J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 1 - 11, doi: 10.2478/johh-2020-0001
Scientific Paper, English

Aleš Klement, Radka Kodešová, Oksana Golovko, Miroslav Fér, Antonín Nikodem, Martin Kočárek, Roman Grabic: Uptake, translocation and transformation of three pharmaceuticals in green pea plants

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• Treated water from wastewater treatment plants that is increasingly used for irrigation may contain pharmaceuticals and, thus, contaminate soils. Therefore, this study focused on the impact of soil conditions on the root uptake of selected pharmaceuticals and their transformation in a chosen soil–plant system. Green pea plants were planted in 3 soils. Plants were initially irrigated with tap water. Next, they were irrigated for 20 days with a solution of either atenolol (ATE), sulfamethoxazole (SUL), carbamazepine (CAR), or all of these three compounds. The concentrations of pharmaceuticals and their metabolites [atenolol acid (AAC), N1-acetyl sulfamethoxazole (N1AS), N4-acetyl sulfamethoxazole (N4AS), carbamazepine 10,11-epoxide (EPC), 10,11-dihydrocarbamazepine (DHC), trans-10,11- dihydro-10,11-dihydroxy carbamazepine (RTC), and oxcarbazepine (OXC)] in soils and plant tissues were evaluated after harvest. The study confirmed high (CAR), moderate (ATE, AAC, SUL), and minor (N4AC) root uptake of the studied compounds by the green pea plants, nonrestricted transfer of the CAR species into the different plant tissues, and a very high efficiency in metabolizing CAR in the stems and leaves. The results showed neither a synergic nor competitive influence of the application of all compounds in the solution on their uptake by plants. The statistical analysis proved the negative relationships between the CAR sorption coefficients and the concentrations of CAR, EPC, and OXC in the roots (R = –0.916, –0.932, and –0.925, respectively) and stems (R = –0.837, –0.844, and –0.847, respectively).
  
  KEY WORDS: Atenolol; Carbamazepine; Sulfamethoxazole; Irrigation with contaminated water; Sorption in soils; Metabolites.
  
  Address:
  - Aleš Klement, Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Dept. of Soil Science and Soil Protection, Kamýcká 129, 16500 Prague 6, Czech Republic. (Corresponding author. Tel.:+420224382757. Fax.: +420234381836. Email: klement@af.czu.cz)
  - Radka Kodešová, Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Dept. of Soil Science and Soil Protection, Kamýcká 129, 16500 Prague 6, Czech Republic.
  - Oksana Golovko, University of South Bohemia in České Budějovice, Faculty of Fisheries and Protection of Waters, South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, Zátiší 728/II, 38925 Vodňany, Czech Republic.
  - Miroslav Fér, Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Dept. of Soil Science and Soil Protection, Kamýcká 129, 16500 Prague 6, Czech Republic.
  - Antonín Nikodem, Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Dept. of Soil Science and Soil Protection, Kamýcká 129, 16500 Prague 6, Czech Republic.
  - Martin Kočárek, Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Dept. of Soil Science and Soil Protection, Kamýcká 129, 16500 Prague 6, Czech Republic.
  - Roman Grabic, University of South Bohemia in České Budějovice, Faculty of Fisheries and Protection of Waters, South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, Zátiší 728/II, 38925 Vodňany, Czech Republic.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 12 - 18, doi: 10.2478/johh-2019-0028
Scientific Paper, English

Ľubomír Lichner, Massimo Iovino, Peter Šurda, Viliam Nagy, Anton Zvala, Jozef Kollár, Jozef Pecho, Vladimír Píš, Nasrollah Sepehrnia, Renáta Sándor: Impact of secondary succession in abandoned fields on some properties of acidic sandy soils

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• Abandonment of agricultural lands in recent decades is occurring mainly in Europe, North America and Oceania, and changing the fate of landscapes as the ecosystem recovers during fallow stage. The objective of this study was to find the impact of secondary succession in abandoned fields on some parameters of acidic sandy soils in the Borská nížina lowland (southwestern Slovakia). We investigated soil chemical (pH and soil organic carbon content), hydrophysical (water sorptivity, and hydraulic conductivity), and water repellency (water drop penetration time, water repellency cessation time, repellency index, and modified repellency index) parameters, as well as the ethanol sorptivity of the studied soils. Both the hydrophysical and chemical parameters decreased significantly during abandonment of the three investigated agricultural fields. On the other hand, the water repellency parameters increased significantly, but the ethanol sorptivity did not change during abandonment. As the ethanol sorptivity depends mainly on soil pore size, the last finding could mean that the pore size of acidic sandy soils did not change during succession.
  
  KEY WORDS: Water repellency; Acidic sandy soil; Land abandonment; Secondary succession; Soil properties.
  
  Address:
  - Ľubomír Lichner, Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia. (Corresponding author. Tel.: Fax.: Email: lichner@uh.savba.sk)
  - Massimo Iovino, Dipartimento di Scienze Agrarie, Alimentari e Forestali, Universita degli Studi di Palermo, Viale delle Scienze, Ed. 4 Ingr. E, 90128 Palermo, Italy.
  - Peter Šurda, Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia.
  - Viliam Nagy, Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia.
  - Anton Zvala, Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia.
  - Jozef Kollár, Institute of Landscape Ecology, Slovak Academy of Sciences, Štefánikova 3, 81499 Bratislava, Slovakia.
  - Jozef Pecho, Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovakia.
  - Vladimír Píš, Soil Science and Conservation Research Institute, Trenčianska 55, 82109 Bratislava, Slovakia.
  - Nasrollah Sepehrnia, Institute of Soil Science, Leibniz Universität Hannover, Herrenhäuser Str. 2, D-30419 Hannover, Germany.
  - Renáta Sándor, Agricultural Institute, Centre for Agricultural Research, Brunszvik u.2, 2462 Martonvásár, Hungary.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 19 - 27, doi: 10.2478/johh-2020-0002
Scientific Paper, English

Giora J. Kidron, Roberto Lázaro: Are coastal deserts necessarily dew deserts? An example from the Tabernas Desert

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• Vapor condensation, whether due to dew or fog, may add a stable and important source of water to deserts. This was also extensively assessed in the Negev, regarded as a dew desert. Dew deserts necessitate a large reservoir of vapor, and are therefore confined to near oceans or seas. Yet, examples of such deserts are scarce. Here we try to assess whether the Tabernas Desert in SE Spain can be regarded as a dew desert, and may therefore facilitate the growth of certain organisms that otherwise would not survive the dry season. We analyze some of the abiotic conditions of four relatively dry months (June, July, August, September) in the Tabernas and Negev deserts (with the Negev taken as an example of a dew desert) during 2003–2012. The analysis showed substantially lower values of relative humidity (by 10–13%) in the Tabernas in comparison to the Negev, with RH ≥95% being on average only 0.9–1.1 days a month in the Tabernas in comparison to 9.7–13.9 days in the Negev. Our findings imply that the Tabernas Desert cannot be regarded as a dew desert, suggesting that rain will be the main factor responsible for the food web chain in the Tabernas.
  
  KEY WORDS: Fog; Negev Desert; Relative humidity; Vapor condensation; Wind speed.
  
  Address:
  - Giora J. Kidron, Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram Campus, Jerusalem 91904, Israel. (Corresponding author. Tel.:+ 972-544-967-271 Fax.: +972-2-566-2581 Email: kidron@mail.huji.ac.il)
  - Roberto Lázaro, Estación Experimental de Zonas Áridas (CSIC), Carretera Sacramento, W/n, La Canada de San Urbano, 04120 Almería, Spain.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 28 - 37, doi: 10.2478/johh-2019-0026
Scientific Paper, English

Michal Mikloš, Dušan Igaz, Karol Šinka, Jana Škvareninová, Martin Jančo, Ilja Vyskot, Jaroslav Škvarenina: Ski piste snow ablation versus potential infiltration (Veporic Unit, Western Carpathians)

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• Snow production results in high volume of snow that is remaining on the low-elevation ski pistes after snowmelt of natural snow on the off-piste sites. The aim of this study was to identify snow/ice depth, snow density, and snow water equivalent of remaining ski piste snowpack to calculate and to compare snow ablation water volume with potential infiltration on the ski piste area at South-Central Slovak ski center Košútka (Inner Western Carpathians; temperate zone). Snow ablation water volume was calculated from manual snow depth and density measurements, which were performed at the end of five winter seasons 2010–2011 to 2015–2016, except for season 2013–2014. The laser diffraction analyzes were carried out to identify soil grain size and subsequently the hydraulic conductivity of soil to calculate the infiltration. The average rate of water movement through soil was seven times as high as five seasons’ average ablation rate of ski piste snowpack; nevertheless, the ski piste area was potentially able to infiltrate only 47% of snow ablation water volume on average. Limitation for infiltration was frozen soil and ice layers below the ski piste snowpack and low snow-free area at the beginning of the studied ablation period.
  
  KEY WORDS: Snow water equivalent; Snow density; Artificial snow; Snow ablation; Soil temperature; Hydraulic conductivity.
  
  Address:
  - Michal Mikloš, Department of Natural Environment, Faculty of Forestry, Technical University in Zvolen, Ul. T. G. Masaryka 24, 960 53 Zvolen, Slovakia.
  - Dušan Igaz, Department of Biometeorology and Hydrology, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, Hospodárska 7, 949 76 Nitra, Slovakia.
  - Karol Šinka, Department of Landscape Planning and Land Consolidation, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, Hospodárska 7, 949 76 Nitra, Slovakia.
  - Jana Škvareninová, Department of Applied Ecology, Faculty of Ecology and Environmental Sciences, Technical University in Zvolen, Ul. T. G. Masaryka 24, 960 53 Zvolen, Slovakia.
  - Martin Jančo, Department of Natural Environment, Faculty of Forestry, Technical University in Zvolen, Ul. T. G. Masaryka 24, 960 53 Zvolen, Slovakia. Faculty of Ecology and Environmental Sciences, Technical University in Zvolen, Ul. T. G. Masaryka 24, 960 53 Zvolen, Slovakia.
  - Ilja Vyskot, Department of Environmentalistics and Natural Resources (FRDIS), Faculty of Regional Development and International Studies, Mendel University in Brno, Tř. Generála Píky 2005/7, 613 00 Brno, Czech Republic.
  - Jaroslav Škvarenina, Department of Natural Environment, Faculty of Forestry, Technical University in Zvolen, Ul. T. G. Masaryka 24, 960 53 Zvolen, Slovakia. (Corresponding author. Tel.:+421455206209 Fax.: Email: skvarenina@tuzvo.sk)

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 38 - 50, doi: 10.2478/johh-2019-0025
Scientific Paper, English

Jan Tadeusz Łukaszewicz, Renata Graf: The variability of ice phenomena on the rivers of the Baltic coastal zone in the Northern Poland

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• The main purpose of the research was to determine the conditions affecting ice phenomena, including the three-phase cycle of ice: expansion, retention and decay of the ice cover on selected rivers of the Baltic coastal zone in the Northern Poland (Przymorze region). The analysis has been elaborated for the years 1951–2010 against the backdrop of currently occurring climatic changes, with particular emphasis on the development and phase variability of the NAO. The article presents the impact of the variability in atmospheric circulation which has manifested in an increase in air temperature, over the last 20 years, on thermal conditions during winter periods in the South Baltic Coastal Strip. The increase in air temperature has contributed to an increase in the temperature of river waters, thus leading to a shortening of the duration of ice phenomena on rivers in the Przymorze region. The article also brings to light an increased occurrence of winter seasons classified as cool, and a disruption in the occurrence of periods classified as normal over the last 30 observed years. The research has demonstrated a significant dependence between the seasonal change in air temperature and the variability of thermal conditions of water, which has a direct impact on the variability of the icing cycle of rivers in the Przymorze region. The authors also show that the variability in forms of ice phenomena for individual river sections is determined by the local factors, i.e. anthropogenic activity, impact of urbanized areas or inflow of pollutants.
  
  KEY WORDS: Ice phenomena; Rivers; Coastal zone; Air temperature; NAO; Water temperature.
  
  Address:
  - Jan Tadeusz Łukaszewicz, Department of Hydrology and Water Management, Institute of Physical Geography and Environmental Planning. Faculty Geographical and Geological Sciences, Adam Mickiewicz University in Poznań, 61-680 Poznań, Poland. (Corresponding author. Tel.:(+48) 693 627 497 Fax.: Email: janluk@amu.edu.pl)
  - Renata Graf, Department of Hydrology and Water Management, Institute of Physical Geography and Environmental Planning. Faculty Geographical and Geological Sciences, Adam Mickiewicz University in Poznań, 61-680 Poznań, Poland.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 51 - 59, doi: 10.2478/johh-2019-0024
Scientific Paper, English

Qian Feng, Dong Linyao, Liu Jigen, Sun Bei, Liu Honghu, Huang Jiesheng, Li Hao: Equations for predicting interrill erosion on steep slopes in the Three Gorges Reservoir, China

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• The Three Gorges Reservoir region suffers from severe soil erosion that leads to serious soil degradation and eutrophication. Interrill erosion models are commonly used in developing soil erosion control measures. Laboratory simulation experiments were conducted to investigate the relationship between interrill erosion rate and three commonly hydraulic parameters (flow velocity V, shear stress τ and stream power W). The slope gradients ranged from 17.6% to 36.4%, and the rainfall intensities varied from 0.6 to 2.54 mm·min–1. The results showed that surface runoff volume and soil loss rates varied greatly with the change of slope and rainfall intensity. Surface runoff accounted for 67.2–85.4% of the precipitation on average. Soil loss rates increased with increases of rainfall intensity and slope gradient, Regression analysis showed that interrill erosion rate could be calculated by a linear function of V and W. Predictions based on V (R2 = 0.843, ME = 0.843) and W (R2 = 0.862, ME = 0.862) were powerful. τ (R2 = 0.721, ME = 0.721) did not seem to be a good predictor for interrill erosion rates. Five ordinarily interrill erosion models were analyzed, the accuracy of the models in predicting soil loss rate was: Model 3 (ME = 0.977) > Model 4 (ME = 0.966) > Model 5 (ME = 0.963) > Model 2 (ME = 0.923) > Model 1 (ME = 0.852). The interrill erodibility used in the model 3 (WEPP) was calculated as 0.332×106 kg·s·m–4. The results can improve the precision of interrill erosion estimation on purple soil slopes in the Three Gorges Reservoir area.
  
  KEY WORDS: Purple soil; Interrill erosion rate; Rainfall intensity; Slope gradient; Hydraulic parameter.
  
  Address:
  - Qian Feng, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China. School of Water Resources and Hydropower Engineering of Wuhan University, Wuhan, Hubei 430010, China. Research Center on Mountain Torrent and Geologic Disaster Prevention, Ministry of Water Resources, Wuhan, Hubei 430010, China. (Corresponding author. Tel.:+86 02782926992. Fax.: +86-02782926357 Email: Qianfeng@whu.edu.cn)
  - Dong Linyao, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China. Research Center on Mountain Torrent and Geologic Disaster Prevention, Ministry of Water Resources, Wuhan, Hubei 430010, China.
  - Liu Jigen, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China.
  - Sun Bei, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China.
  - Liu Honghu, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China.
  - Huang Jiesheng, School of Water Resources and Hydropower Engineering of Wuhan University, Wuhan, Hubei 430010, China.
  - Li Hao, Department of Soil and Water Conservation, Changjiang River Scientific Research Institute, Wuhan, Hubei 430072, China.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 60 - 69, doi: 10.2478/johh-2019-0027
Scientific Paper, English

Václav Matoušek, Štěpán Zrostlík: Collisional transport model for intense bed load

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• In an open channel with a mobile bed, intense transport of bed load is associated with high-concentrated sediment-laden flow over a plane surface of the eroded bed due to high bed shear. Typically, the flow exhibits a layered internal structure in which virtually all sediment grains are transported through a collisional layer above the bed. Our investigation focuses on steady uniform turbulent open-channel flow with a developed collisional transport layer and combines modelling and experiment to relate integral quantities, as the discharge of solids, discharge of mixture, and flow depth with the longitudinal slope of the bed and the internal structure of the flow above the bed. A transport model is presented which considers flow with the internal structure described by linear vertical distributions of granular velocity and concentration across the collisional layer. The model employs constitutive relations based on the classical kinetic theory of granular flows selected by our previous experimental testing as appropriate for the flow and transport conditions under consideration. For given slope and depth of the flow, the model predicts the total discharge and the discharge of sediment. The model also predicts the layered structure of the flow, giving the thickness of the dense layer, collisional layer, and water layer. Model predictions are compared with results of intense bed-load experiment carried out for lightweight sediment in our laboratory tilting flume.
  
  KEY WORDS: Granular flow; Sheet flow; Sediment transport; Grain collision; Tilting flume experiment; Kinetic theory.
  
  Address:
  - Václav Matoušek, Czech Technical University in Prague, Department of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic. (Corresponding author. Tel.: Fax.: Email: v.matousek@fsv.cvut.cz)
  - Štěpán Zrostlík, Czech Technical University in Prague, Department of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 70 - 82, doi: 10.2478/johh-2019-0029
Scientific Paper, English

Mohammad Reza Namaee, Jueyi Sui: Velocity profiles and turbulence intensities around side-by-side bridge piers under ice-covered flow condition

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• Recent studies have shown that the presence of ice cover leads to an intensified local scour pattern in the vicinity of bridge piers. To investigate the local scour pattern in the vicinity of bridge pier under ice-covered flow condition comparing to that under open channel flow condition, it is essential to examine flow field around bridge piers under different flow conditions. In order to do so, after creation of smooth and rough ice covers, three-dimensional timeaveraged velocity components around four pairs of bridge piers were measured using an Acoustic Doppler velocimetry (ADV). The ADV measured velocity profiles describe the difference between the velocity distributions in the vicinity of bridge piers under different covered conditions. Experimental results show that the vertical velocity distribution which represents the strength of downfall velocity is the greatest under rough covered condition which leads to a greater scour depth. Besides, results show that the turbulent intensity increases with pier size regardless of flow cover, which implies that larger scour depth occurs around piers with larger diameter.
  
  KEY WORDS: Bridge pier; Ice cover; Local scour; Acoustic Doppler velocimetry (ADV); Horseshoe vortex.
  
  Address:
  - Mohammad Reza Namaee, Environmental Engineering Program, University of Northern British Columbia, 3333 University Way, Prince George, BC, Canada.
  - Jueyi Sui, Environmental Engineering Program, University of Northern British Columbia, 3333 University Way, Prince George, BC, Canada. (Corresponding author. Tel.: Fax.: Email: jueyi.sui@unbc.ca)

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 83 - 91, doi: 10.2478/johh-2019-0023
Scientific Paper, English

Pavel Vlasák, Václav Matoušek, Zdeněk Chára, Jan Krupička, Jiří Konfršt, Mikoláš Kesely: Concentration distribution and deposition limit of medium-coarse sand-water slurry in inclined pipe

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• Sand-water slurry was investigated on an experimental pipe loop of inner diameter D = 100 mm with the horizontal, inclined, and vertical smooth pipe sections. A narrow particle size distribution silica sand of mean diameter 0.87 mm was used. The experimental investigation focused on the effects of pipe inclination, overall slurry concentration, and mean velocity on concentration distribution and deposition limit velocity. The measured concentration profiles showed different degrees of stratification for the positive and negative pipe inclinations. The degree of stratification depended on the pipe inclination and on overall slurry concentration and velocity. The ascending flow was less stratified than the corresponding descending flow, the difference increasing from horizontal flow up to an inclination angle of about +30°. The deposition limit velocity was sensitive to the pipe inclination, reaching higher values in the ascending than in the horizontal pipe. The maximum deposition limit value was reached for an inclination angle of about +25°, and the limit remained practically constant in value, about 1.25 times higher than that in the horizontal pipe. Conversely, in the descending pipe, the deposition limit decreased significantly with the negative slopes and tended to be zero for an inclination angle of about −30°, where no stationary bed was observed.
  
  KEY WORDS: Sand-water slurry; Pipe inclination; Concentration distribution; Deposition limit; Gamma-ray radiometry.
  
  Address:
  - Pavel Vlasák, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic. (Corresponding author. Tel.:+420 233109092 Fax.: Email: vlasak@ih.cas.cz)
  - Václav Matoušek, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic.
  - Zdeněk Chára, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic.
  - Jan Krupička, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic.
  - Jiří Konfršt, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic.
  - Mikoláš Kesely, Institute of Hydrodynamics of the Czech Academy of Sciences, v. v. i., Pod Patankou 30/5, 160 00, Prague 6, Czech Republic.

J. Hydrol. Hydromech., Vol. 68, No. 1, 2020, p. 92 - 98, doi: 10.2478/johh-2020-0003
Scientific Paper, English

Štěpán Zrostlík, Václav Matoušek: Vertical sorting in collisional layer of bimodal sediment transport

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• Intense collisional transport of bimodal sediment mixture in open-channel turbulent flow with water as carrying liquid is studied. The study focusses on steep inclined flows transporting solids of spherical shape and differing in either size or mass. A process of vertical sorting (segregation) of the two different solids fractions during the transport is analyzed and modelled. A segregation model is presented which is based on the kinetic theory of granular flows and builds on the Larcher-Jenkins segregation model for dry bimodal mixtures. Main modifications of the original model are the carrying medium (water instead of air) and a presence of a non-uniform distribution of sediment across the flow depth. Testing of the modified model reveals that the model is applicable to flow inclination slopes from 20 to 30 degrees approximately, making it appropriate for debris flow conditions. Changing the slope outside the specified range leads to numerical instability of the solution. A use of the bimodal mixture model is restricted to the grain size ratio 1.4 and no restriction is found for the grain mass ratio in a realistic range applicable to natural conditions. The model reveals trends in the vertical sorting under variable conditions showing that the sorting is more intense if flow is steeper and/or the difference in size or mass is bigger between the two sediment fractions in a bimodal mixture.
  
  KEY WORDS: Grain segregation; Bimodal mixture; Granular flow; Sheet flow; Sediment transport.
  
  Address:
  - Štěpán Zrostlík, Czech Technical University in Prague, Department of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic. (Corresponding author. Tel.: Fax.: Email: stepan.zrostlik@fsv.cvut.cz)
  - Václav Matoušek, Czech Technical University in Prague, Department of Civil Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic.