Long Profile Responses of Alpine Braided Rivers in Se France

Abstract

Channel elevation change since the early 20th century was characterized for 31 alpine braided river reaches (total length of 129 km), representing about 20% of the braided river network length in SE France. Present‐day long profiles were surveyed in the field and compared with historical profiles measured between 1894 and 1930. Forty‐four cross sections of active channels including terrace boundaries were also surveyed to study morphological signatures of braided channels characterized by different channel responses (aggradation or degradation). Results show that 56% of the sampling river length is characterized by degradation, 24% by stability and 20% by aggradation. Gravel mining is recognized as the main factor explaining degradation of braided channels. It is also demonstrated that aggrading braided channels are those where sediment supply from active torrents and bank erosion have been best preserved. The comparison of morphological signatures associated with aggrading and degrading channels reveals significant differences in cross‐section morphometry. A normalized bed relief index is proposed as a reliable indicator of the channel response. Field observations show that long‐term aggraded braided channels have a lower bed relief index than degraded channels, as already established by some laboratory experiments. This index can be used to determine the position of the braided channel in the cycle of degradation‐recovery following the cessation of gravel mining. It is also established that aggraded channels develops wider active channels than degraded channels for a same drainage area. This study provides new insights for the assessment of spontaneous recovery potential of braided channels impacted by gravel mining. Copyright © 2012 John Wiley & Sons, Ltd.

What is the Relative Effect of Space and Time to Explain the Braided River Width and Island Patterns at a Regional Scale?

“…Western reaches have a lower slope and altitude values (types 1 and 2) compared to eastern reaches (types 4 and 5 and in part 3).Eastern reaches are strongly influenced by higher activity in terms of sediment delivery. They display higher W* values and a widening pattern, and some of these reaches are also aggraded (Liébault et al ., ). This may also contribute to the temporal gap in vegetation development, for a similar flood history.Regional climatic differences reinforce the west–east trend.…”

“…Liébault et al . () showed that for braided reaches locally affected by gravel mining, the channel readjustment has not been observed in all reaches concerned right after the end of mining activities in the 1990s.…”

“…Indeed, widening patterns correspond regionally to aggraded sectors as demonstrated by Liébault et al . (), who state that such a situation is possible in some areas with low human density and, as a direct consequence, less regulated torrential activity. In contrast, heavily narrowed reaches are located where human impact has been sensibly stronger (i.e.…”

Riparian vegetation as indicator of channel adjustments and environmental conditions: the case of the Panaro River (Northern Italy)

“…A series of geomorphic field surveys was carried out between 2006 and 2007 along the entire study reach, both validating and extending bibliographic information on past bed-elevation changes and present trends of vertical adjustments. Field evidence mainly included the differences in elevation between homologous geomorphic surfaces, such as: (1) between a low terrace identified as being a floodplain from past aerial photographs or other sources and the present active floodplain (Rinaldi 2003); (2) between wooded terraces deriving from abandoned channel surfaces and the top of active channel bars (Liébault et al 2013);and (3) between the top of a gravel lens visible on an eroding bank and the top of gravel in the present bed.…”

The use of remote sensing to characterise hydromorphological properties of European rivers

“…It is worth emphasising, though, that RS data will never substitute the wide range of data sources that currently contribute to the accurate assessment of current river behaviour, geomorphological sensitivity and the evaluation of future trajectories (Simon and Rinaldi 2006;Liébault et al 2013;Grabowski et al 2014). Expert interpretations, field surveys and historical analysis will remain important ways of reading the landscape (Brierley et al 2013), but RS data will support and corroborate conclusions drawn from these sources.…”