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

River confluences are locations in the fluvial network where two incoming flows, often with distinct chemical, thermal, and physical properties, converge and start to mix. The degree of mixing that occurs within a confluence influences ecological and water quality conditions locally as well as farther downstream. Nevertheless, variations in rates and patterns of mixing at confluences under different flow conditions are poorly understood. This study examines the influence of momentum flux ratio, total discharge, and density differences between incoming flows on mixing at a small stream confluence. Temperature data reveal that rates and patterns of mixing depend on event-specific combinations of the three factors. The bottom part of the mixing interface at this asymmetrical confluence is generally distorted towards toward the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it turns to become aligned with the direction of the downstream channel. As the relative contribution of lateral momentum flux from the tributary increases, velocities of flow from the lateral tributary tend to increase, the mixing interface is deflected away from the tributary mouth, and helical motion from the curving tributary flow extends over most of the channel cross section within the confluence. These condition enhance lateral mixing. Total discharge and flow depth are negatively correlated with mixing rates, suggesting that that rates of mixing over the fixed control volume at the confluence is inversely related to the scale (volumetric rate) of flow. Density differences between incoming flows may influence mixing under certain conditions, but overall appear to be less important than momentum ratio and total discharge. Results confirm that although mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions, in general the length scales are small and rates of mixing are large at this small confluence e compared to those typically documented at large river confluences.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Geography

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Lewis, Quinn

Contributors dc:contributor

• Rhoads, Bruce L.

• hydrology
• hydraulics
• temperature mixing
• river confluences

Statement dc:rights

• Copyright 2014 Quinn W. Lewis

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/50737

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/50737