HEC-RAS computes bed change with the "veneer method" by default, raising or lowering all of the wetted nodes within the movable bed limits an equal distance to translate mass change into cross section change. This is often a reasonable 1D assumption but sometimes produces unrealistic cross section change. HEC-RAS includes several features allowing users to diverge from the veneer assumption when advantageous, mostly in the Bed Change Options editor (under the Options menu, see section below). However, one of these features is on the Initial Conditions and Transport Parameters tab of the Sediment Data editor.

The Multiple Movable Bed Limits feature allows user to select several 'active channels' that can erode, leaving 'islands' of 'non-movable- cross section, between them. These 'islands' outside or between the movable bed limits will not erode and will only deposit if the option to deposit outside movable bed limits is selected.
Cross section change for an inundated multi-channel cross section with the default veneer method (top) the multiple movable channel feature with three channels for all XS (middle) and the same cross section with two external channels (bottom).For example, consider the cross section in the top two figures. If capacity exceeds supply, this cross section will erode all of the wetted nodes equally (middle two figures). However if the raised portions of the cross section are stabilized, vegetated islands, that do not scour appreciably even during flood flows, the veneer assumption will not match field observations. The cross section change in bottom two figures will simulate these processes better.

To define multiple mobile channels, select the Number of Mobile Bed Channels drop down box in the upper left of the Initial Conditions and Transport Parameters tab of the Sediment Data editor. Selecting more than one mobile bed channel expands the input table to include more movable bed station pairs, left and right movable bed limits for each discrete channel (figure above). HEC-RAS only requires one set of movable bed limits per cross section, even multiple bed limits are selected. Users can also vary the number of ineffective flow limits between cross sections, ignoring any left undefined after the first pair.

Modeling Note: Multiple Movable Bed Limit Applications

Users have applied this feature to two primary morphological conditions: modeling flow splits and modeling reservoir deltas. First, modeling flow splits or islands with sediment in HEC-RAS can be challenging. So anastomosing or braided channels assumed laterally fixed on simulation time scales can be modeled with multiple movable channels. The limitations of this approach should be taken seriously. HEC-RAS is a 1D model, it does not confine flow or sediment continuity to the sub channels from cross section to cross section, and it still assumes that all the wetted nodes within the movable bed limits change uniformly, even in separate channels.

The multiple movable bed limits method also performed well in reservoir models. Reservoir deltas often form multiple channels that scour during floods, separated by stable vegetated islands. This method captured those processes well. (Gibson and Boyd, 2015)