ocdag meeting two more theory. channel patterns, riffles and pools ocdag first meeting june 5, 2007
TRANSCRIPT
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OCDAG
Meeting Two
More Theory
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Channel patterns,Riffles and Pools
OCDAG first meetingJune 5, 2007
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Downstream changes through a basin
• Downstream in a basin• 3 zones:
– 1 – erosion – Step pool – 2 – transportation– 3 - deposition
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River patterns
• Identified aerial photographs or maps
• Channels with self-similar morphometric characteristic that are different from other patterns
• Alluvial – flow through their own sediments
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River patterns
• Most common river patterns– Straight– Meandering– Braided– Wandering– Anastomosed– Step pool
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Channel patterns
• Rivers can adjust channel patterns to change roughness and sediment transport
• Degree of freedom – along with adjusting grainsize, channel shape, channel
slope
• Valley slope is a boundary condition
• Channel slope related to pattern – meandering channels longer – decreasing slope
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Straight• Uncommon in alluvial settings
• Some channels confined by bedrock are straight
• Low energy distributary channels in deltas
• Most channels tend to meander
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Meandering• Common
– (90% of valley length)
• High sinuosity= length of main channel/
valley length
• Cutbanks on outer bends
• Point bars on inner bends
• Moderate width-depth ratios
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Meandering common
• Water flowing on ice commonly forms meandering forms within the ice
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Meandering types
• Display different geometry depending on local conditions
• From regular to highly irregular
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Itkillik River, Alaska
Figure 14.15
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Meandering Stream Profile
Figure 14.15
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Meandering processes
• Flow faster and deeper closer to bank
• Slower and shallower closer to inside of bend
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Meandering processes
• Causes deposition on inside bank – point bar
• Erosion on outside bank – cut bank
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Lateral accretion (horizontal)• Deposition and erosion occur at similar
rates
• Channel moves but width remains constant – dynamic equilibrium
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Oxbow cutoff
• Lateral migration of meanders cause segments of channel to become close
• Water cuts across neck during a flood
• Channel becomes abandoned to form oxbow lake
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Meander scar
• Old channel location
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Overbank deposition
• During floods, suspended sediment deposited on floodplains
• Greatest amount of sediment deposited next to channel – Forms ridge called a levee
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Floodplain features• Floodplains contains many features that record
past conditions, channel locations and processes
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Confined meanders• Occur where parallel valley
walls block channel migration
• Point bars most common
• Eddy accretions in some confined valleys with valley width between 5-10x channel width
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Braided rivers
• Channels that divide and rejoin at low flows
• Dominated by bedload
• Often gravel but maybe sand
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Braided rivers
• Often in front of glaciers
• High slopes
• Wide and shallow
• Large bars within channel, submerged during high flows
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Braided Stream
Figure 14.14
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Braided Stream
Figure 14.14
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Wandering
• Added as a class between meandering and braiding with characteristics of both
Little Southwest Miramichi
Bella Coola
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Wandering
• Have single and multiple channel sections
• Moderate-high width depth
• Moderate-high sed input
Little Southwest Miramichi
Bella Coola
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Anastomosed rivers
• Originally, braided and anastomosed synonymous
• Anastomosed pattern like varicose veins
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Anastomosed rivers
• Anastomosed reclassed as pattern with:– Interconnected
semi-permanent channels
– With vegetated islands
– Stable banks (DG Smith)
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Anastomosed rivers
• Commonly aggrading
• Channel avulsions and abandonment common
• Many in Australia South Saskatchewan
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Continuum concept
• River patterns are the result of interacting set of continuous variables
• Patterns intergrade
• Each pattern associated with a set of variables
• Problems with classification of rivers
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Classifying river patterns
• Schumm (1981, 85)• Based on sediment load• Bedload
– Braided
• Mixed load– meandering
• Suspended load– Anastomosed and highly
sinuous meandering
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Classifying river patterns
• Based on airphoto interp (Mollard) and previous
• Refinement included 2 axes – Based on sed supply– Sed size and gradient
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River patterns: slope-discharge
• River patterns differentiated on basis of slope + discharge ~ energy– Recall, stream power related to slope and discharge
• In order of decreasing energy– Braided-highest– Meandering-moderate– Anastomosed-low– Straight all over
• Threshold between meandering and braiding found
(Leopold and Wolman 1957)
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Channel patterns: slope-discharge
• Widely used
• But problems:– Used channel slope not valley slope– Therefore, meandering lower slope than
braiding
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Channel patterns: slope-discharge and grain size
• Grain size was added to the slope-discharge plot
• Gravel braided higher slope than sand braided
• Related to sediment trans
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River patterns: stream power and grain size
• Sed trans further considered
• Unit stream power and grain size
• Nice discrimination but– Criticized for use of
estimate for w
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River patterns: bank strength
• If bank erosion– More difficult than downstream trans- straight– Less difficult than downstream trans – braided
• Banks easily eroded• High width-depth and deposition of bars• Causing thalweg shoaling and the deposition of bars
– Meandering in balance• Low width-depth and little mid-channel bar
formation
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Channel migration
• Erosion occurs on cutbanks
• depo occurs on point bars
• Rate of depo and erosion approx equal
• Constant width
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River patterns: processes
• Meandering produces patterns within floodplains– Floodplain – valley bottom
inundated by flood and often produced by alluvial (river) sediments
• Ridges and swales produced during channel migration– Leave traces on floodplain
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Meander geometry
• Wavelength – 10-14 x width
• Radius of curvature– 2-3 x width
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Channel migration rate
• Related to radius of curvature rc
• Max rate 2<rc/w<3
• If rc too small or too large
– Shear stress dist
to obtain rc btwn
2-3
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Flow in meanders
• Flow generally toward outside bank
• Asymmetrical shape – w sloping point bar– Steep cutbank– Max depth near
cutbank
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Secondary flow in meanders• Flow across the channel • Generally observed in curved channels• Created due to super elevation at the outside bank
– Built by centrifugal force – outward force in curve– Builds pressure gradient
- inward force
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Sed trans in meanders - Applying Physics
• Particles on a point bar subject to 3 forces:– Drag force downstream– Gravitational force – down slope– Secondary circulation – upslope
• Finer – – move inwards
• Coarser– move outwards
• Sorts sed on point bar
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Cutoffs – avulsion
• After threshold sinuosity cutoffs common
• Neck type most common• Become oxbow lakes• Increase channel
gradient by decreasing length
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Cutoff
• When a river cannot trans sed and water downstream because of decreased slope (high sinuosity)
• Avulsion develops – cutoff• Bed slope increases following
cutoff • Increasing trans• meanders often regrow
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Riffles and pools• Successive deep pools and shallow riffles
downstream
• Generally form with gravel beds
• Occur in both straight and meandering
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Riffles and pools• Slope <1%
• Pools associated w meander bends– Asymmetric x-section
• Gravel accumulates at riffles
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Pool-riffle spacing
• Spacing between successive downstream pool to pool found to be between – 5-7 x channel width
• Scale related• Pool-pool spacing closer
where large woody debris in channel or bedrock outcrops – forcing pool
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Pool-riffle: grain size
• Pools have smaller grain size than riffles• Due to sorting• Bed topography and grain size interrelated• Some have suggested
pools infill with fine
material at low flows • But fines are flushed
at higher flows
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Pool-riffle: hydraulics
• At low flows:– Riffles have higher velocity are wider and
shallower (high shear stress)
– Pools have low velocity, are narrower and deeper (low shear stress)
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Pool-riffle: hydraulics
• Then how can pools be deeper (scoured) and riffles shallower (deposition)?
• One might expect pools to infill and riffles to be eroded until the bed became flat
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Pool-riffle: hydraulic reversal
• Velocity reversal– As water slope more
similar w increasing stage
– At higher flows the velocity increases faster in pools than riffles
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Pool-riffle: hydraulic reversal
• Velocity reversal– Leads to greater shear
stress in pools than riffles at high flows
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Pool-riffle: hydraulic reversal
• Velocity reversal– Causing pools to be
scoured and deposition on riffles
– Also allows coarser sed to be transported through pools to be deposited on riffles
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Pool-riffle: No hydraulic reversal
• However,
• Studies have found that riffles and pools occur without a velocity or shear stress reversal (Latulippe 2004)
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Sed trans reversal
• Sediment transport reversal occurs (Latulippe 2004)
• Sediment transport increases faster in pools than riffles
• In pools– Smaller sed + less
armouring = greater sed trans – Even with lower shear
stress
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Pool-riffle: formation
• Convergence at pools – Increased:
• shear stress
• scour
• Divergence at riffles– Decreased:
• shear stress
• deposition
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Pool-riffle
• Also related to river meandering
• No one explanation fully satisfactory
• Combination of processes