OCDAG

1. 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 • (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 14.15

12. Meandering Stream Profile Figure 14.15

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. 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

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 14.14

24. Braided Stream Figure 14.14

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 • Moderate-high width depth • Moderate-high sed input Bella Coola 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 • 10-14 x width • Radius of curvature • 2-3 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