1. Modeling of the interactions of the cohesive sediments with the Mangrove
Sylvain Guillou
University of Caen Normandy
Applied sciences laboratory of Cherbourg, LUSAC, EA4253
50130 Cherbourg-Octeville, France
LMDCZ Workshop, April 2017

2. Mangrove slows down the flow inside
Modeling of the interaction of the cohesive sediments with the Mangrove
Mangrove is in mud areas
Mangrove slows down the flow inside
Mangrove are on the edge of the area are more exposed to the attack of the waves and the flow 2

3. Fluid mud Soft mud Compact mud Introduction Transport Flocculation
Cohesive sediment: Physical phenomenon
Transport
Flocculation
Deposition
Erosion
Entraînement
arrachement de copeaux
Fluid mud
Soft mud
Compact mud
Transport
Exchanges
Evolutions 3

4. SPM transportation Surface : bottom : Cohesive sediment:
reservoir
Flocculation
Deposition
Erosion
Transport
bottom
Non erodible bed
: components of the fluid velocity
: vertical and horizontal diffusivity
: Concentration in SPM C
u,v,w
: settling velocity Ws
KMC, AMC
: Erosion and deposition fluxes
Fe, Fd
Surface :
bottom : 4

5. Falling velocity for cohesive sediments
SPM transportation
Cohesive sediment:
reservoir
Flocculation
Deposition
Erosion
Transport
Falling velocity for cohesive sediments
bottom
Non erodible bed
Deposition flux
: Deposit critical shear stress
cd
ce
: Erosion critical shear stress b
: Bed shear stress
Krone(1962)
Erosion flux
Partheniades (1962) 5

6. Consolidation
Cohesive sediment:
𝜏 𝑐𝑒 6

7. Multi-layer model Consolidation Teisson (1993) Cohesive sediment: 7
mud
Flocculation
Deposition
Erosion
Transport
bottom
non erodible bed
Multi-layer model C1 C2 C3 C4 C5 C6 e1 e2 e3 e4 e5 e6 t1 t2 t3 t4 t5 t6 C1 C2 C3 C4 C5 C6
Teisson (1993) 7

8. Multi-layer model Consolidation Teisson (1993) Teisson (1993)
Cohesive sediment:
Model based on the time residence C1 C2 C3 C4 C5 C6 e1 e2 e3 e4 e5 e6 t1 t2 t3 t4 t5 t6 C1 C2 C3 C4 C5 C6
mud
Flocculation
Deposition
Erosion
Transport
bottom
non erodible bed
Teisson (1993)
Teisson (1993) 8

9. Consolidation
Cohesive sediment: Model based on the time residence 9

10. Expulsion of the interstitial water (permeability)
Consolidation
Cohesive sediment: Model coming from soil mechanics Théorie de Kynch, Gibson, Tan, ...
Gibson’s equation :
Expulsion of the interstitial water (permeability)
Structuration of the mud (effective stresses) 10

11. Calculation of the flux between two layers
Consolidation
Cohesive sediment: Multilayer based on the Gibson equation
Thickness of a layer :
Calculation of the flux between two layers
Particles’ velocity :
Darcy’s law, Postulat of Terzaghi, Continuity
Hypothesis :
There exists one transitional concentration Ct between sedimentation and consolidation
(Thiebôt and guillou, 2006) 11

12. Consolidation
Cohesive sediment: Multilayer based on the Gibson equation
Thiébôt and Guillou, 2006. 12

13. Consolidation
Experimental mass concentration profiles (blue line, from Bartholomeeusen et al., 2002 ) and simulated sediment mass concentration profiles (stepped profiles in green from Thiébot, Guillou, Brun-Cottan, CSR, 2011). 13

14. Consolidation Cohesive sediment: Intercomparison of models
Phd of Lan Anh Van (Numerical modelling of sand-mud mixtures settling and transport processes : Application to morphodynamic of the Gironde estuary (France), LHSV, LNHE
Model 1 :
Faster
Development of a process to calculate the transfer coefficient
Model 2 (Gibson eq.):
more physic: Effective stress, permeability
Need the determination of the effective stress and the permeability 14

15. Cohesif/non cohesive Sediment in TELEMAC
Cohesive sediment: The two models have been implemented in SISYPHE and are available in the open source code
Mixture can be considered:
two modes: non cohesive sediment of mean diameter Dns and a cohesive sediment of diameter less than 60 m
Coupling of hydrodynamic, waves and Sediments evolutions
(TELEMAC, TOMAWAC, SISYPHE)
Mixture
Sisyphe user’s manual 15

16. Interactions with the Manvrove
Mangrove is a vegetation with small diameter regarding the size of the area
It could be considered as
a resistant force
a porous media S.
Laboratoire de Construction Hydraulique, EPFL 16

17. Interactions with the Mangrove
Mangrove is a vegetation with small diameter regarding the size of the area
It could be considered as
a resistant force to the flow
a porous media
Laboratoire de Construction Hydraulique, EPFL 17

18. Interactions with the Mangrove
Mangrove is a vegetation with small diameter regarding the size of the area
It could be considered as
a resistant force to the flow
a porous media
Porosity
J. Lhomme, S. Soares-frazão, V. Guinot, Y. Zech, Large scale modelling of urban floods and 2D shallow-water model with porosity, LHB, 2007. 18

19. Interactions with the Mangrove
Mangrove is a vegetation with small diameter regarding the size of the area
It could be considered as
a resistant force to the flow
a porous media
J. Lhomme, S. Soares-frazão, V. Guinot, Y. Zech, Large scale modelling of urban floods and 2D shallow-water model with porosity, LHB, 2007. 19

20. Simulate the impact of the waves on the mud (TOMAWAC)
Modeling of the interactions of the cohesive sediments with the Mangrove
Cohesive sediment with sedimentation-consolidation process (SISYPHE): Model 1
Modelling the impact of the vegetation on the flow in the mangrove with a porous approach (TELEMAC)
Simulate the impact of the waves on the mud (TOMAWAC) 20

21. Modeling of the interaction of the cohesive sediments with the Mangrove
Modelling could be like that:
Mesh produced by Thong
Definition of:
Mud bed composition per zone
Consolidation model: Ci, Ai, Ti, Tauci
SPM model: Settling velocity
Zones of Mangrove and porosity
Tidal flat area 21

22. Thank you for your attention
To be continued
LMDCZ Workshop, April 2017