Combining a thermal tracer with a transport model to estimate shallow flow velocities

For a long time, tracer techniques based on dyes, salts and more recently heat have been used to estimate shallow flow velocities. Traditionally, flow velocity estimation using tracers would consist of tracking the movement of the leading edge or the centroid of the tracer with the flow. An alternative methodology uses an analytical solution of an advection–dispersion transport equation as the mathematical foundation for measuring shallow water flow velocity from tracer measurements. The main goal of the present study was to ascertain whether this alternative numerical methodology can be used with temperature data from thermal tracer measurements. Salt and thermal tracer techniques were applied simultaneously by injecting a double tracer of salted-heated water into different shallow flows simulated in a laboratory hydraulic flume. Simulated flows combined different bed surfaces (smooth acrylic sheet, rough sand board and synthetic grass carpet), flow discharges (from 47 to 1239 ml s -1 ) and bed slopes (0.8, 4.4 and 13.2%), resulting in a wide range of hydraulic conditions. Velocities determined with the abovementioned methodology were compared with those estimated by measuring the tracers’ leading edge and centroid and with mean flow velocity calculated using discharge/depth measurements. Results from combining this alternative numerical methodology with thermal tracer data were similar to results from the salt tracer. Also, the proposed alternative numerical methodology predicted the mean flow velocity calculated from discharge/depth measurements better than the measurements of the leading edge and centroid of the tracers.