<p>To investigate the hydrodynamic challenges associated with the high blockage-ratios encountered during the floating transportation and installation of immersed tubes in restricted inland waterways, a cross-river immersed tunnel project in Dongguan is selected as a case study. A full-scale three-dimensional CFD numerical flume is established using the volume-of-fluid free-surface tracking method, the Reynolds-averaged Navier–Stokes equations, and the RNG <i>k-ε</i> turbulence model, together with nested grids and seven representative working conditions. The effects of foundation-trench excavation, blockage-ratio, and lowering depth on the velocity field, external pressure distribution, streamline and vorticity evolution, and hydrodynamic loads are systematically investigated. The results show that the pressure acting on the tube surface consists of a hydrostatic component and flow-induced pressure perturbations. Its overall vertical distribution is primarily governed by the hydrostatic component, while local non-uniformities are associated with flow disturbances and boundary confinement. The maximum pressure reaches approximately 0.24&#xa0;MPa on the bottom slab at the final seating position. Restricted drainage of the residual water beneath the tube produces a localized squeezing effect during final seating. Under the present geometric and hydraulic conditions, foundation-trench excavation enlarges the local flow cross-section and generates a low-velocity shielding zone, reducing the velocity in representative regions near the trench bottom by more than 90% relative to the incoming flow. The deep trench substantially weakens contraction-induced acceleration around the tube, although localized acceleration remains near the trench edges. The mean drag force reaches its maximum value of approximately 1.91 × 10⁵N when the tube is located at the river center, representing an increase of 18.6% relative to that near the dry-dock exit. During lowering into the trench prior to final seating, the mean drag force decreases by approximately 77.3%, while the magnitude of the mean moment decreases by approximately 73.9% relative to the river-center condition. After final seating, both the directions and magnitudes of the hydrodynamic forces and moment change markedly because of the altered pressure balance and wetted boundary conditions around the tube. The resultant vertical fluid force remains relatively stable during floating and lowering before final seating. Vortex disturbances are strongest near the dry-dock exit, indicating a greater potential for hydrodynamic excitation, and gradually weaken as the tube approaches and enters the trench. The identified mechanisms provide a reference for the floating transportation, lowering control, and final seating of immersed tubes in projects with similar blockage-ratios, relative trench depths, and hydraulic conditions.</p>