Tomáš Galia

Bedload Transport and Morphological Effects of High-Magnitude Floods in Small Headwater Streams - Moravskoslezské Beskydy Mts. (Czech Republic)

Bedload Transport and Morphological Effects of High-Magnitude Floods in Small Headwater Streams - Moravskoslezské Beskydy Mts. (Czech Republic) Bedload transport observed during a flood in May 2010 gave rise to several forms of accumulations in small headwater basins located in the Western Flysch Carpathian Mountains, Czech Republic. We have investigated critical conditions of incipient motion of the largest boulders deposited during a c. Q100 flood event (flood competence method). We have tested several formulas designed for high gradient streams in two small basins in the conditions of local mid-mountain relief. The results show that a flood of such a magnitude is able to transport almost all surface bed material and that bedload transport in steep headwater streams (A ≤ 1 km2) is probably less selective as for the grain size than that in lower gradient gravel-bed streams. The authors discuss the importance of local basin predispostion factors in order to determine critical conditions for the onset of bedload transport. Dnový Transport a Morfologická Odezva Mimořádných Povodní na Malých Pramenných Tocích - Moravskoslezské Beskydy (Česká Republika) Dnový transport sedimentů zapříčinil během květnových povodní v roce 2010 vznik různých typů akumulací v malých pramenných tocích nacházejících se ve flyšových pohořích Západních Karpat. Studie se zaměřuje na určení kritických podmínek nutných pro uvedení největších klastů do pohybu, jež byly následně uloženy do akumulací během této cca Q100 povodně (metoda účinnosti povodně). V rámci výzkumu byly ověřeny některé rovnice vytvořené pro vysokogradientové toky na dvou malých povodích v podmínkách reliéfu hornatin. Výsledky ukazují, že povodeň takové intenzity je schopna transportovat téměř celou povrchovou vrstvu sedimentů a dnový transport je na malých povodích pravděpodobně méně velikostně selektivní než v tocích s nižším gradientem dna. Důraz byl kladen také na lokální predizpoziční faktory ovlivňující kritické podmínky pro uvedení určité velikostní frakce sedimentů do pohybu.

Connectivity of the Coarsest Fraction in Headwater Channels: Imprints of Fluvial Processes and Debris‐Flow Activity

Abstract Some steep headwater streams of the mid‐mountains landscape of the flysch belt of the estern arpathians are affected by debris flows. Connectivity of the largest boulder and cobble fractions has been evaluated in steep streams found in the transitional zones between the hillslopes and alluvial cones of similar lithology and watershed morphometry. Two longitudinal profiles affected by past debris‐flow activity and two longitudinal profiles void of such processes were selected. In the first case, active channels were characterised by the presence of the coarsest fraction, while a downstream trend of sediment coarsening or fining lacked any influence of contemporary fluvial processes and sediment supply. In addition, debris‐flow‐affected streams showed greater differences between the middle axis of the coarsest particle and the particle‐size index including the lengths of all three axes, which is most likely the result of limited active fluvial transport. All the studied streams demonstrated general downstream fining of the largest particles, which resulted from local flysch lithology containing more resistant sandstones in the upper parts of the watershed. The trend of sediment fining was more rapid in the longitudinal profiles unaffected by past debris flows. These types of streams also showed adjustment of the coarsest bed fraction to contemporary processes in channels by sediment coarsening in incised reaches and sediment fining in depositional reaches as well as by downstream response to some lateral sediment inputs. The index of the unit stream power showed no correlation with the considered coarsest fraction in both fluvial‐dominated channels and debris‐flow‐affected channels.

The effects of river patterns on riparian vegetation: A comparison of anabranching and single-thread incised channels

Abstract Riparian vegetation reflects the current conditions and the dynamics of streams. The floodplain vegetation along the watercourse of the Morávka River was subject to study in this project. In some reaches, the river has the natural character of an anabranching gravel-bed stream; in contrast, other Morávka R. reaches are incised into the bedrock. These cases were used to assess potential changes in vegetation conditions as evidence of negative processes taking place in the gravel-bed streams of the Beskydy Mts. The results demonstrate a higher biodiversity in the floodplain along the anabranching river channel. In contrast, the floodplain along the incised river channel shows low biodiversity values. Redundancy analysis was used to determine the relationships between plant species composition, distance from the main channel and relative elevation from the mean water level of the main channel. In addition, the results show a higher degree of change in plant species composition on the floodplain along the incised river channel. The analysis of floodplain groundwater fluctuations shows a decreasing trend in the annual maximum groundwater level.

Sediment Transport in Headwater Streams of the Carpathian Flysch Belt: Its Nature and Recent Effects of Human Interventions

The paper summarizes results of both empirical and modelling research of bedload transport in headwater streams of the Czech part of the Western Carpathians. Flysch lithology (i.e. alternation of less resistant claystones and sandstones) affects bedload transport parameters in view of relatively fine-sized sediment supply resulting in low flow resistance of channels. Flood competence method (Q20 flood) and marked particle displacement method (up to Q1–2 flow) was applied to determine critical conditions for the incipient motion of grains in channel bed. The beginning of bedload transport in flysch headwaters under lower values of critical conditions when compared to other regions was confirmed by application of the criteria of unit stream power and unit discharge. The simulated values of bedload transport intensity (1D transport model TOMSED) during a high-magnitude flood in both supply-limited and transport-limited headwaters are significantly lower than it was observed in torrents of the Alpine environment. In relation to unsuitable contemporary watershed management affecting the sediment transport (large check-dams, removing of large wood from local channels), trends of accelerated incision are observed in most headwater streams as well as in lower-gradient piedmountain gravel-bed rivers of the Flysch Belt of the Western Carpathians. Approaches of contemporary local watershed management are presented and some recommendations for the maintenance of channel stability predisposed by soft lithology (e.g. application of artificial step-pool sequences, management of woody debris in channels) are proposed.

Coarse Bed Sediments in a Headwater Channel as Indicators of Fluvial Processes and Slope-Channel Coupling: A Case Study from the Carpathian Mountains (Czech Republic)

Abstract The character of riverbed sediments usually reflects fluvial processes and the dynamics of sediment transport in fluvial systems. The approach in this study was based on the measurement of the largest boulders located within a bankfull channel, and on the observation of changes in their size in the longitudinal profile of a headwater stream in the Moravian-Silesian Beskids Mountains. The resulting trends in a particle-size index reflect the character of sediment delivery into channel segments and the recent channel-forming processes. The largest boulders were observed in channel sections with a strong interaction of slope and fluvial processes, and a slight coarsening of sediments was recorded in the incised downstream sections of the longitudinal stream profile. In contrast, the refining of bed sediments was typical of the transitional zone between slope-channel coupled reaches and an alluvial cone characterised by a tendency to material aggradation. Shrnutí Charakter dnových sedimentů v říčních systémech obvykle odráží probíhající fluviální procesy a dynamiku transportu sedimentů. Naše metodika byla založena na měření největších valounů a balvanů uvnitř aktivního koryta a sledování velikostních změn v rámci podélného profilu beskydského pramenného toku. Výsledné trendy indexu velikosti klastů reflektují charakter donášky sedimentů do korytových segmentů a současné korytotvorné procesy. Největší balvany byly pozorovány v korytových úsecích se silnou interakcí svahových a fluviálních procesů a mírné hrubnutí sedimentů bylo pozorováno v zahlubujících se úsecích dolní části studovaného podélného profilu toku. Naopak zjemňování dnových sedimentů bylo typické pro přechodnou zónu mezi korytovými úseky přimknutými ke svahům a úseky nacházejícími se na proluviálním kuželu, kde se spíše projevovala agradace materiálu.

Detailed fluvial-geomorphologic mapping of wadeable streams: a proposal of universal map symbology

ABSTRACT Detailed maps are important components of fluvial-geomorphological research, connecting several tools, namely field mapping of presented channel and floodplain forms and the assessment of fluvial processes and hydromorphological conditions of current river management. In this paper, we propose a universal map legend for the complex mapping of small stream channels in a detailed scale, which means including both the channel and adjacent floodplain segments. With the help of the symbology we are able to demonstrate both fluvial forms (i.e. individual features, grain size of bed sediments and fluvial deposits) and fluvial processes (i.e. contemporary trends in channels, character of lateral sediment inputs and flow characteristics) in a single map. In total, nearly 150 symbols were proposed and created as a combination of TrueType font and ArcGIS Style files. However, the principle can be used in various software. The work is accompanied by three map examples from the Nízký Jeseník Mts (the Stará Voda Stream) and the Moravskoslezské Beskydy Mts (the Lubina and Bystrý Streams).

Bedl oad transport and fl ow resistance in steep channels – introduc tion to the issues in the context of mountain basins of the Central Eu ropean region

Although sediment transport in the uppermost parts of mountain basins may be supply- limited, bedload transport in small torrents represents a potential hazard during extraordinary floods. The authors attempt to review recent worldwide research of bedload and hydraulics the results of which could potentially be applied in local steep channels. Approaches to the calculation of flow resistance (Manning relationship, logarithmic and power-law relationships) and critical conditions of the incipient motion of a bed particle (critical shear stress, stream power, flow velocity and discharge) are closely discussed in order to describe hydraulic and bedload transport conditions in steep headwater streams. Difficulties are pointed out in connection with the estimation of critical conditions of bedload transport in headwater streams with poorly sorted bed material and more or less developed bedforms. In the second part of the paper the authors present two pilot studies from the Western Carpathians and test potential applicability of specific formulas in this region. Results of the formulas point to relatively nonselective character of bedload transport during high magnitude floods in Beskydian headwater streams. An important role is played by the intensity of sediment delivery to active channels in limited sediment-supply conditions. The authors also point out that a majority of bedload transport and flow resistance equations were originally created on the basis of flume experiments or they were tested in Alpine torrents and therefore it is supposed that they might work well in local torrents after being calibrated to local conditions.

Biogeomorphological effects of leaf accumulations in stepped-bed channels: Exploratory study, Moravskoslezské Beskydy Mountains, Czech Republic

Abstract The stepped-bed system, with a step-like longitudinal profile, is typical morphology in steep headwater streams. These systems are created by a series of coarse sediments or instream wood (steps with supercritical flows) interspaced with finer material (forming pools with subcritical flows). In the case of well-developed steps and pools, the resulting channel-reach morphology is referred to as “step-pool” morphology. In this study, we identify a previously undescribed type of step-pool formation, the “foliaged step-pool”, in the high-gradient Stoligy Stream of the Moravskoslezské Beskydy Mountains. The defining feature of this formation is the significant presence of leaves in the step structure. The geometry of the steps and pools was measured and the parameters that characterise the distribution, amount and function of leaves acting in these areas were defined. Statistical results showed differences between non-foliaged and foliaged step-pool formations, in which the latter showed a significant increase in storage level, influencing the channel’s hydrodynamics. Particle-size analyses demonstrated that foliaged step-pool formations had finer sediment in the pools, which indicates that there are differences in sediment transport processes between foliaged and non-foliaged formations. These results offer new insights into stepped-bed and step-pool morphology, providing directions for further research on small streams in deciduous forested regions.

Estimation of bedload transport in headwater streams using a numerical model (Moravskoslezké Beskydy Mts, Czech Republic)

The TOMSED numerical model, originally developed for Alpine torrents, was successfully applied on the May, 2010 flood event in headwater streams of the Czech part of the Western Flysch Carpathians. The Manning equation was used together with application of a reduced energy gradient and potential erosion limits. Contributions of form resistance and grain resistance to Manning’s roughness parameter were considered separately. The absence of bedload discharge measurement made it difficult to validate the model for local headwater channels, although channel reaches with recent erosional and depositional trends were recognized fairly well by the simulations. Bedload transport was calculated in the range from 480–1240 m3 for the richly-supplied headwater stream and 380–860 m3 for the stream with limited sediment-supply conditions. It turns out that the channel reaches with higher sediment supply have less developed bedforms (e.g. steps) and thus lower flow resistance resulting in a higher intensity of bedload transport.

Variations in bar material grain-size and hydraulic conditions of managed and re-naturalized reaches of the gravel-bed Bečva River (Czech Republic)

European multi-thread rivers have undergone rapid morphological changes during past centuries due to the extensive direct and indirect human impacts on fluvial systems. As a consequence, we can identify altered patterns of bed sediment calibre reflecting disturbed sediment connectivity and modified flow hydraulics. Changes in the grain-sizes of samples collected on 68 gravel bars in August 2015 were studied along 14.0-km river reach of the Bečva River (Outer Western Carpathian Mts., Czech Republic). The grain-size characteristics obtained were confronted with modeled flow hydraulics and the present stage of the channel. The studied channel reach is presently characterized by several distinctive sections: for a long time (ca. 100 years) regulated single channel sections with artificial bank stabilizations incised several meters in the floodplain and by contrast, multi-thread channel patterns of two sections, which have witnessed retrograde development after large floods in 1997 and 2010 with 100- and 50-year recurrence intervals, respectively. The present channel behaviour of managed (regulated) and re-naturalized (multi-thread) river sections corresponded well with the modeled hydraulic variables for one-year discharge recurrence interval. Especially, re-naturalized river sections showed lower values of flow competence which facilitated the deposition of sediment material in the form of gravel bars. The high occurrence of lateral sediment sources (e.g., tributaries, bank failures) together with sediment disconnectivities (e.g., boulder ramps) in the longitudinal river reach were observed, and grain-size parameters did not particularly reflect the hydraulic conditions. Especially tributaries as sediment inputs had significant effect on bar grain size and increase of channel diversity, although, in general results indicate a gradual downstream fining.