The Transport Methods give you several parameters to influence the Advection-Diffusion sediment transport equation:



The Transport Methods editor is where the user may select the parameters and values for several transport-related variables. These include the Load Correction Factor, Diffusion Coefficient, and adaptation parameters. To open the Transport Methods editor, open the Sediment Data editor, and select open the Options menu and select Transport Methods… (see figure below).


Open the Transport Methods in from the Options menu of the Sediment Data editor.

Load Correction Factor (Relative Particle Velocity)

The load correction factor (beta) is - essentially - the relative velocity of the sediment particles.  The value varies from 0 to 1, as a fraction of the flow velocity (e.g. beta=1 means that the sediment moves at the velocity of flown whild beta=0.1 means that the water velocity is 10X the sediment velocity).

Modeling Note: Particle Velocity Range and Sensitivity

This term influence results more if concentrations are changing dynamically.  Slow changing systems or quasi-steady models will not be sensitive to this parameter.
Because β is a relative velocity, it should not be more than 1 or less than 0 (because sediment does not generally travel faster than water or in the opposite direction).  


HEC-RAS 2D sediment transport has the option to approximate the current velocity and concentration profiles with approximate semi-analytical profiles, as well as to utilize an empirical formula for the bed-load velocity (see figure below). If these options are enabled, a load correction factor is included in the temporal term of the transport equation (i.e. advection-diffusion equation). The load correction factor accounts for non-uniform vertical profiles of the concentration and current velocity as well as the bed-load velocity. Since most of the sediment concentration is typical near the bed where the current velocities are slower, the load correction factor is generally less than one and produces a temporal lag between the flow and the sediment concentrations.

Schematic of sediment and current velocity profiles.

The load correction factor options are specified in the Transport Model and AD Parameters editor which can be accessed by opening the Sediment Data editor and selecting the menu Options and selecting Transport Methods…
The load correction factor options are specified within the section Total-Load Correction Factor section of the first tab of editor called AD Parameters. By default, HEC-RAS does not compute any load correction factors. To turn on the load correction factor the user may check the box labeled Total-load Correction Factor. One this checkbox is selected the user should select the methods used to compute the bed and suspended-load correction factors.


Setting the Total-load correction factor options in the Transport Model editor.

The bed-load correction factor method selects the formula for computing the bed-load velocity. The available bed-load velocity formulas are:

  1. No correction
  2. Van Rijn
  3. Van Rijn-Wu
  4. Phillips and Sutherland

Selecting No Correction assumes the bed-load velocity is equal to the depth-averaged velocity (β=1). The van Rijn and van Rijn-Wu are similar formulas. The only difference is that the van Rijn-Wu formula has coefficients that have been recalibrated with a larger dataset of measurements.


 Setting the bed-load correction factor options in the Transport Model editor.

The suspended load correction factor methods select vertical sediment concentration profile. The three options are:

  1. No correction
  2. Rouse sediment concentration profile
  3. Exponential sediment concentration profile

When selecting the Rouse and Exponential sediment concentration profiles, it is assumed that the current velocity follows a logarithmic profile. If No Correction is selected, the suspended-load correction factor is set to one.


Figure 5. Setting the suspended-load correction factor method in the Transport Model editor.


Generally, for practical applications there is not enough data to calibrate the bed and suspended-load correction factors. Their use is a compromise between accuracy and computational time which are both relatively minor. In general, the morphology change is much more sensitive to other parameters and options such as the transport function and adaptation parameters than the load correction factors.

Diffusion Coefficient

Diffusion in the transport equation includes a diffusion coefficient that you can parameterize.  By default this is zero, computing no diffusion.  HEC-RAS computes the horizontal diffusion coefficients with two methods:

  1. Constant total-load diffusion coefficient
  2. Weighted bed-load and suspended-load diffusion coefficients

Some of these methods require empirical parameters, while others use equations based on hydraulic results.

Modeling Note: Bed Load Diffusion is Usually Small and Can be Can be Ignored


If you partition the suspended and bed load diffusion methods, selecting Eddy Visc/Schmidt # for suspended and None for bed load is a good place to start (that does not require an empirical parameter) because the suspended diffusion is usually much more significant than bed load..


Horizontal turbulent mixing and dispersion is modeled in HEC-RAS with a Fickian diffusion model. Turning on horizontal mixing is important in simulations with high resolution and sharp variations in sediment concentration capacities and concentrations, and when using high-resolution advection schemes. When the computational grid is relatively coarse or when using a first-order advection scheme the numerical diffusion may be so large that adding horizontal mixing is unnecessary. In general, sediment diffusion should not be used in combination with the Diffusion Wave Equation (DWE), since this can result and overly diffusive results.
HEC-RAS computes the horizontal diffusion coefficients with two methods.  The options and parameters are in the AD Parameters tab of the Transport Model and AD Parameters editor which can be opened from the Sediment Data editor under Options and selecting Transport Methods... HEC-RAS has two options for the total-load diffusion coefficient.

The default in HEC-RAS is for total-load diffusion of zero (i.e. no horizontal diffusion).


Constant total-load diffusion coefficient (left) weighted bed and suspended load coefficients (right).

Modeling Note: Suspended Load Diffusion is Larger than Bed Load

Note: The equations for suspended and bed load diffusion coefficients are very similar (Cu*h/~1 vs Cu*'d - see above), except one includes the water depth and the other the particle diameter.  Suspended diffusion is, therefore, orders of magnitude larger than bedload diffusion.  

Erosion Parameters

The Erosion Parameters tab of the Transport Model and AD Parameters editor contains the settings for the noncohesive sediment transport erosion formulation. The total-load sediment erosion is computed as a function of the total-load adaptation coefficient. In HEC-RAS there are two methods for computing total-load adaptation coefficient:

  1. Total-load length
  2. Weighted bed-load and suspended-load adaptation coefficients

Modeling Note: Estimating Adaptation Length

This is an important parameter.  A good initial estimate for the adaptation length is ~1-2X the cell size.  Runs with smaller adaptation lengths will erode and deposit more than while models with larger adaptation lengths.  Larger adaptation lengths will smooth results. 

The total-load adaptation length computes the total-load adaptation length as a function of the unit discharge and sediment fall velocity (see figure below). The total-load adaptation length is the simplest and most computationally efficient of the two options.

Modeling Note: Adaptation Coefficient and Adaption Length

In the Wu (2000) approach erosion and deposition are directly related to the same Adaptation Coefficient () to compute erosion and deposition. 

Both are inversely related to the Adaptation Length ()

Erosion is a function of the equilibrium concentration which comes from the transport function (e.g. Wu or van Rijn)

Deposition is a function of the average concentration in the cell

The weighted bed-load and suspended adaptation coefficients requires specifying methods for the bed-load and suspended-load adaptation coefficients and computing the fraction of suspended sediments. However, it is the most physically accurate method.


If you select a weighted total-load adaptation length you will specify separate suspended and bed load parameters.



Specifying a suspended-load adaptation coefficient.

When first setting up a sediment transport model, it is recommended to use a constant total load adaptation length for simplicity. Once the user has a stable model producing reasonable results, it is recommended to perform a sensitivity of the adaptation length by adjusting the adaptation length. In many cases, the results are not found to be sensitive. This is usually for relatively coarse grid simulations under mild to moderate forcing. However, if the results are found to be sensitive to the adaptation length then more tests are necessary in determining to optimal adaptation method and parameters.

2D Sediment Parameter Quick Start Guide

A printable document that identifies the adjustable parameters and recommendations on ranges, sensitivities, and starting points.