HEC-RAS

HEC-RAS (River Analysis System) is hydraulic modelling software from the US Army Corps of Engineers Hydrologic Engineering Center (HEC). It is free to download and is a standard tool for river and floodplain studies. It tells you how deep and how fast water will be in a channel and on the land beside it for a given flow. HEC-RAS can run in several modes. One-dimensional steady flow uses cross sections along the river and the energy equation, solved step by step from one section to the next, with Manning's equation for friction losses. Unsteady flow, in 1D or 2D, follows a flood wave through time. The 1D unsteady solver uses the Saint-Venant equations. The 2D solver works on a mesh over the terrain and uses either the full shallow water equations or a simpler diffusion wave form. Other modules handle sediment transport and water temperature or quality. Inputs are channel and floodplain geometry (cross sections or a terrain model), Manning's roughness values, bridges, culverts, weirs and gates, and boundary conditions such as a flow hydrograph upstream and a stage or normal depth downstream. Outputs are water surface profiles, depths, velocities, arrival times and inundation maps. For example, to map a 100-year floodplain, an engineer takes the peak flow from a hydrology study, often from HEC-HMS, runs a steady flow model through the town reach, and maps the water surface on a digital elevation model. HEC-RAS is free, well documented and accepted by many regulators. Its limits: 1D models assume flow is at right angles to each cross section, which fails on wide, flat floodplains. 2D runs need a good terrain model and can be slow on large areas. Results are only as good as the roughness values and calibration to observed high water marks.