Dynamique des rivières. res

Transcription

1 Dynamique des rivières res 1

2 Rivers are enormously diverse size: varies by many orders of magnitude geometry: highly variable substrate: bedrock or sediment sediment type: sediment size ranges from mud to gravel stage of development: young, with rugged topography and rapid change, to old, with gentle topography and slow change climate: ephemeral and flashy to very steady 2

3 Morphology of rivers wetted perimeters cross sectional area hydraulic radius = wetted perimeter 3

4 Morphology of rivers 4

5 Morphology of rivers The base level of a river is the elevation of the water surface of the water body, either the world ocean or a lake along the river course, into which the river flows. 5

6 Morphology of rivers Northern Tian Shan, China 6 Poisson, 2002

7 Morphology of rivers The river has some equilibrium longitudinal profile, in the sense that if conditions of tectonic, precipitation, sediment supply, and 7 base level remain constant the longitudinal profile stays the same.

8 Classifying rivers Rivers can be classified in several ways: by the nature of their substrate by the percentage of time they flow by their relationship to the groundwater table by their morphology by the kind of sediment load they carry by the dominant particle size of the bed sediment 8

9 Classifying rivers Nature of substrate detachment limited rivers transport limited rivers 9

10 Classifying rivers Nature of substrate Taiwan, bedrock river Tibet, Alluvial river 10

11 Classifying rivers Percentage of time river flows 11

12 Classifying rivers Percentage of time river flows Runoff cycle At the end of the first dry spell the river level lies below the ground water table in the river banks. After a heavy rainfall the river stage rises rapidly to lie well above the level of the ground water table in the banks. Ground water is stored in the river banks, in the sense that the ground water table is locally and temporarily higher there than in the surroundings. At the end of the rainy period both the river stage and the ground water level are of about the same height and are about at their highest. Then both the river stage and the ground water table fall back to the dry 12 spell situation shown.

13 Classifying rivers Relationship to ground water table (unsaturated zone) (Ground Water Table) (LOSING RIVER) (GAINING RIVER) 13

14 Classifying rivers Morphology 14

15 Classifying rivers Morphology l sinuosity = l L L sinuosity 1 15

16 Classifying rivers Morphology Straight Braided Minnesota, USA Meandered Sunwapta River, Alberta, Canada Anastomosed Yukon, Canada 16 Columbia River in British Columbia, Canada

17 Classifying rivers Morphology wavelength 17 amplitude radius of curvature

18 Classifying rivers Morphology 18

19 Classifying rivers Morphology 2003 McGraw-Hill Higher Education 19

20 Classifying rivers Morphology meander scrolls point bar 20 Laonong river, Taiwan

21 Classifying rivers Morphology narrow neck cutoff oxbow lake sealing channel realignment 21

22 Classifying rivers Morphology Tortous meanders 22 Confined meander scrolls

23 Classifying rivers Morphology ~100 m Torrent St Pierre, Ecrins, France Meunier,

24 Classifying rivers Morphology Meunier,

25 Classifying rivers Morphology 1.5<sinus.<1.8 sinus.>1.8 Burbank & Anderson,

26 Classifying rivers Morphology 26 Church, 2006

27 Measurements of stream flow The stage of a river is the height of the water surface of the stream above an arbitrary datum, usually either sea level or an elevation slightly below the channel bed. Stage is related to depth, but the two are not the same. 27

28 Measurements of stream flow The stage of a river is fairly easy to measure. Various kinds of stream gauges are in use. 28

29 Measurements of stream flow The discharge of a river is the volume rate of flow past a given cross section, measured in cubic meters per second, m 3 /s. Most measurement of river discharge makes use of a simple equation that relates discharge Q past a cross section to the area A of the crosssection and the mean velocity U of flow past that cross section. Q = UA u 29

30 Measurements of stream flow Torrent St Pierre, Ecrins, France 30

31 Measurements of stream flow Hydrographs 31

32 Measurements of stream flow Stage-discharge diagram or rating curve 32

33 Measurements of stream flow 33

34 Measurements of stream flow AB: end of spell without rainfall; all surface runoff has ceased, and groundwater runoff is gradually decreasing. B: surface runoff from a rainstorm reaches the channel. BC: this is the rising limb of the hydrograph; surface runoff increases sharply. 34

35 Measurements of stream flow C: this is the peak or crest of the hydrograph; surface runoff peaks. CD: this is the falling limb or recession limb of the hydrograph. Groundwater runoff peaks here somewhere, then tails off slowly; surface runoff decreases to zero. D: by this time there s no more surface runoff, only decreasing groundwater runoff. 35

36 Measurements of stream flow C A B D 36

37 Measurements of stream flow Isochrons of equal travel times of surface runoff 37

38 The resistance equation for open channel flow 38

39 The resistance equation for open channel flow α α 39

40 The resistance equation for open channel flow α 40

41 The resistance equation for open channel flow α α Hydrostatic fluid pressure on both upstream and downstream are the same and they act opposite to each other. 41

42 The resistance equation for open channel flow W α Frictional force α = τ.( 1).( W o ) Basal shear stress 42

43 The resistance equation for open channel flow A α α Downstream component of the weigth = ρ.(1).( A). g. sinα w 43

44 The resistance equation for open channel flow τ.(1). W o τ o = ρ.(1).( A). g.sinα w ρw. A. g.sinα = W d W A = W. d τ o = ρ. d. g. sinα w 44

45 Stream power river bed Δx Δh From Burbank & Anderson, 2001 Stream power = rate of change of potential energy Stream power Ω = unit length ΔE p = Δt. Δx Recall that change of potential energy Δ E p = m. g. Δh Ω = m. g. Δh Δt. Δx Discharge Q = Vw Δt = m ρ. Δt w m Δt = Q. ρ w Ω = Q. ρw. g. Δh Δx Slope S = Δh Δx Ω = Q.ρ w. g. S 45

46 W Stream power d river bed Δx Δh From Burbank & Anderson, 2001 Specific Stream power = power available per unit area of the bed Ω ω = = W ρ w gqs W But Q = W ( d cosα )( Δx / cosα ) Δt ω = ρ g( d cosα ) S w Δx / cosα Δt Δx / cosα Recall that U =,and that S = tanα Δt ρ w ω =τ U So, ω = dg sinα U o 46

47 Drainage networks divide Drainage basins, watersheds, catchments 47

48 Drainage networks 48

49 Drainage networks Putorana Plateau, Russia 49

50 Drainage networks Canadian shield 50

51 Drainage networks 51 Wyoming, USA

52 Drainage networks Cantal, France 52

53 Drainage networks Central Tibet 53

54 Drainage networks Arthur Newell Strahler, Strahler Stream Order 54

55 Drainage networks 3 Order Number of Segments Bifurcati on Ratio

56 Drainage networks Rb = Order Ra = 4.65 Rl = Order Rs = Order Order Mean Stream Area 10^6 5*10^6 5*10^7 Number of Streams Mean Stream Slope Mean Stream Length

57 Drainage networks Link Slope S = k. A θ avec 0.3 < θ < S ~ A^ Flint s Law 5*10^4 5*10^5 5*10^6 5*10^7 Link Contributing Area Data from Reynolds Creek 30 m DEM, 50 grid cell threshold, points, individual links, big dots, bins of size

58 Drainage networks Montgomery et al.,

59 Drainage networks Longest Upstream Length Lα C l h. A avec 0.5 < h < L = 1.7 A^ Hack s Law 10^3 10^5 10^7 10^9 Link Contributing Area 59