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Geomorphology > Fluvial Geomorphology > Flashcards

Fluvial Geomorphology Flashcards

1
Q

Fluvial geomorphology

A
  • study of running water and its influence in modifying our land surface
  • characteristics of rivers as a function of landscape position
  • how river accomplish ‘work’
  • how rivers respond to change
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2
Q

Geomorphic work

A
  • running water exerts a force = landscape offers resistance
  • fluvial processes dominate in most parts of the world, where precipitation exceeds evaporation
  • except in cold regions (ice dominated), and dry regions (wind = most erosive agent)
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3
Q

Channel

A

the morphological feature within which flow is constrained

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4
Q

Floodplain

A

the flat area surrounding the channel which is inundated with water when the river floods

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5
Q

Bankfull

A

The maximum water level that is achieved before water s disperse onto the floodplain

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6
Q

Catchment/drainage basin

A

the area of land where all surface water will run to a common low point in the landscape

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7
Q

Discharge

A

volume of flow per unit of time

= velocity(average depthchannel width)

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8
Q

Perennial rivers

A
  • flow all year round
  • are maintained by subsurface/groundwater flow
  • typical of humid/sub-humid regions (groundwater maintained by high or constant rainfall)
  • persistent rainfall across the year
  • form is shaped by high frequency, low-magnitude events
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9
Q

Ephemeral rivers

A
  • flow intermittently respond to surface runoff events
  • respond to surface runoff events
  • semi-arid regions (rainfall now high enough to maintain effective subsurface flow)- flow responds to low-frequency, high magnitude events
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10
Q

how climate influences river processes

A
  • amount of water entering the ground controls height of the water table
  • channel flow occurs where land surface intersects the water table
  • in arid/semi-arid water table is so low that it frequently isn’t intersected by landscape
  • consequently no ground water (baseflow) driven river activity
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11
Q

How geology influences river processes

A
  • bedrock permeability has major influence on river flow patterns
  • neighbouring catchments in the thames basin
  • identical climatic conditions
  • different flow regime
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12
Q

Regional drainage networks

A 
Consequent drainage 
- produced from original uplift
- streams follow slope of land over which originally formed 
Subsequent drainage 
- - developed after initial incision
- soft strata, faults
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13
Q

Old terminology

A

assumes ordering of events which may be incorrect

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14
Q

Modern terminology

A

invokes structural control on drainage development

- strike, dip and anti-dip streams

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15
Q

base level in fluvial systems

A
  • the lowest point at which a stream can erode to
  • ultimate base level = sea level
  • falling sea level steepens gradient = incision
  • rising sea level reduces energy = submerged river valleys
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16
Q

regional drainage patterns

A

Dendritic - horizontal, uniform strata/sediment, no structural control
Parallel - strong structural control
Rectangle - primary and secondary drainage directions, alternative soft-hard rock/sediment
Radial - volcanic cones, uplift domes
Centrifugal - high areas on inside of meander bends
Centripetal - central low point, basin floor e.g. caldera subsidence basin

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17
Q

Deranged Drainage

A

develops on newly exposed land

  • following glacial retreat
  • no structural/bedrock control
  • irregular stream courses
  • short tributaries- lakes
  • bogs
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18
Q

Fluvial erosion and transport

A

streams are powerful erosional agents

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19
Q

Stream power

A

the power available to transport sediment

= water densitygravitational accelerationstream discharge*channel slope

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20
Q

Critical power

A

the power required to transport the available sediment load

21
Q

stream load

A

Green River, Colorado River confluence

  • higher suspended sediment load form the green river
  • greater stream power, potential for erosion and transport
22
Q

Hjustrom curve

A

illustrates the water velocity at which particles can be entrained and transport. Consider; cohesion, shape, density and particle size
- applies to alluvial channels only

23
Q

Corrosion

A

chemical weathering of bed and bank material

24
Q

Corrasion/abrasion

A

wearing away by impact or grinding or particles

25
Q

Cavitation

A

high flow velocities at base of waterfalls/rapids = shockwaves from exploding bubbles produced by pressure changes

26
Q

Evorsion

A

force of water smashed bedrock (no solid material involved)

27
Q

Erosional processes

A

Streams may erode their channels

  • downward, net removal of sand and gravels
  • Laterally, undercutting and bank collapse
28
Q

depositional processes

A
  • channel deposits
  • channel margin deposits
  • overbank floodplain deposits
  • valley margin deposits
29
Q

Incision

A

the process downcutting/erosion of a river into its bed (lower of channel height)

30
Q

Aggradation

A

the large scale deposition of sediment causing a vertical increase in floodplain height

31
Q

Channel deposits

A
  • resting bed load, temporary
  • lag deposits, course, heavy deposits
  • channel fills, in abandoned or aggrading channel segments
32
Q

Channel margin deposits

A

Form by lateral accretion and area preserved by channel shifting
- point bars; inside meander loop, slowing water, deposition

33
Q

vertical acceretion

A
  • fine grained material

- natural levees and back swamps

34
Q

splays

A

small accumulations of flood debris

35
Q

valley margin deposits

A
  • colluvium: slope wash and sol creep

- mass movement deposits; debris avalanches and landslides

36
Q

Fluvial erosion landforms

A
  • rills and gullies
  • bedrock channels
  • alluvial channels (straight, meandering, braided, anastromosing, anabranching)
  • valleys
37
Q

rills, gullies and valleys

A
  • increase in scale and energy with time
  • rills <30cm wide, 60cm deep
  • gullies, continuous or discontinuous
38
Q

Straight channels

A
  • uncommon and restricted to V-shaped valleys

- channel migration limited by structural controls

39
Q

meandering channel

A
  • grade 1 of alluvial channel form
  • caused by instabilities in turbulent water on an erodible bank
  • sinuosity of 1.5 (channel length/valley length)
40
Q

Braided channels

A
  • high energy, steep gradients, high sediment supply
  • forms when the stream cannot hold its sediment load and dumps it in the middle of the channel
  • channel bars are collected piles of sediment
  • channel bars force the river to flow around them which separates river into braids
  • typical width of channels is similar to that of bars
41
Q

Anastomosing channels

A
  • multiple interconnected channels
  • floodplain is stabilised
  • typical width of channels is smaller than that of the bars
42
Q

stream ordering

A
  • 2 schemes (Strahler and Shreve)
  • drainage density is the balance between stream erosion and resistance of the surface
  • influenced by; climate, lithology and vegetation
43
Q

balance models

A

how rivers respond to changed in sediment load, base level change and climate
- climate change may lead to alterations in all factors

44
Q

in glacial conditions

A
  • relative valley slope increases
  • channel forming discharge increases
  • lack of vegetation
  • high precip (Seasonality)
  • glacial melt water
45
Q

current rivers

A
  • relatively low energy
  • vegetated catchments and channels
  • less seasonal flow regime
  • not capable of eroding from top of vegetated slope
  • accumulation of fine grained sediments
46
Q

the end of the last glacial

A
  • Periglacial conditions
  • lower sea level = rivers with higher energy regimes
  • sea level ca. 100m below present
  • less vegetation on the landscape
  • energy provided = erosion of Solent river
  • as sea level rose = raised base level = drowned river valleys
47
Q

sediment assemblages

A
  • coarse grained (gravel, pebble transport and deposition)
  • erosional features
  • high energy
  • periglacial processes (ice wedge cast)
48
Q

Thames terrace

A
  • the present day course of the Thames was only arrived at after the Anglian ice overrode its palaeochannels
  • excavation of terraces
  • interglacial deposits contain; pollen, insect fauna, mammalian fauna
  • provide valuable information about the environments of past interglacial periods
  • archaeological material is often found alongside interglacial deposits

Geomorphology (9 decks)

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