Bed Gradations

Instead of specifying the sediment size distribution and potentially the cohesive sediment properties at every 2D computational cell (and cell subarea), HEC-RAS uses a template concept similar to that of the Channel Modification Editor. In HEC-RAS a Bed Gradation Template describes the grain size distribution and cohesive sediment properties of a sediment mixture. Bed Gradation Template are first defined in a database and then associated to Sediment Bed Materials which are specified at each computational cell within RAS Mapper. The Bed Gradation editor, is opened from the Sediment Data editor by clicking on the button . An example grain size distribution in the Bed Gradation editor is shown in the figure below.
To create a new Bed Gradation the user may click on the "New" button . Give it a name (e.g. "Nonuniform"). Enter the gradation information as shown in the figure below.

Figure 1. Bed Gradation editor with an example grain size distribution.

The user may enter the size distribution as percent finer values or grain class percent values. The current mode is indicated by the radio buttons under the table on the left side of the editor names % Finer and Grain Class %. The user may also convert the values in the table from one mode to the other by clicking on the button .
The 2D sediment transport model will only compute grain classes which are utilized in the initial bed gradations or any boundary conditions. This reduces the computational costs and speeds up the model. Consequently, it is important for the user to be mindful of how many grain classes are utilized in the input. For example, HEC-RAS has 5 cohesive grain classes. Except for the particle fall velocity, all the cohesive grain classes are treated the same, therefore using 5 cohesive grain classes or just 1 will have little impact on the results but a big impact on the computational costs.
Optionally, HEC-RAS allows users to associate bulk bed properties with bed gradations. More specifically, the sediment cohesive properties can be specified for each bed gradation. This option is activated by selecting the checkbox at the bottom of the Bed Gradation editor labeled Set Sample Specific Cohesive Parameters (see figure below).

Figure 2. Setting sample specific cohesive parameters in the Bed Gradation editor.