Temporal variability of bedload vs suspended sediment load in a glacier-fed Alpine river

The relative proportion of bedload and suspended sediment transport in rivers features very large variations both in space (within and among rivers) and in time (ordinary vs infrequent floods). However, the current knowledge about the temporal variability in bedload/suspended proportion is limited, as well as of its controlling factors (e.g., water runoff, sediment supply). The present work investigates the partitioning of sediment load into bedload and suspended fractions in the glacier-fed Sulden/Solda River (eastern Italian Alps, drainage area 130 km²), where a monitoring station for water and sediment transport has been operating since 2014.

From 2014 to 2020 the station was equipped with one turbidimeter and 8 geophone plates for monitoring suspended sediment and bedload transport, respectively. Calibration curves for deriving suspended sediment concentrations were derived based on 474 water samples, whereas to convert the geophones signal to bedload mass a portable trap mounted on a crane was used (76 samples collected). Two meteorological stations located at about 2800 m and 1900 m a.s.l. recorded precipitation and air temperature within the catchment.

A 10-minute interval dataset was established, including suspended load, bedload, water discharge, precipitation, and air temperature, measured from May to October 2014-2020 (2018 data had to be excluded for technical problems of the turbidimeter). Hourly intervals characterised by data gaps regarding geophone plates or turbidimeter were excluded from the analysis, and a total of 18,549 hourly data were analysed. The overall range of water discharge observed in the study period was 0.7 – 80.8 m³/s, but reliable suspended sediment data are available only up to about 40 m³/s (RI about 2 yr). The mean annual discharge of the Sulden River was 6.3 m³/s, while the mean discharge from May to October equalled 10.0 m³/s.  

Results show that suspended/bedload partitioning varied with water discharge and time of the year (month) in a complex way. On average, the relative contribution of suspended sediment transport was around 89% for Q<10 m³/s, 96% with 10<Q<20 m³/s, and 97% for Q>20 m³/s. At the lower water discharges (Q<10 m³/s), the suspended sediment fraction diminished over the summer from May/June/July (about 94%) to August/September (88%), and reached its minimum value in October (79%). This trend is possibly due to the intense glacier ablation occurring in August, which increased the coarse sediment supply – transported as bedload in the channels – more markedly than the finer fractions carried in suspension.

At higher flow rates (Q>10 m³/s), the percentage of sediments transported in suspension shows an opposite temporal trend, increasing from May (80%) to October (97%). Such a remarkable difference compared to lower discharges may be due to the strong increase in suspended sediment concentration at higher water discharges multiplied by larger water volumes carrying suspended transport through the entire flow depth, differently from bedload. Such complex dynamics is consistent with previous results in the Sulden River, where a low effective discharge for bedload – driven by the supply of coarse sediments during the glacier melt period – was observed for the same period analysed here.