HEC-RAS requires erosion and deposition in the channel (so you cannot select "none" for either of the channel methods.*  But users can choose if they want to HEC-RAS to erode and/or deposition in the overbank.  This gives users four possible permutations of channel-overbank processes.  These four options are illustrated in the figure below (in order of how often they are used). The first two are the most common and should be used for most applications.

a. Deposition and Erosion in the Channel and No Bed Change in the Overbanks. This is the default approach.  The name of the “movable bed limits” suggests this approach.  The channel can move within the movable bed limits but not outside of them.   However, if the prototype deposits significant sediment outside of the channel, this method can overpredict channel deposition.  It can even lead to the Unrealistic Vertical Adjustmentor “Channel Filled with Sediment” Errors, that can crash the sediment model.  It can also lead to a perched channel or an inverted channel (as depicted in the figure below).  Therefore, option (b) in the figure above (and described below) is very common.  


b. Channel Deposition and Erosion, Deposition in Overbank but no Erosion in the Overbank: If the modeled system deposits in the floodplain, it can be useful for the model to deposit in the floodplain. Floodplain flows can carry (usually fine) sediment and deposit, but rarely scour across the cross section.  So the most common alternative Bed Change Method is option (b) in the figure above, allowing deposition in the overbank, but not erosion.  This approach may, actually, be the appropriate method in more models than the default.


Figure 1-69: Deposition with the default method (Method 1), which confines deposition to movable bed limits (Left) and the method (Method 2) that allows deposition outside the movable bed limits (right).

The Veneer method tends to over-predict floodplain deposition

While method (1) over predicts channel deposition and under predicts floodplain deposition.  Method 2 tends to over-predict overbank deposition (which can lead to artificial channel erosion).  There are a couple reasons that the overbank deposition algorithms over predict floodplain deposition.  First, method (b) does not account for floodplain scour mechanism.  Second, if the time step is too large, the algorithms do not account for the loss of floodplain conveyance quickly enough (see next Modeling Note).

But, by far the most important reason that the 1D model overpredicts overbank deposition is the 1D gradation assumption.  HEC-RAS extends the active layer gradation to the entire mobile, wet, portion of the bed the material it erodes or deposits in the channel has the same gradation as the floodplain.  Therefore, the 1D assumptions tend to deposit coarser sediment in the numerical flood plain than the prototype.  Version 6.6 and beyond include several new floodplain deposition models to try to avoid the under/over prediction issues of these two methods.

c. Deposition and Erosion in All Locations: The current version of HEC-RAS also allows erosion in the overbanks (By selecting Veneer in the Overbank/Erosion quadrant).  This would be similar – in some ways – to setting the movable bed limits to the edge of the cross section.  Floodplain erosion – where it happens – is usually localized or channelized.  The veneer method is rarely an appropriate modeling approach.  So this method should be used with caution, and should be carefully justified with field observations in the model documentation.

d. Deposition and Erosion in the Channel, Only Erosion in the Overbank: This modular approach to bed change allows users to select floodplain erosion and exclude floodplain deposition. This is not a standard morphological process and is almost never appropriate. 

The figure below includes results from the most common channel erosion/deposition and overbank deposition method.

However, HEC-RAS calculates transport based on the channel hydraulic properties, and sometimes the single process bed change can generate unrealistic deposition. Floodplain deposits also tend to be finer than channel sediment in rivers, but HEC-RAS is a 1D model that only tracks a single bed gradation. Minor deposition on very large floodplains during large floods can introduce an artificial sediment sink causing illicit scour in the channel.

Depth Dependent Deposition Method (reservoirs): Some depositional systems, particularly prograding reservoir deltas, depart from the veneer assumption, depositing more in the deeper parts of the channel. The Reservoir Option: Deposit More in the Deeper Parts of the XS method adjusts bed change proportional to water depth.