Grain Class Partition Method

The grain class partition method is the simplest middle-way between under predicting floodplain deposition by ignoring it or over predicting it with the veneer method.

This method is essentially a toggle between "veneer" and "none" based on a user specified grain class.  The veneer method is applied for each grain class independently.   Grain classes finer than the threshold deposit in the channel and floodplain (both sides of the movable bed limits) with the veneer method.  But grain classes including and greater than the specified threshold only deposit between the movable bed limits.  Therefore, the bed change at the floodplain staiton-elevation points will be less than or equal to the station-elevation points in the channel for any time step. (Though, because the channel can also erode, the floodplain elevation change might exceed the channel on the scale of the simulation).

Partitioning by Grain Class Does Not Remove Fine Sediment From the Model

It is important to recognize that just because we partition mass by grain classes, HEC-RAS does not deposit finer sediment on the floodplain.  HEC-RAS 1D only has one active layer and one bed gradation per cross section.  Therefore, even if the floodplain deposits are reduced because coarse particles cannot theoretically leave the channel, the actual, numerical, deposits in the floodplain are identical to the channel gradation (e.g. they have the contra-banned coarse particles in them).  Therefore, this method does not remove disproportionately remove fine particles from the sediment continuity simulations.  HEC is working on other algorithms that can remove finer material.


Rouse-Diffusion Method

The Rouse Diffusion Method is not available in version 6.6

As part of the RAS team's project to update floodplain deposition, we have temporarily suspended the Rouse-Diffusion method.  It tends to chronically under-predict floodplain deposition.  If you have a model that already uses this method, we recommend continuing in version 6.5 (or use the 'None' method which often, functionally, produces the same result).  We do expect to improve this method and bring it back in future releases.

See the detailed description of the Rouse-Diffusion Method in the Technical Reference Manual.

Selecting Rouse-Diffusion Coefficeints

Beta: Floodplain deposition is inversely related to beta.  Lower Beta values make more sediment available for overbank deposition.  Beta<1 puts more sediment in the overbank than the Rouse number predicts.

DT: Lower values of DT spreads sediment over the floodplain more evenly.  DT=0 spreads sediment evenly across the floodplain.  DT=1 concentrates sediment very close to the channel, mostly forming natural levess. 


Place Movable Bed Limits At or Inside Bank Stations For This Method

This method tracks floodplain and channel bed-change separately.  But it uses the bank stations to determine the floodplain deposition and movable bed limits to identify the channel nodes.  Therefore, the movable bed limits should be at or inside the bank stations for this method.  If the movable bed limits are outside the bank stations HEC-RAS will apply both methods to the nodes between them which will cause problems.