J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 1 - 11, doi: 10.2478/johh-2021-0032
Scientific Paper, English

Mariusz Ptak, Mariusz Sojka, Renata Graf, Adam Choiński, Senlin Zhu, Bogumił Nowak: Warming Vistula River – the effects of climate and local conditions on water temperature in one of the largest rivers in Europe

 Full Text in PDF     82 DOWNLOADS

• The paper evaluates changes in the water temperature of the Vistula River – one of the longest rivers in Europe. Mean monthly and annual water temperatures from the period 1971–2017 for 11 stations along the entire length of the river revealed the increasing trends. The mean increase in water temperature in the analysed multi-annual period was 0.31 °C dec–1. In the majority of analysed stations, the key factor determining changes in the water temperature of the river was air temperature. The observed water warming in the Vistula River should be considered an exceptionally unfavourable situation in the context of importance of water temperature for a number of processes and phenomena occurring in river ecosystems. Given the scale of changes, fast measures should be undertaken to slow down the warming.
  
  KEY WORDS: Water temperature; Transit rivers; Trends; Transformation factors; Central Europe.
  
  Address:
  - Mariusz Ptak, Department of Hydrology and Water Management, Adam Mickiewicz University, B. Krygowskiego 10, 61-680 Poznań, Poland. (Corresponding author. Tel.: Fax.: Email: marp114@wp.pl)
  - Mariusz Sojka, Department of Land Improvement, Environmental Development and Spatial Management, Poznań University of Life Sciences, Piątkowska 94E, 60-649 Poznań, Poland.
  - Renata Graf, Department of Hydrology and Water Management, Adam Mickiewicz University, B. Krygowskiego 10, 61-680 Poznań, Poland.
  - Adam Choiński, Department of Hydrology and Water Management, Adam Mickiewicz University, B. Krygowskiego 10, 61-680 Poznań, Poland.
  - Senlin Zhu, College of Hydraulic Science and Engineering, Yangzhou University, China.
  - Bogumił Nowak, Institute of Meteorology and Water Management – National Research Institute, Podleśna 61, 01-673 Warszawa, Poland.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 12 - 29, doi: 10.2478/johh-2021-0033
Scientific Paper, English

Dariusz Wrzesiński, Renata Graf: Temporal and spatial patterns of the river flow and water temperature relations in Poland

 Full Text in PDF     81 DOWNLOADS

• Main aim of the study was to determine the temporal and spatial patterns of relations between monthly and annual average river flow (RF) and water temperature (WT) for 53 rivers in Poland. The research made use of monthly and annual WT and RF for 88 water gauges for the period 1971–2015. Correlations were established using the Spearman’s rank correlation coefficient and the similarity of RF–WT relations was determined using the Ward’s hierarchical grouping. It was demonstrated that correlations between average annual RF and WT were negative (for >85% of water gauges) and statistically significant (p<0.05) only for 30% of water gauges. It was confirmed that the studied RF–WT relations underwent seasonal changes. Positive correlations were clearly predominant in the winter months, while from April to September these relations were negative and statistically significant. The RF–WT relations were also characterized by spatial differences and this had been confirmed by separation of seven groups of water gauge profiles distinguished with the help of the Ward's hierarchical grouping method. The strongest RF–WT relations were apparent in the case of mountainous rivers, for which snow melt supply and summer rainfall supply were predominant, and lakeland rivers, which had a considerable share of groundwater supply. These were classified as cold rivers, as opposed to the cool rivers in the lowland belt, for which the RF–WT relations were the weakest. The results obtained may contribute to the elaboration of an appropriate management strategy for river ecosystems, which are assigned important economic and environmental functions.
  
  KEY WORDS: River flow; Water temperature; Hydrological regime; Correlation coefficient; Hierarchical clustering; Central Europe.
  
  Address:
  - Dariusz Wrzesiński, Adam Mickiewicz University in Poznań, Institute of Physical Geography and Environmental Planning, Department of Hydrology and Water Management, Bogumiła Krygowskiego 10, 61-680 Poznań, Poland.
  - Renata Graf, Adam Mickiewicz University in Poznań, Institute of Physical Geography and Environmental Planning, Department of Hydrology and Water Management, Bogumiła Krygowskiego 10, 61-680 Poznań, Poland. (Corresponding author. Tel.:+4861 829 6259 Fax.: Email: rengraf@amu.edu.pl)

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 30 - 41, doi: 10.2478/johh-2022-0003
Scientific Paper, English

Steffen Beck-Broichsitter, Marisa R. Gerriets, Martin Neumann, Jan-Frantisek Kubat, Jaromir Dusek: Spatial particle size distribution at intact sample surfaces of a Dystric Cambisol under forest use

 Full Text in PDF     21 DOWNLOADS

• The idea of the present study is to describe the spatially varying particle size distribution (PSD) along intact aggregate surfaces with the laser diffraction method (LDM) of four silty-loamy and OC enriched horizons of a Dystric Cambisol from the Uhlířská catchment (Czech Republic) with the laser diffraction method (LDM). Besides, the comparability of the LDM with the sieve and pipette method (SPM), the reproducibility, and the effect of pretreatment on the particle size distribution derived by LDM were analysed. The laser diffraction method enables rapid and continuous particle size distribution measurements with required sample amounts of 0.1–0.2 g for each measurement compared to 5–20 g for SPM. The LDM-derived PSD’s can be directly compared with the standardised SPM-derived PSD’s by using regression analysis with coefficients of determination (r²) between 0.83 and 0.93. Sample pretreatment following standardised proceedings indicates a better comparability between the particle size distributions of both methods. Besides, the highest coefficients of variation of up to 78.6 and therefore the lowest reproducibility were found for the unpretreated PSD of the AE and Bs horizon. Thus, limited evaluability and reproducibility of soil material enriched in organic carbon (OC), used in the current study, needs further analysis. For spatial analysis of PSD’s along intact surfaces of soil aggregates and profiles, spatial data interpolation by inverse distance weighting (IDW), kriging, and triangulated irregular networks (TIN) can be used for detailed measuring, mapping, and spatial extension of the sand, silt, and clay fractions at unsampled locations using a set of samples of known locations. The information offers the possibility of comparing and verifying data obtained by non-invasive midinfrared spectroscopy and Vis–NIR spectroscopy by spatial extension for given soil aggregates and profiles.
  
  KEY WORDS: Laser diffraction method; Sieve and pipette method; Geoprocessing.
  
  Address:
  - Steffen Beck-Broichsitter, Department of Hydraulics and Hydrology, Faculty of Civil Engineering, Czech Technical University in Prague, Thakurova 7, 166 29, Prague, Czech Republic. (Corresponding author. Tel.: Fax.: Email: s.beck-broichsitter@soils.uni-kiel.de)
  - Marisa R. Gerriets, Research Area 1 “Landscape Functioning”, Leibniz Centre for Agricultural Landscape Research (ZALF), Eberswalder Straße 84, 15374 Müncheberg, Germany.
  - Martin Neumann, Department of Landscape Water Conservation, Faculty of Civil Engineering, Czech Technical University in Prague, Thakurova 7, 166 29, Prague, Czech Republic.
  - Jan-Frantisek Kubat, Department of Landscape Water Conservation, Faculty of Civil Engineering, Czech Technical University in Prague, Thakurova 7, 166 29, Prague, Czech Republic.
  - Jaromir Dusek, Department of Hydraulics and Hydrology, Faculty of Civil Engineering, Czech Technical University in Prague, Thakurova 7, 166 29, Prague, Czech Republic.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 42 - 61, doi: 10.2478/johh-2021-0038
Scientific Paper, English

Petra Hečková, Vojtěch Bareš, David Stránský, Michal Sněhota: Performance of experimental bioretention cells during the first year of operation

 Full Text in PDF     42 DOWNLOADS

• Detailed data on the long-term performance of bioretention cells (BC) for stormwater management are sparse. This research aimed at setting up and testing an infrastructure that will provide the data on hydrologic and chemical performance of BC. Two identical experimental BC’s were built. The monitoring methodology monitoring was developed and tested during a first growing season with the first BC supplied with natural rainfall, while the second BC was used for ponding experiments. Key layer of the BCs, a biofilter, was composed of sand, compost and topsoil. Both BCs are equipped with sensors monitoring the components of water balance and the water potential of the biofilter. High levels of total suspended solids were detected in the outflow. The runoff coefficient for the entire period of the growing season was 0.72 in the first BC and 0.86 in the second BC, while the peak outflow reduction for individual rainfall episodes ranged between 75% to 95% for the first BC and 19% to 30% for the second BC. Saturated hydraulic conductivity of the biofilter in the first BC decreased by two orders of magnitudes after the first year of operation. Retention curves of the biofilter changed due to material consolidation.
  
  KEY WORDS: Bioretention; Biofilter; Water retention; Constructed soil; Stormwater; Infiltration.
  
  Address:
  - Petra Hečková, Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, Prague 6, 166 29, Czech Republic. Czech Technical University in Prague, University Centre for Energy Efficient Buildings, Třinecká 1024, Buštěhrad, 273 43, Czech Republic. (Corresponding author. Tel.: Fax.: Email: petra.heckova@cvut.cz)
  - Vojtěch Bareš, Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, Prague 6, 166 29, Czech Republic.
  - David Stránský, Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, Prague 6, 166 29, Czech Republic.
  - Michal Sněhota, Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, Prague 6, 166 29, Czech Republic. Czech Technical University in Prague, University Centre for Energy Efficient Buildings, Třinecká 1024, Buštěhrad, 273 43, Czech Republic.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 62 - 73, doi: 10.2478/johh-2021-0037
Scientific Paper, English

Ivan Magdić, Toni Safner, Vedran Rubinić, Filip Rutić, Stjepan Husnjak, Vilim Filipović: Effect of slope position on soil properties and soil moisture regime of Stagnosol in the vineyard

 Full Text in PDF     33 DOWNLOADS

• Hillslope hydrology in agricultural landscapes is complex due to a variety of hydropedological processes and field management possibilities. The aim was to test if there are any differences in soil properties and water regime along the hillslope and to compare vineyard rows (vine) with inter-rows (grass) area for those properties. The study determined that there are significant differences in the contents of soil particle fractions, pH, and humus content along the slope (P < 0.0001), with lower confidence level in bulk density (P < 0.05). Differences between row and inter-row space were significant for the pH, humus, and silt content, but for sand and clay content, and bulk density differences were not determined. The study determined differences in soil water content among five slope positions (P < 0.0001), and between row and inter-row vineyard space (all with P < 0.05). Where in the upper slope positions (e. g., P1) soil water content was higher than on lower slope positions. Higher soil water content was observed at higher slope positions, associated with clay content. However, it can be concluded that the retention of moisture on the slope is more influenced by local-scale soil properties (primarily soil texture) and variability of the crop (row/inter-row) than the position on the slope.
  
  KEY WORDS: Hillslope; Agriculture; Stagnosol; Soil properties; Soil water content; Bg horizon.
  
  Address:
  - Ivan Magdić, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia. (Corresponding author. Tel.:+385 1 239 3962 Fax.: Email: imagdic@agr.hr)
  - Toni Safner, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia.
  - Vedran Rubinić, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia.
  - Filip Rutić, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia.
  - Stjepan Husnjak, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia.
  - Vilim Filipović, Faculty of Agriculture, University of Zagreb, Svetošimunska cesta 25, 10000 Zagreb, Croatia.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 74 - 88, doi: 10.2478/johh-2022-0004
Scientific Paper, English

Pierre-Adrien Rivier, Dorina Jamniczky, Attila Nemes, András Makó, Gyöngyi Barna, Nikolett Uzinger, Márk Rékási, Csilla Farkas: Short-term effects of compost amendments to soil on soil structure, hydraulic properties, and water regime

 Full Text in PDF     13 DOWNLOADS

• Despite the increasing interest in applying composts as soil amendments worldwide, there is a lack of knowledge on short-term effects of compost amendments on soil structural and hydraulic properties. Our goal was to study the effect of compost and vermicompost-based soil amendments on soil structure, soil water retention characteristics, aggregate stability and plant water use efficiency compared to that of mineral fertilizers and food-waste digestate and examine if these effects are evident within a short time after application. We set up a pot experiment with spring wheat using a sandy and a loamy soil receiving either mineral fertilizer (MF); dewatered digestate from anaerobic digestion of food waste (DG), vermicomposted digestate (VC_DG); sewage sludge-based compost (C_SS) and sewage sludge-based vermicompost (VC_SS). We then monitored and calculated the soil water balance components (irrigation, outflow, evaporation, transpiration, and soil water content). At harvest, we measured shoot biomass, soil texture, bulk density, water retention characteristics and aggregate stability. The irrigation use efficiency (IE) and the plant water use efficiency (WUE) were calculated for each treatment by dividing the transpiration and the dry shoot biomass with the amount of water used for irrigation, respectively. For the sandy soil, we used X-Ray computed tomography to visualise the pore system after applying organic amendments and to derive metrics of the pore-network such as its fractal dimension, imaged macroporosity and critical pore diameter. X-Ray tomography indicated that composting and vermicomposting resulted in more complex and diverse porous system and increased soil macroporosity. The increased fractal dimensions also indicated that compost and vermicompost can contribute to structure formation and stabilization within a short time after their application. Despite the small application rate and short incubation time, the application of organic amendments to the two different soil types resulted in improved soil water holding capacity and water use efficiency. Composting and vermicomposting appeared to have the best effect at reducing the irrigation demand and evaporation losses and increasing the water use efficiency of the plant, likely through their effect on soil structure and the pore-size distribution.
  
  KEY WORDS: Compost; Vermicompost; Soil water retention; Aggregate stability; Water use efficiency; X-CT imaging.
  
  Address:
  - Pierre-Adrien Rivier, NIBIO, Norwegian Institute of Bioeconomy Research, Oluf Thesens vei 43, 1433, Aas, Norway.
  - Dorina Jamniczky, University of Milan, Via Festa del Perdono 7, 20122 Milano MI, Italy.
  - Attila Nemes, NIBIO, Norwegian Institute of Bioeconomy Research, Oluf Thesens vei 43, 1433, Aas, Norway.
  - András Makó, Institute for Soil Sciences, Centre for Agricultural Research of the Eötvös Loránd Research Network, H-1022 Budapest, Herman Ottó út 15, Hungary.
  - Gyöngyi Barna, Institute for Soil Sciences, Centre for Agricultural Research of the Eötvös Loránd Research Network, H-1022 Budapest, Herman Ottó út 15, Hungary.
  - Nikolett Uzinger, Institute for Soil Sciences, Centre for Agricultural Research of the Eötvös Loránd Research Network, H-1022 Budapest, Herman Ottó út 15, Hungary.
  - Márk Rékási, Institute for Soil Sciences, Centre for Agricultural Research of the Eötvös Loránd Research Network, H-1022 Budapest, Herman Ottó út 15, Hungary.
  - Csilla Farkas, NIBIO, Norwegian Institute of Bioeconomy Research, Oluf Thesens vei 43, 1433, Aas, Norway. Institute for Soil Sciences, Centre for Agricultural Research of the Eötvös Loránd Research Network, H-1022 Budapest, Herman Ottó út 15, Hungary. (Corresponding author. Tel.: Fax.: Email: csilla.farkas@nibio.no)

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 89 - 102, doi: 10.2478/johh-2022-0002
Scientific Paper, English

Esmaeil Dodangeh, Hossein Afzalimehr: Incipient motion of sediment particles in the presence of bed forms under decelerating and accelerating flows

 Full Text in PDF     19 DOWNLOADS

• This paper investigates the incipient motion of sediment particles under non-uniform flow in river and laboratory. In rivers, the non-uniform flow is often observed due to the presence of various bed forms. Threshold condition has been examined by using the Shields diagram based on the uniform flow assumption, however, this approach can be led to fallacious results for non-uniform flows where the effect of pressure gradient is significant due to bed forms. This study investigates the chronological order of incipient motion of the particles, the average threshold velocity (Ucr), and Shields parameter for non-uniform flows. River data collection with gravel is used for investigating the incipient motion of surface layer of river bed and the laboratory data collection is considered studying the incipient motion of sub-surface layer of river. Both river and laboratory data collections are conducted in the presence of bed forms. Results reveal that the Shields diagram underestimates the particle incipient motion under accelerating and decelerating flows for the both case of laboratory and river. In both weak and general motion in the laboratory, the values of the critical Shields parameter are located below the Shields diagram, showing no particle motion. Our analysis shows that the incipient motion in river is affected by the presence of bed forms, river width changes, and flow non-uniformity conditions. The results show that in the accelerating flow (the bed form exit with a negative slope), the incipient motion is greater than the decelerating flow (the bed form entrance with a positive slope).
  
  KEY WORDS: Incipient motion; Bed forms; Accelerating and Decelerating flows; Bed shear stress; Shields parameter.
  
  Address:
  - Esmaeil Dodangeh, Natural Disasters Prevention Research Center, School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran.
  - Hossein Afzalimehr, Natural Disasters Prevention Research Center, School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran. (Corresponding author. Tel.:(+98-21) 77240565 Ex: 8131 Fax.: Email: hafzali@iust.ac.ir)

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 103 - 113, doi: 10.2478/johh-2021-0035
Scientific Paper, English

Wyatt D. Lantz, Brian M. Crookston, Michele Palermo: Evolution of local scour downstream of Type A PK weir in non-cohesive sediments

 Full Text in PDF     38 DOWNLOADS

• A large-scale piano key weir laboratory study was conducted to investigate the evolution of the scour process occurring in the downstream basin for two non-cohesive granular bed materials, including the analysis of scour-hole geometry and patterns at equilibrium. It was observed that hydraulic conditions, particularly tailwater level, significantly affect the scour mechanisms and equilibrium morphology, eventually resulting in scour depths that exceeded the weir height. Unprecedented insights on the scour dynamics are also provided, along with tools to estimate the time evolution and maximum scour depth, its location in the streamwise direction, and the maximum scour length.
  
  KEY WORDS: Equilibrium morphology; Granular bed; Piano key weir; Scour evolution.
  
  Address:
  - Wyatt D. Lantz, Utah Water Research Laboratory, Department of Civil and Environmental Engineering, Utah State University, 8200 Old Main Hill, Logan, UT 84322-8200, USA.
  - Brian M. Crookston, Utah Water Research Laboratory, Department of Civil and Environmental Engineering, Utah State University, 8200 Old Main Hill, Logan, UT 84322-8200, USA. (Corresponding author. Tel.:+1 435-797-0247 Fax.: Email: brian.crookston@usu.edu)
  - Michele Palermo, DESTEC-Department of Energy, Systems, Territory and Construction Engineering, University of Pisa, Via Gabba 22, 56122 Pisa, Italy.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 114 - 127, doi: 10.2478/johh-2021-0036
Scientific Paper, English

Saeid Okhravi, Saeed Gohari, Mahdi Alemi, Rodrigo Maia: Effects of bed-material gradation on clear water scour at single and group of piles

 Full Text in PDF     36 DOWNLOADS

• This study presents the results of 32 laboratory experiments on local scour at a single pile and a 1 × 4 pile group for both uniform and non-uniform sediments under clear water conditions. The present study aims to evaluate the effects of different sediment beds made up of mixtures of sand and gravel (four-bed configurations) in d50 (1–3.5 mm) and gradation (1.4–3) ranges on scour depth for different flow discharges and flow depths. Further, the findings of the experiments are deployed to describe the effects of pile spacing and flow conditions on the local pier scour for both uniform and non-uniform bed granulometries. In addition, this study addresses the performance of some existing scourdepth predictors. Also, the corresponding results are suitable for validating the numerical models in local pier scour prediction importantly with non-uniform sediments. In summary, the results show that effects of sediment gradation dampen with increasing flow shallowness. Furthermore, the maximum scour depth at pile groups generally increases as pile spacing decreases for uniform sediments, whereas the mentioned trend was not observed for non-uniform sediments for the same flow and sediment conditions. Moreover, the experimental results revealed that bed sediment gradation is a controlling factor in the pile’s scour. Thus, the existing scour depth predictions could be highly improved by considering sediment gradation in the predictions. Finally, the conclusions drawn from this study provide crucial evidence for the protection of bridge foundations not only at the front pile but also at rear piles.
  
  KEY WORDS: Laboratory tests; Local scour; Pile groups; Pile spacing; Sediment gradation; Uniform and non-uniform sediments. sediments.
  
  Address:
  - Saeid Okhravi, Department of Water Engineering, College of Agriculture, Bu-Ali Sina University, 65178-38695, Hamadan, Iran. Current Address: Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104, Bratislava, Slovak Republic. (Corresponding author. Tel.:+98-9131864149 Fax.: Email: s.okhravi94@basu.ac.ir; saeid.okhravi@savba.sk)
  - Saeed Gohari, Department of Water Engineering, College of Agriculture, Bu-Ali Sina University, 65178-38695, Hamadan, Iran.
  - Mahdi Alemi, Departamento de Engenharia Civil, Faculdadede Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, Porto 4200-465, Portugal. CIIMAR - Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Terminal de Cruzeiros do Porto de Leixoes, Avenida General Norton de Matos, 4450-208, Matosinhos, Portugal.
  - Rodrigo Maia, Departamento de Engenharia Civil, Faculdadede Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, Porto 4200-465, Portugal. CIIMAR - Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Terminal de Cruzeiros do Porto de Leixoes, Avenida General Norton de Matos, 4450-208, Matosinhos, Portugal.

J. Hydrol. Hydromech., Vol. 70, No. 1, 2022, p. 128 - 144, doi: 10.2478/johh-2021-0034
Scientific Paper, English

Hasnain Tariq, Usman Ghani, Naveed Anjum, Ghufran Ahmed Pasha: 3D numerical modeling of flow characteristics in an open channel having in-line circular vegetation patches with varying density under submerged and emergent flow conditions

 Full Text in PDF     36 DOWNLOADS

• In the marine ecological system, the prime role of water management and durability of an ecosystem is being played by the vegetation patches. The vegetation patches in open channels can significantly affect the flow velocity, discharge capacity and hinder energy fluxes, which ultimately helps in controlling catastrophic floods. In this study, the numerical simulation for turbulent flow properties, i.e. velocity distribution, Reynolds stresses and Turbulent Intensities (TI) near the circular vegetation patches with progressively increasing density, were performed using the computational fluid dynamics (CFD) code ANSYS FLUENT. For examination of the turbulent flow features in the presence of circular patches with variable densities, Reynolds averaged Navier-Stokes equations, and Reynolds stress model (RSM) were employed. The numerical investigation was performed in the presence of in-line emergent and submerged patches having variable vegetation density in the downstream direction. Two of the cases were investigated with three circular patches having a clear gap to patch diameter ratio of La/D = 1 (where La is the clear spacing between the vegetation patches and D is the diameter of the circular patch), and the other two cases were analyzed with two patches having a clear gap ratio of La/D = 3. The case with a clear gap ratio (La/D = 3) showed 10.6% and 153% inflation in the magnitude of longitudinal velocity at the downstream of the sparse patch (aD = 0.8) and upstream of the dense patch (aD = 3.54), respectively (where aD is the flow blockage, in which “a” represents the patch frontal area and “D” represents the patch diameter). The velocity was reduced to 94% for emergent and 99% for submerged vegetation due to successive increase in vegetation density made by introducing a middle patch which reduced the clear gap ratio (La/D = 1). For La/D = 1, the longitudinal velocities at depth z = 15cm were increased by 319% than at depth z = 6cm at the downstream of the dense patch (aD = 3.54). Whereas it was observed to 365% higher in the case of La/D = 3. The magnitude of turbulent characteristics was observed 36% higher for submerged vegetation cases having a clear gap ratio of La/D = 1. The successive increase in the patch density reduced the Reynolds stresses, turbulent kinetic energy and turbulent intensities significantly within the gap region. The major reduction in the flow velocities and turbulent properties in the gaps provides a stable environment for aquatic ecosystems nourishment and fosters sediment deposition, and supports further vegetation growth.
  
  KEY WORDS: Vegetation patches; Open channel; Variable patch density; Flow velocity; Turbulence.
  
  Address:
  - Hasnain Tariq, Department of Civil Engineering, University of Engineering and Technology, 47050, Taxila, Pakistan.
  - Usman Ghani, Department of Civil Engineering, University of Engineering and Technology, 47050, Taxila, Pakistan. (Corresponding author. Tel.:+92 51-9047647 Fax.: Email: usman.ghani@uettaxila.edu.pk)
  - Naveed Anjum, Graduate School of Science and Engineering, Saitama University, Saitama, Japan, 255 Shimookubo, Sakura Ward, Saitama, 338-8570, Japan.
  - Ghufran Ahmed Pasha, Department of Civil Engineering, University of Engineering and Technology, 47050, Taxila, Pakistan.