Coastal waters are subject to great environmental and anthropogenic pressures. The diffusion and the transport of these waters are a key element for environmental, ecological and economic management. There are numerous indicators of hydrological characteristics based on theories of transport time scale. However, these indicators strongly depend on the geographical shape of the studied area and tend to give information after long integration time periods, generally on the order of weeks. Here, to qualify a coastal area’s dispersion more precisely, we combined two Lagrangian approaches and estimated a local diffusivity. This paper presents the numerical implementation and the results obtained over a tidally flushed, semi-enclosed water body located at mid-latitude. This new coefficient was estimated using the hydrodynamic model MARS 3D with a barycentric repositioning technique over a tidal period to ensure its reliability. We highlight the existing relationships between local diffusivity and both horizontal and vertical processes. Methodological aspects were analysed based on a reference case (number and distribution of particles, resolution, integration time period). The consistency and sensitivity of the coefficient were studied with different forcing conditions (hydrodynamical and meteorological regimes). In conclusion, our local diffusivity provides a new perspective for understanding the land–sea interface and coastal dispersion and holds potential for future studies of coastal marine ecosystems.