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.

Local Cohesive Parameters

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. These cohesive properties can be defined in the Local Cohesive Data tab.  They will override any global cohesive data for any cells associated with that material type.  Make sure you click the Set Sample Specific Cohesive Parameters check box to activate these.  For more information on these parameters see the Cohesive Material section of the user manual and the section of the 1D manual on estimating cohesive parameters.  (Note: These Local properties only support the dimensionless version of the erodibility parameter (M) not Kd).

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

2D Polygon Sediment Data

In HEC-RAS versions 6.6 and later, HEC added another tab that included bed material properties that the 2D sediment model uses, allowing users to specify it for each bed material type. This input is located in the 2D Polygon Sediment Data tab of the Bed Gradation editor. The 2D sediment specific parameters are the Initial Unit Weight, Erosion Scaling Factor, and bed roughness predictor application options. 


Figure 3. Setting sample specific 2D Sediment Data in the Bed Gradation editor.

Inital Unit Weight: The 2D sediment bed-mixing model can compute density and unit weight based on grain size distribution and other algorithms.  It can also consolidate bed materials.  This feature allows users to deposit new sediment with an "initial" density on top of denser and/or consolidated materials, without the deposits inheriting the density of the previous deposits.

Erosion Scaling Factor: There are modeling situations where 2D models miss three-dimensional processes and consistently underpredict or overpredict deposition or erosion (e.g. behind navigation structures).  An erosion scaling factor can make deposition and erosion asymmetrical.  An erosion scaling factor >1 will increase erosion the erosion term of the bed change equation and an erosion term <1 will reduce erosion in any polygon with this material type.  This is a very direct, empirical approach to account for known erosion or deposition from three-dimensional effects that the model does not capture (e.g. increasing the Erosion Scaling Factor to decrease deposition between navigation structure).  

"Deposition only" simplification

Deposition tends to be more physics based than erosion, which is more empirical.  Therefore, modeling erosion tends to be more difficult.  In modeling situations known to be deposition driven, simplifying a model so that it only deposits can be a useful step of intermediate complexity.  Setting the Erosion Scaling Factor to 0 is a simple way to "turn off" erosion.

The Initial Unit Weight and Erosion Scaling Factors are optional. If the Initial Unit Weight is not specified, then it is computed based on the initial bed gradation. If the Erosion Scaling Factor is not specified, then it is equal to 1.0.

Apply Roughness Computed with Bed Predictor to:  The application of the bed roughness predictor is only relevant if the user has turned on Bed Roughness predictors in the General tab of the Sediment Computation Options and Tolerances editor.  Bed roughness predictors compute bed roughness dynamically based on the sediment and hydraulic properties during each time step.  This option determines which governing equations use those dynamic roughness parameters (both the hydraulics and the sediment or just the sediment).  The hydraulics use the roughness to compute the friction slope, water surface elevation, and velocity.  In the sediment computations, the friction slope is also computed from the roughness, and is then used in the shear stress or stream power to compute transport.