Blair, Executive Director, E-mail: dg stewardshpcentre. Ministry of Environment.
Particular focus will be put on climate change impacts on peak demand. Claude Desjarlais, M. Hydro Electricity Demand en anglais. Government; B. Marco ouranos.
Oil and Gas Commission. Brenda L. Caroline ouranos. Personne-ressource : Moktar Lamari, Ph. Au contraire, il subit des chocs qui sont persistants. Ce dernier point est important. Les raisons en sont maintenant bien connues. La fluctuation de la valeur de ces actifs devrait donc dupliquer celle des revenus, donc du PIB par habitant.
Cet argument ne tient pas. Voir plus loin. Abstract Do we do enough for the future? The rain gauge map presented in Fig. The kriging technique Creutin and Obled ; Lebel et al. The rain climatology of the region Bois et al. In addition to these procedures, a calibration factor derived from radar—rain gauge comparisons performed at the event time scale in the vicinity of the radar sites at ranges less than 30 km , was applied uniformly in time and space to obtain the displayed radar rain amounts.
It obviously accounts for other sources of bias—for example, for an ill-suited Z — R relationship multiplicative coefficient. Note that the two radar maps are pretty consistent regarding the amounts and location of the main precipitation cores. This good overall consistency is an expected result owing to the fact that the rain event affected a region of good visibility Pellarin et al.
More quantitatively, the agreement between the three sets of raw rain data is indeed good with bias values of 0.
A detailed presentation of the mentioned criteria can be found in Berne et al. Figures 6d and 6e present the results obtained by merging the radar and the rain gauge datasets. Inference of the variogram of the ratios is the prerequisite for using this method. The identified variograms present a range of about 60 km for both radar systems. No significant nugget effect i. This significant spatial structure of the ratios indicates that an improvement can be expected by using the spatially variable calibration technique with respect to the rain fields produced by each observation system alone.
A detailed comparison of Figs. Three rain patterns with rain totals greater than mm are visible inside a pattern of rain amounts greater than mm covering most of the considered window. Figure 7 summarizes the results obtained by some of the rain estimation approaches used in terms of exceedance areas; that is, the areas over which a given amount of rainfall was exceeded in the region of interest.
The spatial calibration method provides very consistent results for the two radar systems with areas of about , , and km 2 for the , , and mm thresholds, respectively. As a result of the low density of the daily rain gauge network in some portions of the region of interest, the kriging technique slightly overestimates the exceedance areas in the [— mm] rain threshold range and significantly underestimates them for the rain thresholds greater than mm. Also shown in Fig. The 8—9 September Gard event is therefore particularly remarkable by its spatial extent.
The dynamics of the rain event were analyzed using radar animations of both the instantaneous rain rates at the 5-min time step and the accumulated rain amounts up to a given instant. To summarize this study, Fig. After an initial period — UTC where convection develops inland and starts producing rather uniform rain amounts over the region, the MCS becomes stationary after UTC and takes a southwest—northeast orientation.
It produces sustained rain rates during 6 h, which leads to the well-defined rain pattern shown in Fig 8. During the morning of 9 September , the cold front passes and sweeps the MCS out of the region. As usual, high rain rates are observed during this phase but, resulting from the rapid storm displacement, the total rain amounts remain limited to about mm, with a rather uniform spatial distribution over the region. However, since the storm moves from upstream to downstream over the river network, the additional precipitation may have had a significant hydrological impact.
The consistency of the rain estimates during these three phases with the two spatially calibrated weather radar datasets is good Fig. To further illustrate the time structure of the rain event and assess the quality of the different sources of data, Fig. Then, a brief calm of 1 h is followed by a rainy period of about 2 h with even stronger rain rates e. This final burst corresponds to the passage of the cold front over that area. The Remoulins and Chateauneuf-du-Pape hyetographs are typical of the rainfall that occurred in the Gard plains with two distinct peaks during phases 1 and 3 separated by a calm of about 12 h.
The comparison of the radar and rain gauge hyetographs displayed in Fig. The latter result is certainly in great part due to the different measurement principles in particular in terms of sampling of the radar and rain gauge sensors.
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Such cross-comparisons are useful to detect sensor problems: for instance, since the rain estimates of the two radar systems are very similar, a malfunction e. A detailed radar—rain gauge analysis is beyond the scope of the present paper; however, the results presented here clearly indicate the overall consistency of the three sources of rainfall data. To summarize, the dynamics of the 8—9 September rain event presents the following characteristics: i the MCS typically produced rain amounts of — mm during periods of about 6 h, ii the evolution of the synoptic situation induced a clear shift of the MCS between the Gard plains region toward the upstream part of the Gard watershed during the night of 8—9 September , and iii in a third phase, the passage of the cold front brought additional rain amounts of about mm with brief and violent bursts in the western part of the region.
The rain event therefore presents a marked space and time structure: in the following section , we aim at documenting the hydrological response of the watersheds over all the region and try to find evidence of the rainfall signature in this response.
The operational hydrological data available for these watersheds consists of 17 river stage records that are located on the main streams. Some of the sensors, which are based on pressure measurement principles, were shown to significantly underestimate the observed high water levels. Some of them were destroyed during the flood.
A study has therefore been commissioned by the Gard water authorities to critically analyze and correct these operational measurements as much as possible. In the following, we first present an overall view of the maximum discharge and peak time data collected and its relation with the spatial and temporal distribution of the rainfall. In the second part, attention is given to the study of the distinct hydrological behaviours of two watersheds.
During the postevent investigation, witnesses were interviewed and 93 river cross sections were surveyed and the corresponding flood peaks estimated to assess the contribution of the headwater tributaries. To summarize the postevent investigation results, the estimated peak specific discharges of 17 watersheds with sizes in the range of 10 to km 2 are presented on a map of the region Fig. The spatial distribution of the peak discharges is not surprising and corresponds quite well to the distribution of the total rainfall amounts.
Such totals only partly reflect the rain rates, which are of course influential on the peak discharges. The contributions of most of the tributaries of the Gard and Vidourle Rivers appear very high. The spatial extension of the heavy rainfall event explains the very high total discharge of both the Gard and the Vidourle Rivers.
In terms of dynamics, it is interesting to relate the reported peak times to the rain event space—time analysis presented in section 4b.
The downstream tributaries of the Vidourle River clearly responded early to the rainfall that occurred during phase 1: between e. The response of the downstream tributaries of the Gard River to the phase 1 rain is consistently estimated to have occurred later at UTC for watersheds 14 to 16 for the left bank tributaries. In the southern part of the phase 2 rain core watersheds 4 and 12 , the first peak occurred at about UTC.
In the northern part, due to the proximity of the phase 2 and phase 3 rain peaks watersheds 7, 10, and 11 , only one discharge peak, occurring between and UTC, was reported by the witnesses. The phase 3 rainfall consistently produced the second discharge peak between and UTC in the upstream tributaries and between and UTC in the downstream tributaries on 9 September In the Gard plains, there is evidence of concomitancy between the Gard flood and the secondary flash floods occurring on the downstream Gard tributaries watersheds 14, 15, and 16 during the cold front passage, due to the delayed contribution of the upstream tributaries.
This fact probably increased the hydrological impact of the rain event. The spatial—temporal analysis of the postevent investigation data just presented leads to expected results: the headwater tributaries reacted with a slight delay to the rainfall bursts observed during the three phases of the rain event and the highest specific discharges occurred where the rain event was the most intense.
A complex response, to be analyzed in detail in future work, is likely in the Gard plains during the morning of 9 September as a result of the convolution of the spatial—temporal structure of the rain event with the geomorphologic characteristics of the watersheds. The hydrological model proposed by Gaume et al. To briefly recall the main characteristics of this model: 1 the watershed is represented as a cascade of river reaches having a rectangular cross section, connected to two rectangular slopes; 2 the floods are assumed to be essentially produced by surface or subsurface runoff water; 3 the Soil Conservation Service SCS model is used, because of its simplicity, to evaluate the mean runoff coefficient on each subwatershed and its evolution during the rain event; and 4 the kinematic wave model is used to route the flood flows through the watershed.
The evolution of the runoff coefficient value during the event is derived from the basic SCS equations: where Q t and P t are the accumulated runoff and rainfall amounts expressed in millimeters , respectively, from the beginning of the event up to time t. The model is actually not calibrated; instead, several simulations are performed for several CN values in the range from 50 to Concerning the flow routing model, the number of reaches depends on the morphological complexity of the watershed rather than on its area.
The width and Manning coefficients of each river reach correspond to those evaluated during the field investigation. The Manning coefficent of the slopes is empirically set to 0. The comparison of the simulated hyetographs with the sparse available discharge data only peak discharges and peak times were available at most locations give two types of information concerning the rainfall—runoff process through the watershed: 1 it provides an evaluation of the mean runoff coefficient at various moments during the event, and 2 if the match between the model results and the available data is correct, then the CN value allows an estimation of the watershed retention capacity for this event.
The preliminary analysis reveals different types of hydrological behaviors among the available data. Two examples are presented here for the watersheds of the Crieulon stream 93 km 2 and the upper Vidourle River 86 km 2 —see Fig. Table 3 presents the volumetric balance that could be established for these two watersheds considering the areal rainfall estimated with the spatially calibrated data of the two weather radar systems. The runoff deficit can be estimated to about 95 and mm for the Crieulon and Vidourle watersheds, respectively.
Such deficit values appear relatively low if compared to other studies of recent flash floods Gaume et al. Figure 12 shows the results of the hydrological model implementation. The hydrological model appears to be quite well suited to the Crieulon watershed in terms of synchronization and amplitude of the different peaks.
The curve number optimal to simulate the beginning of the hydrograph CN of about 70, that is a retention capacity of about mm is very consistent with the deficit estimated with the measured hyetographs and hydrograph. The hydrological behavior observed for the Crieulon stream is typical of watersheds mainly on marls and nonkarstified limestone. For the Vidourle watershed, located in a karstic area, the hydrological simulations are much less satisfactory: the watershed clearly has some important rainfall water retention capacities during the flood.
Moreover, a relatively high discharge, not simulated by the rainfall—runoff model, remains after the rain event has ceased. This indicates that a part of the stored rainwater about one third in the presented case is returned during the days following the flood event.
This behavior attributed to the karstic nature of the watershed geology, seems to occur also for steep watersheds on schist often supposed to be impervious. The 8—9 September catastrophic rain event occurred in a meteorological context propitious to heavy precipitation in southeastern France. This warning level was maintained, and the forecasts updated, during the following bulletins of the day of 8 September The fourth-level warning bulletin was issued at UTC 8 September , prior to the passage of the cold front over the region, which provided useful information regarding the expected rain intensification while the situation was already extremely difficult in the region.
Note that, as a result of both coarse grid resolution and parameterized rather than explicit representation of convection, the current operational meteorological models significantly underpredicted the amount of precipitation and incorrectly predicted its position by about km. The mesoscale observations during the event, and notably the satellite and radar observation of the typical V shape of the stationary MCS, were certainly critical in the real-time management of the crisis.
The improvement of quantitative precipitation forecasts is therefore a major objective of the meteorological teams within the OHM-CV. The high-resolution, nonhydrostatic, Meso-NH model Lafore et al. Simulations of the 8—9 September rain event indicate that, compared to the presently operational ARPEGE model, the Meso-NH code, with two nested domains having resolutions of 10 and 2.
However, the forecast location of the rain is still shifted to the north in such simulations. The implementation of the initialization procedure proposed by Ducrocq et al. Another line of research for the meteorological teams is the determination of meteorological precursors that are critically needed to improve the early diagnosis of the potential for extreme weather. In this field, the study of potential vorticity anomalies, already initiated for the Mediterranean heavy rainfall events within the Mesoscale Alpine Programme Bougeault et al.
As a complement to high-resolution meteorological modelling, an analog sorting method, initially proposed by Duband , is being refined Guilbaud ; Bontron Using the analogy between the current and past situations, this method allows probabilistic quantitative precipitation forecasts to be issued in real-time over a set of Mediterranean and Alpine medium-sized — km 2 watersheds. Recent work allowed testing of new analogy criteria and new prognostic variables Obled et al. A real-time prototype has raised the alarm for the 8—9 September rain event more than 24 h in advance.
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Besides the weather radar program the OHM-CV promotes the development, rather than the ongoing decline, of the existing rain gauge networks, notably for the subdaily time steps. The objective is an improved assessment of the water vapor inflow into the region of interest, which certainly represents a crucial point for improving the effectiveness of precipitation forecasting techniques.
Concerning the hydrological impact of such intense rain events, several research actions are also undertaken. The 8—9 September postevent investigation dataset is the subject of a detailed study to confirm the influence of both the space—time structure of the rain event and the geomorphological factors on the subsequent hydrologic response of the watersheds.
A 2D hydraulic model, using the estimated discharge time series as boundary conditions, will be implemented in a final step for simulating the flood propagation and the overflows in the hydrographic network. The procedures used to collect and analyze the postevent investigation data will be progressively standardized and refined with future case studies of major events that are foreseen to occur in the region.
The development of remote sensing techniques for discharge measurement during floods Creutin et al. Besides such observation activities, a continuing effort is dedicated to rainfall—runoff modeling with the dual objectives i of adapting the hydrological model structures to the distributed information that is now available on rainfall and land properties e. The problem of transferring and estimating the parameterization of hydrological models in ungauged basins is especially acute in this context and this subject will deserve specific attention in the future. Finally, a long-term objective is to develop a coupled modelling approach by using high-resolution meteorological models together with distributed hydrological models in order to obtain an integrated forecast of the rainfall and of the subsequent flash floods.
This will allow refinement of flood frequency analysis at the regional scale and, in particular, to assess the 8—9 September catastrophic event from a climatological perspective. Inline with previously published flash-flood monographs, the present paper was aimed at documenting the 8—9 September catastrophic rain event that occurred in the Gard region, France. A description of the synoptic meteorological situation was first given, which showed that all the ingredients required for heavy precipitation in the region were present with, although, no particular precursor suggesting the imminence of such an extreme event.
Radar and rain gauge observations then allowed an assessment of the magnitude of the rain event which is particularly remarkable by its spatial extension with rain amounts greater than mm over km 2 in 24 h. Locally, the MCS produced rain amounts of typically — mm in 6 h; the cold front produced additional rain amounts of typically mm. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds with sizes in the range of 10— km 2 of the Gard and Vidourle Rivers is consistent with the marked space—time structure of the rain event.
Due to the amount and the high quality of the collected information, it is planned to make the 8—9 September OHM-CV datasets available to the international scientific community by the end of The comments of the three anonymous reviewers were very helpful in improving the manuscript. In terms of geology, the mountainous part of the region northwestern portion of the map corresponds to the Primary era formations of the Massif Central e. The light gray box delineates the region affected by the 8—9 Sep rain event. Height in m, with grayscale at the right of the panels of the dynamic tropopause, diagnosed as the isosurface of 1.
The CAPE computation is based on the most unstable parcel. Maps of the total rain amounts observed during the 8—9 Sep rain event in the Gard region the corresponding geographic area is delineated by the light gray box in Fig. Curves of exceedance areas for the 8—9 Sep rain event, i.