Post on 11-Jan-2016
description
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.
CNational 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 SystemObion 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)
Adjustment Processes
Tennessee
NebraskaMississippi
Mississippi
Case Study: Sub-Alpine System
Upstream “natural” disturbance in a coarse-grained, non-cohesive bank
system causing an increase in transport capacity (QS) and a
decrease in resistance (d50)
Adjustment ProcessesAdjustment Processes
Trends of Bed-Level Change
Elk Rock Reach
Salmon B Reach
Mt. St Helens
W. Tennessee
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
Trends of Bed-Level Change
Trends of Bed-Level Change
Coarse-grained material for aggradation derived
from bank sediment.
Widening
But why are they so
different ?
Resistance
Incision creates the conditions for bank
instability and widening by creating higher,
steeper banks
Idealized Adjustment Trends: Mid Continent
2,500 km of streams in W. Iowa (1993-4)
Data from Hadish (1994)
6% stable
80% with unstable
banks
-2
0
2
4
6
8
10
12
0 20 40 60 80 100 120 140 160
DISTANCE FROM MOUTH, IN KILOMETERS
CH
AN
NE
L B
ED
-LE
VE
L L
OW
ER
ING
, IN
ME
TE
RS
Pisa Plain Upper Valdarno
Florence Plain
Lower Valdarno
1845-1987
1845-1960
1960-19871845-1952
1952-1978
1845-1978
Bed-Level Adjustment:
Arno River, ItalyLOWER VALDARNO
Cross-section: 284River k ilometer: 59.01
6
7
8
9
10
11
12
1840 1860 1880 1900 1920 1940 1960 1980 2000
YEARS
EL
EV
AT
ION
, IN
ME
TE
RS
1
2
3
4
Phase II
Phase I
UPPER VALDARNOCross-section: 867
River k ilometer:160.54
128
129
130
131
132
133
134
135
136
1840 1860 1880 1900 1920 1940 1960 1980 2000
YEARS
EL
EV
AT
ION
, IN
ME
TE
RS
1
2
3
4
Phase II
Phase I
Phases of Degradation Since 1900
Phase I: Land use changes with a reduction in sediment supply
Phase II: Gravel mining and upstream dam construction
LOWER VALDARNOCross-section: 284
River k ilometer: 59.01
6
7
8
9
10
11
12
1840 1860 1880 1900 1920 1940 1960 1980 2000
YEARS
EL
EV
AT
ION
, IN
ME
TE
RS
1
2
3
4
Phase II
Phase I
UPPER VALDARNOCross-section: 867
River kilometer:160.54
128
129
130
131
132
133
134
135
136
1840 1860 1880 1900 1920 1940 1960 1980 2000
YEARS
EL
EV
AT
ION
, IN
ME
TE
RS
1
2
3
4
Phase II
Phase I
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 DissipationChannels 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
v12/2g
v22/2g
y1
y2z1
z2
1 2
hf
E = z + y + v2/2g
hf = (z1 + y1 + v12/2g) - (z2 + y2 + v2
2/2g)
Channel bed
Water surface
Energy slope: Se = hf / L
L
Processes That Effect Components of Total Mechanical Energy (E)
• z:
• y:
• v2/2g:
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 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?
Provides Only Limited Insight
QS Qsd50= 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)
Disturbing a Sand-Bed Channel
Bank material Bed d50
(mm) Bank cohesion
(kPa) Friction angle
(o) Sand content
(%) Sand 1.0 4.0 32.5 100 Silt 1.0 7.5 32.5 20 Clay 1.0 40.0 32.5 10
• Slope = 0.005
• Initial width/depth ratio = 13.5
• Assume that QS Qsd50 becomes un-balanced
• Qsd50 = 0.5 * capacity
Energy Dissipation for Different Boundary Materials
Do each of these channels reach equilibrium similarly?
DAYS FROM START OF SIMULATION0 100 200 300 400 0 100 200 300 400 0 100 200 300 400
0
0.0050.00050.00005 @ 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 ClassificationDefinition 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)
CNational Sedimentation Laboratory
From Rosgen (1996); Fig. 5-3
Channel/Bed Material
C
However…
From Rosgen, 1996; Fig. 5-2 C
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
C
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
C
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?
C
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 issuesIgnores bank processes, flow variability and sediment
contribution from banks
• Deterministic: physically based; cause and effectQuantifies 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!
This is Our World of Disturbed Systems
Hotophia Creek, MS1953
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, uw,
k, c
Confining pressure
Tensiometers(pore pressure)
shear surface
12/29/97 01/05/98 01/12/98 01/19/98 01/26/98 02/02/98
FAC
TO
R O
F SA
FETY
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
RIV
ER
STA
GE, I
N M
ETER
S A
BO
VE
SEA
LEV
EL
80
81
82
83
84
85
BFactor of safety
Effect of confining pressure
Bank failures
Stage WA
TE
R L
EV
EL
, M
Hydraulic Bank-Toe Erosion
Click this button to export eroded profile to Option A in Input Geometry worksheet
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00STATION (M)
ELE
VA
TIO
N (
M)
Base of layer 1
Base of layer 2
Base of layer 3
Base of layer 4
Base of layer 5
Eroded Profile
Water Surface
Initial Profile
‘Toe Erosion Step 2’ worksheet
Results
Select material types, vegetation cover and water table depth below bank top(or select "own data" and add values in 'Bank Model Data' worksheet)
Bank top Reach LengthLayer 1 Layer 2 Layer 3 Layer 4 Layer 5 vegetation cover (age) (m)
100Constituent
Vegetation safety margin concentration (kg/kg)
50 0.001
Water table depth (m) below bank top4.00
Own Pore Pressures kPa
Pore Pressure From Water Table
Layer 1 -34.34
Layer 2 -24.53
Layer 3 -14.72
Layer 4 -4.91
Layer 5 4.91
Factor of Safety
1.09Conditionally
stable
57.5 Shear surface angle used Failure width 2.35 m
Failure volume 487 m3
Sediment loading 814176 kgConstituent load 814 kg
Gravel Angular sand Rounded sand Silt Stiff clay
Gravel Angular sand Rounded sand Silt Stiff clay
Gravel Angular sand Rounded sand Silt Stiff clay
Gravel Angular sand Rounded sand Silt Stiff clay
Gravel Angular sand Rounded sand Silt Stiff clay
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
STATION (M)
ELE
VA
TIO
N (
M)
bank profile
base of layer 1
base of layer 2
base of layer 3
base of layer 4
failure plane
water surface
water table
Use water table
Input own pore pressures (kPa)
None
Export Coordinates back into model
Planar FailuresFactor of Safety
1.09Conditionally
stable
Failure width 2.35 m
Failure volume 487 m3
Sediment loading 814176 kgConstituent load 814 kg
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
Process Bank 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
Discretizing the Stream Corridor
Inputs - Cross Sections and Rating Curves
Variable Roughness Elements
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
Sediment Transport
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
S i
N i
Wi
fa ilu re surface andbank profile a fter fa ilure
assum ed groundw ater surface
actua l groundw ater surface
soil
laye
r 1
soil
laye
r 2
soil
laye
r 3
la tera l erosion andbank profile a fter erosion
slice i
cKE
Forces Driving
Forces ResistingFOS
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.