A. G. Roy

Bed morphology and sedimentology at the confluence of unequal depth channels

Abstract Spatial and temporal variations in the bed geometry and bed material size of a sand-bed river confluence with unequal-depth channels were monitored during a sequence of floods. Additionally, two other sand-bed confluences were surveyed to test the replicability of these observations. The confluences studied here have only one avalanche face, which corresponds to the front of a tributary mouth bar in the shallower channel, and do not exhibit a marked scour zone. Three distinct morpho-sedimentological zones are present: (1) an area around the upstream corner of the junction where the sediments are generally finer than the mean, (2) a maximum depth zone with coarser than average particles and (3) a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream. Changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar, but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar. The downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels. The contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone.

Turbulent flow structure at concordant and discordant open-channel confluences

Models of flow at river-channel confluences that consist of two concordant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structure in the streamwise and vertical directions at both concordant and discordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influence on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The spatial distribution of normal stress varies with bed geometry as bed discordance generates a distortion of the mixing layer between the confluent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a decrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resulting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.

Secondary circulation cells in river channel confluences: measurement artefacts or coherent flow structures?

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 flow convergence or divergence, rather than the more traditional recognition of closed helical circulation cells. Copyright

Turbulence at a roughness transition in a depth limited flow over a gravel bed

Turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented. The bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large, protuberant clasts superimposed on the bed in the downstream section. An array of four electromagnetic current meters operating at a frequency of 20 Hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect. Emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as Reynolds shear stress, uv cross-correlation, and quadrant analysis of velocity fluctuations. The results indicate that the presence of protuberant clasts modifies the turbulence structure, increases turbulence intensity and appears to dominate turbulence generation. Distinct flow zones are observed. In particular, a clear distinction is made between (i) a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying, with some vortices or rollers possibly being trapped in this near-bed flow region and (ii) an inner layer, located immediately above the major protuberant clasts, identified by positive vertical velocities and hence upward-directed fluid motion. The dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles. Interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth-limited flows over coarse and irregular bed surfaces.

Time-averaged flow structure in the central region of a stream confluence: a discussion

This paper is a discussion of Rhoads and Kenworthy (1998) ‘Time-averaged flow structure in the central region of a stream confluence’ Earth Surface Processes and Landforms, 23, 171–191, that focuses upon the methods used to identify secondary circulation in river channel confluences. It argues that the Rozovskii method that Rhoads and Kenworthy use to rotate their field data to allow identification of secondary circulation cells is flawed, and can result in misleading conclusions about the nature of flow processes in confluences. It recommends that there is a re-emphasis upon helical as opposed to secondary circulation, and that recent developments in both field monitoring and numerical modelling may help significantly in this respect. Copyright