BSTEM Tutorial

Page Under Construction

This tutorial is under construction in anticipation of the 6.7 Beta release.

We fixed some bugs in BSTEM and thought this would be a good time to roll out a tutorial and a couple videos.

Using the USDA-ARS Bank Stability and Toe Erosion Methods (BSTEM) in HEC-RAS

Early versions of mobile-bed (sediment transport) HEC-RAS limited bed change to uniform vertical variation of the wetted nodes. 
Later versions included bank erosion and toe scour algorithms which can adjust nodes laterally to simulate toe scour process and will compute diagonal failure planes to simulate geotechnical mass wasting processes. 
This workshop will give you experience with the alpha version of these methods.

First, run the sediment transport model and cycle through the cross section outputs to become familiar with how the system responds to the classical sediment transport computations. 
Next you will set up a simple bank erosion model.  There are three ways to define bank material properties.  You will implement all three, adding complexity to your bank erosion model with each step.

1. Add Default Material Type

First, the easiest way to define bank materials is to select a single, default, material type. 
Open your Sediment Data with the Sediment Data Editor  and Save As... a new file.  
Call it something like Add Default Material.
Define BSTEM input data by opening the Sediment Data Editor and clicking on the third tab (USDA-ARS Bank Stability and Toe Erosion Model (BSTEM)).

• Select the Method of Slices from the Bank Failure Method dropdown menu.

• Add default material to the left bank of cross sections 851.142, 714.285, and 571.428
• Click on the Left Bank Material column and select Moderate Silt for the cross sections (including 857) (Figure 1).

• Set the Bank Toe for these cross sections by pressing the Set Toe Station to Bank Station
  • Open up the cross section viewer by selecting the View → Cross Sections… menu or pressing the View XS button from the main HEC-RAS menu
  • Select a good Bank Edge (“Top”) and Toe station for each XS.
    • Note: It is useful to select your movable bed limit as your toe so you can use the button as a first estimate for those.

• Open the Quasi-Unsteady sediment plan editor and Save As... a new plan (e.g. "Add Default Materials").
• Run the new plan by pressing Compute.
• View Results.  From the main HEC-RAS toolbar select View → Sediment Output (at the bottom)
  • Look at the Cross Section Results first.  Choose Cross Section from the result type dropdown box about halfway down the left pane of the viewer.

2. Create Custom Material Type

Lateral erosion and failure processes do not affect Moderate Silt much in this model, but Erodible Silt erodes more than is physically reasonable. 
This is common. 
The parameters in these default materials vary by orders of magnitude and it is unlikely that they represent the materials at any real site.

Therefore, if you have site specific data (which is highly recommended) you can create a soil type that uses those data. 
However, a user-defined BSTEM material, requires a pre-existing grain-size distribution for that material. 
So, you will want to define a gradation for this customized material first.

• Go to the Sediment Data editor and Save As...  Give your new sediment data file a name like "Add Custom Material."
• Select the Initial Conditions and Transport Parameters tab (the first sediment transport data tab).
• Press the Create/Edit Bed Gradation button.  
• Create a new gradation( ) and evenly distribute the gradation between the finest four grain classes (eg. 25%finer than VFM, 50%finer than FM, 75%finer than MM, 100%finer than CM).

Figure: Define material gradation in the main sediment editor before defining BSTEM material.

• Next go back to the USDA-ARS BSTEM Tab and press the Define/Edit BSTEM Sample Parameters button. 
• Adjust the Loam parameters to reflect the following data:
  • Unit Weight = 120 lbf/ft³
  • Friction Angle = 24^o
  • Cohesion = 15 lbf/ft²
  • f_b = 15
  • Gradation- Select the loam gradation you just created
  • Critical Shear = 0.001 lb/ft²
  • Erodibility = 0.003 ft³/lbf-s

• Then add some groundwater data (which we will use later):
  • K = 30 ft/d
  • Reservoir Width = 350 ft
• Then click on the Left Bank material type for one or more of your Cross Sections.
• The user defined material types are listed first (after the DEFINE LAYER method), so you should find the customized Loam material type between DEFINE LAYERS and the first default material type (Boulders). 

• Select it, run, and look at the output.
• To look at sediment results select View → Sediment Output (at the bottom of the menu) from the main HEC-RAS editor.

How many bank failures are computed on the left bank of XS 714.285? How much mass does this contribute?

777 tons
This simulation computes one bank failure, on the falling lime of the hydrograph of the second event. 
This bank failure removes 777 tons of the User Specified bank material and adds it to the sediment load.

Why might the bank failure show up in the cumulative time series but not the instantaneous result?

Note: Intermittent Output can Mask Instantaneous Processes

Cumulative (CUM) results are more reliable than results for each time step, especially for processes that are instantaneous.  By default HEC-RAS only writes sediment results every 10 computational increments.  Even if users decrease this number (requesting more frequent instantaneous outputs) the results can still miss a bank failrue.  The Left Bank Mass (per time step, not cumulative) below does not happen to capture the event when it writes results every other time step.

3. Bank Layers

Finally, river banks are often heterogeneous, composed of layers of different alluvial (or glacial) deposits.  Therefore, it is often advantageous to specify several different materials in a single bank. (see user manual here)

• Open the Sediment Data and Select File → Save Sediment Data As.
• Give the Sediment data file a name like "Add Bank Layers"

Add layers to the left bank of cross section 571, by clicking on the Left Bank Material Type column.  The first item in the drop down list will always be DEFINE LAYERS. Select it (Figure 9).  This will bring up a second table to the left where each layer can be defined with two parameters:

1. A bottom elevation and
2. A material type (which can be either a default or a user defined material type or a mix and match).

Figure 9: Defining bank material layers.

Define three layers.  The bottom of the first layer should be below the bank toe.  So set the bottom elevations to 5, 20 and 35 ft.   Bottom elevations should be defined in increasing order.  The select the material typ the same way you did in the main editor, by clicking on the material column and selecting from the drop down(Figure 10) .  Set the bottom material to the User Specified Loam, and then try other low to moderate strength material for the upper two layers.

Run the simulation and view results.

Figure 10: Defining layers and material types.

Figure 11: Results for the layer method.

[1] The last two workshops have been mostly located in the unsteady sediment tools.  However, BSTEM is currently only in quasi-unsteady.