Rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions

Abstract

Abstract:Confluences are important locations for river mixing within drainage networks, yet few studies have examined in detail the dynamics of mixing within confluences. This study examines the influence of momentum flux ratio, the scale of the flow (crosssectional area) and the density differences between incoming flows on thermal mixing at a small stream confluence. Results reveal that rates and patterns of thermal mixing depend on event-specific combinations of the three factors. The mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel. As the momentum flux from the lateral tributary increases, mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel. The cross-sectional area of the flow is negatively correlated with mixing rates, suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow. Density differences are not strongly related to rates of mixing. Results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions, but that, in general, length scales of mixing are short, and rates of mixing are high at this small confluence compared with those typically documented at large-river confluences.

Resolving two‐dimensional flow structure in rivers using large‐scale particle image velocimetry: An example from a stream confluence

“…The PIV analysis confirms that during conditions at this confluence with a momentum ratio near one, the interaction between the two flows near the junction apex has a wake‐like structure characterized by a stagnation zone immediately downstream from the apex, a zone of velocity deficit extending downstream from the stagnation zone, and shear layers bounding both the stagnation and velocity‐deficit zones. Further LSPIV work under these conditions can help to determine the extent to which interaction between the shear layers influences vortex development downstream of the zone of velocity deficit and the extent to which such interaction influences mixing of the confluent flows [ Lewis and Rhoads , ].…”

Dynamics of Flow at Concordant Gravel Bed River Confluences: Effects of Junction Angle and Momentum Flux Ratio

“…Recent decades have seen an increasingly growing interest in hydrodynamic, morphodynamic, and ecological processes of river confluences, which are vital nodes within fluvial networks and hot spots of biodiversity in lotic freshwater environments (Best, ; Best & Ashworth, ; Lewis & Rhoads, ; Mosley, ; Rice et al, ; Roy & Bergeron, ). These studies explicitly acknowledge the complexity of confluence processes, which are compounded by the spatially variable three‐dimensionality of confluence hydrodynamics (Best, ; Ashmore et al, ; Rhoads & Kenworthy, ; Paola, ; De Serres et al, ; Rhoads & Sukhodolov, ; Constantinescu, ; Sukhodolov et al, ).…”

Exploring Streamwater Mixing Dynamics via Handheld Thermal Infrared Imagery

“…At the NAT site the length to complete mixing increased with increasing stream discharge, while it decreased with increased stream discharge at URB. Nevertheless, the distance to complete mixing can be influenced by a number of factors, such as stream discharge, stream bed morphology and particular features as riffles and pools and water density [43,50].…”

“…Thus far, the potential for handheld TIR imagery to assess water mixing dynamics in small streams has not been fully assessed [23,25,35,39], with the vast majority of the studies relying on airborne TIR observations or reporting only some marginal observations specifically on water mixing. Most studies on water mixing at small scales rely on measurements of water temperature, electrical conductivity and flow velocities with probes placed at different depths in the water column [40][41][42][43]. Ground-based TIR imagery could be a potentially useful method for supporting in-situ selections of representative sampling locations-thereby reducing uncertainties inherent to fundamental mixing assumptions or water quality monitoring campaigns [6,44].…”