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OCDAG Meeting one Theory. Basic concepts OCDAG first meeting June 5, 2007.

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Presentation on theme: "OCDAG Meeting one Theory. Basic concepts OCDAG first meeting June 5, 2007."— Presentation transcript:

1 OCDAG Meeting one Theory

2 Basic concepts OCDAG first meeting June 5, 2007

3 Basic Concepts Discharge Cross sections (hydraulic geometry) Bankfull discharge Sediment Long profiles

4 Measuring surface water Runoff is measured using discharge (Q) –Volume of water passing a cross-section per time (m 3 /s) Discharge = average velocity * cross-section area Q = VA

5 Hydrograph Plot of discharge trough time Has characteristic shape and properties Flood

6 Cross-sections Area of channel occupied by water Changes through hydrograph

7 Cross-sections Display shape Have a width (w) and depth (d) Width Avg Depth

8 Cross-sections Cross-sectional shape related to deposition of bars and bank erosion

9 Cross-sections become more asymmetric with channel meandering Cross-sections

10 May be defined by width to depth ratio w/d Narrow and deep channels = low width to depth ratio Wide and shallow channels = high width to depth ratio

11 Bankfull discharge ( Q bf ) Bankfull level just before flooding occurs

12 Bankfull discharge ( Q bf ) Dominant or channel forming discharge Channel cross-sections adjusted to pass the flow that just fills its banks Minimum width-depth ratio may be used to determine bankfull level

13 Bankfull discharge ( Q bf ) Flow which cumulatively transports the most sediment Range of recurrence intervals but 1.6 or 2.33 years common

14 At a station hydraulic geometry Changes in channel geometry at a cross- section through time w = aQ b avg d = cQ f avg v = kQ m

15 At a station hydraulic geometry Q = vwd = aQ b *cQ f *kQ m –a *c * k = 1 –b+f+m = 1 m>f>b and m>f+b Values of b, f, and m depend on cross-sectional shape f m b

16 Downstream changes through a basin Downstream in a basin 3 zones: –1 – erosion – Step pool –2 – transportation –3 - deposition Increase –Discharge –Width –Depth –Velocity –Stored alluvium (sediment in flood plain) Decrease –Grain size

17 Downstream hydraulic geometry For natural streams Also in the form w = aQ bf b, avg d = cQ bf f, avg v = kQ bf m Q bf is bankfull discharge With increasing discharge –Width and depth increase downstream w = 3.67Q bf 0.45 d = 0.33Q bf 0.35 v = 0.83Q bf 0.20 s = 0.008Q bf -0.20

18 Sediment The unconsolidated grains of minerals, organic matter or pre-existing rocks –transported by water, ice or wind Clastic – from pre-existing rocks Clast – one particle

19 Importance of grain size Grain size influences –Sediment transport –Hydraulic roughness –Hydraulic conductivity (ground water flow) –Aquatic habitat Salmon spawning Feeding locations for some fish Location of benthic aquatic insects

20 Sediment characteristics A, B and C axes Clast shape described as –Elongation –Flatness –Rounded Effects packing of clasts and entrainment A B C

21 Eg. Of rounded cobbles

22 Grain size classes Mesh number Φ = log 2 D Note: large particles are negative in Φ Diameter in mm (D) Wentworth class –descripitve

23 Grain size histogram Provides picture of distribution Cannot be used to easily determine statistical info

24 Bimodal distributions Often occur in fluvial sediments due to depositional processes E.g. sand may infill spaces between gravel Difficult to deal with

25 Cumulative grain size curves Arithmetic ordinate –Most widely used –Curve makes an S shape Probability ordinate –Plotted on probability graph paper –Based on the normal distribution –Results in straight line whose slope depends on sorting –Probability scale is condensed in middle and expanded at the ends 99.9%

26 Grain size curves Relationship between histogram and cumulative frequency

27 Creating grain size curves Obtain weight in each class Determine % in each class (weight in class/total weight) Add percentages from largest to smallest grain size to determine cumulative frequency

28 Reading grain size curves Read specific grain sizes off curve Important grain sizes – Median diameter (D 50 ) = diameter in mm of the 50th percentile on the cumulative curve –D 16 and D 84 the 16 th and 84 th percentile on the grain size curve

29 Mean, median, mode Mode most frequently occurring grain size –not very useful Median diameter (D 50 or Φ50) = Diameter of the 50th percentile on the cumulative curve –50% larger + 50% smaller –Poorly represents bimodal dist Graphic Mean = Φ16 + Φ50 + Φ84 3 Gives better overall picture Values in  (Folk and Ward 1957)

30 Descriptions of grain size Sorting (So) So = Φ84 - Φ16 + Φ95 – Φ5 4 6.6 Values in  Φ (Folk and Ward 1957) Well sorted Very poorly sorted Values from  Equal   very well sorted  well sorted 0.50  moderately well sorted  moderately sorted  poorly sorted  very poorly sorted 4.00  extremely poorly sorted

31 Descriptions of grain size Skewness = Φ16+Φ84-2 Φ50 + Φ5 + Φ95-2 Φ50 2(Φ84-Φ16) 2(Φ95-Φ5) between -10 and +10 normal (>+10 positively skewed) Kurtosis (peakedness) = Φ95-Φ5 2.44(Φ75-Φ25) between 1.11 and 1.50 more peaked (middle of curve more sorted than ends) Values in  (Folk and Ward 1957)

32 Field grain size measurement Four main methods: –Pebble counts –Visual estimation –Photographic techniques –Bulk samples

33 Pebble counts –Also called Wolman (developed by Wolman 1954) and grid by number sample –Pick a patch of bed and randomly pickup and measure the B axis of 100 clasts some say 400 for 95% confidence of +/- 1 Φ –Measures the surface grain size – Quantifies the grain size of an area

34 Visual estimation Determine grain size in 20 plots Calibrate the estimation of D 16, D 50 and D 84 –Visually estimate grain size a number of times until you constantly obtain the correct answer Use your calibrated eye to estimate grain size Disadvantage – less accurate Advantage – fast – can create grain size maps

35 Grain size map from Visual technique

36 Photographic techniques Quadrants are photographed Number of exposed clasts counted Calibration curve is developed for the number of clasts and the D 50 Greater # of particles the lower the D 50 Advantage – fast + more data

37 Bulk samples Dig a volume of sediment Sort sediment into size classes using a plate with square holes of specific size Weigh sediment in each size class Determine the % by weight in each size class Can be done for pavement or sub-pavement Between 256 mm and 32 mm Weight of the heaviest clast cannot be greater than 1-3 % of the total weight –with one 10 kg rock you need 1000 kg sample

38 Lab grain size measurement Sieve –To extend the grain size curve below gravel or to determine for sand and fine gravel For fine sed –Hydrometer –Setigraph –Laser diffraction

39 Sieving Use sieves with different size precisely sized openings Stack with largest on top to smallest on bottom Shake Weigh sediment in each size class remaining on each sieve

40 Settling velocity Settling velocity related to the diameter of the sediment Coarser sediment falls out of suspension first, followed by smaller and smaller grain sizes Defined by stokes law Sphere falling freely through liquid obtains max vel that depends on its diam Also related to the density of the fluid

41 Hydrometer Measures liquid density at given times For fine sand, silt and clay Density of soil - water suspension depends on concentration and specific gravity of soil Suspension allowed to stand –Particles settle Hydrometer used to measure density changes through time Stokes law used to calculate the max particle diameter in suspension

42 Sedigraph Uses same principals as Hydrometer (settling velocity) A cleaned, disaggregated sample is dispersed in fluid Sedimentation rate measured using low energy X- rays through sample to a detector Particles absorb X-rays Percentage of X-ray beams that reach the detector related to grain size

43 Laser diffraction or laser sieve analysis A cleaned, disaggregated sample is dispersed in fluid Grains cause diffraction of a laser beam directed through the fluid. Angle of scattering is inversely proportional to the particle size Intensity of scattering is proportional to the number of particles.

44 Long profile A plot of channel bed elevation and downstream distance Are concave over long sections May be straight or convex over short sections

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