Degradation of multi-thread gravel-bed rivers in medium-high mountain settings: Quantitave analysis and possible solutions

The new article about possible solution of gradually transformed Morávka River channel was published in April 2020. The research was realized with Nature Conservation Agency of the Czech Republic in 2017 (see report here).

The Morávka River was characterised large amount of gravels transported from mountainous area of Beskydy Mts to the forefield. Presently, large volumes of gravels is stopped in the upstream part of the Morávka R. basin by a water reservoir and we should also note the presence of check dams in small tributaries together with increase in forested area during 20th century. A combination of field and laboratory research with modelling of the transport stage and the potential bedload transport to determine the dominant discharge was used to understand the process that is necessary to manage Morávka River system. Artificial disturbance of the vegetation cover (which stabilise gravel material) and the relatively rare high-magnitude floods were identified as important factors that could help to increase and preserve the Morávka River active channel dynamics (in Skalická Morávka nature reserve especially). Because sediment balance of the Morávka River system was negatively affected, the estimation of the sediment material deficit as a tool for support, protection and sustainability of the close-to-natural multi-thread channel reach in the nature reserve was necessary. The estimation of absent sediment volumes during ~1- and 20-year recurrence interval of mean daily discharges based on the analysis of eroded material from repeatedly measured cross sections will help to stop further degradation of other multithread gravel-bed rivers. More about these conclusions are available in article which was published in journal Ecological Engineering.

Schematic showing estimated parameters for eroded material volume calculation of individual segments in the upper part of the nature reserve in the monitored one-year period, showing examples of cross sections at 10.33 r. km; d′i is ½ the length between the given cross section and the adjacent upstream cross section, di is ½ the length between the given cross section and the adjacent downstream cross section, Vi, year is the volume of the eroded/aggraded material of the individual segment in the monitored one-year period, Xi-X'i shows the schematic location of a cross section, (A) Ai, year−1 is the flow area of the channel from the cross section at the beginning of the one-year monitored period in the section where erosion was observed at the end of the monitored period, and (B) Ai, year is the flow area of the channel in the section where erosion was observed from the cross section at the end of the one-year monitored period.

The extension of gravel bars and islands in the active channel of the Morávka River in the nature reserve between 2000 and 2016 and in 1955.

Bed stability of a steep headwater stream during a large flood event

Our new article dealing with dendrogeomorphic methods, hydraulic simulations and large grains located in channel bed of a steep headwater stream was published by Water journal. We focused on the role of a high-magnitude flash flood event (perhaps 100y recurrence interval) and related morphological changes in the channel. Namely, we used an integrated approach that included the dendrogeomorphic reconstruction of past flood activity, hydraulic modelling of the 2014 flash flood parameters using a two-dimensional IBER model, and evaluation of the channel stability using sedimentological parameters. Based on this approach, we observed that the excess critical unit stream power had occurred at only one-quarter of the studied cross-sections. It implies that despite local damage during the last flood, our results suggest relatively stable geomorphic conditions and gradual development of stream channels under discharges similar to that in 2014. 

The article is ready for download here. 

Tichavský, R., Ruman, S., Galia, T. (2020): Hydrogeomorphic impacts of floods in a first-order catchment: Integrated approach based on dendrogeomorphic palaeostage indicators, 2D hydraulic modelling and sedimentological parameters. Water 12(1), 212. https://doi.org/10.3390/w12010212

Results of 2D hydraulic simulations

Ongoing monitoring of tagged bed particles

We continue with the monitoring of bed particles marked by pit-tags, which have been installed into stream beds of two contrast small streams (check-dam impacted and close-to-nature pool-riffle stream) with similar catchment areas. The monitoring covered one bankfull event and several close-to-bankfull discharge events in following months. After ca. 21-month period, up to 350 m transport distance was measured for tagged particles in the check-dam managed stream; some of grains passed through four check dams. Similar transport distance (up to 300 m) was obtained in the pool-riffle stream. However, there were large differences in recovery rates of pit-tag-marked grains. We reached up to 65% recovery rate in the check-dam managed streams, but only 20-25% recovery rate prevailled in the pool-riffle stream. This notably low recovery rate is likely caused by higher morphodynamics of the pool-riffle stream, which caused frequent burying of the marked grains into gravel-bed deposits during sediment transport-effective discharges. We are planning to continue this monitoring also in the following year; this will also include a development of rating curves for both studied streams, which will allow us to create direct relations between the critical discharge and transported distance (or size) of tagged particles. 

Monitored check-dam impacted stream

Monitored pool-riffle stream

Abandoned channelization works in Beskydian steep streams

The major part of the Moravskoslezské Beskydy Mts. was unaffected by human activities until the 17^th century. Since that times, local forests started to be managed as sources of timber and first villages have been built the valleys. At the turn of 19^th and 20^th century, the first large flood-protective structures had been constructed, which mainly consisted of long stone embankments and check dams. Some of these structures were later not needed and local authorities did not attempt to repair them after damages caused by flood events. We are still able to find their remains in some headwater parts of the mountains. Nowadays, they can act as disconnectivities in lateral sediment flux by abruption of effective hillslope-channel coupling or by contrast, they can serve as sources of coarse (for the most time except extraordinary floods immovable) sediments into stream channels. Nevertheless, their presence clearly documents past vital activities of local people in relatively inaccessible headwater parts of the mountain valleys.      

Long stone embankment, the Slavíč Stream

Remains of check dam, the Slavíč Stream

Predictors of low instream wood abundance in Cretan steep streams

Our new paper demonstating the role of low tree density in riparian corridors on low abundance of instream wood in steep nonperennial streams was just published in Water Resources Research. Thus, the paper provides deeper understanding of recruitment and retention procecess of instream wood at the reach scale in vulnerable environment of Mediterranean semiarid landscapes, which was based on our detailled measurements in 37 independent channel reaches in Lefka Ori Mts., Crete, Greece. The parameter of elevation controlled mean annual percipitations, which in turn were reflected in the composition of riparian canopy along the steep mountain streams. It is also interesting that the mobilized wood tended to be stored as individual, dispersed pieces rather than in jams, likely due to overall low instream wood abundance which effectively prevented clustering of wood into large jams.

Deposited instream wood in the Kallikratis gorge. 

The study of river restoration potential of the Černá Ostravice River

Human interactions with mountain streams in Czech part of the flysch Western Carpathians has more than 100 years of history. These are represented by indirect impacts since the 16th century linked with an extensive changes in land use, and by direct human interventions connected with control works, check-dam constructions (e.g., due to timber floating, flood protection, sediment regime control). We finished project which was focused on potential restoration measures in the Černá Ostravice River basin in Moravskoslezské Beskydy Mts in Czech Republic. The project was realized in cooperation with the Union for the Morava River. The study proposes potential measures to improve hydromorphological and habitat conditions in the lower reach of the Černá Ostravice River. The main measures were aimed to increase stream connectivity for aquatic communities or sediment flux, increase morphological heterogeneity and occurrence of instream wood as element affected better stream habitat conditions and increase of water retention in floodplain.

Bank stabilization structures protecting road.

Regulated channel (partially due to timber floating).

Check-dam controlled longitudinal profile of the channel.

Carpatho-Balkan-Dinaric Conference on Geomorphology

Our fluvial group presented research on Carpatho-Balkan-Dinaric Conference on Geomorphology. The Conference was held from June 24 to 27, 2019 at Szeged in Hungary. It was focused on rapidly changing world, various environmental hazards which cause considerable damages and loss of life, or their consequences change the rate of geomorphological processes and forms in the Carpatho-Balkan-Dinaric Region. We participated in sessions which were focused on problematics linked with human impact on fluvial processes and systems. Our contributions were dealing with Morphological response of mountain streams to long-term human interventions in the Czech Carpathians presented by Tomas Galia, Fluvial system diversity in relation to confluence hydrodynamic zone – a review study by Lukáš Vaverka, Morphological effects of long-term low sediment inputs – the Elbe River by Jan Hradecký, Public perception of gravel bed rivers. A case study from the Czech Carpathians by Zuzana Poledniková, Possibilities for stopping of further degradation gravel-bed Morávka River by Tereza Macurová, and Conceptual framework for restoration of Czech Carpathian Rivers by Václav Škarpich.