Andrew Simon, Natasha Pollen-Bankhead, Virginia Mahacek and Eddy Langendoen

1. 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 andrew.simon@ars.usda.gov

2. Lake Tahoe Basin

3. Problem and Objectives • 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?

4. Fine-Sediment Delivery Blackwood Creek Ward Creek Upper Truckee River

5. General Approach 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.

6. 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 • Inputs: gs, c’, f’, fb , h, uw, • k, tc Bank-Stability and Toe-Erosion Model shear surface Tensiometers (pore pressure) Confining pressure WATER LEVEL, M

7. Required Field Data

8. Site Characteristics

9. Quantified Vegetation Characteristics

10. Selection of Annual Hydrograph

11. Discretized Hydrographs

12. Bank-Toe Model 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

13. Toe Erosion ‘Toe Erosion Step 2’ worksheet Results Click this button to export eroded profile to Option A in Input Geometry worksheet

14. Toe Erosion for Initial Flow Event Export new geometry

15. Stability Analysis for First Event

16. Stability Analysis after Second Flow Event

17. Iterative Modeling Scenarios • Existing bank and vegetative conditions • Toe protection (rock) modeled by simulating 256mm clasts 1.0 m up the bank toe. • Addition of top-bank vegetation in some cases

18. Example of Iterative Results

19. Results: Failure Frequency

20. 25th percentile = 80.0 Median = 86.8% 75th percentile = 94.9 Load Reduction

21. Effects of Toe Protection

22. Load Reduction: Other Treatments • Toe protection = 86% (average) • Top-bank vegetation = 53% • Bed-slope reduction (meandering) = 42-54%

23. Extrapolation of Results • Assessments of longitudinal extent of recent failures, and • Loadings rates: High, Medium and Low (values for 100m-long reach) High rate = 36,170 m3/km Medium rate = 4720 m3/km Low rate= 472 m3/km

24. Extrapolation Based on % Reach Failing (Blackwood Creek)

25. Fine-Sediment Loading: 2,511 m3 or 4,432 T

26. Simulated vs. Measured Loadings

27. Cost Basis

28. Summary and Conclusions • Used iteratively, BSTEM is an effective tool to quantify potential load reductions by bank treatments • Toe erosion is a small component (about 13%) of total streambank loadings • However, by reducing toe erosion, mass-failure frequency and associated sediment loadings can be drastically reduced. • Other treatments can be effectively simulated with BSTEM and show significant load reductions.