Quantifying Reductions of Mass- Failure Frequency and Sediment Loadings from Streambanks using Toe Protection and Other Means Andrew Simon, Natasha Pollen-Bankhead, Virginia Mahacek and Eddy Langendoen [email protected]National Sedimentation Laboratory
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Andrew Simon, Natasha Pollen-Bankhead, Virginia Mahacek and Eddy Langendoen
National Sedimentation Laboratory. Quantifying Reductions of Mass- Failure Frequency and Sediment Loadings from Streambanks using Toe Protection and Other Means. Andrew Simon, Natasha Pollen-Bankhead, Virginia Mahacek and Eddy Langendoen [email protected]. Lake Tahoe Basin. - PowerPoint PPT Presentation
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Quantifying Reductions of Mass- Failure Frequency and Sediment
Loadings from Streambanks using Toe Protection and Other Means
• Trend of declining lake clarity for more than 30 years• This has been attributed to the delivery of fine sediment• An estimated 25% of this fine sediment comes from
streambank erosion• About 90% of this emanates from three watersheds
What kind of fine-load reductions can be expected using mitigation measures?
1. Select critical erosion sites within watersheds known to produce substantial quantities of fine-sediment from streambank-erosion processes.
2. Quantify annual loadings from streambank erosion for existing conditions by simulating toe-erosion and bank-stability processes over the course of an annual hydrograph.
3. Quantify annual loadings from streambank erosion for mitigated conditions at these sites by simulating toe-erosion and bank-stability processes over the course of the same annual hydrograph.
4. Compare loadings reductions for the modeled sites and extrapolate results to the remainder of the channel system.
Bank-Stability and Toe-Erosion Model
• 2-D wedge-failure and cantilever model
• Tension cracks
• Hydraulic toe erosion
• Incorporates both positive and negative pore-water pressures
• Simulates confining pressures from stage
• Incorporates layers of different strength and characteristics
By comparing applied shear stress with critical shear stress and erodibility, actual erosion is calculated for each facet, and the profile is redrawn. The new and old profiles can be assessed for bank stability.Layer 1
Layer 2
Layer 3 Toe material
Toe Erosion
Click this button to export eroded profile to Option A in Input Geometry worksheet
Toe Erosion for Initial Flow EventInput bank materialsSpecify the erodibility of the different materials. Use the drop down boxes to select material type or select "Enter own data" and add valuesin the 'Bank Model Data' worksheet. If you select a material, the values shown in the 'Toe Model Data' worksheet will be used. Once youare satisfied that you have completed all necessary inputs, hit the "Run Shear Stress Macro" button (Center Right of this page).
Bank Material Bank Toe Material Bed material
Layer 1 Layer 2 Layer 3 Layer 4 Layer 5
0.36 0.36 0.36 0.28 0.28 0.28 248.83
0.167 0.167 0.167 0.189 0.189 0.189 0.006
Bank Protection
Input bank protection
Bank Toe Protection
Input toe protection
Average applied boundary shear stress 2.61 Pa
Maximum Lateral Retreat 61.65 cm
Mean Eroded Area - Bank 0.14 m2
Mean Eroded Area - Bank Toe 0.12 m2
Mean Eroded Area - Bed 0.00 m2
Mean Eroded Area - Total 0.255 m2
Enter own data Fixed bed
No protection
No protection
Enter own data Enter own data Enter own data Enter own data Enter own data
Stability Analysis for First EventSelect material types, vegetation cover and water table depth below bank top(or select "own data" and add values in 'Bank Model Data' worksheet)
Bank top Reach LengthLayer 1 Layer 2 Layer 3 Layer 4 Layer 5 vegetation cover (age) (m)
Stability Analysis after Second Flow EventSelect material types, vegetation cover and water table depth below bank top(or select "own data" and add values in 'Bank Model Data' worksheet)
Bank top Reach LengthLayer 1 Layer 2 Layer 3 Layer 4 Layer 5 vegetation cover (age) (m)