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Flow, Sediment, and Morpho-Dynamics of River Confluence in Tidal and Non-Tidal Environments

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Journal of Marine Science and Engineering Review Flow, Sediment, and Morpho-Dynamics of River Confluence in Tidal and Non-Tidal Environments 1, 2, , 3, , Ahmed Bilal y , Qiancheng Xie * y and Yanyan Zhai * y 1 College of Water Conservancy and Hydropower Engineering, Hohai University (HHU), Nanjing 210098, China; [email protected] 2 Division of Fluid and Experimental Mechanics, Luleå University of Technology (LTU), 97187 Luleå, Sweden 3 Division of Fluid Mechanics, Coastal and Maritime Engineering, Technical University of Denmark (DTU), 2800 Kgs, 2800 Lyngby, Denmark * Correspondence: [email protected] (Q.X.); [email protected] (Y.Z.); Tel.: +46-7228-703-81 (Q.X.); +45-9145-8313 (Y.Z.) Co-first author: The authors contributed equally to this work. y Received: 9 July 2020; Accepted: 6 August 2020; Published: 7 August 2020 Abstract: River confluences are the key features of the drainage basins, as their hydrological, geomorphological, and ecological nature strongly influences the downstream river characteristics. The river reaches near the coastal zones, which also makes them under the influence of tidal currents in addition to their runoff. This causes a bi-directional flow and makes the study of confluences more interesting and complex in these areas. There is a reciprocal adjustment of flow, sediment, and morphology at a confluence, and its behaviors, differ greatly in tidal and non-tidal environments. Existing studies of the river junctions provide a good account of information about the hydrodynamics and bed morphology of the confluent areas, especially the unidirectional ones. The main factors which affect the flow field include the angle of confluence, flow-related ratios (velocity, discharge, and momentum) of the merging streams, and bed discordance. Hydraulically, six notable zones are identified for unidirectional confluences. However, for bi-directional (tidal) junctions, hydrodynamic zones always remain in transition but repeat in a cycle and make four different arrangements of flow features. This study discusses the hydrodynamics, sediment transport, morphological changes, and the factors affecting these processes and reviews the recent research about the confluences for these issues. All of these studies provide insights into the morpho-dynamics in tidal and non-tidal confluent areas. Keywords: river confluence; morpho-dynamics; tidal effects; flow patterns; sediment transport 1. Introduction River confluence is an essential geomorphological node that controls the downstream routing of flow and sediment. In light of its importance, there has been an increased recognition that more attention needs to be paid to the interaction of flow-sediment-morphology. The study of river confluence has seen significant advances mainly regarding their flow features, or the role of morpho-dynamics here, in influencing such features. Much of the persisting research focuses on the morpho-dynamic evolution of a confluence and their interdependencies on the runoff river. Taylor [1], as a pioneer, worked on the flow characteristics of rectangular channels. Later, Miller [2] investigated, after various field surveys, the relationship between the width, depth, and cross-sectional area of tributaries and post confluence channels for the hilly stream junctions. Mosley [3] discussed the asymptotic behavior of bed features concerning confluence angles and expanded the research topic more scientifically. J. Mar. Sci. Eng. 2020, 8, 591; doi:10.3390/jmse8080591 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 2 of 21 J. Mar. Sci. Eng. 2020, 8, 591 2 of 21 channels for the hilly stream junctions. Mosley [3] discussed the asymptotic behavior of bed features concerning confluence angles and expanded the research topic more scientifically. Best [[4,5]4,5] suggestedsuggested dividing dividing di ffdifferenerent flowt flow features features within within a confluence. a confluence Yuan,.et Yuan, al. [6] et recapped al. [6] recappedthe current the level current of knowledge level of and knowledge development an achievedd development during studiesachieved of uni-directionalduring studies open of uni-directionalchannel confluences. open channel Dixon, confluences. et al. [7] and Dixon, Umar, et etal. al. [7] [ 8and] studied Umar, the et al. river [8] confluencesstudied the usingriver confluencesremote sensing using imagery, remote asensing relatively imagery, new approach, a relatively to new study approach, the behaviors to study in thethe riverbehaviors reach in scale. the riverThese reach studies scale. rea Theseffirm thestudies strength reaffirm of the the hypothesis strength of from the hypothesis Best [4], i.e., from to segregate Best [4], i.e., the to identifiable segregate theconfluent identifiable hydrodynamic confluent patternshydrodynamic into six patterns flow zones. into six Further, flow zones. these worksFurther, also these examined works suchalso examinedmorphological such features morphological as lateral barsfeatures attached as tolatera banks,l bars deep attached scour-holes, to tributary-mouthbanks, deep scour-holes, bars, and a tributary-mouthzone of sediment bars, deposition. and a zone of sediment deposition. In addition to thethe naturalnatural riverriver confluences,confluences, junctionsjunctions thatthat are therethere in urban drainage and the irrigation canalcanal systemssystems are are examples examples of of confluences confluence ins thein the built built infrastructure. infrastructure. Based Based on di onfferent different flow flowand channeland channel characteristics, characteristics, river confluencesriver confluences are diff areerentiated differentiated as those as present those present in (i) upland in (i) reaches,upland (ii)reaches, middle (ii) reaches,middle reaches, which are which represented are represented in the majority in the majority of confluences of confluences studied studied so far, and so far, (iii) and the (iii)reaches the reaches near coastal near coastal areas, whichareas, which are called are tidalcalled confluences. tidal confluences. In terms In terms of flow of direction,flow direction, the tidal the tidalconfluences confluences are distinct are distinct from non-tidalfrom non-tidal ones because ones because they have they a bi-directionalhave a bi-directional continually continually varying varyingflow, while flow, the while others the only others have only a uni-directional have a uni-directional flow. flow. This leads to a general confluence confluence classification classification based based on on flow flow direction, direction, as as shown shown in in Figure Figure 11.. The classificationclassification takes the majority of common confluencesconfluences into account. However, However, there are some special confluences confluences sharing characteristics of two or more categories shown in Figure 11.. For instance, coastal confluences confluences are not necessaril necessarilyy natural and can also be man-made; even for upstream river confluences,confluences, the the flow flow direction is is possibly reve reversedrsed during during compound compound surges surges from from other other rivers. rivers. These exceptional cases are not considered inin thethe presentpresent work.work. Figure 1. ConfluencesConfluences classification. classification. Based onon thethe prevailingprevailing flow flow and and sediment sediment conditions, conditions, ariver a river confluence confluence can can be subdivided be subdivided into intoa non-tidal a non-tidal and tide-drivenand tide-driven one (Table one (Table1). For 1). the Fo former,r the former, reducing reducing the river the sediment river sediment input impacts input impactsthe entire the system entire dynamics. system dynamics. For the latter, For the the sediment latter, the is subjectsediment to bothis subject freshwater to both run-o freshwaterff and the run-offdominating and oceanicthe dominating currents, oceanic showing currents, a bidirectional showing transport a bidirectional feature driven transport by flood feature and driven ebb tides. by floodThe resulting and ebb morpho-dynamic tides. The resulting adaption morpho-dynamic time in a non-tidal adaption river time is usually in a fromnon-tidal months river to years.is usually In a fromtidal environment,months to years. there In exist a tidal two environment, different time ther scales:e exist the firsttwo onedifferent is associated time scales: with thethe tidalfirst periodone is associatedand the second with onethe withtidal theperiod morphodynamic and the second evolution. one with In the a tidalmorphodynamic period, sediment evolution. transport In a varies tidal period,to adapt sediment to the instantaneous transport varies tidal to driven adapt hydrodynamics.to the instantaneous Still, tidal if the driven system hydrodynamics. is not at equilibrium, Still, if thealthough system being is not very at weak,equilibrium, a residual although sediment being transport very weak, may bea residual present atsediment the end oftransport each tidal may cycle. be presentThis latter at contributionthe end of each is responsible tidal cycle. for morphologicalThis latter contribution variation, whichis responsible may be relevantfor morphological in the long variation,term [9,10 ].which may be relevant in the long term [9,10]. J. Mar. Sci. Eng. 2020, 8, 591 3 of 21 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 3 of 21 Table 1. Comparison of the main characteristics between a tidal and non-tidal confluence. Table 1. Comparison of the main characteristics between a tidal and non-tidal confluence. Items Non-Tidal Confluence Tide-Driven Confluence
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