Abstract
The floc size distribution of suspended sediment is a critical driver for in-channel sedimentation and sediment-associated contaminant and nutrient transfer and fate in river catchments. Real-time, in situ, floc size characterisation is possible using available technology but, to date, limited high resolution floc data have been published for fluvial systems draining upland extensive grassland catchments. To that end, suspended sediment floc size distribution and turbidity were characterised at 15-minute intervals using Laser In-Situ Scattering and Transmissometry (LISST) diffraction and a YSI turbidity sonde for six storm events in the upper River Taw (15 km2) catchment in SW England. Maximum event discharges (Q) ranged between 4.3 and 20.0 m3 s−1, with clockwise hysteretic responses (HI = 0.18–0.48) of total suspended solid concentrations (TSS) and Q. The sediment flushing index was highest in the early autumn (0.93) and storm event TSS fluxes varied from 0.04 to 2.9 t km−2. This suggests a change in sources or composition of sediment during higher Q and highly variable patterns of sediment flux from event-to-event. The proportion of particulate organic matter (POM) to TSS was highly variable (5–89%) and did not increase with Q, indicating POM source limitation. The fine-grained tail (D10 and D16) of the floc size distributions decreased during hydrograph rising limbs, with the finest floc sizes associated with the highest TN and TP concentrations at peak Q. The results suggest that dynamic interactions between wet ground and extreme rainfall events can flush significant amounts of sediment from the relatively undisturbed extensive grassland upland catchment. We strongly encourage a sensors-based approach to reveal the spatio-temporal complexity of floc size and associated pollutant export during high Q generated by extreme rainfall since this can help to elucidate processes and mechanisms and generate high-resolution data for water quality modelling without significant user intervention.
Highlights
Elevated suspended sediment transport in rivers is a primary environmental and ecological issue around the world (Hauer et al, 2018; MateoSagasta et al, 2018)
The high Q events significantly increased median total suspended solid concentrations (TSS) and the silt (
Increasing floc diameter during the falling limbs of the storm hydrographs despite the TSS peak being observed before peak Q provide evidence that low-frequency sampling regimes are insufficient to describe the hydrosedimentological dynamics in river systems
Summary
Elevated suspended sediment transport in rivers is a primary environmental and ecological issue around the world (Hauer et al, 2018; MateoSagasta et al, 2018). Understanding the size characteristics of flocs is important for elucidating the fate of sediment and associated contaminants and nutrients in rivers, lakes and reservoirs, since flocculation has important implications for hydraulic behaviour i.e., the deposition in and dispersal through, fluvial systems (Grangeon et al, 2014; Hoffmann et al, 2020). Local floc-size distributions determine the dynamics of suspended sediment fluxes in aquatic systems and associated transport of carbon (C), nutrients and contaminants (Lamb et al, 2020) Despite this important role of flocs, to date, only limited data for the in situ effective particle size (EPS) characteristics of fluvial suspended sediment i.e. flocs have been reported (Williams et al, 2007; Landers and Sturm, 2013; Czuba et al, 2015). Detailed understanding of EPS at storm event scale remains an important evidence gap for fluvial systems and especially over extended time periods encompassing several storm events (Williams et al, 2007)
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