The effect of geological channel structures on floodplain morphodynamics of lowland rivers: A case study from the Bug River, Poland
Introduction
The variable nature of erosional and depositional processes is a hallmark of a fluvial environment (Vandenberghe, 2002, Fryirs and Brierley, 2013) resulting not only from (i) maturing of the valley landscape (Davis, 1899), climate changes (Knox, 2000, Starkel, 2001), tectonic activity (Ouchi, 1985, Wang et al., 2005) but also from (ii) impact of the humans on the catchment area (Falkowski, 1975, Łajczak, 1995, Gaillot and Piègay, 1999, Biedenharn et al., 2000, Brandt, 2000, Keesstra et al., 2005, Gregory, 2006, Hooke, 2006, Martìn-Vide et al., 2010, Miller and Kochel, 2010, Gendaszek et al., 2012, Magliulo et al., 2016)
In the case of lowland river valleys, where river channels have developed within alluvial deposits (so-called alluvial rivers; Leopold et al., 1964), the nature of fluvial processes should depend mainly on the hydrological regime of the catchment. Considerable thickness of alluvial sediments in such cases allows the channel pattern to freely adapt to the flow conditions. As a result, the evolution of the river system is recorded in the morphology and lithology of the floodplain (Falkowski, 1975, Starkel, 1983, Kalicki, 1996).
Floodplain morphology is commonly used in assessment of the morphodynamics of fluvial environments (Vandenberghe, 2002). Individual sets of landforms may not only reflect the hydrological regime of the river (Leopold and Wolman, 1957, Allen, 1970, Schumm, 1971, Starkel, 1995) but also facilitate assessment of the dynamics of individual phenomena accompanying the formation of the floodplain’s surface (Bristow et al., 1999, Arnaud-Fassetta, 2013). Individual sets of landforms may also be associated with successive stages of inundation (Zwoliński, 1992), facilitating assessment of the dynamics of such events and potentially useful for constructing forecasts of floods course (Wierzbicki et al., 2013). Identification of specific morphogenesis of a floodplain surface can serve the use of natural phenomena to manage the floodplain area (Yuill et al., 2016). The concentrations of individual types of landforms may also prove the specificity of the course of fluvial processes in individual sections of the valley (Nelson and Leclair, 2006, Li and Bristow, 2015).
The river valleys of the Polish Lowlands are alike the river valleys of other lowland areas in this temperate zone due to the similar mechanism and course of their Holocene evolution, (Starkel, 1983, Macklin et al., 2006) and their similar contemporary morphodynamics (cf. Schanzer, 1951, Zwoliński, 1992, Wierzbicki et al., 2018). However, the rivers of the Polish Lowlands have some distinctive features resulting from their geological structure (Falkowski, 1990, Falkowski, 1997a, Falkowski, 1997b, Falkowski, 2007b). Many sections of their valleys can be considered as geomorphologically immature and do not have fully developed erosive bases (Falkowska, 2015, Falkowska and Falkowski, 2015). This is due to the fact that valley-forming processes could only start once the last Scandinavian ice sheet had disappeared. In central Poland, this took place after the Odra Glaciation (MIS 8), only approximately 250 ka, whereas in the northwestern part of the Polish Lowlands, this occurred even later, after the Vistulian Glaciation (MIS 2), approximately 22 ka (Lindner and Marks, 1995).
The lack of maturity of many sections of river valleys in the Polish Lowlands is evidenced by the fact that fluvial processes have not obliterated all traces of the complex genesis of the valleys’ reaches. This is often evidenced by two kinds of features: (a) distinctive knickpoints in the longitudinal profiles, marking erosion-resistant outcrop deposits in the channel zone (Falkowski et al., 1988, Falkowski, 1990, Falkowski, 1997a, Falkowski, 2007b; cf. Mackin, 1948, Phillips and Lutz, 2008, Marrucci et al., 2018); and (b) glaciogenic and thermokarst depressions adapted to the river valley reaches (Błaszkiewicz, 2010, Falkowska, 2003, Falkowski, 1975, Falkowski, 1990, Falkowski, 1997a). The former can also often be linked with glaciotectonic disturbances and the latter – with the presence of non-alluvial deposits, such as lacustrine or ice dam, filling parts of many of these valleys’ reaches (Babiński et al., 2014, Falkowski et al., 2017).
Herein, we present our investigation of the effect of erosion-resistant alluvial bedrock on fluvial processes in places where this bedrock forms morphological protrusions. It is relatively easy to determine the influence of this kind of channel geological structure on river behaviour within reaches with permanent bedrock protrusion outcrops. However, in the opposite case, where bedrock protrusions in the channel bottom are hidden beneath the channel alluvial layer during dominating periods of the hydrological year, it is difficult to determine the effect (of great importance for river management). During low and medium flows, resistant deposits do not affect the morphodynamics of the river in a distinct manner (compare Falkowski, 2006, Falkowski, 2007a, Bujakowski and Falkowski, 2019). In contrast, during passage of the flood pulse, the increase in flow energy causes a deepening of the channel bottom (see Leopold et al., 1964) and exposure of the top surface of the channel. Where erosion-resistant sub-alluvial bedrock protrusions are observed (constrained channel, following Meshkova et al., 2012), the channel cannot be deepened and further increases in water flow result in breaking the natural levee barriers and entering the floodplain area. This can be a recurrent phenomenon whenever the highest water stages are reached.
In sections where Polish Lowland rivers are managed and flood protection levees are built, the geomorphological active zone is restricted by these structures to only a narrow section of floodplain. Consequently, the impact of the sub-alluvial bedrock surface morphology on high water current directions is limited, but it is clearly visible in the channel bottom morphology. The arrangement of riffles and pools is related to the bedrock protrusion relief and is stable in time, as far as the shape is concerned (Falkowski et al., 2018). This phenomenon was recorded by the authors multiple times via bathymetric measurements in Polish Lowland rivers, under various flow conditions, including floods (Falkowski, 2006, Falkowski, 2007a, Falkowski, 2007b, Falkowski et al., 2017, Wierzbicki et al., 2018, Bujakowski and Falkowski, 2019). The effect of bedrock protrusion influence on high water current directions is highlighted by the locations where flood-protection embankments have broken (Falkowski, 2006, Falkowski, 2007a, Falkowska and Falkowski, 2015, Wierzbicki et al., 2013, Wierzbicki et al., 2018, Bujakowski and Falkowski, 2019; see also Reid, 1984, Jin and Schumm, 1987, Schumm and Thorne, 1989).
Therefore, peculiar morphologies of the valley bottom and river channel section consisting in the presence of landforms indicating the repeatability of the flood course are believed to be closely related to both the geological structure (Falkowski, 2006, Falkowski, 2007a) and hydrological conditions (see also Liu et al., 2019) (Fig. 1). The objective of the research being presented here is to determine the impact of channel geological structures on floodplain morphodynamics, with the use of very high resolution multispectral satellite imagery. The morphology of a floodplain indicates flood flow dynamics, (Zwoliński, 1992, Wierzbicki et al., 2013) therefore, the distribution of different overbank erosional and depositional landforms in the valley reach should be used as a mark of the impact of geological structures on fluvial processes. If the course of morphogenetic processes in a floodplain could be refined by recognition of valley reach specificity, this would facilitate the design of appropriate hydrotechnical structures, and a general reduction in flood consequences as well as flood risk (Wierzbicki et al., 2020). It could also promote a better understanding of the distribution of depositional environments on the floodplain surface, that influence among others on the special differentiation of pollution of valleys (Myślińska et al., 1993; Falkowska et al., 2016, Falkowska and Falkowski, 2015, Podlasek et al., 2020).
The impact of the geological structures on the course of fluvial processes in the alluvial valley discussed in the article is predicted to increase due to ongoing evolution of fluvial environments (Falkowski, 1975, Kozarski and Rotnicki, 1977, Starkel, 1983, Starkel, 1995, Starkel, 2001, Knox, 2000, Mackin and Lewin, 2008). This evolution manifests itself as an increase in the magnitude of maximum flows and a decrease in the magnitude of minimum flows (Ozga-Zielińska, 1997, Blöschl et al., 2017, Hajdukiewicz et al., 2018). In the case of many fluvial environments this is due to the magnified water and bedload runoff speed in the catchment area resulting from its management. Recorded climate changes in the temperate zone have also influenced the hydrological regime (Knox, 2000, Starkel, 2001, Alfieri et al., 2015, Forzieri et al., 2016, Blöschl et al., 2017). It can be expected that, concurrently with the gradual increase of high water discharges and bed erosion, the sub-alluvial erosion-resistant layer will be exposed at the bottom of river channels more often. This will, in turn, increase geological control over floodplain morphodynamics. Therefore, determination of floodplain morphogenesis for the management of river valleys will become increasingly significant in the near future.
Section snippets
Study area
The Bug River is 755 km long, with a catchment area of 39,420 km2, 49% of which is located in the Ukraine and Belarus (Czarnecka, 2003). The work was conducted in the Podlasie Gorge (eastern Poland) of the Bug River valley, along a nearly 20-km-long reach between the Polish-Belarusian border and the village of Mierzwice (Fig. 2, Fig. 3). The width of the studied valley section ranges from 0.9 to 2.9 km. The average width of the channel during mean water stages ranges from 100 to 200 m. The
Remote sensing
The remote sensing analysis was of key significance in the identification of overbank erosion and depositional landforms. This analysis was conducted using very high resolution (VHR) multispectral satellite imagery. The VHR images, with a spatial resolution of 0.82 m, were taken by the IKONOS 2 satellite between May and August 2007, under moderate water-level conditions. Based on these images, the orthophotomap was prepared using two wavelength bands: the natural colour band (red, green and
Morphology and lithology of the valley bottom
The analysis covered a portion of the Bug River valley bottom that is affected by contemporary flood flows, i.e., the Holocene floodplain zone (FMT and FC). Along the examined section, traces of contemporary flood flows have also been identified within the Pleistocene upper terrace (UTT). Due to this observation, the present form of the UTT surface should be considered as a part of the active floodplain. However, it differs from the typical Holocene floodplain characterizing river valleys of
Discussion of the results
The section of the lower Bug valley analysed in the presented work is an area with a geologically diverse structure, and the specific relief of the floodplain indicates its complex morphodynamics.
The part of the valley where the dynamics of modern channel processes depend on the sub-alluvial bedrock extends beyond the valley reach analysed in this paper (upstream from Niemirów) to the area of Janów Podlaski (Fig. 2). This is indicated by the same irregular channel curvature. Upstream this
Conclusions
The river channel zone in the studied section of the Podlasie Bug Gorge has a complex geological structure. The thickness of its contemporary alluvial deposits is variable, and the channel shows numerous bedrock protrusions composed of high erosion-resistant deposits, often additionally stabilized with a channel lag.
Protrusions of the sub-alluvial bedrock are features that directly affect the morphodynamics and associated floodplain relief. This is highlighted by characteristic landforms
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The findings presented in the article are the result of research conducted by the authors' team under the research grant No. 2 P04E 06929 of the Ministry of Science and Higher Education: Significance of morphogenetic factors in the development of habitat diversity of selected river valley stretches in the Polish Lowlands. Authors would like to thank the anonymous Reviewers and the Editor for their insightful comments and valuable suggestions that helped to enrich the article. Authors also want
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