Flow and longitudinal dispersion in channel with partly rigid floodplain vegetation

The influence of rigid vegetation on the longitudinal dispersion coefficient in a compound open channel was examined using an image processing technique. To simulate floodplain vegetation, cylinders of 5 mm diameter were attached to the floodplain surface. Potassium permanganate solution was used as a conservative tracer. Instantaneous velocity components were measured using particle image velocimetry. The results showed that, compared with non-vegetated conditions, floodplain vegetation decreased the depth-averaged longitudinal velocity and maximum tracer concentration by up to 83% and 12·5%, respectively. It was also found that the magnitude of the longitudinal dispersion coefficient, K, increased with the relative flow depth, Dr (the ratio of the floodplain to main channel flow depth). Furthermore, the value of K increased by up to 39·3% for vegetated tests compared with non-vegetated tests. Moreover, the results were compared with several previous empirical equations and the most appropriate equation for prediction of K in compound channels with partly vegetated floodplain was found to be that proposed by Fischer in 1975.