Mélanie Bertrand

Debris-flow susceptibility of upland catchments

Over the last three decades, many regional studies in mountain ranges under temperate climate revealed that it is possible to discriminate debris-flow and fluvial fans from morphometric indicators measured at the scale of the catchment and the fan itself. The most commonly used indicators are the Melton index (R), a normalized index of the gravitational energy of the catchment, and the fan slope (S). A wide range of thresholds have been proposed for discriminating purpose, but these are generally based on a small population of catchments and may be highly influenced by ambiguous fans included in the data set. A database of 620 upland catchments from several mountain ranges under temperate climate was compiled from the literature to propose robust discriminant morphometric thresholds for debris-flow versus fluvial responses. Linear discriminant analysis (LDA) and logistic regression (LR) were performed using the whole data set, and a leave-one-out cross-validation was used to evaluate performances of the models. Sensitivity and specificity scores obtained for LDA and LR were 0.96 and 0.73, and 0.95 and 0.75, respectively. It is also shown that the channel slope above which debris-flow is observed decreases with the gravitational energy of the catchment. Limitations of the morphometric discrimination are discussed.

Sensitivity analysis of environmental changes associated with riverscape evolutions following sediment reintroduction: geomatic approach on the Drôme River network, France

The present paper aims to put into practice a conceptual framework for gravel-bed river sustainable management previously proposed by Pont et al. [(2009) Conceptual framework and interdisciplinary approach for the sustainable management of gravel-bed rivers: the case of the Drôme River basin (SE France). Aquatic Sciences, 71 (3), 356–370] for the Drôme River Basin (France). It tests the capacity of the functional sector concept (Petts, G.E. and Amoros, C., 1996. Fluvial hydrosystems. Springer) when used to assess the risks of environmental changes. The application of this concept is illustrated by examples focusing on the potential impacts of sediment replenishment on functional sector diversity used as a proxy of habitat diversity, and on trout distribution at a network scale. We used remote sensing and Geographic Information System (GIS) methods to produce original data. A cluster analysis performed on the components of a principal component analysis established a functional sector typology based on channel planform parameters. We calculated an index of present and 1948 functional sector diversity for the entire channel network to highlight past evolutions. A sensitivity analysis was then performed to predict changes in functional sector diversity resulting from defined options of sediment reintroduction through planned forest removal. A similar procedure was developed to evaluate likely changes in brown trout distribution resulting from the impacts of the actions on canopy cover and summer water temperature. The methodological procedure is described, as well as the different assumptions made to move from a theoretical framework to a more practical one. Two examples are used as pilots to evaluate the value of a sensitivity analysis approach based on functional sector types (FSTs) to test management actions aimed at improving aquatic ecology. Limitations and potential improvements are then discussed.

Active channel width as a proxy of sediment supply from mining sites in New Caledonia: Sediment supply from mining sites in New Caledonia

Although the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes, it is still difficult to isolate this effect from the other controlling factors of channel forms, such as the sediment transport capacity (depending notably on the size of the catchment) and local conditions (e.g. confinement, riparian vegetation, valley-floor slope). The rivers in New Caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions. Some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s, which was particularly intensive between the 1940s and 1970s. The propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel-rich peridotite massifs. A first set of 63 undisturbed catchments in peridotite massifs distributed across the Grande Terre was used to fit a classic scaling law between active channel width and drainage area. A second set of 86 impacted sites, where the presence of sediment waves was clearly attested by recent aerial imagery, showed systematically wider active channels, with a width ratio around 5 (established from the intercept ratio of width-area power laws). More importantly, this second set of disturbed sites confirmed that the residual of active channel widths, computed from the scaling law of undisturbed sites, is statistically positively related to the catchment-scale relative area of major mining sediment sources. It is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width, and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes.