Fundamental Concepts in Fluvial Geomorphology

1. National Sedimentation Laboratory Fundamental Concepts in Fluvial Geomorphology Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS asimon@msa-oxford.ars.usda.gov

2. Three Zones of the Fluvial System

3. Force, Resistance, and Form

4. Force and Resistance(Or what it takes to initiate movement (erosion) of material) Think in terms of SPECIFIC PROCESSES • On the stream bed Force/resistance • On the stream banks Force/resistance

5. Stream Types: Schumm

6. Continuity Equation Q = w y V = A V Q = water discharge m3/s w = flow width, in m y = flow depth, in m V = flow velocity, in m/s A = cross-sectional area in m2

7. Stream Power Proportionality gQS a Qsd50 g = unit weight of water Q = water discharge S = bed or energy slope Qs = bed-material discharge d50= median particle size of bed material Thus, streams are open systems with an ability to adjust to altered energy inputs

8. Stream Power: Thresholds

9. Graded time Steady time Time Scales in Geomorphology Gradient Cyclic time Gradient Graded time

10. Variables Change Dependency as a Function of Time Scale!

11. Types of Equilibrium: Thresholds?

12. Why is Fluvial Geomorphology Important?This Will Be on the Test gQS a Qsd50 g = unit weight of water Q = water discharge S = bed or energy slope Qs = bed-material discharge d50= median particle size of bed material Thus, streams are open systems with an ability to adjust to altered energy inputs

13. Graded time Gradient Cyclic time Response to Disturbance Compression of time scales following large-scale disturbances: “natural” or anthropogenic 1,000,000 years = 100 years

14. Trends of Incision: Channelization

15. to = g R S to = mean boundary shear stress g = unit weight of water R = hydraulic radius = A / 2y + w S = channel gradient t* = to / [ (gs – gw) * d] t*= dimension less shear stress gs = unit weight of sediment d = characteristic particle diameter Hydraulic Shear Stress; Force and Resistance

16. Erosion Rate is a Function of Erodibility and Excess Shear Stress te = (to-tc) or e = k (to- tc) e = erosion rate (m/s) k = erodibility coefficient (m3/N-s) to =boundary shear stress (Pa) tc =critical shear stress (Pa) (to-tc) = excess shear stress Critical shear stress is the stress required to initiate erosion.

17. Bed-Level Response

18. Magnitude of Width Adjustments Why are they so different ?

19. Basic Failure Types

20. Normal load – weight of bank increases friction Cohesion– chemical bonds between particles Friction -interparticle roughness Matric suction –apparent cohesion Pore-water pressure – reduces effective friction Forces Affecting Soil Shear Strength Shear surface

21. Bank Stability – The Factor of Safety Resisting Forces Driving Forces If Fs is greater than 1, bank is stable. If Fs is less than 1 bank will fail. (We usually add a safety margin – Fs>1.3 is stable.) Factor of Safety (Fs)= Resisting ForcesDriving Forces (gravity) soil strength bank angle vegetation weight of bank reinforcement water in bank

22. Changes in Width/Depth During Adjustment

23. Idealized Adjustment TrendsFor a given discharge (Q) t gVS Se n tc d

24. This Will Also Be on The Test Applied (Driving) Forces versus Resisting Forces • Hydraulic processes (bed, bank toe) • Geotechnical processes (bank mass)

25. National Sedimentation Laboratory Process, Process, Process Use Form to Tell Us About Process • Channel Evolution Models Use Form to Infer Process • Schumm et al., 1984 • Simon and the Hupp, 1986; Simon, 1989

26. National Sedimentation Laboratory Stages of Channel Evolution(just another empirical model) • References • Stage I • Stage VI

27. Stage and Bed Material Yield

28. Stage and Suspended Sediment Transport

29. Stage of Channel Evolution

30. What Processes are Active?

31. What’s Happenin’ Here?

32. National Sedimentation Laboratory What Processes are Active? Where do they change, and why?

33. National Sedimentation Laboratory What Processes are Active?

34. National Sedimentation Laboratory What Processes are Active?