Claude Bocquillon

Criteria for the choice of flood-routing methods in natural channels

Abstract The Saint-Venant equations are used to describe river waves. Generally, for flood routing in rivers, the Saint-Venant system is reduced to the diffusive wave equation which can be resolved using finite-difference algorithms. The choice of a numerical method, and of the space and time steps to be retained, depends essentially on the form of flood hydrographs and the hydraulic properties of the river. This paper investigates these areas; two sets of criteria are propossed, the first to define parameter ranges representing each wave type and then, in the particular case of the diffusive wave model, to define criteria for the choice of numerical algorithm and appropriate space and time steps. The first analysis was based on the concept that river wave behaviour is determined by the balance between friction and inertia. The conclusions relate to the magnitude of temporal characteristics of flood waves, expressed as a function of the Froude number of the steady uniform flow and a dimensionless wave number of the unsteady component of the motion. The second part discussed questions related to the diffusive wave problem and to numerical instabilities. A technique is proposed to guide the user in the choice of the computational algorithm and specifies the error introduced by numerical methods. The technique was applied to flood-routing simulation for the Loire river in France. In this case, two finite-difference algorithms were compared to the exact solution given by the analytical method. Comparisons between results show the efficiency of the technique to optimise the choice of the finite-difference method and the adequate space and time steps.

Fractal analyses of tree-like channel networks from digital elevation model data

Digital elevation models (DEMs) are generally used to automatically map the channel network and to delineate subbasins. The most common approach to extract a channel network from DEMs consists of specifying a threshold area S which is the minimum area required to drain to a point for a channel to form. This threshold area is usually specified arbitrarily, although it is recognized that different threshold areas will result in substantially different channel networks for the same basin. In this paper, we study the effect of S (that is also the scale of observation) on the morphometric properties (external and internal links, length of drainage paths, mainstream length) and scaling properties (such as Hortons and Strahlers laws, and fractal dimension). Three basins, located in southern France, were extensively studied. The results indicate that morphometric properties vary considerably with S, and thus values reported without their associated S should be used in hydrologic analysis with caution. Then, the fractal geometry is used to take into account the dependence of measured values on observation scales, which is not possible with classical hydrological indexes. The use of fractals allows, first, to point out self-similarity in the structure of channel networks and then to quantify the tree-like organization. New catchment shape indexes, independent of the observation scale S, are defined. These indexes are useful for comparing catchments and for measuring the irregularity level of the channel network.

Hydrological impact simulations of climate change on Lebanese coastal rivers / Simulations des impacts hydrologiques du changement climatique sur les fleuves côtiers Libanais

Abstractn The significance of predicted climatic changes is still uncertain. The hydrological consequences of climatic changes on Lebanese catchments are analysed by means of different scenarios of rainfall variability and temperature increase. The conceptual rainfall–runoff model MEDOR, coupled to a stochastic model of rainfall and temperature, is used to estimate change in runoff by simulation of six scenarios. These test the response to the rainfall structure, to the duration of rainy events, their frequency, and the duration of the rainy season. The climate–runoff model is used to determine the impact of a temperature increase of 2 degrees on the flow characteristics of a watershed affected by seasonal snow cover. The modifications of the hydrological regimes are significant: droughts are predicted to occur 15 days to one month earlier; snowmelt floods are often replaced by rainfall floods; and the peak flow occurs two months earlier. These changes could have a great impact on water resources management in the future. Résumé La significativité des changements climatiques prévus reste incertaine. Les conséquences hydrologiques des changements climatiques sur des bassins versants Libanais sont analysées au moyen de différents scénarios de variabilité de la pluie et daugmentation de la température. Le modèle pluie–débit conceptuel MEDOR, couplé à un modèle stochastique de pluie et de température, est utilisé pour estimer le changement dans lécoulement via la simulation de six scénarios. Ceci permet de tester la réponse à la structure de la pluie, à la durée et la fréquence des événements pluvieux, et à la durée de la saison pluvieuse. Le modèle climat–débit est utilisé pour déterminer limpact dune augmentation de la température de 2 degrés sur les caractéristiques de lécoulement dun bassin versant présentant un couvert neigeux saisonnier. Les modifications des régimes hydrologiques sont significatives: les étiages sont plus précoces de 15 jours à un mois; les crues nivales sont souvent remplacées par des crues pluviales; et les écoulements extrêmes apparaissent deux mois plus tôt. Ces changements pourraient avoir un grand impact sur la gestion des ressources en eau dans le futur.

Morphologie fractale du réseau hydrographique

Resume Le logiciel TraPhyC-BV a ete elabore pour extraire des donnees des modeles numeriques de terrain, les composantes de lhydrologie: reseau de drainage, reseau hydrographique, sous-bassins versants et caracteristiques hydrogeomorphometriques intervenant dans le fonctionnement du bassin versant. A differentes echelles dobservation, le reseau hydrographique etudie presente une structure tres variable. On a fait appel a la geometrie fractale pour identifier des relations “grandeur mesuree/jauge de mesure” afin de quantifier larborescence et detecter des proprietes invariantes dechelle.