1D modelling
A 1D model describes flow along a channel using surveyed or terrain-derived cross-sections. It is fast, well established and works well for confined channels, bridges and culverts, where the flow direction is clear.
2D modelling
A 2D model covers the terrain with a computational mesh and solves for flow in both directions. It handles wide floodplains, flow splitting, complex overland paths and urban flooding better, and it produces depth and velocity maps. It needs good terrain data and more set-up and run time.
Combined 1D/2D
Many studies model the channel in 1D and the floodplain in 2D, getting efficient channel hydraulics and realistic floodplain behaviour in one model.
Choosing an approach
The decision depends on terrain data, the complexity of the flow paths, the level of detail needed and what the regulator expects. A short scoping discussion at the start saves effort later.