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Geog375Gmphlgy > Fluvial Processes > Flashcards

Fluvial Processes Flashcards

1
Q

Some applications of fluvial geomorphology

A
  • Conduits for water and sed movement
  • Conduits for nutrients and contaminants and support biological systems
  • Critical for river ecology, population dynamics, envy chemistry
  • Flood prediction and mitigation
  • Fish habitat
  • Dams
  • Bridges and infrastructure to withstand bankfull (but should consider unusual strong events w/ low recurrence intervals)
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2
Q

How do landscape materials get from valley floors to their ultimate sink (oceans and lakes)?

A
  • Streamflow accounts for 85-90% of total sed transport

- Glaciers 7%

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3
Q

When discharge increases, what also increases? or Decreases?

A
  • Increase: Width, Depth, Velocity

- Decrease: Gradient

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4
Q

What does Q = ?

A

v x A

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5
Q

Hydraulic driving variables

A
  • Discharge
  • Shear velocity
  • Shear stress
  • Stream power
  • Slope
  • Base level
  • Sediment load
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6
Q

Hydraulic Response variables

A
  • Channel width, depth
  • Channel bedrooms
  • Channel patterns (geomorphology)
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7
Q

What are flow rates controlled by?

A
  • Slope
  • Velocity
  • Sediment Size
  • Channel Form
  • Roughness
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8
Q

Streamflow velocity

A
  • Vector quantity with both magnitude (speed) and direction
  • Varies in 4 dimensions (distance from bed, across stream, downstream, time)
  • Highly variable in time and space
  • Effects processes of erosion, transport, and deposition of sed
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9
Q

What is the shape of the velocity profile influenced by?

A
  • Size of roughness elements on streambed
  • Depth of flow
  • Logarithmic velocity profile due to friction of the bed
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10
Q

Where does velocity generally increase?

A
  • Toward stream center
  • but more complex
  • Degree of symmetry can be highly variable, changing with shape of channel
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11
Q

What happens to velocity when Discharge increases?

A
  • Depth increases
  • Reduces influence of roughness elements on the bed
  • So Velocity increases
  • Seasonal freshet and diurnal fluctuations strongly dependent on discharge
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12
Q

Laminar

A
  • Water travels along parallel paths with no significant mixing
  • Relatively rare in low viscosity Newtonian fluids
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13
Q

Turbulent

A
  • Chaotic movement of water
  • Fluctuations in velocity, considerable mixing
  • Irregular paths of fluid flow
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14
Q

Reynolds Number, Re

A
  • Defines the transition between laminar and turbulent flow
  • Ratio between the driving (inertial) forces and resisting (viscous) forces
  • As driving forces (numerator) increases, flow becomes more turbulent
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15
Q

Re <500

A

= Laminar

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16
Q

Re >2000

A

= Turbulent

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17
Q

Froude Number, Fr

A
  • Streamflow, compares inertial and gravitational forces

- = mean velocity/ (square root of gravitational acceleration x channel depth)

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18
Q

Fr < 1

A
  • Subcritical
  • Deep, slow flow (tranquil)
  • Ripples can travel upstream
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19
Q

Fr > 1

A
  • Supercritical
  • Shallow, fast flow
  • Flow velocity is larger than wave velocity
  • Ripples cannot travel upstream
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20
Q

Flow states and energy regime: Uniform

A

Velocity is constant with position

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21
Q

Flow states and energy regime: Non-uniform

A

Velocity is variable with position

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22
Q

Flow states and energy regime: Steady

A

Velocity is constant with time

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23
Q

Flow states and energy regime: Unsteady

A

Velocity is variable with time

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24
Q

Flow states and energy regime: Tranquil

A

Fr < 1

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25
Q

Flow states and energy regime: Rapid (rough)

A

Fr > 1

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26
Q

Where is velocity usually the greatest?

A

Near the surface where channel is deepest

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27
Q

Flow measuring tools

A
  • Ott meter
  • Price-Gurley meter
  • Fish-mounted Price meter
  • Laser Doppler Velocimeter
  • Electromagnetic Current Meter
  • Acoustic Doppler Profiler
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28
Q

Velocity-Area method

A
  • Q, Discharge measurement
  • Volume of water passing through channel cross-section per unit time
  • Q = W x D x V =A x V
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29
Q

Q Rating curves

A
  • Use stage to find Q

- Indirect measurements made using plots of observed Q vs. Stage (height) at a given cross section

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30
Q

Hydraulic geometry

A
  • Relationships btwn the mean stream channel form/dimensions and Q both at-a-station and downstream along a stream network
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31
Q

At-a-station

A
  • Tells us how the channel dimensions change as flow changes at one cross section over time
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32
Q

Hydraulic geometry of rivers: Downstream

A
  • Tells how the channel dimensions change along the channel
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33
Q

Basic definition of flooding

A
  • flow that exceeds channel banks onto the floodplain

-

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34
Q

What are 2 critical stages (water depths)

A
  • Bankfull discharge

- Mean annual flood

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35
Q

Bankfull Discharge

A
  • Flow to bank tops (just before overflow)

- Effective or dominant discharge as largely responsible for altering channel forms (return period of 1-2 years)

36
Q

Mean annual flood

A
  • Flow breaches bank
  • Spill onto floodplain
  • Return period of 2.33 years
37
Q

Recurrence interval

A
  • Return period
  • Average interval of time within which a given flood will be equaled or exceeded once
  • R = (n plus 1)/m
  • n = number of years of record
  • m = rank
38
Q

Probability

A

P = 1/R x 100

- R = recurrence interval

39
Q

Discharge hydrograph

A
  • Q measured over time
40
Q

Event storm hydrographs

A
  • Usually right skewed
  • Steep rising limb reflecting rapid runoff
  • Prolonged decline from gradual depletion of soil water and gw
  • Deforestation and urbanization enhance peak Q
41
Q

Discharge hydrograph

A
  • shape depends on many factors including:
  • Drainage basin shape and size, storm intensity, land use
  • Deforestation and urbanization enhance peak Q
42
Q

Hortonian overland flow

A

Rainfall>infiltration

43
Q

If rainfall < Infiltration

A
  • water moves via interflow (vadose) zone and in gw
44
Q

Rills

A
  • Series of small, parallel channels
  • Sheet flow concentrates into rivulets
  • Cuts small parallel channels
  • Typically several cm wide and deep
  • Best developed where vegetation is lacking
45
Q

Gullies

A
  • Series of large, parallel, v-shaped channels
  • Carved by concentrated runoff
  • Not easily undone
  • Large scale form of rills that have joined
  • Typically several meters deep/wide
  • Prominent in arid areas (badlands), cleared land (cut-blocks, grazing areas, poorly contoured summer fallow fields)
46
Q

Sheet flow

A
  • Overland flow as a shallow layer
  • Removes particles in thin layers
  • Very important process effecting agricultural soil erosion
47
Q

What is the resisting force for channel initiation?

A
  • Given by the weight of the sediment on the slope and cohesive forces (clay, veg, roots)
48
Q

Driving (erosive) force (tractive) is controlled by what?

A
  • Weight of the water (depth and density) and the slope

- Stress = density x gravity x depth x slope

49
Q

LOF

A

Limit of Overland Flow

- Where local depth increases due to various random perturbations

50
Q

Hjulstrom Diagram

A
  • Entrainment vs. grain size
  • Grain-size increases with current velocity for material in transport
  • But erosion of fine sediments requires disproportionally high velocities
51
Q

Drainage Basins

A
  • Includes both streams and the land surface
  • Separated with drainage divide
  • Basins are nested
  • Geomorph and Geology exert strong controls on important effects such as flooding and stream sediment geochem
52
Q

What is Canada’s largest drainage basin

A
  • Mackenzie River
  • 1.8 million km^2
  • 4200km long
53
Q

What are the 3 general sub-regions of drainage basins?

A
  • Colluvial
  • Alluvial
  • Depositional Zones
54
Q

Colluvial zones

A
  • Mass wasting and bedrock channels dominate
  • Headwater channels drain mountainous environments, Typically very steep in bedrock
  • Transport capacity > sed supply (sed input < output)
  • Channels erosive
  • River channels actively eroding
  • Typical V-shape in x-section unless other geomorphic agents active
  • Channels relatively straight, steep, with pools and riffles
55
Q

Alluvial Zone

A
  • Rivers flow through own deposits
  • Lowland river channels flowing in relatively wide valleys (compared to colluvial zone)
  • Wider, flatter valley floors
  • Channel slopes less steep than colluvial streams
  • Low gradient and higher sinuosity
  • Transport Capacity < sed supply (input > output)
  • Rivers erode and deposit at different places and times
  • Point bars
56
Q

Terraces

A
  • Abandoned floodplains formed when river flowed at a higher level than now
57
Q

Strath Terraces

A
  • Bedrock terraces
58
Q

Depositional Zone

A
  • Floodplains adjacent to stream channel, overlap with alluvial zone
  • Dominated by wide, low gradient floodplains, deltas and fans
  • Aggrading and laterally accreting floodplains, well-developed levees, distributary channels
  • Make the depositional zone flood prone
59
Q

Delta

A
  • Classic triangular shape like Nile after Greek letter, or fan-shaped, or birds-foot (series of branching channels)
  • Depositional landform where river enters bodies of water and drop sed load
  • No longer confined so channels expand, velocity drops, sed depositied
  • Geomorph reflects process (tidal, wave, river dominated)
60
Q

Alluvial Fans

A
  • Deposited on land where river leaves confines of narrow constriction (mountains) and flows into broad valley
  • Vary from debris flow dominated colluvial fans to river-dominated fluvial fans
  • Geomorph strongly controlled by depositional processes and events in the basins
  • Steeper and coarser than deltas, larger magnitude flows/floods
61
Q

Temporal changes of alluvial fans

A
  • Past different than present
  • Past: lots of glacial melt
  • Less melt in last 6000 yrs
  • Determined to be only 2-3 m in last 6000 b/c of Mount Mazama ash layer
62
Q

Controls on drainage basin structure

A
  • Bedrock (lithology, structure, tectonics)
  • Vegetation type, density, fire, logging
  • Soil thickness, grain size, composition weathering
  • Slope (tectonics, lanscape development stage)
  • Climate (Precip amount, duration, intensity)
  • Base level: controlled by sea/lake level, tectonics, damming
63
Q

Organization of drainage basin

A
  • 3 systems (Horton, Strahler, Shreve)

- Minor tributaries low order and main trunk highest

64
Q

Strahler Organization

A
  • When 2 first order streams join, forms second order
  • Third order only formed when 2 second order join
  • Downstream order only increases if joined to a branch of the same order
  • Low order streams not always counted, so under represented (Shreve’s accounts for this)
65
Q

Three main categories of drainage basin morphometry

A
  • Linear (avg stream length, bifurcation ration, LOF)
  • Areal (Length-area, drainage density, constant of channel maintenance, mainly dimensionless numbers)
  • Relief (Hypsometric analysis, max basin relief)
66
Q

Bifurcation Ratio

A
  • Ratio of number of streams of a given order to the number in the next highest order (Strahler)
  • Remarkably consistent from one basin to another, except where powerful geological controls dominate
  • Typically 3 -5 and approximates number of second order streams
  • Rb = No/(No plus 1)
  • No = number of streams in each order
67
Q

Drainage density

A
  • Empirical relationship btwn drainage basin area and the total length of streams w/in basin
  • Largely reflects interactions btwn geology and climate
68
Q

Why is Drainage density important

A
  • Real measure of drainage basin structure, not dependent on ordering schemes
  • Drainage effects the time frame btwn concentrated rainfall and river discharge
  • Drainage can be used to define other useful geomorphic properties of the landscape
  • Constant of channel maintenance and length of overland flow
  • High discharge, low time = High drainage density
  • Low discharge, large time = small drainage density
69
Q

How might geology (erosion resistance) and climate effect the constant of channel maintenance and LOF?

A
  • Erosion resistant surfaces exhibit widely spaced streams = low Drainage Density
  • Arid areas lack vegetation = higher drainage densities
  • Humid areas have a good veg cover = Lower drainage density
70
Q

What leads to widely spaced streams (lower drainage density)?

A
  • Resistant bedrock surfaces
  • High infiltration capacity
  • Thick vegetation cover
71
Q

Hack’s Law

A
  • Relationship btwn length of streams and area of their basins
72
Q

Dendritic drainage

A
  • Common
  • Homogeneous materials
  • Governed mostly by slope
  • Confluences of tributaries at acute angles
73
Q

Parallel Drainage

A
  • System develops where slope is uniform and pronounced
  • May be fault controlled
  • Sometimes indicated presence of major faults
  • Confluences of tributaries at very small angles
74
Q

Trellis Drainage

A
  • Systems develop in folded topography (synforms)
  • Geologically controlled
  • Short tributary streams enter main channel at sharp right angles
  • Flow in 2 directions
75
Q

Rectangular Drainage

A
  • Systems develop in regions of faulting or have 2 main directions of jointing but are of otherwise similar resistance
  • Confluences at right angles
76
Q

Radial Drainage

A
  • Systems develop around a central elevated point (volcanoes, domes)
  • Can also develop in closed basins (centripetal drainage)
77
Q

Deranged Drainage

A
  • No coherent pattern
  • Result from disruptions to existing drainage patterns (glaciated, karst, permafrost terrains)
  • Short streams enter trunk streams at all angles
  • Hollows in quasi-random locations get filled w/ water, commonly become wetlands
78
Q

Straight Channel

A
  • Uncommon
  • Distinguished from meandering when sinuosity < 1.5
  • Structurally controlled or artificially straightened
  • Few straight for entire length
  • Steep headwater streams
  • Pool-riffle and step-pool sequences
79
Q

Sinuosity

A

Channel L/Valley L

80
Q

Pools

A
  • Areas of active erosion
81
Q

Riffles

A
  • Regions of shallower flow over gravel
82
Q

Length of pool-riffle sequence

A
  • Generally 5-7 stream widths

- Only in gravel-bed rivers

83
Q

Step-pools

A
  • Often replace pool-riffles in steep bedrock channels
  • Also common in bouldery creeks
  • Steps comprised of pebbles, cobbles, or boulders (in alluvial) or resistant bedrock outcrops in bedrock channels
84
Q

Meandering Streams

A
  • Series of curves of similar shape and amplitude
  • Much higher suspended load than bedload
  • Thalweg alternates from side to side creating the typical cut-bank/point-bar geomorphology
85
Q

Braided Channels

A
  • Multiple small channels and islands (braid bars)
  • Steep gradients, broad valleys
  • High and variable Q, qsed, and stream power
  • Erodible banks (non-cohesivue sediment)
  • Unstable bars
  • High bedload transport, with rapid and frequent Q changes
  • Glaciofluvial systems (huge sed and variable dirurnal discharge or seasonal)
86
Q

Anastamosing/ Anabranching rivers

A
  • Multiple, deep, narrow, stable channels with vegetated islands
  • Shallow gradients, low stream power
  • Most sed transported as suspended lot (clays, silts)
  • Vertically aggressing systems
87
Q

Graded river

A
  • Water and sed pass w/o net change in form
  • Channel characteristics adjust until slope and discharge provide just enough velocity to transport sediment supplied from the basin and no more

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