Réf. Staffler & al. 2008 - A

Référence bibliographique complète
STAFFLER H., POLLINGER R., ZISCHG A., MANI P. Spatial variability and potential impacts of climate change on flood and debris flow hazard zone mapping and implications for risk management. Nat. Hazards Earth Syst. Sci., 2008, Vol. 8, p. 539-558.

Abstract: The main goals of this study were to identify the alpine torrent catchments that are sensitive to climatic changes and to assess the robustness of the methods for the elaboration of flood and debris flow hazard zone maps to specific effects of climate changes. In this study, a procedure for the identification and localization of torrent catchments in which the climate scenarios will modify the hazard situation was developed. In two case studies, the impacts of a potential increase of precipitation intensities to the delimited hazard zones were studied. The results showed a high spatial variability of the sensitivity of catchments to climate changes. In sensitive catchments, the sediment management in alpine torrents will meet future challenges due to a higher rate for sediment removal from retention basins. The case studies showed a remarkable increase of the areas affected by floods and debris flow when considering possible future precipitation intensities in hazard mapping. But, the calculated increase in extent of future hazard zones lay within the uncertainty of the methods used today for the delimitation of the hazard zones. Thus, the consideration of the uncertainties laying in the methods for the elaboration of hazard zone maps in the torrent and river catchments sensitive to climate changes would provide a useful instrument for the consideration of potential future climate conditions. The study demonstrated that weak points in protection structures in future will become more important in risk management activities.

Mots-clés
Climate change, floods, debris flows, risk management, hazard maps, Alps, Bolzano - South Tyrol.

Organismes / Contact
Department of Civil Protection, Autonomous Province of Bolzano South Tyrol, Bolzano, Italy.
Department of Hydraulic Engineering, Autonomous Province of Bolzano South Tyrol, Bolzano, Italy.
Abenis AG, Chur, Switzerland. a.zischg@abenis.ch.
Abenis Alpinexpert srl, Bolzano, Italy.
geo7 AG, Berne, Switzerland.

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Precipitation Rivers Floods, Torrential events  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italian Alps Autonomous Province of Bolzano – South Tyrol Rio Ridanna/Mareiter Bach and Rio Cengles/Tschenglser Bach     2050-2100

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
Modélisations
Seasonal and regional changes in precipitation patterns are to be expected as follows:
In Autumn, extreme values for daily precipitations are expected to increase by 10% in the Northern Alps and by 20% in the Southern Alps. In winter and spring, an increase between 0% and 20% is expected for both regions (KOHS, 2007). Brunetti et al. (2001) observed a trend for an increase in frequency of extreme precipitation events in Northeastern Italy. Under the most unfavourable conditions, a 100-year event of today could in the future become a 20-year event (Frei et al., 2006).
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
Caspary (2004) underlines that the discharge regimes of the rivers Donau, Enz, Kocher and Alp in South West Germany show statistical instationarities in their time series because of the relative accumulation of extreme events since the 1990s. E.g. a discharge event with a reoccurrence interval of 100 years in the reference period 1932-1976 of the river Enz at the gauge of Pforzheim equals a discharge event with a reoccurrence interval of 30 years in the reference period 1932-2002.
Modélisations
Remarkably increases in runoff and discharge volumes were also computed for the Lavanttal region (Austria) when considering possible effects of climate changes (Regional Office of Carinthia, Department of Water Economy 2008).

An indirect effect of the increase of mean temperature is the rising altitude level for the limit between rainfall and snowfall. In areas of the Northern Alps below 1500 m a.s.l., an increase of flood peaks is expected in winter due to higher soil water contents, the rising of the rainfall/snowfall limit level and due to an increased liquid precipitation (KOHS, 2007).
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
   

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
Identification and localisation of alpine torrent and river catchments sensitive to climate changes
A high spatial variability of the sensitivity of the torrent catchments to specific impacts of climatic changes has been highlighted:
- The runoff of nearly all torrent catchments is expected to increase in summer;
- The runoff in winter is expected to increase only in torrent catchments having a high percentage of their total surface area below 2000 m;
- The bed load transport in summer is expected to increase in high mountain areas and is expected to decrease in catchments at submontane levels;
- The bed load transport in winter increases in a few mountain torrent catchments and does not change in the most catchments;
- In some catchments eroding younger deposits (weathered material), a decrease in extreme events is highlighted;
- In some catchments eroding older deposits an increase in extreme events is pointed out;
- The frequency of small scale debris flow and sediment transport processes is expected to increase in most of the torrent catchments.

Potential impacts of climate changes to the delimitation of flood hazard zones, case study Rio Ridanna/Mareiter Bach
The analyses of the possible impacts of climate changes showed that the flooded areas of a design event with a return period of 30 years representing the assumed future climate conditions (scenario +20%) have a larger extent than the flooded areas of a design event with a return period of 100 years representing the actual climate conditions (scenario 2000). The hazard zones delimited and classified following the guidelines for hazard zone mapping (Autonome Provinz Bozen – Südtirol, 2006) show remarkable changes if considering the assumed changes in precipitation intensities due to climate changes. The hazard zones representing the assumed future climate conditions show a shift from the yellow zones (hazards with low intensities prevail) to the blue zones (the construction of new buildings is regulated), which have significantly increased extent. The potential shifts from blue hazard zones to red hazard zones (the construction of new buildings is restricted) do not show significant consequences for buildings. The expected damages of a flood event with a return period of 30 years (scenario +20%) increased up to 1700% (in comparison to the scenario 2000). The expected damages of a flood event with a return period of 100 years increased up to 207% and up to 117% for an event with a return period of 200 years.

Potential impacts of climate changes to the delimitation of debris flow hazard zones, case study Rio Cengles/Tschenglser Bach
The assumed increase of 20% of the input parameter rainfall intensity for the design events (scenario +20%) lead to an increase of the water discharge of about 37% for a return period of 30 years, of about 45% for a return period of 100 years and of about 31% for a return period of 300 years. The transported volumes increased about 36% for a return period of 30 years, about 51% for a return period of 100 years and of about 43% for a return period of 300 years relative to the design events representing the actual climate conditions. The peak discharge of a design event with a return period of 30 years representing the assumed future climate conditions has nearly the same dimension as a design event with a return period of 100 years representing the actual climate conditions. The areas affected by debris flows increases of about 4–30%. The changes in the extent of the hazard zones do not have consequences for the settlements and do not influence the risk situation.
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Flood and torrential events intensity
Precipitation intensity
The study was made in three main steps. Firstly, the sensitivity of the alpine torrent catchments to climate changes was analysed qualitatively on the regional scale. Secondly, the possible effects of climate changes to the delimitation of flood hazard zones were analysed in a case study. In another case study, the possible effects of climate changes to the delimitation of debris flow hazard zones were analysed. In this study only the impacts of a potential increase in the intensities of extreme precipitation events (>50 mm/d) to the delimitation of hazard zones were analyzed.

Identification and localisation of alpine torrent and river catchments sensitive to climate changes
The sensitivity analysis was made in a pilot area of the Autonomous Province of Bolzano – South Tyrol. The catchments were classified into three catchment classes: mountain torrents, torrential rivers and alpine rivers. The basic assumptions for potential future climate conditions were the following:
– The daily mean temperatures in summer and winter are increasing (Heimann and Sept, 2000; OcCC, 2007);
– The mean sum of precipitation in summer is decreasing or remains constant;
– The mean sum of precipitation in winter is increasing (OcCC, 2007);
– The intensity and frequency of short extreme rainfall events in summer and autumn is increasing (Christensen and Christensen, 2003).

It was assumed that the following factors are varying spatially and are relevant for the sensitivity of the torrent catchments to climate changes:
– Percentage of areas located between 1000 and 2000 m a.s.l.: It is expected that the snow cover in these areas will be reduced and the frequency of combined snowmelt/rainfall events will increase (KOHS, 2006). A threshold value of 50% of these areas respective to the total catchment area was chosen.
– Characteristics of bed load source areas: Bed load source areas could be divided into recent (recent weathering and denudation processes) and older deposits (relict geomorphologic deposition processes). If the percentage of areas with older deposits to the total bed load source area exceeds 30% of the total catchment area, the torrent catchments were classified as torrents eroding older deposits. If the percentage of landslide areas with respective to the total bed load source area exceeds 30%, the torrent catchments were classified as torrents mainly influenced by landslide activity.
– Available bed load source areas: The sensitivity of torrent activity against climate changes increases with a higher proportion of bed load source areas respective to the total catchment area.
– Permafrost degradation: Permafrost influences the hydrology and stability of steep scree slopes. Catchments were classified as sensitive, if more than 30% of the total catchment area is subjected to permafrost degradation. This information layer was created by modelling the permafrost distribution of 1850, 1990 and 2100 (after Stötter, 1994; Stötter & Zischg, 2007). The difference between the datasets of the permafrost distribution of 1850 and 2100 was classified as permafrost degradation areas.
– Areas with elevated surface runoff: Areas with reduced water storage capacities increase the surface runoff. The sensitivity of torrential rivers and rivers to climate changes increases with a higher proportion of areas with reduced water storage capacities respective to the total catchments area.

The delimitated catchment areas were classified by the combination of these factors. The classification was made by means of a decision tree implemented into a GIS-based procedure.


Potential impacts of climate changes to the delimitation of flood hazard zones, case study Rio Ridanna/Mareiter Bach
The focus of this case study lied on testing the robustness of the methods and procedures for hazard mapping to changes of the needed input parameters. On the basis of a literature review, a possible increase of 20% (by 2050-2100) of the precipitation intensity for each design event (reoccurrence interval 30, 100, 200 years), as indicated by Frei et al. (2006) for the Southern Alps, was assumed for this sensitivity analysis. The hazard induced by bed load transport and overbank sedimentation was not considered.

The Rio Ridanna/Mareiter Bach basin lies in the north of the Province. The catchment area is 210 km2. This study area is a representative example for an alpine river with hazard potential for settlements. For the assessment of the present flood hazard situation, this procedure was followed:
– statistical analyses of the precipitation time series of the measurement stations in the study area and calculation of the characteristics of precipitation events relevant for the hazard scenarios with a return period of 30, 100 and 200 years;
– preparation and calibration of the rainfall-runoff model;
– simulation of the inundation processes for each return period;
– delimitation of the hazard zone map;
– analysis of the exposed buildings.

The statistical analysis of the precipitation time series was based on the measurement stations of Ridanna/Ridnaun (31 measurement years). The precipitation values of a rainfall event with a duration of 24 hours representing reoccurrence intervals of 30, 100 and 200 years have been calculated. For the representation of the design precipitation events under future climate conditions (scenario +20%) 20% of these calculated values were added. For the discharge prediction, the rainfall-runoff model Hec-HMS and the SCSapproach was used. For the simulation of the inundation process, the simulation model SOBEK of WL Delft Hydraulics was used.


Potential impacts of climate changes to the delimitation of debris flow hazard zones, case study Rio Cengles/Tschenglser Bach
The Rio Cengles/Tschenglser Bach torrent lies in the western part of the Province. The catchment area is 11 km2. This study area is a representative example for systematized alpine torrents eroding older deposits in permafrost degradation areas. For the assessment of the actual situation of debris flow hazards, the following procedure was followed (IPP 2007):
– characteristics of precipitation events relevant for the hazard scenarios with a return period of 30, 100 and 300 years;
– preparation and verification of the rainfall-runoff model;
– simulation of the bed load transport in the transit area and in the sediment retention basins;
– simulation of the debris flow processes in the deposition area for each return period;
– delimitation of the hazard zone map;
– analysis of the exposed buildings.

The calculation of the rainfall characteristics representing the present climate conditions (scenario 2000) was made following the procedures of VAPI (Villi and Bacchi 2001). For the representation of future climate conditions (scenario +20%), 20% of these precipitation values were added. For the discharge prediction, the rainfall-runoff model Hec-HMS and the SCS-approach was used. For the simulation of the bed load transport in the transit area and in the sediment retention basins, the simulation model DAMBRK of the US National Weather Service was used. For the simulation of the debris flow processes in the runout area, the simulation model Flow-2D (O'Brian, 2001) was used.

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Autonome Provinz Bozen – Südtirol: Richtlinien für die Erstellung von Gefahrenzonenplänen und zur Klassifizierung des spezifischen Risikos, Bozen, 2006.

Brunetti, M., Maugeri, M., and Nanni, T.: Changes in total precipitation, rainy days and extreme events in northeastern Italy, Pure and Applied Geophysics, 21, 861–871, 2001.

Caspary, H. J.: Zunahme “kritischer” Wetterlagen als Ursache für die Entstehung extremer Hochwasser in Sdwestdeutschland, in: Klimaveränderung und Konsequenzen für die Wasserwirtschaft – Fachvorträge beim KLIWA-Symposium am 3. und 4.5.2004 in Würzburg, 135–151, 2004.

Christensen, J. H. and Christensen, O. B.: Climate modelling: Severe summertime flooding in Europe, Nature, 421, 805–806, 2003. [Fiche biblio]

Frei, C., Schöll, R., Fukutome, S., Schmidli, J., and Vidale, P. L.: Future change of precipitation extremes in Europe: Intercomparison of scenarios from regional climate models, J. Geophys. Res., 111, D06105, doi:10.1029/2005JD005965, 2006.

Heimann, D. and Sept, V.: Climate change estimates of summer temperature and precipitation in the Alpine region. Theor. Appl. Climatol., 66, 1–12, 2000. [Fiche biblio]

IPP Ingenieure Patscheider and Partner GmbH: Untersuchungen zur Sensitivität der Murganggefahr des Tschenglser Baches gegenüber möglichen Klimaveränderungen. Unpublished report within the project Interreg IIIB Alpine Space “ClimChAlp – Climate Change, Impacts and Adaptation Strategies in the Alpine Space”, Bozen, 2007.

KOHS – Kommission Hochwasserschutz im Schweizerischen Wasserwirtschaftsverband: Auswirkungen der Klimaänderung auf den Hochwasserschutz in der Schweiz. Ein Standortpapier der Kommission Hochwasserschutz im Schweizerischen Wasserwirtschaftsverband (KOHS), Wasser, Energie, Luft, 99(1), 55– 59, 2007.

O'Brien, J. S. and Julien, P. Y.: FLO-2D, Users Manual, Version 2001.06, 2001.

OcCC: Klimaänderung und die Schweiz 2050, Erwartete Auswirkungen auf Umwelt, Gesellschaft und Wirtschaft, Bern, 172 pp., 2007.

Regional Office of Carinthia, Department ofWater Economy: Rainfall/Runoff Model for small catchment areas in the Lavant Valley for determination of potential future effects through assessment of regional climate change scenario. Interreg IIB Alpine Space project ClimChAlp – Climate Change, Impacts and Adaptation Strategies in the Alpine Space, Project report of WP 5 – Climate Change and Resulting Natural Hazards, Klagenfurt, 2008.

Stötter, J.: Veränderungen der Kryosphäre in Vergangenheit und Zukunft sowie Folgeerscheinungen. Habilitation thesis, University of Munich, Munich, 264 pp., 1994.

Stötter, J. and Zischg, A.: Alpines Risikomanagement – theoretische Ansätze, erste Umsetzungen, in: Naturrisiken und Sozialkatastrophen, edited by: Felgentreff, C. and Glade, T., Berlin, Heidelberg, 297–310, 2007.

Villi, V. and Bacchi, B.: Valutazione delle piene nel Triveneto, CNR-GNDCI Consiglio nazionale delle ricerche – Gruppo nazionale per la difesa dalle catastrofi idrogeologiche, Padova, Brescia, 2001.