Open channel confluences are where rivers and canals meet. They are complex with variable flow patterns that can cause flooding, scouring, and sediment accumulation. The document presents a conceptual model of a 90-degree asymmetrical confluence with rectangular channels to systematically study key parameters like discharge ratio. Laboratory experiments are conducted using measurement techniques like ADV and LSSPIV to analyze velocities and turbulence. Numerical modeling is also used to efficiently study parameters and provide additional data for validation. The overall aim is to improve understanding of confluence hydrodynamics to help address engineering issues.

1. Open channel confluence
hydrodynamics
Stéphan Creëlle(1), Laurent Schindfessel(2) & Tom De Mulder(3)
(1) PhD fellow Special Research Fund (BOF) of Ghent University – (2) PhD fellow Research Foundation-Flanders (FWO) – (3) Supervisor
Ghent University – Dep. of Civil Engineering
Hydraulics Laboratory
Sint-Pietersnieuwstraat 41
B-9000 Ghent, Belgium
e-mail: Stephan.Creelle@UGent.be
River and canal systems fulfill important societal, economical and ecological functions, such as discharge of water and sediments, waterways for navigation and habitats for unique
ecosystems. In a river and canal network, open channel confluences are omnipresent.
Confluences are characterized by complex flow patterns, which often induce important water level elevations (flood risks), scour holes (risk of instabilities of banks and engineering
structures) and accumulation of sediments in bars (risk of maintenance dredging). Hence, knowledge of the parameters that govern flow and river bed dynamics at confluences is of great
practical interest.
A conceptual model of the complex flow behaviour at confluences is presented in Figure 1. One of the key variables for confluence flow patterns, is the discharge ratio q, defined as the
ratio between incoming discharge of the tributary channel (Qt) and main channel outflow(Qd):
𝑞 =
𝑄𝑡
𝑄 𝑑
=
𝑄𝑡
𝑄 𝑚 + 𝑄𝑡
Other important parameters are the angle between the main and tributary flow, the shape of the channel cross-sections, the concordance or discordance of the channel beds, the bed
roughness and upstream flow disturbances.
In order to allow a thorough and systematic study of these parameters, a 90° asymmetrical confluence, with a fixed bed and rectangular channel cross-sections is chosen.
Contact details
Laboratory experiments Numerical modelling Research objectives
Figure 1 : Illustration of a conceptual model for an open channel confluence
Flow stagnation point
Separation zone
Mixing layer
between separation zone and tributary flow
Mixing layer
between tributary flow and main channel flow
A zone of flow separation is formed at the
inner wall of the downstream section of the
main channel. This is the result of a local
streamwise momentum deficit, caused by the
tributary inflow, forcing the flow to the outer
wall. In this zone of flow separation low flow
velocities exist, resulting in a settling
opportunity in case one of the incoming
flows carries sediment.
The large velocity difference between the
separation zone and the tributary flow gives
rise to high shear forces, resulting in highly
turbulent flow. This shear creates coherent
flow structures, traveling along a mixing layer,
which can be seen as a virtual
interface between the flow from
the separation zone and the tributary.
Tributary channel inflow (Qt)
Flow coming from the tributary contains a
momentum perpendicular to the main
channel direction. This momentum forces
flow in the main channel to contract towards
the outer bank , resulting in a contracted flow
section just downstream of the confluence.
At the wall, near the upstream corner of the
confluence, a flow stagnation point arises. No
flow velocities at this point exist. The exact
location varies with the discharge ratio q. For
high values of q this point is located on the
main channel wall, whereas for low values of
q it moves into the tributary.
Velocity differences between the two
incoming flows induce shear forces, resulting
in a mixing layer between the tributary and
main channel flow. Since the flow rearranges
itself depending on the discharge ratio,
velocity differences are small when compared
to the other mixing layer. Presence of
coherent structures on this mixing layer is
dependent on the flow conditions. Fluid
mixing and lateral redistribution of
streamwise momentum is highly dependent
on these structures.
Contracted flow
Just downstream of the confluence, the free
flow path is reduced because of the
appearance of the separation zone. This
results in a local zone of high flow velocities
near the outer bank.
Main channel inflow (Qm)
Flow in the main channel is oriented in the
streamwise direction. Because of the
downstream flow contraction, flow patterns
in the upstream section are affected. In case
of significant tributary inflow, the flow
contraction can result in high energy losses,
leading to important increases in upstream
water levels.
Downstream outflow (Qd)
At the downstream end, the combined
flow flows out of the confluence zone. The
degree of mixing and uniformity in flow
velocity is highly variable and depending
on all the upstream flow features.
Figure 2 : Surface tracers for visualisation of both mixing layers Figure 3 : LES simulation of flow at q=0.75 [-]
The aim of the above presented research is to
contribute to the insight into open channel confluence
hydrodynamics, which is a prerequisite for more
efficient and effective problem mitigation or problem
solving in river and canal engineering issues related to
confluences.
In addition to this, the fundamental study of these
types of complex 3D flows allows to gain insight into
the capabilities and constraints of state-of-the-art
advanced measurement and flow simulation
techniques, which is needed to advance the nowadays
commonly applied tools in hydraulic consultancy.
Laboratory experiments are very valuable when
studying open channel confluences because of the
complexity of the associated flow features .
Temporal measurements of water heights, surface and
subsurface velocities and turbulence characteristics are
required.
Applied measurement techniques comprise of acoustic
and capacitive water level measurement, pressure
measurements, velocity measurements by both
Acoustic Doppler Velocimetry (ADV) and Large Scale
Surface Particle Image Velocimetry (LSSPIV).
Numerical modelling of the flow at the confluence
provides means to efficiently perform parameter
studies and to obtain more extensive data sets.
Modelling the complex flow at the open channel
confluence provides a challenging test case to explore
the capabilities of a numerical package.
Once validated with measurement data from the
laboratory experiments, the model can be applied to
provide extensive datasets which are impossible to
obtain from laboratory work because of technical
incompatibilities or because the work load is
prohibitively large.