Norbert Landon

Le phenomene d'incision des cours d'eau est bien connu dans les Alpes du nord et sur certains cours d'eau des Alpes du sud mais qu'en est-il des affluents dromois et ardechois du Rhone ? Nous pouvons nous demander si ces cours d'eau, dont les caracteristiques hydroclimatiques et les usages sont plus proches des systemes mediterraneens sont marque par cette tendance. Effectuer une analyse synthetique de la situation, tel est le premier objet de notre travail (partie 1). Apres avoir caracterise les contrastes entre les piemonts des Prealpes dromoise et du massif central ardechois (chap. L), nous etablissons un diagnostic a propos de l'etat physique des cours d'eau retenus (chap. 2) : le Doux, l'Eyrieux, l'Escoutay, l'Ardeche, la Galaure, l'Herbasse, la Veore, la Drome, le Roubion, le Jabron, le Lez, l'Aygues. Outre les specificites de leur bassin, l'evolution de leur profil en long et les contraintes subit par leurs plaines alluviales sont etudiees. Typique et representatif de notre terrain d'etude, nous avons consacre une grande partie du travail a la Drome. Du constat de l'incision et de ses impacts (partie 2), a la recherche des causes (partie 3), nous avons cherche a comprendre les changements enregistres. Par volonte de rendre operationnel ce travail de recherche et apres avoir determine quels sont les facteurs de l'evolution de la Drome, nous abordons le probleme de la gestion des sediments. Elle se pose vis-a-vis des potentialites de recharge du cours d'eau et de sa capacite a vehiculer la charge de fond. La quatrieme partie de notre travail permet d'apporter des elements de reponses. Elle s'interesse aux potentialites de recharge a l'echelle du bassin versant (chap. 1) et permet de dresser un premier bilan sedimentaire qui s'affine dans le second chapitre consacre a la recharge sedimentaire a l'echelle de la plaine alluviale de la Drome. Ce bilan est ensuite complete par une analyse de la mobilite des materiaux a l'echelle du troncon afin d'apprecier les conditions de reversibilite du processus d'incision. A chaque etape du bilan, des regles de gestion sont presentees. En dernier lieu, nous posons le probleme de la transposabilite de cette recherche aux autres cours d'eau et bassins versants etudies dans le cadre de notre premiere partie.

ABSTRACT Erosion of the river beds linked to human activities is currently a crucial issue. By inducing strong changes in river dynamics, this erosion alters many of the related ecological processes. The modification of the relationships between the river and its aquifer due to erosion is not really understood. The large scale consequences of erosion for the interactions between river and the underlying aquifer must be clarified, together with the small scale effects of river erosion on hyporheic flowpaths. The role of water exchange within the hyporheic zone on the distribution of interstitial invertebrates is universally recognized, but nothing is known on the potential links between riverbed erosion, the subsequent modification of water exchange at a local scale and their consequences on the distribution and biomass of hyporheic invertebrates.Two sections, stable versus eroded, were studied on the Herbasse, a third-order stream, by sampling hyporheic water and invertebrates at two depths within the substrate (−15 and −50 cm deep), upstream and downstream of ten riffles. Seven physical and chemical parameters were used to identify the hydrological flowpaths at both the riffle (up and downwelling areas) and the section scales (eroded vs stable) and to specify the level of nutrients in the hyporheic habitat.Significant differences were observed between the stable and eroded sections both in the vertical and longitudinal dimensions, with for example available oxygen decreasing with depth and along hyporheic flowpaths, and revealing more obvious differences in the eroded section gaining groundwater than in the stable section. Most of the hypogean organisms (Niphargus sp., Salentinella juberthiei, and Proasellus sp.) were most abundant at the deepest level, in the upwelling zones and the eroded section, while typical epigean taxa (Gammarus sp., Plecoptera and Ephemeroptera) were mostly present near the surface and in the stable section, suggesting that erosion processes may influence the development of hyporheic assemblages at riffle and reach scales.

Soumis à une pression humaine de plus en plus importante depuis le XVIIIe siècle, les talwegs des cours d'eau ardéchois et drômois se sont profondément modifiés. Déprise agropastorale, bassins versants stabilisés, endiguements, extraction de granulats sont autant de facteurs explicatifs de l'incision observée sur de nombreux tronçons. L'impact cumulé des politiques d'aménagement et de gestion est en grande partie responsable de cette évolution. Si la loi sur l'eau de 1992 affirme la nécessité d'une gestion intégrée permettant de satisfaire les usages dans le respect des équilibres naturels, il n'en a pas toujours été de même et certains intervenants préfèrent encore favoriser l'évolution en cours. Malgré tout, de nouveaux outils de gestion, comme les schémas d'aménagement et de gestion des eaux (S.A.G.E.) devraient permettre aux gestionnaires de promouvoir la restauration des profils en long et, plus généralement, celle du milieu physique.

... their ecological functions (Forman and Godron, 1986) managers should attempt to rehabilitate affected areas that show good ecological potential. In disconnected sections (notably downstream of Livron: Figure 7) characterized by ageing vegetation 296 H. PIEÂ GAY AND N ...

Soumis à une pression humaine de plus en plus importante depuis le XVIIIe siècle, les talwegs des cours d'eau ardéchois et drômois se sont profondément modifiés. Déprise agropastorale, bassins versants stabilisés, endiguements, extraction de granulats sont autant de facteurs explicatifs de l'incision observée sur de nombreux tronçons. L'impact cumulé des politiques d'aménagement et de gestion est en grande partie responsable de cette évolution. Si la loi sur l'eau de 1992 affirme la nécessité d'une gestion intégrée permettant de satisfaire les usages dans le respect des équilibres naturels, il n'en a pas toujours été de même et certains intervenants préfèrent encore favoriser l'évolution en cours. Malgré tout, de nouveaux outils de gestion, comme les schémas d'aménagement et de gestion des eaux (S.A.G.E.) devraient permettre aux gestionnaires de promouvoir la restauration des profils en long et, plus généralement, celle du milieu physique.

: This paper shows how written archives, historical cross-sections and longitudinal profiles, old maps, aerial photographs and supplementary field measurements can be used together to reconstruct the evolution of a river's channel geometry in relation to changes affecting its catchment area during the contemporary period. A reach of the Drôme river upstream of Luc en Diois was chosen for study because of its original geomorphological features and the fact that abundant and complementary data were available. The written archives were used to obtain a precise date of different events such as floods and works carried out within the valley (railway and road construction and restoration work). These elements were used to define the chronology of geomorphological changes identified from analyses of topographical information. These data showed that sedimentation in the Grand Lac plain was active during the modern period but slowed during the 19th century and was almost negligible after this period. This observation is explained by works conducted within the plain which reduced its efficiency to trap sediments, but also climate and landuse changes which took place during the contemporary period. Spontaneous catchment afforestation, mainly observed between 1945 and 1970, is a key-factor which limits the present bedload input within the river system.

Résumé/Abstract La notion d'espace de liberté, mise en avant lors des Assises Nationales de l'Eau en mars 1991, est aujourd'hui reprise par les gestionnaires qui souhaitent promouvoir son application dans le cadre de la Loi sur l'eau du 3 janvier 1992 (Schémas départementaux des carrières et Schémas d'Aménagement et de Gestion des Eaux-SDAGE). Le SDAGE Rhône-Méditerranée-Corse la définit comme l'espace du lit majeur dans lequel le chenal ou les chenaux fluviaux assurent des translations latérales ...

La plupart des scientifiques qui ont etudie l'effet des accumulations de bois mort dans les cours d'eau de tete de bassin versant ont fait reference a des cours d'eau nord-americains. Dans le cadre de cet article, nous analysons les caracteristiques des elements ligneux et leurs effets sur les conditions hydrauliques, granulometriques et topographiques du lit dans un contexte geographique europeen, celui d'un cours d'eau situe en tete de bassin versant, le Doulon, bassin de l'Allier, dont les rives ne sont plus entretenues depuis le milieu du 20ieme siecle. Nous avons mis en evidence une origine essentiellement ripicole des structures ligneuses presentes dans le lit et une diversification des conditions hydro-geomorphologiques liees a la presence de ces structures malgre des quantites relativement faibles (7 a 32 m3.ha-1). Ces structures ne sont pas equi-reparties sur un continuum, leur distribution etant regie par differents facteurs. De fait, les mecanismes...

Du constat recurrent de l'incision des lits fluviaux aux actions entreprises actuellement pour favoriser la recharge et le transit des materiaux dans les cours d'eau francais de montagne et de piemont, cet article a pour objectif de dresser un etat de la question et de temoigner des actions entreprises dans le cas de la riviere Drome (premier S.A.G.E. mise en place en France par arrete prefectoral en decembre 1997).

Le phenomene d'incision des cours d'eau est bien connu dans les Alpes du nord et sur certains cours d'eau des Alpes du sud mais qu'en est-il des affluents dromois et ardechois du Rhone ? Nous pouvons nous demander si ces cours d'eau, dont les caracteristiques hydroclimatiques et les usages sont plus proches des systemes mediterraneens sont marque par cette tendance. Effectuer une analyse synthetique de la situation, tel est le premier objet de notre travail (partie 1). Apres avoir caracterise les contrastes entre les piemonts des Prealpes dromoise et du massif central ardechois (chap. L), nous etablissons un diagnostic a propos de l'etat physique des cours d'eau retenus (chap. 2) : le Doux, l'Eyrieux, l'Escoutay, l'Ardeche, la Galaure, l'Herbasse, la Veore, la Drome, le Roubion, le Jabron, le Lez, l'Aygues. Outre les specificites de leur bassin, l'evolution de leur profil en long et les contraintes subit par leurs plaines alluviale...

Abstract This work maps and interprets the evolution of the urban footprint of Phnom Penh from 1973 to 2015 and reviews its main socio-ecological impacts. The quantified patterns of urban growth and land-use change are based on the processing and analysis of Landsat satellite images (MSS, TM, and OLI sensors) and are enhanced by observation- and interview-based information obtained in the field. The growth of Phnom Penh is shown to have encroached initially on the fertile agricultural lands of the Mekong River floodplain, but since 2006 the city has been sprawling predominantly over natural lakes and wetlands that until then were functional components of the urban mosaic and underpinned the livelihoods of its population. Urban land areas increased from 3000 ha in 1973 to 4000 ha in 1990, subsequently soaring to 25,000 ha in 2015, i.e. an average annual increase of 850–1000 ha. The discussion of these changes focuses on (i) changing livelihoods on the urban fringe; (ii) the numerous, and sometimes large, real-estate projects that have sprung up around the city centre on former wetland areas and are funded by strongly imbricated capital interests between the nation’s elites and foreign investors; (iii) a comparison of the urban growth figures with other Asian cities; and (iv) current land policies in Cambodia. The results call for urgent policymaking to address the diseconomies that arise from the impending loss of ecosystem services; from the accentuation of flood hazards caused by the rapid, ongoing suppression of wetland habitats at the delta head of the eighth largest river in the world; and from the unequal distribution of the costs and benefits of urban growth among the metropolitan population.

ABSTRACT Erosion of the river beds linked to human activities is currently a crucial issue. By inducing strong changes in river dynamics, this erosion alters many of the related ecological processes. The modification of the relationships between the river and its aquifer due to erosion is not really understood. The large scale consequences of erosion for the interactions between river and the underlying aquifer must be clarified, together with the small scale effects of river erosion on hyporheic flowpaths. The role of water exchange within the hyporheic zone on the distribution of interstitial invertebrates is universally recognized, but nothing is known on the potential links between riverbed erosion, the subsequent modification of water exchange at a local scale and their consequences on the distribution and biomass of hyporheic invertebrates.Two sections, stable versus eroded, were studied on the Herbasse, a third-order stream, by sampling hyporheic water and invertebrates at two depths within the substrate (−15 and −50 cm deep), upstream and downstream of ten riffles. Seven physical and chemical parameters were used to identify the hydrological flowpaths at both the riffle (up and downwelling areas) and the section scales (eroded vs stable) and to specify the level of nutrients in the hyporheic habitat.Significant differences were observed between the stable and eroded sections both in the vertical and longitudinal dimensions, with for example available oxygen decreasing with depth and along hyporheic flowpaths, and revealing more obvious differences in the eroded section gaining groundwater than in the stable section. Most of the hypogean organisms (Niphargus sp., Salentinella juberthiei, and Proasellus sp.) were most abundant at the deepest level, in the upwelling zones and the eroded section, while typical epigean taxa (Gammarus sp., Plecoptera and Ephemeroptera) were mostly present near the surface and in the stable section, suggesting that erosion processes may influence the development of hyporheic assemblages at riffle and reach scales.

ABSTRACT Erosion of the river beds linked to human activities is currently a crucial issue. By inducing strong changes in river dynamics, this erosion alters many of the related ecological processes. The modification of the relationships between the river and its aquifer due to erosion is not really understood. The large scale consequences of erosion for the interactions between river and the underlying aquifer must be clarified, together with the small scale effects of river erosion on hyporheic flowpaths. The role of water exchange within the hyporheic zone on the distribution of interstitial invertebrates is universally recognized, but nothing is known on the potential links between riverbed erosion, the subsequent modification of water exchange at a local scale and their consequences on the distribution and biomass of hyporheic invertebrates.Two sections, stable versus eroded, were studied on the Herbasse, a third-order stream, by sampling hyporheic water and invertebrates at two depths within the substrate (−15 and −50 cm deep), upstream and downstream of ten riffles. Seven physical and chemical parameters were used to identify the hydrological flowpaths at both the riffle (up and downwelling areas) and the section scales (eroded vs stable) and to specify the level of nutrients in the hyporheic habitat.Significant differences were observed between the stable and eroded sections both in the vertical and longitudinal dimensions, with for example available oxygen decreasing with depth and along hyporheic flowpaths, and revealing more obvious differences in the eroded section gaining groundwater than in the stable section. Most of the hypogean organisms (Niphargus sp., Salentinella juberthiei, and Proasellus sp.) were most abundant at the deepest level, in the upwelling zones and the eroded section, while typical epigean taxa (Gammarus sp., Plecoptera and Ephemeroptera) were mostly present near the surface and in the stable section, suggesting that erosion processes may influence the development of hyporheic assemblages at riffle and reach scales.

Du constat récurrent de l’incision des lits fluviaux aux actions entreprises actuellement pour favoriser la recharge et le transit des matériaux dans les cours d’eau français de montagne et de piémont, cet article à pour objectif de dresser un état de la question et de témoigner des actions entreprises dans le cas de la rivière Drôme (premier S.A.G.E. mise en place en France par arrêté préfectoral en décembre 1997).

... their ecological functions (Forman and Godron, 1986) managers should attempt to rehabilitate affected areas that show good ecological potential. In disconnected sections (notably downstream of Livron: Figure 7) characterized by ageing vegetation 296 H. PIEÂ GAY AND N ...

French rivers have been transformed by centuries of development to satisfy various social demands such as navigation, hydro-electric power production, flood control, or checking erosion. These modifications have led to impacts that are detrimental to ecology, to resources, and to human interests. In recent years, increased public and management awareness of these problems has given way to new conceptions and objectives in the field of river engineering, among which the goal of establishing sustainable long-term management strategies is of the highest importance. Because many of these problems are directly or indirectly related to changes in the geomorphological functioning of fluvial systems, there is a need to integrate a geomorphological approach into river engineering practices. The principles of “engineering geomorphology” are developed here and are illustrated by examples taken from rivers that drain the French Alps and their piedmont.