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