OCDAG Meeting Two More Theory. Channel patterns, Riffles and Pools OCDAG first meeting June 5, 2007.
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Transcript of OCDAG Meeting Two More Theory. Channel patterns, Riffles and Pools OCDAG first meeting June 5, 2007.
OCDAG
Meeting Two
More Theory
Channel patterns,Riffles and Pools
OCDAG first meetingJune 5, 2007
Downstream changes through a basin
• Downstream in a basin• 3 zones:
– 1 – erosion – Step pool – 2 – transportation– 3 - deposition
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
River patterns
• Most common river patterns– Straight– Meandering– Braided– Wandering– Anastomosed– Step pool
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
Straight• Uncommon in alluvial settings
• Some channels confined by bedrock are straight
• Low energy distributary channels in deltas
• Most channels tend to meander
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
Meandering common
• Water flowing on ice commonly forms meandering forms within the ice
Meandering types
• Display different geometry depending on local conditions
• From regular to highly irregular
Itkillik River, Alaska
Figure 14.15
Meandering Stream Profile
Figure 14.15
Meandering processes
• Flow faster and deeper closer to bank
• Slower and shallower closer to inside of bend
Meandering processes
• Causes deposition on inside bank – point bar
• Erosion on outside bank – cut bank
Lateral accretion (horizontal)• Deposition and erosion occur at similar
rates
• Channel moves but width remains constant – dynamic equilibrium
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
Meander scar
• Old channel location
Overbank deposition
• During floods, suspended sediment deposited on floodplains
• Greatest amount of sediment deposited next to channel – Forms ridge called a levee
Floodplain features• Floodplains contains many features that record
past conditions, channel locations and processes
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
Braided rivers
• Channels that divide and rejoin at low flows
• Dominated by bedload
• Often gravel but maybe sand
Braided rivers
• Often in front of glaciers
• High slopes
• Wide and shallow
• Large bars within channel, submerged during high flows
Braided Stream
Figure 14.14
Braided Stream
Figure 14.14
Wandering
• Added as a class between meandering and braiding with characteristics of both
Little Southwest Miramichi
Bella Coola
Wandering
• Have single and multiple channel sections
• Moderate-high width depth
• Moderate-high sed input
Little Southwest Miramichi
Bella Coola
Anastomosed rivers
• Originally, braided and anastomosed synonymous
• Anastomosed pattern like varicose veins
Anastomosed rivers
• Anastomosed reclassed as pattern with:– Interconnected
semi-permanent channels
– With vegetated islands
– Stable banks (DG Smith)
Anastomosed rivers
• Commonly aggrading
• Channel avulsions and abandonment common
• Many in Australia South Saskatchewan
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
Classifying river patterns
• Schumm (1981, 85)• Based on sediment load• Bedload
– Braided
• Mixed load– meandering
• Suspended load– Anastomosed and highly
sinuous meandering
Classifying river patterns
• Based on airphoto interp (Mollard) and previous
• Refinement included 2 axes – Based on sed supply– Sed size and gradient
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)
Channel patterns: slope-discharge
• Widely used
• But problems:– Used channel slope not valley slope– Therefore, meandering lower slope than
braiding
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
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
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
Channel migration
• Erosion occurs on cutbanks
• depo occurs on point bars
• Rate of depo and erosion approx equal
• Constant width
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
Meander geometry
• Wavelength – 10-14 x width
• Radius of curvature– 2-3 x width
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
Flow in meanders
• Flow generally toward outside bank
• Asymmetrical shape – w sloping point bar– Steep cutbank– Max depth near
cutbank
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
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
Cutoffs – avulsion
• After threshold sinuosity cutoffs common
• Neck type most common• Become oxbow lakes• Increase channel
gradient by decreasing length
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
Riffles and pools• Successive deep pools and shallow riffles
downstream
• Generally form with gravel beds
• Occur in both straight and meandering
Riffles and pools• Slope <1%
• Pools associated w meander bends– Asymmetric x-section
• Gravel accumulates at riffles
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
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
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)
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
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
Pool-riffle: hydraulic reversal
• Velocity reversal– Leads to greater shear
stress in pools than riffles at high flows
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
Pool-riffle: No hydraulic reversal
• However,
• Studies have found that riffles and pools occur without a velocity or shear stress reversal (Latulippe 2004)
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
Pool-riffle: formation
• Convergence at pools – Increased:
• shear stress
• scour
• Divergence at riffles– Decreased:
• shear stress
• deposition
Pool-riffle
• Also related to river meandering
• No one explanation fully satisfactory
• Combination of processes