Volume 13, numéro 1, 2000

FR :

La gestion de l'eau dans les systèmes de culture hors-sol fait apparaître deux problèmes distincts. D'une part, les ressources en eau doivent être de bonne qualité et ne pas contenir de pesticides ou de germes pathogènes. D'autre part, les rejets fortement " chargés " en nutriments (NO₃-, PO₄³-) polluants pour l'environnement, doivent être limités par le biais de leur recyclage ce qui implique nécessairement la désinfection des effluents.

La technique mise en œuvre pour obtenir cette maîtrise de la qualité tant chimique que microbiologique des solutions circulantes en culture hors-sol est celle d'une oxydation à l'ozone seul et couplé au peroxyde d'hydrogène dans des réacteurs constitués de mélangeurs statiques. Les conditions de traitement sont une dose d'oxydant de 10 g O₃/m³ d'effluent à traiter, un rapport H₂O₂/O₃ de 0,15 g/g pour un temps de contact dans le réacteur de l'ordre de la seconde. Etudié sur site dans le cadre du traitement de effluents de serre réels, le procédé s'est révélé tout à fait adapté pour abattre les pesticides (# 90 % pour l'atrazine), maîtriser la prolifération des micro-organismes (Flore aérobie mésophile, flore fongique) et en particulier des germes pathogènes (Clavibacter michiganensis, Fusarium, Pythium sp ).

Le procédé novateur O₃/H₂O₂ sur mélangeurs statiques constitue donc pour les serristes une réponse nouvelle dont l'un des intérêts est de combiner les effets " détoxiquant " et désinfectant.

EN :

The management of water resources in soil-less cultures presents two difficulties. On one hand, the quality of these resources has to be good, that is to say without pesticides or pathogens. On the other hand, the effluents contain high concentrations of nutrients (NO₃-, PO₄³-), damageable for the environment, and should be recycled. Thus, recycling has to include necessarily a disinfection step to satisfy the quality requirement. The main disinfection treatments used in soil-less cultures are slow sand filtration, ultraviolet treatment, heat treatment, nanofiltration, ozone or hydrogen peroxide oxidation, iodine or chlorine treatment.

In order to control the chemical as well as the microbiological quality of the recycled nutrient solution, we suggest oxidation (O₃) and advanced oxidation (O₃/H₂O₂) processes, carried out in static mixers as chemical reactors instead of bubble columns. We have been studying this process in situ for the treatment of a 1-hectare greenhouse. The pilot plant unit can be configured under three setups (Figure 2) according to the aim to favor either the molecular action of ozone or the formation of very reactive radical species such as the hydroxyl radical. In this second case, the mechanism of ozone decomposition is given by Figure 1.

The first step of the study was to measure the influence of the nutrient solution to be recycled on the efficiency of atrazine removal (Figures 3 and 4). In comparison with tap water, the percentage of pesticide removal is lower by about 10 to 20 %. Solutions with nutrients do not drastically change the process efficiency. The experiments were carried out with various ozone dosages and various ozone / hydrogen peroxide mass ratios, using the three configurations (Figures 5 and 6). With these results, the best operating conditions for micropollutant removal are a treatment rate of about 10 g O₃ /m³ of treated solution, a H₂O₂/O₃ ratio equal to 0.15 g/g and a contact time in the reactor in the range of 1 to 2 seconds. The influence of the configuration type is not really marked. The results show that, under these conditions, this technique leads to good pesticide removal efficiencies (about 90 % for atrazine).

In a second step, experiments were carried out on real solutions containing microorganisms from the greenhouse, sometimes spiked with special bacteria (Clavibacter) or fungi (Fusarium). Some results are reported in Figures 7, 8 and 9. With the same oxidant dosage conditions, the role of the configuration is clearly demonstrated. The best results are obtained with a molecular action of ozone in the first static mixed reactor followed by a free-radical action within the second reactor. Thus, it is possible to prevent germ proliferation (aerobic mesophilic flora and fungi flora) and particularly pathogenic species. The abatement of Clavibacter michiganensis reaches 3.5 to 4 logarithmic units, 1 to 1.5 units for Pythium and 2 to 4 units for Fusarium. The treatment does not effect a complete sterilization, e.g., the beneficial bacterium Pseudomonas fluorescens survives. The global impact of the treatment on the nutritive quality of the treated solution is negligible. Nevertheless, we can note that the process induces a decrease of the ion concentrations of Fe (II) (- 5 to 30 %) and Mn (II) (-10 to 15 %) as a result of the oxidation of the EDTA chelate. In fact, this problem is observed with all oxidation and UV treatments. The residual oxidant (O₃, H₂O₂) concentrations are low and do not induce obvious toxic effects on the cultures.

Thus, the technique is consistent with a recycling of the treated effluents. The advantages of the process include very short contact times, compactness of the equipment, no need for pretreatment, reasonable investment and operating costs, an increase of the oxygen concentration in the treated effluent, and possible curative effects on the culture's germ contamination due to the residual concentration of hydrogen peroxide. The disinfection efficiency of this suggested process is similar to those obtained with more common techniques like UV irradiation. Moreover, the studied process can also reduce, for example, an eventual chemical pollution of the water resource. In conclusion, the O₃, H₂O₂ process in static mixers appears to be a new solution for greenhouse farmers.

FR :

Les stations d'épuration des eaux usées municipales du Québec, comme ailleurs au Canada et aux États Unis, sont en général peu efficaces sur le plan énergétique. Il est donc possible de concevoir des hypothèses de chaînes épuratoires améliorées au plan de leur efficacité énergétique et de leur performance globale en y introduisant, d'une part, diverses MEEE et, d'autre part, certains procédés unitaires améliorant l'épuration. Cependant, le problème de choisir, parmi ces hypothèses de chaînes d'épuration, celle correspondant au procédé le plus adéquat pour prendre en charge une situation donnée, demeure entier. Cet article analyse ce problème de choix pour les stations d'épuration de capacité comprise entre 5,000 m³/d (≈ 5000 personnes) et 100,000 m³/d (≈ 100,000 personnes). En faisant l'hypothèse, d'un côté, que l'ajout des mesures d'efficacité énergétique peut améliorer la consommation énergétique d'une chaîne d'épuration, et d'un autre côté, que l'insertion de segments de procédés peut contribuer à améliorer leur performance globale, nous avons élaboré, à partir des stations d'épuration de types Biofiltration, Physico-chimique, Réacteurs Biologiques séquentiels (deux variantes), Boues activées et Étangs aérés, six (6) hypothèses de chaînes épuratoires (chaînes 1 à 6) respectant les exigences opérationnelles et épuratoires et nous les avons comparées entre elles, sur la base d'une analyse multicritère d'aide à la décision, en vue d'en déterminer les plus performantes. Cette analyse multicritère intègre les aspects techniques, énergétiques, économiques, etc. et prend en compte les préférences du(des) décideur(s) dans le processus de choix. Les résultats obtenus montrent que, parmi les six hypothèses de chaînes étudiées, les trois premières positions sont occupées par les chaînes 3, 1 et 4 respectivement. Ce type d'analyse pourrait jouer un rôle complémentaire à une étude technico-économique visant le choix de technologies d'épuration.

EN :

Quebec municipal wastewater treatment facilities, like those elsewhere in Canada and the United States, generally are low efficiency energy consumers (ELECTRIC POWER RESEARCH INSTITUTE (EPRI), 1993; OWEN, 1982; ONTARIO-HYDRO, 1993; METCALF and EDDY, INC., 1992, SASSEVILLE et al., 1995). The work of METCALF and EDDY, INC. (1992) and of EPRI (1993) concluded that it would be possible to substantially reduce electricity demand and to improve the utilization of electrical energy in the municipal wastewater treatment processes by introducing Electricity Saving Measures (ESMs) in the processes and their management.

In the province of Quebec, given the potential savings linked to the reduction of electricity consumption in municipal wastewater treatment facilities, and the progressive expansion of the province's wastewater treatment facilities, the adoption of energetically efficient wastewater treatment technologies is particularly timely. SASSEVILLE et al. (1995) estimated that it would be possible to save 5 M $ at the present level of wastewater treatment, based on a cost of about 24 M $ for the 400 GWh of electricity annually consumed in the municipal wastewater treatment facilities. This saving would come solely from the implementation of appropriate energy-saving measures.

In hypothesizing on one hand that adding energy-saving measures can improve the energy consumption of a wastewater treatment chain, and on the other hand that the introduction of segments of processes can contribute in improving overall performance, we have elaborated from existing wastewater treatment facilities six hypotheses of liquid treatment chains that respect operational and regulation requirements, on the basis of the experience developed in the operation of municipal wastewater treatment facilities. The six hypothetical treatment chains were elaborated from facilities of the following types: biofiltration (chain 1), physico-chemical (chain 2), sequencing biological (batch) reactors system A (chain 3), sequencing biological (batch) reactors system B (chain 4), activated sludge (chain 5) and aerated lagoons (chain 6). The energy-saving measures utilized in the elaboration of these hypothetical treatment chains were chosen on the basis of a conjunctive analysis (KIBI et al., 1997). However, the problem of choosing among these treatment chains the one corresponding to the most adequate process for a particular situation is still present. The present article analyzes this choice for wastewater treatment facilities of a capacity between 5 000 m³/d (≈ 5 000 persons) and 100 000 m³/d (≈ 100 000 persons).

How then to choose the most efficient of these hypothetical municipal wastewater treatment chains?

Generally, the choice of the treatment technologies is done on the basis of single-criterion mathematical models: for example, the reduction of construction costs, or of exploitation and maintenance costs (ECKENFELDER, 1982; PINEAU et al., 1985; WANG and WANG, 1979; TYTECA et al., 1977). There are other approaches based on dynamic simulation models or on technological and econometric analyses (HYDRO-QUÉBEC, 1993; MACRAE 1989; LESSARD, 1989; BROCKTON, 1987; HOLDREN, 1987; FOSBERG and MUKHOPADHYAY, 1981; REID, CROWTHER and PARTNERS, 1978; KLEMETSON and GRENNEY, 1976).

These different approaches are often insufficient to distinguish the real value of the different technological options. Furthermore, they do not take into account many important factors (technological, economic, financial and environmental, ergonomic and socio-political) that affect their implementation and should be considered in identifying an acceptable and viable solution.

The multicriteria approach of decision-making advocated in this article can mitigate this difficulty. It will take into account key factors in the conception and operation of treatment technologies, especially energy and environmental factors, likely to give rise to efficient treatment facilities from both an energy and a treatment point of view. However the analysis of decisional factors to consider in this multicriteria analysis gives rise to a particular problem. They affect the decision that can be appreciated by deterministic relationships, offering a high level of certainty as to its evaluation, while others have a non-deterministic nature (uncertainty and imprecision). Research using the multicriteria analysis approach has been performed in similar situations over the last twenty years, in some cases applied to the environmental and energy sectors (e.g. KEENEY and NAIR 1977; ROY and VINCKE 1981; TEGHEM and KUNSCH 1985; SIMOS 1990; HANSON 1991; ROUSSEAU and MARTEL 1994). Here again, these approaches have limits since the cases were treated either in a situation of certainty or a situation of uncertainty and imprecision. The proposed model deals with the case of certainty and uncertainty at the same time, therefore improving the applicability of the multicriteria approach in the situation under study.

The solution retained consists of applying this type of modelling in order to classify from the best to the worst, the six hypothetical treatment chains. This approach utilizes in the modelling process, fourteen evaluation criteria, various criteria weights, quantitative and qualitative evaluations, as well as the indifference, preference and veto thresholds. The main steps of the model are the construction of evaluated outclass relationships and the exploitation of these outclass relationships. The multicriteria aggregation procedure utilizes an elaborated mathematical model based upon the methods of ELECTRE III (Roy, 1978) and PROMETHEE II (BRANS et al., 1984), as well as the works of DÉROT et al. (1994).

The ranking of these treatment chain hypotheses, elaborated on an empirical level in consultation with the operators and others involved in wastewater treatment and obtained on the basis of this procedure, can discriminate among their overall performance characteristics rather well. It also emphasizes their energy efficiency, since the energy criteria have on average in the analysis a weight that is 28% higher than the other evaluation criteria. The results obtained show that the hypothetical chain 3 is ranked first, chain 1 occupies the second rank, whereas chain 4 is in the third rank. The last three ranks are occupied respectively by chains 6, 5 and 2.

In a decisional and strategic approach, the first three treatment chain hypotheses can be considered overall as being the highest achievers. This result signifies that in the scope of investments related to the expansion of treatment facilities and the construction of facilities with a flow rate contained within the considered range, these three treatment chains (when considering different modification hypotheses), should be preferred over the other chains when the emphasis is on their overall performance including energy efficiency. However, other analyses would be necessary in the case of the construction of a new wastewater treatment facility with a flow rate above the level considered in this study.

Generally, the results of this analysis can assist in discriminating among the behaviors of the technologies considered, and in judging their relative performance in the investments of the construction of new wastewater treatment facilities, in addition to a technico-economic analysis. Overall, the multicriteria model described in this study identified a compromise solution between evaluations of a different and conflicting nature. This result demonstrates that this type of analysis is appropriate for tackling multidimensional problems.

FR :

L'indice pluviométrique mensuel proposé par le National Climatic Data Center (NCDC) est égal à la moyenne arithmétique des précipitations observées à certaines stations de la division climatique. Les différents problèmes d'homogénéité des données prises en compte par le NCDC pour le calcul de l'indice sont quantifiés pour la région climatique centrale de l'Oklahoma. Une amélioration de la méthode de calcul est proposée. Le calcul de ce nouvel indice utilise un nombre fixe de stations et fait appel à une méthode d'estimation des données manquantes. L'estimation des valeurs manquantes permet de disposer d'un jeu de données complet, ce qui augmente la représentativité de l'indice. Les moyennes mensuelles des valeurs absolues des différences entre l'indice NCDC et l'indice proposé sont comprises entre 6 % (mai) et 13 % (août) des précipitations moyennes et entre 9 % (novembre) et 24 % (août) des écart-types mensuels. Ces valeurs démontrent que les problèmes liés à la méthode de calcul d'un indice pluviométrique mensuel régional utilisée par le NCDC peuvent être importants. Les résultats de recherches sur la variabilité temporelle des précipitations utilisant un indice pluviométrique régional devraient être interprétés en connaissance de ces différences.

EN :

The NCDC monthly precipitation index is computed as a simple average of the monthly precipitation at several stations within a climate division. The influences of the discontinuities in records used by NCDC are quantified for the central climate division of Oklahoma. An improvement of the calculation method is proposed. The number of stations used is fixed and the missing monthly data values are filled. The monthly averages of the absolute values of the differences between the two indices vary from 6 % (May) to 13 % (August) of the mean monthly precipitation and from 9 % (November) to 24 % (August) of their mean temporal variations. These values demonstrate that the discontinuities in station records of the NCDC divisional precipitation index can be relevant and that research results on regional precipitation variability should be interpreted with consideration of the approximation errors introduced.

FR :

Dans cette étude, des mesures électrochimiques ont été utilisées pour caractériser l'efficacité inhibitrice du produit commercial Kemazur 1620, employé pour le traitement des eaux des circuits de refroidissement. L'influence de la concentration de ce composé ainsi que l'effet de la température du milieu ont été étudiés.Les courbes de polarisation ont été obtenues à l'aide d'un montage à trois électrodes. La méthode électrochimique a permis de déterminer la vitesse de corrosion en l'absence et en présence d'inhibiteur et par conséquent, le taux de protection. Ainsi, le Kemazur 1620 présente une très bonne efficacité pour une concentration de 2000 ppm. Entre 1000 et 2000 ppm, l'efficacité inhibitrice augmente de 56 à 91 %. En outre, l'efficacité inhibitrice du composé a été comparée à celle du nitrite de sodium.

EN :

In this study, electrochemical measurements were carried out to characterize the inhibition efficiency of the commercial product Kemazur 1620, employed for water treatment in cooling circuits. The influence of parameters such as the concentration of the compound and the effect of the temperature on the behavior of the carbon steel / corrosion medium interface was studied. Polarization curves have been obtained by a three-electrode device. In the first part of this study, only the evolution of the corrosion rate as a function of concentration was examined. It has been shown from the polarization curves that the Kemazur 1620 presents a very good efficiency for a concentration of 2000 ppm. Between 1000 and 2000 ppm, the inhibition efficiency increases from 56 to 91 %. In a second part, the inhibition efficiency of the compound has been compared to an inhibitor used for similar uses.

FR :

Les travaux présentés dans cet article ont pour objet l'étude de l'influence de la distance parcourue par les écoulements de surface sur l'intensité des transferts par ruissellement des produits phytosanitaires. Une première série d'essais réalisée sur des parcelles expérimentales labourées mais non cultivées montre une rapide décroissance des quantités exportées avec l'augmentation de la distance de transfert. La quantité cumulée de matières transférées peut alors être décrite par une fonction exponentielle décroissante de la distance. Une seconde série d'expérimentations sur des parcelles cultivées en blé en hiver puis au printemps en maïs montre que l'intensité de cette réduction dépend pour une large part de la pénétration des molécules dans le sol, qui les soustrait aux entraînements de surface. Cette migration verticale est favorisée par la faible affinité des matières avec les composants du sol en particulier la matière organique. Elle est également plus forte quand la réserve en eau du sol est élevée, ce qui augmente la migration par diffusion des composés et la descente lors des pluies des tranches d'eau les plus contaminées. Par ailleurs des pratiques culturales laissant peu de sol nu limitent le ruissellement au profit de l'infiltration. Ces divers mécanismes expliquent que l'effet distance est moins sensible pour une molécule peu mobile comme le lindane que pour l'atrazine, dont la mobilité conduit à des exportations élevées lors des premières pluies qui suivent les traitements.

EN :

The aim of this study was to investigate how the distance covered by runoff water containing pesticides influenced the resulting quantity of pesticides liable to enter neighbouring waters courses. We carried out primary experiments on a bare soil, equipped with runoff water collection equipment placed at the following distances from the area to which the pesticide was applied: 1.5 m, 5 m, 9 m and 13 m. Each 10 m² test surface was treated with several pesticides: atrazine, simazine, cyanazine, 2,4-D and 2,4-DB. Results show that the pesticide quantities found in the water collectors decreased exponentially with distance. Under the experimental conditions, a distance of about 4 m led to a 10-fold reduction in pesticide runoff transfer rates.

Further similar experiments were carried out on soil cultivated with wheat and with maize. Two different pesticides, atrazine and lindane, exhibiting different behaviour in soil and different chemical and physical properties, were chosen. Observed results were similar and the total pesticide quantity was again found to be a decreasing exponential function of the distance from the treated plot. Reduction in the amount transferred, as a function of runoff distance, depends mainly on vertical migration, which in turn reduces the quantity of surface pesticide available. This vertical migration is favoured by a low affinity between pesticide and soil. This phenomenon is amplified with increasing soil water reserve. It is due to both pesticide diffusion and vertical displacement of water. The state of the surface due to arable farming practices and vegetation also plays a determining role in the vertical and horizontal displacement of water, and consequently affects pesticide transfer by runoff. Therefore the distance factor was less prominent in spring for soils cultivated with maize than in winter for the same soil cultivated with wheat. However, in all cases, the influence of runoff distance upon the quantities of pesticide transferred to surface waters remains crucial.

FR :

Les transferts d'eau dans le sol sont généralement, pour des raisons de facilité, supposés se dérouler dans des conditions isothermes. Les modèles proposés couplant les transferts hydriques et thermiques se heurtent aux difficultés inhérentes à la détermination des coefficients de transfert. Disposant de l'évolution de la température, de la succion et de la teneur en eau dans les profils du sol de lysimètre, une comparaison portant sur l'importance des gradients hydriques et thermiques dans le transfert d'eau en phases liquide et vapeur a été menée. Il se dégage clairement, que dans le domaine de teneurs en eau qui intéressent l'agronome (teneur en eau supérieure à celle au point de flétrissement), l'essentiel des transferts se fait via la phase liquide. En effet, les flux dus aux gradients de teneur en eau dépassent d'au moins deux ordres de grandeur les flux induits par les gradients de température. Ce qui confirme bien la validité des équations de Darcy-Richards (DARCY, 1856; RICHARDS, 1931) où l'effet de température est négligé.

EN :

In most studies of liquid water flow, isothermal conditions are assumed. Since the beginning of the last century, several models and studies concerning mass and heat transfer have been conducted, but results didn't agree on the effect of temperature and hydraulic gradients on water movement. Also, the relative importance of these different gradients is not well known. Models used to quantify the simultaneous transfer of water due to temperature and water content gradients require a predetermination of thermal and iso-thermal coefficients of water diffusivity. Moreover soil characteristics must be known, mainly the relationships between hydraulic conductivity (K), pressure head (h), and water content (Ɵ). Measurements of water content, pressure head and temperature were done in a loamy sandy clay soil within a weighable lysimeter. The relationships between hydraulic conductivity, pressure head and water content were obtained from field and laboratory data.

The water content gradient is generally less than those of temperature. Even with the maximum temperature gradient, the flux due to the water content gradient is more important than those due to the temperature gradient; the difference can reach 200 %. Various field data (temperature, pressure head and water content) for different field conditions concerning the mass transfer in both liquid and vapor phases resulting from hydraulic and temperature gradients were analyzed. Results show that when soil moisture is between the wilting point and field capacity, mass transfer occurs mostly in the liquid phase. This result confirms the validity of Darcy-Richards (DARCY, 1856; RICHARDS, 1931) equation where the effect of temperature is neglected. Also, the isothermal diffusivity of liquid water is very important when compared to all other diffusivities.

FR :

La prévision des inondations urbaines et de leur impact sur le milieu passe par la modélisation précise et lisible des flux inondants. Leur représentation est cependant rendue difficile par le caractère transitoire et multidirectionnel des écoulements, dans un milieu dont la géométrie est très irrégulière. Cet article traite plus spécifiquement du comportement du bâti africain vis à vis des écoulements, en situation inondante, et des lois de stockage et de vidange que l'on peut définir à différentes échelles représentatives de l'habitat : concession, bloc de concessions. Nous présentons trois propriétés du bâti nécessaires et suffisantes pour décrire le comportement hydraulique du milieu à ces échelles : sa pénétrabilité, sa stockabilité et sa transmissivité. L'étude du comportement hydraulique de l'objet bâti élémentaire, la concession, nous permet de relier ces propriétés à des caractéristiques géométriques de cet objet. Une approche agrégative conduit ensuite à définir un indicateur de la structure géométrique du bâti, l'HistoSeuil, équivalent à une densité d'ouvertures et caractéristique de la pénétrabilité du bâti. L'étude de sa pertinence géométrique, i.e. sa variabilité intra- et inter-quartiers a été réalisée dans le cas particulier de la ville de Ouagadougou (Burkina Faso) ; elle est basée sur le relevé systématique des ouvertures observables sur différentes façades de voiries de trois quartiers de types différents, (habitat individuel et spontané). Sa pertinence hydraulique, i.e. sa capacité à reproduire le comportement hydraulique moyen de l'objet urbain modélisé, est enfin abordée. Développée dans le contexte particulier de Ouagadougou, cette approche est généralisable à des configurations urbaines très diverses.

EN :

Stormwater runoff generates one of the most critical natural risks in urban environments: impervious surfaces and high drainage network densities lead to frequent urban flooding events, with short process times and within small urban areas. In all parts of the world, urbanisation is growing, and urban flood hazards consequently occur more and more frequently. Examples of important flood damages suffered by urban populations are numerous, especially in tropical regions where the violence and rapidity of tropical storms often lead to an overloading of the drainage system and to the flooding of adjacent built-up areas. Prediction and evaluation of these damages require the determination of some important hydraulic characteristics of the flood, such as maximum water depth or flooding duration. Currently-used models are generally limited to checking the sewer system efficiency. Therefore new models are now expected to represent with accuracy and reliability the stormwater runoff, which can result from sewer system overloading. However, this kind of modelling is hard to carry out because of the geometric complexity of the urban media and because of the rapidity of urban storms and their associated flooding. Moreover, the modelling of the behaviour of the flooded built-up areas should sometimes be integrated into complete models of urban flooding, given their important influence on the hydrodynamics of the flood. However the geometric complexity of these built-up areas prevents us from a complete and accurate description of the different obstacles and water ways encompassed in such areas. Simplified descriptions at a larger scale are consequently to be found.

This paper highlights the important physical characteristics that determine the hydraulic behaviour of every hydraulically-independent urban cell, and suggests a way to represent the exchange and storage laws of built-up areas at different scales: individual plots and blocks of plots. The study was performed in the particular case of Ouagadougou's areas. The hydraulic behaviour of every built-up area can be modelled with three important physical characteristics, the two first of which are related to the structure of the surrounding walls:

- water perviousness : the ease with which the passing flood can enter or exit the plot. This is dependent upon the aperture density, which can vary according to the façade;

- transmissivity: the ease with which water can pass through the plot. It depends on the perviousness of the different external or internal façades of the individual plot. If one façade is waterproof, the transmissivity becomes nil in the perpendicular direction;

- storativity : determined with the storage capacity of an individual plot. It is a function of the internal surface area of the plot.

These three characteristics are functions of height. Moreover, they are essential and sufficient to describe the behaviour of every basic or global urban object (plot, block of plots...). At the "block of plots" level, the transmissivity and storativity concepts are comparable to the hydraulic roughness and urban porosity concepts that have already been proposed in scientific papers (e.g., Braschi et al. 1991). These two characteristics are sufficient for modelling the hydraulic behaviour of every open urban medium. Nevertheless, some urban media are non-transmissive because of a high connection level between the different obstacles. In these cases the perviousness property is very useful for modelling the different exchanges between the built-up areas and the adjacent flooded roads. The residential urban areas of Ouagadougou, used as an illustration for this study (Figure 1), correspond to this case of partitioned urban areas. The structure of the Ouagadougou's residential districts is standard and is organised around the individual plot, a parcel shielding one or several families: the individual plot is isolated from other plots and from the roads by a surrounding wall that constitutes one of the elementary hydraulic objects of the urban environment. The evolution of the flood water depth in a plot adjacent to a flooded road, determined by equation 1, depends on its floodable surface Sc and on its perviousness. Its perviousness is defined by the geometric characteristics of the apertures present in the wall: the type of aperture, weir or orifice; its height, hs, its length, Ls, and its opening if an orifice, a. Measures of exchanges between roads and plots made during some flood events in Ouagadougou (Hingray 1999) showed that the classical discharge laws for weirs or trough orifices can be used to model these exchanges (equations 2, 3 and 4).

We suggest a way to simulate the exchange and storage laws of these built-up areas at a larger scale: the block of plots. An aggregation approach enables us to define a structure indicator: " l'HistoSeuil " (Figure 2). It is based on the description of the lengths of weirs and apertures found in road façades, and is equivalent to an aperture density function. The exchange discharge between the block of plots and the flooded adjacent road can be computed with a simply convolution (equation 6) between this HistoSeuil and the reference discharge laws for broad-crested weirs (equations 7 and 8). The geometric relevance of the indicator is next discussed: it seems to be a relatively stable geometric characteristic of an urban area (Figure 3). This result is given by a systematic survey of the apertures observed in 24 road façades belonging to 3 different districts of Ouagadougou. The two first are traditional residential districts, more and less developed. The "Patte d'Oie" district is fairly old and was established in the 1970's (numerous well developed plots). The second one (Wemtenga 1) is a recent housing estate (1988) (numerous unfinished or empty plots). The final one (Wemtenga 2) is a very recent district of spontaneous development (disorganised built-up area structure). Furthermore, the hydraulic relevance of this indicator, its ability to reproduce the average hydraulic behaviour of a block of plots, is approached. Initial results seem to be positive. If both the hydraulic and geometric relevance of the structure indicator presented in this paper are validated by the additional work that we are carrying out at this present time, this approach may prove to be useful for the hydraulic modelling of built-up areas. Moreover the study of other types of built-up areas could lead to the determination of a hydraulic typology of urban areas. In particular, this study, performed in the case of Ouagadougou, a big city in a developing country, seems to be valid for every city where the built-up areas are highly partitioned.