadian Geotechnical Journal, 21, pp. 663-677.
Spreafico M., Lehmann Ch., Naef O. (1999). Recommandations concernant l’estimation de la charge sédimentaire dans les torrents. Berne: Groupe de travail pour l’hydrologie opérationnelle.
Keywords: debris-flow, hazard mapping, alluvium fan, standard methodology
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