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Geo: Hydrology and Fluvial Geomomrphology > 1.3 river channel processes and landforms > Flashcards

1.3 river channel processes and landforms Flashcards

1
Q

hydraulic action

A
  • sheer force of water hits banks and bed
  • forces water into cracks
  • repeated pressure changes weaken channel
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2
Q

cavitation

A
  • air bubbles trapped in water get compressed into cracks
  • implode releasing a shockwave
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3
Q

abrasion

A
  • small boulders /stones scratch and scrape their way down wearing away river bed
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4
Q

attrition

A
  • stones collide with each other
  • gradually smooth/ round
  • smash into pieces
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5
Q

corrosion

A
  • dissolving of rocks because of chemical reactions in water
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6
Q

factors of erosion

A
  • ph, velocity, geology, human activity, gradient, load
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7
Q

traction

A
  • moves large boulders and rocks along riverbed
  • material too large for current
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8
Q

saltation

A
  • small pebbles and stones are bounced along with local rise and fall of velocity
  • too large for suspension, not enough for traction
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9
Q

suspension

A
  • fine mat (silt) such as alluvium are held up and carried with the flow
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10
Q

solution

A
  • being worn down as it’s being carried
  • solute load
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11
Q

deposition

A
  • river drops load (sediment) as velocity falls
  • water looses energy
  • boulders first
  • in drought / low discharge
  • shallowing of gradient, increasing friction
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12
Q

critical erosion velocity

A
  • lowest velocity at within grains can be moved
  • smaller, unconsolidated move easier than clay
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13
Q

sedimentation

A

particles in a liquid settle

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14
Q

entrainment

A

sediment in incorporated into fluid flow as a part of erosion

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15
Q

straight channel

A
  • rare: hard engineering
  • thalweg moves side to side
  • often: a central ridge of deposited material
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16
Q

braided channel

A
  • channel broken up by islands and bars
  • island: vegetated, bars: less stable
    happens because:
  • steep channel gradient
  • easily erodible bank mat
  • highly variable discharge
  • monsoonal climate
  • abundant sediment
  • no capacity to transport in straight channel
17
Q

meandering channel

A
  • sinuosity greater than 1.5
  • channel slope, discharge, load combine to create a situation where meandering is the only way for a river to use up energy equally through the channel reach
18
Q

factors of flow

A
  • gradient
  • volume of water, ppt in basin
  • channel slope
  • channel roughness and friction
19
Q

laminar flow

A
  • requires a smooth, straight channel, low velocity
  • flow in sheets (laminae): illusion of uniform movement
  • not often in lower reaches
  • groundwater, glaciers
20
Q

turbulent flow

A
  • higher velocity, uneven channel morphology, rough
  • associated with hydraulic action: large variations in pressure
  • vertical turbulence can create hollows in bed (potholes)
  • may form gulley/gorge
21
Q

helicoidal flow

A
  • horizontal turbulence can result in corkscrew effect
  • lot of material carried
  • alternating pools and riffles
  • alternating deposition and erosion: meanders
22
Q

waterfall formation

A
    • alternating layers of MR and LR
    • vertical erosion
      2.
    • LR eroded
    • overhang, undercutting
    • plunge pool: abrasion
    • unsupported cap rock falls
    • splashback: headward erosion
    • plunge pool deepened
23
Q

gorge

A
  • created as retreat of waterfall leaves a narrow, steep sided valley
24
Q

oxbow lake formation

A
  • thalweg moves side to side, alt dep and erosion
  • gap narrowed by erosion: river breaks gap in flood
  • current along straighter route becomes dominant: old path run dry
  • outside bank undercut by lateral erosion: river cliff
  • inside bend: slow shallow: friction: deposition
25
Q

types of meanders

A
  • intrenched: symmetrical in cross section, vertical erosion is aster than lateral, where there is a significant fall in base level
  • ingrown : asymmetrical, lateral erosion : lateral migration
26
Q

pools and riffles

A
  • turbulence in river
  • eddies (swirls) cause deposition of coarse sediment:riffles
  • where there is a high velocity : small ridges of material mid-stream where velocity decreases
    -pools cause deposition of fine sediment where low velocity, in deeper parts of meanders
  • pool: deeper area of channel, less turbulent, fine sediment
  • riffle: shallow area of channel, turbulent, coarse sediment
27
Q

formation of levees

A
  • during flood, biggest coarse material is deposited first, fine material like alluvium outer on plain
  • after many floods natural levees build up
28
Q

floodplains

A
  • prone to flooding
  • width due to meander migration
  • river bluff: area of higher ground or edge
  • fertile land (nutrient in sediment)
  • point bars
29
Q

deltas

A
  • where river meets a body of standing water: deposition of sediment
  • deposition increased with salinity as small particles : floccuate
30
Q

cuspate delta

A
  • pointed
  • shaped by regular but opposite movement
  • Tiber delta
31
Q

bird’s foot delta

A
  • river brings large amount of fine silt and alluvium
  • deposition along edges of tributaries
32
Q

arcuate delta

A
  • fan shaped areas
  • LSD or other currents that keep sea-ward edges smooth
  • Nile delta
33
Q

lacustrine delta

A
  • can also from inland
  • where river meets lake (may fill lake)
34
Q

meander formation

A
  • starts in relatively straight channel
  • helicoidal flow when rough riverbed
  • pools and riffle sequence developing
  • thalweg produced point bar deposit and cut bank
  • sinuousity increases over time
  • meander migration
35
Q

bottomset beds

A

created from the lightest particles that settle furthest from active delta front

36
Q

foreset beds

A

deposited in inclined layers over the bottomset beds as the active lobe advances

37
Q

topset beds

A

of an advancing delta are deposited over previously laid foreset bed and form an extension of the landward alluvial plain
- distributaries occur between the accreting islands in the delta

Geo: Hydrology and Fluvial Geomomrphology (5 decks)

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