Bartłomiej Wyżga

Impact of the channelization-induced incision of the Skawa and Wisloka Rivers, southern Poland, on the conditions of overbank deposition

The impact of river incision in response to channelization on the conditions of overbank deposition is shown by the study of two montane rivers from the upper Vistula drainage basin, southern Poland. The Wisl/oka River had insufficient energy to destroy the river-control structures and remained laterally stable in the course of the channel downcutting. Under such conditions, the incision has raised the relative elevation of the floodplain above the river bed, thereby reducing considerably the frequency of overbank flows, and increasing concentration of suspended sediment transport within the incised channel. On the high-energy Skawa, the long periods of incision of the channelized river alternated with the shorter periods of lateral channel migration over the twentieth century. This has led to the formation of an incised meander belt, within which flood flows are constricted, and where the high velocities of the floodplain flows inhibit overbank deposition. Field observations confirm an insignificant role played nowadays by floodplain sedimentation in the valleys of both rivers. This study shows that the potential of the floodplains of the Carpathian tributaries to the Vistula for sediment storage has been dramatically reduced over the few past decades as a result of the channelization-induced incision of the rivers. The frequency of overbank flows has decreased considerably on the rivers draining the eastern part of the Polish Carpathians, and the majority of the suspended sediment is routed within the resultant enlarged channels. In the western part of the mountains, high velocities of the floodplain flows restrict overbank deposition on the narrow floodplains developed along incised channels. Copyright

Methods for studying the response of flood flows to channel change

Abstract The effect of a change in channel morphology of a stream upon flood flows may be obliterated by simultaneous changes in meteorologic factors or in another physiographic factor and thus flood-frequency analysis and statistical tests cannot be seen as definitely indicating the lack or existence of a genetic non-stationarity of flow record related to the channel change. In a study of two mountain rivers from the upper Vistula drainage basin, southern Poland, three alternative procedures are presented which analyse temporal trends in the relationship between inflow and outflow peak discharges of flood waves passing the modified reach. While these techniques examine various aspects of the record of flood discharges, all of them separate changes in flows accomplished in a reach under investigation from those born in the upstream part of the catchment. Annual maxima analysis allows direct comparison of the timing and relations between changes in channel morphology and their response in flood flows. The high consistency of changes in vertical channel position of the analysed rivers and of variations in transformation of their flood flows shows the importance of channel incision for increasing flood hazard on channelized streams.

Floods at the northern foothills of the Tatra Mountains — A Polish-Swiss research project

The present paper introduces the topical area of the Polish-Swiss research project FLORIST (Flood risk on the northern foothills of the Tatra Mountains), informs on its objectives, and reports on initial results. The Tatra Mountains are the area of the highest precipitation in Poland and largely contribute to flood generation. The project is focused around four competence clusters: observation-based climatology, model-based climate change projections and impact assessment, dendrogeomorphology, and impact of large wood debris on fluvial processes. The knowledge generated in the FLORIST project is likely to have impact on understanding and interpretation of flood risk on the northern foothills of the Tatra Mountains, in the past, present, and future. It can help solving important practical problems related to flood risk reduction strategies and flood preparedness.

Hydromorphological complexity as a driver of the diversity of benthic invertebrate communities in the Czarny Dunajec River, Polish Carpathians

To verify whether the variability in benthic invertebrate communities along the mountainous Czarny Dunajec River is mainly driven by the variation in hydromorphological or water quality, diversity of the communities was determined for 18 cross-sections with 1–5 low-flow channels and compared with the complexity of physical habitat conditions and with physico-chemical water quality. An increase in the complexity of flow pattern in the river was associated with increasing cross-sectional variability in physical habitat parameters. Distinct hydromorphological characteristics of the cross-sections with a given number of low-flow channels were especially pronounced if the analysis was limited to the parameters measured directly, whereas calculated complex hydraulic and sedimentary variables represented information overload. Taxonomic richness of the invertebrate communities was unrelated to physico-chemical water parameters, which consistently pointed to the high water quality. Instead, the richness positively correlated with a degree of variation in physical habitat parameters and was best predicted by the number of low-flow channels in a river cross-section. This study indicates that physical habitat complexity in a mountain river can be considered a proxy to the diversity of its invertebrate communities and that restoration of such complexity will be necessary for future recovery of invertebrate communities in impacted river sections.

Estimating mean flow velocity in channel and floodplain areas and its use for explaining the pattern of overbank deposition and floodplain retention

Abstract Stage-discharge curves for out-of-bank flows are derived from direct discharge measurements and theoretical calculations of channel and floodplain conveyance at a station, and comparisons of volumes of flood waves at successive stations. A method was developed which permits estimation of the mean flow velocity in the channel and floodplain zones at a given total discharge taken from a rating curve. Out-of-bank channel flows are estimated under the assumption that the interface planes between the channel zone and the floodplain are included in the wetted perimeter for the channel subarea. The floodplain flows are then calculated as the residual values after subtracting the flow in the channel zone from the total discharge. Results obtained using the method are presented for three cross-sections located on mountain, foothill and piedmont rivers within the upper Vistula drainage basin, southern Poland, and are considered in relation with the pattern of floodplain sedimentation caused by a major flood of July 1997. High rates of lateral thinning and fining of overbank deposits were observed for the piedmont Vistula River which is characterized by a marked contrast in mean velocity between the channel and floodplain flows. On the mountain Skawa River and the foothill Skawinka River mean velocity of the floodplain flow was high, both in absolute values and when compared with that of the channel flow; overbank deposits were generally thin there and showed minor changes in thickness, mean size and sorting across the floodplains. This study shows that the proposed method of estimating mean flow velocity in channel and floodplain areas may yield valuable information about hydraulics of out-of-bank flows, which may be used for explaining the observed pattern of floodplain deposition. The results also demonstrate that the floodplain of the Vistula acts mainly as a storage reservoir at low out-of-bank flows but its efficiency in carrying flood water grows when flow exceeds that of 12-year frequency. On the other hand, retention potential of the Skawa floodplain is very low but flow becomes increasingly stored after water starts to spread rapidly onto the higher terrace at a discharge of 14-year recurrence interval.

Present-day downcutting of the Raba River channel (western Carpathians, Poland) and its environmental effects

Abstract 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.

Interpretation of the invertebrate-based BMWP-PL index in a gravel-bed river: insight from the Polish Carpathians

Like its British prototype (Biological Monitoring Working Party score system), the Polish benthic invertebrate-based BMWP-PL index is commonly regarded as an indicator of river water quality. This interpretation of the index has been verified in a study of the gravel-bed Biała River. Benthic macroinvertebrates were sampled at 10 sites and compared in one channelized and one unmanaged cross-section per site. The resulting taxa richness and BMWP-PL index scores were compared with water quality and physical habitat characteristics in the cross-sections. Channelized and unmanaged cross-sections clearly differed in their physical habitat conditions, and water quality characteristics mostly varied in the downstream direction. Particular cross-sections hosted between 3 and 26 invertebrate taxa, with the respective BMWP-PL scores indicating the water in the surveyed cross-sections varied between high and poor quality. However, the BMWP-PL scores were unrelated to physicochemical characteristics of the river water, which consistently pointed to high water quality. Instead, the scores were significantly related to several physical habitat variables, with the number of low-flow channels in a cross-section explaining the largest proportion of the variance in the index values. The relationship of the scores with the complexity of flow pattern in the river and a lack of their dependence on physicochemical water characteristics show that the BMWP-PL index should not be regarded as an indicator of water quality but rather as an indicator of the ecological status of rivers, dependent both on their hydromorphological and water-quality characteristics.

Decadal variability of floods in the northern foreland of the Tatra Mountains

Floods in the northern foreland of the Tatra Mountains considerably contribute to the total flood damage in Poland. Therefore, the question whether the magnitude and frequency of floods have changed in this region is of high interest. This study aims at investigating the inter-decadal variability of magnitude, frequency and seasonality of floods since the mid-twentieth century, to better understand regional changes. The analysis was accomplished in a multi-temporal approach whereby trends are fitted to every possible combination of start and end years in a record. Detected trends were explained by estimating correlations between the investigated flood parameters and different large-scale climate indices for the northern hemisphere, and by trends found in intense precipitation indices, number of days with snow cover, cyclonic circulation types, temperature and moisture conditions. Catchment and channel changes that occurred in the region over the past decades were also considered. Results show that rivers in the area exhibit considerable inter-decadal variability of flows. The magnitude and direction of short-term trends are heavily influenced by this inter-decadal variability; however, certain patterns are apparent. More extreme, although perhaps less frequent floods are now likely to occur, with a shift in the seasonality, decreasing flood magnitudes in winter and increasing during autumn and spring. The identification of the factors contributing to the occurrence of flood events and their potential changes is valuable to enhance the flood management in the region and to improve the resilience of the population in this mountainous area.

The Carpathians: Integrating Nature and Society Towards Sustainability

The Carpathian Mountains: challenges for the Central and Eastern European landmark.- Science for the Carpathians: using regional capacity to cope with global change.- Progress in understanding the dynamics of Carpathian abiotic environment.- Progress in understanding the dynamics of Carpathian abiotic environment.- Landslide susceptibility assessment: GIS application to a complex mountainous environment.- GIS evaluation of erosion-sedimentation risk caused by extreme convective rainstorms: case study of the Stonavka River catchment, Czech Republic.- Assessment of the impact of agricultural land utilization practices on soil losses.- The effects of climate change on element content and soil pH (Sikfokut DIRT Project, northern Hungary).- Climate changes in the vertical zones of the Polish Carpathians in the last 50 years.- Air temperature variability in the High Tatra Mountains.- Climate trends in the Slovak part of the Carpathians.

Large wood clogging during floods in a gravel-bed river: the Długopole bridge in the Czarny Dunajec River, Poland

During floods, large quantities of wood can be mobilized and transported downstream. At critical sections, such as bridges, the transported wood might be entrapped and a quick succession of backwater effects can occur as a result of the reduction of the cross-sectional area. The aim of this work is to explore large wood-related hazards during floods in the gravel-bed river Czarny Dunajec (Polish Carpathians), where the river flows through the village of Dlugopole. This work is based on the numerical modelling of large wood transport together with flow dynamics in which inlet and boundary conditions were designed based on field observations. The exploratory approach developed in this study uses multiple scenarios (193) to analyse the factors controlling bridge clogging: wood size, wood supply, flow conditions, morphology and obstacles in the riverbed. Results highlighted the strong control of log length (stronger than that of log diameter) on potential blockage probability; however, according to our results the main factor controlling the bridge clogging was the flood discharge. River morphology and wood supply play an important role as well. Especially the river morphology may reduce bridge blockage, as it influences flow velocity and depth, and creates natural retention zones for wood. In addition, the impacts of the bridge blockage were analysed in terms of afflux depth and length, and flooded area. Results showed that the bridge blockage may result in a significant increase in the water depth (up to 0.7 m) and flooded area (up to 33% more), therefore increasing flood risk in the village. This article is protected by copyright. All rights reserved.