OCDAG

Meeting one

Theory

Basic concepts

OCDAG first meetingJune 5, 2007

Basic Concepts

• Discharge

• Cross sections (hydraulic geometry)

• Bankfull discharge

• Sediment

• Long profiles

Measuring surface water

• Runoff is measured using discharge (Q) – Volume of water passing a

cross-section per time (m3/s)

Discharge = average velocity * cross-section area

Q = VA

Hydrograph

• Plot of discharge trough time

• Has characteristic shape and propertiesFlood

Cross-sections

• Area of channel occupied by water

• Changes through hydrograph

Cross-sections

• Display shape

• Have a width (w) and depth (d)

Width

AvgDepth

Cross-sections

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

• Cross-sections become more asymmetric with channel meandering

Cross-sections

Cross-sections

• 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

Bankfull discharge (Qbf)

• Bankfull level just before flooding occurs

Bankfull discharge (Qbf)

• Dominant or channel forming discharge

• Channel cross-sections adjusted to pass the flow that just fills its banks

• Minimum width-depth ratiomay be used to determinebankfull level

Bankfull discharge (Qbf)

• Flow which cumulatively transports the most sediment

• Range of recurrence intervals but 1.6 or 2.33 years common

At a station hydraulic geometry

• Changes in channel geometry at a cross-section through timew = aQb

avg d = cQf

avg v = kQm

At a station hydraulic geometry

• Q = vwd = aQb *cQf *kQm

– 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

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

Downstream hydraulic geometry

• For natural streams

• Also in the form w = aQbfb, avg d = cQbf

f, avg v = kQbfm

• Qbf is bankfull discharge

• With increasing discharge– Width and depth

increase downstream

w = 3.67Qbf0.45

d = 0.33Qbf0.35

v = 0.83Qbf0.20

s = 0.008Qbf-0.20

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

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

Sediment characteristics

• A, B and C axes• Clast shape described as

– Elongation

– Flatness

– Rounded

• Effects packing of clasts and entrainment

A

B

C

Eg. Of rounded cobbles

Grain size

classes• Mesh number• Φ = log2 D • Note: large particles are negative in Φ• Diameter in mm (D)• Wentworth class

– descripitve

Grain size histogram• Provides picture of

distribution• Cannot be used to easily

determine statistical info

Bimodal distributions

• Often occur in fluvial sediments due to depositional processes

• E.g. sand may infill spaces between gravel

• Difficult to deal with

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%

Grain size curves

• Relationship between histogram and cumulative frequency

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

Reading grain size curves

• Read specific grain sizes off curve

• Important grain sizes– Median diameter (D50) =

diameter in mm of the 50th percentile on the cumulative curve

– D16 and D84 the 16th and 84th percentile on the grain size curve

Mean, median, mode

• Mode most frequently occurring grain size– not very useful

• Median diameter (D50 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 pictureValues in (Folk and Ward 1957)

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

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)

Field grain size measurement

• Four main methods:– Pebble counts– Visual estimation– Photographic techniques– Bulk samples

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

Visual estimation

• Determine grain size in 20 plots

• Calibrate the estimation of D16, D50 and D84

– 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

Grain size map from Visual technique

Photographic techniques

• Quadrants are photographed

• Number of exposed clasts counted

• Calibration curve is developed for the number of clasts and the D50

• Greater # of particles the lower the D50

• Advantage – fast + more data

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

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

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

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

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

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

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.

Long profile

• A plot of channel bed elevation and downstream distance

• Are concave over long sections

• May be straight or convex over short sections