This paper is concerned with the representation of secondary
circulation in river channel confluences. Recent research has
emphasized the complex three-dimensional flow fields that exist where
two river channels join. Field and laboratory measurements have been
developed to describe time-averaged flow fields in terms of primary and
secondary circulation, and to interpret these in terms of key
generating processes. Central to this research is the need to
understand the effect that flow structures have upon both mixing
processes and confluence geomorphology, notably the development of
scour-holes within the junction zone. One of the common problems faced
by this research is the dependence of observed secondary flow
structures upon the rotation plane for which they are determined.
Different researchers have used different rotation planes, such that
intercomparison of results from different field sites is difficult.
Problems also arise when only two-dimensional measurements (e.g.
downstream and cross-stream) are available, and vertical velocities
need to be inferred from analysis of secondary circulation patterns. If
different analytical methods produce different patterns, so different
inferences could be reached. This paper uses a numerical model to show:
(i) that different analytical methods do result in very different
estimates of the strength of secondary circulation; (ii) that there are
problems in inferring vertical velocities from secondary circulation
cells identified using these methods in confluences. most notably as a
result of the effects of planform acceleration and deceleration; and
(iii) that field and laboratory measurements suffer from being unable
to measure the three-dimensional flow field instantaneously, and hence
allow understanding of the evolution of flow structures through time. A
three-dimensional solution of the Navier-Stokes equations for open
channel flow, combined with a free surface approximation and an
unsteady turbulence model, allows representation of the
three-dimensional time-averaged flow field, and some aspects of the
unsteady evolution of these flow structures. Hence, the researcher can
be freed from the dependence of results obtained upon the analytical
method chosen. This emphasizes the downstream transport of mass in the
form of a helix, which will be central in zones of how convergence or
divergence, rather than the more traditional recognition of closed
helical circulation cells.