Evaluation of slopes delivery to catchment sediment budget for a low-energy water system: a case study from the Lingèvres catchment (Normandy, western France)

Abstract

Quantification and characterization of hydrological and sedimentary transfers are now major challenges to preserving European soils and water resources (Law on Water, Water Framework Directive). In northwestern France, stream water quality is currently reduced by suspended sediment. However, in these low‐energy water systems, sediment origin and dynamics into the catchment remain difficult to analyze from a local to a regional scale because of intermediary traps caused by landscape structure (hedgerow areas) and land use organization on the slopes. To provide a better understanding of sediment dynamics in these systems we proposed to analyze runoff dynamics on slopes and to assess slopes' contribution to the sediment budget of a catchment. This study is based on sediment connectivity analysis, used to accurately determine runoff dynamics between producing areas and the outlet of the catchment. This work is focused on the Lingèvres catchment (15 km2), a typical feature of the low‐energy water system located in the western part of France, in between open‐field and bocage landscapes. The results demonstrate that in a hedgerow landscape, known for its protective role against runoff connection to the river, sediment delivery from slopes is more important and efficient than in an open‐field area where no obstacles are observed. This is explained by diverse drainage densities and spatial organization of roads, and ditch and plot entries which induced differentiated sediment delivery from agricultural plots and the river. However, they also underlined the weakness of sediment delivery from slopes compared with sediment fluxes exported from the catchment. Conversely, they highlight the fundamental role of erosion processes due to agricultural practices and landscape organization in headwater systems, which seem to be the main functional area at the sub‐catchment scale.

How might sediment connectivity change in space and time?

“…These networks may exhibit a spatial pattern that would efficiently connect runoff from the hillslopes to the outlet of the catchment. Following this hypothesis, remote agricultural plots (i.e., far from the river) can be artificially connected (Bocher, ; Moussa, Voltz, & Andrieux, ; Viel et al, ; Viel, Delahaye, & Reulier, ). It has indeed been demonstrated that land cover and various linear features blocking or connecting the runoff to the stream are coupled to create a complex artificial drainage network that deeply modifies the topographic dynamics of runoff (usually observed in openfield landscapes; Reulier, Delahaye, Viel, & Davidson, ; Viel et al, ).…”

“…Following this hypothesis, remote agricultural plots (i.e., far from the river) can be artificially connected (Bocher, ; Moussa, Voltz, & Andrieux, ; Viel et al, ; Viel, Delahaye, & Reulier, ). It has indeed been demonstrated that land cover and various linear features blocking or connecting the runoff to the stream are coupled to create a complex artificial drainage network that deeply modifies the topographic dynamics of runoff (usually observed in openfield landscapes; Reulier, Delahaye, Viel, & Davidson, ; Viel et al, ). Consequently, the local mutual interference between the sediment cascade and anthropogenic infrastructure may alternately improve or reduce the local connectivity; therefore, the processes observed at the local scale do not always explain the global response of the catchment.…”

“…The results highlight that headwater systems (i.e., where the drainage network is initiated) are the more contributive areas (75% of sediment exported from the catchment). Conversely, slopes and river channels contribute to 5% and 20% of sediment exports, respectively (Viel et al, ). Nevertheless these ratios hide the spatial unsteadiness of sediment delivery at a local scale.…”

The anthropogenic nature of present-day low energy rivers in western France and implications for current restoration projects

“…2). The modern hydrological and sedimentary dynamics of this catchment are now well-known due to continuous monitoring (2009)(2010)(2011)(2012) of hydrological and suspended sediment flow recorded at four points of the catchment every 6 min (Viel, 2012;Viel et al, 2013aViel et al, , 2014aFig. 1).…”

“…Research in the Seulles catchment shows that today half of the sediments annually exported by the river come mainly from the headwater systems (Strahler first-order catchments) in the initiation of the drainage network, where runoff processes are moving from unconcentrated to concentrated flows (Viel et al, 2014a). The very limited direct contribution by the erosion of cultivated soils is notable, especially in relation to what occurred over the last three millennia.…”