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