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This paper is concerned with the representation of secondary circulation in river channel confluences. Recent research has emphasized thecomplex 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 methodsdo 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 ofthe Navier-Stokes equations for open channel flow, combined with a free surface approximation and an unsteady turbulence model, allows repres [ABSTRACT FROM AUTHOR]

This paper describes and assesses: (i) the use of a new instrument for the determination of three-dimensional flow velocities in natural rivers, the acoustic Doppler velocimeter (ADV); and (ii) a method forpositioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed andwater surface. The ADV uses the Doppler shift principle to measure the velocity of small particles, assuming to be moving at velocities similar to the fluid. Velocity is resolved into three orthogonal components, and measured in a volume 5 cm below the sensor head, minimizing interference of the flow field, and allowing measurements to be made close to the bed. A simple method for positioning and orienting theinstrument using digital tacheometry is described, and is used to obtain velocity measurements concurrently with measurements of both bedand water surface topography. The paper includes a preliminary fieldassessment of the ADV by comparing velocity profiles with those generated from Marsh McBirney electromagnetic current meters, and a full field assessment of the position and orientation methodology. These results suggest that the recommended methods in combination with an ADV are able to provide reliable mean three-dimensional velocity field information and accurate bed and surface topography. [ABSTRACT FROM AUTHOR]

Bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile. However, most measuring devicesused in the field do not allow for flow velocity to be measured veryclose to the bed. The lack of near-bed measurements is a critical loss of information which may affect bed shear stress estimates. Detailed velocity profiles obtained from a field acoustic Doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near-bed points is critical for the estimation of bed shear stress in a shallow river environment. Moreover, the results indicate that using the full flow depth instead of the bottom 20 per cent of theprofile generates an underestimation of the shear stress when flow is uniform. [ABSTRACT FROM AUTHOR]