To select Bed Change Options select the Options->Bed Change Options menu in the Sediment Data editor.

The 1D bed change methods are relatively simple. They are easy to understand. However, selecting the right bed change approach can be one of the most sensitive and difficult parts of 1D sediment modeling. The 1D bed change algorithms often attempt to simplify laterally heterogeneous processes into cross-section averaged behavior. These assumptions can lead to numerical artifacts and unintended consequence. Modelers must select their bed change model carefully and experiment with their sensitivity, to select the one that introduces the least error.

Selecting a Modular, Global Bed Change Method

The default approach to bed change in 1D sediment transport is always the “veneer method.” The veneer method adjusts each wet, movable node the same vertical distance to deposit or erode the computed mass at each cross section.^[1] But the new editor gives users more flexibility to mix-and-match appropriate bed change methods in the channel and floodplains.

The Global Bed Change Editor allows user to select a bed change method in a 2X2 matrix, making selections by process (erosion or deposition) and location (channel and overbank).

Global Bed Change Options (with default values)

"Channel" and "Overbank" Based on Movable Bed Limits

“Channel” and “Overbank” in these methods are determined by Movable Bed Limits NOT Bank Stations. In most places in HEC-RAS, the bank stations mark the transition between the channel and the overbank of floodplain. That is also true in the sediment model, where transport capacity is based on the results from the hydraulic channel (the portion of the model between the hydraulic bank stations). But in the bed change options, the channel and the overbanks transition at the Movable Bed Limits.

HEC-RAS requires erosion and deposition in the channel. But users can select erosion 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 aplications.

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 Adjustment or “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 6.2 includes a new Rouse-Diffusion method 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.

The Veneer Method is the primary approach to cross section change in 1D models. But several of these global drop down menus include other options. HEC-RAS includes alternate erosion and deposition methods that can depart from the veneer assumptions and use differnt algorithms to change the cross section node elevation, when appropriate.

These include:

Alternate Erosion Methods

Alternate Deposition Methods

Selecting Local Bed Change Methods

Area-Volume Conversion Method

*       Backwards compatible - uses Simpson for bed sorting, rectangle for bed change
*       Average end area - uses HEC-6 method for bed sorting and bed change
*       Simpson - uses Simpson for bed sorting and bed change
*       Single –V - uses rectangle for bed sorting and bed change
See the section of the technical reference manual on Volume-Area conversions for technical detail on these methods.

^[1]The veneer method is sometimes - colloquially - called the “peanut butter method” because it is like trying to spread a uniform layer of sediment across all of the movable cross-section points.