THERMAL MIXING AT VEGETATED STREAM CONFLUENCES

Thermal pollution affects biological and chemical processes in stream ecosystems. Since some species are more susceptible to changes in temperature than others, the rate at which thermal mixing occurs can drive habitat changes at various temporal and spatial scales. We conducted a series of laboratory experiments in a racetrack flume to characterize thermal mixing at a confluence of two parallel streams in the presence of various types of vegetation. We used surface particle image velocimetry (SPIV) and quantitative thermal imaging (QTI) to measure instantaneous velocity and temperature fields under four scenarios: 1) flat bed without vegetation, 2) sparse rigid array, 3) dense rigid array, and 4) dense flexible array. All cases were conducted under emergent conditions with an initial temperature difference of at least 3 degree Celsius between the two merging streams. Data collected with an infrared camera revealed the spatial evolution of the temperature field through the different patches. The results revealed a clear effect of patch density and plant flexibility in: a) the temperature dissipation throughout the arrays, b) penetration (excursion) length into (out of) the patch, and c) lateral thermal diffusion and longitudinal thermal dispersion. The results of our analysis can be used to predict the effect of various sources of thermal pollution in streams, such as thermoelectric power plant discharges in streams, as well as to develop new guidelines for the design of such effluents to adapt existing regulations.