Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Increasing the forecasting lead-time of
Weather Driven Flash-floods
Edited by
S. ANQUETIN, J.-D. CREUTIN, G. DELRIEU, V. DUCROCQ, E. GAUME, I. RUIN
Laboratoire d’étude des Transferts en Hydrologie et Environnement
Grenoble, France
In response to the request
H01/812/02/D9056/AG/ct
Institute for Environment and Sustainability
Joint Research Centre
Final report : April, 2004
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
TABLE OF CONTENTS
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I. INTRODUCTION
II. SCIENTIFIC AND TECHNICAL ISSUES TO BE ADDRESSED
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A Flash Flood Producing Storms
1) Mesoscale convective systems
2) Heavy orographic rainfall climatology
3) Synoptic and mesoscale environments conductive to flash flood
producing thunderstorms
4) Topographic factors acting in the genesis and evolution of the
quasi-stationary MCS
5) Extremes and climate change
10
B Hydrology of the flash flood
1) Physical processes
2) Disparity of watershed hydrological behaviours
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C Observation Strategy
1) Environment data base
2) Meteorological observations
3) Hydrological observations
4) Natural laboratories
5) Special Observation Periods
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D Hydrological predictability issues
1) Meteorological predictability issues
2) Hydrological predictability issues
3) The scientific challenge of the coupling approach
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III. TOWARDS EUROPEAN CONCERTED ACTIONS
A. Motivation to tackle the problem at the European level
B. Scenarios for a possible research activity dealing with flash
flood
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IV. REFERENCES
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ANNEX
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Table 1 : Summary of recent flash-floods in Mediterranean Europe
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Figure 1: Number of days with daily precipitation greater than 200 mm
34
over southern France
Figure 2: monthly distribution of the number of days with rain gauge
35
precipitation above 200 mm from 1958 to 2000 for the Lozère, Hérault,
Gard and Ardèche departments.
Figure 3: The Cévennes-Vivarais Mediterranean Hydro-meteorological
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Observatory window.
Figure 4: Hydro meteorological observatory of the Region of
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Catalunya - Spain
Figure 5 : Topographic map of the Barcelona area showing the extension
38
of the Besòs River and its instrumentation.
Figure 6 : The Adige River Basin Hydrometeorological Laboratory - Italy
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Table 2 : Previous European programmes that supported flash flood
40
research
Detailed on the European programs listed Table 2.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
I. INTRODUCTION
In this report, the wording "storm driven floods" designates fast rising floods generated by intense and
long lasting stormy showers.
The term is meant to cover both “flash floods” produced by rain accumulations of typically more than
200 mm during less than 6 hours over natural watersheds ranging in area from 25 to 2500 km2 and “urban
floods” produced over built-up areas of typically 1 to 100 km2 by even shorter storms accumulating over
50 mm in less than 1 hour.
Like for all floods, the generating factors of flash-floods are meteorological - moisture convergence,
orography and convection, and hydrological - soil saturation and runoff concentration. The specificity of
flash-flood generating factors is in relation with the scales and intensities mentioned above.
¾ Stationary meso-scale convective systems are almost the only type of atmospheric situation that
can produce the above mentioned amounts of precipitation over areas of sufficient extent.
¾ Antecedent soil moisture conditions and land-use can play a role in the triggering of such floods.
¾ Basin morphology with steep slopes in the upper basins and flat areas downstream favours fast
runoff concentration and flooding.
Flash-floods differ markedly from floods occurring on larger basins with larger time characteristics. The
rising rate of waters of several m.h-1 and the flow velocities of several m.s-1 make these floods far more
dangerous for human lives than large river floods (excluding dam breaks). The intense erosion and solid
transport associated with these extreme events add to the hazard and strongly influence the quality of
soils, waters and ecosystems.
Topography or land use intimately links the structure of the storm (triggering factors) and the underlying
water drainage network (flow concentration).
In Mediterranean Europe as well as in many other temperate areas in the world, flash-flooding is one of
the most devastating natural hazards in terms of human life loss (Table 1 – ANNEX). For memory, the
storm flooding in Alger on 10 November 2001 caused 886 victims. In France over the last two decades
more than 100 deaths and several billion of Euros of damages. High-profile natural catastrophes in 2002
included the two flooding events in Europe in July and August, which caused insured losses of 3.2 billion
€. In September, flash flood in France brought additional losses of 440 million €. In comparison, a series
of tornadoes, in US, in April cost insurers USD 1.5 billion, while Hurricane Lili in the Caribbean and the
US caused losses of USD 650 million. In fact, the flood losses in 2002 highlight the potential threat
presented by risk concentrations. Losses arising out of the 2002 floods totalled USD 3.9 billion, which is
higher than the average recorded since 1990 (USD 1.1 billion) and eight times higher than the average
recorded since 1970 (USD 0.5 billion).
At the same time, new technologies offer real-time observations that allow faster and more accurate
meteorological and hydrological model developments. The uncertainty of flash flood predictions is
decreasing and forecasting lead-times are increasing.
At the European level, there is almost no legislation on floods. The EU commission (Colombo et al.,
2002) recommends that flash flood zoning should be integrated to legislation and building in flood-prone
zones should be better controlled. The difficulty is to determine which area is prone to flash flood and
what flooding scenario should be taken into account to map flood zones.
The aim of the proposed study on "Increasing the forecasting lead-time of weather driven flash-floods" is
twofold:
i.
to summarise the current state of the art of flash-flood prediction, and
ii.
to identify possible future co-ordinated research activities on flash-floods in Europe.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
The main objective is to establish in Europe a consistent strategy to improve our understanding of flashfloods under changing climate and land-use pressures and to adapt engineering methods of now-casting
and long-term planning in consequence.
The open questions that have motivated this study include:
i.
How does it happen and more specifically what is the relative importance of the different logical
and hydrometeorological factors controlling flash-floods?
ii.
How to develop observation strategies and modelling strategies to deal with the large range of
time and space scales of the flash flood event?
iii.
Would regional modelling strategy be an option to handle the large range of time and space scales
associated to the flash flood?
iv.
Is there a climatic trend in the occurrence of flash-floods in Europe making them more frequent
than in the past?
This document is organized in two main sections.
In the first section we define the problem of flash-flood forecasting and we list the main scientific issues
that need to be addressed.
In the second section we briefly describe the previous European programmes devoted to flash flood. The
dispersion of the efforts within programmes that were not only concerned by flash flood lead to the
difficulty to highlight major results. Suggestions for future improvements in flash flood risk reduction are
then proposed through research proposals and possible actions at European level.
Figures and tables are put together in the ANNEX Section at the end of the document.
This report constitutes a first step of the writing of a blueprint dedicated to the storm driven floods in the
Mediterranean. This blueprint will be used at national and European level in support to
hydrometeorological laboratories and associated research projects.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
II. SCIENTIFIC AND TECHNICAL ISSUES TO BE ADDRESSED
Flash floods are phenomena in which the important hydrologic processes occur at the spatial and
temporal scales of stormy precipitations. Flash flood understanding is therefore at the interaction of the
atmospheric and hydrologic sciences, namely hydrometeorological science.
Flash flood research focuses on diagnosing and forecasting excessive precipitation accumulation in terms
of spatial and temporal distribution at very fine scales. Nevertheless, flash flood forecasting is also linked
to the understanding of the hydrology of the phenomena. Thus, observation (geomorphologic data, active
soil processes) as well as modeling needs to be better documented. Forecasting potentiality is limited by
both the fast response of the catchment area and the uncertainty in the temporal and spatial variability of
the soil properties and the rainfall.
This part proposes a state of the art of the observation and the understanding of flash floods. Four major
fields of studies are considered in order to get a full and integrated picture of the meteorological and
hydrological processes leading to flash-flood events:
¾ The flash-flood producing storm;
¾ The hydrology of flash floods;
¾ The Observation Strategy;
¾ The Modelling strategy and the predictability issues.
Proposals and recommendations aim at highlighting the needs in each field in order to better understand
their interaction.
A. The flash flood producing storms
The Mesoscale Alpine Programme (MAP) had for objectives to improve the understanding and
prediction of intense weather in mountainous areas, such as the Alps region (Bougeault et al, 1998).
Major scientific objectives were, in particular, to gain a better understanding of orographically influenced
precipitation events related to flooding episodes (Bougeault et al, 2001). Whereas MAP has significantly
contributed to our knowledge on precipitating events over the Alps region during fall (special issue of the
Quaterly Journal of the Royal Meteorological Society, January 2003), there still remain scientific
questions to be addressed in particular for the heavy precipitating events over the other western
Mediterranean regions.
1) The mesoscale convective system
Heavy precipitation is the result of either convective or non convective processes, or combination
of both. Over southern France, large amount of precipitation might be cumulated during several dayperiods when one or several frontal perturbations are slowed down and enhanced by the Massif Central
and the Alps relief. But in most cases, large amount of precipitation is cumulated in less than one day
when a mesoscale convective system (MCS) stays over the area during several hours (Rivrain, 1997).
Riosalido (1990) has also shown that most of the flash-flood events in the Eastern Spain can be attributed
to quasi-stationary MCSs. Frequently, these quasi-stationary MCSs are backward regenerative systems
that take a V-shape in the infrared satellite imagery (Scofield, 1985) and in some cases in the radar
reflectivity images. Backward regeneration is obtained by a continuous generation of new cells at the tip
of the V, whereas the mature and old cells are advected toward the V branches (Rivrain, 1997, Benech et
al, 1993, Ducrocq et al, 2003). The V-shape resulting from the interaction of the divergent convective
motions at the top of the anvil with the upper south to south-westerly diffluent environmental flow that
prevails generally during these heavy precipitation episodes.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Needs and proposals:
ƒ
ƒ
ƒ
By the use of long term meteorological data base (NCEP/NCAR, ERA40), determine the percentage
of heavy precipitating events that can be attributed to quasi-stationary MCSs?
To document the characteristics of the MCS during its life cycle from satellite and radar data. How
frequent the V-shape in the infrared satellite imagery is associated with quasi-stationary MCSs?
Using high resolution mesoscale modelling, identify the factors that promote the stationarity of MCSs
(stationary large-scale systems, orography, evaporative cooling, density currents)
2) Heavy orographic rainfall climatology
On a climatic point of view, Frei and Schär (1998) have gathered high resolution rain gauge
observations of daily surface rainfall to produce a precipitation climatology covering a large part of the
Mediterranean arc. This climatology shows the enhancement of precipitation along the Alpine foothills,
with dryer conditions in the mountain range. It also reveals clearly that the Cévennes-Vivarais region, the
southern part of the Massif Central, is one of the five rainiest areas of the region. This region is
particularly prone to heavy rainfall (i.e. more than 100 mm in 24 hours) as shown by the climatology of
precipitation over the southern France established by METEO-FRANCE and MATE1 for the 1958-2000
period.
Figure 1 shows the number of days for which the daily surface rainfall were above 200 mm between 1958
and 2000 over the southern France: the Southern Alps, the Eastern Pyrenees, the Eastern Corsica and the
Cévennes-Vivarais region are the areas the most frequently concerned by heavy precipitation; the
Cévennes-Vivarais region being the area where the highest number of heavy precipitation events have
been recorded. From Figure 1, it is clear that the heavy precipitation events occur mainly over the southeastern flank of the mountainous areas, facing the moist low-level south to south-easterly flows over the
Mediterranean Sea which generally prevails during these flash-flooding episodes.
Figure 2 presents the monthly distribution of number of days with precipitation above 200 mm for the
four rainiest departments of the Cévennes-Vivarais region. It shows clearly that heavy precipitation
climatology is characterized by an autumn maximum. This is a common feature of the heavy rainfalls in
the western Mediterranean regions. At that period of year, the Mediterranean Sea is still quite warm after
the long sunshine periods of the summer, whereas upper cold air, advected from northern Europe, begins
to concern the area, thereby producing propitious conditions to lower static stability and to ensure a
sustained moisture supply.
Need and proposal:
ƒ
Using long term data base and mesoscale meteorological models, establish a climatology, for the
Mediterranean region, of the location where convective cells initiate repeatedly, in particular with
respect to the underlying orography and to the low-level incident flow.
3) Synoptic and mesoscale environments conductive to flash flood producing thunderstorms
Lin et al. (2001) have synthesized some common synoptic and mesoscale environments
conductive to heavy orographic rainfall, based on US, Alpine and East Asian cases. The common
environmental ingredients of theses cases have been identified as follows:
i.
a conditionally or potentially unstable air stream impinging on the mountain;
ii.
a very moist low-level jet;
iii.
a steep mountain to help release the conditional instability ;
iv.
a quasi-stationary synoptic system to slow the convective system over the flash-flood area.
1
MATE: French government Department of town and country Planning and of the Environment
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Increasing the forecasting lead-time of Weather Driven Flash-floods
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For some cases, a deep short wave trough or positive potential vorticity (PV) anomaly is found to
approach the threat areas. The approaching trough tends
i.
to induce a low-level jet perpendicular to the mountain range,
ii.
to reduce the static stability beneath the upper-level PV filament, and
iii.
to provide upper-level divergence for additional upward motion over the upslopping topography
to enhance convection (Massacand et al, 1998).
The PV streamer, considered as a dynamical precursor of heavy precipitation over the southern flank of
the Alps (Doswell et al, 1998), may be absent as for example for the East Asian cases studied by Lin et al
(2001). For these cases, a high convective available potential energy (CAPE) value is observed and seems
to compensate the increase of instability beneath the PV streamer for the other dynamically forced cases.
One of the projects of MAP, the P2 project2, focused on the impact of upper-tropospheric
potential-vorticity streamers approaching the Alps on the generation of heavy precipitation. It appears that
the approach of considering PV streamer as a precursor of heavy precipitating events has to be modulated
by:
i.
the precipitation fields are rather sensitive to the fine scale features of the upper-level PV
filament (Fehlman et al , 2000);
ii. the orography, as well as diabatic processes associated with convection, can altered the
streamer’s PV evolution (Morgenstern and Davies, 1999; Hoinka et al, 2003);
iii.
a precursor upper-level trough and an associated moist southerly flow at low-levels do not
necessary induce an heavy rainfall event (Rotunno and Ferreti, 2001).
A study of heavy precipitation over the Cévennes - Vivarais region has shown that the
environment of this case exhibits the common ingredients to heavy orographic rainfall as those previously
mentioned (Ricard, 2002). A short-wave trough associated to a southerly flow over the Massif Central
was also identified. For convection over this area, the low-level south to southeasterly flows from the
Mediterranean sea provide the moisture for the heavy precipitation events, and convection may be
triggered over the Massif Central crests. In addition, Alps or Pyrenees round-over low-level flows can
generate low-level convergence over the near Mediterranean Sea which help to trigger convection.
Moreover, for some heavy precipitation events, as for example the Gard disaster in September 2002, the
maximum of precipitation was not located over the mountainous areas but in the upwind lower
mountainous areas. Therefore, upslope triggering is not the only process involved in the conditional
instability release in this region. Romero et al (2000), by studying two cases of extreme precipitation
over eastern Spain, have also pointed out the role of the Atlas ridge to enhance the low-level easterly flow
toward eastern Spain and the horizontal convergence over the Mediterranean sea.
Need and proposal:
ƒ
Try to identify some precursors in order to better predict the events leading to flash-flooding. Using
long term data base and ANALOG method to extract specific meteorological situations, a
climatology of the key parameters (PV anomaly, CAPE, Precipitable Water, intensity and direction of
low-level flow) has to be established in order to better understand why a heavy precipitation event
becomes an extreme precipitation event. Mesoscale meteorological modeling can also be used to
highlight the small scale processes in such events. These constitute one of the major objectives.
2
The MAP Design Proposal identified 8 projects (Binder and Schär, 1996) :
P1 : “ Orographic precipitation mechanisms”
P2 : “ Incident upper-tropospheric PV anomalies “
P3 : “Hydrological measurements for flood forecasting”
P4 : “Dynamics of gap flow”
P5 : “Unstationnary aspects of Föhn in the Rhine valley”
P6 : “Three-dimensional gravity waves”
P7 : “Potential –vorticity banners”
P8 : “ Structure of the planetary boundary layer over steep orography”
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Increasing the forecasting lead-time of Weather Driven Flash-floods
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4) Topographic factors acting in the genesis and evolution of the quasi-stationary MCS
In the western Mediterranean region, the Mediterranean Sea and the orography form a strong
topographic component acting on the genesis and evolution of the quasi-stationary convective systems.
Mediterranean Sea provides the moisture supply to the strong low-level southerly flow that feeds up the
heavy precipitating events. The role of the Mediterranean Sea, through sensible and latent heat fluxes,
have been studied by Buzzi et al (1998) for one case of heavy precipitating event. They have found that
the Mediterranean Sea acts essentially on the intensity of the convective precipitation and not on the
location for their studied case.
For both synoptic-scale forced and unforced cases, recent numerical studies (Buzzi et al, 1998;
Ferreti et al, 2002; Ricard, 2002) have shown that suppression of orography significantly reduces total
simulated rainfall, clearly indicating a major role of the orography in producing flooding rain.
Inside the P1 project of MAP, “Orographic precipitation mechanisms”, the basic mechanisms of
production or enhancement of precipitation by topography have been addressed.
¾ Based on a climatology of radar data for the 1998 and 1999 autumns, Houze et al (2001) have
shown that most of the precipitation growth in the Mediterranean side of the Alps occurs below
the Alpine crest height.
¾ They showed also that the nature of the precipitation was a strong function of the Froude Number
(Fr) of the flow (Durran, 1990): for high Fr (i.e., un-blocked or flow-over regime), the low-level
flow rose up over the terrain and most of the precipitation enhancement occurred directly over the
lower slopes, whereas for low Fr (i.e., blocked or flow-around regime), the low-level flow turned
cyclonically as it approached the Alpine barrier, instead of rising over the terrain.
¾ Medina and Houze (2003) has extended the work of Houze et al (2001), by exploring the
microphysical processes conductive to an orographic enhancement of the precipitation associated
with baroclinic systems for these two flow regimes.
i.
For the blocked and stable case, the rain was mainly produced by a simple stratiform
process over the windward slopes. Precipitation amounts may be large as a result of the
persistence, although rainfall intensity is limited as the lower layer of air is blocked and
do not contribute to the lifting of moist air and also as convective cells are infrequent
under these stable conditions.
ii.
For the unblocked and unstable case, the general background of precipitation was also
stratiform. But for this high Fr case, the low-level air rose together with the upper level
air, so that a larger amount of moisture was transported compared with the blocked case.
Also, as the air was slightly unstable, embedded convective cells formed and enhanced
the formation of precipitation on the windward slopes.
The two combined effects add significantly to the precipitation production in the unblocked
case. Rotunno and Ferretti (2001) have shown that the two types of flow (blocked and
unblocked) can cohabit within the same situation, so that the variations along the mountain
barrier in the orographic-flow response can also induce convergence that enhances precipitation
in an area that may not be collocated with the steepest mountain slopes.
The shape and fine-scale structure of the mountain range play also a role in modulating the
precipitation. Scheidereit and Schär (2000) have shown that the specific arc-shape of the Alpine
topography may intensify the Coriolis-induced asymmetry of the flow and concentrate precipitation in
some small areas. Miniscloux et al (2001), Cosma et al (2002) and Anquetin et al. (2003), for shallow
convection organized in bands, and Ricard (2002), for a quasi-stationary convective system over the
Cévennes-Vivarais, have shown that the small-scale orographic features of the Massif Central, focus and
intensify the precipitation due to the convergence of low level air masses within the succession of
oriented east-west ridges and penetrating valleys.
There is also evidence of precipitation feedbacks which interact with orography to contribute to
the rainfall enhancement. During persistent rainy periods, the subsidence caused by evaporation and
melting of precipitation particles may induce the formation of downslope and down-valley flows (Steiner
et al, 2003). The subsiding air concentrates in river valleys which act as air drainage channels. This
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drainage flow is similar to the nocturnal drainage flow that occurs during clear sky and weak synoptic
conditions. This down-valley flow can develop underneath an opposite-directed moist southerly flow
aloft that is forced to lift over the mountain and in which precipitation forms. However, atmospheric
instability, through vertical overturning of air, may prohibit such development of down-valley flow.
Besides, for convective systems, the density current produced by the evaporation of falling precipitation
in the sub cloud layer may serve as a formation mechanism for convective cells and both the combined
effects of cold-air outflows and orography forcing has to be taken into account in the generation and
evolution of quasi-stationary convective systems (Chu and Lin, 2000). In particular, evaporative cooling
may trigger new cells far upstream of the mountain.
Soil moisture has been found to have an impact on the development and evolution of convection
over continental areas (Clark and Arrit, 1995; Pan et al, 1996; Gallus and Segal, 2000). It is not certain
that the quasi-stationary convection over the western Mediterranean region is sensible to soil moisture, as
the low-level flows that feed up the convective systems don’t cover a long distance over the continent.
Needs and proposals:
ƒ
ƒ
Using high resolution mesoscale meteorological models, study the role of the underlying orography at
different scales as well as the role of the proxy mountain ridges.
Based on long term data base (radar, rain gauge) and simulated rain fields, identify the contribution of
the shallow convection enhanced by the topography within the general pluviometric system of a
region prone to regular flash-flood events.
5) Extremes and climate change
Precipitation is the main contributor to the variability in the water balance and changes in surface
rainfall have large implications for hydrology. Flood frequency is altered by changes in the year-to-year
variability in precipitation and by changes in short-term rainfall properties, such as for example
thunderstorm rainfall intensity. Extreme climate events receive increased attention. The main focus,
motivated by the increase in deaths and in economic losses associated with the extreme events, is to
identify if extreme weather events, including heavy precipitation events, are increasing in frequency. One
of the major problems to answer to this question is the lack of long-term, high-quality data. Most of the
countries have only reliable data since World War II. Moreover, potential changes in heavy rainfall
frequency are difficult to infer from global climate models (GCM), as they have still some difficulties in
reproducing the observed patterns of variability and especially in simulating precipitation at small scales.
First and foremost, trends in heavy precipitation events must be examined in the context of trends
in average annual precipitation. Nicholls et al (1996) concluded from the observed available record that
global mean precipitation has increased since the start of the twentieth century. However, there are
different trends in different parts of the world, as reported by the Third Assessment Report (TAR) of the
IPCC3 which had summarized studies into trends in precipitation from long series of data. These studies
show that there is a general increase of annual land precipitation in the middle and high latitudes of the
Northern Hemisphere (estimated between 0.5 to 1% by decade), while land-surface rainfall has decreased
on average over the tropics and subtropics in both hemispheres (Houghton et al, 2001). Moisselin et al
(2002), by analyzing long series of homogenized monthly precipitation at 40 sites over France, tends to
confirm such an increase trend of precipitation, even though a large part of the precipitation rises are not
statistically significant. It is not possible to infer any tendency to an increase or decrease of the
precipitation over southern France during fall. Climate models tend also to simulate an increase of
precipitation in winter over northern Europe and a decrease of surface rainfall in summer over Southern
Europe.
Considering the increase/decrease in global precipitation, one would expect an increase/decrease in
extreme events (Mearns et al., 1984). There is effectively some evidence that the increase in precipitation
is reflected in the increase of extreme precipitation events over the United States (Karl and Knight, 1998).
3
IPCC : Intergovernmental Panel on Climate Change
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In the United Kingdom, there has been also an increased frequency of heavy precipitation events in
winter (Osborn et al, 2000). Easterling et al (2000) showed that in most cases when a country
experienced a significant increase or decrease in seasonal precipitation, the change in the amount of
precipitation falling during the heavy precipitation events is of the same sign (increasing or decreasing),
but the variations in frequency of heavy precipitation events are not so directly linked to the variations in
seasonal precipitation. Climate model results tend also to indicate an increase in the relative variability of
seasonal and annual precipitation as well as an increase of frequency of heavy precipitation events with
global warming. Confidence on these results must however be considered with respect to the current
limitations of the climate models. So that, it is still beyond our reach to conclude to an increase or
decrease of extreme precipitation events due to global warming for the western Mediterranean regions.
Need and proposal:
ƒ
To increase confidence in climate model results for the rainfall over the western Mediterranean
regions, through the use of climate regional models or of a precursor-base approach.
B. Hydrology of flash flood
As seen before, flash-floods can occur in urban and rural settings. Over small natural river basins
of typically 25 to 2500 km2, rain accumulations of typically more than 200mm in 6 hours produce flashfloods within a few hours. Over built-up areas of typically 1 to 100 km2, urban flash-floods are produced
even faster by shorter storms (over 50mm in less than 1 hour). The time and space scales of these floods
as well as their intensity make their study very different from the study of classical large river floods
(basins of 2500 km² and over) that forged the hydrology science during the last century. The classical
strategy in hydrology is to identify target points like cities or dams where a flood prediction must be
established. The upstream watersheds are equipped with rain gages and the outflow is routinely measured
at the target point. Provided a long series of rainfall-runoff data is taken, models of prediction can be
fitted to predict runoff from rainfall measurements. In the case of flash-floods, this strategy fails at least
for two reasons – these phenomena are difficult to observe and, consequently, it is difficult to identify the
dominant generating processes and to model them.
The main obstacle to study flash flood is clearly the lack of reliable measurements. The difficulty
of building a scientific argumentation on inaccurate data probably explains why the papers reporting on
extreme hydrological events are sparse and generally published in reviews of limited audience (Gilard et
al., 1995; Gutknecht et al., 1994; Cosandey, 1993; Hemain et al., 1989; Dacharry et al., 1988, Kolla et al.,
1987). Moreover, most of the publications in international reviews are focused on specific and limited
issues: peak discharge and return period estimation methods (Rico et al., 2001; Alcoverro et al., 1999;
House et al., 1995, Costa et al., 1987), and sometimes on the flash floods geomorphic consequences
(Alcoverro et al., 1999). An analysis of the rainfall-runoff relationship is seldom proposed (Ogden et al.,
2000, Belmonte et al., 2001; Gaume et al., 2002).
In conclusion the main need regarding the study of flash-floods is certainly to develop
reliable high resolution observation strategies covering regions large enough to be exposed to
extreme storms with a reasonable frequency (see the previous chapter on observation). In other
words flash-floods are at the very heart of the fashionable question of ungauged basins.
The lacks in the understanding of flash-floods are summarized by two main questions:
¾ What are the dominant processes that produce runoff under extreme rainfall?
¾ Is there a commonality of behaviour of the watersheds under these extreme conditions?
1) Dominant processes
As surprising as it can appear there is still no unique and simple theory about the runoff production on
watersheds during flood events. The main reason is that a variety of processes can be involved which are
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usually grouped in two categories - saturation excess (Dunne process) or infiltration excess (Horton
runoff). Due to the high space and time variability of the watershed’s characteristics (land use, soil type
and depth and wetness, subsoil, local slope, usptream contributing area), these processes can be active at
the same time in various combinations (Ambroise, 1998). Dealing with extreme rainfall accumulations
(i.e. typically more than 200mm in a few hours), the question is then to understand what dominant
processes govern the triggering of the fast horizontal flows and how they develop into a flash-flood.
Needs and proposals:
To identify the processes triggering the runoff at the agricultural surface unit the following aspects must
be investigated:
ƒ To determine the relationship between the soil and subsoil conditions with the formation of runoff
and preferential flow in macro pores (non-destructive geophysical techniques must be developed and
applied).
ƒ To analyze the impact of vegetation and more generally soil occupation on the above mentioned
relationship.
To understand how fast horizontal flows develop the following aspects must be investigated:
ƒ To determine the structure of the intermittent hydrographic network.
ƒ To understand the role of man made networks like roads or drains.
2) Disparity of behaviours
The disparity of the watershed hydrological behaviours especially during extreme flood events is an
important research issue. What are the important watershed characteristics (land use, soil properties,
geology, morphology, initial wetness conditions) which have a real influence and should be particularly
measured and well taken into account in the models? To tackle this question of disparity, available
measurements on gauged watersheds are not sufficient. Being very localized in space, flash-flood seldom
occur on gauged watersheds, and being very violent in intensity they frequently damage the equipments
along the rivers. The surest way to turn around this difficulty is to conduct post-flood investigations able
to collect data like flood marks, witness accounts, films and photos). Post-flood investigation is an arid
and time consuming activity. It is nevertheless an absolute necessity, firstly because the scientific
questions can only raise from a detailed observation of the studied phenomenon and secondly because it
is the only way to multiply the case studies and to show evidence of an eventual impact of the watershed
characteristics (land use, soil properties, geology, morphology) on its hydrological behaviour during flash
flood events. Some first investigations were conducted during the past years by the research teams
contributing to the OHM-CV, especially on the 1999 Aude event (Gaume et al. 2003) and the 2002 Gard
event.
Needs and proposals:
ƒ
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To study series of extreme rainfall runoff data in order to identify eventual commonalities of
behaviour.
To define the effect of the initial distribution of soil moisture on the development of surface runoff.
To develop a new model concept for extreme flood response that can be parameterized for ungauged
basins (i.e. without calibration).
C. The observation strategy
Understanding the genesis and dynamics of storm-driven floods needs observation resources over
a wide range of resolutions (time scales ranging from a few minutes up to a few hours, and space scales
from one to few hundreds of km2).
Country planning issues, as well as climate studies, imposes to work at the regional scale over periods as
long as centuries in order to observe a statistically significant number of extreme events. There is
undoubtedly a scientific challenge to develop observation strategies and tools able to deal with such a
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variety of space and time scales. Working with heterogeneous spatial and time series is certainly an
inherent feature of the problem of interest, if we consider the following points:
a) observation in the mountainous and maritime environments is both essential and difficult;
b) the development of operational weather and hydrological networks is recent (networks
developed essentially after World War II and telemetry appeared during the 70s) and strongly
evolves, not always in the good direction for in situ instrumentation.
It is obvious that research observation systems must take advantage of the existing operational
observation systems and, in return, that research must contribute to improve operational observation
systems. In the last decades, space-borne and ground-based remote sensing techniques represent a major
advent in terms of space-time resolution for observation of both hydro-meteorological (cloud height and
thickness, water vapor, cloud water content, rainfall rate, sea surface temperature, etc) and surface
(topography, land use, etc) variables. Finding a complementary, rather than a concurrent, use of remote
sensing and in situ observation techniques is certainly an important objective in order to take the best
benefit of both measurement principles (high space-time resolution versus accuracy). The river discharge
measurement is of primary importance for the considered thematic and still poses very difficult problems,
especially during floods.
Needs and proposals:
Based on these preliminary remarks, a three fold observation strategy is needed and proposed within
research platforms, detailed in the next section:
ƒ
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Develop a number of research hydro-meteorological observatories where detailed observation is
performed over a significant period (the current decade, at least) and a significant spatial domain.
Develop a methodology and organize the research/operational communities in order to perform,
under a common format, hydro-meteorological post-flood investigations for the extreme stormdriven floods occurring all over the Mediterranean region.
Increase for some selected sites the length of flood records using historical archives and field
evidence of past floods (historical data, paleo-hydrological indicators).
In this document, focus is given to the first observation strategy and we propose to highlight how
operational observation practices in the Mediterranean and new research possibilities could be used in
synergy to improve the hydrometeorological observation.
1) Environment data base
Digital terrain models (DTM), geographical information systems (GIS) and in particular urban
data banks (UDB) are becoming largely available for the description of the surface topography (with a
typical horizontal resolution of 100 m), land use (agriculture, transport infrastructures, urbanization) and
drainage networks (river and channel networks, urban drainage systems, etc). The time evolution of the
environmental data is difficult to handle and to maintain up to date. For some specific applications (e.g.
hydraulic modeling), the accuracy of the standard topographical data is, therefore, not sufficient. The
geometrical and hydraulic description of rivers and urban hydroworks (dams, storage reservoirs…) and
their operating rules are sometimes difficult to obtain from operational bodies. Moreover in some
mountainous areas, the flow regulation may be dominant for the low hydrological regime, the
hydroworks becoming “transparent” only during floods. Finally, it is well known that the macro
heterogeneities of the ground act in the storage and transport capacities of the soil, and are therefore
important on the hydrological regime, in particular during the floods.
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Needs and proposals:
ƒ
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Perform if required topographical campaigns to collect environment data (land use, river beds
morphology (description of minor and major river beds)) at adequate accuracy.
Improve and develop a centralized access strategy of an environment data base (geological and
pedological maps (thickness and type of the soils), maps of the macro-heterogeneities (e.g., the
karstic networks)) and keep it open to the research community (at least, be aware of an updated
catalogue of collected data and where to find it).
2) Meteorological observations
We present first a brief review of the operational networks available within the Mediterranean
region. Then, we propose needs and potential evolution of these networks within research natural
laboratories, described in section 2). We highlight the need for European concerted actions at the
operational and research levels.
Concerning surface measurements and in the particular case of France, the project RADOME is
currently running at Météo France. It aims at renewing its network of automated weather stations. The
project will be completed in 2005 resulting in a network of 550 high-quality stations deployed over the
entire country. This will lead to an average resolution of 1 station every 1000 km², which will be quite
unique in Europe. For obvious meteorological reasons, some regions such as the south-east of France –
prone to intense weather events – have a higher density. Each station provides real-time measurements of
wind, temperature, pressure, precipitation and humidity. Among these parameters, only the temperature,
the pressure and the humidity are assimilated by the current French operational prediction model
(ARPEGE). The expected resolution of one station every 1000 km ² will however not be sufficient for all
parameters and all geographical environments. For instance the humidity and precipitation fields over
hilly regions have a much higher spatial (3D) variability than the pressure field over the plains.
Surface measurements over the Mediterranean Sea are provided by a very limited number of ships and
buoys. This is recognized as an important weakness of the current observing network as the wind and
humidity (surface) fields are essential to account for (and predict) the occurrence of flash-floods in the
coastal mountains bordering the Mediterranean Sea. The recent MAP experiment has shown that the
exact location and intensity of precipitating systems in the Southern Alps depend upon the stability
conditions well upstream of the mountain range.
In a context of flash flood mitigation, real-time monitoring of rainfall at the regional scale is essential to
provide information for heavy precipitation warnings and the assessment of the hydrologic impact of such
rain events using rainfall-runoff models.
Rain gauge represents the reference sensor for measuring rainfall at ground level. Nevertheless, their use
for the spatial estimation of rainfall in mountainous regions is not straightforward. First, the rain gauge
network density needs to be adapted to the required time resolution. Based on a series of rain events
observed over urban Mediterranean watersheds, Berne et al. (2004) show that the required time resolution
of the rain measurement should be a fraction of ¼ to 1/3 of the average lag time of the catchment. For
natural Mediterranean catchments, Lebel et al. (1987) estimated the spatial structure of the rain fields for
different time steps ranging from 1 to 24 hours. For watersheds of 100, 500, 1000 km2, the temporal
resolution is typically of about 25, 60 and 100 min, respectively, and the spatial resolution of 6.4, 8.5 and
10 km, respectively. Except in some urban areas, actual rain gauge network densities are in the best cases
of 10 km for the daily time step and drop to about 15 km for the infra-daily time steps. Therefore, the rain
gauge network spatial resolution is not adequate for real-time monitoring of watersheds smaller than 1000
km2. Moreover, there is generally a lower density of the network in altitude since the installation and
maintenance of the gauges are easier in the valleys. A further difficulty is related to the evaluation of the
respective contributions of liquid and solid precipitation.
In summary, rain gauge networks provide valuable point references for checking the quality of radar
estimates or precipitation forecasts based on numerical model. The density of the networks is more
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appropriate to the event time scale than to capture the dynamics of the storms variability leading to flash
floods at fine rural and urban scales.
Complementary networks of daily rain gauge are essential for post flood investigation.
Weather radar systems offer a number of advantages in the real-time monitoring context with spatial
and temporal resolutions of typically 1 km2 and 5 min, a large spatial coverage and an immediate
availability. However, in mountainous regions, the measurement of rainfall is complex and the quality of
radar estimates varies strongly, depending on the location.
The interactions of the electro-magnetic waves with the relief (ground clutter and screening effects) and
the vertical structure of the atmosphere (reflectivity enhancement or decrease below the sampled volume,
bright band effects, partial beam filling at cloud tops, etc) explain a large part of this range-dependent
spatial variability (Joss and Waldvogel 1990; Andrieu et al. 1997; Berne et al. 2004). Other error sources
can be evoked, such as calibration defaults, attenuation (a wavelength dependent effect), uncertainty in
the Z-R relationship(s), anomalous propagation. During the last two decades, research efforts were
devoted to develop identification methods and correction algorithms for the various error sources (e.g.,
Andrieu et al. 1995; Delrieu et al. 1997; Vignal et al. 1999; Nicol et al. 2004) and procedures for
optimizing the radar siting and the operating protocols (Pellarin et al. 2002). Radar hydrology case studies
(e.g. Andrieu et al. 1997; Creutin et al. 1997) and more comprehensive radar rain gauge evaluations (e.g.
Joss and Lee 1995; Joss et al. 1998; Young et al. 1999; Vignal and Krajewski 2001) have allowed to
better quantify the potential of weather radar for the quantitative estimation of rainfall.
Moreover, an important operational effort has been dedicated to the development of weather radar
networks in the Mediterranean region. For example, this is the case in France with the deployment of four
S-band radar systems at Bollène, Oppoul, Collobrières and Aléria within the "Arc Méditerranéen" project
of the ARAMIS network (Météo France) funded by the Ministry of Ecology and Sustainable
Development. These radar systems complement the existing systems in Nîmes (S-band) and Sembadel
(C-band).
In regions of very low visibility and high vulnerability (e.g. cities located in mountainous settings), the
use of radar systems working at attenuated frequencies (X- band) has been proposed (Delrieu and Creutin,
1991) to locally complement the conventional radar networks. The required attenuation corrections could
be based on the use of mountain returns (mountain reference technique; Delrieu et al 1997; Serrar et al.
1999) and/or on polarization and phase diversity techniques (Testud et al. 1999).
There are unfortunately very few operational microphysical measurements: a number of ceilometers
and telemeters have been deployed in some airports (e.g. Nice) allowing the estimation of the visibility,
the cloud base height only for aircraft safety purposes. Accurate knowledge of the drop size distribution
and/or discrimination between snow, rain and hail is crucial for QPE with radars but there are no
disdrometers or such instruments operated in real-time by Météo France and the conversion of radar
reflectivity into rain rates is performed with a standard Z-R relationship (Marshall-Palmer).
Apart from geo-stationary satellite measurements (Meteosat and MSG - Meteosat Second Generation which will soon be declared operational) that only provide an integrated view on the vertical structure of
the atmosphere and cloud winds, there are actually very few altitude measurements in the
Mediterranean region. The operational radio-sounding stations in the French-Spanish-Italian
Mediterranean region are Nîmes, Ajaccio, Palma de Mallorca, Cuneo (Italy), Geneva. Aircraft
observations (AMDAR) of wind and temperature during the take-off and landing phases are now also
assimilated by operational GCMs but most of them are concentrated in Northern Europe (Paris, London,
Frankfort). A number of UHF / VHF wind profilers are operated by National Weather Services and some
of them are assimilated by GCMs with a slightly positive impact. In the French Mediterranean region, the
Nice airport is equipped with a UHF profiler. All in all, not to mention the spatial resolution, the upper-air
meteorological conditions are clearly not well documented in the Mediterranean region. Modellers hope
that this gap will be filled in the near future with the MSG products, GPS technology and an increase in
the number of ship measurements.
Needs and proposals:
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Editors: S. Anquetin et al.
Establish a community infrastructure for collecting and making available large datasets of remote and
in situ hydro meteorological observations and products,
Increase the quality of the radar estimation of rainfall at the regional scale in mountainous regions:
1) characterize the “hydrologic visibility” of the existing networks,
2) improve algorithms for the radar-based estimation of quantitative precipitation and establish
measures to quantify uncertainties,
3) develop algorithms for the optimal combination of multi-radar data,
4) Reinforce the radar network densities,
5) Develop the concept of dual-polarization radars, whether at X-band, C-band or S-band, to
improve the microphysical description of precipitating systems.
Intensify the transfer of specific radar data processing techniques proposed by the research
communities towards the operational radar services (e.g. the use of volume scanning strategies for
identifying radar waves - relief interactions and the vertical structure of precipitation).
Develop disdrometer network that might be used in two ways: i) to calibrate accurately weather
radars at the time scale of the episode; ii) based on long-term statistics of disdrometer data, it can be
used to refine locally the Z-R relationship used for hydrological purposes.
Follow and be linked to the Global Precipitation Mission (see the NASA website:
http://gpm.gsfc.nasa.gov/) that would be of special interest for monitoring and understanding the
flood-producing storms along the Mediterranean coasts, and to improve the skills of global and
regional weather prediction models through assimilation of such precipitation measurements. The
first workshop on the ground validation of GPM took place in Abington (UK) in 2003
(http://www.rcru.rl.ac.uk/GPMGV/).
3) Hydrological observations
Soil moisture monitoring can help to understand flood triggering. The reaction of a watershed to rain
depends on the initial soil moisture conditions. Antecedent precipitation indexes and base flow conditions
are indicators of the hydric state of the watershed that are commonly used for runoff prediction. A more
assessment of soil moisture would improve our understanding of flood triggering in complementing the
above indicators. Means to monitor soil moisture at the watershed scale must be promoted. Promising
avenues are open by microwave radiometry born by satellites (SMOS project) but also by planes or
simply installed at the ground in place offering a wide view over the watershed. Alternative avenues are
offered by the monitoring of air moisture in the boundary layer that could indirectly give upper soil
moisture conditions.
River discharge observation and quantification remain a difficult task to handle.
The most classical technique for discharge measurement is based on the water stage measurement in a
river section not prone to backwater effects, coupled with a calibration of the stage-discharge relation.
The rating curve is established point by point by means of a gauging (i.e. a flow velocity sampling over
the river section). The limits of this technique are the cost and the difficulty to gauge rivers during
highflows for opportunity and safety reasons. This results in a poor accuracy of river discharges at
highflows which are extrapolated through hydraulic formulas.
Hydroworks sometimes offer more satisfactory discharge estimation methods based on the hydraulics
laws of the regulating works (weirs, sills, sluice gates, etc). Other specific difficulties for measuring high
discharges are linked to the characterization of overflows in the major beds, solid transport that modifies
the water viscosity, the possible modification of the river beds and the often-observed destruction of the
stage equipment during floods.
There, there is an important miss of information, particularly for rivers prone to flash flood.
New efforts are, therefore, devoted to the development of remote sensing techniques for discharge
measurement (Creutin et al. 2003). Such techniques are already employed for height measurements (e.g.
ultra-sonic probes fixed on bridges, video camera for a global surveillance of the river) and should
complement and/or replace in-situ sensors often based on pressure measurement principles. Moreover,
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complementary information is then possible to be derived from these observations and would be
important to assimilate in hydraulic models (e.g. surface velocity, bathymetry of the changing river beds).
Needs and proposals:
ƒ
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Develop remote sensing techniques for discharge measurement relying on river surface velocity
measurements (PIV technique based on video imagery and Doppler radar/Lidar techniques) and
estimation of river bathymetry (use of ground penetrating radar (GPR) or Lidar).
Develop airborne and/or satellite photogrammetric surveys to characterize flooded areas and evaluate
the geomorphological changes in the river beds and their re-balancing in the months following the
event.
Develop remote sensing of soil moisture (local, air and space borne radiometry, indirect measurement
using scintillometry detector of latent heat flux).
4) Natural laboratories – Proposition of Pilot sites
The scientific objectives are linked to:
¾ the development of modern observation techniques that could be generalized in a next step to the
whole Mediterranean region,
¾ the validation of the available or newly proposed meteorological and hydrological models in the
perspective of their further coupling.
These sites must rely on existing operational systems to be reinforced by means of concerted instrumental
actions associating the operational and the research communities. The creation and maintenance of
research databases is, therefore, an important goal to achieve for such observatories.
Recommendations based on the report of the 9th prospectus development team of the US weather research
program (Droegemeier et al., 2000), emphasized the need to create such natural laboratories to tackle the
research on flash flood mitigation.
In this document, we present three complementary existing sites prone to flash-floods within the
Mediterranean region:
¾ the Cévennes-Vivarais area in France corresponding to a medium-elevation mountainous area,
¾ the region of Barcelona in Spain representative of a big urbanized area crossed by fast reacting
coastal rivers,
¾ the watershed of the Adige river in Italy representative of an Alpine high-mountainous area.
a) The Cévennes-Vivarais Mediterranean Hydro-meteorological Observatory
(OHM-CV), France
The OHM-CV initiative (http://www.lthe.hmg.inpg.fr/OHM-CV/index.html) started in 2000 and
has received the label of "Environment Research Observatory" (ORE is the French acronym) from the
Ministry of Research in 2002.
One of the OHM-CV objectives is to develop a natural laboratory in the Cévennes-Vivarais region,
described hereafter.
Figure 3 presents the general view of the OHM-CV (radar, rain gauges) and a short description of the
region (topography, geology) that is regularly prone to flash floods especially in autumn.
Several historical major floods (Jacq 1994; Deblaere et Fabry 1997) can be mentioned: 1858, 1933,
September 1958 (Cévennes region), October 1988 (Nîmes), September 1992 (Ardèche area), September
2002 (Gard region), and December 2003 for all the right bank tributaries of the Rhône River.
The punctual 10 year return period rainfall is greater or equal to 50 mm and 200 mm for the hourly and
daily time steps, respectively, over most of the region (Bois et al 1997). Two Cévennes hydrological
watersheds (Gardon d’Anduze river at Anduze 550 km2 and the Ardèche river at Vogüé 635 km2) were
especially studied in the last three decades and may continue to be used as reference basins for detailed
research projects. The problem of prediction on un-gauged (or poorly gauged) basins is particularly acute
in this region and should also be addressed.
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This region is already well instrumented with operational observation systems. Nevertheless, the
operational instrumentation is managed by several weakly connected meteorological and hydrological
services having their own metrological objectives and practices.
A first on-going action of the OHM-CV concerns the creation of a research data base by gathering,
normalizing, examining and archiving the operational data. This effort started in 2000 and is planned to
last for at least ten years in order to document a broad sample of rain events with a rich and rather stable
observation system. The rain gauge and water level networks belong to no less than five services
specialized in weather and environmental surveying and flood alerts. During the period 2000-2002, 22
rain events, with rain amounts greater than 50 mm per day at some locations in the region of interest,
were processed and archived. Obviously, besides the rainfall and hydrological datasets, the OHM-CV
also benefits from the Météo-France meteorological datasets (radio-soundings, analyses of the operational
NWP model, etc).
Ongoing actions, needs and proposals :
The OHM-CV operational observation system is being progressively upgraded and complemented by
means of concerted operational and research actions:
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GPS meteorology: The objective is to assess the potential of the Global Positioning System for
measuring the atmospheric water vapor content. Four permanent GPS are located within the OHMCV window.
o Perform specific campaigns with additional GPS sensors to :
¾ Estimate the 3D water vapor field and its evolution above the network (30 x 30
km2) through a tomographic inversion techniques,
¾ Characterize the entering water vapor fluxes from the Mediterranean sea shore
and to start establishing a climatology.
Improve the quantitative use of the ARAMIS weather radar network for meteorological and
hydrological applications in mountainous regions. The case of the two S-band radars (Nîmes and
Bollène) separated by only 60 km is especially suited
o to assess the value of a radar volume scanning strategy to cope with radar waves - relief
interactions and to identify and account for the vertical structure of precipitation,
o to develop the radar networking techniques to estimate rain fields at the regional scale.
o Install a permanent DSD (Drop Size Distribution) instrumentation to improve the inversion of the
radar measurements.
Prepare the data assimilation into the future French operational NWP model.
o Develop a radar observation operator within the Météo France AROME project, in order to
assimilate the ARAMIS network 3D reflectivity data,
o Develop GPS observation operator such as the tropospheric slant delays.
Test new discharge measurement techniques: compare a video-camera based prototype for
measuring both the river stage and surface velocities to classical measurement techniques at the
EDF/INPG discharge station on the Isère river at Grenoble, France (Fourquet and Saulnier, 2004).
b) The Barcelona region, Spain
The region of Catalunya (Spain) (Figure 4) is known to be prone to severe storm-floods,
especially during the autumn period, when primary or secondary cyclonic perturbations drain moist and
unstable air masses coming from the Mediterranean sea. This region is drained by a set of coastal rivers.
Many of them cross densely urbanised and industrialised zones. Among them, the Besòs River and the
Llobregat River pass north and south through the conurbation of Barcelona (more than 3 millions of
inhabitants). Of special interest is the Besòs watershed (1020 km2, Figure 5) which was affected in 1962
by a catastrophic flood event that caused about 800 casualties and exceptional economical damages.
During last decades, the river bed has been degraded and canalised by means of big concrete protection
structures. Considerable EU and Spanish investments have been devoted in recent years to rehabilitate
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this area into a modern urban sector and create a fluvial park. The city of Barcelona organizes in 2004 an
International Forum of the Cultures in a newly urbanised area precisely built in the Besòs river delta.
The radar observation network (Figure 4) allows a remarkable coverage of the Barcelona area and its
main coastal rivers. The 1962 event made the Besòs watershed to be extensively instrumented and studied
in recent years with exceptional of hydrological time series compared to the Spanish standards (Figure 5).
The creation of the fluvial park has motivated the development of a flood forecasting centre operated by
CLABSA (the Sewer Management Company of Barcelona City). CLABSA has implemented new
instruments (stage record stations) and control structures (inflated dams) along the park. Up to now this
system relies on very simple hydrometeorological models, and the warning thresholds are based on
conservative assumptions, but research efforts are made to improve this system. An on-line alert system
based on hydro-meteorological data and hydrological models is being developed to monitor and forecast
the combination of the flows coming from the semi-urbanised Besòs basin and the flows produced by the
urban drainage network of the City.
The Hydrometeorological Observatory of Catalunya, and more specifically the Besòs River Project, is
supported by CLABSA (Clavegueram de Barcelona, S.A.), the SMC (Servei de Meteorologia de
Catalunya) and the ACA (Agència Catalana de l’Aigua).
c) The Adige region, Italy
The “Adige River Basin Hydrometeorological Laboratory” (LIBA) started in 2000. Its objective
is to develop a framework aimed at the effective utilization of radar rainfall estimates for the
identification and prediction of storm-flood events in a region characterized by rugged topography.
A number of different agencies are responsible for the operation of the hydrometeorological data (Figure
6) gathering and analysis: HYDROBZ in Provincia of Bolzano, METEOTRENTINO in Provincia of
Trento, CSIM in the Veneto Region and the Adige River Basin Water Authority. The Spino d’Adda radar
system is managed and maintened by Nuova Telespazio (TELECOM group). The real-time
interconnection among the three different radar is operated within METEONET, the organisation which is
taking care of the connection and the integration among weather radars in northern Italy to obtain
composite radar field and integrated radar products.
On-line flood forecasting in the Adige River basin is based on two different rainfall – runoff models (e.g.
at the event scale with a unit hydrograph model and continuously with a conceptual model). These
models can be operated in real-time, with on-line provision of relevant data of precipitation forecasting,
and have the capability of on-line adjustement of the parameters.
The LIBA project is supported by local governments (Provincia Autonoma di Bolzano and Provincia
Autonoma di Trento), by the Adige River Water Authority and by the Italian National Research Council.
5) Defining a Special Observing Period
In complement to the data routinely collected by these laboratories, a Special Observing Period (SOP)
should take place before of the decade. It could address different meteorological and hydrological issues
at different scales. Examples of currently explored items of this SOP are the following:
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To perform airborne drop-soundings over the Mediterranean Sea in order to complement the
operational sounding network during Mesoscale Convective System situation.
To strengthen GPS observing systems to characterize water vapor over the Mediterranean sea
To use light-configuration radars close to mountainous research watersheds to characterize both
orographic rain triggering and its hydrological impact
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High-resolution height/discharge measurements (e.g. for all sub-watersheds of about 50 km2 or less
for the Ardèche or the Gardon rivers) for assimilation in rainfall-runoff modeling
D. The modelling strategy and predictability issues
Numerous scientific and technological developments during the past several years have
positioned the research community to make significant advances not only on the understanding and
quantitative forecasting of precipitation, but also in determining the fate of precipitation upon its entry
into hydrological system. They include, for example, the deployment of in situ ground monitoring sites at
high spatial density, the emplacement of intensive radar network and the development of meso-scale
models for quantitative precipitation forecast.
To provide an initial soil wetness condition before the storms and a high resolution rain field, MAP
experiments (Ahrens, 2003) reveal that the scale gap between the hydrological model and precipitation
forecasts of present day Limited Area Models (LAM) introduces significant errors.
Hydrological performance with coarse grid input is better than with non robust and uncertain highresolution input. This implies that resolution enhancement of LAMs is useful only if the quality of fine
precipitation forecasts is at least of the order of quality of present LAM forecast.
Nevertheless, there are noticeable advances both in meteorology and hydrology that make possible the
use of a coupling approach to understand the water cycle in the framework of flash flood. The
atmospheric and hydrological systems are obviously linked and any effort in research plan related to flash
flood prediction must consider this coupling.
1) Meteorological predictability issues
Predictability limits result from the nonlinearity and instability of the dynamics of the
atmosphere, together with the lack of a precise knowledge of the atmospheric state at any time and
location. The atmospheric predictability depends significantly on flow regime. Therefore, some
phenomena are more predictable than others. Synoptic and large mesoscale systems possess more
intrinsic predictability than cloud-scale convective systems (Tennekes, 1978). There are no known
estimates of predictability limits for many small-scale phenomena, including thunderstorms. However,
one can suggest that there are some factors that can increase the predictability on the mesoscale, such as
surface heating, synoptic-scale disturbance or topography forcing. This is particularly the case for the
quasi-stationary MCSs of the western Mediterranean region, for which the relief of this region can extend
the period of predictability associated with convective phenomena. Indeed, the topographically driven
mesoscale circulations often result from interaction of the synoptic-scale flow with the orography, and
consequently the mesoscale predictability is controlled by the synoptic-scale predictability (Mass et al,
2002).
Non-hydrostatic models employing grid-spacing of about several kilometers have shown
substantial success in simulating realistic heavy precipitation systems of the western Mediterranean
region (Stein et al, 2000; Ducrocq et al, 2002; Richard et al, 2003, Asencio et al, 2003). Ducrocq et al
(2002) have shown that the success of the high-resolution model may depend strongly of the initial
conditions. Using high-resolution observations to produce detailed initialization results in more realistic
simulations.
Recognition of the uncertainties in the initial conditions and in the model physics, as well as the
inherent predictability of the atmosphere at the mesoscale, has led to suggest an alternative strategy to
deterministic high-resolution forecast, namely ensemble forecasts at lesser resolution to produce
probabilistic predictions (Brooks et al, 1993). Greater knowledge of characteristics of the predictability
limits of heavy precipitation events that occur over the western Mediterranean region is critical for
choosing between these two NWP4 strategies.
4
NWP : Numerical Weather Prediction
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Needs and proposals:
ƒ
ƒ
ƒ
ƒ
ƒ
To characterize the predictability of heavy precipitation events. This includes investigation of the
characteristics of initial conditions errors (including surface forcing such soil moisture and sea
surface temperature) that influence predictability and of the effects of uncertainties in model physics
(e.g., microphysics, turbulence,...) on perturbation growth.
To estimate the feasibility and the improvement of high resolution non-hydrostatic modelling for the
prevision of flash flood event by the operational Numerical Weather Prediction (NWP) models.
To develop strategies to assimilate operational radar data within operational NWP models.
To use research radars in addition to operational radars to allow more detailed and comprehensive
analysis of microphysical and kinematic processes in storms and their relation to precipitation
production.
To develop forecasting techniques based on probabilistic approaches that will help to produce high
spatial and temporal resolution forecasts of storm development, evolution, dissipation and rainfall
amount.
2) Hydrological predictability issues
A variety of tools have to be developed or improved to test hypothesis concerning the involved
hydrological processes and their dynamics: numerical and physical simplified watershed or hill-slope
models and field experiments. Only few recent scientific works have really tried to use physically based
small scale hydrological models (Abdul and Gilham 1984, Ogden et al. 2000). The results obtained
concerning the understanding of the flood producing processes, the influence of the slope, the soil
properties and thickness are encouraging but partial: three-dimensional non-stationary models must be
used and surface and subsurface flows really coupled in the models, complex soil and surface geometry
tested … Thanks to the improvements of the numerical techniques it is now feasible. Recent
developments are encouraging in this sense (POWER (Haverkamp et al., 2004), LISFLOOD (De Roo,
1999)); nevertheless their feasibility to reproduce the hydrological answer of a small watershed (~ few
hundreds of km2) faced to a flash flood event is not proved yet. Concerning the field studies, the main
difficulty lies in the measurement of the soil and subsoil properties, their spatial variability and in the
supervision of the temporal and spatial evolution of their water content. New measurement technologies
like the non-destructive geophysical techniques must therefore be tested and used.
The real predictive power of the existing rainfall-runoff models must be assessed and their
implementation for an operational flood forecasting issue prepared. Today, very few rainfall-runoff
models are used in an operational flood forecasting context. Three main reasons can be put forward to
explain this state of fact.
i.
A minimum requirement for a model, if it is used to forecast the evolution of river discharges,
is that it leads to better results than simply reproducing the last observed discharge value. If
the objective is to minimise the variance of the prediction errors, the variance of the model
simulation errors should be lower than the variance of the discharge fluctuations over the
forecasting horizon. Experience shows that "Nash" criteria in rainfall-runoff modelling
applications are generally lower than 80%: i.e. the variance of the modelling errors represents
generally more than 20% of the overall variance of the considered discharge series. But, the
variance of the discharge fluctuations over a given forecasting horizon is usually much lower
than the overall variance of a discharge series and tends to decrease when the forecasting
horizon is shortened. Therefore, rainfall-runoff simulations can generally not be directly used
as flood forecasts: existing rainfall-runoff models must be adapted using data assimilation
procedures for instance, or specific forecasting models must be developed.
ii.
Flood warning and forecasting systems have been mainly developed for large rivers and
upstream great agglomerations. The forecasts are based on anticipation of the flood wave
propagation in the downstream part of the river. Depending on the size and shape of the
21
Increasing the forecasting lead-time of Weather Driven Flash-floods
iii.
Editors: S. Anquetin et al.
watershed, such techniques can provide accurate forecasts for relatively short lead times: a
few hours to a few days, typically 1/5 of the time of concentration of the watershed.
A conceptual breakthrough has been made by the meteorological community with the
ensemble weather forecasts; the same effort has to be done in hydrology. The forecasts users
must be aware that forecasting tools do generally not produce the certain future situation but
define the field of the possible. A forecast is not a prediction. This field of the possible can be
wide if the forecasting horizon is large and should diminish as this horizon is reduced if the
forecasting tool works well.
Due to the high spatial heterogeneity of the rainfall events producing flash floods, distributed models,
more complex to implement and validate, may be necessary. The operational implementation of the
rainfall-runoff models must be prepared. This means that their sensitivity to errors and to the level of
temporal and spatial resolution of their parameters and input data and in particular rainfall data must be
assessed. This will define the minimum level of accuracy of atmospheric model results necessary for a
future efficient coupling of hydrological and atmospheric models. This means also that data assimilation
strategies, and in particular real time strategies, must be tested.
Beside this deterministic approach, French research team associated to the Electricité de France has
developed a simple probabilistic flood forecasting chain. This chain is devoted to flood forecasting in
medium-sized catchments (some 100 km2 to 1000 km2) and is adapted to work with different input data
and information. The main idea is to use simple hydrological model to transform probabilistic rainfall
scenarios into discharge scenarios. The resulting “spaghetti-like” plots are interpreted into probabilistic
forecast ranges for discharges at different lead-times.
Needs and proposals:
ƒ
ƒ
ƒ
ƒ
To develop an efficient flash flood forecasting tools using rainfall measurements or forecasts: the
output variable would not be necessarily a discharge, it could be a forecasted discharge evolution, or
a real-time guidance for flash flood risk, for example;
To quantify forecast uncertainty by providing probabilistic forecast guidance.
To organize a rainfall-runoff test and inter-comparison programs based on small watersheds exposed
to flash flood (objectives of IASH PUB project).
Data assimilation procedures should be tested to take benefit from the available measured discharge
data.
3) The scientific challenges of the coupling approach
The interest of the coupling between the atmospheric and the hydrological systems is twofold:
i.
ii.
The one way coupling (i.e. the precipitation field is used as the hydrological model input)
allows to improve the prediction of the river discharge by the use of the spatially
representation of the rain field given by the atmospheric model. This first approach needs to
better qualify the simulated rains field, especially at the hydrological scale of interest (i.e. for
flash flood purpose, space scale is on the order of few square kilometres and the time scale of
the order of one hour).
The two-way coupling (i.e., there is a complete interaction between the soil and the
atmosphere, the surface fluxes (sensible and latent) are the coupling variables) allows to
improve the hydrological answer. Most Soil Vegetation Atmosphere Transfer models (used in
atmospheric model) simulate the exchange of heat and water fluxes well, but many have the
disadvantage of not sufficiently simulating soil hydrologic processes at basin scales,
essentially neglecting lateral water lateral transport and its effect on the groundwater table.
Therefore, the two-way coupling may improve the soil moisture characterization and increase
the quality of the initialization of the hydrological models especially for non gauged
catchments.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
The coupling approach is now well established especially in its one-way formulation (Westrick and Mass,
2001; Warner et al., 1991; Benoit et al., 2000; MAP, 2003). Although precipitation is the main forcing
variable for surface hydrologic processes, it is still poorly predicted by meteorological models, even at
space and time scales much coarser that necessary for flash flood forecasting. Therefore, this first step
needs still to be investigated to improve the representation of the atmospheric processes that governs the
intense precipitation.
The two-way coupling is probably the wave of the future and has been already tested for ideal simulations
(Walko and al., 2000) and/or to improve local weather forecast (Seuffert and al., 2002). Both studies
show that the two-way coupling system improves the soil moisture field due to the lateral water transport
processes that are taking into account. Therefore, the energy fluxes, the boundary layer structure, and the
precipitation are better reproduced.
Needs and proposals:
ƒ
ƒ
ƒ
ƒ
ƒ
Identify the degree of coupling (one-way or two-way) required and strategies for dealing with the
different timescales inherent in atmospheric and hydrologic systems in the framework of flash flood.
Conduct sensitivity and parameter estimation studies of hydrologic and atmospheric models run
individually and in coupled manner in order to determine which aspects exhibit the greatest
sensitivity as a means for identifying those components and physical processes that should receive the
most attention.
Investigate the use of statistical downscaling techniques to see if they provide useful information at
the flash flood scales.
To estimate uncertainties of flash flood forecast, uncertainties in atmospheric model precipitation
output must be complemented with a characterization of their errors.
Combine statistical and deterministic approaches in modelling atmospheric and hydrological
processes
23
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
III. TOWARDS EUROPEAN CONCERTED ACTIONS
A. Motivations to tackle the problem at the European level
Research on flash-floods requires a mobilisation of activities and resources across Europe for at least four
reasons:
i.
Flash-floods are rare events. In Europe one or two flash-floods per year have dramatic
consequences. Probably ten times more cases of storms of comparable severity occur during
the same period. The only way i) to capitalise knowledge about these phenomena and ii) to
assess an hypothetical trend due to climate factors is to lead a study at European scale.
ii.
Flash-floods are ill-documented events. Each country has its own investigation and archiving
rules frequently separating the meteorological and hydrological aspects and seldom
considering socio-economic consequences in a detailed quantitative manner. Only a common
observation strategy across Europe can make the appropriate information homogenous and
available at the required level.
iii.
The forcing meteorological situations and their climatic trend develop at the continental
scale.
iv.
Socio-economic short and long-term strategies mitigating flash-floods need to be harmonised
across Europe. In particular the socio-economical value of the different protection strategies,
warning and planning in particular, needs to be re-assessed given the hypothesis of a change
in the forcing conditions like climate and landscape.
Usually, flash-flood researches have been funded within programmes dealing with “floods” in general.
Very few programmes were concentrated only on flash-flood. Therefore, results, recommendations, and
needs on flash-flood are difficult to handle and to summarize due to the dispersion within many reports.
Table 2 presents a list of the main programmes where flash-flood studies were supported by the EU.
More information on these programmes, in particular the main results dealing with flash flood, is given in
ANNEX.
B. Scenarios for a possible research activity dealing with flash-flood
Based on the previous European projects and the actions defined in this document by Needs and
Proposals, we think that specific field experiments targeted to improve both the physical processes and
model forecasts are now needed to observe a statistically significant number of intense and/or extreme
cases. Modelling projects and observation strategies should rely on the three natural laboratories (II.C.4).
They are representative of the Mediterranean region (medium elevation mountainous area, Alpine
high mountainous area, urbanized area). To address the issues of numerous space-time scales involved in
the flash flood forecasting, research activity should be designed as a multi-year program defined by the
following ideas:
o
o
There is a real necessity to maintain a strong European research focussed on the understanding
of the hydrometeorological processes leading to flash-flood. As shown (II-A and II-B),
atmospherical processes associated to the quasi-stationary MCSs as well as the relationship between
the soil and subsoil conditions with the formation of runoff are still open scientific questions.
Therefore, such research must be conducted within a multidisciplinary framework associating
meteorology, hydrology and soil sciences.
To assess this interdisciplinary research, complementary approaches and tools must be developed
and maintained at the same time, and must be done at the European level to support the costs
and the personal efforts. Observations, theoretical developments, modelling are necessary to
24
Increasing the forecasting lead-time of Weather Driven Flash-floods
o
o
o
o
Editors: S. Anquetin et al.
understand, to model and then to built new generation of flash flood forecasting system. Needs and
proposals defined in II-A, II-B and II-C are strongly linked all together and must be studied in this
sense.
To improve the lead time of flash flood prediction in Europe, several modelling strategies might
be considered and compared at different scale (regional scale to catchment scale). We should
combine statistical and deterministic approaches in modelling atmospheric and hydrological
processes. Future actions should be part of HEPEX, the recent initiative proposed jointly by NOAA,
ECMWF, WCRP/GEWEX, IAHS/PUB.
The link between operational services, end-users and the research community must be
reinforced to define of a real-time flash flood forecasting system. The large range of time and space
scales of such event lets open the question of modelling strategy and should be assessed in a
concerted action.
There is a real need to centralize and capitalize works and data on a same region of interest.
The MEDEX initiative is, in this sense, a challenging project that has been endorsed by the World
Weather Research Project of the World Meteorological Organisation. A large number of participating
institutes from the Mediterranean Area focus their studies on Cyclones that produce high impact
weather in the Mediterranean region. The same initiative should take place around the study of
Mediterranean flash floods.
From all these points, long term and concerted actions must be recognized at the European level.
It also reveals the necessary to focus the work on natural laboratories (Pilot sites) prone to flash
floods that will be representative of the diversity of the Mediterranean region (medium elevation
mountainous area, Alpine high mountainous area, urbanized area).
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Editors: S. Anquetin et al.
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République
Française.
133
pp.
Available
at
the
Internet
site:
http://www.environnement.gouv.fr/infoprat/Publications/publi-ige.htm.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
ANNEX
32
Editors: S. Anquetin et al.
Increasing the forecasting lead-time of Weather Driven Flash-floods
33
Editors: S. Anquetin et al.
Increasing the forecasting lead-time of Weather Driven Flash-floods
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Table 1: Summary of recent flash-floods in Mediterranean Europe [the chosen period is roughly the period when the European radar network
appeared]
Place
Barcelona area
(Spain)
Date
25/9/1962
Nîmes (France)
4 - 5/10/1988
Vaison-laRomaine (France)
26/9/1992
Brig (Switzerland) 22 - 24/9/1993
Versilia (Italy)
19/6/1996
Biescas (Spain)
7/8/1996
Corbières (France) 12-13/11/1999
Soverato (Italy)
9-10/9/2000
Gard (France)
8 - 9/9/2002
Comments
Human losses
Insured costs
The Besos river inundates the Terrasa sector after a storm that produced overMore
than
100080 millions US $
250mm of rainfall in 2 hours.
human lives
(about 500 from
CRED source)
Urban flood caused by the Cadereaux watersheds (less than 50km2). Peak flow11 victimes
610 millions € (local
in the city of about 1000 m3/s.
insurers)
Peak flow of about 1000 m3/s at Vaison la Romaine, over 300 mm of rainfall in58 human lives
24 hours. Outflow coming essentially from a 50 to 100 km2 sub-catchment of
the Ouvèze river.
The average 24h precipitation was 40 mm the 23rd and 65 mm the 24th and2 victims
5 millions US $
maxima at Simplon (in Saltina river's catchment above Brig) of 120 mm the
23rd and 220 mm the 24th were recorded. Water crashes through the town at a
rate of 3m/h for 12 hours
400mm of rainfall accumulation in less than 6 hours with maximum rainfall26 victims
33 millions US $
intensity of about 88mm in 30 minutes.
Small upstream catchment of the Aras River (18 km2) at the outlet of which a87 human lives in a
camping site was installed. Over 250 mm of rainfall accumulation in 6 hours.camping site
Peak flow of 400 to 600 m3/s (i.e. about 30 m3/s/km2)
Rural flood of the Aude River (4840 km2), 30 to 50% of the peak flow being35 victims
3 millions US $
produced by a 123 km2 watershed (Gaume et al. 2003)
350mm of rainfall accumulation in 24 hours with a peak at 185mm in 6 hours,16 victims (12 from a
causing landslides.
camp ground)
Over 600 mm of rainfall in 24 hours at Anduze. Peak flow of the Gard river of25 victims
440 millions € (Swiss5000 to 7000 m3/s for a 1400 km2 basin. Specific outflow of 30 m3/s/km2 on(The same river killedRe Insured Losses)
small tributaries (S<50 km2).
35 persons in 1958) 750 millions € (Local
Insurers)
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Number of days
Figure 1 : Number of days with daily precipitation greater than 200 mm over southern
France (from CDROM-pluies extrêmes sur le sud de la FRANCE, METEO-FRANCE and
MATE, 2002).
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Monthly distribution of daily rainfall > 200 mm
(1958-2000)
Number of days
Figure 2: monthly distribution of the number of days with raingauge precipitation above 200 mm
from 1958 to 2000 for the Lozère, Hérault, Gard and Ardèche departments. (from CDROM-pluies
extrêmes sur le sud de la FRANCE, METEO-FRANCE and MATE, 2002).
36
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Sembadel
2000
1960
)
m
k(
u
d
n
te
é
II
tr 1920
e
b
m
a
L
1600
1400
1200
1000
1880
800
600
400
1840
200
660
700
740
780
0
Extended Lambert II
Figure 3: The Cévennes-Vivarais Mediterranean Hydro-meteorological Observatory window.
The coloured map represents the topography (m asl) which culminates at the Mount Lozère site (1699 m).
The main Cévennes rivers (Cance, Doux, Eyrieux, Ardèche, Cèze, Gard and Vidourle) are right bank
tributaries of the Rhône river with a typical Mediterranean hydrological regime (very low waters during
summer, floods occurring mainly during the autumn). They are characterized by steep slopes in the head
tributaries of the Cévennes mountains. In terms of geology, the mountainous part of the region (northwest of the map) corresponds to the Primary era formations of the Massif Central (granite, schists) while
sedimentary and detrital formations dominate in the Rhone valley region (south-eastern part of the map)
with, in places, karstified limestones. Many villages and several small to medium-sized cities exist in the
region (the main city, Nîmes, counts 200 000 inhabitants).
The three weather radar sites are indicated by the crosses and the 40 km range indicators. The black
circles and triangles give the locations of the hourly rain gauge network.
Within the 160 x 200 km2 Cévennes-Vivarais window, the observation system is comprised at the
moment of: (i) three weather radars of the Météo-France ARAMIS network located at Nîmes (S-band),
Bollène (S-band) and Sembadel (C-band), (ii) two networks of about 400 daily rain gauges and 160
hourly rain gauges and (iii) a network of about 45 water level stations.
The “green tourism” is very developed, with in particular the famous Ardèche gorges site, leading to a
spatially diffuse vulnerability to flash-floods.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Figure 4: Hydro meteorological observatory of the Region of Catalunya – Spain
Catalunya covers roughly 30000 km2 of hilly terrain between the sea and the Pyreneos mountain ridge.
This region is drained by a set of coastal rivers. Many of them cross densely urbanised and industrialised
zones. Among them, the Besòs River and the Llobregat River pass north and south through the
conurbation of Barcelona (more than 3 millions of inhabitants).
Figure 2 shows the SAIH rain gauge network (red triangles) and the deployment of the radar network
(indicated by the small pictures).
This region is covered by a C-band radar of Corbera installed in 1992 by the Spanish Meteorology
Institute (INM). A second radar system belonging to regional Meteorological Service of Catalunya has
been installed in the Gavarres mountain in 2001. It is situated about 100 km north-east from Corbera.
This allows a remarkable coverage of the Barcelona area and its main coastal rivers. Two additional radar
systems have been installed recently to cover the western part of the Catalunya region draining to the
Ebro
watershed.
38
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Figure 5: Topographic map of the Barcelona area showing the extension of the Besòs River and its
instrumentation.
In the last 1980s, an important renovation of its instrumental equipment was done in the framework of the
national SAIH program. Today the Catalan Agency for Water Management (ACA) operates a telemetered
network of 7 stage record stations and 13 rain-gauges inside the watershed. Forty additional rain-gauges
are available in the Barcelona region. The data is available every 5 minutes in real time. The watershed
also benefits of an exceptional radar coverage since the Corbera radar is located within range distances of
20 to 60 km and the Gavarres radar within range distances of 40 to 80 km.
39
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Figure 6: The Adige River Basin Hydrometeorological Laboratory - Italy
The Adige river watershed (12000 km2 at Verona) lies in north-eastern Italy. The altitudes range from
100 m a.s.l. up to 4000 m a.s.l.. The region, and especially the upper Adige river basin, is frequently hit
by flash flood events (Termeno Rio Inferno, June 1987; Fortezza, July 1999, Cortaccia, 26 June 2001).
The operational observation system includes :
(1) a network of three weather radar systems. Two are C-band Doppler weather radars (Mt. Macaion and
Mt. Grande) and one is a S-band Doppler radar (Spino d’Adda). They are used for the
hydrometeorological surveillance of the region. The Mt. Macaion weather radar (1860 m a.s.l.) has been
recently installed (1999). The Mt. Grande radar system (420 m a.s.l.) is in operation since 1989 and was
recently improved. The S-band weather radar in Spino d’Adda, in the Po valley, has been modified during
1995, enabling coherent (Doppler) measurements;
(2) a network of ground-based instruments: 140 tipping bucket rain gauges and 30 hydrometric stations.
(3) the Meteosat imagery...
40
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
Table 2: Previous European programs that supported flash flood research.
Project
A comprehensive forecasting system for
AFORISM
flood risk mitigation and control
Anomalies induced by mountains and sea in ANOMALIA
rainfall over land in the western
Mediterranean area
River basin modelling, management and
RIBAMOD
flood mitigation
Applied research on a transferable FLOODAWARE
methodology, devoted to flood awareness
and mitigation, helping the decision and
negotiation processes, adapted to a changing
environment, and respecting the water
resources
European river flood occurrence and total
EUROTAS
risk assessment system
Flash-flood risk assessment under the impact FRAMEWORK
of land use changes an d river engineering
works
Forecasting
floods
in
urban
areas
TELFLOOD
downstream of steep catchments
Risk of inundation - planning and response
RIPARIUS
interactive user system
Runoff and atmospheric processes for flood
RAPHAEL
hazard forecasting and control
Satellite and combined satellite-radar
MEFFE
techniques in meteorological forecasting for
flood events
Telematics-assisted handling of flood TELEFLEUR
emergencies in urban areas
The development of active on-line HYDROMET
hydrological and meteorological models to
minimise impact of flooding
Systematic, palaeoflood and historical data
SPHERE
for the improvement of flood risk estimation
Prevention in the Mountains for Protection of
PREMO 98
the valley
Guidelines on flash flood prevention and
mitigation
Optimisation of the hydro meteorological HYDROPTIMET
forecast tools
Integrated Flood Risk Analysis and
FLOODsite
Management Methodologies.
41
Framework
Programme
FP2
1991 - 1994
FP3
1994 - 1997
EC Programme
FP4
1996 - 1998
FP4
1996 - 1998
ENV 2C
FP41
998 - 2000
FP4
1998 - 2000
ENV 2C
FP4
ENV 2C
FP4
1998 - 2000
FP4
1998 - 2000
FP4
1998 - 2000
TELEMATICS 2C
FP4
1998 - 2000
FP4
1998 - 2000
TELEMATICS 2C
FP5
EESD
EPOCH
ENV 1C
ENV 2C
ENV 2C
ENV 2C
ENV 2C
ENV 2C
FP5
FP5
NEDIES
2002 - 2004
INTERREG-IIIb
MEDOC
Global Change and
Ecosystems
FP6
2004 - 2008
Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
DETAILED ON THE EUROPEAN PROGRAMMES LISTED TABLE 2.
ƒ
A comprehensive forecasting system for flood risk mitigation and control (AFORISM)
The aim of this project was the development of a comprehensive flood forecasting system and the study
of alternative management policies intending to flood risk mitigation. The research was organized
through the following tasks: (a) the analysis of intense rainfall events and their classification by weather
type as well as the modelling of intense rainfall and the production of alternative hyetographs of temporal
evolution of rainfall; (b) the comparison of the alternative rainfall-runoff models, using multiple time
steps in modelling rainfall-runoff and applying it to hydrological basins, (c) the forecasting of spatialtemporal evolution of rainfall using limited area models; (d) the development of optimisation models in
order to mitigate flood risks; (e) the development of an expert system for flood management; (f) the
development of a geographical information system for visualisation of the evaluation of flood and its
consequences; and (g) the integration of the forecast and control system in the Reno basin (Italy).
ƒ
Anomalies induced by mountains and sea in rainfall over land in the western Mediterranean
are (ANOMALIA)
ANOMALIA was funded by the 3rd Framework Programme, lasted between May 1994 to January 1997,
and was coordinated by Andrea Buzzi, CNR Bologna.
The aim of this project was to improve the basic knowledge of the meteorological processes that lead to
the formation of storms and floods in the Mediterranean area. Particular emphasis was placed on the
study of the scale interaction processes, where synoptic and mesoscale phenomena play a synergistic role,
in connection with orographic forcing and air-sea energy exchanges, in the generation of severe weather
events.
Some of the objectives of the ANOMALIA project coincided with the objectives set for MAP. These
include: the effect of orography on deep convection and frontal precipitation that lead to flash flooding,
the numerical prediction of moist processes over complex terrain and the better understanding of local
factors influencing severe and quasi-stationary events.
ƒ
River Basin Modelling, Management and Flood Mitigation (RIBAMOD)
The RIBAMOD concerted action was funded by the European Commission Directorate General for
Science, Research and Development (DG XII) within the 4th Framework Programme and lasted from May
1996 to October 1998. Paul Samuel from the HR Wallingford was the project coordinator.
The main objectives were:
Facilitate understanding of technical and policy issues in flood management
Examine how advanced modelling should support planning of flood defence
Identify methods and procedures for sustainable development, management and use of the river and its
catchments
The main issues linked to flash floods can be summarized by:
o There is no universal model applicable in all circumstances.
o The hydrological model is tied to the study objectives.
o Flow simulation in steep mountainous rivers is needed.
o Design of the hydrometeorological data network with sufficient redundancy to achieve the required
accuracy and the security of information for forecasting in the most severe conditions is required.
o A better understanding and quantification of the uncertainty in the forecasting process is needed.
o The dominant runoff generation processes in severe storms is needed
o Better communication between meteorologists and hydrologists is needed to improve flood
forecasting.
o Better communication between the research community and operational agencies in the
implementation of research advances is needed.
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Increasing the forecasting lead-time of Weather Driven Flash-floods
Editors: S. Anquetin et al.
The main references are:
o RIBAMOD, Proceedings of the 1st workshop on Current Policy and Practice, Eds Casale, Pedroli and
Samuel, 1998
o RIBAMOD, Proceedings of the 2d workshop on Impact of Climate Change on flooding and
Sustainable River Management, Eds Casale, Samuel and Bronstert, 1999.
o RIBAMOD, Proceedings of the workshop and 2d expert meeting on integrated systems for real time
flood forecasting and warning, Eds Casale, Borga, Baltas and Samuel, 1999.
ƒ
Prevention and forecast of flood (FLOODaware)
The coordinator was Nicolas Gendreau, Cemagref France.
This project aimed to build a European methodology for flood management and damage mitigation with
accepted standards, especially on vulnerabilities and risk maps implementation. The objective was to
implement into models and tools new synthetic approaches, such as the “Inondabilité” method, developed
in water sciences and management. This methodology deals with synthetic models in hydrology,
hydraulic modelling, hazards parameters, vulnerabilities, crossed maps …
Results have already been obtained for a quantification of the hazard and works are done for an estimate
of the objectives of protection against floods and are presented in the final report:
Floodaware final report. Programme Climate and Environment 1994-1998. Area 2.3.1: Hydrological and
hydrogeological risks. Contract: ENV4-CT96-0293
GENDREAU N., 2000, 240 p., ENG., Cote : 00/0038. ISBN 2-85362-525-7
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Risk of Inundation – Planning
And Response Interactive User System (RIPARIUS)
The concerted action RIPARIUS, 1998 – 2000, took place in the continuity of the RIBAMOD project and
was supported by the European Commission (DG XIII) within the TELEMATICS APPLICATIONS
Programme. Ken Blyth, Center for Ecology and Hydrology, Wallingford, was the project coordinator.
This project has provided a broad input of user requirements for flood information in relation to the
climatic, physical and cultural differences that exist within Europe. The consortium tried to provide a
good representation of the geographic factors affecting flooding within the EU: one group had experience
of northern maritime climates, one group of European continental climate, and one of Alpine and/or
Mediterranean influence.
Recommendations and needs were highlighted for:
o Flood alerts
o Flood information systems
o Crisis management
o Long term actions ( flood awareness, education, flood risk mapping
The main references are:
o Environment agency, 1998, Airbone light detection and ranging feasibility study, R&D Technical
Summary ES41, National Centre for Environmental Data and Surveillance, Environment Agency,
Bath, UK.
o Ministry of the Interior, 1999, Rescue Services in Finland, Ministry of the Interior, Department for
Rescue Services, Kirkkokatu, 12, FIN-00170, Helsinki, Finland.
o Moses, 2000. Methods of improving public warnings of emergencies. Report of RIPARIUS Second
Workshop, York, 2-3 March 2000.
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European river flood occurrence and total risk assessment system (EUROTAS)
The EUROTAS project was built on the conclusions of the RIBAMOD concerted action. It was funded
by the 4th Framework Programme and lasted between 1998 and 2000. Paul Samuel, HR Wallingford, was
the coordinator.
The EUROTAS project led to the following advances and achievements:
o Development and demonstration of the integration of several existing hydrological and
hydrodynamics simulation models within and open-systems catchment scale framework,
o Development and demonstration of a land-use change scenario builder,
o Development of a methodology for downscaling precipitation scenarios at the catchment scale from
GCM simulations
o Illustration of the sensitivity of flood flows to urbanisations, land-use change and climate change at a
variety of scale and European climatic zones.
o Illustration of the uncertainty in flood risk assessment based upon observations and the need for
improved confidence in GCM simulations before firm conclusions can be drawn on the impact of
climate change on flood risks.
Main reference:
European River flood occurrence and total risk assessment system. Scientific report ENV4-CT-0535.
Published by HR Wallingford May, 2001.
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Flash-Flood risk assessment under the impact of land use changes and river engineering
works (FRAMEWORK)
FRAMEWORK was funded under the 4th Framework programme, lasted between 1998 and 2000, and
was coordinated by Paolo Burlando, ETH Zurich.
The main focus of FRAMEWORK is on the explanation of nonstationary effects on the occurrence and
intensity of extreme floods in flashy streams. These are produced by climate forcing and anthropogenic
changes of the river basin system, which involve land use, watershed management practices and river
engineering works. FRAMEWORK aims at providing a revision of the current methods for flood
prediction and prevention in flashy streams under the improved knowledge of the river basin system and
its interaction with climate, land use and watershed management practices.
FRAMEWORK provided methodological advances including
o the development of methods for physically-based regionalization of flood flows in the probability
domain: these are useful to approach the flood hazard at the regional scale;
o the assessment of the capabilities of derived distribution methods as a useful diagnostic tool for flood
frequency analysis at the basin scale;
o the development of simulation methods for flood frequency estimation throughout the river network,
which provide a comprehensive technology to investigate anthropogenic effects on the flood hazard
using Monte Carlo experiments under a physically based skeleton.
The demonstration of these methodologies was performed through the application of the methods and
models at different and complementary spatial scales, that is, through
o regional case studies including Austria, Switzerland and North-Western Italy,
o basin case studies, including selected flashy streams of Germany, Italy, Spain, Switzerland and the
United Kingdom.
The final reports of the different partners are on the web site:
http://www.diiar.polimi.it/framework/Partner%20Report%20Summary.htm
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Runoff and atmospheric processes for flood hazard forecasting and control (RAPHAEL)
The project coordinator was Baldassare Bacchi from the University of Brescia, Italy.
The basic objective of the RAPHAEL project, as indicated by the acronym was to develop, implement
and demonstrate the use of coupled meteorological and hydrological models at the regional scale in order
to improve flood forecasting and management in complex mountain watersheds
With this guideline further objectives were delineated as follows:
o
to apply coupled atmospheric-hydrological models and carry out a multi-scenario modelling
experiment to show the potential use of advanced flood forecasts in view of the reservoir regulations
during hazardous flood events;
o
to investigate the benefits achievable in atmospheric models by introducing hydrological feedback
with detailed land-surface schemes,
o
to investigate the benefits achievable in atmospheric models by introducing hydrological feedback
with detailed land-surface schemes, including snow and ice dynamics;
o
to validate meteorological data generated by Numerical Weather Prediction (NWP) models and
meteorological observations by means of runoff measurements and distributed hydrologic water
balance calculations;
o
to investigate the benefits of remotely-sensed land surface parameters, state variables and fluxes (e.g.,
land cover, soil moisture, snow cover, evapotranspiration) as related to sub-grid parameterization of
both atmospheric and hydrologic models;
o
to improve techniques and tools for scale-adaptation of observed and simulated variables, with
particular reference to the areal distribution of rainfall, snow cover, and land-surface-atmosphere
fluxes.
The final report can be loaded from:
http://www.map.ethz.ch/map-doc/raphael/deliverables/deliv.htm
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Satellite and combined satellite-radar techniques in meteorological forecasting for flood
events (MEFFE)
This project, coordinated by Franco Prodi, CNR Bologna, aimed at improvements in rainfall
intensity estimates for mitigating the risk of flood events using nowcasting techniques (meteorological
satellites, combined satellite-radar data and numerical models).
The main object of the project was achieved by:
o
better knowledge of meteorological systems generating different flood events;
o
coupling satellite data, radar data and numerical Limited Area Models;
o
improving MW and Vis-IR algorithms for precipitation retrieval;
o
improving weather numerical models (LAM and Cloud Mesoscale Models) that combine surface and
upper air measurements, and radar-satellite data;
o
defining the characteristics of Nowcasting procedures for rainfall rate intensity.
The final report can be downloaded from:
http://www.isao.bo.cnr.it/~meffe/sum/summary.htm
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The Development of On-Line Hydrological and Meteorological Models to Minimise the
Impact of Flooding (HYDROMET)
HYDROMET, coordinated by Ian Cluckie, University of Bristol, sought to develop real time flood
forecasting systems utilising state of the art weather radar data for the provision of quantitative rainfall
forecasts and distributed rainfall inputs and a variety of rainfall - runoff models for the conversion of
these inputs into flows at key points on dendritic river networks. It also aimed to improve the quality of
the initial radar data through the use of novel calibration, adjustment and correction techniques and then
to test the models developed on a number of river basins across Europe.
The ultimate objective is the construction of operational flood forecasting systems which are capable of
providing forecast information with sufficient accuracy and lead time to help mitigate the impact of
floods.
The HYDROMET Integrated Radar Experiment (HIRE) aimed at providing the HYDROMET Partners
with the opportunity to aggregate and integrate their respective work on improving the quality of radar
data for hydrological modelling.
The two scientific objectives of the Experiment were:
o To achieve a significant improvement in the estimation of rainfall over a large Mediterranean urban
area by integrating a complimentary set of radar equipment supported by conventional surface
instrumentation.
o To carry out a preliminary study of the generation of flow simulations using the improved rainfall
estimates for a discrete sub-catchment within the metropolitan area of Marseilles.
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Systematic, Paleoflood and Historical data for the improvement of flood risk estimation
(SPHERE)
The catastrophic floods that have occurred recently in Europe warn of the critical need for hydrologic
data on floods over long-time scales. Paleohydrological techniques provide information on hydrologic
variability and extreme floods over long-time intervals (100 to 10,000 yr.) and may be used in
combination with historical flood data (last 1,000 yr.) and the gauge record (last 30-50 yr.). In SPHERE,
new scientific frameworks and technical tools integrating multidisciplinary approaches (geologic,
historical, hydraulic, statistical and GIS) on extreme flood risk assessment was generated. These was
tested in case study areas in Spain and France. New methods of reconstructing paleofloods and the
historical flood record of these basins lead to the elaboration of a complete catalogue of major past floods.
Systematic and non-systematic data was combined for flood frequency analysis, using improved methods
for the adjustment of distribution functions.
More information is available on:
http://www.ccma.csic.es/dpts/suelos/hidro/sphere/enter.html
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NEDIES PROJECT: Guidelines on Flash Flood Prevention and Mitigation
The NEDIES project is being conducted at ISPRA by the Institute for the Protection and Security of the
Citizen. The objective of the project is to support the Commission Services of the European
Communities, Member State Authorities and EU organizations in their efforts to prevent and prepare for
natural disasters and accidents, and to manage their consequences.
A main NEDIES activity is to produce “guidelines” on natural disasters and accidents. Flash floods are
one of the priority areas within the DG Environment Major Project on Prevention of Natural and
Technological Disasters. These guidelines were mainly envisaged for decision-makers in the field of flash
flood management, but can also be of interest to practitioners and the general public.
Final report:
NEDIES PROJECT : Guidelines on Flash Flood Prevention and Mitigation, by A.G. Colombo, J. Hervas
and A.L. Vetere Arellano, EUR 20386 EN, JRC Ispra.
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Optimisation of the hydrometeorological forecast tools (HYDROPTIMET)
The Interreg IIIB MEDOC project, called HYDROPTIMET (2002-2004) managed by the Piemonte
Region (Italy), is currently dedicated to the optimisation of the hydrometeorological forecast tools. This
project has for objectives to experiment new tools and methodologies for quantitative precipitation
forecast in some specific areas of the northern part of the Mediterranean arc. One achievement of this
project will be to draw some recommendations on the use of high resolution numerical forecast for flashflood forecasting; the scope of these recommendations will be, however, limited due to the little number
of case studies (only four flash-flood events).
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(FLOODsite)
The funded European FP6 Integrated FLOODsite project (2004-2008), coordinated by Paul Samuel, HR
Wallingford, has for ambition to develop an integrated methodology for flood risk analysis and
management. Only a small part of the project is devoted to the flash-floods. The use of advanced
atmospheric models to forecast flash-floods is not considered in FLOODsite, runoff models are planned
to be forced by radar data.
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