Jean-Paul Bravard

Channel metamorphosis, floodplain disturbance, and vegetation development : Ain River, France

Abstract The purpose of this paper is to describe and explain channel metamorphosis of the Ain River in east-central France and the effects of this metamorphosis on floodplain disturbance and vegetation development. The Ain River is a 195 km long stream originating in the Jura Mountains which flows into the Rhone River between Lyon, France, and Geneva, Switzerland. The lower 40 km of the Ain River, beyond the mountain front, are situated in a valley of outwash deposits where the floodplain is 0.2 to 1.2 km wide. A complex mosaic of floodplain landscape units has developed. Maps dating back to 1766 and six sets of aerial photographs dated between 1945 and 1991 were used to document changes in channel pattern. Aerial photos and field surveys were used to compile maps of landscape units based on dominant vegetation life-forms, species, and substrate. Six maps dated between 1945 and 1991 were digitized in ARC/INFO and an overlay was generated to determine the changes in landscape units as related to channel disturbance. Change from a braided to a single-thread meandering channel probably took place in the period 1930–1950. The process of river entrenchment has occurred throughout the Holocene but has accelerated in the present century due to shortening of the river course, construction of lateral embankments, and vegetation encroachment following reservoir construction and cessation of wood-cutting and grazing. The increase in horizontal channel stability coupled with channel entrenchment have decreased floodplain disturbance and lowered the water table by approximately one meter. Pioneer and disturbance-dependent landscape units have experienced a more terrestrial-like succession to an alluvial forest. Abandoned channels have also been replaced by alluvial forests. On poorly drained soils, shrub-swamp communities of willow and hydrophytic herbaceous plants have been replaced by mixed forests of ash, alder, black poplar, and oak. On well drained alluvial soils, ash and oak dominated hardwood forests have declined in favor of mesophytic stands of black poplar. All types of vegetation, but particularly dry grasslands-shrublands, have been cleared for mines, campgrounds, agriculture, and other types of development. Using several measures, landscape diversity decreased between 1945 and 1991.

Impacts of reforestation and gravel mining on the Malnant River, Haute-Savoie, French Alps

The Malnant River is a rapidly incising river with a French name that translates as ‘‘bad creek,’’ reflecting local opinion of the hazards from dramatic channel changes that have occurred in the last few centuries. Downcutting in the last three decades has created severe problems for farmers in this small watershed (16 km 2 ) as bridges are undermined, streamside roads are threatened, and irrigation diversion structures are rendered unusable. The purpose of our study was to determine the extent and causes of downcutting. A detailed landcover map dated 1732 revealed that forest cover had been reduced by that time to 10% of the present-day cover. The Malnant was strongly affected by floods and debris torrents during the 18th and 19th centuries that delivered massive amounts of sediment. During the 20th century, reforestation reduced the sediment delivery from hillslopes. In addition, gravel extraction in the Malnant and in the Fier River (of which the Malnant is a tributary) has lowered the base level for the river. This initiated a knickpoint that moved upstream. Weirs placed in the Malnant in 1968 were used to measure rates of bed incision in the field. With a mean width of 4.0 m and degradation up to 36 cubic meters per meter channel length, the lower 4.5 km of the Malnant has experienced a net loss of approximately 163,000 m 3 of bed material. Above the 4.5-km point on the

Channel incision, gravel mining and bedload transport in the Rhône river upstream of Lyon, France ("Canal de Miribel")

The Miribel canal is a former arm of the Rhone embanked between 1848 and 1857 over a length of 18 km to improve navigation at low discharges. The impact of this was a hydraulic tilting of the long profile characterised by 4 m of degradation upstream and 4–6 m of aggradation of bedload downstream. This phenomenon increased downstream flooding. Since 1937 a diversion dam has controlled upstream water input, thus reducing the transit of the pebble bedload. However, excessive harvesting of sands and gravels occurred between 1970 and 1980, resulting in a general lowering of the river bed and the accompanying water table with ecological consequences in the alluvial plain and for water supply. This development made it all the more necessary to obtain knowledge about the bedload discharges passing through the Miribel canal, and more broadly about the hydraulic conditions as functions of the varying discharge. Calculation of shear stresses and grain size measurements on the lateral bars after several floods in 1989-90 show that movement of bed-material is initiated at a discharge of 440 m3 · s−1 (equalled or exceeded 40 days · year−1), and becomes general at 550 m3 · s−1 (equalled or exceeded 30 days · year−1). Transport discharge is thus relatively frequent and involves distal fluvio-glacial deposits composed of fine-grained materials. Potential transport calculated by the Meyer-Peter formula is around 60,000 t · year−1 for the range of discharges between 440 and 850 m3 · s−1. For these discharge values, the canal experiences a loss of materials without replacement from upstream; for higher rates of discharge, the floodgates let through an unknown quantity of materials which partially make up the loss. Gravel harvesting ceased in 1991 but the diversion dam will have to be operated in a different way in order to increase the input of bedload into the canal.

Principles of engineering geomorphology for managing channel erosion and bedload transport, examples from French rivers

Abstract 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.

The Pliocene-Quaternary tecto-sedimentary evolution of the Larissa Plain (Eastern Thessaly, Greece)

In order to understand the present-day morphological, geographical and environmental patterns of the Larissa Plain, a large amount of historical, archaeological, sedimentologi-cal, stratigraphic, t...

Radar signatures and structure of an avulsed channel: Rhône River, Aoste, France

Near Aoste, France, an avulsion shifted the Rhone River from a southwesterly course through the valley of the Marais des Avenieres to its present northwesterly course in the Basses Terres. Ground penetrating radar (GPR) reflection paterns from the modern and avulsed channel are used to reconstruct and interpret the fluvial architecture of the avulsed channel deposits. A grid of GPR lines was run across and along the avulsed course of the Rhone. The radar reflections have impressive continuity and, for the most part, horizontal to gentle dips (1–2°); although, in places, the reflections have steeper dips (averaging 6°), which are often accompanied by mound-like reflection patterns. Draped over or stacked adjacent to the mounds are packages of dipping reflections which form an en echelon stacking pattern, which is interpreted as being produced by the downstream growth of gravel bars. The cross-valley profiles are composed of stacked concave reflections which are laterally continuous with dips that vary from 1° to a maximum of 8° along the flanks of the concavities. The complete assemblage of these multistoried concave reflection patterns is interpreted as a suite of complete or partial channel fills. The coarse-grained sediments of the pre-avulsion Rhone probably represents a transitional phase between a coarse-grained, chute-modified meandering depositional system and a wandering gravel-bed river depositional style.

Energy gradient and geomorphological processes along a river influenced by neotectonics (the Saône river, France)

Abstract The purpose of this study is to link the sediment transit and the flood plain storage of the Saone to hydromorphological characteristics of the hydrosystem, which exemplifies a new approach to sediment dynamics. The study of suspended sediment concentration in terms of temporal evolution, together with sediment deposition in terms of spatial variability, is a way to record the longitudinal evolution of the sediment load, which expresses the available energy gradient from upstream to downstream in hydrosystem. The Saone river is a 480-km-long Rhone tributary, with an oceanic pluvial regime, and an average yearly discharge of 440 m s−1 at Lyons. The watercourse is characterised by very gentle slopes controlled by the neotectonics of the Bresse trough and by Holocene fluvial dynamics. Sediments were sampled during the December 1993–January 1994 flood (2 375 m3 s−1) and the 1995 January–February flood (1 826 m3 s−1). A fine partition into homogeneous sectors, using stream power as well as shear stress, has been realized on a 400 km reach using longitudinal and cross-sections at one kilometre intervals. This partition, compared with the results of the field sampling, shows that the amount of energy is closely connected to the hydromorphological characteristics of the river.