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
2
Q
cavitation
A
- air bubbles trapped in water get compressed into cracks
- implode releasing a shockwave
3
Q
abrasion
A
- small boulders /stones scratch and scrape their way down wearing away river bed
4
Q
attrition
A
- stones collide with each other
- gradually smooth/ round
- smash into pieces
5
Q
corrosion
A
- dissolving of rocks because of chemical reactions in water
6
Q
factors of erosion
A
- ph, velocity, geology, human activity, gradient, load
7
Q
traction
A
- moves large boulders and rocks along riverbed
- material too large for current
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
9
Q
suspension
A
- fine mat (silt) such as alluvium are held up and carried with the flow
10
Q
solution
A
- being worn down as it’s being carried
- solute load
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
12
Q
critical erosion velocity
A
- lowest velocity at within grains can be moved
- smaller, unconsolidated move easier than clay
13
Q
sedimentation
A
particles in a liquid settle
14
Q
entrainment
A
sediment in incorporated into fluid flow as a part of erosion
15
Q
straight channel
A
- rare: hard engineering
- thalweg moves side to side
- often: a central ridge of deposited material
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
