fluvial processes “the great sculptor of the landscape”

Fluvial Processes

“the great sculptor of the landscape”

I. The River ChannelA. Basic Mechanics

1. Laminar Flow2. Turbulent Flow

I. The River ChannelA. Basic Mechanics

1. Laminar Flow2. Turbulent Flow3. Reynolds Number

Re = driving forces = V D p = (velocity * depth * fluid density) resisting forces u (fluid viscosity)

laminar < 500 700 < turbulent \

I. The River ChannelB. Flow Equations and Resisting Forces

Discharge = velocity * depth * widthQ = V*A

1. Manning Equation

1.Manning Equation

v = R 2/3 S ½nWhere

v = average flow velocityr = hydraulic radiuss = channel slope (unitless)n = Manning roughness coefficient

R = A/P A = Area P = Wetted Perimeter

Q = A R 2/3 S ½

N

WhereQ = average flow dischargeA = area of channelR = hydraulic radiusS = channel slope (unitless)n = Manning roughness coefficient

R = A/P A = Area P = Wetted Perimeter

I. The River ChannelB. Flow Equations and Resisting Forces

1. Manning Equation2. Chezy Equation

V = C *(RS)1/2

II. Sediment in ChannelsA. Transportation1. Suspended load2. Bedload

B. Entrainment and Erosion

II. Sediment in ChannelsA. Transportation1. Suspended load 2. Bedload3. Washload

B. Entrainment and ErosionC. Deposition

II. Sediment in ChannelsA. Transportation1. Suspended load 2. Bedload3. Washload

B. Entrainment and ErosionC. Deposition

“ a battle between velocity and gravity”

III. The Quasi-Equilibrium Condition

III. The Quasi-Equilibrium Condition A. Hydraulic Geometry


Q = V*A


Q = V*AQ = V * w * d


Q = V*AQ = V * w * d

w = aQb

d = cQ f

v = kQ m

A. Hydraulic Geometry

“at a station trends”

M = 0.26

M = 0.4

M = 0.34

A. Hydraulic Geometry

“distance downstream trends”

M = 0.5

M = 0.1

M = 0.4

Q W d V10.1 33.3 0.71 0.4311.3 31.5 0.67 0.54

15 38 0.79 0.528.9 38 0.94 0.8156.8 40.7 1.1 1.27106 29.1 2.14 1.7119 44.5 1.95 1.37125 42.5 1.58 1.86132 42 1.9 1.66133 30 2.08 2.13181 43 1.9 2.22201 43 2.04 2.29312 55 2.24 2.54494 70 6.04 1.17503 66 3.47 2.2629 73 3.7 2.33674 71 4.55 2.09

1100 72.5 4.97 3.061740 75 5.56 4.172930 215 3.38 4.04

Distance Downstream

y = 14.904x0.2375

y = 0.2754x0.395

y = 0.2448x0.3669

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000 3500

Q - Discharge

wid

th, d

epth

and

vel

ociti

es

y = 0.2754x0.395

y = 0.2448x0.3669

y = 14.904x0.2375

0.1

1

10

100

1000

1 10 100 1000 10000Discharge (cfs)

wid

th,

dept

h an

d ve

loci

ty (

ft,s)

W

D

V

B. The Influence of Slope

Slope(ft/mi)

B. The Influence of Slope

III. The Quasi-Equilibrium ConditionC. Channel Shape ….in cross section:

F = 255M-1.08

Where F = width to depth ratio (W/D)M = % silt and clay in channel

IV. Channel Patterns

….in plan view (bird’s eye)

StraightMeanderingBraided

Transition between StraightAnd Meandering is whenSinuosity is 1.5

IV. Channel Patterns

From: Montgomery and Buffington, 1997

(pools and riffles)

(pools and riffles)Riffles are spaced ~ 5-7 timesthe channel width

(pools and riffles)

(pools and riffles)

`

IV. Channel PatternsMeanders…….

http://www.where-rv-now.com/Log/2004/Jasper/BlueMeander.jpg

IV. Channel Patterns Meanders…….

Meanders…….

A few final words on stream form….

braided


braided

Anastomosing channels

A few final words on stream form….The factors responsible are……


Why do channels take on a certain pattern?????



It’s primarily due to the relationship between slope and discharge(or velocity)



It’s primarily due to the relationship between slope and discharge(or velocity)

The ole’ Chezy Equ:V = C *(RS)1/2

V = C *(DS)1/2

or


V = C *(DS)1/2

It’s primarily due to the relationship between slope and discharge (or velocity)

The ole Chezy Equ:

V = velocityC = roughnessD = depth of flowS = slope of channel

V = C *(DS)1/2

V = velocityC = roughnessD = depth of flowS = slope of channel

The change in slope is a RESPONSE to changes in channel shape,NOT a cause of braiding

Increasing the slope of a stream DOES NOT cause it to braid.

V. Rivers, Equilibrium, and Time“the profile of streams”

knickpoints

V. Rivers, Equilibrium, and Timethe graded river: (page 227)

V. Rivers, Equilibrium, and Time

the graded river: (page 227)“one in which, over a period of years, slope is delicately

adjusted to provide, with available discharge and with prevailing channel characteristics, just the velocity required for the transportation of the load supplied from the drainage basin. The graded stream is a system in equilibrium; its diagnostic characteristic is that any change in any of the controlling factors will cause a displacement of the equilibrium in a direction that will tend to absorb the effect of the change.” Mackin, 1948

the graded river: Lane Diagram

Factors affecting stream morphology

• Width• Depth• Slope• Velocity• Discharge• Flow resistance• Sediment size• Sediment load

Leopold et al (1964)


Responses from adjusting load and discharge…



ACTIVITY RESPONSE SLOPE




Increase in load ??? ???Decrease in load ??? ???Increase in discharge ??? ???Decrease in discharge ??? ???




Increase in load Aggradation increaseDecrease in loadIncrease in dischargeDecrease in discharge




Increase in load Aggradation increaseDecrease in load Degradation decreaseIncrease in dischargeDecrease in discharge




Increase in load Aggradation increaseDecrease in load Degradation decreaseIncrease in discharge Degradation decreaseDecrease in discharge




Increase in load Aggradation increaseDecrease in load Degradation decreaseIncrease in discharge Degradation decreaseDecrease in discharge Aggradation increase


The reservoir problem…..



Chris Greene LakeCharlottesville

fluvial processes “the great sculptor of the landscape”

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