1. 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 National Sedimentation Laboratory

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 Ward Creek Upper Truckee River Blackwood Creek

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. 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 Inputs:  s, c’,  ’,  b, h, u w, k,  c Confining pressure Tensiometers (pore pressure) shear surface WATER LEVEL, M

7. Required Field Data

8. Site Characteristics Stream Location (km) Bank height (m) Special characteristics Blackwood Creek 1.943.0No top-bank vegetation 2.392.4Lemmon’s willow (moderate) Upper Truckee River 4.512.6Meadow vegetation 8.451.9Mixed meadow and woody vegetation 13.12.7Golf course with lodgepole pine Ward Creek 2.4814.9 14.9 m steep, terrace slope adjacent to channel; coarse material at toe; Mature conifers 3.601.3Meadow vegetation

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

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 1.Existing bank and vegetative conditions 2.Toe protection (rock) modeled by simulating 256mm clasts 1.0 m up the bank toe. 3.Addition of top-bank vegetation in some cases

18. Example of Iterative Results

19. RiverStationRkmCondition Blackwood1.94 Existing Blackwood1.94 Toe Protection Blackwood2.39 Existing Blackwood2.39 Toe Protection Upper Truckee50+344.51Existing, No Vegetation Upper Truckee50+344.51Toe Protection Upper TruckeeHole 613.1Existing Upper TruckeeHole 613.1Toe Protection Upper Truckee50+344.51Existing-Veg Upper Truckee50+344.51Toe Protection-Veg Upper Truckee179+748.45Existing Upper Truckee179+748.45Toe Protection Ward22+352.48Existing-No Toe Slope Ward22+352.48Toe Protection-No Toe Slope Ward 26+44 3.6 Existing Toe Protection Failure events at peakdrawdownTotal 437 011 213 101 336 022 314 011 213 011 7310 000 101 000 505 011 Results: Failure Frequency

20. RiverStationRkmCondition Blackwood1.94 Existing Blackwood1.94 Toe Protection Blackwood2.39 Existing Blackwood2.39 Toe Protection Upper Truckee50+344.51Existing, No Vegetation Upper Truckee50+344.51Toe Protection Upper TruckeeHole 613.1Existing Upper TruckeeHole 613.1Toe Protection Upper Truckee50+344.51Existing-Veg Upper Truckee50+344.51Toe Protection-Veg Upper Truckee179+748.45Existing Upper Truckee179+748.45Toe Protection Ward22+352.48Existing-No Toe Slope Ward22+352.48Toe Protection-No Toe Slope Ward 26+44 3.6 Existing Toe Protection Eroded FinesLoad reduction m3m3 m 3 /kmm3m3 % 8978970 337393.3 61610 79.7797 39884.3 12.5125 1471470 73370.7 43430 2342340 115189.4 25250 70700 33668.6 22220 5355350 379299.9 0.3103.102 3363360 5256.2100.0 00 35.0350 50283.1 5.959.2 Load Reduction 25 th percentile = 80.0 Median = 86.8% 75 th percentile = 94.9

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 1.Assessments of longitudinal extent of recent failures, and 2.Loadings rates: High, Medium and Low (values for 100m-long reach) High rate = 36,170 m 3 /km Medium rate = 4720 m 3 /km Low rate= 472 m 3 /km

24. DistanceExtent of failures (km) (%) LeftRightAverage 8.2900-10% 5 8.1900-10%26-50%21.5 7.69011-25% 18 7.18011-25% 18 7.17011-25%76-100%53 6.8400-10%11-25%11.5 6.5100-10%51-75%34 6.0300-10%26-50%21.5 5.5500-10%26-50%21.5 5.0800-10%51-75%34 4.15026-50%11-25%25.5 3.9500-10%76-100%46.5 2.80051-75%0-10%34 1.97026-50%11-25%25.5 1.77011-25%51-75%40.5 0.32051-75%0-10%34 Extrapolation Based on % Reach Failing (Blackwood Creek)

25. Distance (km) Extent of failures (%) Reach Length (km) Reach Failing (%) Weighting factor Total volume (m 3 ) Fraction <0.063 mm (%) Fines volume (m 3 ) LeftRightMean12(1)*(2)/100 8.290-10 5.0------ 8.190-1026-5021.50.1013.250.013362.55.83.6 7.6911-25 18.00.5019.750.098746.60.00 7.1811-25 18.00.51180.091843.326.011.3 7.1711-2576-10053.00.0135.50.003512826.033.4 6.840-1011-2511.50.3332.250.106450.226.613.4 6.510-1051-7534.00.3322.750.075135422.178.3 6.030-1026-5021.50.4827.750.133262920.0125.7 5.550-1026-5021.50.4821.50.10324877.938.5 5.080-1051-7534.00.4727.750.130461623.5144.7 4.1526-5011-2525.50.9329.750.276713063.647.0 3.950-1076-10046.50.20360.0720260421.4557.3 2.8051-750-1034.01.1540.250.4629218512.3268.7 1.9726-5011-2525.50.8329.750.2469116524.8289 1.7711-2551-7540.50.20330.0660238716.6396.3 0.3251-750-1034.01.4537.250.5401254916.3415.6 0.0026-50 38.00.32360.115254416.388.6 Fine-Sediment Loading: 2,511 m 3 or 4,432 T

26. Simulated vs. Measured Loadings

27. Cost Basis

28. Summary and Conclusions 1.Used iteratively, BSTEM is an effective tool to quantify potential load reductions by bank treatments 2.Toe erosion is a small component (about 13%) of total streambank loadings 3.However, by reducing toe erosion, mass-failure frequency and associated sediment loadings can be drastically reduced. 4.Other treatments can be effectively simulated with BSTEM and show significant load reductions.