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.
