Finally, each sediment control volume requires width. Specify lateral 'Movable Bed Limits,' in the Sta Left and Sta Right columns, which constrain erosion and deposition to the cross-section nodes between them (see other options in the Bed Change Options section below). Movable Bed Limits are inclusive. If the movable bed limits have the same station as a station-elevation point, that point will move. They are the last nodes to move, not the first points that can't move (this worked differently in very early versions of HEC-RAS sediment).

HEC-RAS will only deposit or erode wet cross section points between these lateral limits by default. Select lateral limits carefully. The wetted top width between the Movable Bed Limits is also the width used to scale some transport functions that compute transport per-unit-width. So if inundated, the movable bed limits selected can also influence transport capacity.

Movable bed limits should be selected carefully and do not always, or even often correspond to channel banks. However, for the cases in which they do, the Use Banks for Extents button sets all erodible bed limits to the main channel bank stations as an initial estimate.

Movable Bed Limit Tools

HEC-RAS includes two tools to help modelers define movable bed limits quickly. The first is the Use Banks for Movable Bed button at the bottom of the Sediment Data editor. This button automatically sets all of the movable bed limits to the bank stations from the geometry file. Bank stations are not necessarily the best choice for movable bed limits. It is usually appropriate to adjust movable bed limits based on bank stations. But this button helps populate the data quickly for exploratory runs and to initialize these cross-section parameters before adjusting them at each cross sections.

Modelers should review movable bed limits at each cross section and make intentional decisions about where they should be located. Current versions of HEC-RAS make this more convenient, including buttons from the Graphical Cross Section Editor to adjust these nodes. The and buttons move the left movable bed limit and the and buttons adjust the right movable bed limit (each by one cross section node per button-push).

Using these two tools together, setting all the movable bed limits to the bank stations and then using the buttons to adjust them at each cross section is often the most efficient and reliable way to choose these parameters.

Modeling Note: Movable Bed Limit Sensitivity

Select movable bed limits carefully. Model results can be sensitive to this parameter. They affect deposition rate, converting mass change to bed change. However, they can also affect transport. Several of the transport functions compute transport per unit width. The unit transport is applied to the "movable" portion of the cross section (i.e. G_s=f(W), where G_s is transport and W is the distance between movable bed limits). Therefore, moving the movable bed limits out can have a complicated effect on transport, slowing the invert change by distributing mass change over a larger area, but also increasing transport capacity by increasing the width that HEC-RAS applies the unit transport rate to.

Modeling Note: Identifying the Movable Bed

Just because HEC-RAS offers a convenient tool to set the movable bed limits to the channel banks, does not mean that that is always the best place for them. Movable bed limits are often more appropriate at the toe of the banks. Carefully consider the morphology of the system you are modeling and decide which portion of the cross section is actually vertically active (figure below-left).

Modeling Note: Avoiding Cross Section "Inversion"

Selecting movable bed limits close to the bank toe (figure below-left) can lead to unrealistic cross section shapes in depositional conditions. In the figure below, the movable nodes inside the movable bed limits deposit enough that they end up significantly higher than the immediately adjacent nodes. This cross-section inversion is common when the MBL is inside of a similar station-elevation point and the model only deposits between the movable bed limits.

When placing the movable bed limits at the bank toe, deposition can cause “inverted” cross section shapes, where the depositing nodes rise above those that do not move.

This condition has a few reasonable fixes:

• First, the modeler could turn on the option to allow deposition outside of the movable bed limits (figure below-left).

• The modeler could experiment with moving the movable bed limits out to include all of the movable nodes.

• Or the modeler could simplify the cross section to get rid of the intermediate nodes between the movable bed limits and the banks (figure below-right). Filtering cross sections to <100 station elevation nodes (and often <60) is usually good modeling practice for 1D sediment models.

When placing the movable bed limits at the bank toe, deposition can cause “inverted” cross section shapes, where the depositing nodes rise above those that do not move.

These approaches all embed assumptions that may or may not be acceptable for the analysis and objective. Of course, the first question the modeler would want to ask in this situation is: "is this result credible"? Is the modeled period of record likely to deposit 5m of sediment or is this a symptom of a larger mode problem (e.g. overloaded boundary condition or upstream erosion)?