The escalating demand for port, waterway, and marine infrastructure development necessitates significant volumes of dredged soils requiring offshore disposal. Discharge into planned marine disposal sites induces multifaceted impacts on local and adjacent marine ecosystems, altering habitat conditions and the community structures of plankton, benthic organisms, and associated biota. This study presents a computationally efficient model for simulating the diffusion and deposition processes associated with open sea dredged soil disposal. The model is founded on 2D instantaneous point-source diffusion theory and sediment settling equations. It conceptualizes the high-concentration sediment-water mixture discharged as an instantaneous point source and explicitly incorporates the lag of sediment particles relative to water motion. Governing equations are solved via spatio-temporal discretization. Computational efficiency is markedly enhanced through convolution calculation and fast Fourier transform. This approach enables rapid prediction of suspended sediment concentration fields and subsequent seabed morphological changes. The model was validated using measurement data from the dredged soil handling process off the coast of Skikda Port, Algeria, and demonstrated positive validation results. Sensitivity analysis further confirmed the robustness of the model. This model facilitates rapid environmental impact assessment of dredged soil disposal and optimization of disposal schemes, offering significant practical utility for balancing the objectives of engineering economics and marine ecological conservation.