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Metamorfózy korýt štrkonosných vodných tokov Ondavskej vrchoviny
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The article presents the postflood period coarse sediment connectivity (CSC) assessment approach on the basis of the holistic ideas of the interpretation of the long-term CSC in river channels. Seven sets of remote sensing data (1949–2009) were used as the basic information sources to study the CSC along the braided-wandering Belá River by using the patch-graph approach for assessing the structural connectivity. The bar areas (patches), with 200 m direct links between them, and the attribute of a deflection angle of a link to the flow direction were parameters used to estimate the integral index of connectivity (IIC) and bar area and bar link importance indices. The higher values of the IIC fit with a well-developed bar system. Conversely, the values of the IIC decrease because of the progressive degradation of the braided pattern. The potential functional CSC was inferred from the variability of the values obtained by the balance indices representing the balance of the channel–floodplain connectivity (B1) and the bar–bar connectivity (B2). Eight types of the potential functional CSC based on the balance indices and the deduction of processes that conditioned the channel–floodplain and in-channel CSC linkages were identified. In order to assess the development of the braidplain pattern in terms of the structural connectivity during the study time span, the linear trendline analysis of IIC associated with flood periods was applied. On the basis of the coefficient of determination, all reaches exhibit a decreasing trend in IIC values but different response to flood periods.
The bank erosion area, rate of bank retreat and overall geomorphological and financial effects of channel migration due to recent flood events (over the time span 1987–2009) are identified using remotely sensed data and GIS. A 39 km-long reach of the meandering, gravel bed Topľa River (Flysch Slovak Carpathians) was selected as the study area. Based on the analysis of culmination discharges, two different flood periods are identified. The first one (1987–2002) is characterised by the dominance of low magnitude flood events, compared to the second one (2002–2009) with higher magnitude floods. Aerial photographs from 2002 and 2009 were chosen as a way to capture the morphological changes that occurred after the flood periods, while those from 1987 served as the reference point. In total, an area of 85.2 ha was eroded and 60.1 ha were deposited. The average channel shift per year doubled from 0.8 m/year (1987–2002) to 1.6 m/year (2002–2009). The most eroded land cover category in the riparian zone is floodplain forest, followed by arable land, grasslands and pastures and shrubs. From an economic point of view, the eroded floodplain with arable land and grassland (€ 29,924.02 in total) is a negative consequence of channel migration.
Timing and causes of the present-day channel downcutting of Carpathian tributaries to the Vistula River are exemplified by a case study of the Raba River, a gravel-bed stream characterized by great variability of water stage and discharge. Up to 3 m of river incision has occurred since the beginning of the present century
associated with an increase in stream power caused by river-control works, and reduction in bed-material load linked to changes in basin management. Gravel extraction from the channel and modifications in flood flows have increased rates of incision over the last thirty years.
Lowering of ground-water levels on the valley floor, increase in river-bank susceptibility to erosion, decrease or cessation of overbank-sediment accretion, and impoverishment of plant and animal communities of riverside biotopes are the principal environmental effects of channel downcutting.
Changes in the channel geometry and facies pattern of the Raba River, a mountain gravel-bed stream of the temperate zone, have been analysed to reconstruct transformations of the hydrologic regime during the last 200 years. The river straightened, became shallower and widened during the nineteenth century. Point bar deposits showing a diversified facies pattern were then replaced with and overlain by very poorly sorted channel bar deposits of a low-sinuosity river. These changes are attributed to a marked increase in bed load due to the repeated occurrence of flash floods of high magnitude. Channel degradation and coarsening of bed material during the twentieth century have resulted from increased stream power (channelization effect) and reduced sediment supply, due to variations in basin management and a change in flood hydrographs. A tendency to meander has reappeared but, as point bar deposits are much coarser now, the nineteenth century aggradational phase has had a persistent effect upon the river.