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
Flow states and energy regime: Rapid (rough)
Fr > 1
26
Where is velocity usually the greatest?
Near the surface where channel is deepest
27
Flow measuring tools
- Ott meter - Price-Gurley meter - Fish-mounted Price meter - Laser Doppler Velocimeter - Electromagnetic Current Meter - Acoustic Doppler Profiler
28
Velocity-Area method
- Q, Discharge measurement - Volume of water passing through channel cross-section per unit time - Q = W x D x V =A x V
29
Q Rating curves
- Use stage to find Q | - Indirect measurements made using plots of observed Q vs. Stage (height) at a given cross section
30
Hydraulic geometry
- Relationships btwn the mean stream channel form/dimensions and Q both at-a-station and downstream along a stream network
31
At-a-station
- Tells us how the channel dimensions change as flow changes at one cross section over time
32
Hydraulic geometry of rivers: Downstream
- Tells how the channel dimensions change along the channel
33
Basic definition of flooding
- flow that exceeds channel banks onto the floodplain | -
34
What are 2 critical stages (water depths)
- Bankfull discharge | - Mean annual flood
35
Bankfull Discharge
- 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
Mean annual flood
- Flow breaches bank - Spill onto floodplain - Return period of 2.33 years
37
Recurrence interval
- 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
Probability
P = 1/R x 100 | - R = recurrence interval
39
Discharge hydrograph
- Q measured over time
40
Event storm hydrographs
- 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
Discharge hydrograph
- shape depends on many factors including: - Drainage basin shape and size, storm intensity, land use - Deforestation and urbanization enhance peak Q
42
Hortonian overland flow
Rainfall>infiltration
43
If rainfall < Infiltration
- water moves via interflow (vadose) zone and in gw
44
Rills
- 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
Gullies
- 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
Sheet flow
- Overland flow as a shallow layer - Removes particles in thin layers - Very important process effecting agricultural soil erosion
47
What is the resisting force for channel initiation?
- Given by the weight of the sediment on the slope and cohesive forces (clay, veg, roots)
48
Driving (erosive) force (tractive) is controlled by what?
- Weight of the water (depth and density) and the slope | - Stress = density x gravity x depth x slope
49
LOF
Limit of Overland Flow | - Where local depth increases due to various random perturbations
50
Hjulstrom Diagram
- Entrainment vs. grain size - Grain-size increases with current velocity for material in transport - But erosion of fine sediments requires disproportionally high velocities
51
Drainage Basins
- 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
What is Canada's largest drainage basin
- Mackenzie River - 1.8 million km^2 - 4200km long
53
What are the 3 general sub-regions of drainage basins?
- Colluvial - Alluvial - Depositional Zones
54
Colluvial zones
- 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
Alluvial Zone
- 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
Terraces
- Abandoned floodplains formed when river flowed at a higher level than now
57
Strath Terraces
- Bedrock terraces
58
Depositional Zone
- 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
Delta
- 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
Alluvial Fans
- 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
Temporal changes of alluvial fans
- 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
Controls on drainage basin structure
- 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
Organization of drainage basin
- 3 systems (Horton, Strahler, Shreve) | - Minor tributaries low order and main trunk highest
64
Strahler Organization
- 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
Three main categories of drainage basin morphometry
- 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
Bifurcation Ratio
- 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
Drainage density
- Empirical relationship btwn drainage basin area and the total length of streams w/in basin - Largely reflects interactions btwn geology and climate
68
Why is Drainage density important
- 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
How might geology (erosion resistance) and climate effect the constant of channel maintenance and LOF?
- 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
What leads to widely spaced streams (lower drainage density)?
- Resistant bedrock surfaces - High infiltration capacity - Thick vegetation cover
71
Hack's Law
- Relationship btwn length of streams and area of their basins
72
Dendritic drainage
- Common - Homogeneous materials - Governed mostly by slope - Confluences of tributaries at acute angles
73
Parallel Drainage
- 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
Trellis Drainage
- Systems develop in folded topography (synforms) - Geologically controlled - Short tributary streams enter main channel at sharp right angles - Flow in 2 directions
75
Rectangular Drainage
- Systems develop in regions of faulting or have 2 main directions of jointing but are of otherwise similar resistance - Confluences at right angles
76
Radial Drainage
- Systems develop around a central elevated point (volcanoes, domes) - Can also develop in closed basins (centripetal drainage)
77
Deranged Drainage
- 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
Straight Channel
- 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
Sinuosity
Channel L/Valley L
80
Pools
- Areas of active erosion
81
Riffles
- Regions of shallower flow over gravel
82
Length of pool-riffle sequence
- Generally 5-7 stream widths | - Only in gravel-bed rivers
83
Step-pools
- 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
Meandering Streams
- 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
Braided Channels
- 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
Anastamosing/ Anabranching rivers
- 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
Graded river
- 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