Channel Disturbance and Evolution: Controls and Implications for Stream Restoration

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 ReachSalmon 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

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10

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0 20 40 60 80 100 120 140 160DISTANCE FROM MOUTH, IN KILOMETERS

CH

AN

NEL

BED

-LEV

EL L

OW

ERIN

G,

IN M

ETER

S

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 kilometer: 59.01

6

7

8

9

10

11

12

1840 1860 1880 1900 1920 1940 1960 1980 2000

YEARS

ELE

VA

TIO

N, I

N M

ETE

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

Phases of Degradation Since 1900Phase I: Land use changes with a reduction in sediment supply

Phase II: Gravel mining and upstream dam constructionLOWER 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

ELE

VA

TIO

N, I

N M

ETER

S

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

ELEV

ATI

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, IN

MET

ERS

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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 waterQ = water dischargeS = bed or energy slopeQs = bed-material discharged50= 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 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!

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 ApproachIf 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

FACT

OR O

F SAF

ETY

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

RIVE

R ST

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IN M

ETER

S ABO

VE

SEA

LEVE

L

80

81

82

83

84

85

BFactor of safety

Effect of confining pressure

Bank failures

Stage WA

TER

LEV

EL, M

Hydraulic Bank-Toe Erosion

Click this button to export eroded profile to Option A in Input Geometry worksheet

-1.00

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5.00

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0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00STATION (M)

ELEV

ATIO

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

1.09 Conditionally stable

57.5 Shear surface angle used Failure width 2.35 mFailure 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

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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 tableInput own pore pressures (kPa)

None

Export Coordinates back into model

Planar FailuresFactor of Safety

1.09 Conditionally stable

Failure width 2.35 mFailure 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 CapabilitiesProcess Bank Stability-

Toe ErosionCONCEPTS

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 FateSediment 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 after fa ilure

assum ed groundw ater su rface

actual g roundwater surface

soil

laye

r 1so

il la

yer 2

soil

laye

r 3

la te ra l eros ion andbank p rofile a fter erosion

slice i

cKE

Forces DrivingForces 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.