Wednesday, 24 October 2018

Effect of hydraulics of regulated and re-naturalized channels on grain-size parameters

With co-operation with our colleague Assoc. Prof. Dr. Zdeněk Máčka from Masaryk University, we published article about effect of hydraulics of regulated and re-naturalized channels on grain-size parameters in Science of the Total Environment multidisciplinary journal.

The grain-size characteristics obtained on 68 gravel bars were confronted with modelled flow hydraulics and information obtained by fluvial-geomorphic mapping along 14.0-km river reach of the Bečva River (Outer Western Carpathian Mts., Czech Republic). 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.
The mapping of sediment (dis)connectivity brought important findings about the character of sediment flux in the studied reach, which were confronted with simulated cross-sectional hydraulics. We demonstrated that in the case of a high occurrence of lateral sediment inputs (tributaries, bank failures) and longitudinal sediment flux disconnectivities (weirs or boulder ramps), the assessment of the longitudinal distance, bar grain size and simulated hydraulics submerging bars did not produce any clear relationships. Although the sections with re-naturalized multi-thread patterns showed distinctive hydraulic variables (i.e., larger wetted width or lower unit stream power), we did not observe direct relationships with their bar sediment sizes. This implies that for complex fluvial systems of multi-thread rivers as the transition reaches connecting mountainous and lowland areas, even those in unconfined valley settings out of the primary sediment sources, additional factors (i.e., effect of bank failures and especially tributaries as sediment inputs, weirs or boulder ramps as sediment flux disconnectivities) beyond local flow hydraulics and distance from the main sediment sources contribute to better explanation of the downstream evolution of grain-size patterns.

View of the Bečva River channelized (managed) single channel sections (A, C, E) and retrogradually developed (re-naturalized) multi-thread channel sections (B, D).

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