HEC-RAS is an integrated package of hydraulic analysis programs, in which the user interacts with the system using a Graphical User Interface (GUI). The system can perform Steady Flow water surface profile calculations, One and Two-dimensional unsteady flow computations, Sediment Transport/movable boundary computations, water quality analysis and several hydraulic design computations.

In HEC-RAS terminology, a Project is a set of data files associated with a particular river system. The modeler can perform any or all of the various types of analyses, included in the HEC-RAS package, as part of the project. The data files for a project are categorized as follows: plan data, geometric data, steady flow data, unsteady flow data, quasi-steady flow data, sediment data, water quality data, and hydraulic design data.

During the course of a study the modeler may want to formulate several different Plans. Each plan represents a specific set of geometric data and flow data (and possibly sediment data, water quality data, and hydraulic design information). Once the basic data are entered into the HEC-RAS, the modeler can easily formulate new plans. After simulations are made for the various plans, the results can be compared simultaneously in both tabular and graphical form.

HEC-RAS has the ability to lay out a model based on a geospatial coordinate system, or it can be developed without one. The very first versions of HEC-RAS did not utilize geospatial data, and all geometric data was developed from the HEC-RAS Geometry editor. However, if you want to bring in terrain data, extract model elevation information from that terrain, and perform inundation mapping of the model results, you must establish a geospatial coordinate system and develop a terrain model using HEC-RAS Mapper. HEC-RAS Mapper is a geospatial data development system in results mapping module contained directly from within HEC-RAS. HEC-RAS Mapper has an entirely separate user's manual that describes the following: how to establish a geospatial coordinate system for the project; developing a terrain model for use within HEC-RAS; laying out the HEC-RAS model geometry and extracting the terrain elevation data for model components; and visualizing results through maps, graphics, and tables.

In the current version of HEC-RAS, there are some duplication of tools for laying out the hydraulic model geometry between the Geometric data editor and HEC-RAS Mapper. If the user wants to work directly with a terrain model in a horizontal coordinate system, it is suggested to do most of that work in HEC-RAS Mapper. Once the model is laid out and terrain information is extracted, the user will still need to use the model component data editors (i.e. cross section editor, bridge/culvert editor, etc…) to parameterize the model fully.
This chapter provides an overview of how a hydraulic analysis study is performed with the HEC-RAS software.