Jean Baptiste Filippi

Frédéric Bosseur

Xavier Pialat

Suzanna Stradda

Simulation of coupled fire/atmosphere interaction with the MesoNH-ForeFire models

Keywords: Fire spread, wildland, fire, coupled atmosphere-fire numerical model 1

Fire behaviour is dependent of many physical processes and modelling interaction between all these processes requires a highly detailed and computationally intensive model. In this paper we propose an approach that couples a fire area simulator to a mesoscale weather numerical model in order to simulate local fire/atmosphere interaction. Five idealized simulation cases are analysed showing strong interaction between topography and the fire front induced wind, interactions that could not be simulated in non-coupled simulations. The same approach applied to a real case scenario also shows results that are qualitatively comparable to the observed case. All of these results were obtained in less than a day of calculation on a dual processor computer, leaving room for improvement in grid resolution that is currently limited to fifty meter.

Introduction

Fire behaviour is dependent of many physical processes; modelling interaction between all these processes would require a highly detailed and computationally intensive model. Moreover, it is rarely possible to gather sufficient data to initiate a simulation at the level of detail required for such simulations. Nevertheless, fire area simulator, such as FARSITE, are of a prime interest to the people who fight wildfires, and taking into account more of these coupled physical effects may permit to enhance the accuracy of such models. The proposed approach has been developed to add local fire/atmosphere interaction to the family of fire area simulators. Numerical coupling of a fire model with an atmospheric model has already been the subject of numerous studies, starting from the work (with static fire) of [START_REF] Heilman | Simulations of Horizontal Roll Vortex Development Above Lines of Extreme Surface Heating[END_REF] to the more recent work of [START_REF] Clark | Description of a coupled atmosphere-fire model[END_REF], that proposes a simplified model of fire spread tailored for a Canadian forest [START_REF] Rothermel | A mathematical model for predicting fire spread in wildland fuels[END_REF], coupled with the WRF meso-scale model [START_REF] Skamarock | A Time-Split Nonhydrostatic Atmospheric Model for Research and NWP Applications[END_REF]. While these efforts are effective at simulating the coupled effects at the scale of a large fire (several square kilometres) with a high degree of fire front precision, the use of Rothermel model may be subject to caution as effects of wind and slope on the rate of spread is expressed through coefficients that are experimentally fitted to wind values. Wind predictions are then to be issued as if the fire was not there and no local heterogeneous change in the fire/atmosphere coupling can be taken into account.

Other studies are more focused on combustion processes with a detailed physical formulation of the fire front. With WFDS, Mell et al. (2006) obtained a good numerical correspondence with real prescribed burning experiment of Australian grassland (Cheney and Gould, 1995). HIGRAD/FIRETEC, [START_REF] Linn | Studying Wildfire Using FIRETEC[END_REF] is able to perform several numerical investigations with different topography and wind conditions. These efforts are necessary to understand the mechanisms driving the fire spread and to evaluate fire suppression practices. Nevertheless the real-time tracking analysis of large fires would require access to large computing facilities and detailed ground data, which are difficult to gather because of the scales on which the simulation would be run.

In an effort to tackle these problems, Meso-NH and ForeFire have been developed to serve research purposes for operational models. In an approach similar to [START_REF] Clark | Description of a coupled atmosphere-fire model[END_REF], this meso-scale atmospheric model and the reduced physical front tracking wildfire model are coupled to investigate the differences induced by the atmospheric feedback in terms of propagation speed and behaviour. The main originalities of this combination resides in the fact that Meso-NH is run in a Large Eddy Simulation (LES) configuration and that the rate of spread model used in ForeFire provides a physical formulation to take into account effect of wind and slope.

Numerical models and coupling method

The coupled code is decomposed into three model components and a coupling component.

The atmospheric model is responding to energy fluxes from the fire front. The fire rate of spread model and the front tracking method are used to simulate the fire front at a higher resolution than the atmospheric model. The coupling component performs the simulation synchronisation, the data transformation and interpolation.

Fire propagation model

The fire rate of spread (ROS) model is based on the assumption that the flame is acting lake a radiant tilted panel that is heating the vegetation in front of it (see Balbi et al., 2009). It has been developed to provide an analytical formulation of the propagation speed given slope, wind speed, and fuel parameters.

A front tracking method is used for the simulation of the fire area evolution. The fire front line is decomposed into a set of connected points, or markers. Each marker has a specific propagation direction and speed, such as shown in figure 1. The velocity of each marker is given by the rate of spread model and the direction that coincides with the normal to the fire front at the location of the marker. This method has been selected due to its computational efficiency, and the ability to simulate the propagation of an interface at high resolution (a few meters) needed to take into account different vegetations, roads, houses and fire breaks over a large area typical of a wildfire accident (hundreds of square kilometres).

P r P o P l P l P o P r Fig. 1. Front tracking and markers. Circles represent markers along the firefront line. Arrows show the propagation vector (bisector of the local angle at the marker P 0 between the point at left, P l and point at right, P r ). Grey area represents the burned fuel.

The fire front thickness is constructed by projecting the location of the marker along the propagation vector after the burning duration of the fire, noted R T for "Residence time" (see fig. 3).

Fig. 2. Integration of burning area.

Red shape represents the fire front. Integration is performed on each atmospheric cell to compute the ratio of the burning area over the cell area.

Wind and elevation fields are interpolated at the location of the marker using a bi-cubic method at the very location of the markers. The wind is estimated from the value interpolated at the marker location, while the slope angle in the fire propagation direction is estimated from the elevation difference between the elevation at the fire marker and the elevation at the location projected after R T .

Meso-NH atmospheric model

Meso-NH is an anelastic non hydrostatic mesoscale model [START_REF] Lafore | The Meso-NH Atmospheric Simulation System. Part I: Adiabatic formulation and control simulations[END_REF], intended to be applicable to all scales ranging from large (synoptic) scales to small (large eddy) scales and can be coupled with an on-line atmospheric chemistry module. For the fire coupling application Meso-NH is run in Large Eddy Simulation configuration (Δx <= 50m) mode without chemistry. Turbulence parameterization is based on a 1.5-order closure [START_REF] Cuxart | A turbulence scheme allowing for mesoscale and large-eddy simulations[END_REF], with a prognostic equation for turbulent kinetic energy in 3D. We selected open boundary condition for all tests. Momentum variables are advected with a centered 4 th order scheme, while scalar and other meteorological variables are advected with a so-called monotonic Piecewise Parabolic Method (Woodward and Colella, 1984). An externalised surface module is used for the fire feedback in the simulation.

Coupling atmospheric and wildfire model

The wildfire model forces the atmospheric model at the first (ground) level injecting heat fluxes in W.m -2 , flux of water vapour in kg.m -2 and radiant temperature in K. Polygon clipping is used to derive the burning surface of an atmospheric cell (noted S b ) over the total cell area noted S c (ΔxΔy) (Figure 2). The burning ratio for each atmospheric grid cell is noted

R b , = S b / S c.
As only a portion of the cell is burning, an equivalent radiant temperature (T e ) for the whole cell is averaged from a nominal flame temperature (T n ) and the soil temperature from the atmospheric model (T s ). T e is given by: The operation is performed for all atmospheric grid point at ground level, allowing to construct three matrices that are passed to the atmospheric model just before updating the wind matrix used by the fire simulation. Wind matrices forcing the fire model are updated at each atmospheric time step and wind is assumed to be constant during the entire duration of the time step.

4 4 4 ) 1 ( n b s b e T R T R T + ! = , Equivalent heat fluxes (Q e ) in W.m -2 ,

Idealised experimental set-up

In order to evaluate the ability of the coupled code to estimate the coupled influences of topography and wind on fire spread, five tests were run corresponding to a partial set of the set-ups proposed by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF]. Base functions used to create the different topographies are taken from [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF], those functions are used to create an idealized flat, canyon, hill ridge and up can terrains. For all cases the domain size has been set to 640*320*500m with horizontal spacing of 16m and an average vertical spacing of 20m. Unlike the test cases in [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF], vegetation is assumed homogeneous in the domain with parameters given in Table 1 for all simulations. These values are based on mean values deduced from experimental studies [START_REF] Santoni | Instrumentation of wildland fire: characterisation of a fire spreading through a Mediterranean shrub[END_REF]. In this experiment, vegetation was shrubs with an average dry fuel load of 7kg.m -2 .

A R0 r0 u0 R T . Q n .
Wv n T n 1.5 0.1m.s -1 0.01m.s -1 5m.s -1 30s 250kW.m -2 0.1kg.m -2 1000K Table 1. Experimental parameters, with A: Radiant factor, R 0 : rate of spread without wind and slope, r 0 flame thickness speed factor, u 0 : flame gas velocity, R T : fire residence time, Q n : nominal heat flux, Wv n : nominal water vapour flux and T n : nominal radiant temperature.

Atmospheric model background wind field was of 6m.s -1 constant in height (with a maximum simulation height of 500m). A passive scalar tracer with a distribution set to the burning ratio of each grid point and for each atmospheric time step is used as a marker for smoke injection.

Figures 3,4, 5, 6, and 7 present the simulation results for the flat, canyon, hill, ridge and upcan cases 120s after ignition.

In the flat Case (Fig 3a), the flow remains largely unaffected behind the fire. The simulation reveals an area of confluence ahead of the front with some recirculation that is located at the base of the fire plume (Fig 3b). The plume is relatively weak, affecting the flow to an altitude of 60m over ground. Overall flow speed does not greatly differ from the original flow speed of 6m.s -1 . However, local enhancement of the surface velocity due to the coupling between the fire and the atmosphere leads to a greater ROS at the head of the fire compared to the noncoupling case. This effect can be attributed to the induced wind being taken into account in the coupled simulation. The canyon case (figure 4) clearly enlightens the strong influence of taking into account the coupling between fire and atmosphere in the simulation of the fire dynamics. In that case the surface wind is strongly decreased in the canyon by topographic effects. These effects are not fully compensated by the increased slope and we observe weaker ROS than in the flat case. In such scenario the induced wind plays a major role in the dynamics of the fire spread and the use of a coupled model results in increased ROS and better accounting of the physics. With the same slope and same wind speed, the Hill case (Figure 5) presents a slightly different behaviour. The area of confluence is located here ahead of the fire front, so the maximum wind speed are just over the fire head. The resulting tilt angle results in a stronger ROS, and a larger burning injection area. Figure 6 presents the results for the ridge test case. The topographic effects results in a widening of the burning area in the transverse direction of the wind due to slope gradient in that direction. This topographic/fire effect seems to be quantitatively superior as observed in [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF]. Results for the upcan test are shown in figure 7. Once again the topography has more influence in our simulations than in that of [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF]. The narrowing of the fire head compared to the ridge case is of factor 3 in our case whereas [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF] results show a factor around 2. Finally figure 8 presents the propagation distance of the fire front in the wind direction for all five cases. Though quantitatively different from the results of [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF] due to different fuel properties, our coupled model exhibits the same behaviour in simulating fire/atmosphere interaction. Our models also retrieves the grouping of behaviour between, on one hand, the flat and canyon cases, and on the other end the hill, ridge and upcan cases.

Real-case simulation

Case description

Our coupled model has also been applied to a past real case fire that occurred on the 16 th of October 2007 near Ajaccio at location called Vazzio. The fire ignited around 14:30 and experienced almost free propagation till 15:40 under a sustained and whirling wind of about 4 to 5m.s -1 and gusts of about the same magnitude. Finally the fire was stopped around 18:30 and burned up to 0.60km 2 of land with the burned area contour reported in figure 10.

Simulation setup

The coupled simulation was run on a 2.5km×2.5km×1.5km domain discretized on a 50×50×30 mesh for the atmospheric model simulation (Δx = Δy = Δz = 50m). The wind is given by a radiosounding made at 12:00 at the Ajaccio airport which is located three kilometers away. Topography is given by the BDTOPO (IGN database) with a precision of 50m. Vegetation is extracted from the IFN database and classified between a homogeneous Mediterranean maquis where fuel is present and non-burnable areas representing roads and buildings.

The simulation was run on a Xeon 3.0 Ghz processor (4 cores) and took approximately 23 hours to perform 6h of real-time fire propagation.

Results and discussion

The aim of this preliminary study is to apply the presented couple model to a real fire and compare qualitatively the results with observations concerning the coupling between the fire and the atmosphere.

The authors are well aware of the incertitude on all the relevant parameters like wind magnitude and direction, vegetation properties and humidity. This simulation is thus not intended for quantitative comparison with observations but as a preliminary test to assess the proposed coupled model in simulating fire/atmosphere interaction.

Figure 9 presents some simulated contours of the fire compared to the observed final contour. The global shape of the simulated final contour is, in good agreement with the observed contour, especially as a change of wind direction from west to north-west during the afternoon was not taken into account in the simulation. Concerning the fire/atmosphere coupling figure 10 presents a comparison between the simulated plume after one hour of fire and a photography of the fire which was supposedly taken around 15:30. The direction of observation in the simulation case was taken as close as possible from the one in the photography.

Once again qualitative agreement between simulation and observation is observed. The structures of the simulated plume are not as refined as in the real one but this is mainly due to the relatively low refinement of the grid for the atmospheric simulation (50m). Simulated direction and height of the plume are similar to the observed ones. Nevertheless dispersion seems to be underestimated in our simulation as the plume expansion is slightly lower in the simulation. This drawback supposedly mainly stems from the coupling fluxes injected by the fire simulation. As explained earlier in this article the forcing fluxes from the fire are the heat flux, the flux of water vapour and the radiant temperature. Thus no turbulent kinetic energy is injected in the atmospheric simulation whereas fine structures of characteristic length less than 50m are observed in the fire and are assumed to contribute to the agitation of the atmosphere.

Fig. 10. Simulated and observed plume. Simulated plume is given 1h after the fire ignition (the blue contour represents the fire front at that time); the observation was taken between 30 minutes and 1h30 after the ignition.

Conclusions

MesoNH/ForeFire coupled model of wildland fire spread is used to investigate the effect of topography on fire induced winds. With a straightforward coupling method, the atmospheric model is able to simulate the atmosphere dynamic induced by the fire and the subsequent effects on the RoS with meaningful results. The five idealized scenarios allowed simulating induced flow patterns similar to those observed from the simulation by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF] with HIGRAD/FIRETEC. Transverse topological effects seem to be of more importance in our model as the widening/narrowing of the head fire is significantly greater in our simulations. The main feature of these simulations still remains that the fire head spread rate in the wind direction exhibits similar behaviours to those found by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF]. This behaviour is of particular interest as performing HIGRAD/FIRETEC simulations of the flow and fire patterns over a complex vegetation distribution with high resolution are nowadays computationally unreachable for large scale wildland fires.

The proposed coupled model was then applied to a real-case scenario and compared with observations. The model's behaviour is qualitatively similar to the real fire in simulating the fire propagation as well as the fire/atmosphere interaction though some issues still remains such as the physics of the coupling or the collecting of the data necessary for such simulations.

The objective was here to move from fire area model with forced wind fields to coupled wind field that could represent the local perturbations that may greatly affect the fire behaviour. Further enhancements are planned to perform simulation of large past fire and

  corresponding to the energy of the hot gaseous column over an atmospheric cell, is approximated from a nominal convective heat flux (Q n ) with water vapour fluxes (Wv e ) in kg.m -2 , representing the amount of water vapour evaporated from the vegetation is interpolated over an atmospheric cell from nominal water vapour content (Wv n ) with

Fig. 3 .

 3 Fig. 3. FLAT (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds for the coupled (red) and non-coupled (grey) simulations. Arrows denote the wind vectors at ground level for the coupled-case. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer. (c) 3d wind field and passive tracer concentration isocontours.

Fig. 4 .

 4 Fig. 4. CANYON (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds for the coupled (red) and non-coupled (grey) simulations. Arrows denote the wind vectors at ground level for the coupled-case. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer. (c) 3d wind field and passive tracer concentration isocontours.

Fig. 5 .

 5 Fig. 5. HILL (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds for the coupled (red) and non-coupled (grey) simulations. Arrows denote the wind vectors at ground level for the coupled-case. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer. (c) 3d wind field and passive tracer concentration isocontours.

Fig. 6 .

 6 Fig. 6. RIDGE (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds for the coupled (red) and non-coupled (grey) simulations. Arrows denote the wind vectors at ground level for the coupled-case. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer. (c) 3d wind field and passive tracer concentration isocontours.

Fig. 7 .

 7 Fig. 7. UPCAN (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds for the coupled (red) and non-coupled (grey) simulations. Arrows denote the wind vectors at ground level for the coupled-case. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer.

Fig. 8 .

 8 Fig. 8. Propagation distance of the fire front in function of time in the five cases.

Fig. 9 .

 9 Fig. 9. Simulations results and observations for the Vazzio fire. Blue: simulated fire contour at 15:30 (after 1h), Green: simulated contour at 18:30, Red: final observed contour of the fire.

Acknowledgements

This research is developed within the IDEA project ANR-09-COSI-006-01


Jean Baptiste Filippi

Frédéric Bosseur

Céline Mari

Suzanna Stradda

NUMERICAL EXPERIMENTS USING MESONH/FOREFIRE COUPLED ATMOSPHERIC-FIRE MODEL

Keywords: Fire spread, wildland, fire, coupled atmosphere-fire numerical model

In this study we attempt to couple the MesoNH atmospheric model in its large eddy simulation configuration with a fire contour model, ForeFire. Coupling is performed at each atmospheric time step, with the fire propagation model inputting the wind fields and outputting heat and vapour fluxes to the atmospheric model. ForeFire model is a Lagrangian front tracking model that runs at a typical front resolution of 1 meter. If the approach is similar to other successful attempts of fire-atmosphere coupled models, the use of MesoNH and ForeFire implied the development of an original coupling method. Fluxes outputted to the atmospheric models are integrated using polygon clipping method between the fire front position and the atmospheric mesh. Another originality of the approach is the fire rate of spread model that integrates wind effect by calculating the flame tilt. This reduced physical model is based on the radiating panel hypothesis. A set of idealized simulation are presented to illustrate the coupled effects between fire and the atmosphere. Preliminary results show that the coupled model is able to reproduce results that are comparable to other existing numerical experiments with a relatively small computational cost (one hour for a typical idealized case on a 200 GFlops capable computer). MesoNH serves as a research model for the meteorological systems in France and Europe, and is well integrated within the operational tool chain. Future validation scenarios will be performed on nested simulations of real large wildfires.

Introduction

Fire behaviour is dependent of many physical processes; modelling interaction between all these processes would require a highly detailed and computationally intensive model. Moreover, it is rarely possible to gather sufficient data to initiate a simulation at the level of detail required for such simulations. Nevertheless, fire area simulator, such as FARSITE, are of a prime interest to the people who fight wildfires, and taking into account more of these coupled physical effects may permit to enhance the performance of such models. The proposed approach of has been developed to add locale atmosphere interaction to the family of fire area simulators. Numerical coupling of a fire model with an atmospheric model has already been the subject of numerous studies, starting from the work (with static fire) of [START_REF] Heilman | Simulations of Horizontal Roll Vortex Development Above Lines of Extreme Surface Heating[END_REF] to the more recent work of [START_REF] Clark | Description of a coupled atmosphere-fire model[END_REF], that proposes a simplified model of fire spread tailored for a Canadian forest [START_REF] Rothermel | A mathematical model for predicting fire spread in wildland fuels[END_REF], coupled with the WRF meso-scale model [START_REF] Skamarock | A Time-Split Nonhydrostatic Atmospheric Model for Research and NWP Applications[END_REF]. While these efforts are effective at simulating the coupled effects at the scale of a large fire (several square kilometres) with a high degree of fire front precision, the use of Rothermel model may be subject to caution as effects of wind and slope to the rate of spread is expressed as coefficients that are experimentally fitted to wind values as if the fire was not there.

Other studies are more focused on combustion processes with a detailed physical formulation of the fire front. With WFDS, Mell et al. (2006) obtained a good numerical correspondence with real prescribed burning experiment of Australian grassland (Cheney and Gould, 1995). HIGRAD/FIRETEC, [START_REF] Linn | Studying Wildfire Using FIRETEC[END_REF] is able to perform several numerical investigations with different topography and wind conditions. These efforts are necessary to understand the mechanisms driving the fire spread and to evaluate fire suppression practices. Nevertheless the real-time tracking analysis of large fires would require access to large computing facilities and detailed ground data, which are difficult to gather because of the scales on which the simulation would be run. Méso-NH and ForeFire that have been developed to serve research purposes for operational models. In an approach similar to [START_REF] Clark | Description of a coupled atmosphere-fire model[END_REF], this meso-scale atmospheric model and the reduced physical front tracking wildfire model are coupled to investigate the differences induced by the atmospheric feedback in terms of propagation speed and behaviour. The main originalities of this combination resides in the fact that Méso-NH is run in a Large Eddy Simulation (LES) configuration and that the rate of spread model used in ForeFire provides a physical formulation to take into account effect of wind and slope.

Numerical models and coupling method

The coupled code is decomposed into three model components and a coupling component. The atmospheric model is responding to energy fluxes from the fire front. The fire rate of spread model and the front tracking method are used to simulate the fire front at a higher resolution than the atmospheric model. The coupling component performs the simulation synchronisation and the data transformation and interpolation.

Fire propagation model

The fire ROS model is based under the assumption that the flame is acting lake a radiant tilted panel that is heating the vegetation in front of it (see Balbi et al., 2009). It has been developed to provide an analytical formulation of the propagation speed given a slope, wind speed, and fuel parameters. This hypothesis is similar to the hypothesis made by Rothermel model [START_REF] Rothermel | A mathematical model for predicting fire spread in wildland fuels[END_REF]. Calculations of the flame tilt angle , given by Figure 1 presents the flame tilt calculation, is the local slope angle in degrees, U the normal wind velocity in the propagation direction (in m.s -1 ). R, the RoS (in m.s -1 ), is obtained by adding the nominal speed of the fire, to the accelerated speed due to the tilted flame radiating in the direction of the propagation. R is given by:

g g g cos 1 ) cos sin 1 ( 0 0 r R R A R R + - + + = .
A is a radiant factor (A is high if much of the energy generated by combustion is radiated by the front). Parameter r 0 is a speed factor due to radiation (in m.s -1 ), that is dependent on the flame thickness. Parameter R 0 is the rate of spread without wind and slope (in m.s -1 ) and u 0 , the vertical gas velocity in the flame without wind and slope (in m.s -1 ).

A front tracking method of markers is used for the simulation of the fire area evolution. The fire line is decomposed into a set of connected points, or markers. Each marker has a specific propagation direction and speed, such as shown in figure 2. The speed at which the marker is travelling along its propagation vector is given by the rate of spread model. The direction of the propagation vector is taken as the bisector of the angle formed by the marker, and the location of the immediate left and right markers. Markers are redistributed along the front if separated by more than the resolution distance r and removed if separated by less than r/4. A fireline is defined as a full set of interconnected markers. Id two points of different firelines are separated by less than r/4 the two fronts are merging. The integration of a marker advance is performed in a discrete event fashion, with no global time step but specific activation time for markers. Every marker is always advancing by the same distance q estimating from the propagation speed when the marker would have moved by this distance. The timed activations events are placed in a sorted event list, and the simulation is performed by activating the most imminent event. The method has been selected because of its computational efficiency, and the ability to simulate the propagation of an interface at the high resolution (a few meters) needed to take into account different vegetations, roads, houses and fire breaks over a large area typical of a wildfire accident (hundreds of square kilometres). The fire front thickness is constructed by projecting the projecting the location of the marker along the propagation vector after the burning duration of the fire, noted R T for Residence time (see fig. 3). Wind and elevation fields are interpolated at the location of the marker using a bi-cubic method at the very location of the markers. The wind is estimated from the value interpolated at the marker location, while the slope angle in the fire propagation direction is estimated from the elevation difference between the elevation at the fire marker and the elevation at the location projected after R T .

Meso-NH atmospheric model

This anaelastic non hydrostatic mesoscale model [START_REF] Lafore | The Meso-NH Atmospheric Simulation System. Part I: Adiabatic formulation and control simulations[END_REF] is intended to be applicable to all scales ranging from large (synoptic) scales to small (large eddy) scales and can be coupled with an on-line atmospheric chemistry module. For the fire coupling application Méso-NH is run in Large Eddy Simulation configuration ( x <= 50m) mode without chemistry. Turbulence parameterization is based on a 1.5-order closure [START_REF] Cuxart | A turbulence scheme allowing for mesoscale and large-eddy simulations[END_REF], with a prognostic equation for turbulent kinetic energy in 3D. We selected open boundary condition for all tests. Momentum variables are advected with a centered 4 th order scheme, while scalar and other meteorological variables are advected with a socalled monotonic Piecewise Parabolic Method (Woodward and Colella, 1984). An externalised surface module is used for the fire feedback in the simulation.

Coupling atmospheric and wildfire model

The wildfire model force the atmospheric model at the first (ground) level injecting heat fluxes in W.m - 2 , flux of water vapour in kg.m -2 and radiant temperature in K. Polygon clipping is used to derive the burning surface of an atmospheric cell, (noted S b ) over the total cell area noted S c ( x y) (Figure 3). The burning ratio for each atmospheric grid cell is noted R b , = S b / S c.

As only a portion of the cell is burning, an equivalent radiant temperature (T e ) for the whole cell is averaged from a nominal flame temperature (T n ) and the soil temperature from the atmospheric model (T s ). T e is given by: The operation is performed for all atmospheric grid point at ground level, allowing to construct three matrices that are passed to the atmospheric model just before just before updating the wind matrix used by the fire simulation. Wind matrices forcing the fire model are updated at each atmospheric time step and wind is assumed to be constant during the entire duration of the step.

Experimental Set-up and simulation results.

In order to evaluate the ability of the coupled code to estimate the coupled influences of topography and wind on fire spread, 5 tests were ran corresponding to a partial set of the set-ups proposed by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF]. Base functions used to create the different topographies are from [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF], those functions are used to create an idealized flat, canyon, hill ridge and up can terrain. For all cases the domain size has been set to 640*320*500m with horizontal spacing of 8m and an average vertical spacing of 10m. Vegetation is assumed homogeneous, with parameters (Table 1) for all simulations, based on mean values deduced from experimental studies [START_REF] Santoni | Instrumentation of wildland fire: characterisation of a fire spreading through a Mediterranean shrub[END_REF]. In this experiment, vegetation was shrubs with an average dry fuel load of 7 kg.m -2 .

A R0 r0 u0 R T . Q e .
Wv e T n 1.5 0.1m.s -1 0.01m.s -1 5m.s -1 30s 250kW.m -2 0.1kg.m -2 1000K Table 1. Experimental parameters, with A: Radiant factor, R 0 : rate of spread without wind and slope, r 0 flame thickness speed factor, u 0 : flame gas velocity, R T : fire residence time, Q n : nominal heat flux, Wv n : nominal water vapour flux and T n : nominal radiant temperature.

Atmospheric model background wind field was of 6m.s -1 constant in height (with a maximum simulation height of 500m). A passive scalar tracer with a distribution set to the burning ratio of each grid point and for each atmospheric time step is used as a marker for smoke injection. Situation is different in the canyon case (Figure 5) where the flow speed at ground level is accelerated to13m.s -1 . The accelerated flow, combined with a steep slope does greatly accelerate the fire rate of spread. Nevertheless, as can be seen in Figure 4(b) the area of convergence is situated just over the fire head, providing weaker winds to tilt the flame. It appears that the canyon topography is creating a recirculation, visible Figure 4(b) that is weakening the winds at ground level near the front. Wit the same slope and same wind speed, the Hill case (Figure 6) presents a different behaviour. The area of confluence is located here ahead of the fire front, so the maximum wind speed are just over the fire head. The resulting tilt angle results in a stronger ROS, and a larger burning injection area. The ridge case (Figure 7) is similar to the hill case; a specificity of this case is that locally, over the ridge, wind is accelerated over the flank, creating bursts of fire and a less regular fire shape. Figure 9 presents the position of the fire head over time. We can observe that for the Hill and Ridge cases, a strong acceleration is happening 60 meters after the starting point, corresponding to the start of the steep slope, this acceleration ends at 180m, corresponding here to the end of the steep slope. The Upcan case is less affected by the slope effect, as the slope is less abrupt in this case, nevertheless, in this case, the propagation speed does continue to stay at almost its maximum level until the end of the domain, suggesting that the flow, accelerated by the fire, is still influencing heavily the fire spread (figure 8 a). The Canyon case is exactly the same slope as the Hill case; however, Figure 9 clearly shows that the spread behaviour is different here. In Figure 5 we can observe that the canyon topography generates a horizontal recirculation that is preventing strong flow to reach the fire head and creates a reverse flow near the bottom of the Canyon. Similar behaviour is present in the simulation from [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF] with HIGRAD/FIRETEC, although it is not possible to state that the same effects are involved. Finally, the flat case spread rate is almost constant during the whole simulation run, and less affected by the coupled atmospheric effects. All simulations were performed on a bi dual-core Intel Xeon processor running at 3 Ghz with 8Gb of memory. The calculation time for each run was about 4 hours for 300 seconds of simulation with a time step of 0.1s. The calculation of the fire front displacement only accounts for 10 seconds of computer time, while the coupling requires 200 seconds of mainly input/output operations for the whole duration of the run.

Conclusions

MésoNH-ForeFire coupled model of wildland fire spread is used to investigate the effect of topography on fire induced winds. With a straightforward coupling method, the atmospheric model is able to simulate the atmosphere dynamic induced by the fire and the subsequent effects on the RoS with meaningful results. The five idealized scenarios allowed simulating induced flow patterns similar to those observed from the simulation by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF] with HIGRAD/FIRETEC. Another important aspect of these simulations is that the fire head spread rate is also comparable to those found by [START_REF] Linn | Coupled influences of topography and wind on wildland fire behaviour[END_REF], significantly lower in the Flat scenario, and slightly lower in the canyon case. Nevertheless, unlike HIGRAD/FIRETEC, performing simulation of the flow and fire patterns over a complex vegetation distribution at a high resolution is impossible with the proposed coupled model. The objective was here to move from fire area model with forced wind fields to coupled wind field that could represent the local perturbations that may greatly affect fire behaviour. Further enhancements are planned to perform simulation of large past fire and simulation with the online chemistry module of Méso-NH to investigate fire smoke and particle transport.

  Is a key point in the model formulation as angle provides the accelerated ROS.

Fig. 1 .

 1 Fig. 1. Calculation of the flame tilt angle.

Fig. 2 .

 2 Fig. 2. Front tracking and markers. Circles represent markers along the fire line. Arrows show the propagation vector (bisector of the local angle at the marker P 0 between the point at left, P l and point at right, P r ). Grey area represents the burned fuel.

Fig. 3 .

 3 Fig. 3. Integration of burning area. Red shape represents the fire front. Integration is performed on each atmospheric cell (shades of grey correspond to burning (S b ) to total cell area (S c = x y) ratio, black is S b max).

  fluxes (Q e ) in W.m -2 , corresponding to the energy of the hot gaseous column over an atmospheric cell, is approximated from a nominal convective heat flux (Q n ) with water vapour fluxes (Wv e ) in kg.m -2 , representing the amount of water vapour evaporated from the vegetation is interpolated over an atmospheric cell from nominal water vapour content (Wv n ) with

Figures

  Figures 4, 5, 6, 7, and 8 present the simulation results for the flat, canyon, hill, ridge and upcan cases 120s after ignition. In the flat Case (Fig 4 a) the flow remains largely unaffected behind the fire. The simulation reveals an area of confluence ahead of the front with some recirculation that is located at the base of the fire plume (Fig 4 b). The plume is relatively weak, affecting the flow to an altitude of 100m over ground. Overall flow speed does not greatly differ from the original flow speed of 6m.s -1 .

Fig. 4 .

 4 Fig. 4. FLAT (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds of simulation. Arrows denote the wind vectors at ground level. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer.

Fig. 5 .

 5 Fig. 5. CANYON (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds of simulation. Arrows denote the wind vectors at ground level. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer.

Fig. 6 .

 6 Fig. 6. HILL (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds of simulation. Arrows denote the wind vectors at ground level. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer.

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. RIDGE (a) Horizontal section (x/y) at Z=10m, fire lines after 120 seconds of simulation. Arrows denote the wind vectors at ground level. (b) Cross section (x/z) of the coupled case at Y=160m, shading represents concentration of the injected passive tracer.

Acknowledgements

This research was supported by PEPS-07_36 from the French National Centre of Scientific Research (CNRS).


Rebeca Vieira De Mello
email: rebeca.mello@gmail.com

Representation of a

Keywords: Urban water; resources, risk assessment, politics, sustainable development

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

It is not new to say, after stated at the United Nations conference meeting about the environment of Rio de Janeiro 1992 (UNCED), that the implementation of a sustainable development in large cities has been introduced as one of the main commitments on their municipal managements. "The idea of sustainability recalls the logic of practice, where practical effects viewed as desirable are made to happen. […] When applied to urban space, the idea of sustainability has generated different representations and perspectives for managing cities" [START_REF] Acselrad | Discursos da Sustentabilidade Urbana[END_REF]. An example of it in Recife is the incorporation of municipal Agendas 21, which holds as one of its main principle the line: "think globally, act locally" with leaflets and methodological guides that assist the town planners to put sustainable development into practice.

In spite of the fact that the rhetoric on those urban projects is in accordance with the speech of the urban sustainability -following the three dimensions: social, economic and the environmental -does it in practice, really represent a true change on the way of conceiving urban projects? Or may it only be some kind of "greenwashing", or even the vulgarisation of the term in itself, turning it banal and meaningless?

In the City of Recife, Northeast of Brazil, after the 90's, urban projects have begun to integrate the perspective of a sustainable town development. Before it, urban planning was based on the preservation of the cultural heritage. This idea of the conservation of the city heritage for future generations was the first avatar on the urban sustainability. But, recently the city have begun integrate the environment perspective. Recife is conquering the spaces along the waters as the borders, rivers and humid zones. Although they have been situated in the common rivers' estuary, the borders of water were until then ignored and had little integration with the city's activities.

In this search for the implementation of the sustainable city, urban rivers and wetlands begin to have the meaning of natural patrimony to be preserved. "Water, it's not only considers like an element of the environment, but as an essential component of the ecosystems to be protected, restores and emphasize" [START_REF] Gauthier | La mise en oeuvre de la ville viable: une problematique d'action publique[END_REF]. By the simple adequacy of the waters' flow with urban necessities (rectification of banks, canalization of the streams, draining of the swampy zones, waterproofing of the borders), these projects try to integrate water and to respect its natural flow.

This paper intends to analyze the representation of a sustainable development in the recent urban planning of the City of Recife and the vulgarization of the green speech. In this way, we will verify a example of occupation of borders on superficial waters.

PROBLEMS AND CONTEXT

For a better understanding of the case of Recife, it is necessary to highlight some social and natural characteristics of the place. Recife is situated in the tropical zone of Brazil, on the sea cost, common estuary of three river basins. This forms a complex fluvial system composed by five rivers and 66 canals. In low lands of the estuaries, there are the mangroves, that are swampy areas of transition between the river and the sea. The mangrove is a rich ecosystem, where we can find you a great variety of clams, fishes, crabs and birds. It is an important area for the ecosystem balance. Besides being a zone of reproduction of innumerable species, the zone is fundamental for the purification of the water and the absorption of rain waters, thus preventing floods and cyclic inundations. However, the mangrove is threatened by the water pollution and the constant embankment due to the urban growth towards the flooded zones.

It is in these muddy areas of the rivers banks where the poorest population installs their habitation. The sprouting of slum in these places comes to the fact that these uncertain zones between water and land do not possess commercial value. There are the last areas vacated by the city center, where this population can find work. With the highest GIP (2,04 to euros/per capita) of a metropolitan region of approximately 3,5 million inhabitants, Recife exerts a strong demographic attraction in the region. The flow of immigrants has two main origins: they run away from drought of the arid countryside or they come from other cities near the littoral to try their luck in the capital. Arriving in Recife, the majority find themselves without work. Thus they have no choice but improvise precarious shacks in the most devaluated areas. The slum shacks on the borders of the river are called palafitas, wooden houses on stilt. The conflict between social dynamics and the environment preservation is a major problem of the great Brazilian cities. This conflict has its origins in the relation between the urban growth and the social inequality. The substandard housing in the cities is a frequent alternative of settlement of the population with low purchase power. The social inequality is consequence of the disparity in the income distribution. Without access to the formal market of the urban property, this part of the population occupies areas of environmental preservation, as fens and river sides. This type of housing generates two different situations. The first one would be the life risk that this part of the population is submitted, due to the fragility of the habitations against high tides and the exposition to illnesses because of a very unsatisfactory sanitary condition. The second deals with environment preservation, that generates the conflict between the individual right to occupy these spaces and the collective interest for preservation and protection of this environment.

The Town Hall estimates that more than 70 thousand people live on the rivers banks and canals in Recife in precarious ways. Out of this total, 50,000 people live in palafitas, (JC, 2006). The houses of palafitas are spread in 32 areas along the rivers and the estuary. Without access to the service of neither garbage collection nor water supply, the irregular habitations contribute for the water pollution. The families who inhabit the borders of the rivers are at the same time cause of the degradation of the quality of waters and victims. They represent the part of the population that is more exposed to the illnesses of water propagation, as leptospirosis.

Even if the town is surrounded by the water, the access to drinking water is becoming more and more difficult, which proves the deficiency of its supplying system. The Instituto Brasileiro de Geografia e Estatística (IBGE, Brazilian Institute of Geography and Statistics) estimates that 88% of the domiciles are connected to the water supply net, but in the great majority of areas, water arrives to the taps each two days. Moreover, only 30% are connect to the used water evacuation net. Only 20% of this volume is treated, and the rest of it is rejected in rivers and canals without any treatment. The population perceives the river as an open air sewer that stinks and causes illnesses. Thus the city lives a paradox where the water is a major urban problem for its presence (polished state) or for its absence (potable state).

METHOD AND MATERIAL

The method of research used understands the survey of the projects destined to the banks river and flooded areas, the study of the urban evolution and the analysis of urban projects dealing with the removal of the palafitas. From this survey, we could compare the official documentation (theoreticians) with the data observed in the local.

The projects that aim to withdrawal the families living in substandard housings in flooded areas in Recife is not properly a new kind of intervention inspired by an holistic vision of the sustainable development. The first projects for the flooded areas in the town date from the beginning of 20th century, motivated for the hygienists ideas. These projects consisted on the destruction of the precarious shacks, fill the flooded zones with earth and in the relocation of families for houses in the periphery.

In 2003, the city hall launches an ambitious project having as objective the eradication of the palafitas in ten years, called "Recife sem palafitas" Recife without palafitas. Such project of the city hall and the Ministry of the City acts in field of urban renewal of the flooded areas with palafitas, with the objective to relocated and build proper houses to this population. The city hall presents the project as being a set of urban, environmental, socio-economic and cultural actions.

The project of the eradications of the palafitas is integrated to other central project office called "Capibaribe Melhor" A better Capibaribe River. This project counts on the support of the World Bank. It's a integrated actions of urbanism that aims to promote the environmental requalification of the Capibaribe river basin.

RESULTS

Through the confrontation between the project and the actions to eradicate the palafitas, it was possible to get to some results. Talking still about the project "A better Capibaribe" we can distinguish some positive points:

• The attempt to integrate the territory of the river basin with the a area of intervention of the urban planning seems to be an innovative action and a perspective of water resources management integrated in urban areas;

• The withdraw and transposition of the population from the border permit then to access a better conditions of housing;

• The implementation of the project of integrated sanitation: integrated actions of water supply, pavement, draining, improvement of sanitary services, intervention, sanitary and environmental education, garbage collection, control of illness and permanent maintenance of the systems.

However, with the relocation of a significant number of families who lived in risk areas, the free spaces have been transformed into ways for automobiles, instead of having its natural characteristics restored. No further action in the direction of ecological requalification of the ecosystem of the fen has been observed.

On the other hand, we have observed:

• -The increase of the waterproofing by the rivers banks;

• -The lack of elements that could facilitate the identity of the inhabitants with the river.

In the case of the project withdrawing its precarious habitations we have noticed two models of intervention. The first one is the relocation of the population to better places next to the old housings. The second one is about the urbanization of some of these slum areas. In this case, the biggest challenge is to find technical solutions for sanitary supplying, sanitation and garbage collection, adaptable to a flooded area.

The solutions techniques used in the construction of the urban project count on embankments and waterproofing of the ground, without presenting any innovation to reduce the ecological impact. From these observations, we can raise some perspectives for further research. That would be interesting to investigate the vision of the authors of the project on the water system. What reasons determined the choices for these techniques of urban management not very respectful to the environment.

The free space on the fens due to the relocation of these inhabitants could represent an opportunity to construction of a city in which the environment is dealt in a sustainable way. The rivers banks and swamps could be used for prevention of floods, for toxic retention of sediments, substances, for stabilization of the micro-weather, instead of the role as cultural patrimony. (Dunga, 1992, cit. in Coelho 2003) Despite the sensible environmental speech the analyzed projects do not present a regard for the insertion of these potential functions of the urban environment in its projects.

Finally, we can notice that in both projects the main interest seems to be the amplification of the road system of the city. The population of Recife counts on a deficient public transport service, and the use of the individual transport is the cause of continue traffic obstruction. The improvement of the road system is one of the main complaints of the elite. We can conclude that the term of sustainable development in the ecological speech of these projects is used only to legitimize elitist aims and ends out being little eco friendly.

CONCLUSIONS

This paper underlines the vulgarization of the concept of sustainable development in the urban projects. In the case of Recife, we could observe an apparently strong effort of the municipality to construct a sustainable city. However this is not reflected on the usual techniques practices. As points Yves Guermond, the sustainable urbanism "is not what it believes to be (a new response for new problems), but essentially the resumption, the repetition of unconscious discursive configurations born at the previous century» [START_REF] Guermond | Repenser l'urbanisme par le développement durable[END_REF]. Thus, it makes us to think that urban planners and managers lack a more holistic understanding of urban environment. At Recife, the sustainable development is used to legitimize projects that does not represent a deep change on classical employed solutions and that are far to have any real ecological consistency.

Figure

  Figure 1: Localization plan Figure 2: Recife, plan of river basins

Figure 3 ; 4 :

 34 Figure 3; 4: Palafitas in the rivers banks of Capibaribe (image of: Renato Spencer)

Table 1 : projects Theory referential Projects objects Actions

 1

			To improve social,	Integrated Urbanization: a better	Parks and green areas
			economic and	quality of urban space along the
			environmental	Capibaribe River by creating and	Water and sewage macro-
			conditions of the low-	recovering the physical infrastructure	system
	Project :Capibaribe Melhor	Prefeitura da Cidade do Recife, URB, MCconsulnting	income population groups that live in the Capibaribe River Basin, with a view to ensuring the sustainability of their development and reduce environmental degradation of the canals and of the Capibaribe River itself caused by inappropriate use (sewage and garbage disposal) and human occupation of the river banks	associated with leisure, sanitation, macro and micro drainage, access and mobility. Social and economic development: development of participatory and economically entrepreneurial communities, and at the same time to instill in these communities the need to preserve and conserve the natural and built environment in places where they live and, by extension, throughout the city of Recife. Institutional Development: assisting the various departments within the Prefecture of Recife with direct responsibilities in the field of action	Recovery of macro-drainage system Increased urban accessibility and mobility Development of health and environmental education Support for employment and income Development of sporting and cultural potential Fostering urban operations Popular participation and social control Municipal environmental management; Management, monitoring and evaluation of the project.
				of this Project to exercise their
				functions more efficiently and
				effectively.

/ 7

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Arona Cissé
email: aronacisse@gmail.com

Decentralized cooperation and management of urban services in southern countries: an application on water supply services in Senegal

Keywords: Urban services, drinkable water policies, decentralized cooperation

Decentralized cooperation and management of urban services in southern countries: an application on water supply services in Senegal

Arona Cissé

INTRODUCTION

This thesis work follows our master's dissertation in which we analysed ten years of publicprivate partnership in urban water management in Senegal. We shown that even if the efficiency's goal was attempt, social and spatial discrimination increased during this period. In this frame our interrogation was how authorities can improve water access for poor people living in poor parts (not always illegally) of developing countries cities? Therefore, decentralized cooperation and its proximity-based model interested us as a new way to reinforce solidarities and knowledge transfers as stated in 3 rd World Water Forum in Kyoto.

Since the late 70's the issue of urban planning and services access in developing countries is at the heart preoccupations of various agencies and actors at diversified levels of interventions from national to the international area.

Today, the issue continues to interest political and theoretical debates because cities in developing countries records most raised demographic rates in the world (today's tendencies are around 4% per year). Because of their high population growth these countries are facing institutional and material difficulties to assure access to urban services to the people, drinkable water1 especially. According UNDP projections, by the year 2020 more than the half of African population will live in urban areas, and authorities will face growing problems if nothing is done.

According to some authors [START_REF] Jaglin | Pouvoirs et cités en Afrique noire[END_REF][START_REF] Jaglin | Les services d'eau urbains en Afrique subsaharienne : vers une ingénierie spatiale de la diversité ?[END_REF]Dubresson 2004) problems that faces cities in developing countries can be understood under two ways: demographic pressure, due to both natural growth rate and the phenomenon of rural population exodus to cities on one hand, on the other to the urban territories sprawling as a result of population growth which that the size of the African cities does not stop increasing. These phenomena coupled with the lack of regulatory authorities in these areas often located in suburb facilitate for us, the disorganization of urban space and network services in developing countries.

To face the issue of network services access in developing countries, drinkable water and sanitation in particular, several programmes were thought by countries and international organizations. In the early 80's, an ambitious programme (International Drinking Water Supply and Sanitation Decade) involving states and several international organizations (governmental and non governmental) tried to ensure access to water "for all" by the year 1990. However, the results of this vast programme have not been commensurate with the investments made, what some authors [START_REF] Breuil | Renouveler le partenariat public-privé pour les services d'eau dans les pays en développement » (Comment conjuguer les dimensions contractuelles, institutionnelles et participatives de la gouvernance ?[END_REF] attribute to a bad coordination between the evolution of structures and institutional reform within the framework of this project and those who follow.

By the early 90s, the speech of international organization is going to change; the objective is not any more to assure the access to water for all, but trying to insure access for the largest number [START_REF] Baron | Mutations institutionnelles et recomposition des territoires urbains en Afrique : une analyse à travers la problématique de l'accès à l'eau[END_REF]. This situation constitutes according to us a backing on behalf of states and international organizations in front of the scale of the problem. The failure of the drinking water and sanitation decade goals' and the disappointment of developing countries national government, subjected to rigorous structural adjustment programmes 2 , changed urban policies in developing countries.

Since the 90s, Bretton-Woods institutions (IMF and World bank) and other partners of developing countries focused their speeches around decentralization. It's in this context that will be launched in 1992, the Municipal Development Programme between the developing countries and donor organizations. This new strategy oriented towards decentralization is seen by some (Stern, 1991) as a way to find new financial resources and a more efficient scale of intervention after the depletion of previous strategies. This work's aim is to analyse the interaction between local policies of water accessibility management and international decentralized cooperation and its consequences on urban territories organization and water access improvement. Decentralized cooperation refers to the international action of local governments in North/South or South/South exchanges and projects run by international NGO subsidized by local governments.

Decentralized cooperation and management of urban services in southern countries: an application on water supply services in Senegal -Arona CISSE 3 / 6

METHOD AND HYPOTHESIS

Our work will consist in analysing this frame of action where speeches and practices on urban areas are in perpetual evolution, to see how the actors situated at different levels of intervention, going of the international to the premises, coordinate their strategies to produce a policy tending to resolve the imbalance existing in the access to the drinking water on urban segregated territories [START_REF] Baron | Mutations institutionnelles et recomposition des territoires urbains en Afrique : une analyse à travers la problématique de l'accès à l'eau[END_REF]. It will also ask how local actors appropriate, adapt or throw back imported models so called " ready to manage " by [START_REF] Jaglin | Pouvoirs et cités en Afrique noire[END_REF], proposed by donors (IMF, World Bank) or local partners in the North, but their efficiency leaves sceptic some (Damiba, 1996).

The research method includes a bibliographic review, official documents analysis, interviews and questionnaires to relevant actors. The expected result is an evaluation of decentralized cooperation stakes and above all a discussion on new opportunities of collaboration between developed countries local governments and those of developing countries in urban services improvement, especially water accessibility.

We shall try to report this phenomenon through the literature of these last twenty years on the problem of the access to the urban services in developing countries, in Africa in particular, their stakes in spatial engineering [START_REF] Jaglin | Les services d'eau urbains en Afrique subsaharienne : vers une ingénierie spatiale de la diversité ?[END_REF] and of reorganization of the urban space in search of a " good urban governance " [START_REF] Baron | Mutations institutionnelles et recomposition des territoires urbains en Afrique : une analyse à travers la problématique de l'accès à l'eau[END_REF]; but also the modalities of price determination of the water which, for some, has to be made by the market to reflect the real cost of the water and allow an effective allocation of the resources [START_REF] Thobani | Tradable Property Rights to Water. How to improve water use and resolve water conflicts[END_REF]Camedessus, 2003), whereas for the others (Shiva, 2003) this trade drift is on the base of the disparities in access to water services which must be considered as a public good (Patrella, on 2003;on 2004).

Participatory mechanisms will be analysed in their implementation in the context of the daily ways that communities (NGO's and users association) organize themselves and local governments international cooperation practices for water management. The goal is to identify new ways of collective action and new institutional organization that make it possible to integrate decentralized cooperation in water issues solution.

We shall try to confirm or deny certain assumptions in this work:

-Does decentralized cooperation at the same time it allows communities to improve the water access management is not going to be a way of accentuation of existing disparities between urban spaces and populations, on territories already characterized by an mosaic form of organization [START_REF] Baron | Mutations institutionnelles et recomposition des territoires urbains en Afrique : une analyse à travers la problématique de l'accès à l'eau[END_REF])? -How the decentralized governments, within the support of international cooperation, can improve the participation of the local actors (who have a deeper insight of populations' difficulties) in establishing a solution to the problems of access to water in a " bottom up " model?

Evolution of water sector policies

The reforms that followed the structural adjustment policies lead to the end of free service in water and national government's subsidizes. This change in the management of drinking water services in developing countries joined the concerns of the World Bank which considers within the framework of Global Water Partnership that water is an economic good that it is necessary to value and to distribute by taking into account granted investments and profits removed from equipments.

The International Conference on Water and Environment (Dublin 1992) stated: "for all its different uses, water has an economic value and should be recognized as an economic good". According to this report, "the non-recognition in the past of the economic value of water drove to wasting and harmful practices for the environment of the resource…." It's on the basis of this principle that developing countries led changes in their water policies with the establishment of public-private partnerships (PPP).

The emergence of a new model of public management

The "depressed" context of post-Washington consensus (Petiteville, 1995) in developing countries allowed to local authorities the opportunity to put in profit their new attributions in foreign actions and engage North/South decentralized cooperation relationships.

The increasing openness of the international relations to non-governmental actors (after the fall of Berlin Wall) and the broadcasting of decentralization model as well as in northern and southern countries participated to the spread of decentralized cooperation [example: French Law of 1992 that legitimised international actions of local authorities and Britain's "Local government overseas assistance" Law in 1993]. These legal devices will permit local governments to become more emancipated from national authorities and to set up economic strategies in their field of competence.

In Africa, the trend towards decentralization is carried by International organizations such as World bank to ensure on one hand a more efficient management of public services and on the other hand to reduce public deficits by transferring at the same moment some revenue and expenses to local authorities. In a context of spreading of "local governance" concepts (World Bank, on 1992), several African countries under structural adjustments constraints subscribed to this model heavily recommended by international financial partners; that will be the case of Mali (by a law of February 11th, 1993 determining conditions of the free administration of regions and their autonomy) and Senegal by the adoption of Local Government Code in 1996.

It is important to wonder about the impact of transformations intervened since the early 90's (which are ongoing) both in speeches and practices on the urban territories in developing countries, especially dealing with water sector. This period (the 90s) is also that of the circulation of a "hybrid" model of public management: the New public management, which associates public and private stakeholders in public-private partnerships (PPP). The generalization of this model is going to follow in the importance, which was given -at the end of the year's 80-to the participation of users in costs recovering of water services and in commons generally. Thus, international community in New Delhi (1990) and Dublin (1992) conferences on water and environmental stated:

• The diversification of services (a little for all is better than a lot for little)

• The association of users and decision-makers at all levels of decision • The end of the free services, giving the water an economic value… It's in this circumstance that at the World Water Forum in Kyoto (2003) public assistance is put at the centre of improvement of water policies, especially in developing countries. After the market regulation and less state model advocated in the 90s shown its limits, it was necessary to find a balance between the market and the State (or the decentralized communities) to assure fair access water for populations, particularly for the poor. According to the recommendations of the Forum, this solution has to go through a strengthening of international solidarity that must be manifested by the development of decentralized cooperation in water sector, support NGOs and associations' actions, and Private sector engaged in developing countries.

Decentralized cooperation and water services management improvement

To us, decentralized cooperation can help local authorities of developing countries to reinforce their management capacities, especially in water services supply for poorest citizens. For example in France, local governments have already competences in the management of urban services and space too. And in the frame of Oudin Law (2005), which allow them to engage international actions in water access and sanitation programmes, they can help developing countries local authorities to improve their technical and political skills in urban territories management. We will try to show how decentralized cooperation can help northern countries, local authorities especially, to anticipate on problems that they might face in the future, as water resources allocation will generate more and more competition between territories and populations usage (household, industries, agriculture…).

According to Graham and Marvin (2001) splintering urbanism is a consequence of the less state model that strengthens urban networks fragmentation and the failure of state regulation. Decentralized cooperation, bye the close relations it creates between partners can help developing countries to overcome these issues and lead toward social and spatial equilibrium in cities services access, especially drinkable water.

CONCLUSION

Water access services is one of the most discussed issue in international meetings nowadays. This research tries to bring a new comprehension of international practices in water management, especially how developing countries can improve their urban management to guarantee a fair access to water services. By the combination of theoretic framework (collective action and institutions economics) we will discuss the consequences of decentralised cooperation in urban services management in developing countries. We also try to show how decentralized cooperation can favour the emergence of new collective action both at local and international scales.

In this research we assume that to ensure fair access to drinkable water at low cost (as declined in the Millennium Development Goals) to urban population, it should be necessary to strength local actors "capabilities" (in the sense of Amartya [START_REF] Sen | Un nouveau modèle économique -développement, justice, liberté[END_REF] so that local governments would be able to regulate socio-spatial organization of urban services, access to water in particular. For us, public-private partnership model has shown its limits and a new model with a stronger role for public institutions is essential to ensure social investment and spatial equilibrium.

The results of ongoing research (literature review, interviews and land practices observation) will allow us to confirm or deny our presumptions.

Access to drinkable water is characterized by domestic connection to the network or modes of collective supplies (standpipes or other water sources) available on a reasonable distance from home, estimated at

meters[START_REF] Jaglin | L'eau potable dans les villes en développement : les modèles marchands face à la pauvreté, In « Les nouvelles politiques de l'eau : enjeux urbains, ruraux, régionaux[END_REF].2 Period of budgetary austerity imposed on developing countries by the Bretton Woods institutions to "consolidate public finances and bring fiscal stability in highly indebted states. Some authors These programmes are institutional process resulting from the adoption of economic and financial agreements by developing countries with Bretton-Woods institutions. Developing countries accept reforms programme in exchange of financial assistance...