Transport functions compute transport potential without accounting for sediment 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.  "Armoring" is the most common impact of bed sorting, where a coarse cover layer forms above a subsurface layer that is more representative of the river's bed material load (see example below).

Select a bed mixing and armoring algorithm from the drop-down box titled Sorting Method below the transport function.

Three methods are currently available:

    • Thomas (Ex5) – This method uses a three-layer bed model that forms an independent coarse armor layer which limits erosion of deeper layers. HEC-RAS 4.1 and earlier called this method Exner 5 and was the default method in HEC-6.  
    • Copeland (Ex7) – Copeland (1993) developed an alternate version of the Thomas mixing algorithm (Exner 7). This method was designed for sand bed rivers because it forms armor layers more slowly and computes more erosion.
    • Active Layer – The two armoring methods above have been successfully applied on many river systems. However, they are complex and can suffer from 'black box' effects, producing results that users have trouble interpreting. Both armoring methods also have numerical thresholds that generate gradational non-linearities introducing noise in the gradational results. Therefore, HEC-RAS also includes a simple two-layer active layer method. The active layer thickness is set equal to the d90 by default (an assumption is only appropriate for gravel beds) but is editable since version 5.0 ({*}Sediment OptionsBed Mixing Options{*}).

See the Sorting and Armoring Section in the Technical Reference Manual for detailed descriptions of the three sorting methods.

Users can specify separate gradations for the active or cover layers, either manually or with a model hotstart (using Bed Mixing Options or Gradational Hotstarts).

Modeling Note: Mixing Method Sensitivity

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

Modeling Note: Hiding

HEC-RAS assumes grain class independence. HEC-RAS 5.1 and earlier did not include any hiding functions except for those embedded in the Wilcock and Crowe transport function. Version 6.0 and later include additional hiding functions that can be used with other transport functions.

Use Active Layer with Wilcock Crowe

The Wilcock and Crowe sediment transport function is a surface based method, which accounts for armoring implicitly. Therefore, selecting both Wilcock and Crowe and the Thomas/Copland armoring methods would double-count armoring effects. Select the Active Layer mixing method with the default active layer thickness when using Wilcock and Crowe.

Modeling Note: Thomas Method Erodes Less

The Thomas method tends to form a more effective armor layer which tends to reduce or slow erosion compared to the other two.