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Geomorphic/Sedimentary Responses of Rivers to Floods: Case Studies from Slovakia
Abstract
The aim of this study is to outline the geomorphic/sedimentary responses of three Slovak river systems of different character (a non-channelized meandering gravel-bed river, a mixed-bedrock headwater river, and a channelized section of a large alluvial river) to extreme flood events. Lateral channel shifts and the spatial variability of channel landforms as responses of a non-channelized gravel-bed stream to flood events were studied on a 13.2 km long reach of the Ondava River in two time horizons, 2002 and 2009 (reference year: 1987). Two different methods were used to quantify the geomorphic effect of floods on the Topl’a River. First is the analysis of the remotely sensed imagery before (September 2006) and after (October 2009) the July 2008 flood. Second is the analysis of representative cross sections measured on each of the 78 delimited channel reaches. Lithofacies studies of the Danube River overbank deposits were conducted on the right-bank inter-dike inundation area (active floodplain) of 300–600 m width. Case studies prove that the responses of different river systems to floods are governed by a combination of ‘global’ laws and ‘local’ spatial and/or temporal factors (different settings and scales).
... The application of remote sensing and GIS in interpretation of river geomorphic response to floods is presented in two case studies (the braided and meandering gravel bed rivers) where the basic source for the extraction of geomorphic objects were orthophotographs and aerial photographs. In case of the braided stream (the River Belá, Slovakia), it was from seven time horizons (1949, 1961, 1973, 1986, 1992, 2003, and 2009) and in case of the gravel beds and meandering streams (the River Topľa, Slovakia), it was from six time horizons (1949,1961,1981,1987,2002,2009) (Kidová and Lehotský 2012;Kidová et al. 2016;Lehotský et al. 2013;Rusnák and Lehotský 2014). Time horizons of object extraction were chosen based on the analysis of hydrographs in a way that captured the state of the river system after periods with occurrence of specific N-year floods. ...
... On the contrary, extreme flood events with 5-10 year recurrence intervals led to a pronounced increase of channel dynamics. In accord with the results of Lehotský et al. (2013) or Rusnák and Lehotský (2014) obtained from other east-Slovakian River Ondava, the lower discharge is precisely responsible for the formation of the channel, ground plan pattern, and relatively slow bank erosion connected with the overall behaviour of the river. ...
... These extreme floods result in significant sediment transport and cause widening or incision of the river bed [19]. In the lowland areas, heavy flooding could lead to intense bed migration and avulsions [20,21], and intense aggradation in floodplains and some segments of the channel [22]. A decrease in annual precipitation and discharge triggers opposite processes. ...
In the 21st century, climate change and its consequences are getting more serious. The changes in temperature and precipitation alter the run-off conditions, subsequently influencing the channel processes of rivers. The study aims to analyse the hydrological changes of a small, sub-alpine river (Rába River, Central Europe) and the bank erosional processes since the 1950s. The bank erosion was determined based on topographical maps, aerial photographs, and RTK–GPS surveys. Short (2–3 days) floods were common between 1950–1980, and low stages occurred in 65–81% of a year. However, in the 21st century, extreme regimes developed, as record-high, flashy floods alter with long low stages (91–96% of a year). The bank erosion shows a cyclic temporal pattern, gradually increasing until it reaches a high value (4.1–4.9 m/y), followed by a limited erosional rate (2.2–2.8 m/y). However, the magnitude of the bank erosion is decreasing. It could be explained by (1) the lower transport capacity of the more common low stages and (2) the seasonal shift of the flood waves, appearing in the vegetational period when the riparian vegetation can more effectively protect the banks from erosion.
... These catchments have many common features, including the mountainous character of their upper parts manifesting itself in a quick reaction to rainfall and high dynamics of surface runoff (Korpak et al., 2021;Wyżga et al., 2021). The similar characteristic is also shared by the catchments on the Slovak side [Rusnák et al., 2018;Barabas et al., 2017;Lehotský et al., 2013]. ...
Although the Carpathian Mts. area is considered as extremely prone to sur-face erosion which results in capacity loss of the dammed reservoirs, a lack of data to follow details of this process is perceivable. The research of the se-lected sediment fractions transport tracking was conducted using the capa-bilities of the digital platform—Macromodel DNS (Discharge- Nutrient- Sea) for the catchment with drinking water reservoir in the Polish part of Western Carpathian. The continuity of sediment transport simulation in two hydrologi-cally different elements of the catchment—the river and the reservoir—was possible due to consolidation of two models in the platform—SWAT (Soil & Water Assessment Tool) and AdH/PTM (Adaptive Hydraulics Model/Particle Tracking Model). The result of those modules' integration was a database for tracking the individual sediment fractions delivered to the reservoir and de-posited in specific reservoir zones. The implementation of climate and land use change scenarios allowed additionally to analyse the estimation of those processes in the future. The simulation outcomes consist of daily flows and monthly sediment loads at the reservoir inflow and the individual sediment particle fractions deposition location inside of the reservoir.
... The last study period (2009) was specific by stabilization of central bars and their transformation to islands what affected the overall simplification trend of the Belá River pattern from braided to transitional wandering-braided one [8,9,10]. Lately gravel mining and river sediment replacing definitely support this trend in time when the opposite nature-based management approaches and challenges (channel renaturalization of the rare and unique river system in Slovakia -NATURA 2000, protected area) are the most desirable [ 7,11,12,13,14]. Similar development on the east Slovak the river Ondava and the Topľa with decrease channel width from 87.6m to 32.6m and from 62.1m to 27.2m confirms the necessity to protect so vulnerable natural elements [15,16,17,18]. Responsible river management with protection from gravel mining and green approaches to reduction flood risk are essential for sustainable development of natural active channels. ...
Mountain stream gravel is very often legally and illegally mined and gravel is removed from river beds sometimes on the very large scale which is disastrous for fluvial situation of rivers, for river ecology and river engineering works done for flood protection such as river revetments, bridges and all hydraulic structures. This situation makes a big problem for all river managers. Thus gravel mining of the mountain streams in the Polish Carpathians is the subject of many scientific studies when we observe river problems, but also it has a place in Slovakia. This paper deals with such problems additionally showing examples of such bad practices.
... In mountain streams where the streambed consists mostly of gravel and coarse sands, the bed load is reported to constitute in some cases even up to 70 per cent of the total bed load [15]. Mountain stream gravel is very often both legally and illegally mined from riverbeds, which is disastrous for the fluvial state of rivers and for river ecology; furthermore, it causes the destruction of flood protection strategies, river revetments, bridges and all hydraulic structures [16][17][18][19][20]. This situation presents a major problem for all river managers. ...
The prediction and calculation of the volume of gravel and/or sand transported down streams and rivers—called bed-load transport is one of the most difficult things for river engineers and designers because, in addition to field measurements, personnel involved in such activities need to be highly experienced. Bed-load transport treated by many engineers marginally or omitted and often receives only minor consideration from engineers or may be entirely disregarded simply because they do not know how to address the issue—in many cases, this is a fundamental problem in river management tasks such as: flood protection works; river bank protection works against erosion; building bridges and culverts; building water reservoirs and dams; checking dams and any other hydraulic structures. Thus, to share our experience in our paper, bed-load transport was calculated in two river/stream mountain catchments, which are different in terms of the characteristics of the catchment area and the level of river engineering works performed along the stream channel—both are tributaries of the Dunajec River and have similar Carpathian flysh geology. The studies were performed in the Mlyne stream and in the Lososina River in Polish Carpathians. Mlynne is one of the streams in the Gorce Mountains—it is prone to flash flooding events and has caused many problems with floods in the past. It flows partially in the natural river channel and partially in a trained river channel lined with concrete revetments. The stream bed load is accumulated in the reservoir upstream of the check dam. The Lososina River is one of the Polish Carpathian mountainous streams which crosses the south of the Beskid Wyspowy Mountains. It mostly has a gravel bed and it is flashy and experiences frequent flooding spring. At the mouth of the Lososina River, there is one of the largest Polish Carpathian artificial lakes—the Czchow lake. The Lososina mostly transports gravel as the bed load to the Czchow water reservoir where the sediment is deposited. In the early seventies, the Lososina was partly canalised, especially in places where passes inhabited areas. The paper compares the situation of bed-load transport in the Lososina River before and after engineering training works showing how much sediment is transported downstream along the river channel to the Czchow artificial lake. Also compared is the Mlynne bed load transport upstream and downstream from the check dam showing how much sediment might be transported and deposited in the reservoir upstream from the check dam and when one could expect this reservoir to be clogged.
The central part of Ondavská vrchovina upland represents
a medium-altitude and moderately dissected relief with
narrow elongated valleys. The Topľa and Ondava rivers
are the main axis of flysch valleys and their flat bottoms
are modelled by dynamic river processes acting upon thin
layers of Quaternary sediments. They bear a record of all
historical changes of the channel pattern, from a wide
wandering gravel-bed wild river to a narrow and sinuous
channel. Both rivers are characterized by distinct bank
erosion, resulting in bank movement and bank retreat
within meander bends. From an economic point of view,
erosion of arable land and grassland is a negative
consequence of channel migration. Vice versa, from an
ecological point of view we can consider bank erosion as
a natural process that leads to the increase of geo- and
bio-diversity of the riparian landscape. Nowadays, ‘green
approaches’ are applicable in the river management,
aiming to eliminate technical interventions in the river
basins and allowing for free channel migration. All these
processes create valuable ecosystems with natural flood
regime and floodplain forest.
The changes in river morphology result from several factors such as climate, human interventions, lithology or land use and land cover changes. This paper shows morphological changes of two different river systems (braided-wandering of Belá and sinuous gravel bed of Ondava) over the last 60 years as a response to changes of land use, flood events distribution changes and human impact in the basins. Methodological framework is based on analysis of aerial photographs from seven time horizons during the 1949 – 2009 period, and analysis of hydrological data (maximal annual discharges). General trend of evolution of the river planforms is characterised by sinuous channel degradation, i.e. narrowing and straightening as well as multi-thread planform simplification. The possible hypothesis was that the behaviour of the braided-wandering Belá River represents a situation close to a threshold – degrading behaviour of a multichannel river system. In the long-term consideration, simplification of the originally unfolded braided channel pattern and progressive transformation into the wandering type was expected. Extreme discharges in the sinuous gravel-bed river induce considerable shifts of river banks and channel migration. Gradual loss and destruction of the area along the concave bank and deposition of material on the opposite bank were the processes that control the river flow along the stream. Flood events with recurrence interval of 10-years are significant for local geomorphic reworking of the studied river systems.
article i nfo Article history: Accepted 4 January 2009 Available online xxxx The paper outlines contemporary geomorphic and sedimentary changes (1949-2007) in the suburban inter- dike reach of the Danube River in Bratislava underpinning requirements of its management. The studied river reach represents the right bank inter-dike area of a tectonically determined bend of the Danube River which is about 5 km long with a radius of 1.5 km. The landform and floodplain roughness classification schemes show variations in hydraulic and depositional conditions during flooding. Investigation of landform and floodplain roughness changes and the bank retreat rate is based on the multitemporal interpretation of aerial photographs. Sedimentological analysis of 10 borings and 20 pit exposures helped to delimit the morphostratigraphic units. The process and the rate of the vertical accretion of the modern floodplain were investigated by sedimentological and dendrochronological methods. The behaviour of the study reach is dependent on upstream dams in Austria. Dams produce changes in the suspended load regime and a steady riverbed erosion of 2 to 3.5 cm occurs upstream of the study river reach near the Slovak-Austrian border. The Čunovo dam downstream expediates upstream channel filling in spite of the ongoing channel gravel mining in the Danube channel. This has resulted in channel bed aggradation of about 1 m since 1992. The bank retreat of about 100 m during 1949-2007 was caused by the natural bank erosion processes as well as by channel straightening as a flood control measure for Bratislava. The bank shift resulted in the development of the new levee. The current overbank deposits differ from the older gravels, consisting of a fine-grained alluvium varying in thickness from 0.5 m to 1 m. The lithofacies of three flood deposits (March 2002, August 2002, and September 2007) were examined. Overbank sediments differentiate with distance from the bank and vertically. Suburban settings of the study reach and the flood control measures along with the external forcing (dams) induced changes in channel morphology as well as in floodplain roughness variability. This has resulted in changes in the present floodplain sedimentary conditions, channel forms and biodiversity. The landscape evolved through three distinct evolutionary phases.
This book outlines a generic set of procedures, termed the River Styles Framework, which provides a set of tools for interpreting river character, behavior, condition, and recovery potential. Applications of the framework generate a coherent package of geomorphic information, providing a physical template for river rehabilitation activities. management and restoration of rivers is a rapidly growing topic for environmental scientists, geologists and ecologists - this book provides a learning tool with which to approach geomorphic applications to river management describes the essential geomorphological principles underlying river behaviour and evolution demonstrates how the River Styles Framework can turn geomorphic theory into practice, to develop workable strategies for restoration and management based on real case studies and authors extensive experience applicable to river systems worldwide synthesises fluvial geomorphology, ecology and management.
Along floodplains there occurs an ordered sequence of geomorphic events during floods. The following sequence occurs for individual flood events: (1) the rise of floodwater and of groundwater, (2) the inundation of the floodplain; (3a) adjustments of the overbank flow pattern to the floodplain environment; (3b) the flood peak; (4) the initial fall of floodwaters and changes in the overbank flow pattern; (5) a gradual cessation of the overbank flow; (6a) infiltration, evaporation and a wave-like motion of stagnant water; and (6b) postflood transformation under subaerial conditions. Field observations support this model for meandering rivers in the Polish Lowland.
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
Channel adjustments of the Danube River near Bratislava, Slovakia, are reconstructed from historical river maps for 1712–1886, the period preceding mid-course channelization. The study reach comprised an anastomosing–meandering planform, characteristic of the upper area of a large alluvial fan. The key mechanisms of channel change were avulsions through channel switching into chute channels, meander development through progression and cutoffs, and abandonment of secondary channels. The gravel-bed Danube River was characterized by rapid rates of lateral erosion (maxima from 7·5 to 37 m a−1), and extensive areas of point bars and gravel bars. Within three to seven years, new bars were stabilized by Salici–Populetum woodlands which subsequently developed into more stable vegetated islands. Human interventions were relatively common and frequent in the past, though increasing fluvial activity during the 18th century was responsible for their limited effectiveness. Uncontrollable lateral erosion at Petržalka village, terminated only by a switch in position of the Danube River upstream from Bratislava (1766–1774), appears to be closely related to increasing ice-floods and other high magnitude flood events in the latter half of the 18th century. These events were coincident with the peak of the Little Ice Age. The effect of floods was probably amplified by human works which resulted in stabilization of secondary channels, simplification of the river pattern, and concentration and widening of the main channel. A further sign of channel readjustment to new conditions was the evolution of new large meanders, a tendency for channel switching and production of new anastomosing–meandering parallel channels. Copyright © 2002 John Wiley & Sons, Ltd.
This paper investigates the impact of a 1000-year flood in August 2002 on floodplains and valley morphology of an Austrian mixed alluvial bed rock river. Discharges with a recurrence interval between 500 and 2000 years caused distinctive overbank scouring and material deposition in the floodplains. After the 1000-year flood, those morphologically affected areas were at random intervals documented over the whole longitudinal profile. In addition to overbank erosion in curved sections (cut-offs), the river bed locally widened, floodplain stripping occurred and local overbank scours were documented along straight parts of the river. A hydrodynamic-numerical model, combined with field measurements, was used to analyse the cause of these erosional landforms. Based on the modelled hydraulic conditions for a one-year flood (30–78 m3 s–1) and the catastrophic 2002 event (700–800 m3 s–1), the numerical results allowed a cause-effect study with 19 parameters. Deterministic and statistical analysis (ANOVA, discriminant analysis) showed that the morphodynamic effects of the 2002 flood were influenced by the variability of valley morphology of the Kamp River, which led partially to supercritical flow during flood constriction. These processes were in some cases also anthropogenically influenced. Lateral constriction and expansion of the valley geometry over short distances led to scouring and aggradation within the inundated areas during the event. These morphological features were therefore responsible for the elongated scour holes in the floodplains. Copyright © 2009 John Wiley & Sons, Ltd.