Critical conditions for beginning of bedload transport in Beskydian headwater channels

Bedload transport in view of its beginning was evaluated during years 2010 and 2011 in Beskydian headwater channels, namely Malá Ráztoka and Lubina streams (Fig. 1). After Q20-25 flood event in May 2010, flood competence method was applied in nine channel-reaches (7 in Lubina Stream and 2 in Malá Ráztoka Stream), when we measured diameters of the largest transported boulders (ca. 200-400 mm) transported and stored in to fluvial accumulations during such event (Fig. 2). Also, geometry parameters and grain size distributions were obtained from the selected reaches upstream the fluvial accumulations. Movement of marked particles (18-152 mm) was evaluated during smaller discharges up to bankfull stage between November 2010 and July 2011 in channel-reaches nearby a gauging station in the Malá Ráztoka basin (Fig. 3). 

Fig. 1. Location of the studied torrents: Lubina (L) and Malá Ráztoka (MR).

 Fig. 2. Measurement of the boulder stored during May 2010 flood event in the Lubina Stream.

Fig. 3. Position of marked particles on the crest of step in step-pool channel (Malá Ráztoka).

Derived critical values (unit stream power and unit discharge) of the movement of certain grain diameters were fitted to relationships ωci=0.445di^1.06 (di in mm) and qci=0.8di^0.51 (di in m) (Fig. 4 and Fig. 5). Obtained critical values were generally lower than those coming from Alpine and Andine environments (see Lenzi et al., 2006; Mao et al., 2008). This fact may have been caused by lower bed armouring or higher sediment supply in local torrents, when compared to the magnitude of discharges in Alpine and Andine torrents. In step-pool morphology systems, the dynamics of sediment transport was higher in pools than on steps at bankfull flow (38% of grains, respectively 12% of grains, were moved), whereas the opposite was observed at lower flow.

Fig. 4. Derived critical unit stream powers from both flood competence and marked particle transport methods.

Fig. 5. Derived critical unit discharges from both flood competence and marked particle transport methods.

The results of the research were recently published in the Carpathian Journal of Earth and Environmental Sciences: link  

• Lenzi, M.A., Mao, L., Comiti, F. (2006): When does bedload transport begin in steep boulder-bed stream? Hydrologic. Process. 20, 3517-3533.
• Mao, L., Uyttendaele, G.P., Iroumé, A. & Lenzi, M.A., 2008. Field based analysis of sediment entrainment in two high gradient streams located in Alpine and Andine environments. Geomorphology 93, 368-383.