 Various reach models are available as of 2015, either 1D models (flood levels measured in the channel) or 2D models (variable flood depths measured across the extent of a floodplain).HEC-RAS, the Hydraulic Engineering Center model, is among the most popular software, if only because it is available free of charge.Other models such as TUFLOW combine 1D and 2D components to derive flood depths across both river channels and the entire floodplain.