CRITICAL CONDITIONS FOR THE BEGINNING OF COARSE SEDIMENT TRANSPORT IN THE TORRENTS OF THE MORAVSKOSLEZSKÉ BESKYDY MTS (WESTERN CARPATHIANS) - Carpathian Journal of Earth And Environmental Sciences

Two methods were applied in small headwater streams in the region of the Moravskoslezké Beskydy Mts: flood competence method and marked particle displacement method. The first method consisted in the measurement of the diameters of the largest boulders (200-400 mm) that had deposited in fluvial accumulations as a consequence of a Q25 flood event (5/2010). Transport of marked particles (18-152 mm) was studied in the period between 11/2010 and 7/2011, during which several minor events occurred in local torrents and maximal observed discharge was equal to the bankfull stage (Q1-Q2). The latter method was also used in two selected gullies to evaluate the intensity of coarse sediment transport in initial channels. Derived critical values (unit stream power and unit discharge) of the movement of certain grain diameters were fitted to relationships ω_ci= 0.445d_i^1.06 (d_i in mm) and q_ci=0.8d_i^0.51 (d_i in m). Obtained critical values were generally lower than those coming from Alpine and Andine environments. The same situation was observed using a relationship obtained for dimensionless shear stress, which takes form: τ*_ci=0.097(d₉₀/d_i)^-0.502 (d_i in m). 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. Unlike the gully more or less armoured with vegetation, the active gully void of any vegetation cover demonstrated a much higher intensity of sediment transport during both fluvial and colluvial processes.