Réf. Hennegriff & al 2006 - A

Référence bibliographique complète
HENNEGRIFF, W., KOLOKOTRONIS, V., WEBER, H., BARTELS, H.,Climate Change and Floods – Findings and Adaptation Strategies for Flood Protection. KA - Abwasser, Abfall, 2006, 53, Nr. 8.

Abstract: The climatic conditions in Southern Germany have changed noticeably in the 20th century, especially during the last three decades. Both in specific regions and interannually, the trends found exceed the natural margins of deviation previously known from long measurement series for some measured quantities. The mean and also the extreme floods are expected to increase significantly, although the results of the model chain global model – regional climate models – water balance models are still uncertain. As a precaution an adaptation strategy has been developed for the field of flood protection which takes into consideration the possible development for the next decades and also takes into account the uncertainties.

Mots-clés
Climate Change, floods, recent trends, future scenario, adaptation strategies

Organismes / Contacts

Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg (LUBW), Griesbachstraße 1 76185 Karlsruhe
Bayerisches Landesamt für Umwelt, Lazarettstraße 67, 80636 München
Deutscher Wetterdienst, Kaiserleistraße 44, 63067 Offenbach


(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
Temperature, precipitation (rainfall / snowfall) Flood runoffs Flood Hazard  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Southern Germany Bavaria and Baden-Württemberg Länder      

• 1931–2000: air temperature & precipitation
• 1951/52 - 1995/96: snow cover duration
• Since at least 1931: trends investigation on long-term behaviour of flood runoffs
• 2021-2050: scenario period


(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
Air temperature:
• Increase in the annual mean temperature: between 0.5 and 1.2°C;
• Rise in the monthly mean temperature for August: between 0.7 and 1.7°C;
• Rise in the monthly mean temperature for December: between 1.8 and 2.7°C;
• Increases in temperature – although less significant than in August and December – were also found in the months January, February, March and October;
• Region-specific distinctive features: e.g. main focus of the temperature increase in winter at lower altitudes; higher temperature increase in the west of Baden-Württemberg;
• The greatest increase in the monthly mean temperature occurs in most regions in December.

Precipitation behaviour:

• Significant decrease in the catchment precipitation in the summer half year, especially in North Württemberg and Lower Franconia; in Eastern Bavaria the decrease shows only a low level of significance; in Southern Bavaria a slight increase can be noted;
• Increase, in most cases significant, in the catchment precipitation in the winter half year; the foothills of the Alps are an exception, where the slight increase is not statistically significant;
• Regional clear increase in heavy precipitation by 30-35% in the winter half year; however, in summer only small changes;
• Regional foci of heavy precipitation in winter can be found in the Black Forest, in the Northeast of Baden-Württemberg and in the Northern Bavarian Region;
• Winter half year more humid, summer half year drier.

In the winter half year, the precipitation-bearing Western weather fronts have increased in Southern Germany. These fronts that are particularly important for the formation of floods which may go some way towards explaining the changes found.
Modélisations
Air temperature:
The air temperature will continue to increase clearly in future in Baden-Württemberg and Bavaria. The annual mean temperature increase amounts to 1.7°C. In winter, the increase of approx. 2°C is highest, in summer the increase amounts to 1.4°C. The expected temperature increase in winter is of special importance, as the temperature has great influence on the temporary storage of precipitation as snow and thus may be decisive for the expected flow conditions in future.

Summer days and hot days:

The number of summer days (days with Tmax >= 25°C) and hot days (days with Tmax >= 30°C) will rise significantly in Southern Germany. The average number of summer days is approx. 17 more per year in the future scenario than in the current state at all climate stations. The maximum duration of periods with summer days, however, increases for most of the climate stations. The number of hot days will nearly double at single climate stations in the area examined for the current period (1971-2000) and for the future (scenario 2021-2050). At the same time the frequency of occurence of extremely high temperatures will increase. The example of the station in Karlsruhe shows that in the case of an increase in the mean maximum air temperature, which was found in the scenario to be 2.3 °C, it is more than twice as likely that temperatures of more than 30°C will occur.

Frosty and icy days:

As a result of global warming the number of frosty days (days with Tmin < 0°C) and also the number of icy days (days with Tmax < 0°C) will decrease significantly. In the future scenario the average number of frosty days is approx. 30 less than in the current state at all weather-stations. The maximum duration of periods with frosty days also decreases significantly. The number of icy days in the future scenario is in most cases less than half that in the current state.

Precipitation:

For the selected future scenario the increase in the mean annual values of precipitation amounts to approx. 8% with a bandwidth of approx. 4% to approx. 17%. The large-scale precipitation will change by maximum -4 % in summer in Southern Germany. On the other hand, it is to be expected that precipitation in winter will increase significantly. Depending on the region, the varying increase will amount to up to 35 %. One area showing striking annual precipitation totals with relatively high changes is that on the southwestern-northeastern half-circle (Black Forest-Odenwald-Spessart-Franconian Forest). However the available regional climate models cannot currently provide quantitative data for the future development of convective short-period precipitation (thunderstorms), which are of importance for urban drainage and for floods in smaller catchment areas.

Weather conditions:

In winter, an increase is to be expected in the frequency and duration of west weather conditions (west condition cyclonic), which are important for flood formation. In summer, great changes are not to be expected.
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Air temperature:
After detailed examination of all available time series in the examined period from 1931–2000, 354 weather-stations proved to be suited for a further regionalisation of the air temperature. Time series of the daily mean temperatures in the catchment areas were determined from the regionalised measurement series for 33 investigation catchment areas, which cover Baden-Württemberg and Bavaria.


Precipitation:

The long precipitation series on all available stations in Southern Germany were interpolated with a geostatistical method to yield grid point precipitation and daily catchment precipitation heights were calculated. Statistical parameters for the daily values for the catchment area in one month were then analysed as representative partial samples. The series of the monthly values formed in this way were analysed in detail in a time series analysis. Station time series were employed to examine heavy precipitation.


Modelling climate development up to 2050:

In order to make statements about possible climate changes in Southern Germany for the next decades, regional climate scenarios had to be developed. As an optimum method has not yet been devised for this purpose, different institutions were given the task of establishing regional climate scenarios as a part of KLIWA. They were required to develop three different methods, namely:
• a statistical downscaling method using cluster analysis (Potsdam Institut für Klimafolgenforschung/PIK),
• a statistical dynamic downscaling using classification of weather conditions (Fa. Meteo-Research /MR) and
• a regional dynamic climate model (REMO) (Max-Planck-Institut für Meteorologie /MPI).


In order to achieve comparable results, the KLIWA partners established conditions that were to a large extent identical: measurement data 1951-2000, verification period 1971-2000, global model ECHAM 4, IPCC emission scenario B2, scenario period 2021-2050. After comparison and evaluation of the results of the three methods, which, as expected, delivered a certain range of results, further evaluations were primarily made on the basis of the results of the Meteo research method.

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
Snow cover duration:
The trend towards winters with less snowfall with less lasting snow cover is definitely apparent.
Up to moderate altitudes the durations decrease markedly in general. In the observed period, however, some regional distinctive features can be seen. In the eastern parts of the examined region (eastern part of the Alps and the Bavarian Forest) the decrease in the lower altitudes is from 20 % to 30 %. This trend weakens with increasing altitude and reverses (positive trend) on higher ground. In the western parts of the regions (Upper-Rhine plain and the western declivity of the Black Forest) the duration of snow cover decreases approx. 50 % and more on lower ground and decreases at moderate altitudes to 10 to 20%. In the higher regions mean values under 10 % are observed. Here, too, the trend weakens with increasing altitude. However, only in isolated cases are values observed where the trend is reversed.
At lower and moderate altitudes the number of days with snow cover has decreased markedly:
• approx. 30 – 50% in lower regions (< approx. 300 m above sea level);
• approx. 10 – 20% at moderate altitudes (between 300 and 800 m above sea level);
• less than 10% on high ground, or in some cases even increasing at higher altitudes (> approx. 800 m above sea level).

Long-term behaviour of flood runoffs:

The long-term behaviour of the highest runoffs can be characterised as follows:
• When examining the annual series from 70 to 150 years duration, the majority of annual highest runoff levels do not show significant changes;
• When examining the last approx. 30 years the highest runoffs show increasing trends at many gauges;
• The frequency of winter floods has increased since the 70s with the exception of Southern Bavaria;
• The monthly flood runoffs in the winter half year since the 70s are higher than in the time before the 70s.
In summary, it can be established that only in the last 30 to 40 years do the examined runoff time series demonstrate regional increases in flood runoffs.
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Groundwater recharge & Catchment runoff / Snow cover:
Changes in snow cover regimes and their parameters have effects on water balance, especially on the soil water balance, the groundwater recharge and the regime of the catchment runoff (flood formation). The parameters snow cover period, snow cover time, longest snow cover period (winter cover), start of the maximum snow cover height, constancy of snow cover, conservation of winter cover and maximum water equivalent values are best suited to describe the snow cover conditions and the water reserves stored in the snow cover. All the snow cover parameters mentioned correlate strictly with the elevation of the ground.
Climatic development in the 20th century:
The investigation of long series of hydrometeorological and hydrological measurements available provides information about the natural variations observed to date and any noticeable changes. These investigations were systematically carried out for Baden-Württemberg and Bavaria on the basis of a large data set within the context of KLIWA. Here the long-term behaviour of the flood runoffs, the mean runoffs, the regional and heavy precipitation, the air temperature, the evaporation and the snow cover period were analysed for time periods in the 20th century.


Long-term behaviour of flood runoffs:

The investigation of the long-term behaviour (of flood runoffs) included determination of any linear trends present in the time series of the annual and monthly highest runoffs. The annual and monthly highest runoff values at 107 gauges, which have long observation series since at least 1931, formed the basis for the trend investigations. Furthermore 51 gauges with shorter time series, i.e. with observation start after 1932, were included in the analysis.

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
The results predict a marked increase in mean flood levels (MHQ), and also in the extreme runoff. Although the results of the model chain (global model – regional climate model – water balance models) and the model assumptions contain uncertainties, the results all point in the same direction. Thus for the period considered up to the year 2050 it can be assumed that floods will intensify due to climate change in Baden-Württemberg and Bavaria.
Hypothèses
 

Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu / Facteurs de contrôle
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Flood runoff

The data of the regional climate scenarios were employed as input quantities for the water balance models (WHM), in order to make statements on the impact of climate change on water balance (e.g. runoffs into flowing waters). The water balance models on the basis of LARSIM are available as a 1-km grid for the whole of Baden-Württemberg; in Bavaria the model system ASGi has, up to now, been adapted for the river regions north of the Danube. The modelling of water balance focused at first on the possible future changes in runoff, with initial consideration of the effects of flood runoff . For this purpose the runoff values obtained from water balance modelling were analysed with methods of extreme value statistics.

(4) - Remarques générales

The climate conditions in Southern Germany, which have an impact on the entire water balance, have changed noticeably in the past century, especially during the last three decades. In specific regions the trends found exceed the natural margin of deviation, derived from long measurement time series, for some of the variables examined. The results suggest the explanation that the global and regional climate is human-induced, a basic premise which the international climate research community no longer questions. The investigations carried out for Southern Germany in the context of KLIWA agree in this respect with the trends stated in comparable studies. In particular, the KLIWA results on the changes in heavy precipitation have caused the German Meterological Service to put the KOSTRA values on a new basis as KOSTRA-DWD- 2000. This will be of importance for the assessment practice in water management.

(5) - Syntèses et préconisations
Adaptation of flood protection planning:
Against this background, it was necessary to take precautions and develop an adaptation strategy which takes into consideration the possible development for the next decades, but also takes into account the existing uncertainties. For this reason decisions should, at their core, be harmless in the long run while at the same time remaining adaptable should the need arise (e.g. as a result of new findings in climate research) according to the ”flexible and no regret“ strategy.

The evaluations provided reasons to modify the method previously used to determine design runoff and, as a result of the climate change, to consider a ”load case climate change“. When planning new flood protection measures in future, the load case climate change should also be examined.

Increase in the design runoff:

Increased design runoff has to be taken as the basis for the load case climate change. This is carried out with a supplement (“climate change factor“) to the currently valid design value. In Baden-Württemberg, the climate change factors for the runoffs differ between regions depending on the recurrence time.

In order to assess the magnitude of the climate change factors, the results of the regional climate scenarios established in the context of KLIWA, were employed as input quantities for water balance models and the runoff determined in water balance modelling was evaluated using extreme value statistics. The results for the future scenarios were compared with those of the current state. This was used to determine regional climate change factors for runoff for different recurrence intervals (five areas for Baden-Württemberg).

The runoff resulting from flood regionalisation or hydrological model calculation can be directly increased with the climate change factor for the runoff in the load case climate change.

Examples:
The following examples should explain how plans can be implemented in case of increased design values, i.e. taking into consideration the load case climate change.

• Planning of a flood protection dam: The dam is built according to current guidelines. However, additional measures are taken, which would not be required according to previous planning regulations. For example, an additional strip of ground on the valley side is reserved, enabling a future increase of the dam, if necessary, without additional problems.

• New constructions where a future alteration or adaptation is not possible or is only very expensive (e.g. bridges), should immediately be planned to take account of future increased calculation parameters relating to the level of water, if necessary.

• New constructions where a future adaptation is less difficult (e.g. river walls) should in view of the construction features (e.g. statics) be planned to a higher specification than currently required so that any further adaptation necessary (e.g. increase in height using stationary or mobile elements) would be possible without high costs.

Références citées :

IPCC (2001): Climate change 2001, Summary for Policymakers, Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge

Katzenberger B.: Bisherige Erkenntnisse aus KLIWA-Handlungsempfehlungen, KLIWA-Berichte, No. 4, München

KLIWA (2002): Langzeitverhalten der Hochwasserabflüsse in Baden-Württemberg und Bayern, KLIWA-Berichte, No. 2, Karlsruhe

KLIWA (2005): Langzeitverhalten der Lufttemperatur in Baden-Württemberg und Bayern, KLIWA-Berichte, No. 5, München

KLIWA (2005): Langzeitverhalten der Schneedecke in Baden-Württemberg und Bayern KLIWA-Berichte, No. 6, München

Ministerium für Umwelt und Verkehr Baden-Württemberg (2005): Klimawandel in Baden-Württemberg, Press release, Minsterium für Umwelt und Verkehr, 04.03.2005

Bremicker M. (2000): Das Wasserhaushaltsmodell LARSIM – Modellgrundlagen und An¬wendungsbeispiele, Freiburger Schriften zur Hydrologie, Vol. 11, Institut für Hydrologie der Universität Freiburg

Landesanstalt für Umweltschutz Baden-Württemberg (2005): Leitfaden „Festlegung des Bemessungshochwassers für Anlagen des technischen Hochwasserschutzes“, Karlsruhe

Straub H.: Langzeitverhalten von hydrologischen Größen, KLIWA-Berichte, No. 4, München