Existing flow routing models (except vector-based) are limited to the rectangle mesh system because they are based on the flow direction models that only support the rectangle mesh system.

In this study, we introduce a new flow direction model, which supports both structured (e.g., rectangle and hexagon) and unstructured Voronoi-type (e.g., Model for Prediction Across Scales, MPAS) meshes.

Our model uses a topology relationship-based method to model the river network, implement depression filling, and produce consistent flow direction field and routing parameters.

We applied and evaluated this new model at a coastal watershed in the eastern United States. Our simulations show that our model produces robust results (e.g., river channel slope and drainage area) in different model configurations.

Model outputs from this study could be used to improve both regional and global scale hydrologic models as well as Earth system models that have a river routing component, especially when unstructured meshes are used.