Assessment of injected warm plumes along a free surface flow channelusing fiber-optic distributed temperature sensing and numericalsimulations

International audienceUnderstanding and predicting stream thermal regimes is a key goal for aquatic ecosystems resiliency to climatechange. Mapping thermal anomalies finely becomes feasible thanks to methods such as fiber-optic distributedtemperature sensing (FO-DTS). Despite being the main thermal anomalies in stream, groundwater inflows aredifficult to detect because of high water stages and turbulent stream flow. We hypothesized that thresholds in flowregime and hydraulic parameters may affect thermal regime characterization. Our main objective was to test andvalidate the use of FO-DTS for the quantification of inflows in order to determine the physical processes behindthese thresholds.Experiments were carried out outdoor, using an open flow hydraulic channel. A warm water tank was usedto simulate groundwater inflows with known discharge rates and temperatures. These discharge rates variedbetween 4 and 72% of the channel flow. Numerical experiments were also conducted to test the consistency of ourexperimental results and discriminate the effect of inflow rate and hydraulic parameters. The water temperaturein the channel was monitored by Fiber-Optic Distributed Temperature Sensing with cables set on two lines, overthree depths.The injected warm plume was tracked along the channel and across the water stage to estimate temperatureincreases it induced. A relationship was found between these thermal anomalies and flow dynamic, definingdifferent types of flow configurations. For given channel flow rate and water stage, a threshold for the inflow ratewas identified at which the injected plume is not detectable by our means. The effect of the channel flow velocityover the plume spreading appears clearly with a dominance of advection for high flow rate. In addition, outdoorexperiments were affected by atmospheric conditions (air temperature, solar radiation, etc.) while simulationsallowed refining results without external artefacts and showed a good fit with measurements. This study shows howFO-DTS allows spatio-temporal characterization of a thermal anomaly to determine threshold values for its detec-tion at given hydraulic conditions, with broad applications for hydrology, hydroecology and environmental sensin