Channel Disturbance and Evolution: Controls and Implications for Stream Restoration

1. National Sedimentation Laboratory Channel Disturbance and Evolution: Controls and Implications for Stream Restoration Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS, USA

2. National Sedimentation Laboratory Introductory Points • Alluvial streams are open systems that dynamically adjust to variations in flow energy and sediment supply. • Streams adjust their morphology to imbalances between available force and sediment supply as a function of the resistance of the boundary sediments to hydraulic and geotechnical forces. • Thus, two channels of similarmorphology disturbed by an identical perturbation can attain different equilibrium morphologies • Also, diverse streams subject to diverse perturbations can respond similarly. CW

3. Impetus for Restoration Efforts • Major land-clearing activities between the mid-1850’s and early 1900’s to bring land into agricultural production. • Soil-conservation techniques were not used/available. • Massive erosion from fields and uplands in many areas, particularly the mid continent. • Channels filled with eroded sediment causing reduced conveyance and increases in the frequency and duration of flooding. • Large-scale programs to dredge and straighten many fluvial systems to improve land drainage and reduce flooding. • Resulting channel instabilities caused incision and massive erosion of main stem and tributary streams (valley-fill deposits ). • Erosion from channel systems has become the dominant source of sediment in many (most) of these watersheds. • Clean-Water Act, TMDLs, Rosgen

4. Gravity is A Constant!! • The physics of erosion are the same wherever you are…no matter what hydro-physiographic province you are in…whatever the stream type may be. • Channel adjustment is driven by the imbalance between the driving and resisting forces • Differences in rates and magnitudes of adjustment, sediment transport rates and ultimate channel forms are a matter of defining those forces…deterministically or empirically Incision enhances channel response by creating flows with greater transporting power

5. Obion Forked-Deer River Basin Case Study: Coastal-Plain System Downstream, anthropogenic disturbance in a sand-bed, cohesive- bank system causing an increase in transport capacity (gQS) Modified from Lutenegger (1987)

6. Adjustment Processes Mississippi Tennessee Mississippi Nebraska

7. Case Study: Sub-Alpine System Upstream “natural” disturbance in a coarse-grained, non-cohesive bank system causing an increase in transport capacity (gQS) and a decrease in resistance (d50)

8. Adjustment Processes

9. Trends of Bed-Level Change Elk Rock Reach Salmon B Reach Mt. St Helens W. Tennessee

10. Function for Degradation/Aggradation z / zo = a + (1-a) e(-k t) z = elevation of the channel bed at time t, z0 = elevation of the channel bed at t0 = 0, a = dimensionless coefficient determined by regression equal to z/z0 when equation becomes asymptotic, 1-a = total change in dimensionless elevation, k = coefficient, determined by regression and indicative of the rate of change on the channel bed per unit time, t = time in years since the onset of the adjustment process. a > 1, aggradation; a < 1, degradation

11. Trends of Bed-Level Change

12. Trends of Bed-Level Change Coarse-grained material for aggradation derived from bank sediment.

13. Widening Incision creates the conditions for bank instability and widening by creating higher, steeper banks But why are they so different ? Resistance

14. Idealized Adjustment Trends: Mid Continent 6% stable 80% with unstable banks 2,500 km of streams in W. Iowa (1993-4) Data from Hadish (1994)

15. Bed-Level Adjustment: Arno River, Italy

16. Phases of Degradation Since 1900 Phase I: Land use changes with a reduction in sediment supply Phase II: Gravel mining and upstream dam construction

17. Boundary Resistance and Channel Response • General trends of channel response to disturbance (channelization and reduction of sediment supply) provide only a semi- quantitative view of how different disturbances can cause similar responses. • Similar channels may respond differently as a function of the relative and absolute resistance of the boundary (bed and banks) to hydraulic AND geotechnical forces • Alluvial-channel response has been defined by many with non- linear decay functions that become asymptotic and reach minimum variance with time.

18. Minimization of Energy Dissipation Channels adjust such that their geometry provides for a minimum rate of energy dissipation given the constraints of the upstream sediment load, roughness and resistance of the boundary materials If this holds true, then we should be able to track this over time in disturbed, adjusting streams

19. hf Flow Energy and Energy Dissipation E = z + y + v2/2g hf= (z1 + y1 + v12/2g) - (z2 + y2 + v22/2g) v12/2g Water surface v22/2g y1 y2 z1 Channel bed z2 L 1 2 Energy slope: Se = hf / L

20. Processes That Effect Components of Total Mechanical Energy (E) For each parameter comprising E, what processes would result in a reduction in those values? • z: • y: • v2/2g: degradation widening, aggradation widening, increase in relative roughness, growth of vegetation, aggradation, Thus, different and often opposite processes can have the same result

21. Aggradation and widening Degradation and widening Adjustment by Different Processes

22. Differences in sediment supply Importance of Widening in Energy Dissipation • Salmon B reach: aggradation and widening • Elk Rock reach: degradation and widening • Reduces flow depth (pressure head) for a given flow; • Increases relative roughness, and therefore, • Reduces flow velocity (kinetic energy); • Combined with degradation (potential energy) is the most efficient means of energy reduction because all components of E are reduced; • Counteracts increase in potential energy from aggradation

23. Effect of Bank Materials on Incision • Assume that gQS a Qsd50 is balanced • How does a channel respond if disturbed? • Will the channel incise? • Will the channel fill? • Will the channel widen? • Will the channel narrow? • Will it equilibrate to the same geometry?

24. Provides Only Limited Insight 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 Where will erosion occur? How will channel form change? Simulated using a numerical model of bed deformation and channel widening (Darby, 1994; Darby et al., 1996)

25. Disturbing a Sand-Bed Channel • Assume that gQS a Qsd50 becomes un-balanced • Qsd50 = 0.5 * capacity • Slope = 0.005 • Initial width/depth ratio = 13.5

26. 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 DAYS FROM START OF SIMULATION @ 0.90 0.00005 @ 0.87 0.0005 @ 0.80 0.005 Energy Dissipation for Different Boundary Materials 0 Energy adjustment is similar at given slope because of an equal, but excessive amount of flow energy relative to sediment supply. Do each of these channels reach equilibrium similarly?

27. Styles of Adjustment for Different Boundary Materials DAYS FROM START OF SIMULATION Disturbance: Upstream sediment supply cut in half

28. Adjustments for Different Boundary Materials Response to similar disturbance: Sediment supply = 0.5 * capacity From Simon and Darby (1997)

29. Response with Different Bank Materials • How does the channel respond? • How much will the channel incise*? • How much will the channel widen*? • What is the stable W/D ratio*? It depends! 0.4 – 3.5 m 0 – 13 m 5.6 – 16.4 * For a given initial slope of 0.005 m/m

30. Can A Form-Based Design System Address these Issues? • To many, the Rosgen classification and associated “natural channel design” have become synonomous with the terms “stream restoration” and “fluvial geomorphology” • Is required for many restoration proposals, job applications etc. • Empowerment of individuals, groups and agencies with limited experience in watershed sciences to engineer wholesale re-patterning of stream reaches using technology never intended for engineering design • Many of these projects are being implemented across the country with varying degrees of success…

31. Misuses • Predicting river behavior…processes • Engineering design in disturbed systems • Use of a single discharge (bankfull) • Ignores temporal and spatial scales • Ignores processes and the concept that rivers are dynamic and part of open systems Uses and Misuses: Classification vs. “Natural Channel Design” Uses • Characterize the SHAPE and the average composition of boundary sediments of stream reaches • As a communication tool for the above • Classification is rapid and easy to perform

32. Problems with Application of Classification Definition of bankfull level, particularly in unstable systems. “Bankfull” discharge and the dimensions represented by hydraulic geometry refer to stablechannels. In unstable channels, they are changing with time.

33. Problems with Application of Classification Inconsistent determination of stream type among observers with no clear guidance for determining stream type when more than one was possible “…the classification system … appears to do little to improve communication among practitioners beyond what the raw measures of channel attributes would have done.” Roper et al., (2008), in press, JAWRA “Rosgen A stream type that 5 out of 8 times was misclassified by observer measurements as a B channel type” From Roper et al., (2008), in press

34. National Sedimentation Laboratory Implications for a Form-Based System: Bed or Channel Material?(two different populations) CW

35. Channel/Bed Material CW From Rosgen (1996); Fig. 5-3

36. However… CW From Rosgen, 1996; Fig. 5-2

37. Avoiding the Problem?? Rosgen (2006) “Streambanks generally make up five percent or less of the channel boundary…This would avoid the problem…” True if width/depth (W/D) ratio is about 40 or greater. However, they comprise 29% for example if W/D = 5. For example, usingguidance to proportionately sample pools and riffles (p. 5-27, Rosgen, 1996):

38. C5 These two C5’s represent very different transport regimes Why is this Important? • Sites may not classified correctly: Example: • “C” channel shape: gravel bed, silt/clay banks • “C” channel shape: sand bed, sand banks • As we have seen differences in bank materials are critical to predicting channel response and stable geometries • Particle-size data cannot be used for incipient motion or transport analysis • Extensive data sets collected by various agencies cannot be used for analysis of hydraulic erosion, geotechnical stability, or channel response CW

39. Perhaps Explains Why… “The consequence of a wide range of stream channel instability can be described and quantified through an evolution of stream types (Figure 1).”Rosgen (2001) • E to E • C to C; C to Bc; C to D • B to B • Eb to B From Rosgen (2001) CW

40. Forcing a Form-Based System to Describe Process “The consequence of a wide range of stream channel instability can be described and quantified through an evolution of stream types (Figure 1).”Rosgen (2001) • E to E • C to C; C to Bc; C to D • B to B • Eb to B Can this be predicted a priori? Can this truly be quantified? If not, what does this mean for the “Natural Channel Design” approach since “…stream classification does not attempt to predict…stability…” Rosgen (2006) From Rosgen (2001) CW

41. And Aren’t Most of These Similar? + Incision Widening Filling From Rosgen (2001) From Rosgen (2006)

42. Implications for Sediment TMDLs • What are background, natural, stable rates of sediment transport/ bed-material characterisitics? • We must be able to discriminate between stable and unstable conditions to determine departure from natural or background conditions

43. A Rapid Means of Evaluating Thousands of Streams was Needed The very popular Rosgen Classification offers one such means of rapidly classifying streams • easy to understand • novices can perform • excellent communication tool about channel form We don’t have the time or the money to perform detailed analyses at every site that needs to be evaluated and that may require a TMDL Still, a scientifically defensible procedure is required

44. Stream Types With No “Reference” Condition for Sediment TMDLs • D:Active lateral adjustment with abundant sediment supply….aggradational processes…high bedload and bank erosion (Rosgen, 1996). • F: Entrenched…laterally unstable with high bank-erosion rates (Rosgen, 1996). • G: Gullies…deeply incised…unstable with grade control problems and high bank erosion rates (Rosgen, 1996). • Thus, alluvial stream types D, F and G have no REFERENCE condition for sediment transport and sediment TMDLs

45. “…stream classification does not attempt to predict…stability… Rosgen (2006) “A two-week course is required to teach professionals (including individuals who have graduated from college with advanced degrees in engineering, geology, hydrology, fisheries, etc.) how to conduct a watershed and stream channel stability analysis”. Wow…

46. Consider This… “Concepts that have proved useful in ordering things easily achieve such authority over us that we forget their earthly origins and accept them as unalterable givens…The path of scientific progress is often made impassable for a long time by such errors.” Einstein, 1916

47. So, What to Do? Potential Approaches • Empirical (including “Natural Channel Design”): regime equations; not cause and effect; time independent Morphology related to discharge (hydraulic geometry) etc. Can address tractive force and bed-entrainment issues Ignores bank processes, flow variability and sediment contribution from banks • Deterministic:physically based; cause and effect Quantifies driving forces and resistance of boundary sediments to the appropriate processes and functionally linked to upland delivery of flow and sediment. It’s a big toolbox! Use what is appropriate for the scale and objective of the project. Approaches are NOT mutually exclusive!

48. This is Our World of Disturbed Systems

49. 1953 1977 Hotophia Creek, MS System-wide disturbance to channel system caused by lowering of the water surface of the trunk stream due to dam closure How do you analyze this system? Do you need to consider dynamic processes with time? Will a “reference reach” approach be appropriate?

50. Unstable reach “Reference reach” Dynamic System? Unstable reach is 100 m from “reference reach