Modeling Low‐Flow Mixing through Pools and Riffles

The complex nature of low‐flow mixing in natural channels has been investigated using both laboratory experiments and the numerical solution of a proposed mathematical model that is based on a set of mass balance equations describing the mixing and mass exchange mechanisms. Laboratory experiments, which involved collection of channel geometry, hydraulic, and dye dispersion test data, were conducted in a model of four pool‐and‐riffle sequences in a 49‐m (161‐ft) long tilting flume. The experimental results show that flow over the model pool‐riffle sequences is highly nonuniform. Concentration‐time curves are significantly skewed with long tails. Comparison between measured and predicted concentration‐time curves shows good agreement in the general shape, peak concentration, and time to peak. The proposed model shows significant improvement over the conventional one‐dimensional dispersion model in predicting natural mixing process in open channels under low‐flow conditions through pools and riffles.