The nature of cohesive sediments Lecture 13

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The nature of cohesive sedimentsLecture 13

• What are cohesive sediments ?
• Where are they found ? Why are they important ?
  How do they behave ?
• Properties of cohesive sediments
• cohesion, adhesion, bulk density, water content,
  composition,
• the cohesive sediment cycle
• erosion, entrainment, transport, mobile
  suspensions, stationary suspensions,
  flocculation, aggregation, free settling, primary
  consolidation, secondary consolidation
• physical manifestation of cohesive sediments

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What do cohesive sediments look like ?
from space
from the ground
from the air
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Cohesive sediment transport

• cohesive sediments - clays ( d50 lt 2-5 microns)
• plate-like with ionic charges
• positive on the faces
• negative on the edges
• the ionic charges of attraction exceed the
  gravitation force
• Cohesion the bonding of like substances (clay
  minerals)
• glacial flour (no cohesion)
• kaolinite (low cohesive)
• illite, chlorite (moderate cohesion)
• Smectite (high cohesion)
• Adhesion the bonding of unlike substances
  (organic matter)
• most natural marine sediments rich in organic
  matter

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The cycle of fine-grained sediments

• settled bed
• bed erosion
• mobile suspension
• entrainment
• settling
• stationary suspension
• primary consolidation
• settled bed
• secondary consolidation

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Settling of fine-grained sediments(1)

• Sol no settling of suspension (d50 lt 1 micron)
• molecular diffusion and Brownian motion dominates
• Mobile suspension settling due to
• flocculation (electro-chemical) ? floccules
• aggregation (biological) ? aggregates, pellets
• Character of floccules
• size much greater than individual particle size
  (30-50 microns)
• density lower that individual particles
  (1050-1100 kg/m3)
• the are spherule in shape, not plates
• they are easily broken by shear
• Floccule settling rate gtgtgt particle settling rate

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Settling rate as a function of concentration (C)
Stokes settling or Impact Law
The Amazon river
Southampton Water
Settling velocity
Bristol Channel
Pyroclastic flows (Pompei)
Floccule settling
No settling (fluid mud)
Hindered settling
Log concentration
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Settling of fine-grained sediments(2)

• Floc(cule) settling falls within three distinct
  ranges
• Free settling (C lt 300 mg/L)
• floccule settling (300 lt C lt 2000 mg/L)
• hindered settling ( C gt 2000 mg/L)
• Free settling

Impact Law (Cd constant)
Stokes Law (Cd 24/Re)
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Settling rate as a function of concentration (C)
The Amazon river
Southampton Water
Settling velocity
Bristol Channel
Pyroclastic flows (Pompei)
Floccule settling
Log concentration
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Settling of fine-grained sediments(3)

• Flocculation settling as a function of
  concentration (C)
• where K1 and n are empirical coefficients
• Inter-particle collisions are high, water escape
  opposite to settling direction

C in kg/m3 Ws in m/s
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Settling rate as a function of concentration (C)
Wp peak seetling rate Cp equivalent
concentration
The Amazon river
Southampton Water
Settling velocity
Bristol Channel
Pyroclastic flows (Pompei)
Hindered settling
Log concentration
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Settling of fine-grained sediments(4)

• Hindered settling C is so high that fluid escape
  is inhibited by the numbers of particles leading
  to pore pressure build up.
• The downward sediment flux (WsC) is balanced by
  the upward flux of turbid water (WsC)
• where n 5.0, and Cp is the value at Wp

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Size of flocs

• Floc size (df) is a balance between break-up and
  formation
• Floc size is governed by the size of dissipating
  eddies of flow (i.e. the Kolmogorov microscale,
  ?)
• Where df gtgt ? inertial effects dominant
  (break-up)
• Where df ltlt ? viscous effects dominant
  (formation)
• G root mean square velocity gradient (s-1)
• W total power dissipated /unit vol/unit time
• µ absolute viscosity (kgm-1s-1)

Kinematic velocity (m2s-1)
Energy dissipation/unit mass (Nms-1kg-1)
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The stages of bed development through settling(1)

• Mixed mobile suspension
• stratified mobile suspension
• lutocline
• fluid mud layer
• stationary mud layer
• deforming cohesive bed
• stationary cohesive bed

Newtonian fluid transitional density front highly
non-Newtonian flow (hindered settling) elasto-plas
tic bed elastic bed
depth
concentration
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The nature of settling

• Ws dependent on
• salinity
• concentration
• Ws is in mm/s, and is
• maximum at 20-30 psu
• maximum at 2-4 g/L
• turbidity maximum
• k - concentration decay constant (C(t))

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The stages of bed development through settling(2)

• Bed level rises with 1/t
• the lutocline (gradient in sediment
  concentration) falls as
• where h is be height at time t, h8 is
  consolidated bed level, and k is an
  empirically-derived decay constant
• nominal bed density 1090 kg/m3
• nominal sediment density 1200 kg/m3

Einstein and Krone relationship
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The water column through settling

• Initial stage well mixed
• (20 mins)
• surface clearing,
• development of lutocline
• (4 hours)
• fluid mud (6 - 24 hours)
• stationary bed (gt 24 hours)
• consolidation profile

Sills and Been (1981)
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The stages of bed development through settling(3)

• sediment bulk density (?b)
• water content ()