Revealing the River Network Structure and Pollutant Distribution Patterns in a Braided River Using Graph Theory and Numerical Simulation

Abstract

Braided rivers, a distinctive landform typically found in the upper reaches and alluvial fans of river systems, are characterised by fragmented riverbeds and complex flow structures. With the continuous increase in urban pollution, water quality in braided rivers has progressively deteriorated. Due to their intricate flow dynamics and geomorphological complexity, understanding pollutant transport in braided rivers requires a comprehensive analysis of hydrodynamic behaviour, hydrogeomorphic features, and their combined influence on pollutant distribution. In this study, a depth–averaged two–dimensional hydrodynamic model and a mass transport model were developed to simulate the flow conditions and pollutant dispersion in a braided reach of the Lhasa River. Furthermore, the river network structure of this reach was characterised using a graph–theoretical approach. Pearson correlation analysis was applied to examine the interrelationships among hydrodynamics, water quality, and hydrogeomorphology to explore the links between topological structure and pollutant behaviour. The results revealed a network–like flow structure within the study reach, where large sandbars further fragmented the flow, resulting in pronounced spatial heterogeneity and localised water quality deterioration. As discharge increased, the area of shallow water remained relatively unchanged, suggesting that the additional flow primarily spilled laterally toward both banks. Consequently, the channel's confining capacity decreased, as indicated by an 81.58% reduction in the confinement index, and the braiding index declined by 51.66%. In contrast, lateral connectivity increased by 11.57%, thereby enhancing the lateral mixing of flow. Furthermore, the study found the lateral flow exchange rate and upstream inflow discharge were the main influences on the chemical oxygen demand (COD) concentration variation. Based on these findings, a river connectivity enhancement measure was implemented to leverage the river's self–purification capacity to alleviate pollution pressure in the study reach. Notably, the measure resulted in a maximum reduction of 8.36% in COD concentration in the downstream reach under a 90% guaranteed flow during the driest months, as well as a 67.65% decrease in the variation between maximum and average COD concentrations. These findings provide important scientific and technical guidance for the effective management and improvement of water quality in braided river systems.