Determination of the length of water function transition zones of braided rivers

Abstract

Water environment function zones are designated for water environment management, among which transition zones are set to connect between different zones that implement different water quality standards. The method by which we effectively set the length of each transition zone is crucial for most rivers, particularly for complex topographic conditions, such as braided rivers. In this study, using the water transition zone of the Lhasa River, a depth-averaged planar two-dimensional hydrodynamic water quality model was established to simulate hydrodynamic conditions and pollutant transportation under different discharge rates. The length of the transition zone was then analyzed. Our results showed the river braid index changed with different discharge rates. When discharge increased from 41.8 to 273 m3/s, the exposed beach area decreased by approximately 36% and the braid index decreased by approximately 20%, from 3.63 to 2.90, resulting in the length of the transition zone to increase by 109% (from 11 to 23 km). Further analysis showed that the length of transition zone increases exponentially with the increase in flow and decreases logarithmically with the increase in braid index.