The Bed Mixing Options editor contains both 1D and 2D sediment bed mixing options and other parameters. The editor is opened from the Sediment Data editor by going to the Options menu and selecting Bed Mixing Options … (see figure below).


Figure 1. Accessing the Bed Mixing Options editor from the Sediment Data editor.

An example of the Bed Mixing Options editor is shown in the figure below. The sections which are applicable to 2D sediment are Hiding Functions and Active Layer Options.


Figure 2. Bed Mixing Options editor.

Hiding Functions

In non-uniformly sized sediment beds, smaller particles are hidden from the flow and physically trapped by larger particles whereas larger particles are more exposed to the flow and less impeded to move freely. The hiding (and exposure) function computes a correction to the incipient motion variable such as a shear stress or velocity to account for the hiding and exposure of particles. The hiding functions available in HEC-RAS are:

  1. None (no correction)
  2. Ashida and Michiue (1971)
  3. Day (1980)
  4. Egiazaroff (1965)
  5. Hayashi et al. (1980)
  6. Parker et al. (1982)
  7. Proffitt and Sutherland (1983)
  8. Wilcock and Crowe (2003)
  9. Wu et al. (2000)


The hiding and exposure function can have a big impact on the results. Some of the hiding and exposure functions were developed for in conjunction with specific transport potential functions. For example, the Wu et al. (2000) hiding function was developed for use with bed- and suspended-load transport potential functions published in the same paper. Similarly, the Day (1980) and Proffitt and Sutherland (1983) hiding function where developed specifically for the Ackers and White (1973) transport potential formula. Lastly the Wilcock and Crowe (2003) hiding function was developed specifically for the Wicock and Crowe formula (Wilcock 2001; Wilcock and Crowe 2003).

Active Layer Options

Within the Active Layer Options section of the Bed Mixing Options editor are the input options controlling the active layer thickness and the Exchange Increment Method. The exchange increment is not utilized in the HEC-RAS 2D sediment transport model.

Active Layer Thickness

There are two methods for computing the active layer thickness: (1) d90, and (2) X d90. The  methods are selected within the Active Layer Options section of the Bed Mixing Options editor. The option d90 , the active layer is set to the 90th percentile diameter of the active times a user-specified multiplier. If the multiplier is set to 1, then the two methods produce the same results. In future versions, the method d90 will be dropped in favor of the X d90 method with a multiplier for simplicity. The

Minimum Active Layer Thickness

The option is provided to specify a minimum active layer thickness within the Active Layer Options section of the Bed Mixing Options editor. This option is useful when simulating very fine material as the active layer thickness can become unreasonably small. Increasing the minimum active layer thickness may also reduce instability problems when the active layer gradation is changing very quickly.