Changeability of Movable Bed-Surface Particles in Natural, Gravel-Bed Channels and Its Relation to Bedload Grain Size Distribution (Scott River, Svalbard)

The presented paper analyses the variability of grain size distribution parameters of bedload transported by the gravel-bed Scott River (Svalbard) draining a glacier catchment with an area of 10km(2). The grain size distribution analysis is one of the basic elements of identification of the fluvial transport mechanisms in gravel-bed rivers. It is used for the determination of threshold values for bedload movement. It is also treated as an important indicator of the origin, routes of distribution, and conditions of transport and deposition of fluvial bedload. The field study in a natural proglacial gravel-bed channel was carried out at two reaches in the mouth section of Scott River. The study revealed relatively high temporal variability and similar mean parameters of grain size distribution in conditions of low discharges. Bedload transport rates reached a mean of 71.9-76.0kgd(-1) in channel cross-section. Bedload texture was dominated by gravels with a proportional contribution of the fine-grained fraction along with very fine-grained gravels (8-2mm) of 38.8%. The medium-grained fraction (16-8mm) constituted 33.7%, with a lower contribution from the coarse-grained fraction (32-16mm) of 23.2%, and the very coarse-grained fraction (64-32mm) of 4.4%. Two periods in the course of bedload transport and distribution of grain size distribution parameters were distinguished based on variation of hydro-meteorological conditions. The first half of the measurement period was distinguished by significantly higher values of daily loads and increased contribution of the coarse-grained and very coarse-grained fraction (28-31% and 6.2-6.6%, respectively). During this time, the river discharged up to 94% of bedload. This resulted in a clear tendency for riverbed scouring. The second half was distinguished by generally low daily bedload transport rates (<10kgd(-1)), an increase in contribution of fine- and very fine-grained gravels (42-55.6%), and a change in the tendency to aggradation. Grain size indices were more varied, and grains were usually finer and better sorted. Selective transport processes, often related to redeposition, were dominant in the channel. Along with an increase in flow velocity, conditions for material deposition became more variable. This was manifested in weaker sorting and an increase in grain diameter.