1. An introduction to cohesive sediment transport processes
Bas van Maren

2. Contents (1) Fine sediment transport processes
(erosion, flocculation, consolidation)
(2) Large-scale transport
(estuaries and coastal seas)
(3) Implications for & effect of ecology
E = Erosion rate [kg/m^2/s]
M = Erosion parameter [kg/m^2/s]
Τb = bed shear stress [Pa]
Τe = erosion treshold shear stress [Pa]
Destabilizers => Arenicola / Lugworm: lives in burrows in the sediment at depths of 20-40cm. It feeds on organic matter in the sediment by drawing water into the burrow and filtering organic particles.

3. 1) Fine sediment processes
Fine sediment characteristics
Flocculation (& consolidation)
Erosion

4. Sand, silt and clay fractions
English
Min. size (µm)
Max. size (µm)
Sand 63
2000
Silt 2
Clay
Mud
Sand, silt and clay fractions

5. Flocculation / (Hindered) settling / Bed formation / Erosion
1) Fine sediment processes
Flocculation / (Hindered) settling / Bed formation / Erosion
E = Erosion rate [kg/m^2/s]
M = Erosion parameter [kg/m^2/s]
Τb = bed shear stress [Pa]
Τe = erosion treshold shear stress [Pa]
Destabilizers => Arenicola / Lugworm: lives in burrows in the sediment at depths of 20-40cm. It feeds on organic matter in the sediment by drawing water into the burrow and filtering organic particles.

6. Flocculation
Single particle velocity of mud is to 1 mm/s (clay: – mm/s)

7. Flocculation
Single particle velocity of mud is to 1 mm/s (clay: – mm/s)
But observed settling velocity between 0.1 and 10 mm/s

8. Flocculation
Single particle velocity of mud is to 1 mm/s (clay: – mm/s)
But observed settling velocity between 0.1 and 10 mm/s
 Flocculation due to
salinity
turbulence
concentration pH

9. Flocculation Salinity & Ph
1) Clay particles are negatively charged (red)
=> repulsive force

10. Flocculation Salinity & Ph
1) Clay particles are negatively charged => repulsive force
2) Van der Waals force (green)
=> attractive force

11. Flocculation Salinity & Ph
1) Clay particles are negatively charged => repulsive force
2) Van der Waals force => attractive force
3) Double diffusive layer
=> neutralizes the negative charge, enhancing attraction
=> thickness depends on availability ions
=> attraction depends on ion concentration

12. Ion concentration depends on salinity & pH
very easy to flocculate, strong flocs
difficult to flocculate, weak flocs
very easy to flocculate, compact and strong core
easy to flocculate, compact but weak flocs
STABLE
Ion concentration depends on salinity & pH

13. Ion concentration depends on salinity & pH
very easy to flocculate, strong flocs
difficult to flocculate, weak flocs
very easy to flocculate, compact and strong core
easy to flocculate, compact but weak flocs
STABLE
Sea
Estaries
Rivers
Ion concentration depends on salinity & pH

14. Flocculation Floc size and structure depends on: pH and Salinity:
Floc strength decreases with pH
Floc compaction increases with salinity
Turbulent energy and sediment concentration
Floc size increase with concentration
Equilibrium floc size decreases with turbulence (floc break-up)
In reality, the floc size at low turbulence rate is low because of long timescales.

16. Flocculation Floc size and structure depends on: pH and Salinity:
Floc strength decreases with pH
Floc compaction increases with salinity
Turbulent energy and sediment concentration
Floc size increase with concentration
Equilibrium floc size decreases with turbulence (floc break-up)
In reality, the floc size at low turbulence rate is low because of long timescales.
Organic matter
Floc size increases with the amount of organic material (polymers)

17. + = Significant influence of organic matter:
Organic matter in mud consists mainly of polymers
(Winterwerp & Van Kesteren, 2005)
Polymers adsorb to clay and enhance flocculation by:
- Neutralizing the particle charge
- Bridging between particles + =
Mud flocs Polymers Larger flocs

18. Hindered settling and consolidation
Settling primary particles
Flocculation
Hindered Settling = sedimentation of high-concentrated suspensions where particles are retarding each other. Reduction settling velocity 10-90%
Sedimentation
Consolidation = expulsion of excess pore water by the weight of the sediment

19. Erosion Erosion types Sand-silt-clay mixtures
Floc erosion, mass erosion
Sand-silt-clay mixtures
Effect clay
Effect silt

20. Van Kesteren, WL|DelftHydraulics Van Kesteren, WL|DelftHydraulics
Erosion types
Van Kesteren, WL|DelftHydraulics
Surface erosion
= drained process
E = Erosion rate [kg/m^2/s]
ME = Erodibility parameter [m/Pa*s] => [m] relates to the vertical decrease of the bed height: erosion depth
Cv = consolidation parameter=> permeability
φs = volume concentration (phi) sediment t = 0
Rho dry = dry density
D50 = median grain size
Cu = undrained shear strength
Why dry density, relative water content
This formula is promising and that is why we continue on this way/direction
(Winterwerp & Van Kesteren, 2005)
Mass erosion
= undrained process
Van Kesteren, WL|DelftHydraulics

21. Cohesive strength / plasticity
Erosion type
Cohesive strength / plasticity
Rate of pore water dissipation: permeability and capacity to deform
E = Erosion rate [kg/m^2/s]
M = Erosion parameter [kg/m^2/s]
Τb = bed shear stress [Pa]
Τe = erosion treshold shear stress [Pa]
Destabilizers => Arenicola / Lugworm: lives in burrows in the sediment at depths of 20-40cm. It feeds on organic matter in the sediment by drawing water into the burrow and filtering organic particles.
Low permeability High permeability

22. Low permeability sediment, undeformed swelling fracturing
Erosion type
Low permeability sediment, undeformed
swelling
fracturing
Van Kesteren
E = Erosion rate [kg/m^2/s]
M = Erosion parameter [kg/m^2/s]
Τb = bed shear stress [Pa]
Τe = erosion treshold shear stress [Pa]
Destabilizers => Arenicola / Lugworm: lives in burrows in the sediment at depths of 20-40cm. It feeds on organic matter in the sediment by drawing water into the burrow and filtering organic particles.
Deformations and pore water pressures

23. Clay / sand mixtures (Van Ledden, 2003)

24. Effect silt & clay Less pore volume, therefore reduced permeability
Reduction in erosion rate
Increased undrained erosion with clay content in clay-sand mixtures
Also undrained erosion in silt beds
Floc erosion  mass erosion

26. 3) Large-scale transport
Estuarine Turbidity Maximum

27. ETM formation 1) Upstream transport by tidal asymmetry
Sand: maximum flow asymmetry

28. ETM formation 1) Upstream transport by tidal asymmetry
Sand: maximum flow asymmetry

29. ETM formation 1) Upstream transport by tidal asymmetry
Sand: maximum flow asymmetry
Mud: also / mainly slack tide asymmetry
(combination with settling lag)

30. ETM formation
(1)
(1) beginning of flood: high flow velocities and fully mixed concentration profile  upstream transport

31. ETM formation
(2)
(1) beginning of flood: high flow velocities and fully mixed concentration profile  upstream transport
(2) end of flood: low flow velocities, sediment settling from suspension  upstream transport

32. ETM formation
(3)
(1) beginning of flood: high flow velocities and fully mixed concentration profile  upstream transport
(2) end of flood: low flow velocities, sediment settling from suspension  upstream transport
(3) beginning of ebb: low flow velocities, no sediment in suspension  no transport

33. ETM formation
(4)
(1) beginning of flood: high flow velocities and fully mixed concentration profile  upstream transport
(2) end of flood: low flow velocities, sediment settling from suspension  upstream transport
(3) beginning of ebb: low flow velocities, no sediment in suspension  no transport
(4) end of ebb: low flow velocities, sediment settling from suspension  downstream transport

34. ETM formation
Residual transport by slack tide asymmetry due to settling lag:
- always transport during flood
- partly transport during ebb

35. ETM formation 1) Upstream transport by tidal asymmetry
2) Upstream transport by settling lags and scour lags
Fine sediment is transported from areas with high flow velocity to low flow velocity

36. ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport

37. u u 1) u(A) < ucr, no transport 2) u(A) > ucr, upflat transport
ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport
2) u(A) > ucr, upflat transport

38. u u 1) u(A) < ucr, no transport
ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport
2) u(A) > ucr, upstream transport
3 u(B) < ucr, sediment transported upflat while settling

39. u u 1) u(A) < ucr, no transport
ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport
2) u(A) > ucr, upstream transport
3) u(B) > ucr, sediment transported upstream while settling
4) u(B) < ucr, no transport

40. u u 1) u(A) < ucr, no transport
ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport
2) u(A) > ucr, upstream transport
3) u(B) < ucr, sediment transported upstream while settling
4) u(B) < ucr, no transport
5) u(B) > ucr, downflat transport

41. u u 1) u(A) < ucr, no transport
ebb
ebb u u
flood
flood
1) u(A) < ucr, no transport
2) u(A) > ucr, upstream transport
6) u(A) < ucr, sediment transported downflat while settling
3) u(B) < ucr, sediment transported upstream while settling
4) u(B) < ucr, no transport
5) u(B) > ucr, downstream transport

42. ebb
ebb u u
flood
flood
Settling lag effect: sediment is transported landward because u(A) > u(B),
Therefore: the period between u = 0 and u = ucr is longer at B than at A.

43. ETM formation 2) Upstream transport by settling lags and scour lags
Fine sediment is transported from a high energy environment to a low energy environment
Upstream transport by gravitational circulation
4) Downstream transport by river flow
Hence: estuary will fill in in absence of river flow (effect dams!)

44. 3) Large-scale transport
Estuarine Turbidity Maximum
River plumes

45. 4 Interaction mud & ecology
Effect ecology on mud
Settling velocity (increasing flocculation rate)
Biostabilisation (reduced erosion rate)
Bioturbation (increased erosion rate)
Sediment trapping (mangroves, salt marshes, mussel beds)

46. Bioturbation Biostabilisation E = Erosion rate [kg/m^2/s]
M = Erosion parameter [kg/m^2/s]
Τb = bed shear stress [Pa]
Τe = erosion treshold shear stress [Pa]
Destabilizers => Arenicola / Lugworm: lives in burrows in the sediment at depths of 20-40cm. It feeds on organic matter in the sediment by drawing water into the burrow and filtering organic particles.
Bioturbation
Biostabilisation

47. Effect mangroves on mud
Coastal protection  loss of mangroves results in erosion
Sediment trap  loss of mangroves results in
increased turbidity which harm corals and sea grass
Increased sedimentation in tidal channels
Salinity filter  loss of mangroves results in increased salinity

48. Interaction mud & ecology
Effect ecology on mud
Settling velocity (increasing flocculation rate)
Biostabilisation (reduced erosion rate)
Bioturbation (increased erosion rate)
Sediment trapping (mangroves, salt marshes, mussel beds)
Effect mud on ecology
High C reduces primary production (making of organic compounds in the water column)
High sedimentation rates suffocates sea grass and corals

49. Effect mud on corals & sea grass
Effect mud on corals: Sediment particles stick on corals and choke them
High sedimentation usually because of
Increased mud supply by land clearance (destruction of forests)
Dredging
Loss of mangroves (sediment sink)
Effect loss of corals & sea grass:
Nursery room for fish
Coastal defense