Spatial and temporal dynamics of suspended sediment fluxes in an alpine
river: the Arc and Isère rivers, France
Abstract
Fine sediment dynamics is an important issue in alpine rivers since it
impacts both dam management (decrease of the reservoir capacity,
flushing strategy) and river management (interaction between fine
sediment and vegetation, gravel bar stabilisation, flood risk).
Suspended Particle Matter (SPM) is generally assumed to be transported
as washload although significant exchanges with the river bed have been
detected in several cases. As a consequence but also due to the variable
SPM sources and differences in the discharge and sediment wave
velocities, one can often observe a hysteresis behaviour during
hydrological events that evolves spatially along the system. In this
study, we will present results from a network of hydro-sedimentary
stations (measuring both water discharge and sediment concentration)
along a reach of 120 km starting from the Arvan River at its confluence
and the Arc and Isère rivers, that is part of the ZABR (or Rhône Basin
Long Term Environmental Research Observatory). The SPM dynamics is
analysed for different types of events (large flood, spring flood,
flushing event, debris flow). In particular, we focus on the potential
of erosion or deposition between each station by calculating the total
SPM flux during each event at each station as well as the hysteresis
behaviour using a hysteresis index. The evolution of these parameters is
related to input parameters such as discharge and sources and grain size
characteristics of the suspension. Indeed, most observed sediment
exchanges (erosion or deposition) occur on the gravel bars when
inundated, and depend on settling characteristics of the SPM. Also the
experimental site provides some useful data for estimating advection and
dispersion characteristics of various SPM events that could be used to
validate numerical models.