Effect of harbour siltation and dredged material release

1. Delft3D user conference, 2015
Effect of harbour siltation and
dredged material release on
suspended sediment levels in the
marine environment using D-WAQ
Thijs van Kessel (Deltares)

2. Frame of reference 1
Plume
Natural background
SPM = suspended
particulate matter

3. Frame of reference 2
• Natural background
• Maintenance dredging
• Capital dredging
• MV2 construction
• Examples from North Sea,
Scheldt and Ems

4. Different types of mud models
Natural mud dynamics
1. Prescribe bed sediment availability, compute SPM (may be 2D)
2. Compute both bed sediment availability and SPM (3D)
- First type easier to calibrate, but may give wrong answers at long
time scale or for scenario studies (e.g. change in bathymetry)
- Direct impact: hydrodynamics → SPM
- Indirect impact: hydrodynamics → mud distribution → SPM
- At Deltares we aim for 2 (in principle)
Plume studies
1. Excess (one or few plumes)
2. Including background (many plumes)
1 december 2015

5. Coupled versus uncoupled computation
Coupled Delft3D-FLOW
+ Effect of SPM on water density and hydrodynamics
+ Effect of erosion and deposition on bathymetry and hydrodynamics
 Heavy computations
Uncoupled Delft3D-WAQ
+ Fast computations,
+ Many mud runs for one hydrodynamic run
+ Longer timeframe than hydrodynamic timeframe possible (rewind)
 No effect of SPM and erosion, deposition on hydrodynamics
 So confined to low C and morphologically stable areas

6. Bed module
- 2L bed module (buffer concept)
- nL mixed Eulerian-Lagrangian module (sand-mud morphology)

7. Residence time
• probability of particle in suspension:
P = Ch / (Ch + m +p d2 ρbed)
• local residence time:
RTl = mbuf / (αws C)
• area residence time:
RTa = L/(Pures)
Problem: SPM observations insufficient to calibrate
residence time

8. Measured and observed Cd content
1986 2000
Ref Validatiesom fase 1
BufHi BufLo

9. Contaminants Transport: Numerical Experiment

10. Illustration: effect of buffer capacity
without buffer layer with buffer layer
Where does the plume go? Does it deposit (permanently)?
Or does it resusped and is it flushed out of the system?

11. Impact of sand mining on SPM concentration
variable years 1996-2003
0
5
10
15
20
25
30
35
40
45
50
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
year
SPM(mg/l)
background
background+sand mining
Problem: how to measure a small change compared to a large variability?
Kessel, T, van, J.C. Winterwerp, B. van Prooijen, M. van Ledden, W. Borst.
(2011). Modelling the seasonal dynamics of SPM with a simple algorithm for
the buffering of fines in a sandy seabed. Continental Shelf Research 31 S124–
S134. DOI:10.1016/j.csr.2010.04.008.

12. Scheldt estuary
Wal D. van der, T. van Kessel,
M.A. Eleveld, J. Vanlede (2010).
Spatial heterogeneity in estuarine
mud dynamics. . Ocean
Dynamics doi:10.1007/s10236-
010-0271-9.
Kessel, T. van, J. Vanlede, J.M.
de Kok (2011). Development of a
mud transport model for the
Scheldt estuary.
Continental Shelf Research 31
S165–S181 DOI:
10.1016/j.csr.2010.12.006.

13. Salinity and SPM along thalweg
1 december 2015

14. Effect of Sloe harbour (1)
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15. Effect of Sloe harbour (2)
1 december 2015
Nice feature: dredging and release module
including (re)labelling

16. Mud origins near Vlissingen
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17. Mud origins near Antwerp
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18. Explanation of issues at Antwerp
• Narrower estuary
• ETM formation, location dependent on freshwater discharge
• Construction of new tidal dock (DGD)
• Deepening
• Possibly increasing trend SPM
• High maintenance volumes and cost
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19. Shift of release location Antwerp
1 december 2015
Kessel, T. van, Vanlede, J., Van Holland, G. (2015). Human versus natural mud fluxes in
the Scheldt Estuary: are they significant and if so, how can they best be optimised? E-
proceedings of the 36th IAHR World Congress, The Hague, the Netherlands.

20. The Ems Estuary: modeled SSC
1 december 2015
Maren, D.S., T. van Kessel, K.
Cronin, L. Sittoni (2015). The impact
of channel deepening and dredging
on estuarine sediment concentration.
Continental Shelf Research 95, 1-14.
doi:10.1016/j.csr.2014.12.010.

21. Mud balance
• Ratio between dredging and deposition (by nature, not maintained)
• R → 0 : Safe! Natural mud balance. Small recirculation
• R → 1 : Danger! Human-dominated mud balance. Large
recirculation, dumping location critical
• No ‘safety valve’ means: sedimentation pressure builds up!
• Solution:
• create sedimentation space
• or release dredged material at sea or store on land
• or reduce tidal asymmetry, estuarine circulation (depth)
1 december 2015

22. Think outside-in
Plume
Natural background

23. Conclusions
• Different approaches presented
background ↔ excess, coupled ↔ uncoupled, SPM only ↔ bed
• With focus on LoT and LaS behaviour
• Capital dredging plumes enhance SPM levels
• LoT and LaS impacts much depend on exchange with seabed
• Maintenance dreding plume not necessarily enhance SPM levels
• Balance between siltation (decrease) and release (increase)
• If siltation in harbours and access channels >> accretion tidal flats
and saltmarshes: engineered system
• In that case: a large part of the mud is ‘second hand’
• Both concentration in water column and siltation rate can be
influenced significantly by adapting the maintenance strategy
• Choose your model approach accordingly
1 december 2015