Numerical Modeling for Hydrodynamics and Near-Surface Flow Patterns of a Tidal Confluence

Abstract

Because of the flow influx of tributaries, a confluence forms a unique environment carrying interesting hydrodynamic features and other attributes. The understanding of flow behavior here is important, particularly if it is on a tidally influenced channel in a harbor metropolitan. Because of communal requirements, there is a possibility of building wading structures, which may interplay with the flow in this zone. The knowledge of unidirectional river or flume confluences so far is not readily applicable for similar features in channels near coastal areas that have tidal flow in addition to river runoff. In this study, a tidal confluence that has a highly dynamic bidirectional flow is investigated. Near-surface flow patterns in a tidal cycle are simulated by using a numerical model. A field survey provides the bathymetry, time-series boundary conditions, and corresponding verification data. Good agreement is reached between calculated and measured results. Based on the condition of tidal current, four scenarios are selected for which confluence flow patterns are observed, both spatially and temporally. The results indicate that at least one recirculation is always in the tidal confluence for all flow conditions, which rotates counterclockwise for the ebb flow and clockwise for the flood flow. In addition, there is no absolute slack water condition at the tidal junction in the study area. The study also finds that the flows of all three connected channels at the confluence change in a looped pattern with respect to one another. Furthermore, the study reports unique relationships among the ratios of different flows.

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

“…Both of these conceptual models are physically combined, as shown in Figure 4. Field investigations [5,21,22], laboratory experiments [6,[23][24][25], and numerical simulations [14,[26][27][28][29][30], complementing each other, are common methods to help understand the morpho-dynamics of a confluence. Bradbrook [31] suggested that combining all three methods would help to understand the confluences more comprehensively.…”

“…As a result, the flow patterns in terms of the conventional flow zone definitions are different. Unlike unidirectional confluences, there are limited references available regarding the tidal confluence flow patterns, and its complexity has not drawn much attention [14,18,28].…”

“…A recent study performed 3D simulations for a tidal confluence in China [28] to examine the surface flow features based on extensive field measurements. It is noticed that the typical confluent flow patterns change more gradually and repeat in a cycle with the progression of a tidal phase.…”

“…These four flow scenarios with respect to the discharge conditions are shown in Figure 6. Figure 7 illustrates the four corresponding flow patterns during a full tidal cycle, i.e., the maximum flood tide, flood to ebb transition, the maximum ebb tide, and finally, ebb to flood transition [28]. Figure 7a corresponds to the hydraulic condition when the discharge near the confluence is at its lower peak.…”

“…At the ebb tide, a zone of flow separation also exists close to the left river In addition to the tidal flow features, the study captures some interesting observations regarding the hydraulic behavior of the tidal confluence. The most observable behavior of the transition cases is that the change of flow direction in both tributaries does not occur at the same time due to their geometric and hydraulic asymmetry [28]. This non-coherent flow reversal results in a "no slack water" condition at tidal channel intersections.…”