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Basins and Fluvial Process

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Title: Basins and Fluvial Process


1
Basins and Fluvial Process
  • Drainage Basins and Networks
  • Channel Initiation
  • Basin Denudation
  • Fluvial Process
  • feedback and response
  • geomorphic work
  • bankfull channel
  • hydraulic geometry
  • open channel toolbox

2
Watershed Networks
  • Watershed network comprised of
  • headwater and network systems
  • First and second order streams often comprise 70
    of the stream network (Benda et al, 1992)
  • High ecological value
  • Stream networks defined by
  • Natl Hydrography Dataset (1100,00)
  • Terrain analysis
  • (area, area-slope area-length thresholds)

3
Effects of low order channels on downstream
reaches in the network
  • Synchronous (or asynchronous) inflows of water,
    sediment, nutrients, and organic matter create a
    variety of channel conditions and biological
    assemblages
  • Connectivity of headwater systems to downstream
    reaches affects the cumulative and dispersed
    nature of material transport processes
  • Gomi, et al, Understanding processes and
    downstream linkages of headwater systems,
    BioScience, Oct. 2002, vol. 52, no. 10

4
Drainage Basins
  • Stream order (Strahler)
  • 1st order no tributaries
  • 2nd two 1st

5
Limitations of Horton-Strahler ordering
  • Order does not express the intuitive size of a
    catchment very well

6
Magnitude an alternative approach (Shreve)
  • Magnitude may give a better idea of the size of
    the network
  • Shreve explored all the possible network
    topologies for a given magnitude

7
Magnitude and stream order Magnitude of a basin
is the number of first order or exterior links
in a catchment. Magnitude correctly emphasizes
identifying where the channel begins. Stream
order is commonly done on nearly arbitrary
network scales, and therefore means little.
Hortons laws, which are derived from analysis
of stream orders, have no physical meaning.
M 9
N 2M-1 N number of links (exterior interior)
For channel head theory, see
Istanbulluoglu et al. JGR 2005 for gully head
theory Tucker and Bras, 1998, WRR for theory to
landscape evolution
8
14 m across
3200 m across
Landscapes consist of ridge and valley topography
at all scales, but only finest scale reveals the
actual valley network and defines the transition
between hillslopes and valley
Montgomery and Dietrich, 1992, Science
9
c
c
a
b
b
d
Channel networks are of finite extent. The
spacing of the finest-scale valleys depends on
the competition of valley cutting and hillslope
eroding processes. Fractal analysis breaks down
at the channel-hillslope transition.
10
Drainage Density
  • Low (x km/km2)
  • Moderate
  • Why?
  • Climate
  • Substrate
  • Slope

11
Badlands
  • Very high drainage density!
  • Why?

12
Channel Initiation
  • Channel head the upstream limit of concentrated
    water flow between banks (Dietrich and Dunne,
    1993)
  • a major boundary between hillslopes and channels
  • pivot point in sediment transport between
    diffusive process and incisive process
  • Channel initiation requires runoff
  • Channel initiation occurs by
  • saturated overland flow
  • seepage erosion
  • shallow landsliding

13
Channel Head Location and Topography
Montgomery and Dietrich, 1989
14
Channel head threshold transition between
hillslope and channel processes
15
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16
Basin Denudation
  • Three processes
  • weathering physical/chemical breakdown of
    bedrock and partially weathered material
  • slope weathered products are moved downslope by
    gravity and slope wash
  • fluid-transfer transport by water, air, or ice

17
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18
Denudation
  • Denudation is the result of the linkage between
    sediment yield and hillslope weathering
  • Denudation ? Erosion
  • f (precipitation, vegetation, basin size,
    elevation, relief, rock type, human intervention)

19
Denudation Rates
units of denudation rate m/yr units of sediment
yield kg/m2yr
20
Denudation
  • How measured
  • Reservoir volume
  • Stream measurements of suspended, dissolved, and
    bedload
  • Why the difference between reservoir and gage
    data?
  • Effects of land use?
  • Universal Soil Loss Equation
  • RUSLE

21
Sediment Delivery
  • Efficiency
  • Decreases with increasing basin area why?
  • Evolution
  • Effects of land use
  • How will this affect streams?

22
Fluvial Process
  • Although the river and hillslope do not resemble
    each other at first sight, they are only extreme
    members of a continuous series and when this
    generalization is appreciated one may fairly
    extend the river all over its basin and up to its
    very divide. Ordinarily treated the river is
    like the veins of a leaf broadly viewed it is
    the entire leaf
  • William Morris Davis (1899)

Channel morphology f(rate and magnitude of
delivery of water, sediment, wood) Process
creates form!
23
Matter and Energy Exchange in Geomorphic Systems
Credit Ian Walker, Univ. of Victoria
24
Potential Energy and Kinetic Energy
  • Bernoulli energy equation
  • H d Z V2/2g losses
  • d depth
  • Z elevation above datum, e.g. sea level
  • V velocity of flow
  • g gravity

H1
25
Driving and Resisting Forces
  • Driving forces gravity propelling water
    downslope
  • Resisting forces friction within the fluid
    (water), and friction between water and the
    channel boundary
  • Work Force x distance
  • Power Rate at which work is done
  • Unit Stream Power
  • Watts/m2

26
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27
Adjustments in the Fluvial System
28
Example of process linkage and complex response
1959 Hebgen Lake earthquake-induced landslide
t0, x0
SPACE
Deposition
t1, x2
TIME
Deposition
t3, x4
29
Geomorphic WorkFrequency and Magnitude
30
Effective Discharge transports the most sediment
31
RIVERS ARE THE AUTHORS OF THEIR OWN GEOMETRY
  • Given enough time, rivers construct their own
    channels.
  • A river channel is characterized in terms of
    its bankfull geometry.
  • Bankfull geometry is defined in terms of river
    width and average depth at bankfull discharge.
  • Bankfull discharge is the flow discharge when
    the river is just about to spill onto its
    floodplain.

32
Dominant, Bankfull, Effective Discharge
  • Dominant discharge
  • discharge (Q) largely to partially responsible
    for an equilibrium or mean state
  • mean state adjustment of channel geometry to
    imposed conditions
  • discharge that does the most work

33
Bankfull Discharge
Bankfull discharge Discharge where flow just
starts to flow on the floodplain
Bankfull discharge occurs on average every
1.52.3 years For that reason, approximated by
Q1.5-Q2
34
BANKFULL PARAMETERS THE RIVER AND ITS FLOODPLAIN
floodplain
A river constructs its own channel and floodplain.
channel
At bankfull flow the river is on the verge of
spilling out onto its floodplain.
35
CAVEAT NOT ALL RIVERS HAVE A DEFINABLE BANKFULL
GEOMETRY!
Rivers in bedrock often have no active
floodplain, and thus no definable bankfull
geometry.
Wilson Creek, Kentucky a bedrock stream. Image
courtesy A. Parola.
Highly disturbed alluvial rivers are often
undergoing rapid downcutting. What used to be
the floodplain becomes a terrace that is almost
never flooded. Time is required for the river to
construct a new equilibrium channel and
floodplain.
Reach of the East Prairie Creek, Alberta, Canada
undergoing rapid
downcutting due to stream straightening. Image
courtesy D. Andres.
36
THRESHOLD CHANNELS
Threshold gravel-bed channels are channels which
are barely not able to move the gravel on their
beds, even during high flows. These channels
form e.g. immediately downstream of dams, where
their sediment supply is cut off. They also
often form in urban settings, where paving and
revetment have cut off the supply of sediment.
Threshold channels are not the authors of their
own geometry. The relations presented in this
tool do not apply to them.
Trinity Dam on the Trinity River, California,
USA. A threshold channel forms immediately
downstream.
37
Hydraulic Geometry
38
At-a-Station and Downstream Hydraulic Geometry
at-a-station
downstream
39
Downstream hydraulic geometry relations (Leopold
and Maddock,1953)
40
Parker, 2004
41
Fonstad and Marcus, 2003
42
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43
Lanes balanceInteraction between water and
sediment
  • Qs d50 Qw S
  • Qs sediment discharge (kg/s)
  • Qw water discharge (cm/s)
  • d50 sediment size (m)
  • S slope (m/m)

44
The Open-Channel Toolbox TM Peter Wilcock
  • Conservation Relations
  • Conservation of Mass (Continuity)
  • Conservation of Energy
  • Conservation of Momentum
  • Constitutive Relations
  • Flow Resistance
  • Sediment Transport

45
Conservation of Mass (Continuity)
  • Mass is neither created nor destroyed
  • Inputs outputs
  • Inputs and outputs for fluid flow are discharge
  • Vel x Flow Area

U1A1 U2A2
46
Conservation of Momentum (Force-balance)
  • Newtons Second Law
  • In steady, uniform flow,
  • Depth-slope product

47
Conservation of Energy
  • Energy is neither created nor destroyed
  • Two components
  • kinetic ( )
  • potential (zh)
  • Energy is also converted to heat, hf
  • H1 H2 hf

48
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50
Flow Resistance
  • Relation between velocity, flow depth, basal
    shear stress, and hydraulic roughness
  • A variety of relations exist including
  • Mannings
  • Chezy
  • Empirical
  • The big unknown n

Using continuity,
51
Flow Resistance Eqns.
  • Chezy
  • V CvRS
  • Where
  • CChezy roughness (22-220)
  • V velocity
  • Rhydraulic radius
  • Schannel slope
  • Manning
  • V(1.486/n) R2/3 S1/2
  • Where
  • n Mannings roughness coefficient (0.02-006)

52
Sediment Load
  • Sources
  • Chemical weathering (dissolved)
  • Human activity
  • Mass wasting
  • Slopewash
  • Rill and gully formation
  • Channel scour
  • Bed
  • Cutbanks

53
Sediment Transport and Incipient Motion
  • They are not the same
  • sed trans mass flow rate per unit time
  • incipient motion moves or not moves (binary 0
    or 1)
  • What they share
  • f(shear stress)
  • transport depends on the fluid force applied to
    the bed

54
Incipient Motion
  • Whether a stone on the channel bed moves f(t)
  • t dimensionless shear stress
  • ratio of flow force per area acting on the bed to
    grain weight per area

Shields number
55
Tractive Force
Shields equation
  • Grain motion is driven by shear stress, t
  • Units of force/unit area psf, psi, Pa
  • Critical shear stress, tc
  • Shear stress needed to get a grain of a given
    size moving
  • Shields number or
  • dimensionless shear stress t

56
Shields diagram
motion
no motion
(Wilcock, 2006)
57
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58
What does transport depend on?
  • The strength of the flow, the fluid, and the
    sediment
  • Strength of the flow shear stress
  • The sediment grain size and density
  • The fluid water density and water viscosity
    (its resistance to deformation)

59
Sediment transport
Emmett and Wolman (2001)
  • Directly expressed in terms of sediment supply
    and water supply
  • Shear stress is a descriptor of transport rate

Meyer-Peter and Muller
General Form
60
Sunset Creek, Bellevue, WA
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66
Questions?
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