Census and typology of braided rivers in the French Alps

Abstract

Following the implementation of the European Water Framework Directive (WFD) and the need to reach a "good ecological status" for rivers, key-questions are being raised about braided rivers. Before any environmental policy can be drawn up, these rivers need to be located, long term changes must be evaluated, and the regional diversity of such systems must be understood, as their inner complexity has not yet been well studied. Therefore, the aim of this work is to carry out a census of the braided channels of the French Alps and to establish a typology based on basic geomorphic indicators. A minimum estimate of the cumulative length of braided rivers prior to major infrastructure construction amounted to 1214 km. Around 53% of these rivers have disappeared during the last two centuries in relation to embankment or channelization, but a loss of 17% is still unexplained. The range in catchment size, mean slope and active channel width has been determined for the Western Alpine braided channels as well as the range in changes due to narrowing, widening and shifting. Seven types of braided rivers have been distinguished based on geographical settings (climate conditions and geology) and differences in terms of adjustment to human pressure on peak flow and sediment delivery. The percentage area of islands in the active channel and the relative length of banks also show a regional difference. Maximum and minimum thresholds of braided activity have been established taking into account the active channel width and the catchment area. The position of the studied reaches between these two thresholds are discussed in relation to position of rivers known in the literature, considering both longterm trends and short-term fluctuations in channel width.

Geomorphological Approaches for River Management and Restoration in Italian and French Rivers

“…This period, characterized by floods with a similar intensity, mirrors those observed at the end of the nineteenth century, notably on the Rhine and the Rhône, for which we have long hydrological series, showing that these events are not unusual within this period of time. As shown by Piégay et al [2009], however, the magnitude of this widening is low if compared to the magnitude of narrowing observed at the contemporary time scale and concurs with the intensity of channel width fluctuations related to flood series, as we have observed on different rivers and notably on the Drôme. Moreover, a new narrowing trend is observed following these 1990s/early 2000s events, notably on the Drôme and other braided rivers located in the French Alps [Hervouet, 2010], and interpreted as a recovery process acting in a fairly stationary trend.…”

Evolución morfo-sedimentaria del tramo medio del río Segre

“…trenzada) a ríos de cauce único y de menor sinuosidad; esta actividad facilita el acorazamiento del lecho y produce incisión y, consecuentemente, elimina además hábitats acuáticos CIG 41 (1), 2015, p. 23-62, ISSN 0211-6820 y riparios de gran interés (por ej. Piégay et al, 2009;Rinaldi, 2003). El impacto no se limita al tramo original de la extracción sino que los efectos pueden desplazarse aguas arriba y abajo (por ej.…”

Variabilidad espacial de los cambios de cauce en una rambla mediterránea en las seis últimas décadas (1946-2006)

“…10), causing an enlargement of the gravel cannel. These destructive processes related to the occurrence of high magnitude floods or to a high frequency of significant flow events have been documented in other Mediterranean rivers (Zanoni et al, 2008;Piégay et al, 2009;Rinaldi et al, 2009;Surian et al, 2009Surian et al, ). -1978Surian et al, -1991.…”