Analysis of transverse mixing in natural streams under slug tests

To investigate the effects of the stream geometry on the transverse mixing of the pollutant, tracer tests were conducted at seven different sites in tributaries of the Han River in Korea. The routing procedure combined with the stream-tube concept was developed to analyze the dispersion data collected in a natural stream with irregular geometry under slug tests. The routing procedure was used to calculate the transverse dispersion coefficient from the observed concentration data, and the result was compared with the results by the moment methods. The field data showed that, in most tests on the curved reaches of the streams, the high tracer concentration as well as the high velocity occurred following the thalweg line. However, the local irregularities inside the stream cross-section significantly affected the transverse distributions of stream velocity and tracer concentration as much as the channel meander did. The observed transverse dispersion coefficients by the stream-tube routing procedure were in good agreement with those by the stream-tube moment method. The analysis of the relations between the transverse dispersion coefficients and the basic hydraulic parameters showed that as the ratio of width to depth (W/H) increased, the transverse dispersion coefficients tended to increase. The transverse dispersion coefficients were also proportional to the sinuosity of the stream. The transverse dispersion coefficients also tended to increase as the ratio of mean velocity to shear velocity (U/U *), which is the friction term, increased.