Experimental investigation on flow structure and discharge distribution in T-shape open channel bifurcations of variable widths

Abstract

In this research, open-channel bifurcations, defined as geometrical singularities of a hydrographic network, where the inflow separates into two downstream branches are investigated. Bifurcations are naturally present in river networks, mostly upstream from islands and in deltas, where the main flow does not necessarily continue in a straight direction but may form a dimensionally asymmetrical Y-shaped diverging channel. Sediment transport in bifurcations are increasingly becoming a major issue when extensive suspended as well as bed material conveyed by rivers regularly encroach into side or lateral (i.e. bifurcated) channels utilized for abstracting raw water thus affecting optimal intake for potable consumption. These divisions in flow are also observed in human-intervened conditions such as wastewater networks where the combined sewer overflows evacuate part of the exceeding sewer discharge capacity toward the surrounding environment. In addition, flooded streets reaching three-branch crossroads generate bifurcated channel flows. For most design or man-made cases, the main flow continues straight ahead, while the side branch, generally narrower, makes an angle close to 90° with the main channel, forming a junction known as T-shape bifurcation. Many researchers have completed studies on T-shape bifurcations to understand the behaviour of water flow and sediment transport. However, a complete understanding of the phenomenon, especially in relation to 2D or 3D secondary flows and vortices, is lacking up to this day. Moreover, the distribution of flow discharges in both main and lateral/branch channels requires further detailed investigation. As such, the objectives of this research on open-channel flow in 90° bifurcations are to determine experimental techniques of flow visualization, investigate methods to identify three-dimensional flow patterns, improve empirical discharge distribution relationships, and establish the effect of reduced side branch width on flow distribution. A total of 668 experimental runs have been conducted and completed in the open-channel flume system at Laboratoire de Mécanique des Fluides et d’Acoustique (LMFA) of Institut National des Sciences Appliquées (INSA) in Lyon, France. From the results, it is observed that flow visualization techniques were generally not effective nor adequately consistent to differentiate between 2D standard recirculation or 3D helical flow structures. Therefore, another approach based on analysis of fluid power gain or loss plotted against inflow discharge in the main channel is introduced as an alternative. As for assessing flow division through the bifurcation, the results are organized into sections to progressively address discharge distributions in the subcritical or free recirculation transcritical regimes. Consequently, a new predictive set of empirical equations for flow distribution is derived based on the known upstream discharge plus branch-width ratio and stage-discharge relationships in the downstream or lateral/branch channels. This discharge distribution formulation is accurate, as indicated by determination coefficients of at least 0.98 when compared to measurements, without even identifying beforehand whether the flow is in fully subcritical or free recirculation transcritical regime through the lateral/branch channel. Based on the successful accomplishment of stipulated objectives, recommendations for further work to include establishment of a flow-structure identification system, as well as extending the experiments to bifurcations with significantly narrower side branches. The angle of flow divergence could also be varied to other than 90°.