These methods determine the pattern of deposition outside the movable bed limits. Access them by selecting the Options→ Bed Change Options (1D) menu in the Sediment Data editor. They are only available if a Floodplain Partition method is selected, but they do not determine how much mass HEC-RAS will deposit in the floodplain, only how it is distributed.
Veneer Method
The veneer method is the simplest approach to depositional cross-section modification. The deposited mass is converted to a volume with the unit weight or dry density, which is translated to an area change using the end-area method or Simpson rule. Then that area change is applied to the cross section by evenly distributing it over every wet station-elevation point (including those within ineffective flow areas and blocked obstructions*).
*The 1D sediment transport algorithms convert blocked obstructions to station-elevation points in the first time step, so they can deposit or erode.
Linear Floodplain Deposition Method
Floodplains tend to deposit more sediment near the channel that away from it. The Linear deposition method is "linear" with distance from the movable bed limit. It computes zero bed change at the farthest, wet, station-elevation point from the channel and then a maximum at the nearest station-elevation point to the channel in order to reduce deposition linearly with distance from the movable bed limit.
These methods only affect the depositional pattern in the floodplain, not the magnitude (mostly)
HEC-RAS does not invoke these floodplain deposition methods until after it computes the mass deposited outside the movable bed limits. These methods only affect the pattern of floodplain deposition. Therefore, in the figure below, there is no difference between the three methods in the first time step. However, as the methods change the shape of the cross section, they can have diverging effects on the hydrodynamics later in the simulation, which can result in some difference in the final depositional mass by the end of a model run.
Distance-Decay Floodplain Deposition Method
The distance decay method is also designed to concentrate deposition near the channel. It can generate a non-linear depositional pattern which simulates the shape of natural levees. Walling and He (1998) and others have demonstrated that distance-decay models can estimate this distance-dependence of floodplain deposits. Therefore, HEC has included a distance-decay floodplain deposition model in HEC-RAS. This is more sophisticated and, in some cases, more realistic than the linear method, but requires a user specified parameter (
) that controls the shape of the depositional surface.
Lower lambda values flatten the floodplain deposition while higher lambdas exaggerate the decay. The figure below includes the veneer result (brown) as well as distance decay (green) with values of 0.3, 1 (default) and 5. As these values approach 0, they converge on the veneer method (equal bed change at each wet node). Even the default =1 is closer to the veneer method than the linear distribution. But =5 starts to build substantial natural levees and floodplain deposition asymptotes to near-zero about 2/3rds of the way along the floodplain.
Add near-bank station elevation points to your floodplain with this method
Because this method can concentrate deposition close to the channel, denser node spacing near the channel can improve the floodplain deposition results.
For the equations used and more examples see: Distance Decay Equations and Examples
