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Communication dans un congrès

Quantification of sediment fluxes of small to medium catchments over the Loire Brittany river basin (France)

Abstract : Quantifying volumes of sediments produced on hillslope or in channel and transported within river systems is necessary to understand basin specific sediment yield and establish sediment budgets. Several research efforts have already investigated either global budgets at the river basin to continental scale or detailed budgets at the plot to field scale. However, very few studies have proposed estimation of sediment yields (SY) for small to medium catchments (10 to 105 km²) and tried to draw the link between sediment yields and basin properties. In this context, the present study aims at bridging this gap and we focused on the Loire Brittany river basin (France) for which mean annual sediment loads at river outlets have recently been estimated (Delmas et al. 2012). 110 small to medium watersheds spread over this territory were chosen covering 67% of the total basin area. 60 watersheds are nested in at least one other watershed. We first present a large database of SY which were estimated from suspended sediment concentration and flow discharge data measured at gauging stations with survey length ranging from ten to forty years. Rating curves taken from the literature (Delmas et al. 2011) were chosen for the estimation of sediment fluxes. Secondly, sixteen variables recognised in the literature as controlling factors of basin hydrosedimentary response (drainage area, mean slope, mean and maximum altitude, percentage of vegetated areas, basin shape, drainage density, number of obstacles to flow, river mean slope, mean annual rainfall, precipitation variability, water flow discharge, infiltration, sediment load, mean annual erosion) were chosen for confrontation in a correlation matrix. Investigations on basin erosion and sediment export capacity relied on the use of three indexes: SDR (Walling 1983), FssT (Ludwig and Probst, 1998), Fss (Delmas et al. 2009). Finally, a semi-distributed approach for the development of local sediment budgets is investigated. The resulting estimated annual sediment loads at basin outlet range from 2.5 x102 to 1.4x106 t.yr-1 and specific sediment yields (SSY) range from 2.9 to 32.4 t.km-2.yr-1 (Figure 1) with an associated mean uncertainty of 20%. Estimated values are identical to those found for catchments presented in the literature with similar global characteristics. Mean annual sediment loads from the three principal Loire tributaries (Allier, Cher, Vienne) contribute to 66.1% of the total load of the Loire river at its outlet while accounting for 40.6% of its surface. Relation between SSY and basin area were investigated. Though it is common to expect a negative correlation between both variables due to an increase in deposition areas as catchment size increases, no clear relationship was found in the present study. Moreover, despite model uncertainties in the prediction of sediment loads, no clear relationship between specific sediment yields and chosen variables could be established. Calculated index were also confronted to SSY without clear correlation. These findings confirm those of previous researches and show that relationships between SSY and controlling factors are complex and non-linear. Expected results from a semi-distributed model should therefore provide a clearer insight into sediment transport and redistribution processes. Figure 1: The 110 watersheds and associated specific sediment yields. References Delmas M., Cerdan O., Mouchel J.M., Garcin M. (2009) A method for developing a large-scale sediment yield index for european river basins. Journal of Soils and Sediments, 9(6), 613-626. Delmas M., Cerdan O., Cheviron B., Mouchel J.M. (2011) River basin sediment flux assessments. Hydrological Processes, 25(10), 1587-1596. Delmas M., Cerdan O., Cheviron B., Mouchel J.M., Eyrolle F. (2012) Sediment exports from french rivers to the sea. Earth Surface Processes and Landforms, 37(7), 754-762. Ludwig W. and Probst J.L. (1998) River sediment discharge to the oceans; present-day controls and global budgets. American Journal of Science, 298(4), 265-295. Walling DE (1983) The sediment delivery problem. Journal of Hydrology, 65(1), 209-237.
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https://hal-brgm.archives-ouvertes.fr/hal-00799743
Contributeur : Aurore Gay <>
Soumis le : mardi 12 mars 2013 - 15:43:57
Dernière modification le : mercredi 16 septembre 2020 - 16:57:29

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  • HAL Id : hal-00799743, version 1
  • PRODINRA : 316182

Citation

Aurore Gay, Olivier Cerdan, Magalie Delmas, Marc Desmet. Quantification of sediment fluxes of small to medium catchments over the Loire Brittany river basin (France). 10th International Conference on Fluvial Sedimentology, Jul 2013, Leeds, United Kingdom. ⟨hal-00799743⟩

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