Wednesday, 21 May 2014

Conference report: State of Geomorphological Research in 2014 (Czech Republic)

April 23rd – 25th, 2014

We participated on the conference State of Geomorphological Research in 2014. It was realized under the patronage of Czech Association of Geomorphologists by the University of J. E. Purkyně in Ústí nad Labem and the Institute of Rock Structure and Mechanics of the ASCR, v.v.i. It was opened to all geomorphology scientists from the Czech Republic, Slovakia and Poland. Conference included 5 scientific sessions. We presented our conclusions from research through the session Fluvial processes and landforms.
The session included wide range of aspects of current research from Czech Republic, Slovakia and Poland. It comprised of fluvial processes and dynamics, morphological spatial variability and changes over various timescales, and fluvial interactions with other processes such as tectonics or ecological dynamics.

We presented our conclusions about:

Contemporary state and river processes of the Olše River channel (Outer Western Carpathian, Czech Republic)
Authors: Václav Škarpich, Tomáš Galia, Jan Hradecký

In the last decades, intense and temporally variable anthropogenic disturbances in the Czech Carpathians basins continuously adjusted new constrains of rivers. Especially afforestation and bank stabilisation caused reduction of sediment supply. Originally gravel-bed character of rivers with high amount of transportable material was changed. The effect of hungry waters (Kondolf 1997) induced progressive channel transformation. Present-day state of channels shows a tendency towards the acceleration of processes linked with river bed deepening. Focusing on the Olše River in the Czech part of flysch Carpathians, the paper summarizes results of energetic potential aspects of contemporary Carpathian river channels. New internal conditions of channels have been affecting acceleration of erosion processes. The main cause is an adjustment of flow dynamics. The unit stream power values (after Bagnold 1966) increased two or three times between the years from 1960 to 2003. Other characteristics, used for evaluation of factors identifying the energetic potential of channels (as hydraulic radius etc.), show increase of values in the year 2003 compared with the year 1960. Contemporary state of the Olše River channel predicts that future development will lead to progressive incision.

Channel patterns and connectivity in Beskydian headwater channels
Authors: Tomáš Galia, Václav Škarpich and Jan Hradecký

The contribution summarizes investigations in small flysch-based mountain streams of Moravskoslezské Beskydy Mts. Continuity in channel-reach morphologies were assesed in longitduinal stream profiles with respect to intensity of bedload transport and thus, (dis)equilibrium between potential sediment supply and transport capacity during flood events. Parameters of observed channel-reach morphologies were statistically evaluated by PCA analysis to obtain leading predispositing factors. Bedload transport was evaluated by simulations in 1D transport model TomSED and by observations of marked particle movement. Moreover, measurements of largest boulders occurred in channels gave information about competence of stream power during high-magnitude flood events to transport this coarsest fraction.
There was evident, that streams under low sediment-supply or high transport capacity were prone to occurrence of bedrock, bedrock-cascade and also step-pool morphology on the lower gradients. On the other hand, cascades and step-rapids without well-developed step-pool morphology were typical rather for high sediment-supply conditions. Differeneces in size of coarsest sediment fraction were assessed in longitudinal profiles under past debris-flow events and under prevailling fluvial processes. Occurrence of past debris-flows indicated much higher sized of largest boulders related to transport power of these colluvial events. Longitudinal profiles without debris-flow evidence in same cases indicated changes in boulder size trends with respect to fluvial processes, especially erosion (coarsening) and deposition (fining). Nevertheless, this fact was not evident in all investigated stream profiles. In addition, role of stream power was not important at all, when there was not observed any dependence between computed stream power and size of largest boulders.

The artificial boulder steps as a modern control works structure in mountain streams
Authors: Zdeněk Přibyla, Tomáš Galia

Torrent and gully control (currently regulated by ČSN 75 2106) in the Czech Republic landscape has influenced mountain streams since the 19th century. Mainly traditional control works are implemented, particularly staircase-like sequences of concrete check dams or bed sills. The main objective is to reduce the channel gradient, inhibit erosion of the channel bed and banks and limit sediment transport into the lower parts of the basin. Modern trends of control works abroad tend to conservation of natural channel morphology. In last two decades, sequences of artificial boulder steps alternating with pools are widely used in mountain streams restoration. Natural step and pool systems in mountain streams show high values of ​​flow resistance reducing sediment transport and they does not affect the channel connectivity and stream ecology by contrast to artificial check-dams. Often regulated stepped-bed channels in the Moravskoslezské Beskydy Mts. are characterized by lower size of bed sediments fraction, high transport capacity and bed incision. The natural stream morphology is usually presented by disordered channel bed with frequently alternating channel units such as cascades, steps, pools and bedrock.
An experiment of manual rebuilding of the channel bed was performed in the Malá Ráztoka stream. The 40 m channel reach (channel slope 0.09) is created by disorganized cascade and step units was rebuilt to regularly alternating units of steps and pools. Designated dimensions of steps high H (0.44 m) and steps spacing L (4.1 m) were calculated according to suggestions of foreign studies and considering the local channel slope S: 1<H/L/S>2 (Abrahamas et. al. 1995; Lenzi, 2002). Only boulders available in the channel or deposited along the banks were used for the steps construction. The key stones in boulder steps exceed 0.4 m and this corresponds to stability for the flood event >Q20 (Galia and Hradecký 2012). The rebuilt reach will be subjected to a detailed long-term monitoring focusing on effects of flood flows on sediment transport and changes in channel geometry (step and pool geometry). Construction of several other sections and starting their monitoring is planned for spring 2014.

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