The Initial Conditions and Transport Parameters is the first tab in the Sediment Data editor and opens by default when the editor launches. From this editor the user can specify the transport function, sorting method, fall velocity method for the entire model. It is also used to specify sediment data for 1D cross-sections. However, if the model does not include cross-sections, then that information may be left empty.

Transport Function

Select a Transport Function from the drop-down box near the top of the editor. HEC-RAS 6.0 includes the following eleven transport functions:

1. Ackers and White (Ackers and White 1973; Day 1980; Proffitt and Sutherland 1983)
2. England and Hansen (Engelund-Hansen 1967)
3. Laursen-Copeland formula (Laursen 1968)
4. Meyer-Peter and Müller (1948)
5. Toffaleti (1968)
6. MPM-Toffaleti (Meyer-Peter and Müller 1948; Toffaleti 1968)
7. Yang (sand and gravel eqns.)
8. Wilcock and Crowe (2003)
9. Soulsby-van Rijn (Soulsby 1997)
10. van Rijn (1984a,b; 2007a,b)
11. Wu et al. (2000)

Modeling Note – Transport Function Sensitivity: Sediment transport functions simulate non-linear transport processes and produce very different results. Model results are very sensitive to selected function. Carefully review the range of assumptions, hydraulic conditions, and grain sizes for which each method was developed. Select the method developed under conditions that most closely represent the system of interest and calibrate results to actual bed change.

Note: Transport Function Sensitivity

Sediment transport functions simulate non-linear transport processes and produce very different results. Model results are very sensitive to selected function. Carefully review the range of assumptions, hydraulic conditions, and grain sizes for which each method was developed. Select the method developed under conditions that most closely represent the system of interest and calibrate results to actual bed change.

Sorting Method

Transport functions compute transport potential without accounting for availability. The bed Sorting Method (sometimes called the mixing or armoring method) keeps track of the bed gradation which HEC-RAS uses to compute grain-class specific transport capacities and can also simulate armoring processes which regulate supply. The Sorting Method drop down menu in the Sediment Data editor only applies to 1D sediment transport. The 2D sediment transport model always uses a method similar to the Active Layer method for 1D except that it can have any arbitrary number of layers. The active layer thickness is set equal to the D90 by default or a factor of the D90. 

Note: Sorting Method Model Sensitivity

Sediment transport results can be as sensitive to the sorting method parameters selected as the transport function.

Fall Velocity Methods

The same sediment fall velocity formulas are available for 2D and 1D sediment transport. The options include:

1. Rubey (1933)
2. Toffaleti (1968)
3. Van Rijn (1993)
4. Report 12 (Default method in HEC-6)
5. Dietrich (1982)
6. Soulsby (1997)
7. Wu and Wang (2006)

The fall velocity formula used to compute free particle settling velocity for both cohesive and noncohesive sediments. Depending on the concentration and whether the user has selected flocculation, cohesive sediments may also settle as flocs. Hindered settling of noncohesive particles may also be simulated if selected by the user. In general, the fall velocity formula should not used as a model calibration parameter for morphology change, since the results are not very sensitive to the formula. Preference for the formulas is generally based on the transport formula used, the range of grain classes, and/or the parameters utilized by the fall velocity formulas. For example, the Wu and Wang formula takes into account the particle shape.