Effect of bed elevation discordance in the main river on the confluence hydrodynamics in 90o straight-channels’ confluences

A role of the difference in bed elevations in the main-river (ΔzMR) at its entrance to a 90º straight-channels' confluence is studied numerically using a 3D finite-volume model. Three characteristic hydrological scenarios with DR = {0.250, 0.583, 0.750} are analysed in four confluence layouts having ΔzMR / hd [0.0, 0.50] (where hd stands for the flow depth in the deeper canal at the confluence). It is found that: 1) a portion of the tribu-tary flow in the bottom layers is entrained by the bed-step, 2) the w- velocity has the same order of magnitude as Vxy for ΔzMR ≥ 0.25hd and DR > 0.5, 3) the opposite bank is endangered by erosion due to strong upward motion along this bank, 4) additional shear layer develops on the outer side of the distorted umax-core and 5) the presence of bed-step favours bed-load transport both in and far downstream of the confluence

Estimation of 1D-confluence model parameters in right-angled discordant beds’ confluences using 3D numerical model

Parameters of 1D confluence models are originally defined for concordant beds’ (CB’s) confluences. This paper aims at estimating these parameters for discordant beds’ confluences using numerical simulation results of 3D flow. A 3D finite-volume based model SSIIM2 that was successfully validated in CB’s confluences is applied in this study, too. Confluences with the low,moderate and maximal observed bed elevation discordance ratio values are analysed for three characteristic hydrological scenarios: dominance of the tributary flow, equal contributions of the combining flows and dominance of the main-river flow. It is shown that: 1) the mean flow angle δ approaches junction angle α with the increase in bed elevation discordance, especially when tributary flow dominates, 2) the value of Hager’s correction coefficient σ is not constant and 3) the contribution of the tributary flow to the 1D momentum equation is under predicted when either parameter δ or σ is used for its estimation

Pregled i ocena analitičkih modela ušća

Assumptions upon which development of 1D models of free-surface flow rest are not valid at open-channel jun­ctions. Thus, the 1D flow equations cannot be used for calculation of upstream flow depths, or water levels in a dendritic channel network. To facilitate design of river confluences and/or networks of drainage canals, about ten analytical models have been proposed since 1940-ties. Development of these models was either based on the theoretical considerations, or on the combination of the theoretical considerations and laboratory experiments. The models are valid only for the confluences with equal channel-bed elevations (concordant beds confluences), the majority being derived for the subcritical flow. In this paper three models are examined in detail, two of which are based on the assumption of equal upstream flow depths (the models of Gurram et al. and Hsu et al.), whereas the third one allows different upstream flow depths and different channel widths of the combining channels (the model of Shabayek et al.). The paper in­dicates some inconsistency in derivation of the equal-depths models and assesses the influence of the incon­sistent terms through comparison with the existing ex­perimental data. It is found that, in the steady flow con­ditions, taking into account the junction angle instead of the flow angle at the tributary entrance to the confluence results in negligible difference in the prediction of upstream flow depth. The future research should reveal how the flow angle b varies with the junction angle α and the discharge ratio of the tributary and main river flows - δ=f (α, q).Na spojevima otvorenih tokova strujnice postaju zakrivljene, jer vertikalno ubrzanje fluidnih delića nije zanemarljivo u odnosu na ubrzanje zemljine teže, pa su to mesta gde ne važe osnovne pretpostavke na kojima se zasnivaju jednačine linijskog tečenja. Stoga se te jednačine ne mogu koristiti za određivanje dubina uzvodno od ušća koje su neophodne za uređenje rečnih ušća ili projektovanje mreža drenažnih kanala. Od sredine 1940-tih do danas za ove potrebe izvedeno je, bilo isključivo teorijskim razmatranjima, bilo kombinacijom teorijskih razmatranja i laboratorijskih ogleda, oko desetak analitičkih izraza različitog nivoa detaljnosti. Svi izrazi su izvedeni za ušća sa jednakim kotama dna svih kanala na mestu njihovog spajanja, a većina njih važi za uslove mirnog tečenja. U ovom radu detaljno se razmatraju tri novija modela izvedena za uslove mirnog tečenja od kojih se dva (modeli Gurama i ost. i Hsua i ost.) zasnivaju na Tejlorovoj pretpostavci o jednakosti uzvodnih dubina i važe za ušća korita jednakih širina, dok treći model (model Šabajeka i ost.) dozvoljava da uzvodne dubine i širine korita budu različite. Ukazuje se na nedostatke pri izvođenju modela zasnovanih na pretpostavci o jednakosti uzvodnih dubina i ocenjuje se uticaj spornih članova kroz poređenje sa rezultatima postojećih merenja. Pokazuje se da su uz odgovarajuće izmene pojedinih modela, relativne greške svih modela nalaze u granicama greške merenja, pri čemu se najmanja greška dobija primenom modela Šabajeka i ost. Utvrđeno je da je greška koja se čini uzimanjem ugla uliva a umesto ugla skretanja toka na ušću δ u uslovima ustaljenog tečenja zanemarljiva. Buduća istraživanja treba da utvrde vezu δ=f (α, q), gde je q odnos protoka pritoke i reke

Determination of a stone particle trajectory in the flowing water

U ovom radu prikazana je metodologija proračuna trajektorije kamena u tekućoj vodi. Analizirani su faktori koji utiču na dužinu trajektorije, odnosno na vreme tonjenja i domet ispuštenog zrna. To su: početna dubina sa koje se kamen ispušta u vodeni tok da slobodno pada, krupnoća kamena, brzina toka (ambijentalnog fluida) i faktor oblika zrna.Trajectory of a stone particle released into the flowing water is determined. The length of the particle trajectory, i.e. the duration of sinking and the distance traveled by the released particle, is analysed in terms of the parameters considered: the initial depth at which the stone particle is released into the flowing water, the particle diameter, the flow velocity and the shape factor

Can a 3D-Numerical Model be a Substitute to a Physical Model in Estimating Parameters of 1D-Confluence Models?

A possibility of using a 3D-numerical model instead of a more expensive physical one for estimation of parameters in 1D-confluence models is considered. A set of existing experimental data from 90º confluence is used for the 3D-model validation. A satisfactory agreement for measured velocities, mean flow angle δ and correction coefficient σ is achieved. It is also shown that the use of a mean flow angle δ in 1D-model provides a better prediction of the component ITx of the tributary force of inertia.http://webpages.fe.up.pt/iahr2014/default.asp?m=3 http://webpages.fe.up.pt/iahr2014/Final_Programme_IAHR_Porto_11042014.pd

Novel approach to stream corridor restoration

U radu se prikazuje jedan od mogućih modela organizacije obnavljanja narušenih režima prirodnih vodotoka, njihovog priobalja i pribrežja. Navode se i opisuju ključni organizacioni koraci kao što su: pokretanje predloga za donošenje plana obnavljanja rečnog pojasa, definisanje razmera i granica projekta i uspostavljanje okvira za planiranje i upravljanje kako bi se omogućila međusobna komunikacija učesnika i komunikacija sa svim zainteresovanim institucijama, organizacijama i pojedincima. Na kraju se komentariše neophodnost uvođenja prikazanog koncepta organizacije u našu hidrotehničku praksu kako u oblasti obnavljanja, tako i u oblastima korišćenja i zaštite vodenih resursa.The paper presents an organisational model for stream corridor restoration. The key steps, such as: launching a restoration initiative, defining its scope, setting project boundaries and establishing planning and management framework to facilitate communication among all involved and interested parties, are listed and described. Finally, the necessity of introducing the proposed organisational concept both into the water resource restoration, and utilisation and conservation thereof is discussed.Rad je dostupan i na internet stranici časopisa Vodoprivreda https://www.vodoprivreda.net/savremen-nacin-organizovanja-u-oblasti-uredenja-vodotoka-i-obnavljanja-recnih-pojaseva

Hydrodynamics of 90o concordant beds’ confluences of straight-channels with unequal channel widths

With the exception of confluences of large alluvial rivers, tributary channel is usually narrower that the channel of the main-river. hydrodynamics of confluences of equal width channels has been thoroughly studied using 3D numerical models and to a certain extent it was studied in laboratory confluences. Hydrodynamics and morphodynamics of confluences with unequal channel widths were recently studied experimentally for mountainous rivers, and there are limited experimental data on hydrodynamic characteristics for horizontal bed confluences. This study aims at analysing hydrodynamics of alluvial river confluences with unequal channel widths and concordant beds. They are analysed for the three typical hydrological scenarios at the confluence (defined by the discharge ratio DR = QMR / (QMR + QT )): 1) the dominance of the tributary flow (DR = 0.250), 2) equal contributions of the combining flows (DR = 0.583) and 3) the dominance of the main-river flow (DR = 0.750). A confluence with the 90o junction angle is chosen for the study, since this angle allows for the development of all six subzones within the confluence hydrodynamics zone that were recognised by Best in 1980s. Two values of the channel-width-ratio of the tributary ("T") and main-channels ("MR") (BT / BMR = {0.75, 0.50}) are analysed in addition to the case of equal width channels (BT / BMR = 1.00). As it was expected, the flow deflection on the horizontal plane (defined by the flow angle δ = arc tg (v/ u)) reduced with the narrowing of the tributary channel, due to increase in the value of the momentum-flux ratio. For DR=0.250, the momentum-flux ratio increases by 22 and 73% for BT /BMR = {0.75, 0.50}, respectively, whereas for DR=0.583 and DR = 0.750, this increase ranges between 10 and 40%. The greatest effect on the reduction of the flow deflection (increase in the average flow angle (δav)) is achieved in the case when the main-river flow dominates. The increase is between 28 and 33% for BT /BMR = {0.75, 0.50}, respectively. The reduced flow deflection results in deeper penetration of the tributary flow into the main channel and consequent widening of the recirculation zone (RZ) in the post-confluence channel. The maximal RZ width is increased by 50% for BT /BMR = 0.75 and it is almost doubled for BT / BMR = 0.50 when DR = 0.250. The increase does not exceed 50% for DR=0.583, whereas for DR = 0.750, it reaches 35% when BT /BMR = 0.75 and it is again doubled for BT / BMR = 0.50. The maximal increase in the stream-wise velocity magnitude of almost 45% is attained for BT / BMR = 0.50 when DR = 0.250. The RZ widening and the increase in the stream-wise velocity magnitude result in the increased transport capacity within the maximal velocity and flow recovery zones. An indicator of this increase is the value of non-dimensional bed shear stress (τ0 / τ0,cr) for the grain of certain size. For example, τ0 / τ0,cr for the sand grain of 1 mm size increases approximately 4.5 times in the confluence with BT / BMR = 0.50 when the tributary flow dominates. Under the remaining two scenarios, the rise in τ0 / τ0,cr is significantly smaller regardless of the BT / BMR -value, i.e. it does not exceed 30%

Uporedna analiza rezultata dobijenih primenom ortogonalnih ineortogonalnih mreža u modelima prostornog tečenja - deo l

Application of orthogonal and non-orthogonal grids in numerical models of the three-dimensional flow is considered. A river confluence is chosen as an example due to strong three-dimensional flow effects occurring at the confluence and the complex bathymetry of the river bed. In addition to the type of numerical method/scheme used, the quality of the numerical simulation results depends on the choice of the computational grid, too. In contrast to orthogonal grids, the non-orthogonal ones are better aligned with the boundary. Hence, they are more suitable for discretisation of the complex flow domains such as the river confluence than the orthogonal ones. The paper aims at investigating the difference in the numerical simulation results obtained with the two types of grids. The investigation is rather qualitative than the quantitative one. The results are discussed in terms of the type of the secondary flow patterns developed in the confluence and the post-confluence channel under different flow conditions at the confluence. .Razmatra se primena ortogonalnih i neortogonalnih mreža u modelima prostornog tečenja. Za analizu je izabrano rečno ušće, jer je na ušću tečenje izrazito prostorno, a korito ima složen oblik. Na kvalitet rezultata dobijenih numeričkim simulacijama pored pravilnog izbora numeričke metode, bitno utiče i izbor tipa računske mreže. Neortogonalna mreža se svojim oblikom bolje prilagođava čvrstoj granici od ortogonalne, pa je pogodnija za predstavljanje računskih oblasti složenog oblika kakvo je ušće. Cilj rada je da se kroz uporednu kvalitativnu analizu rezultata utvrde razlike između dva tipa računskih mreža. Poređenje rezultata je obavljeno na osnovu tipa sekundarnog strujanja koje nastaje na ušću i nizvodno od ušća pri različitim odnosima inercijalnih sila reke i pritoke.

Numerical Simulation Of 3D Flow In Right-Angled Confluences With Bed Elevation Discordance In The Main River

Two major morphological phenomena in alluvial river confluences are the development of an avalanche face or faces at the entrance of one or both converging channels to the confluence, due to deposition of coarser sediments, and deepening of the scour hole in the post-confluence channel. The development of an avalanche face creates the difference in bed elevations between incoming and outgoing channels. Laboratory experiments in movable bed models of confluences showed, and bathimetric surveys in field confluences confirmed, that avalanche faces develop in both converging channels only in alluvial river confluences with large junction angles (α ≥ 45o). The effect of bed elevation discordance between the tributary and main channels was studied in the author’s previous papers. Another step in recognising controls to the complicated flow pattern in a river confluence is the analysis of the role of bed elevation discordance in the main river. This paper, thus, continues the line of previous studies by considering the effect of different extents of bed elevation discordance in the main river on the flow characteristics in the confluence hydrodynamics zone, namely: 1) on the flow deflection at the tributary entrance to the confluence, 2) on cross-sectional distributions of the three velocity components in the post-confluence channel and 3) on variations of the recirculation zone size throughout the flow depth. To facilitate comparison with previous studies and to follow the above mentioned findings regarding the limit junction angle, only the confluence with α = 90o is considered. Four values of the bed elevation discordance ratio ΔzMR / hd in the range [0.0; 0.5] are analysed (where ΔzMR stands for the difference in bed elevations between the stretches of the main river upstream and downstream of the upstream junction corner and hd stands for the flow depth in the main river at the confluence)