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Kamal EL KADI ABDERREZZAK

EDF-R&D, Laboratoire National d’Hydraulique et Environnement (LNHE)

PHYSICAL PROPERTIES OF SEDIMENT PARTICLES

17-19 September 2009 | UNL, Santa Fe, Argentina

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OUTLINE

I. Non-cohesive sediment particles

II. Cohesive sediment particles

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OUTLINE

I. Non-cohesive sediment particles

II. Cohesive sediment particles

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SINGLE PARTICLE (1/2)

• Sediment density ρs: ≈ 2,650 kg/m3, assumed to be constant in most rivers

• Particle size and grade scale

• Nominal diameter d: the diameter of a sphere that has the same volume as thegiven particle

• If the sediment particle is considered as an ellipsoid (a, b, and c as its diametersin the longest, the intermediate, and the shortest mutually perpendicular axes, respectively)

• Sieve diameter: the length of the side of a square sieve opening through whichthe given particle will just pass ≈ b

• Fall diameter: the diameter of a sphere that has a specific gravity of 2.65 andhas the same settling velocity as the given particle in quiescent, distilled water ata temperature of 24 °C

3 abcd = a b

c

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SINGLE PARTICLE (2/2)

• Corey Shape factor: = 0.7 for naturally particles, =1 for spherical ones

• Cohesive particles: d<60 µm

• How to measure the diameter

• For coarse particles (boulders, cobbles, and coarse gravel): direct measurements of the volume or the diameters a, b, and c

• For fine gravel and sand: sieving or visual accumulation tube • For cohesive particle (silt, clay): hydraulic settling methods (pipet method,

hydrometer method)

abcS p /=

Wu (2007)

Cobble

Sand

Gravel

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SEDIMENT MIXTURE (1/2)

• A mixture that consists of sediment particleswith non-uniform sizes

• Represented by a number of size classes

• Represented by the frequency histogramand cumulative size frequency curve

• Characteristic diameters

• Mean diameter

• Median diameter

• Cumulative percentile values

• Geometric diameter

• Effective diameter

∑=N

kkm dpd1

50d

∑=

N

kk

e

dpd

1

)/(

1

NpN

ppg dddd ....21

21=

dk=grain size for which k% of sediment is finer by weight, and pk=fraction of bed material, by dry weight, corresponding to the diameter dk, N=total number of size classes

90752510 dddd

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SEDIMENT MIXTURE (2/2)

• Uniformity

• A smaller value of σ (or Gr) corresponds to a uniform sediment mixture

• Porosity: measure of the volume of voids per unit volume of the deposit

• Uniform sediment mixture

• Non-uniform sediment mixture

• Angle of repose

• may range from 30◦ to 42◦ for non-cohesive sediment particles

16

84

d

d=σStandard deviation Gradation coefficient

+=

16

50

50

84

2

1

d

d

d

dGr

( )

≥−−+

<

+−

=

mm

mm

1)/095.0exp(175.03.0

14

525.01

00

3

1

dddd

dd

dp δ

21.050 )1.0(

0864.0245.0

dp +=

d=sediment size in mm; d0= a reference size, set at 1 mm, δ1= thickness of the water film attaching to sediment particles, set 0.0004 mm, φ in degrees

5027.15.32 d+=φ

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GRAIN SIZE DITRSIBUTION (1/2)

• Why characterize grain size distributions in terms of a logarithmic scale

• Plotted using a linear grain size scale, all the information about the finest grain sizes are crowded off the scale

Grain Size Distribution: Half Sand, Half Gravel0.0625 mm ~ 64 mm, Logarithmic Scale

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

D mm

Per

cen

t F

iner sand gravel

Grain Size Distribution: Half Sand, Half Gravel0.0625 ~ 64 mm, linear scale

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70

D mm

Per

cen

t F

iner

sand

gravel

Parker (2004)

Logarithmic scale for grain size Linear scale for grain size

Parker (2004)

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GRAIN SIZE DITRSIBUTION (2/2)

• Unimodal distributions

• The function p(d) has a single mode (or peak)

• The grain size distributions of most sand-bed streams are unimodal, and can often be approximated with a Gaussian function

• Bimodal distributions

• The function p(d) has two modes

• The grain size distributions of many gravel-bed river show bimodal grain size distributions (a sand mode and a gravelmode)

Unimodal

Bimodal

Plateau

f denotes the mass fraction of a sample that is finer than size d, p is the probability density of size d

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GRADISTAT: A GRAIN SIZE DISTRIBUTION TOOL

• GRADISTAT: a free computer programfor grain size distribution of unconsolidated sediments (by Blott(2000))

• Analysis of grain size statistics fromany of the standard measuringtechniques, such as sieving and laser granulometry

• Mean, mode, sorting, skewness andother statistics are calculatedarithmetically, geometrically andlogarithmically using moment and Folk and Ward graphical methods.

• The program runs within Excel

• producing a range of graphical outputs including frequency and ternary plots

• To download:www.kpal.co.uk/gradistat_abstract.htm

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OUTLINE

I. Non-cohesive sediment particles

II. Cohesive sediment particles

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GENERAL TRANSPORT PATTERNS

• Cohesive particles: d<60 µm

• Cohesive sediments (clay, mud, fine silt) widely exist in rivers, lakes, reservoirs, estuaries, and coastal waters

• Cohesive sediments may stick together due to the action of electrostaticalforces (flocculation)

• Flocs may be transported by convection, turbulent diffusion, and gravitational settling

• Suspension is usually the main transport mode

• Variations in flow conditions may cause sediment erosion and deposition

• The settled cohesive deposits may consolidate, due to gravity and the overlying water pressure

Wu (2007)Tsai et al. (1987)

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FLOCCULATION (1/2)

• Flocculation is affected by sediment size, concentration, salinity, turbulence, temperature…

• Sediment size

• Flocculation is negligeable for d>30µm, but becomes stronger as d reduces

• The flocculation factor varies with the median size ofthe dispersed sediment according to (Migniot, 1968)

• Concentration of sediment

• As the sediment concentration increases, the flocsettling velocity increases

• As the sediment concentration increases further, the floc settling velocity decreases

• At very large concentrations, a large number of

• particles form large-scale floc matrices; the flocsettling velocity becomes very small

Migniot (1968)

n

rw

d

dF

=

50

ωsf and ωsd=median settling velocities of flocs and the corresponding dispersed sediment particles, respectivelyn = 1.8, and dr = a reference diameter, about 0.0215 mm

Mehta (1986)

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FLOCCULATION (2/2)

• Salinity

• When salinity is low, the floc settling velocity increases rapidly as salinity When salinity exceeds a certain value, its influence on flocsettling becomes very slight

• Turbulence intensity

• For low shears, turbulence increases the chance of collision among sediment particles and thus strengthens flocculation

• For high shears, strong turbulence may break apart the flocs and attentuate flocculation

• Formulas of floc settling velocity

(Chien and Wan, 1983)

4/150

3/14/36/118.0 dCCsasf−= ςβω

2/1

20

80

= d

dς 2/15014.01 −+= dβ

Csa is the salinity

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DEPOSITION AND EROSION

• Deposition rate: Krone (1962) and Mehta and Partheniades (1975) proposed formulas to determine the deposition rate

τ=shear stress, τmin=critical bed shear stress below which all sediment particles have a full probability to deposit on the bed, τmax=critical bed shear stress above which all sediment particles remain in suspension

• According to Krone (1962), τmin = 0, whereas Mehta and Partheniades (1975) found that τmin might be larger than zero

• Erosion rate: According to Partheniades (1965), the surface erosion rate is a linear function of the dimensionless excess shear stress

M=erodibility coefficient, and τce=critical shear stress for erosion

−=

ce

ceb ME

τττ

maxmin

max

min,0max,0min

min

01

)/()(1

1

τττττ

ττττττ

≤≤

≥−

−−−≤

if

if

if

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CONSOLIDATION

• Consolidation is a compaction process of deposited materials under the influence of gravity and water pressure with a simultaneous expulsion of pore water and a gain in strength of bed materials

• Variation of bed density

• The dry bed density varies along the depth below the bed surface (Hayter, 1983)

• Lane and Koelzer (1953) proposed a formula to determine the dry density of bed material in the consolidation process

ρd=dry density of bed, H=bed thickness, z=depth below the bed surface, and a and m= coefficients dependent

ρd0 = dry density after 1 year of consolidation