Thermal Mixing at Vegetated Stream Confluences

Thermal pollution affects important 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 in natural streams 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 stream confluence in the presence of various types of vegetation patches. We used surface particle image velocimetry (SPIV) and quantitative thermal imaging (QTI) to measure instantaneous velocity and temperature fields at a confluence of two parallel streams under four scenarios: 1) flat bed without vegetation, 2) sparse array of rigid vegetation, 3) dense array of rigid vegetation, and 4) dense array of flexible vegetation. All cases were conducted under emergent conditions (plants protruding through the free surface), 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. Coupled with the SPIV-measured velocity fields, it allowed us to solve the advection-diffusion equation to characterize both longitudinal thermal dispersion in the direction of the flow, as well as lateral thermal diffusion under each scenario. 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 develop new guidelines for the design of such effluents to adapt existing regulations based on the actual conditions of vegetated streams.