New methods for the identification of fine sediment sources: from the discharge-turbidity relation to a reach scale understanding

Fine sediment transported in suspension is an important part of the total sediment yield in most rivers with erodible upland sediment sources. Fine sediment has positive effects on the stabilization of riverbanks, the accretion of floodplains, nutrient transport, and carbon sequestration. However, when fine sediment load is excessive, it can also clog the streambed, reduce invertebrate and fish habitat, prevent river-aquifer exchange and hyporheic flows, and damage hydropower infrastructure.

 To effectively design sediment management policies in rivers, it is fundamental to understand the fine sediment dynamics at the catchment scale. This study focuses on the washload, the fine sediment fraction that, once entrained, remains in suspension until its deposition.

Washload dynamics are typically quantified by concurrently measuring stage or discharge (Q) and turbidity, from which suspended sediment concentration (SSC) is derived. Q-SSC pairs often create a hysteretic relationship, allowing us to infer the distance of fine sediment sources upstream of the station.

This contribution adopts a reach-scale perspective on Q-SSC analysis, moving beyond single-station hysteresis loops and leveraging Q-SSC data from two stations, one upstream and one downstream. The core idea is that we can study washload as a passive tracer to gain further information about the hydraulic variables of roughness and water velocity, for each event separately. We can then integrate this information to further describe the fine sediment sources dynamics and the washload regime of the studied reach. Combining the subsequent reach-scale information we can completely reconstruct the washload production timing and yields across the whole catchment.

For this purpose, we developed new tools which ought to become the new standard for Q-SSC analysis. First, we identify in the discharge timeseries the flood and sediment pulse events through a new algorithm based on Empirical Mode Decomposition. Second, we study the virtual velocity of the flood and sediment signal by a new definition of cross correlation, analysing the Hilber transforms of the signal. Third, we study the presence and the nature (e.g. intensity and seasonality) of suspended sediment sources and tributaries through a time dependant boundary condition analytical solution of the advection-diffusion equation, both for discharge and washload concentration.

 The development of these three new tools and their application to the Arc-Isere (France) with six stations and four reaches, allowed us to identify the fine sediment sources and sinks in the river network. We also gained insights into seasonal fine sediment yields, the deposition and re-suspension dynamics of riverbed sediment stocks, and their progressive depletion during the spring-summer season. These methods are generalizable and applicable wherever discharge (Q) or stage and SSC data are available at two or more locations.