Le réseau hydrographique théorique (RHT) français et ses attributs environnementaux

Hervé Pella, Jérôme Lejot, Nicolas Lamouroux, Ton Snelder
2012 Géomorphologie relief processus environnement  
Géomorphologie : relief, processus, environnement vol. 18 -n° 3 | 2012 Varia Le réseau hydrographique théorique (RHT) français et ses attributs environnementaux The theoretical hydrographical network (RHT) for France and its environmental attributes Hervé Pella, Jérôme Lejot, Nicolas Lamouroux et Ton Snelder Édition électronique Référence électronique Hervé Pella, Jérôme Lejot, Nicolas Lamouroux et Ton Snelder, « Le réseau hydrographique théorique (RHT) français et ses attributs
more » ... uts environnementaux », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 -n° 3 | 2012, mis en ligne le 04 novembre 2014, consulté le 29 avril 2019. URL : http://journals.openedition.org/geomorphologie/9933 ; DOI : 10.4000/geomorphologie.9933 © Groupe français de géomorphologie Abridged English version The assessment and management of streams at the catchment and larger scales involves description of the environmental characteristics of stream segments over entire river networks (Snelder and Biggs, 2002; Snelder et al., 2005). Such assessments require the use of connected and oriented hydrographical networks that enable the computation of catchment environmental attributes. This need is reinforced with the multiplication of integrated large-scale environmental policies such as the Water Framework Directive in Europe. This paper describes the derivation, the properties and the potential of a theoretical hydrographical network for France, the RHT. The RHT is an oriented network along which a number of reach-scale and catchment-scale environmental attributes were estimated. The network is derived from the BD Alti ® 50-m digital elevation model (DEM) of the French Geographic National Institute (IGN). Before deriving the RHT, the DEM was modified using the RHE hydrographical network, the latter being a simplification of the referential hydrographical network of the IGN, BD Carthage ® . The RHE is an oriented hydrographical network that includes 221,077 joined segments (Pella et al., 2008; tab. 1 and fig. 1 ). In contrast to the BD Carthage ® , the oriented and joined segments of the RHE enable the navigation of the network in the upstream and downstream direction, and the accumulation of distances and reach attributes from river sources to mouths (tab. 2). However, it is impossible to associate unique drainage Géomorphologie : relief, processus, environnement, 2012, n°3, p. 317-336 Le réseau hydrographique théorique (RHT) français et ses attributs environnementaux The theoretical hydrographical network (RHT) for France and its environmental attributes Résumé Le réseau hydrographique théorique (RHT) est un nouveau réseau hydrographique dérivé de la BD Alti ® de l'Institut Géographique National (IGN). La méthode « Agree » utilisée pour construire ce réseau permet de modifier un modèle numérique de terrain à partir d'un réseau d'écoulement pré-défini. Ainsi, le RHT est développé à partir du modèle numérique de terrain de la BD Alti ® re-conditionné par le RHE (Réseau Hydrographique Étendu), qui est lui-même une simplification du réseau hydrographique de référence de l'IGN, BD Carthage ® . La compatibilité entre le RHT et la BD Alti ® permet d'identifier les bassins versants et de simuler des écoulements avec une meilleure précision. Cette approche permet d'intégrer un ensemble d'attributs spatialisés et de les cumuler le long du réseau. Ainsi, des attributs topographiques, hydrologiques et climatiques sont calculés et intégrés dans un système d'information géographique. Des comparaisons et des tests ont été réalisés pour décrire la couverture spatiale du nouveau réseau et valider le calcul des attributs environnementaux. Les perspectives concernent de multiples aspects de la gestion intégrée des bassins. Mots clés : attributs environnementaux, SIG, MNT, hydrologie, gestion des cours d'eau. Abstract The theoretical hydrographical network (RHT) is a new digital hydrographical network derived from the BD Alti ® digital elevation model of the French Geographic National Institute (IGN). We used the 'Agree' method to build the RHT network. This method modifies the topography of the digital elevation model to make it consistent with a vector coverage corresponding to the observed stream network. Specifically, the RHT is developed from the BD Alti ® modified by the RHE network, the latter being a joined and oriented simplification of the observed hydrographical network of the IGN, BD Carthage ® . The consistency between the RHT and the BD Alti ® enables a precise identification of catchments and improved simulations of flows. This approach enables the integration of several spatial attributes and their accumulation along the network. We calculated various topologic, hydrologic and climatic attributes for the RHT and integrated them in a geographic information system. Comparisons and tests have been made to evaluate the new hydrographical network and its environmental attributes. The network has many potential applications associated with integrated basin management. basins with each segment of the RHE due to many inconsistencies between the RHE and the DEM. Therefore, the accumulation of basin characteristics for this network is not possible. This means that catchment characteristics, e.g. cumulative specific discharges, average rainfall patterns or catchment slopes, cannot be derived for the RHE. Because it is derived from the DEM, the RHT (fig . 3 and fig. 4 ) resolves these problems and enables the accumulation of catchment properties along the network. The 'Agree' method (Hellweger, 1997; Maidment and Djokic, 2000) was used to modify the DEM by the observed RHE network. This method lowers the altitudes of all DEM cells corresponding to the RHE and smoothes the DEM (fig. 2) . A theoretical network was then derived from the modified DEM using the D8 conventional method (O'Callaghan and Mark, 1984) . The resulting network is both consistent with the DEM (enabling aeral calculations) and close to the observed hydrographical network. The automatic derivation of the network by the D8 method was not perfect and required corrections. In particular, topological errors and inconsistent segments appeared in flat areas (fig. 5). They were automatically detected and manually corrected ( fig. 6 ). The final RHT represented 283,639 km and comprised 114,601 segments, which were associated with various topographic, climatic and hydrologic attributes (tab. 3). Fig. 7 describes the methods used to derive the attributes and fig. 8 shows some results in a selected catchment. The calculation of environmental attributes for the RHT was verified by comparing the results with independent calculations made for a simpler hydrographical network of France representing 92,838 km, the DP (Pella et al., 2006) . Because the DP was derived from the partitioning of France into major sub-basins provided by the IGN, the calculation of aeral attributes was also possible on this simplified network. After identifying corresponding pairs of segments in the RHT and the DP, we compared their altitude, slope, distance to the source, basin-area, temperature, and discharge characteristics. All attributes except segment slope were closely related between the two networks (R2 > 0.9; P < 0.001; fig. 10 ). Segment slope calculations were consistent between the two networks (R2 = 0.42; P < 0.001), but their correspondence was less accurate due to fact that the paired segments could have different lengths and different upstream and downstream altitudes. Additional environmental attributes and a variety of ecological models can be applied on the RHT. For example, additional rainfall data was calculated on the network to derive a typology of flow regimes and generalised models of bed sediment size (Snelder et al., 2008 (Snelder et al., , 2011. Ongoing works allow the calculation of hydraulic characteristics, habitat variables (Lamouroux, 2008) and the theoretical distribution of aquatic species (Oberdorff et al., 2001). Together with national data on weirs and dams, these models can provide an objective basis for assessments associated with integrated basin management including issues dealing with e.g. migratory species, connectivity restoration and sediment fluxes (Norris et al., 2007).
doi:10.4000/geomorphologie.9933 fatcat:fwsd4yjafza6dc2aovffrv5k54