Numerical assessment of hydro-morphodynamics affected by altered upstream discharge in the Dujiangyan reach of the Min River, China

The Dujiangyan Irrigation Project, a globally well-known hydraulic engineering project, is significant for the Chengdu Plain. In recent years, due to water supply scarcity in the Chengdu Plain and increasing flood disasters in the Min River, the Moertan Reservoir was constructed to alleviate these pressures. The construction resulted in a mid-reach embankment along the Min River that forced channel narrowing and triggered changes in channel morphology, flow and bedload transport. This study utilised field measurements coupled with physical and numerical models to simulate the resulting characteristics of flow, bedload transport and topographic change for different flood frequencies in the Dujiangyan reach following emplacement of the Moertan Reservoir. The results indicated that following extensive hydraulic regulation, the flow-bedload transport characteristics and morphodynamics exhibited similar behaviours across different flood frequencies. The presence of a mid-channel gravel-cobble shoal (GCS) divided the flow into multiple branches, with the right branch systematically dominating flow and bedload transport across all flood frequencies. Further downstream, the Fish Mouth Levee effectively diverted nearly equal amounts of flow through the Inner and Outer rivers. In contrast, approximately 20% and 80% of the bedload was transported through the Inner and Outer rivers, respectively. The findings were beneficial in providing a reference for the flood season management of the Dujiangyan Irrigation Project and the maintenance of associated hydraulic engineering projects. Coupling field measurements with physical and numerical models, the hydro-morphological characteristics during floods in the Dujiangyan Reach involving the famous Dujiangyan Irrigation Project are simulated. Highly regulating the riverbank causes the flow fields, bedload transport and bed morphology to behave similarly for different flood frequencies. Understanding the hydro-morphological response under the current engineered structure of the river can suggest effective strategies for the management of hydrological and sediment processes of the Dujiangyan Irrigation Project. image