Volume 14, numéro 3, 2001

FR :

Depuis plusieurs années, la Zone Non Saturée d'une parcelle expérimentale au nord de l'Italie (plaine du Pô) a fait l'objet de nombreuses études sur les caractéristiques physiques et géologiques du milieu, ainsi que sur les modalités du mouvement des eaux d'infiltration. Le site a été équipé de 6 tensiomètres à mercure, 6 bougies poreuses pour l'échantillonnage de l'eau et d'un tube d'accès pour la sonde à neutrons.

L'application des différentes méthodes d'évaluation a fourni, pour la période d'étude (Septembre 1994-Août 1995), des valeurs de l'infiltration sensiblement différentes. La méthode physique, basée sur le calcul du bilan hydrique annuel, et considérant le sol comme un unique réservoir monocouche, a indiqué que 19% du total des précipitations arrive à s'infiltrer. D'autre part, un modèle de simulation intégrant les caractéristiques hydrodynamiques du sol a permis d'évaluer la recharge à 29% des précipitations annuelles. Le calcul de la répartition de la teneur en eau sur le profil porte sur la description des fluctuations temporelles du plan de flux nul, en relation avec les épisodes pluvieux et l'intensité de l'évaporation.

La méthode géochimique du "profil en ions conservés" entre l'eau de pluie et du sol a permis, quant à elle, d'évaluer la lame d'eau infiltrée à environ 12% des précipitations annuelles.

La composition isotopique des eaux extraites du sol manifeste un enrichissement global en isotopes lourds (²H et ¹⁸O) par rapport aux eaux météoriques, conséquence d'une évaporation intense. L'infiltration semble être de type "piston flow" dispersif. Elle suit, seulement pendant l'hiver, un flux où la composante convective dans la microporosité est prédominante sur la composante diffusive qui a lieu, de préférence, dans la macroporosité.

EN :

In groundwater management, it is necessary to define water movement, evaluate infiltration and evapotranspiration rates, and quantify the physico-chemical evolution of transported solutes throughout the unsaturated zone (UZ). Traditionally, in temperate regions, recharge rate is evaluated by the comparison between physical methods (based on direct measurements of hydrological parameters with lysimeters, tensiometers and neutron gauges) and geochemical approaches (conservative ions, stable isotope profiles and artificial tracers).

In the Po Plain (Northern Italy) intensive agricultural irrigation and overexploitation of groundwater profoundly affect the quality and availability of shallow groundwater resources. The region has been intensively cultivated with corn and rice during the last 5 centuries. Irrigation canals have been constructed in order to distribute water from alpine rivers to areas more favourable to agriculture. In the past, only the water balance method has been applied to obtain recharge rates in these situations.

An experimental field site has been operational since 1987 in the ENEA-EUREX Nuclear Centre of Saluggia. The experimental plot represents an unsaturated zone in fluvio-glacial deposits (Holocene) of the Dora Baltea River, formed by gravel and sand, interlayered with silt levels. The water table is usually at a depth of 200 cm but it varies as a function of the river level. The objectives of the present study were to describe water movement throughout the UZ and to evaluate infiltration and evapotranspiration rates using different physico-chemical methodologies. The Saluggia plot was equipped with six tensiometers, a neutron gauge hole and 6 porous cups, up to a depth of 160 cm.

During 10 years of monitoring, the most favourable period for understanding infiltration processes and water movement through the UZ is September 1994 - August 1995. This period is characterised by an exceptional rain event (300 mm in 48 hours), followed by a river flood. Water samples collected with porous cups at various depths have been analysed for chemistry and stable isotope composition, together with rainfall and Dora Baltea River water.

In October 1994 (beginning of the rainy period) and May 1995 (beginning of the dry period) soil samples were collected at different depths and water was extracted under vacuum conditions. Stable isotope analyses were performed on extracted waters. Soil samples were also analysed for granulometry, chemical and mineralogical composition.

Reducing soil to an unique monolayer aquifer, the water balance method suggests that only 19% of the total precipitation infiltrates, whereas a simulation model taking into account soil hydrodynamic characteristics estimates an infiltration rate of 29% of the rainfall. The position of the Zero Flux Plane (ZFP) fluctuates seasonally between the surface and the maximum studied depth, as a consequence of precipitation events and evaporation fluxes. In some periods, the ZFP is level with the water table and direct discharge may occur.

Measurements of the stable isotopic composition (δ²H and δ¹⁸O) of soil water allows a quantitative estimate of direct groundwater discharge. On a δ²H versusδ¹⁸O plot, pore waters from the UZ have an isotopic composition that differs from that of the majority of groundwater samples, plotting below the local rain water line and indicating some degree of evaporation during the recharge process. The isotopic enrichment is particularly significant at the evaporation front, suggesting that pore water in soil reflects a different recharge regime from those of the regional ground waters. Water isotope and solute composition were substantially modified from their original composition during the infiltration process within the soil, via mechanisms such as anion exchange with soil particles, salt precipitation/dissolution or isotopic fractionation.

Arial recharge was also evaluated using the depth distribution of a conservative solute. Assuming that chloride is derived from precipitation alone, Cl- content in soil and in rain water suggests that 12% of total rainfall infiltrates. Therefore, the recharge rate is estimated to be less than 100 mm/a. These data are in agreement with other results obtained by the chloride concentration profile method, in areas of the Po Valley, but are considerably different from those evaluated by the traditional physical methods.

FR :

Sur le littoral du Nord - Pas-de-Calais (France), la Craie d'âge Crétacé supérieur constitue l'aquifère le plus exploité régionalement pour l'alimentation en eau tant potable qu'industrielle. Sur la frange la plus littorale, l'aquifère crayeux, semi-captif est affecté par de nombreuses failles. Dans certains secteurs, la profondeur de la craie et la localisation des failles, restent aléatoires suite au manque de forages et d'affleurements. Afin de palier à ces lacunes, trois méthodes géophysiques (sondages électriques et sismiques, profilage électromagnétique) ont été appliquées.

La synthèse des données obtenues a permis de démontrer que la craie est découpée en une série de compartiments par des accidents tectoniques présentant un affaissement vers le Sud et vers l'Ouest. L'interprétation des données géophysiques a également permis d'obtenir des informations sur la répartition de l'interface eau douce - eau salée au sein de l'aquifère crayeux.

Dans la région, le concept classique de biseau salé ne permet pas d'expliquer l'irrégularité spatiale des intrusions. L'étude semble démontrer que la répartition et l'extension des intrusions d'eau salée peuvent être corrélées avec la localisation des accidents tectoniques, les intrusions les plus éloignées du rivage se faisant au droit des zones faillées.

Ceci amène à proposer une nouvelle approche concernant l'extension des intrusions d'eau salée : elles seraient directement tributaires de la géométrie et des caractéristiques hydrodynamiques de l'aquifère crayeux, ces dernières étant fortement influencées par la présence des failles.

EN :

Along the North Pas-de-Calais coast between a point south of Boutonnière du Boulonnais and the Authie River, Cretaceous rocks are limited to the east by a paleocliff that overlooks the Quaternary coastal and dune deposits. The chalk aquifer is connected to the Quaternary aquifers and further west to the sea water. Close to the seashore the top of the chalk plateau falls rapidly from 20 m above sea level to 20 m below sea level, causing the aquifer to grade from being unconfined in the east to confined in the west. This chalky aquifer is cut by several faults. A structural survey (LOUCHE et al., 1997) based on borehole data was carried out to evaluate whether the rate and direction of groundwater flow are controlled by the different faults. Two major fault groups striking N030° and N110° have been revealed by the structural map, with a third minor group (N 160- 170°) to the north of the Canche. The two major groups, created during tectonic activity from the Palaeozoic to the present, are composed respectively of structures with general faulting directions of N 100-110° and N 020-040°, previously described by COLBEAUX et al. (1993). Chalk depth and fault location are unknown in certain areas due to the lack of bore holes and outcrops. With the aim of overcoming these deficiencies, three traditional geophysical techniques (electrical sounding, seismic logging and electromagnetic profiling) were carried out between the Canche and Authie rivers in the coastal area, west of the chalky cliffs.

The interpretation of the geophysical results allows us to confirm the presence of previously located faults and identify new ones in the area where the chalk is overlain by Quaternary cover. The chalky aquifer is hence divided by the faults into a series of compartments that are inclined to the south or to the west. In addition, anomalies have been recorded in the fresh water - salt water interface on the Picardy continental shelf (MERCIER & BACROT, 1990). These anomalous zones are aligned along the northern extension of the sub N-S faults proposed by MENNESSIER & BEUN (1980), located a few kilometres to the south. Based on indirect evidence, these latter authors interpreted these faults as being responsible for considerable displacement of a major part of the Quaternary deposits. Given the lack of direct observations on salt-water distribution, geophysical results, and more specifically the electrical soundings, are required to provide supplementary information in order to better understand this phenomenon. Furthermore, the study of the salt water location, using electrical soundings, points out the juxtaposition of deep saline intrusions, demonstrating that the concept of a homogeneous wedge-shaped body parallel to the seashore can no longer be used to describe this chalky aquifer.

In the Bas-Champs area, electromagnetic profiling and three electrical soundings provided additional information on the relation between tectonic events and saline water intrusion (Fig. 3). The electromagnetic profiling shows the presence of two faults (Fig. 6). The vertical faults have an apparent low electrical resistivity (2.5 Ω.m) indicating the presence of brackish or salt water. Electrical sounding carried out to the south of the electromagnetic profile (SE9) shows a decrease in the electrical resistivity from 12.5 m depth, also confirming the presence of salt water. The results of an electrical sounding located to the north of the electromagnetic profile (SE10) show that the ground is saturated with fresh water up to at least 27.2 m (resistivity of 90 Ω.m). At 1 km distance, there is a juxtaposition of three sectors separated by faults filled up with salt water. However, an electrical sounding (SE2, Fig. 2) located near the sea showed that the ground is saturated with fresh water up to at least 52.7 m deep.

Using geological, hydrogeological and geophysical data correlations, between structural context and saltwater intrusion could be established and a new hypothesis for saltwater intrusion for this study area could be proposed. Intrusion is directly dependent on the geometry and the hydrodynamic characteristics of the chalky aquifer, as reflected by its heterogeneous nature.

Salt water seems to follow preferential intrusion directions corresponding to faults rather than the classic model of a salt wedge. These results suggest the hypothesis that the seawater intrusion distribution relates to the tectonic configuration of the aquifer. The chalky series is cut into hydrogeological compartments individualised by tectonic accidents, which are preferential axes for sea intrusion. The presence of a saltwater front located far from the coast, as indicated by electrical soundings, could be explained by the geological history of the maritime plain, where the Pleistocene and Holocene seas covered the maritime plain up to the fossil cliff (LOUCHE et al., 1998).

FR :

La connaissance des champs pluviométriques annuels est importante dans les zones arides et semi-arides où la gestion de l'eau est un exercice permanent d'aménagement de la pénurie, comme cela est le cas au Nord du Mexique.

On se propose de montrer ici qu'une meilleure connaissance des disponibilités en eau peut s'appuyer sur la détermination de régions pluviométriquement homogènes à partir de diverses méthodes complémentaires ou convergentes.

Pour définir des régions homogènes, on part de la répartition des stations par rapport à la régression pluviométrie/altitude. Des analyses factorielles en composantes principales et des correspondances permettent également de proposer des régions homogènes suivant des variables définies et disponibles pour toutes les stations. On s'appuie aussi sur les régimes pluviométriques pour déterminer d'autres régionalisations. Parallèlement on a pu utiliser la répartition des stations par rapport au gradient altitudinal pour créer des régions dont l'homogénéité vis-à-vis des précipitations a pu être vérifiée par la Méthode du Vecteur Régional (MVR), basée sur le principe de la pseudo-proportionnalité des données de postes proches.

La comparaison des résultats obtenus par chacune des méthodes permet de constater que dans la région traitée, les limites entre régions " homogènes " sont souvent les mêmes, bien que les modes de détermination soient différents. Enfin, les différences apportent une information supplémentaire pour la compréhension des mécanismes locaux ou régionaux de la répartition des champs de pluie.

EN :

Knowledge of annual rainfall is of great importance in arid and semi-arid areas, because water management is dominated by scarcity. The Nazas-Aguanaval river basin constitutes one of the main endoreic basins in Mexico (92 000 km²). It extends from the crests of the Western SierraMadre to as far as the Chihuahuan desert, in the states of Durango, Coahuila and Zacatecas. Spatial variability of rainfall is significant with annual rainfall amounts ranging from 900 mm in the higher areas of the Sierra Madre to 180 mm at the centre of the Laguna de Mayran. However, temporal variability of the precipitation amount is also appreciable, and it increases from the sub-humid areas of the mountains to the desert. The coefficient of variation for annual precipitation ranges from 0.2 in the mountains to 0.4 in Chihuahuan desert. Furthermore, in 1992, 1994, 1995, and from 1997 to 2000 severe rainfall deficits forced farmers to reduce strongly irrigated areas, thus leading to socio-economic development problems in this region. It is shown in this paper that an improvement in water availability knowledge is attainable by the determination of homogeneous rainfall regions, based on complementary or convergent methods.

Rainfall distribution is a result of many factors, including the atmospheric circulation, the continental pattern, the coastal design, the location of major mountainous massifs, the distance from the ocean, and other site factors. The regionalisation of precipitation has been the subject of much research for almost all types of climates. The influence of zonal and regional factors is also determined in regional monographs where the role of local variables (relief, vegetation, general roughness of landscape, etc.) is described in relation to the large-scale circulation scheme. In most of the cases, the interpolation of values between two observations is necessary and quite difficult. Kriging is widely used for this purpose, as is co-kriging, which takes into account the topography or some other local factors and frequently gives better reconstitution of rainfall data. In order to determine the first set of homogeneous regions in northwestern Mexico, the elevation gradient of the rainfall amount was defined by a simple regression. All the stations were located with respect to the regression line and they can form apparent groups. The following relation was obtained :

P=0.31 H - 133

r²=0.73; n=84)

(where P is annual rainfall in mm and H the altitude in m).

In the same way, various statistical analysis were performed using all data available from the rainfall measurement stations, such as elevation, distance from the Pacific Ocean, exposure, annual rainfall amount, and the type of topography and vegetation cover surrounding the station. An Empirical Orthogonal Function Analysis (EOF) and a Factorial Analysis of Correspondences (FAC) revealed other kinds of regionalisation. The precipitation regime is tropical-like in spite of the latitude (25° N), but the percentage of annual precipitation in winter appeared as a segregating factor and thus was used to define the climatic geography. This was determined by a stepwise discriminant analysis, which allowed the segregation of the north-eastern area of the Nazas-Aguanaval basin. This is the dryer region of the basin because it is less exposed to monsoon fluxes and the proportion of winter rain is higher there than in the remaining basin.

The main variables explaining the spatial distribution of precipitation are altitude and distance from the Pacific Ocean, as determined by both the EOF analysis and the analysis of correspondences. The grouping of stations segregated by the elevation gradient regression led to regions where the homogeneity in relation to the annual rainfall amount was tested and verified by the Regional Vector Method (RVM). This method is based on the principle of pseudo-proportionality between annual rainfall amounts at close stations.

The Nazas-Aguanaval basin is divided into three climatic regions defined by precipitation: the Western Sierra Madre, the Chihuahuan desert, and a semi-arid area that is divided into two sub-regions (Middle Nazas basin and Aguanaval plateau) by the analysis of correspondences. As a result of the regional rainfall analysis, some variograms were performed to determine the length of the validity of the rainfall data. However, it appeared that a multidirectional variogram did not explain these data. The role that relief (mainly the Western Sierra Madre) plays in the spatial distribution of precipitation does not explain the length of rainfall data. Introducing the direction of mountain range into the variogram demonstrated that the relief played a significant role, and in this case the length of the rainfall variogram data was 180 km.

A comparison of results obtained using each method led to the conclusion that the boundaries between homogeneous regions are often the same while the determination processes are different. Finally, all the proposed methods are complementary and the differences between all characterisations give additional information regarding the local and regional processes that explain the annual rainfall spatial distribution. Simple tools have been used to acquire a better knowledge of rainfall spatial distribution.

In the case of Northern Mexico, the low density of a measurement network (rain gauges), particularly in mountainous or arid zones, is partially attenuated by the possibility of evaluating the main climatic characteristics for the different regions defined in terms of rainfall.

FR :

De l'urée et du super phosphate triple ont été appliqués aux étangs d'alevinage de carpes chinoises de la station Deroua pour stimuler la croissance des algues. Toutefois, une forte proportion de ce phosphore est fixée par les sédiments et sera libérée en grande quantité dans les étangs lorsque les sédiments sont en conditions anaérobies.

Nous avons étudié le relargage des orthophosphates par les sédiments provenant de deux étangs différents. L'un (A₁) en terre ce qui permet la percolation des éléments nutritifs vers les couches profondes, l'autre (C₁) dont le fond recouvert d'une fine membrane en polyéthylène, est parfaitement imperméable et empêche toute perte. Par ailleurs, nous avons étudié les effets de l'oxygène dissous, du pH, des nitrates et du glucose sur le relargage des orthophosphates par les sédiments.

Les résultats obtenus montrent qu'en anaérobiose, la solubilité du phosphore augmente dans l'eau interstitielle. Les orthophosphates libérés, dont la concentration peut atteindre 873 µg/l, proviennent de la fraction minérale en particulier du Fe(OOH)=P. Le sédiment C₁ qui accumule de grandes quantités de matière organique libère plus de phosphore que le sédiment A₁.

L'addition des nitrates limite la mobilisation du phosphore à partir des sédiments. Le pH acide provoque la dissolution du phosphore lié au calcium alors qu'un pH alcalin provoque celle du phosphore lié aux hydroxydes de fer et sa précipitation partielle sur les carbonates présents dans les sédiments.

EN :

In the Deroua fish farm (Béni-Mellal, Morocco), a series of ponds was lined with polyethylene to prevent water seepage (lined ponds, C). Another series of ponds was not lined (unlined ponds, A). The lined ponds do not need much fertiliser to enhance phytoplankton growth. They receive a small quantity of urea and triple super phosphate and accumulate high levels of organic matter in their sediments. The unlined ponds receive a large quantity of fertilisers. In the ponds, an important increase in phytoplankton growth occurred particularly after fertilisation with triple super phosphate. The phosphate (P) released from the sediment is the essential source of P for phytoplankton when the ponds are not fertilised.

Sediment samples used in this study were collected in June 1999 from the A₁ and C₁ ponds of the Deroua fish farm. The bottom sediments were collected with a core sampler to provide samples from the 0-5 cm layer. The samples were mixed and analysed for pH, total Kjeldahl nitrogen (NTK), nitrates (N-NO₃^-), ammonium (N-NH₄ ⁺), total iron (tot-Fe) and P-fractionation in eight replicates, using the Golterman method (1996).

In this work, experiments were conducted to evaluate the P released from sediments of lined and unlined ponds under controlled conditions. The samples included: untreated sediments, sediments treated with hydrogen peroxide (H₂O₂), and sediments treated with chloroform (CHCl₃). H₂O₂ was used to destroy the organic matter of sediment. To inactivate bacteria, CHCl₃ was added to the sediments. CHCl₃ does not modify the mineral and organic structures of sediments but does reduce the bacterial biomass. Urea and triple super phosphate were added to Chinese carp rearing ponds at Deroua fisheries farm to stimulate algal growth. However, phosphate bound to the sediments was released into the water column during anaerobic conditions. The released phosphate from lined pond (C₁) enhanced the algal production.

We have tested the fertilisation capacity of A₁ and C₁ ponds as well as the effects of different factors such as dissolved oxygen, pH, nitrate, and organic matter (glucose) on the phosphate released from the sediments. The results show that under anoxic conditions, phosphate solubility increased in the interstitial water. The released phosphate (873 µg/l) comes from the mineral fraction, particularly from Fe(OOH)=P. The P released from the A₁ untreated sediment under oxic and anoxic conditions was low in comparison to the P released from the C₁ untreated sediment and did not exceed 9.1 µg.g-¹ d.w. (Figure 1). The removal of organic matter with H₂O₂ had a positive effect on the P released from the C₁ sediments only after the second day of incubation. The inactivation of bacteria with CHCl₃ had an inhibitory effect on the P released from the C₁ sediments under oxic and anoxic conditions; however for the A₁ sediments, this inactivation of bacteria had no effect on the kinetics of P release. Phosphorus release from the C₁ sediments was important under anoxic conditions; the P came from Fe(OOH)=P after the reduction of Fe(OOH).

The addition of nitrates decreased the mobilisation of phosphate from the sediments. Acid conditions increased the dissolution of calcium-bound phosphate, while alkaline conditions increased the dissolution of iron-bound phosphate and its re-precipitation onto carbonate present in sediments. High concentrations of organic matter (glucose) increased oxygen demand and favoured the development of reducing conditions. The phosphate was released from Fe(OOH)=P.

The conceptual model of P released in fish ponds shows that, in semiarid climates and in calcareous areas, the polyethylene lining of ponds prevents not only the seepage of water but also the percolation of nutrients. Consequently, the P was more available for algal uptake in the lined ponds. More phosphate will be released concomitant with the process of the ponds becoming anoxic, as nitrate concentrations and the pH of sediments decrease.

FR :

Nous nous intéressons dans ce travail de recherche à la modélisation d'une série d'événements par la théorie des processus ponctuels temporels. Un processus ponctuel est défini comme étant un processus stochastique pour lequel chaque réalisation constitue une collection de points. Un grand nombre d'ouvrages traitent particulièrement de ces processus, cependant, il existe dans la littérature peu de travaux qui se préoccupent de l'analyse de séries d'événements. On identifie deux catégories de séries d'événements : une série d'un seul type d'événements et une série de plusieurs types d'événements.

L'objectif de ce travail est de mettre en évidence les différents tests statistiques appliqués aux séries d'un seul ou de plusieurs types d'événements et de proposer une classification de ces tests. Nous présentons d'abord une revue de littérature des processus ponctuels temporels, accompagnée d'une classification de ces modèles. Par la suite, nous identifions les tests statistiques de séries d'un seul type d'événements et nous examinons leur applicabilité pour une série de deux ou de plusieurs types d'événements. Les tests statistiques identifiés sont répartis en quatre classes : analyse graphique, tests appliqués au processus de Poisson homogène et non homogène, tests appliqués au processus de renouvellement homogène et les tests de discrimination entre deux processus ponctuels. Ce travail est réalisé avec l'idée d'une application ultérieure dans le cadre de l'analyse du risque.

Les résultats de cette recherche ont montré qu'il n'existe dans la littérature que des tests d'une série d'un seul type d'événements et ils sont, généralement, valables pour les processus ponctuels suivants : Poisson homogène et renouvellement homogène. L'application de ces tests aux séries de deux ou de plusieurs types d'événements est possible dans le cas où les événements sont définis par leurs nombres et leurs temps d'occurrence seulement, i.e. la durée de chaque événement n'est pas prise en considération.

EN :

The design and management of hydraulic structures require a good knowledge of the characteristics of extreme hydrologic events such as floods and droughts, that may occur at the site of interest. Occurrences of such events may be modelled as temporal point processes. This modelling approach allows the derivation of various performance indices related to the design and operation of this infrastructure, as well as to the quantification and management of the associated risks. In this paper, we present statistical tests that may be applied for the modelling of a series of events by temporal point processes. A point process is defined as a stochastic process for which each realisation constitutes a series of points. Although a large body of literature dealt with temporal point processes, very few focused on the analysis of a series of events.

In the present paper we identify two types of series of events: the first represents a series of only one type of event, and the second represents a series of several types of events. The main objective of this research is to comprehensively review the statistical tests applied to the series of one or several types of events and to propose a classification of these tests. This comprehensive review of statistical tests applied to point processes is carried out with the ultimate objective of applying these tests to real case studies within the framework of risk analysis. For example, an extended low-flow event constitutes a risk that may place a water resources system in a state of failure. Thus, it's important to identify and quantify this risk in order to ensure the optimal management of water resources. The modelling of the observed series of events by point processes can provide some statistical results, such as the distribution of number of events or the shape of the intensity function. These results are useful in a risk analysis framework, which includes two steps: risk evaluation and risk management. In the first part of the paper, a review and classification of the various temporal point processes are presented. These include the homogeneous and nonhomogeneous Poisson processes, the Negative Binomial process, the cluster point processes (such as the Neyman-Scott and the Bartlett-Lewis processes), the doubly stochastic Poisson processes, the self-exciting point processes, the homogeneous and nonhomogeneous renewal processes and the semi-markov processes. Also, we illustrate the various links and relationships that exist between these point processes. This classification is elaborated by considering the homogeneous Poisson process as the starting point. The simplicity and the wide use of this process in the statistical and hydrological literature justify this choice.

In the second part of the paper, statistical tests of a series of one type of event are identified. A series of events may be characterised by the number of events, the occurrence times of the events or by the duration of each event. These characteristics are considered as random variables that must be represented by suitable statistical distributions. A series of events may also be characterised by the intensity function, which represents the instantaneous average rate of occurrence of an event. Clearly, the choice of the statistical distribution to model the number of events in a series or the intensity function depends on the nature of the observed data. For example, a stationary series of events may be represented by a constant intensity function. Thus, it is necessary to conduct an analysis of the observed series of the events, such as graphical analysis and statistical testing in order to select and validate the hypothesis underlying the point process model. The hypotheses that may be verified include trend analysis, homogeneity analysis, periodicity analysis, independence of intervals between events, and the adequacy of a given distribution for the number of events and for the time intervals separating events.

In the third part, the applicability of the tests identified in the second part to the case of a series of two or more types of events is examined. In this part, our goal is to analyse the global point process (or the pooled output) obtained by the superposition of the p subsidiary point processes. The decomposition of the global process into p point processes necessitates an identification of each type of event, characterised generally by the number of occurrences and by the intervals between the successive events of the same type. We also examine the applicability of the statistical tests identified in the second part to the case where the global point process is characterised by the duration of each type of event. We investigate more specifically the case of two subsidiary point processes (p=2) where the two event types alternate in the time (an alternating point process). Finally, statistical tests identified in the second part are classified into four categories: tests based on graphical analysis; tests applied to the homogeneous and nonhomogeneous Poisson processes; tests applied to the homogeneous renewal process; and finally tests of discrimination between two specific processes. Theses tests of discrimination include the selection among the Poisson process and the renewal process, the Poisson process and the Binomial point process, and finally, the selection among these three point processes: Cox process, Neyman-Scott process and renewal process.

The results of this research indicate that, in the past, mostly tests for a series of one type of event were presented in the literature. These tests are only valid for the following point processes: a homogenous Poisson process or a homogenous renewal process. The application of these tests to a series of two or several types of events is possible as long as these events are only described by their number and time of occurrence i.e. the duration of each event can not be taken into consideration. Otherwise, these tests are applicable to the alternating point process, which is characterised only by the number and the duration of the two types of events.

FR :

Cet article a été réalisé dans le cadre d'un projet de trois ans visant à développer une méthodologie d'analyse, de prévision et de contrôle des risques d'inondation au Québec. Il présente une approche concrète pour calculer les impacts potentiels et le risque d'inondation et utiliser ces résultats afin d'évaluer la situation du risque local, de décider si les impacts doivent être minimisés et de choisir les moyens d'intervention appropriés. Le risque d'inondation est considéré comme étant le produit de la probabilité d'occurrence des crues et des conséquences occasionnées par ces événements. Les pertes de vies potentielles et les dommages directs sont évalués en simulant les niveaux d'eau de différents scénarios d'inondation à partir d'un modèle d'écoulement unidimensionnel non permanent, et en intégrant ces résultats à un logiciel géoréférencé de calcul des dommages d'inondation. L'analyse des impacts et du risque calculé permet de dresser un portrait du montant des dommages annuels potentiels sur les sites habités le long du cours d'eau et un portrait de l'évolution des impacts en fonction de l'amplitude des crues. Cette analyse mène à l'identification des sites où existe un risque jugé inacceptable selon des critères préétablis. Pour chaque site où des interventions sont justifiées par le niveau de risque, des scénarios de minimisation des impacts tenant compte des mécanismes d'inondation sont élaborés et ensuite simulés afin d'en mesurer l'efficacité. Un exemple d'application à un site de la rivière Châteauguay illustre la méthode et les gains pouvant découler de son utilisation.

EN :

Each year, several rivers in Quebec are responsible for severe flooding and these events generate major socio-economic impacts. The frequency and magnitude of these episodes highlight the existence of a real flood risk. Using global information concerning level and extent of flood risk, authorities would be more likely to make appropriate decisions in the management of flood risk. This article results from a three year project aimed at developing a methodology for the analysis, forecasting and control of flood risk in Quebec. It suggests a concrete approach for the evaluation of the potential impact of floods in order to obtain a better knowledge of local risk in inhabited areas and exploits there results to evaluate the acceptability of the calculated risk and to plan appropriate risk minimisation interventions.

Risk is defined as the product of the mathematical expectation of a specified occurrence with the expected consequences of the event. In floodplain studies, flood risk is the probability of the occurrence at a given flood multiplied by the expected consequences resulting from this event. Different types of consequences may be observed, clearly the easiest to evaluate being direct or material damages and potential loss of life. The risk calculated using the proposed definition is attributable in variable proportions to the frequency of the floods and the amount of damages. A given calculated global risk on a site could be the result of frequent floods, each causing moderate damage or of a single (or more) extreme event, with very low probability of occurrence, but causing severe damage. Risk associated with rare events could be considered as an acceptable risk, a risk we decide to live with, since the resources available to prevent flood damage are often limited and a decision is taken to optimise the allocation of these resources. The flood level corresponding to the limit between acceptable and unacceptable risk must be determined by the population concerned and be based on a good knowledge of the risk situation.

The proposed methodology to evaluate and minimise flood risk for a site localised in a river flood plain involves six steps:

1. the realisation of a hydrologic frequency analysis to determine the amplitude of the floods associated with the flood frequency,

2. the hydraulic simulation of floods to predict water level and velocity in the stream for each scenario,

3. the assessment of direct damage and potential loss of life for each flood simulated,

4. the calculation of risk,

5. the risk analysis considering the limit of acceptable risk and

6. the choice and planning of appropriate intervention to eliminate unacceptable risk.

This approach has been applied to the study of a site along the Châteauguay River, a tributary of the St Laurence River, a river that experiences flood events every two years or so. Seven flood scenarios (the 2, 3, 10, 20, 100, 1 000, and 10 000 year flood) are used to evaluate the risk for a site localised in the village of Huntingdon. Hydraulic characteristics, water level and velocity, associated with each flood scenario are determined using the DAMBRK model, a one-dimensional unsteady flow model. The results are incorporated in DOMINO, a geo-referenced software calculating flood impacts. This software allows the user to create a three-dimensional numerical model of the site based on topographic information. The superposition of hydraulic results provides the flow depth at any point within the site. Damage is evaluated by integrating the municipal roll number of Huntingdon, which provide the site location and value of each building, and gives an estimate of the population threatened by each flood event at the site. These results of direct damages are used to calculate the risk related to each flood event simulated on the Huntingdon site. For this application, the unacceptable risk has generally been agreed to be the risk resulting from the 20 year flood, or more frequent floods, for the material damages and to the number of potential losses of life associated with the 100 year flood or more frequent flood. The analysis indicates that an unacceptable risk of 23 993 $ per year for material damages and potential loss of life of 50 persons exist. Different site scale interventions to eliminate this risk have been simulated and proven to be efficient only if complemented with a few local modifications to the more exposed buildings.

This approach may be extended to the study of any river because it takes into consideration local hydrologic and hydraulic conditions. It has the advantage of being based on existing information and to be automated, which limits the time and resources required to obtain the base data and perform the necessary simulations.

FR :

Dans les rivières graveleuses, il est établi que les structures d'échappement formées dans la zone de recirculation à l'aval d'amas de galets génèrent d'intenses échanges turbulents. Le mécanisme responsable de l'échappement demeure par contre mal connu. Peu d'études sur la dynamique des structures d'échappement ont été réalisées dans des écoulements où le nombre de Reynolds est élevé comme c'est le cas en rivières. De plus, les connaissances actuelles ne tiennent pas compte des découvertes récentes sur la turbulence en rivière à lit de graviers où on a observé des structures de forte et de faible vitesse occupant toute la profondeur de l'écoulement et pouvant durer plusieurs secondes. Ces structures à grande échelle devraient jouer un rôle sur le mécanisme d'échappement étant donné l'influence de la vitesse ambiante sur la dynamique de la zone de recirculation. Nous rapportons les résultats de deux expériences originales sur les liens dynamiques entre les structures à grande échelle et le mécanisme d'échappement en aval d'un amas de galets. La première expérience repose sur l'analyse de corrélations croisées entre des séries de vitesses obtenues au sommet et à l'aval proximal d'un amas de galets. Les résultats montrent que les fortes fluctuations dans le sens de l'écoulement au sommet de l'obstacle sont liées, quelques instants plus tard, à de fortes fluctuations vers l'amont dans la zone de recirculation. La seconde expérience utilise la visualisation des structures d'échappement et la mesure simultanée des vitesses de l'écoulement. L'analyse combinée des images vidéo et de séries de vitesse suggère une relation entre le passage des structures à grande échelle et les manifestations de l'échappement. Ces résultats nous permettent de présenter un modèle où, lors du passage d'un front de haute vitesse, une structure d'échappement se développe et prend de l'expansion vers le lit et vers la surface en se propageant vers l'aval alors que, lors du passage d'un front de faible vitesse, elle s'élève vers la surface de manière plus cohérente. Cette étude propose un nouveau mécanisme d'échappement et révèle le rôle que joue la structure de l'écoulement ambiant sur le développement de structures dans les cours d'eau à lit graveleux.

EN :

The flow structure in a gravel-bed river is closely related to the presence of protruding clasts and of pebble clusters. It is well known that shedding motions from the lee side of large clasts and clusters are a recurrent process that explains the strong exchanges of momentum in river flows. However, shedding has yet to be fully characterised for high Reynolds number flows such as those found in gravel-bed rivers. Moreover, our current understanding of shedding mechanisms does not include the recent discovery that large-scale flow structures in the form of high- and low-speed wedges occupy the entire flow depth over a gravel-bed river. From two original experiments, this paper investigates the influence of these wedges on the nature of shedding in the lee of a pebble cluster. The interactions between the large-scale wedges and shedding may be a key element for understanding flow organisation at the river reach scale. The first experiment provides an analysis of the space-time correlation of velocity time series obtained downstream from a pebble cluster in a natural river. Two pairs of one-minute time series were sampled. The first series of each pair was located in the region of flow separation downstream from the obstacle whereas the second was located at its crest. Results show that a significant negative correlation occurs with a negative time lag for the downstream velocity component. This reveals that a strong downstream velocity vector at the crest of the obstacle is followed 1 to 4 seconds later by a strong upstream velocity vector in the region of flow separation. The strength of the recirculation motion responds to the velocity fluctuations above the cluster. This is a crucial process in the development of vortex shedding. The second experiment aimed at visualising the shedding motion downstream from an obstacle. An underwater camera was used to obtain images of fluid motion in the lee of a pebble cluster while three electromagnetic current meters measured streamwise and vertical velocity fluctuations along a vertical profile downstream from the obstacle. A white tracer was injected in the region of flow separation to depict the development of flow structures that are shed into the flow. Despite the high Reynolds number of the flow, we have obtained good quality images revealing the presence of different modes of vortex shedding initiated in the region of flow separation. From the velocity records, it was possible to identify the large-scale flow wedges and to show that the type of vortex shedding is controlled by high- and low-speed wedges.

Based on these results, we propose a model having two steps: when a high-speed wedge approaches the pebble cluster, the shedding motion develops vertically both towards the water surface and towards the bed as the structures convect downstream; when a low-speed wedge passes, the shedding motion advects mainly towards the surface and it conserves a stronger coherence. This response of the shedding motion to the type of flow wedge is a recurrent and fundamental phenomenon. The results and the model presented herein shed light on the complex nature of vortex shedding in flows at high Reynolds number such as those found in rivers.