Presentation on theme: "Channel Disturbance and Evolution: Controls and Implications for Stream Restoration Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS,"— Presentation transcript:
Channel Disturbance and Evolution: Controls and Implications for Stream Restoration Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS, USA 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 similar morphology disturbed by an identical perturbation can attain different equilibrium morphologies Also, diverse streams subject to diverse perturbations can respond similarly. CC National Sedimentation Laboratory
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
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
Case Study: Coastal-Plain System Obion Forked-Deer River Basin Modified from Lutenegger (1987) Downstream, anthropogenic disturbance in a sand-bed, cohesive- bank system causing an increase in transport capacity ( QS)
Trends of Bed-Level Change Elk Rock Reach Salmon B Reach Mt. St Helens W. Tennessee
Function for Degradation/Aggradation z / z o = a + (1-a) e (-k t) z = elevation of the channel bed at time t, z 0 = elevation of the channel bed at t 0 = 0, a = dimensionless coefficient determined by regression equal to z/z 0 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
Phases of Degradation Since 1900 Phase I: Land use changes with a reduction in sediment supply Phase II: Gravel mining and upstream dam construction
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.
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
Flow Energy and Energy Dissipation v 1 2 /2g v 2 2 /2g y1y1 y2y2 z1z1 z2z2 12 hfhf E = z + y + v 2 /2g h f = (z 1 + y 1 + v 1 2 /2g) - (z 2 + y 2 + v 2 2 /2g) Channel bed Water surface Energy slope: S e = h f / L L
Processes That Effect Components of Total Mechanical Energy (E) z: y: v 2 /2g: For each parameter comprising E, what processes would result in a reduction in those values? degradation widening, aggradation widening, increase in relative roughness, growth of vegetation, aggradation, Thus, different and often opposite processes can have the same result
Adjustment by Different Processes Degradation and widening Aggradation and widening
Importance of Widening in Energy Dissipation Salmon B reach: aggradation and widening Elk Rock reach: degradation and widening 1.Reduces flow depth (pressure head) for a given flow; 2.Increases relative roughness, and therefore, 3.Reduces flow velocity (kinetic energy); 4.Combined with degradation (potential energy) is the most efficient means of energy reduction because all components of E are reduced; 5.Counteracts increase in potential energy from aggradation Differences in sediment supply
Effect of Bank Materials on Incision Assume that QS Q s d 50 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?
Provides Only Limited Insight QS Q s d 50 = unit weight of water Q = water discharge S = bed or energy slope Q s = bed-material discharge d 50 = 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)
Disturbing a Sand-Bed Channel Slope = 0.005 Initial width/depth ratio = 13.5 Assume that QS Q s d 50 becomes un-balanced Q s d 50 = 0.5 * capacity
Energy Dissipation for Different Boundary Materials Do each of these channels reach equilibrium similarly? DAYS FROM START OF SIMULATION 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 0 0.005 0.0005 0.00005 @ 0.90 @ 0.87 @ 0.80 Energy adjustment is similar at given slope because of an equal, but excessive amount of flow energy relative to sediment supply.
Styles of Adjustment for Different Boundary Materials DAYS FROM START OF SIMULATION Disturbance: Upstream sediment supply cut in half
Adjustments for Different Boundary Materials Response to similar disturbance: Sediment supply = 0.5 * capacity From Simon and Darby (1997)
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*? * For a given initial slope of 0.005 m/m It depends! 0.4 – 3.5 m 0 – 13 m 5.6 – 16.4
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…
Uses and Misuses: Classification vs. “Natural Channel Design” 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 Uses 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 Misuses
Problems with Application of Classification Definition of bankfull level, particularly in unstable systems. “Bankfull” discharge and the dimensions represented by hydraulic geometry refer to stable channels. In unstable channels, they are changing with time.
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
Implications for a Form-Based System: Bed or Channel Material? (two different populations) CC National Sedimentation Laboratory
From Rosgen (1996); Fig. 5-3 Channel/Bed Material CC
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, using guidance to proportionately sample pools and riffles (p. 5-27, Rosgen, 1996):
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 C5 These two C5’s represent very different transport regimes CC
Perhaps Explains Why… From Rosgen (2001) “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 CC
Forcing a Form-Based System to Describe Process From Rosgen (2001) E to E C to C; C to Bc; C to D B to B Eb to B If not, what does this mean for the “Natural Channel Design” approach since “…stream classification does not attempt to predict…stability…” Rosgen (2006) “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) Can this be predicted a priori? CC Can this truly be quantified?
From Rosgen (2001) From Rosgen (2006) And Aren’t Most of These Similar? Widening Filling Incision +
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
A Rapid Means of Evaluating Thousands of Streams was Needed 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 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
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
“…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…
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
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!
Hotophia Creek, MS 1953 1977 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?
Dynamic System? How would a practitioner address this situation for a potential mitigation project? “Reference reach”Unstable reach Unstable reach is 100 m from “reference reach
A Tiered Approach Reconnaisance Level: 1. Use form to define dominant processes and relative stability. Determine if the instability is localized or systemwide (scope) from rapid geomorphic assessments (RGAs), BEHI, gauging station records, dendro- chronology, air photos. Identify the problem not just the symptom. 2. Use regional, historical flow and sediment transport data to define transport conditions (rates) for stable streams in the region.
A Tiered Approach If the problem is localized (ie. Bridge constriction; local structure; livestock impacts; deflected flow) the practitioner has more options, including a “reference-reach” approach. But you can just as easily use a deterministic approach that is based on implicitly analyzing the specific processes (ie. bank instability).
Applied (Driving) Forces versus Resisting Forces Hydraulic processes (bed, bank toe) Geotechnical processes (bank mass) Quantify the Variables that Control the Processes We have the tools. It’s not hard or very time consuming!
Bank-Stability Model 2-D wedge-failure model 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
Hydraulic Bank-Toe Erosion Click this button to export eroded profile to Option A in Input Geometry worksheet ‘Toe Erosion Step 2’ worksheet Results
Planar Failures Partly controlled by failure plane angle Based on reach length Based on constituent concentration
Cantilever Failures National Sedimentation Laboratory
A Tiered Approach However, If the problem is systemwide instability, or in an urban setting, the practitioner had better obtain a complete quantitative understanding of hydrology, magnitudes and trends of adjustment processes, as well as the absolute and relative resistance of the boundary materials to erosion by hydraulic and geotechnical forces. If this is the case, then the practitioner needs to rely on validated numerical models, populated with field data to predict response and stable geometries.
A Tiered Approach Analytic Level: Static and Dynamic Numerical Modeling 1. Collect data to define the variables that control processes (force and resistance) 2. Use the best available numerical models for prediction We cannot ignore the watershed and its delivery of energy and materials to the channel system. In fact, changes to the watershed may be the cause (problem) of the channel instability. An upland model that provides flow and sediment loadings as the upstream boundary condition should be coupled with a deterministic channel-process model (that also handles mass failures). This way changes at the watershed level can be incorporated into potential channel effects
NSL Channel-Model Capabilities ProcessBank Stability- Toe Erosion CONCEPTS Hydraulic bank erosion Bank stability Bed erosion Sediment transport Vegetation effects ‘Hard’ engineering Channel evolution Rapid Assessments
CONCEPTS Hydraulics –Unsteady, one-dimensional, i.e. dynamic inputs Sediment transport and bed adjustment –Non-equilibrium sediment transport – excess settles Streambank erosion –Fluvial erosion and mass failure simulated Instream structures: culverts, bridge crossings, grade control structures HEC 6 Hydraulics –Steady, one-dimensional, i.e. steady-state only Sediment transport and bed adjustment –Equilibrium sediment transport – excess dumped Streambank erosion –No bank processes simulated Instream structures: culverts, bridge crossings, grade control structures
Sediment Sources and Fate Sediment can come from the surrounding area, banks, bed or upstream When excess sediment is entrained, the surplus settles out based on particle size, rather than being simply instantaneously dumped
Streambank-Erosion Modeling Combination of hydraulic erosion and mass failure Hydraulic erosion of cohesive soils is expressed by an excess shear stress relation Bank stability is expressed by a Factor of Safety
Summary and Conclusions Whether disturbances are “natural” or anthropogenic, occur at slow rates over long periods of time or are catastrophic and instantaneous, incision occurs because of an imbalance between sediment supply and transporting power. Resistance of the boundary to hydraulic and geotechnical forces provide partial control of adjustment processes and stable channel morphologies. Restoration has to be in the context of the condition of the watershed and the channel system… spatial and temporal aspects of the instability Restoration of an unstable reach within an dynamic, unstable system will not likely be successful An energy- or deterministically-based approach provides a reliable means of analyzing adjustment processes and predicting stable-channel geometries in unstable systems.