Scour

1. Scour • Scour Overview • Types of scour • How to estimate scour • What to do about it • Demo/Example Material in this section is drawn primarily from NEH 654 TS 14B Jon FrippNDCSMCFt. Worth, TX

2. Scour:Downward vertical erosion in a channel bed.

3. Scour:Causes • Watershed changes • Site Changes • Natural Conditions Degradation: Long-term sediment removal occurring through increased erosion from the channel bed. NEH 654.13

4. Scour:Rate of change Alluvial Channels:Maximum depth may be reached in hours Cohesive Channels: Maximum depth could be reached in days Glacial tills, sand stones, shales: Maximum depth may be reached in months Limestone: Years may be needed to reach maximum depth Granite: Centuries may pass before maximum depth

5. Scour – Why Worry? Answer: No matter what is designed for the bank, if the toe is undermined, it will not work well. Photograph from Stephen T. Maynord Example of local failure of riprap at toe of slope

6. Scour – Why Worry? • Answer: Scour along the toe can result in geotechnical instability on the bank.

7. Scour – Why Worry? • Answer: No matter what is designed that crosses the bed, if it is undermined, it will not work well. The material used for the structure is not the issue – the foundation is what is important

8. Scour – Why Worry? • Answer: We need to estimate how much of an impact scour might have on the project and provide a design that can accommodate.

9. Types of Scour • General: Commonly affects the entire channel cross section, but general scour may affect one side or reach more than another • Bedform:Usually found in sand-bed streams, this is the troughs between crests of bedforms • Local: Commonly affects the streambed immediately adjacent to some obstruction to flow Scour is scary

10. Types of Scour • General Scour • Contraction Scour • Bend scour • Bedform Scour • Dunes, antidunes, sand beds • Local Scour • Drop structures • Grade control • Step Pools • Sills • Bridge Piers and Abutments • Vanes, barbs, deflectors, etc • Other?

11. Total Scour It all adds up!

12. How do you estimate total scour? Analytical techniques vs. “Rules of thumb” for scour assessments

13. How do you estimate scour: “Rules of thumb” for scour assessments Examples • Two feet below existing channel grade • Three feet below existing channel grade • Four feet below existing channel grade • 30 feet below existing channel grade • Two feet below the deepest pool depth observed in the channel • A depth equal to twice the maximum particle observed in the channel bed • Others?

14. How do you estimate scour: “Rules of thumb” for scour assessments Good judgment comes from experience, and a lot of that comes from bad judgment! (Tex Bender)

15. How do you estimate scour: Analytical techniques • Allows designer to consider harsh conditions • Many different formulas are available • Use the right formula in the right application Many streams can scour during high flows and fill during flood hydrograph recession

16. Scour resulting from structures that span or partially span the channel • Check Dams and Constructed Riffles • Stream Barbs and Deflectors

17. Approaches to prevent undermining of the structure • Provide a foundation at depth below anticipated scour • Provide sufficient material to launch into anticipated scour hole • Provide a pre excavated, armored scour hole

18. Scour at grade controls

19. Scour at step pools

20. Scour at structures that partially span the channel

21. Scour resulting from hydraulics of flow around bends A major cause of bank failure A major cause of the failure of bank protection projects to perform as designed

22. Approaches to Prevent Undermining of the Toe • Excavate below the anticipated scour or continue revetment to a hard layer • Reduce stress • Provide launchable material • Sheet pile toe to below scour or hard layer • Pave bed

23. Scourresulting from hydraulics of flow around bends Davg Dmax Flow through channel meander bends results in water moving in a corkscrew or helical pattern. Velocity components not in the streamwise direction are referred to as secondary currents. The bank toe is often the locus of maximum stress and erosion. Thalweg Dmax/Davg>1

24. Many of these formulas have an empirical basis

25. Bend Scour Many formulas are available

26. Bend Scour Bend scour can be higher when banks are armored.

27. Location of maximum scour Toe scour can extend through a bend

28. Example Problem • Given: Bank protection is needed for a road located adjacent to the outer bend of an eroding bank • Channel Bed is erodable • Bend Radius is 60 feet • Channel width is 20 feet • Design depth is 5 feet • Current Maximum depth along eroded area is 10 feet • Estimate the maximum depth in the bend. (use eqns TS14B-33/34)

29. W/Rc = 20 / 60 = 0.33 Ymax/ Y = 1.5 + 4.5 x 0.33 = 3 Ymax = 5 x 3 = 15 feet Q: is this typically the end of the question? Check with another equation Rc/W = 3; W / Y = 4 (use 20) Ymax/ Y = FS x (1.8 - .051 Rc/W + 0.0084 x W / Y) = 2.7 Ymax = 13.6 feet …. say 14 feet, fairly close, note there are some different assumptions for each formula, more information in NEH 654

30. Example Problem Continued • Given: Project is going to include a riprap toe • How does this estimate of maximum depth affect the design?

31. Zb = Y (Y max/ Y -1) = 5x(3–1) = 10 ft of potential scour over average depth So, if the Zb is estimated to be 10 ft and Ymax is 15 ft Look at the current depth along the outer bend as 10 ft Design protection must be able to withstand at least 5 feet of additional scour below current grade Note: concept designs from different sites • Example Options: • Provide a launch able rock toe • Use sheet pile to secure toe • Excavate below anticipated scour depth and secure toe

32. Flow Rock installed to launch into toe scour Note steep angle where stone was undercut and launched (self-adjusted) Original Installation Launched angle Photograph from Dave Derrick

33. The END When you find yourself in a hole, stop digging!(Tex Bender)