Overview

HEC-RAS has a wide range of boundary and initial conditions that can be applied to a model.  Boundary conditions consist of external boundary conditions along the perimeter of the 2D area, internal boundary conditions, and global boundary conditions (Meteorological Data) that are applied to the entire model (i.e., precipitation, wind, etc.). 

There are four types of external boundary conditions that can be linked directly to the boundary of 2D Flow Areas, these are:

  • Flow Hydrograph
  • Stage Hydrograph
  • Normal Depth
  • Rating Curve

The Normal Depth and Rating Curve boundary conditions can only be used at locations where flow will leave a 2D flow area.  The flow and stage hydrograph boundary conditions can be used for putting flow into or taking flow out of a 2D flow area.  For a Flow Hydrograph, positive flow values will send flow into a 2D flow area, and negative flow values will take flow out of a 2D area.  For the Stage Hydrograph, stages higher than the ground or water surface in a 2D flow area will send flow in, and stages lower than the water surface in the 2D flow area will send flow out.  If a cell is dry and the stage boundary condition is lower than the 2D flow area cell minimum elevation, then flow will not transfer. 

There are two types of internal boundary conditions that can be used in HEC-RAS:

  • Flow Hydrograph
  • Precipitation

The Precipitation boundary condition can be applied directly to any 2D flow area as a time series of rainfall excesses.  This method is limiting in that it applies the same precipitation spatially to the entire 2D Flow Area.  A newer way of applying precipitation has been added to HEC-RAS Version 6.0. 

Global boundary conditions are boundary conditions that get applied to the entire model.  The current global boundary condition types in HEC-RAS Version 6.0 are:

  • Precipitation
  • Evapotranspiration
  • Wind

The new methods for entering global precipitation and wind boundary conditions can be in the form of gridded data or point gage data.  If point gage data are used, then the information is interpolated between the gages using various optional methods.