Turbidity currents are a major cause of structural damage induced by submarine landslides. Numerical modeling serves as a powerful tool for predicting and evaluating the associated fluid-structure interaction (FSI) problems. However, turbidity currents often originate from debris flows, requiring specialized models to accurately capture the transition processes. In this study, we propose a debris flow-to-turbidity current transition model based on the smoothed particle hydrodynamics (SPH) method and further investigate the FSI processes between the resulting complex flow and structures. The transition model, which functions as a particle splitting algorithm, along with the FSI model, is implemented on a CUDA-GPU platform to enable efficient large-scale 3D simulations. The study includes verification and validation of the proposed model. Importantly, we also examine how accounting for the debris flow-to-turbidity current transition affects the impact forces exerted on structures, providing critical insights for engineering risk assessment.