HEC-RAS includes two channel deposition options but more floodplain deposition algorithms than any of the other bed change approaches.  The number of floodplain deposition methods scales to the difficulty of the problem of 1D floodplain deposition. The historic methods posed a "Goldilocks problem."   Choosing to limit deposition to the channel (e.g. Floodplain Deposition = None) overloaded the channel in systems with floodplain deposition, either overpredicting deposition or underpredicting erosion (or, often, causing to the "Unrealistic Vertical Adjustment" error).  But depositing in floodplains (especially broad floodplains) with the distance-weighting of the veneer method over-predicts floodplain deposition with the laterally and vertically naïve 1D algorithms, which tends to substantially overpredict erosion or under predict deposition in the channel.

The other methods take a range of approaches to improve estimates of floodplain deposition, computing intermediate overbank deposition rates between the underprediction of "none" and the overprediction of the 1D veneer assumption.

In versions 6.6 and beyond, the floodplain deposition algorithms are divided into two components: the Floodplain Partition option that determines how much mass the model will deposit in each floodplain and a Floodplain Distribution algorithm, that determines how that mass will be applied to the wet, floodplain, station-elevation points. 

Floodplain Mass Partition Methods

Liabilities of the Veneer Method for Floodplain Deposition 

The Veneer Method has several liabilities for floodplain deposition, including:

1. It deposits the same grain classes in the floodplain as the channel (based on channel hydraulics), which is connected to...
2. It allows bed load to deposit on the floodplain when floodplain deposition is usually limited to wash load or, at least, suspended bed-material load. 
3. It deposits the same sediment thickness on the floodplain as the channel, which is rarely observed.
4. It does not conserve mass within the grain classes, converting floodplain deposits to the channel active-layer gradations when the water falls to only inundate the channel. 
5. It only deposits in the floodplain if the channel deposits (when these processes are often separate).
6. It assigns mass to the floodplain based on the total wetted width of the floodplain, despite the fact that rivers constrain floodplain deposition close to the channel.

The Floodplain Mass Partition Methods 

The conceptual approach and interface of the floodplain mass partition methods are described in the user guide here, and more rigorous mathematical descriptions are included in the technical reference documentation here.

The methods are also illustrated with links to the user and tech ref documentation in the chart below.

Floodplain Mass Distribution Methods

In almost all cases (with the exception of the Rouse-Diffusion method) HEC-RAS invokes the Floodplain Distribution methods after it computes the mass deposited in each overbank.  The distribution methods control how HEC-RAS spreads the deposited sediment out over 

*It may seem odd to partition with the veneer method but distribute with a different method.  As a mass partition method, the veneer method is basically a distance weighted algorithm (e.g. the floodplain gets mass proportional to the ratio of its wetted width relative to the total wet width).

Example Files

Floodplain Deposition Example Data Set.zip