Changing perspectives: the river as a place ... the river as a thing What kind of thing? Rivers are: LARGE DYNAMIC •Geomorphic systems ...because they move.

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Transcript Changing perspectives: the river as a place ... the river as a thing What kind of thing? Rivers are: LARGE DYNAMIC •Geomorphic systems ...because they move.

Changing perspectives: the river as a place ... the river as a thing

What kind of thing?

Rivers are: LARGE DYNAMIC •

Geomorphic systems

•

Hydrologic systems

...

because they move and shape the landscape they flow through.

...

because they participate in regional water cycling.

•

Ecological systems

...

because they support a diverse and highly adapted living community

CONSEQUENCE 1:

Rivers and the regional landscape are intimately intertwined

•River basins are larger than most elements of the landscape mosaics they drain. Channel systems cross and integrate many different terretrial ecosystem, climatic, geologic, land-use and political boundaries.

•For us this means we will always need to be aware of how regional physiography, landuse patterns, and climate shape local river environments.

CONSEQUENCE 3:

Rivers have their own distinctive, self-generating behavior.

•Rivers aren’t physically passive systems, they continually do “work” on their own channels and on the landscape they flow through.

•Whether we choose to work with or against the natural behavior of a river, understanding how a river behaves (and why) is a necessary ingredient for success. “Managing” rivers without an understanding of how they work is a recipe for disaster.

CONSEQUENCE 4.

A systems perspective (i.e. seeing the river as a complex whole) and a willingness to hop disciplinary boundaries is absolutely essential to really understanding rivers.

•What we know about rivers has been historically divided-up between a number of technical disciplines. Traditional disciplinary boundaries frequently fragment our view and limit our understanding. Understanding river ecosystems requires bridging multiple fields each with a slightly different technical jargon….but its worth it!!

Regional Climate Basin Hydrology Fluvial Geomorphology Channel Hydraulics Channel Structure Regional Landscape Structure Ecology

Topic 1.

Fluvial Geomorphology

•Rivers (channel networks) are both water and earth moving systems: By eroding, transporting and depositing sediment they shape: (a) the landscape they occur in, and (b) their own channels •To accomplish geomorphic work rivers use

energy

The amount of

energy

available for geomorphic work is proportional to both the amount (mass) of water being moved in the channel (discharge: Q), and to the rate of fall (slope) of the channel.

10QS

Hydraulics

vel 1 depth 1 elev 1 vel 2 depth 2 elev 2 Energy loss =

total energy 1 - total energy 2

= potential energy available to do work (power) Power generated (watts/meter) =

Specific or

unit stream power

Total stream power 10QS [ kilowatts m -1 (length)] specific stream power 10QS/w [kilowatts m -2 (area)] Stream A Q=100 cms S= .01 m/m Power tot W= 20 m = 10 kilowatts m -1 Power unit =.5 kilowatts m -2 Stream B Q=10000 cms S= .0001 m/m Power tot W= 2000 m = 10 kilowatts m -1 Power unit =.005 kilowatts m -2 M.J. Wiley 1999 all rights reserved

How is power utilized in the channel?

• overcoming internal and external frictional resistance to water flow • sediment erosion and transport (usually < 5%)

Fluid Flow Basics: internal friction Viscosity is a fluid’s internal •dynamic viscosity of water (m) = .0018/ (1+.0337*T+.00022*T 2 ) T= water temp Celsius •kinematic viscosity ( n ) = m/ density •eddy viscosity ( n e ) M.J. Wiley 1999 all rights reserved

Fluid Flow Basics: open channel Reynold’s # Re# = (Velocity * Depth) / ( n ) transition from laminar to turbulent generally in the range: Re# = 500-2000

No slip

Where ( n ) is kinematic viscosity all length units equal can be used to classify/describe

Fluid Flow Basics: external friction

In addition to internal resistance (friction)

(external) frictional resistance along the channel perimeter

Velocity

Fluid Flow Basics: external friction

Chezy Equation (1769)

V =

 RS where

is a system-specific constant S = slope and R = hydraulic radius R = Wetted Perimeter For 2 channels of equal Cross-sectional Area and Slope: wetted perimeter High area to perimeter ratio (R)

Low Friction

= Higher Velocity Low area to perimeter ratio (R)

Higher Friction

Fluid Flow Basics: external friction

Manning’s Equation (1889) Velocity = 1 n

2 3

S

1 2 R = Wetted Perimeter For 2 channels of equal Cross-sectional Area and Slope: High area to perimeter ratio (R) hydraulically smooth (n is little) = Highest Velocity High area to perimeter ratio (R) Hydraulically rough (n is large) = Higher Velocity Low area to perimeter ratio (R) hydraulically rough (n is large) =Lowest Velocity M.J. Wiley 1999 all rights reserved

Velocity = 1 n

2 3

S

1 2 R = Wetted Perimeter For 2 channels of equal Cross-sectional Area and Slope:

Some typical Manning's coefficient values:

Smoothed concrete channels: .012 Vitrified clay channels .015

Straight unlined earth channels .020

Larger Rivers and earth channels with some vegetation .025

Winding natural streams and weedy artificial channels .035

Winding streams with rocky beds;rivers with vegetated banks .040

Natural streams with very weedy channels and vegetated banks .075 - .150

Darcy-Weisbach Eq

V =  8 gRS/ff 1/  ff = - c log a (R/D x ) where a,c are coefs and D x is the diameter exceeded x % of the time in a random bottom sample ( usually 84-90%) (R/D x ) is called the “relative roughness”; ff is a roughness coef.

R is the hydraulic radius.

Fluid Flow Basics: Summary

Flow velocity ~ RS n

Erosion: basic concepts

lift force

flow

gravitational force shearing force frictional force

Erosion: basic concepts

gravitational force shear force frictional force

Particle erosion and deposition depends upon the balance of relevant forces

simplify by focusing on shear force (

)

t crit is defined as the threshold force which exceeds gravitational and frictional resistance and results in

incipient motion

Erosion: basic concepts

gravitational force shear force frictional force

Particle erosion and deposition depends upon the balance of relevant forces If t If t > t crit particle will be transported < t crit particle will not be transported t/t crit :: mode of transport (suspended/bed) M.J. Wiley 1999 all rights reserved

Shields critical shear equation

t cr = Q 9.807 ( density part where - density water ) d Q t cr is in N/m2 d is particle diameter in mm is a dimensionless shear parameter for hydraulically rough channels Q is most often estimated as 0.06

But varies with hydraulic roughness Normal beds: “settled” bed with uniform or random arrangement of grain sizes: .035-.065

Loose beds: quick sands and gravels with large water-filled voids: .01-.035

Packed beds: smaller material filling voids between larger components: .065-.1

Highly embedded with fines: >.1

(Carson & Griffiths 1987) for hydraulically smooth channels Q is much higher: 0.8-3.0

Erosion: basic concepts

Shields critical shear equation

Erosion: basic concepts Shear stress (

tractive force

) on channel walls and floor are reflected in velocity gradients dy t dv = ( ) 

y dy dv low shear high shear

Shear force is highest where water velocity increases rapidly

*Note that velocity is always zero at the channel wall: this is called the no-slip condition and is a fundamental assumption of fluid dynamics m =dyn. And  =eddy viscosity M.J. Wiley 1999 all rights reserved

Erosion: basic concepts Shear stress (

tractive force

) at specific points on the bed can be measured using an accurate current meter. The velocity profile is directly measured and t calculated from a derived parameter called the

shear velocity

Erosion: basic concepts Average Shear stress (

tractive force

) on channel walls and floor are usually estimated using Leopold’s formula: t=r gRS [N/m 2 ] r= density of water (varies with temp.) g = gravitational accerlation [9.807 m/s] R = hydraulic radius [m] S = reach energy slope [~ channel slope]

Erosion Basics: How much force is available to move sediment?

Shear force is proportional to the near bed velocity gradient

Erosive force

t

~ Velocity

Ultimately 10QS places the upper limits on energy available to push water through the channel reach g r R bf S ~ 10Q 05 S/w

Incipient thresholds for movement

Unit power dissipation (Kw/m 2 )

Erosion: basic concepts •Sediment is eroded when there is sufficient power (and shear stress) to move available sediment particles. •Large particles (e.g. rocks) require substantial energy to move because they have greater mass. •Very small particles (clays and silts) also require alot of energy because of electrostatic cohesion between particles. •Mid-sized particles (e.g. sand) require the least energy and are threfore most easily eroded. •Once suspended in the river transport energy is roughly proportional to mass.

basic hydraulic concepts While 10QS places the upper limits on energy available to push water through the channel reach and Manning’s Eq expresses reach-scale constraints related to bed friction local velocity and therefore bed erosion is also shaped by: (1) channel shape effects (2) continuity constraints (3) specific energy constraints (4) down stream hydraulic controls (slope constraints)

basic hydraulic concepts: X-section constraints Channel shape itself has strong effects on shear force distributions Shape (1) focuses shear differentially across the channel wall (2) affects the frictional resistance of the channel [R,Manning’sEq] (3) generates local continuity effects

basic hydraulic concepts: continuity constraints

Hydraulic Continuity

Q = W x D x V = Velocity x Cross-sectional Area

V V V+

*cross-section mean values Q Q

A A+ A-

basic hydraulic concepts: specific energy constraints Local conservation of specific energy constraint: •E total = Z + D + V 2 /2g { ~ Bernoulli’s Eq.

}

•E

specific

= D + V

/2g

doesn’t change rapidly* If we treat E specific as a constant and solve for D or V note that for any discharge we get two alternate solutions (called

alternate depths/velocities

Deep & Slow or Shallow & Fast

subcritical or supercritical tranquil or shooting

*Conservation of momentum

basic hydraulic concepts: specific energy constraints

This result leads to a classification of flow with respect to Velocity

Froude number = V/  gD wave propagation velocity =  gD subcritical: Fr# < 1 supercritical: Fr# > 1 When Fr# =1, flow is said to be

critical

Under conditions of critical flow: • V 2 /2g = 1/3E specific • D=2/3 E specific • V=  gD • E specific is at a minimum possible for that Q • Q = A  gD

basic hydraulic concepts: specific energy constraints vel 1 depth 1 vel 2 elev 1 depth 2 elev 2 sub-critical critical/transition super-critical lower shears higher turbulent shear very high shear less erosive depth/direction unstable very erosive M.J. Wiley 1999 all rights reserved

basic hydraulic concepts: slope constraints vel 1 depth 1 Hydraulic control vel 2 elev 1 depth 2 elev 2

Backwater effects

Introducing the concept of Channel Equilibrium

A river is a complex system in which watershed hydrology(Q), channel hydraulics and sediment transport all interact dynamically. The tendency for these processes to achieve a balance (dynamic • equilibrium) is observed both:

locally-

where the channel cross section and bed form can be viewed as being the result of a balance between erosive power and channel wall resistance.

•and

at larger scales

- where channel planform [sinuosity, slope] and network pattern [topology] are adjusted to achieve a balance between available power generated by water transport, sediment erosion and transport, and channel margin resistance.

Channel equilibrium: channel Deposition point bar extension sedimentation pool filling Sediment Load Qs* d 50 Equilibrium produces a stable channel configuration channel Erosion bank erosion meander cutting vertical incision Stream Power

10QS

The balance between stream power and sediment load determines the stability of the river channel by favoring either bank erosion or sediment deposition (after Lane 1959).

Qs*d 50 ~ QS

Channel equilibrium: A

graded river

has just the right channel slope and cross-section shape ( W D V) to move the water & sediment load it carries. Its channel form is considered to be adjusted to the available water and sediment load and more or less stable in configuration. This implies a balance between available erosive force (power), work being done (sediment transport) and channel wall resistance (friction) M.J. Wiley 1999 all rights reserved

Channel equilibrium: What kinds of responses are possible in a river? How precisely might the river system respond to changes in discharge, or sediment load, or to channel alterations [e.g. dredging or straightening]? There are

three primary

ways in which a river Channel can adjust to an imbalance between power and load:

Adjustment of cross-section shape.

adjusted within the continuity constraint. Width, depth, area, radius can (each/all) be

Adjustments in horizontal channel pattern (planform).

Horizontal channel structure influences energy dissipation by controlling slope and hydraulic radius. Meandering, braiding, bar deposition, etc. are all possible responses when power and load are out of balance.

Adjustments in bed form/ roughness

. Another way to adjust local velocity is to make the channel more or less rough. Adjustments in the river bedform, e.g. dunes (which increase surface roughness and slow down discharge), riffle-pool sequences (elevational adjustments in bed and sites of energy dissipation) M.J. Wiley 1999 all rights reserved

Channel equilibrium:

widening vertical incision blowouts

Channel equilibrium: Basic forces relevant to particle erosion

lift force gravitational force shearing force frictional force

At a larger scale, channel erosion reflects the same basic principal: Where shear and lift forces exceed channel wall resistance...

Cross-section adjustment

Planform adjustment Planform refers to the 2-dimensional structure of a channel reach Basic channel planform types: Straight Meandering Braided Anastomosing

Planform adjustment Presque Isle 1.1 Iron 1.2 Sturgeon 2.0 Manistique 2.7

Lower Raisin drains 1.0

Manistee 3.2

Sinuosity

= channel length / valley length

Planform adjustment Meandering is a form of slope adjustment original fall: 3.5 ft original distance: 900 ft original slope: 0.0038 (ft/ft) channelized fall: 3.5 ft channelized distance: 300 ft new slope: 0.0167

Power increases by a factor of 3

Planform adjustment S = channel length / valley length S = 1.27

S = 1.15

S t r ai g h t = 1 M e a n d e r i n g > 1 . 5 T o r t u o u s M e a n d e r i n g = 4 S = 2.03

S = 1.80

1839 1888 1903 River Sid, Devon (Hooke 1977) 1958

Planform adjustment

Channel equilibrium: the fluvial system

Process Response Theory

From the idea of graded rivers evolved Process Response Theory (PRT). If a river’s channel form is at or near a dynamic equilibrium with its load, any alteration of Q, sediment load, or channel form will trigger a complex channel response to move the river towards a new equilibrium. PRT has become the foundation for modern channel engineering and restoration in the last 20 years +

discharge

(Q) climate -

hydraulic radius sinuosity

x-area + slope sediment erosion

transport

+ + +

stream power

Negative feedbacks within the

fluvial system

between

hydraulic

(blue),

channel shape graded river

(green) and

sediment transport

The rivers we see represent quasi-equilibrium solutions to the Qs*D50 ~ QS interaction there are limits to what is possible…. .2

.05

.04

.03

.02

.01

.005

.004

.003

.002

Too much power excessive degradation slope adjustment downwards .001

.0005

.0004

.0003

.0002

.0001

.00005

.00004

.00003

.00002

1 Too little power excessive aggradation slope adjustment upward 2 3 4 5 10 20 30 40 50 100 200 300 400 500 1000 2000 3000 Slope and stormflow relationships in natural Michigan river channels.

The Fluvial System: Do we understand enough to make predictions?

Schumm’s Model for channel response

Channel parameters:

Q=discharge w=width d=depth, L= meander wavelength S=slope Qs=sediment discharge Sin=sinuosity

Predictive relationships

Q+= w+, d+, L+, S Q- = w-, d-, L-, S+ Qs+= w+, d-, L+, S+, Sin-, (w/d)+ Qs-= w-, d+, L-, S-, Sin+ Q+, Qs+ = w+, d±, L+, S±, Sin-, (w/d)+ Q-, Qs- = w-, d±, L-, S±, Sin+, (w/d) Q+, Qs- = w±, d+, L±, S-, Sin+, (w/d) Q-, Qs+ = w±, d-, L±, S+, Sin-, (w/d)+

Primary relationships:

Schumm and Khan 1972 Experimental study of stream channels Geol. Soc. Amer. Bull. 83:1755-1770.

A Slope = .0008

Q=.15 cfs Meadering

thalweg

B Point bar deposition after fine sediment additions

C Slope = .0008

Point bars at lower Q D Braided channel

River adjustment : the issue of time

A more wholistic view of adjustment processes: Channel Evolution Model (Elliot1979,Schumm et al. 1984, Simon 1989) • •

headcutting nickpoints

Channel adjustment is a spatially & temporally complex process Characterized by cycles of migrating degradation & aggradation: •Trigger event •Base level lowering •Network rejuvenation •Re-equilibration of sediment discharge, stream power, and transport competence

C.E.M.

Basic stages

Longitudinally (Schumm et al.1984 ) Serially (Rosgen 1996)

I II III IV V

Channel equilibrium:

Adjustments in bed form/ roughness. Another way to adjust local velocity is to make the channel more or less rough. Adjustments in the river bedform, e.g. dunes (which increase surface roughness and slow down discharge), riffle-pool sequences (elevational adjustments in bed and sites of energy dissipation)

Adjustment of cross-section form.

Width, depth, area, radius can (each/all) be adjusted within the continuity constraint.

Adjustments in horizontal channel pattern (planform).

Adjustments in longitudinal profile (profile form).

The long-term processes of landscape erosion and sediment transport shape the river profile and control large scale patterns in power distribution.

Adjustments in River Network Structure.

Network adjustment

3 3 1 1 2 Describing the Network Position of channel units 2 1 1 2 2 2

(Horton-Strahler) Stream Order

4 2 6

Link number

Network adjustment Horton’s Basin Laws N x+1 =

L x+1 =

N x (Horton 1945) L x (Hortom 1945) S x+1 =

S x (Horton 1945 ) DA x+1 =

Da x (Schumm 1956) [ linearized forms ] ln N = b SO; where exp( b) ln L = b SO; where exp( b) is

ln S = b SO; where exp( b) is

ln DA = b SO; where exp( b) is

Network adjustment area intercept Horton’s Basin Laws -related concepts •

bifurcation ratio

= 1 /

DYNAMIC •

drainage density

= S L/ Area ( units: l/l 2 ) = S (N i * L i ) / total area •

constant of channel maintenace

= intercept on log-log plot of L vs log A

River adjustment : some real-world examples….

Hunt Creek Sand Experiment treated section- slope response Bedload increment 4x daily for five years from ~20ppm to 80-110 ppm

year

1972 1976 1980

up elev

rel ft

0.5

1.25

0.25

dn elev

rel ft

0 0 -0.25

length

4000 4000 4000

Slope

0.000125

0.000313

0.000125

change in slope x depth (R)

1 2.5

1 1.5

1.19

1.6

tractive force

N/m2

0.55934911

1.109375735

0.596639051

Insect standing crop Brook trout standing crop Control Aggradation Degradation Post-Tretmentt Control Aggradation Degradation Post-Tretmentt 2.0

1.5

1.0

1972 pre-treatment 1976 after 5 years treatment 1980 after 5 years recovery

year

1972 1976 1980

up elev

rel ft

0.5

1.25

0.25

dn elev

rel ft

0 0 -0.25

length

4000 4000 4000

Slope

0.000125

0.000313

0.000125

change in slope x depth (R)

1 2.5

1.5

1.19

1.6

tractive force

N/m2

0.55934911

1.109375735

0.596639051

0.5

0.0

DEPOSITION SCOUR -0.5

1000 2000 3000 4000 5000 DISTANCE DOWN STREAM [feet]

Urban Storm Drainage & channel design .2

.05

.04

.03

.02

.01

.005

.004

.003

.002

.001

.0005

.0004

.0003

.0002

.0001

.00005

.00004

.00003

.00002

1 2 3 4 5 Glendale channel seg upstream of intake 10 20 30 40 50 100 200 300 400 1000 2000 3000 ANN10

Changing perspectives: the river as a place ... the river as a thing What kind of thing? Rivers are: LARGE DYNAMIC •Geomorphic systems ...because they move.

Transcript Changing perspectives: the river as a place ... the river as a thing What kind of thing? Rivers are: LARGE DYNAMIC •Geomorphic systems ...because they move.

Fluvial Geomorphology

energy

Hydraulics

How is power utilized in the channel?

Velocity

Chezy Equation (1769)

Low Friction

Higher Friction

Manning’s Equation (1889) Velocity = 1 n

S

Velocity = 1 n

S

Some typical Manning's coefficient values:

Darcy-Weisbach Eq

Flow velocity ~ RS n

Erosion: basic concepts

Shields critical shear equation

Shields critical shear equation

Erosive force

~ Velocity

Hydraulic Continuity

•E

= D + V

/2g

Deep & Slow or Shallow & Fast

graded river

widening vertical incision blowouts

C.E.M.

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