Maximum Depth or Minimum Elevation

The Initial Conditions and Transport Parameters tab includes a list of model cross sections where users specify initial sediment conditions for each cross section. The editor will filter sections with the River: and Reach: drop down lists to focus the display on a particular study river or reach, however the sediment model will not run unless the user specifies the control volume (two movable bed limits and either a depth or elevation) and bed gradation for each cross section.

Modeling Note: Geometry Changes and Sediment Files

Sediment files depend on the geometry and hydrodynamic files for their data structure. Each cross section requires data and the boundary condition locations inherit from the flow boundary conditions. Like other files in HEC-RAS, users can mix and match sediment files in a plan. However, if users make major changes to geometry files (e.g. adding or deleting cross sections, interpolating, or moving the boundary conditions) it will change the sediment file structure. Two models with different cross section layouts or boundary condition locations cannot share a sediment file. If you make significant changes to the cross sections in your model and – particularly – if you change the boundary condition locations, the Sediment Editor will try to adjust. But it is often easier to just create a new sediment file which will start with a clean data structure that matches the active geometry and hydraulic boundary conditions than to try to edit an old sediment file that is tied to different cross sections and boundary conditions.

HEC-RAS sediment control volumes are 'centered' around each cross section as depicted in the figure below. Control volumes extend from the midpoint between the cross section and the next one upstream to the same midpoint downstream. The user must specify the width and vertical thickness of the control volume, however. The dotted line in the Cross Section Plot traces the vertical and lateral control volume extents. The control volume contains the available erodible sediment, which can be thought of as a 'sediment reservoir' (see Entering and Editing Sediment Data).

Specify the sediment control volume's vertical dimension in either Max Depth or Min Elev column. (Note: The editor will only allow one of these per cross section, deleting the first if the user specifies both.) The Max Depth (see Entering and Editing Sediment Data) approach is more common. With Max Depth the user sets the bottom of the control volume to a distance below the original invert of the channel.

The second option, Min Elev, sets a 'hard' elevation below which the model cannot erode. This option often simulates known bedrock control, grade control structure, a flume bottom, or a concrete channel lining. The model allows erosion as long as the thalweg is higher than this elevation, but will not scour below it.

Modeling Note: When to use Min Elev

Good data on non-erodible depth is rare. Therefore, most models use the Max Depth option, and increase the max depth if credible simulation results erode all the way through it. Min Elev is used in two primary cases: known bed rock or hard pan elevations from blow-count or geophysical data and 2) concrete channels, where the Min Elev is the same as the invert (which would be equivalent to a Max Depth = 0) or buried beneath deposition but built at a known slope and elevation.