EFFECT OF BED MORPHOLOGY ON FLOW MIXING LENGTH AT RIVER CONFLUENCES

MIXING processes at river confluences have an important bearing on pro blems such as pollutant dispersal and the management of river systems, but remain poorly understood. Previous studies(1-11) have indicated t hat flow mixing downstream of a confluence is a slow process, typicall y completed at distances greater than 100 times the channel width. Bed morphology is now emerging as a critical factor: flume experiments ha ve shown that the width to depth ratios(12) and height discordance(13, 14) between the confluent channels should influence mixing rates(12-14 ). But the magnitude of these effects in real rivers is not known. Her e we report measurements from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates. F or the rivers studied, mixing is complete at distances five to ten tim es shorter than those reported previously.