# Presentation: 466745

## Bank-Stability and Toe-Erosion Model

**	**** **Andrew Simon, Andrea Curini, Eddy Langendoen, and Robert Thomas
- USDA-ARS National Sedimentation Laboratory, Oxford, MS
**	****	**

## Bank-Stability Model

- 2-D wedge-failure model
- Incorporates both positive and negative pore-water pressures
- Simulates confining pressures from stage
- Incorporates layers of different strength and characteristics
- Inputs: s, *c*’, ’, b , *h*, *u*w

- Confining pressure

- Tensiometers
- (pore pressure)

- shear surface

- WATER LEVEL, M

## Web Address

## Model Structure

-  **Introduction page**: provides general background
-  **Technical Background page**: provides equations for stability analysis including positive and negative pore-water pressures, effects of vegetation, and the toe-erosion algorithm.
-  **Model Use and FAQ page**: provides methodology for application of model features including hints for working with bank geometry, selecting the shear surface, soil layers, pore-water pressure/water table, vegetation, and the toe-erosion algorithm.

## Model Structure (cont’d)

**Input Geometry page**: Enter coordinates for bank profile, soil layer thickness, and flow parameters.
**Toe Model Step 2 page**: Enter erodibility data for bank toe and soil layers, and run shear-stress calculations.
**Toe Model Data page**: Enter non-default values for erodibility.
**Bank Model Step 2 page**: Enter bank-material properties (geotechnical), water table/pore-water pressure information, and obtain results.
**Bank Model Data page**: Enter non-default values for bank-material (geotechnical) properties.

## Modeling Steps

- Model the current bank profile by first evaluating the effect of hydraulic erosion at the bank toe.
- Take the resulting new profile and run this in the bank- stability model to see if the eroded bank is stable.
- Devise environmentally-sensitive schemes to protect the bank from both erosion and instability.
- Test these proposed schemes for erosion resistance and bank stability in the two models.

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**

## Introduction Sheet

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**

## Input Geometry Sheet

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****for***** *****bank geometry and input geometry data. For this first example select Option *****B*****.**

## Input Geometry Sheet

**Input Geometry Sheet**

## Starting with Option B (P. Downs version)

- If you don’t know failure-plane angle

**Select: Option B**
** ****5m high bank**
** ****85 degree angle**
** ****1m toe length**
** ****25 degree toe angle**
** ****Friction angle 30 degrees**
** ****Enter shear surface angle**

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****to input bank geometry**
**Enter Bank-layer Thickness**

## Enter Bank Layer Thickness

**Enter Bank Layer Thickness**

## Enter Bank Layer Thickness: Detail

**Enter Bank Layer Thickness: Detail**

**For this example, enter 1m thicknesses for all five layers**

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****to input bank geometry**
**Enter bank-layer Thickness**
**Enter channel-flow parameters**

## Flow Parameters for Toe-Erosion Model

**Flow Parameters for Toe-Erosion Model**

## Flow Parameters for Toe-Erosion Model

**Input the above values for this example**

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****to input bank geometry**
**Enter Bank-layer Thickness**
**Enter channel-flow parameters**
**Select model component: **_**Toe Erosion**_** and click “Run Bank Geometry Macro” - You are directed to the appropriate “Material Types” worksheet.**

## Select the Component to Model

**Select the Component to Model**

## Toe Erosion: Input Bank Materials

**Select bank layer materials shown below from drop down boxes :**
**Layer 1** = Erodible cohesive, **Layer 2** = Moderate cohesive,
**Layer 3** = Moderate cohesive, **Layer 4** = Erodible cohesive,
**Layer 5** = Moderate cohesive, **Bank Toe Material** = own data

## Toe Erosion: Input Bank Materials

- Click on the “Toe model data” sheet to enter your own data for the bank toe.

- For this example, enter values of   c = 1.5               k = 0.082          for  toe material

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****to input bank geometry**
**Enter Bank-layer Thickness**
**Enter channel-flow parameters**
**Select model component: **_**Toe Erosion**_** and click “Run Bank Geometry Macro” - You are directed to the appropriate “Material Types” worksheet.**
**Return to “Toe Erosion Model Step 2” worksheet. Click on “Run Shear Stress Macro”. Note undercutting. Click on “Export coordinates back into model”**

## Toe Erosion

**Click this button to export eroded profile to Option *****A***** in Input Geometry worksheet**

**‘****Toe Erosion Step 2’ worksheet**

**Results**

## Profile Exported into Option A

**Profile Exported into Option *****A***

**Model redirects you back to the “Input geometry” sheet. You can run another flow event or run the Bank-Stability model.**
**We will choose to run the Bank-Stability model.**
**To run Bank-Stability Component you must first select elevation of shear-surface emergence and shear-surface angle.**

**Use 1.0 and 57.5**

## Operational Steps

**Open Excel file “bsandtem4.1”**
**Click on “Enable Macros”...to “Introduction” sheet**
**Click on “Input Geometry” sheet**
**Select EITHER Option *****A***** or Option *****B *****to input bank geometry**
**Enter Bank-layer Thickness**
**Enter channel-flow parameters**
**Select model component: **_**Toe Erosion**_** and click “Run Bank Geometry Macro” - You are directed to the appropriate “Material Types” worksheet.**
**Click on “Run Shear Stress Macro” then click on “Export coordinates back into model”**
**Enter shear-plane emergence elevation and angle, then click on “Bank Model Step 2” worksheet**

## Material Types: Stability Model

## Operational Steps

**Enter Bank-layer Thickness**
**Enter channel-flow parameters**
**Select model component: **_**Toe Erosion**_** and click “Run Bank Geometry Macro” - You are directed to the appropriate “Material Types” worksheet.**
**Click on “Run Shear Stress Macro” then click on “Export coordinates back into model”**
**Enter shear-plane emergence elevation and angle, then click on “Bank Model Step 2” worksheet**
**Select bank-material types to assign geotechnical values**

## Bank material properties

- In this example start by selecting ‘silt’ for all five soil layers, from the drop down boxes

## Bank-Material Properties (cont’d)

- If you wanted to add your own geotechnical data you could select “own data” from the drop down boxes and go to “Bank Model data” sheet to enter your own values

**Again, For this example choose silt for all layers**

## Go back to “Input Geometry” worksheet.Select ‘Bank Stability Component’ and then click on “Run Bank Geometry Macro” button

## Running bank stability macro...

- First you are asked if you want to select a cantilever failure:

- For this example, select “No”

## Running bank stability macro...

- If you choose not to select a cantilever failure, as in this case, another message box will appear, asking if you want to insert a tension crack.

- Again, for this first example select “No”         (we will use this feature in a later example)

## Operational Steps

**Enter Bank-layer Thickness**
**Enter channel-flow parameters**
**Select model component: **_**Toe Erosion**_** and click “Run Bank Geometry Macro” - You are directed to the appropriate “Material Types” worksheet.**
**Click on “Run Shear Stress Macro” then click on “Export coordinates back into model”**
**Enter shear-plane emergence elevation and angle, then select Bank-Stability model and click on “Run bank geometry macro”. Model redirects you to “Select material types”**
**Select bank-material types to assign geotechnical values or select “enter own data”**
**Select type of pore-water pressure data (water-table elevation or measured values).**

## Data for Pore-Water Pressure

**Data for Pore-Water Pressure**

**Or**

**In “Bank Model Step 2” worksheet**

- In this case select option to use water table depth, and enter a value of 4.0m below the bank top

## Results: Factor of Safety

**Results: Factor of Safety**

**Partly controlled by failure plane angle**
**Based on reach length**
**Based on constituent concentration**

## How can you make this bank more stable or more unstable?

- Try experimenting with the following parameters to get a feel for the model:
- Water surface elevation (Input Geometry Sheet)
- Shear angle (Input Geometry Sheet)
- Water table height (Bank Model Step 2 sheet)
- Bank material types (Bank Model Step 2 sheet)
- Vegetation component (Bank Model Step 2 sheet)

## Further Simulations...

-  **Once stability has been determined, the coordinates may be exported back into the model (“Initial Geometry” sheet) **_**IF**_** the modeller deems that the bank has failed. This is done by clicking the “Export Coordinates back into model” button. **
_**IF**_** the bank remains stable, return to the “Initial Geometry” sheet to simulate another flow event or another pore-water pressure condition.**

## Example 2

- Go back to “input geometry” worksheet
- Make sure Option A is still selected.
- We are going to enter a new bank profile. Enter the coordinates opposite

## Example 2

- Set your water surface elevation to 2m

- Set your shear emergence elevation to 1.32 and failure surface angle to 57.5 :

## Run bank stability macro again...

## Run bank stability macro...

- This time select “Yes” to run a cantilever failure

## Enter data for Pore-Water Pressure...

**Enter data for Pore-Water Pressure...**

**“****Bank Model Step 2” worksheet – initially select a value of 4.0 m below bank top for this example**

## Example 2 results...

- Under these conditions the bank is stable

## What happens if......

- You increase the height of the water table ( and hence, pore-water pressures) in the bank?
- Increase water table height to 3m below surface

## Example 2....

- Bank stability is reduced, but bank is still stable.

## Example 2

- Now assume that the flow level recedes to 1m.

- This is the “typical” drawdown case and often represents
-  the most critical condition

## Example 2...

- Bank is now unstable (*F**s* = 0.93)

## Example 2...

- Again, try adjusting variables, for example:
- Bank materials
- Width of undercut block
- Water table height (what is the critical water table height for a given water surface elevation?)
- Vegetation component

## Example 3...

- This time we are going to look at a bank with a tension crack...
- Set up the following bank and shear profiles in Option A:

## Example 3...

- Next go to the “Bank model step 2” worksheet and select a water table depth 3m below the bank surface
- Return to “Input Geometry” and run bank stability macro.

## Example 3...

- This time select “No” for cantilever failure, and “Yes” to insert a tension crack

## Example 3...

- You are now prompted to add a tension crack depth (maximum and minimum estimated values are indicated in the prompt box). For this example type in the largest suggested value: 0.87m, and click ‘OK’

## Example 3...

- In this case bank Fs with the tension crack is 0.94, and without the tension crack is 1.21

## Example 3...

- As the *F**s *with the tension crack is <1, and is considered unstable, click on the button to ‘Export coordinates back into model’

## Example 3...

- Rerun the bank stability macro again..... This time your bank should be stable with a failure angle of 55 degrees, even under worst case conditions (fully saturated bank, with low flow to provide confining force)

## Testing for Bank Stabilization

- Now, try creating your own bank profiles and experimenting with bank stabilization by adjusting input parameters in the toe erosion and bank stability macros.

**Hydraulic vs. Geotechnical Processes**

## Distinguish Between Hydraulic and Geotechnical Bank Protection

- Toe armoringrock, LWD, live vegetation, fiberschines
- Bank face armoringmattresses, vertical bundles, geotextiles
- Bank reinforcementpole and post plantings, bank top vegetation, brush layers, drainage

## Distinguish Between Hydraulic and Geotechnical Bank Protection

- Hydraulic protection reduces the available boundary hydraulic shear stress, and increases the shear resistance to particle detachment

- Geotechnical protection increases soil shear strength and decreases driving forces

## Adding vegetation effects

-     Select type/age of vegetation from drop down box
-     Select vegetation safety margin (0 – 100 %)

## Adding vegetation effects.... An example

- No vegetation – unstable

- Switchgrass (5 years-old @ 50% safety margin) – conditionally stable

- Switchgrass (5 years-old @ 100 % safety margin)

## Comparing Bio-engineering With Hard Engineering

- Comparing Bio-engineering With Hard Engineering

- 2m high vertical silt bank
- Fs bare = 0.31 worst case
- Fs regraded to 1 in 1.5 = 1.33
- Fs with cottonwood = 1.33

- Costs:
- Regrading - 3 m3 per m channel plus cost of land
- Bioengineering – plant materials plus maintenance

- Excavated material 3 m3/m

- 2m

- 3m

## Factor of Safety v. Bank Angle

- Planting vegetation on a 90°, 1m high silt bank is the equivalent of cutting back a bare slope to; 5 yr old Black willow  72°7 yr old River birch  48°5 yr old Switch grass  38°

## Bank Stabilization Techniques

- Brush layer and brush trench

- Plant bundles of willow cuttings in trenches on bank face or top.  Brush layer reinforces bank face, and reduces scour and surface erosion.

## Bank Toe Protection

- Attach fiber roll or tree stumps and root wads to bank toe and fill in behind with soil and willows

- Design question:
- How much effect will the LWD have on bank toe erosion rates?

- Fiberschines and large woody debris (LWD)