Experimental and numerical investigation on hydraulic characteristics of the water-retaining weir in the long-distance water conveyances system

Water-retaining weirs are key local flow-control structures in long-distance water conveyance systems, and their hydraulic performance is of great significance for maintaining flow depth, improving conveyance conditions, and ensuring operational safety. This study aimed to optimize the structural design of water-retaining weirs in a long-distance water conveyance system to improve their hydraulic performance. Physical model tests were first conducted using a 1:20-scale model to investigate four distinct water-retaining weir structures. Based on the experimental results, the variation of key hydraulic parameters, namely the local head loss coefficient (ζ), submergence coefficient (σ) and the discharge coefficient (m), were systematically investigated. To compensate for the limitations of the physical model tests and to further clarify the complex flow-field characteristics, three-dimensional numerical simulations were subsequently performed. Experimental data were combined with simulated flow patterns to conduct a comparative analysis of the two weir types with and without a dissipation weir. This study demonstrated that the structure without an energy dissipation pool, combined with a higher weir crest, yielded a smaller local head-loss coefficient and a larger submergence coefficient. Additionally, removing the energy dissipation pool resulted in a more stable flow regime with fewer fragmented vortices under well-flow. The installation of a downstream energy dissipation pool had a minor effect on the discharge coefficient. On this basis, the weir structure was further optimized, resulting in a hydraulically efficient design that was better suited to practical engineering applications. These findings from this research enhance the understanding of the hydraulic behaviour of water-retaining weirs and provide a useful basis for the design and optimization of long-distance water conveyance systems.