Effect of Buoyancy on Transverse Mixing in Streams

A dye tracer study has been conducted in a large recirculating flume for the purpose of examining the influence of buoyancy on transverse mixing in streams. Using Rhodamine WT dye, the transverse spread of a freshwater discharge into a saline receiving stream was measured downstream from the release point. The discharge emanated from a circular pipe 2.54 cm in diameter, aligned parallel to the stream flow and located at mid‐depth at the flume sidewall. Fluo‐rometer samples were obtained throughout the cross section at downstream stations spaced 18‐24 m apart. The transverse variance of the depth‐averaged concentration distribution was determined at each station. Using a dimensionless parameter reflecting the ratio of the buoyancy input to the stream turbulent intensity, we present findings as to when the effects of buoyancy must be included in the analysis of transverse mixing. These findings illustrate the potential for greatly enhanced transverse mixing in cases of buoyant discharges, with a good deal of variability depending on discharge and stream characteristics.