1. Meeting Two More Theory
OCDAG
Meeting Two
More Theory

2. Channel patterns, Riffles and Pools
OCDAG first meeting
June 5, 2007

3. Downstream changes through a basin
Downstream in a basin
3 zones:
1 – erosion – Step pool
2 – transportation
3 - deposition

4. 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

5. River patterns Most common river patterns Straight Meandering Braided
Wandering
Anastomosed
Step pool

6. 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

7. Straight Uncommon in alluvial settings
Some channels confined by bedrock are straight
Low energy distributary channels in deltas
Most channels tend to meander

8. Meandering Common High sinuosity Cutbanks on outer bends
(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

9. Meandering common
Water flowing on ice commonly forms meandering forms within the ice

10. Meandering types
Display different geometry depending on local conditions
From regular to highly irregular

11. Itkillik River, Alaska
Figure

12. Meandering Stream Profile
Figure

13. Meandering processes Flow faster and deeper closer to bank
Slower and shallower closer to inside of bend

14. Meandering processes Causes deposition on inside bank
point bar
Erosion on outside bank
cut bank

15. Lateral accretion (horizontal)
Deposition and erosion occur at similar rates
Channel moves but width remains constant – dynamic equilibrium

16. 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
Oxbow cutoff

17. Meander scar
Old channel location

18. Overbank deposition
During floods, suspended sediment deposited on floodplains
Greatest amount of sediment deposited next to channel
Forms ridge called a levee

19. Floodplain features
Floodplains contains many features that record past conditions, channel locations and processes

20. 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

21. Braided rivers Channels that divide and rejoin at low flows
Dominated by bedload
Often gravel but maybe sand

22. Braided rivers Often in front of glaciers High slopes Wide and shallow
Large bars within channel, submerged during high flows

23. Braided Stream
Figure

24. Braided Stream
Figure

25. Wandering
Bella Coola
Added as a class between meandering and braiding with characteristics of both
Little Southwest Miramichi

26. Wandering Have single and multiple channel sections
Bella Coola
Have single and multiple channel sections
Moderate-high width depth
Moderate-high sed input
Little Southwest Miramichi

27. Anastomosed rivers Originally, braided and anastomosed synonymous
Anastomosed pattern like varicose veins

28. Anastomosed rivers Anastomosed reclassed as pattern with:
Interconnected semi-permanent channels
With vegetated islands
Stable banks (DG Smith)

29. Anastomosed rivers Commonly aggrading
Channel avulsions and abandonment common
Many in Australia
South Saskatchewan

30. 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

31. Classifying river patterns
Schumm (1981, 85)
Based on sediment load
Bedload
Braided
Mixed load
meandering
Suspended load
Anastomosed and highly sinuous meandering

32. Classifying river patterns
Based on airphoto interp (Mollard) and previous
Refinement included 2 axes
Based on sed supply
Sed size and gradient

33. 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)

34. Channel patterns: slope-discharge
Widely used
But problems:
Used channel slope not valley slope
Therefore, meandering lower slope than braiding

35. 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

36. 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

37. 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

38. Channel migration Erosion occurs on cutbanks depo occurs on point bars
Rate of depo and erosion approx equal
Constant width

39. 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

40. Meander geometry Wavelength Radius of curvature 10-14 x width

41. 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

42. Flow in meanders Flow generally toward outside bank Asymmetrical shape
w sloping point bar
Steep cutbank
Max depth near
cutbank

43. 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

44. 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

45. Cutoffs – avulsion After threshold sinuosity cutoffs common
Neck type most common
Become oxbow lakes
Increase channel gradient by decreasing length

46. 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

47. Riffles and pools Successive deep pools and shallow riffles downstream
Generally form with gravel beds
Occur in both straight and meandering

48. Riffles and pools Slope <1% Pools associated w meander bends
Asymmetric x-section
Gravel accumulates at riffles

49. 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

50. 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

51. 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)

52. 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

53. 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

54. Pool-riffle: hydraulic reversal
Velocity reversal
Leads to greater shear stress in pools than riffles at high flows

55. 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

56. Pool-riffle: No hydraulic reversal
However,
Studies have found that riffles and pools occur without a velocity or shear stress reversal (Latulippe 2004)

57. 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

58. Pool-riffle: formation
Convergence at pools
Increased:
shear stress
scour
Divergence at riffles
Decreased:
deposition

59. Pool-riffle Also related to river meandering
No one explanation fully satisfactory
Combination of processes