Hydraulic complexity at a laboratory-scale confluence: Experimental distribution of bioenergetics metrics and its relation to Yangtze finless porpoise habitats.

This article focuses on investigating hydraulic complexity metrics—M1 (kinetic energy gradient), M2 (velocity gradient), and M4 (horizontal vorticity)—within a laboratory-scale river confluence using Particle Image Velocimetry (PIV) to understand their spatial distribution under steady and unsteady flow conditions. The study links these hydrodynamic features to ecological patterns, particularly habitat preferences of the endangered Yangtze finless porpoise, which favors low M1 zones such as separation and stagnation areas likely associated with feeding opportunities. Results demonstrate that M1 peaks in shear layers and separation zone boundaries, M2 is elevated in stagnation and tributary inflow regions, and M4 highlights turbulent zones, collectively providing a comprehensive framework for assessing habitat suitability. The research underscores the value of PIV in capturing detailed flow structures and recommends incorporating vertical velocity data (M3 metric) in future studies to enhance ecological habitat characterization and conservation planning.