Investigating the temporal dynamics of suspended sediment during flood events with 7Be and 210Pbxs measurements in a drained lowland catchment

Abstract

Soil erosion is recognized as one of the main processes of land degradation in agricultural areas. High suspended sediment loads, often generated from eroding agricultural landscapes, are known to degrade downstream environments. Accordingly, there is a need to understand soil erosion dynamics during flood events. Suspended sediment was therefore sampled in the river network and at tile drain outlets during five flood events in a lowland drained catchment in France. Source and sediment fallout radionuclide concentrations (7Be, 210Pbxs) were measured to quantify both the fraction of recently eroded particles transported during flood events and their residence time. Results indicate that the mean fraction of recently eroded sediment, estimated for the entire Louroux catchment, increased from 45 ± 20% to 80 ± 20% between December 2013 and February 2014, and from 65 ± 20% to 80 ± 20% in January 2016. These results demonstrate an initial flush of sediment previously accumulated in the river channel before the increasing supply of sediment recently eroded from the hillslopes during subsequent events. This research highlights the utility of coupling continuous river monitoring and fallout radionuclide measurements to increase our understanding of sediment dynamics and improve the management of soil and water resources in agricultural catchments.

Direct Channel Precipitation and Storm Characteristics Influence Short‐Term Fallout Radionuclide Assessment of Sediment Source

“…Full details and assumptions are discussed in Matisoff et al (). This approach is not without drawbacks, as outlined in Le Gall et al () and Walling (). We use it in a limited and relative way and compute sediment age in order to evaluate the similarity between radionuclide activities and their ratios in precipitation and on suspended sediment.…”

“…Scientists and resource managers are interested in the sediment fluxes on relatively short time scales, for example, storm events to seasons and years, often with an underlying motivation to know if a significant portion of sediment exported results from recent landscape surface erosion versus gully and streambank erosion (Bonniwell et al, ; Le Gall et al, ; Walling, ; Whiting et al, ). This requires an understanding of the sources and flow paths of both sediment and the water that erodes and carries it.…”

“…Hence, the FRN activity in channel‐intercepted portion of wet deposition is often ignored or assumed negligible in fingerprinting and age dating applications (e.g., (Bonniwell et al, ; Matisoff et al, ), following a conceptual model depicted in Figure a. In watersheds with considerable overland flow this assumption has been supported (e.g., Jweda & Baskaran, ; Le Gall et al, ). However, hydrologic investigations in forested settings have long discussed and quantified the importance of direct precipitation onto the wetted stream channel and connected saturated areas for contributing to stream channel discharge during events (Dunne & Black, ).…”

“…In this paper, we examine the relevance of channel interception and other watershed hydrologic dynamics (e.g., presence or absence of overland flow) in relationship to the watershed export of sediment‐associated 7 Be and 210 Pb xs . Specifically, we examine the following hypotheses that currently provide the foundation for many fingerprinting and sediment age dating studies (e.g., Gellis et al, ; Le Gall et al, ; Matisoff et al, ): (1) deposition of short‐term FRNs onto the channel area is negligible when compared to the flux exiting the stream channel reach, and (2) young sediment, defined as having a 7 Be/ 210 Pb xs ratio similar to that of precipitation, originates from recent landscape surface erosion and, therefore, not from exposure of sediments within the fluvial environment to event rainfall. Upon testing these hypotheses (and finding them poorly supported by our data), we propose a revised conceptual model in which channel deposition of FRNs is responsible for sediment‐associated 7 Be and 210 Pb xs via resuspension and tagging of suspended sediment within the storm event.…”

Hydro‐sedimentary Dynamics of a Drained Agricultural Headwater Catchment: A Nested Monitoring Approach

“…Although suspended sediment yield was shown to globally decrease with slope (Milliman and Syvitski, 1992), small headwater lowland catchments may exhibit significant sediment exports (Vanmaercke et al, 2011). Previous studies investigated sediment transfer in these catchments by constructing sediment budgets (Walling and Collins, 2008) or conducting sediment fingerprinting (Smith and Blake, 2014; Foucher et al, 2015; Le Gall et al, 2017) approaches. Russell et al (2001) and Walling et al (2002) demonstrated the need to take into account tile drain networks in lowland catchments equipped with these systems.…”