Sedimentation in the Botlek Harbour

opossumoozeMécanique

21 févr. 2014 (il y a 3 années et 7 mois)

87 vue(s)

Port Research Centre
Sedimentation in
the Botlek Harbour
A research into driving water exchange mechanisms
Sedimentation in
the Botlek Harbour
A research into driving water exchange mechanisms
January 2013, Adil El Hamdi
ISBN/EAN: 978-94-6186-126-9
NUR-code: 950
Reeksnummer: 41
© Port Research Centre Rotterdam-Delft.
Gebruik van gegevens en teksten is met bronvermelding vrijelijk toegestaan. Commercieel gebruik van deze gegevens is niet toegestaan.
Voor informatie over de publicaties neem contact op met het secretariaat van het Port Research Centre Rotterdam-Delft:
Technische Universiteit Delft
T.a.v. dr.ir. R.M. Stikkelman
Postbus 5015
2600 GA Delft
Nederland
T +31 (0)15 278 72 36
E r.m.stikkelman@tudelft.nl
I www.prc.tudelft.nl
Havenbedrijf Rotterdam
Postbus 6622
3002 AP Rotterdam
Nederland
T +31 (0)10 252 10 10
F +31 (0)10 252 10 20
E info@portofrotterdam.com
I www.portofrotterdam.com
Bezoekadres
World Port Center
Wilhelminakade 909
3072 AP Rotterdam
Havennummer 1247


2






Sedimentation in
the Botlek Harbour

A research into driving water exchange mechanisms
.




Master Thesis

Final Report



Adil EL HAMDI

Jan 2011









3






4


Thesis Committee
:


Prof. ir. T. Vellinga



Technical University of Delft / Port of Rotterdam

A. Noordijk




Port of
Rotterdam

Prof. dr. ir. H. Winterwerp


Technical University of Delft / Deltares

P. Taneja




Technical University of Delft


Prof. Cheong Hin Fatt


National University of Singapore

Dr. M. Chui Ting Fong


National University of Singapore
























Contact:



Port of Rotterdam

Postbus 6622

3002 AP Rotterdam

The Netherlands


T
+31 (0)10 252 10 10

E

info@portofrotterdam.com

I

www.portofrotterdam.com

Technical University of Delft

Postbus 5

2600 AA Delft

The Netherlands


T

+31 (0)15 27 89 111

E

info@tudelft.nl

I

www.tudelft.nl

National University of Singapore

21 Lower Kent Ridge Road

Singapore 119077

Singapore


T

+65 6516 6666

E

askiro@nus.edu.sg

I

www.nus.edu.sg





5






6


A
CKNO
WLED
GEMENT



In front of you is the final report of my Master thesis. The thesis forms the final part

of my Double
Master Programme ‘
Hydraulic Engineerin
g and Water Resource Management’
. I took this
programme at the Technical University of Delft and the National University of Singapore.
The
research itself was done the Port of Rotterdam. Hereby I

want to thank the colleagues of the Port of
Rotterdam for supporting me during my research.


However I want to thank the thesis committee in particular, for their

guidance and help
. The
committee members:


Prof. ir. T. Vellinga ( Head committee)


Technical University of Delft / Port of Rotterdam

A. Noordijk





Port of Rotterdam

Prof. dr. ir. H.
Winterwerp



Technical University of Delft / Deltares

P. Taneja





Technical University of Delft

Prof. Cheong Hin Fatt




National University of Singapore

Dr. M. Chui Ting Fong




National University of Singapore


And of course
I would like to thank my fa
mily and my friends
too
for their support.



Rotterdam, jan 2012




Adil El Hamdi




7






8


S
UMMARY


Siltation of harbour basins and navigation channels is a serious problem in the port of Rotterdam as well
in many other harbours all over the world. Due to
siltation, basins and channels require frequent
maintenance dredging to guarantee safe navigational depths. The costs associated with these dredging
activities are quite high.


To keep the channels and harbours in Rotterdam navigable, Rijkswaterstaat and t
he Port of Rotterdam
are dredging approximately 15 million m
3

of sediment a

year. The
dredging

cost

of the
Botlek Harbour
only

is already about 3 million Euros a year.
It is a task to keep the costs in the Port of Rotterdam as low
as possible to compete wi
th other ports. Reducing maintenance dredging costs is in line with the goal of
the Port of Rotterdam to be the most competitive, innovative and sustainable port in the world.


Most sedimentation within the maintenance area of the Port of Rotterdam occurs
in the Botlek.
According data, between 1.5 and 3 million m
3
/year is dredged in the Botlek Harbour. Although the
current dredging philosophy more or less works, the question arises whether there are solutions that
are more cost
-
effective.
However
,

the problem is so complex that it narrowed down
for the sake of
research quality.



The main causes of siltation in general and s
pecifically in the Botlek Area form an important part

of the
study
.
Hyd
rodymical models (SIMONA & Delft3D), were
used to gain
insight in the
s
edimentation

problem.
The focus in this the
sis
was

more on the hydrodynamics
.
The exchange mechanisms between
the river and Botlek Harbour were

investigated, which were

needed to examine the effectiveness of
certain solutions.

I
n practic
e a

lot of solutions are proposed

in literature
, however in th
is study only a
couple of ‘hard’

measures are investigated. The first possible solution that was examined was the
us
e

of
a
Current Deflecting Wall.
It turned out that the hydrodynamics were very se
nsitive to the configuration
of the CDW. While sometimes it would lower the exchange flow, at other cases it would even make the
problem worse.

The second solution was to make a gap in the
Geulhaven dam.

However this was not a
good solution
as high exchang
e flows occurred.
The last proposed solution
, the filling of the underwater
dam,

seemed more feasible as it would decrease the exchange flow according the numerical models.


The research has first order results which can be used in further studies. Accordi
ng to this results,
certain solution
s

will decrease the exchange flows
. O
n turn
it
would

very likely

result in lower
sedimentation rates in the Botlek Area. It is
expected

that some CDW configurations and the filling of
the underwater dam would have a posi
tive effect when it comes to sedimentation.
However
,

t
his
resea
rch is the first step of an extensive

study that must made to deal with the problem.


First of all many thing
s

can b
e done to improve

the models, for example by using a higher spatial
resolutio
n.

Secondly, other sets of con
ditions must be modelled

to see what

kind

of effect this has on
exchange flows. In addition, sediment must be included in the models to have more insight on the
sedimentation itself. The next step would
be a feasibility study,

including a cost benefit analysis
.
I
t would
be
wise
to improve the models further and to make a scale model

for the most feasible solution
.

In the
ideal case, were all
steps

are positive and hard conclusion can be made, it would be a good idea for the
Por
t of Rotterdam

to start a

pilot.



9






10


T
ABLE OF CONTENT


Acknowledgement

................................
................................
................................
........................

6

Summary

................................
................................
................................
................................
......

8

List of figures

................................
................................
................................
...............................
14

List of tabl
es

................................
................................
................................
................................
18


1

Introduction

................................
................................
................................
.........................
20

1.1

Problem description

................................
................................
................................
......
20

1.1.
1

General

................................
................................
................................
.................
20

1.1.2

Botlek Harbour

................................
................................
................................
......
20

1.2

Current approach

................................
................................
................................
..........
22

1.3

Research goal and questions

................................
................................
..........................
23


2

System

................................
................................
................................
................................
.
26

2.1

System boundaries

................................
................................
................................
........
26

2.2

Site Conditions

................................
................................
................................
..............
26


3

Literature review

................................
................................
................................
..................
28

3.1

Estu
arine hydrodynamics

................................
................................
...............................
28

3.2

Sediment transport and morphology

................................
................................
..............
30

3.2.1

Sediment properties
................................
................................
...............................
30

3.
2.2

Non
-
cohesive sediment properties
................................
................................
..........
31

3.2.3

Motion and transport

................................
................................
.............................
31

3.2.4

Cohesive sediment and fluid mud

................................
................................
...........
32

3.3

Turbidity maximum

................................
................................
................................
.......
34

3.4

Re
ducing costs

................................
................................
................................
..............
35

3.5

Measures applied in the past

................................
................................
.........................
36

3.5.1

Siltation trap

................................
................................
................................
..........
36

3.5.2

Silt screen

................................
................................
................................
..............
37

3.6

Minimising harbour siltation

................................
................................
..........................
39

3.6.1

Strategies

................................
................................
................................
..............
39

3.6.2

Exchange mechanisms
................................
................................
............................
39

3.6.3

Possible solutions
................................
................................
................................
...
46



11


4

Dr
edge data analysis

................................
................................
................................
.............
50


5

Research Strategies and approach

................................
................................
.........................
52

5.1

Strategies
................................
................................
................................
......................
52

5.2

Use of models

................................
................................
................................
...............
52


6

Hydrody
namical models
................................
................................
................................
........
54

6.1

Software

................................
................................
................................
.......................
54

6.2

Model equations and assumptions

................................
................................
.................
55

6.3

Available models

................................
................................
................................
...........
55

6.4

Adjusted model

................................
................................
................................
.............
57

6.4.1

Boundaries

................................
................................
................................
............
57

6.4.2

Hydrodynamical boundary conditions

................................
................................
.....
60

6.4.3

Resolution choice
................................
................................
................................
...
60


7

Determination of circulations

................................
................................
................................
66

7.1

Horizontal vs. vertical circulation

................................
................................
....................
66

7.2

Formulation

................................
................................
................................
..................
66

7.3

Cross se
ctions

................................
................................
................................
...............
68

7.4

Data handling and script

................................
................................
................................
69

7.5

Model input

................................
................................
................................
..................
70

7.5.1

SIMONA

................................
................................
................................
................
70

7.5.2

Delft
-
3D

................................
................................
................................
................
70

7.6

First results

................................
................................
................................
...................
71

7.6.1

Comparison


water level

................................
................................
.......................
71

7.6.2

Comparison


salinity

................................
................................
.............................
72

7.6.3

Initial field

................................
................................
................................
.............
73

7.6.4

Compar
ison with measurements


flow velocities
................................
.....................
74

7.6.5

Flow pattern

................................
................................
................................
..........
74

7.6.6

Horizontal circulation


qualitative

................................
................................
..........
81

7.6.7

Circulation


quantitative

................................
................................
.......................
82


8

Contribution exchange mechanisms

................................
................................
......................
84

8.1

Decomposition method

................................
................................
................................
.
84

8.2

Contribution of the three mechanisms

................................
................................
...........
84



12


8.2.1

Cross section A

................................
................................
................................
......
86

8.2.2

Cross section B
................................
................................
................................
.......
87

8.2.3

Cross section C
................................
................................
................................
.......
88

8.3

An
alysis

................................
................................
................................
........................
89


9

Exchange mechanisms and sedimentation

................................
................................
.............
90

9.1

Sediment concentration measurements

................................
................................
.........
90

9.2

Combination of results
................................
................................
................................
...
91


10

Measures

................................
................................
................................
.........................
96

10.1

Solutions to be investigated

................................
................................
...........................
96

10.2

Current Deflecting Wall

................................
................................
................................
.
98

10.2.1

Variant 1

................................
................................
................................
...............
98

10.2.2

Variant 2a
................................
................................
................................
..............
98

10.2.3

Variant 2b
................................
................................
................................
..............
99

10.2.4

Var
iant 3

................................
................................
................................
...............
99

10.3

Hole in Dam

................................
................................
................................
................

100

10.3.1

Variant 1

................................
................................
................................
.............

100

10.3.2

Variant 2

................................
................................
................................
.............

100

10.4

Filli
ng underwater dam

................................
................................
................................

101

10.4.1

Variant 1

................................
................................
................................
.............

101


11

Results

................................
................................
................................
...........................

102

11.1

No measures

................................
................................
................................
...............

102

11.2

Curre
nt Deflecting Wall

................................
................................
...............................

103

11.2.1

Variant 1

................................
................................
................................
.............

103

11.2.2

Variant 2a
................................
................................
................................
............

104

11.2.3

Var
iant 2b
................................
................................
................................
............

105

11.2.4

Variant 3

................................
................................
................................
.............

106

11.3

Hole in Dam

................................
................................
................................
................

107

11.3.1

Variant 1

................................
................................
................................
.............

107

11.3.2

Var
iant 2

................................
................................
................................
.............

108

11.4

Filling underwater dam

................................
................................
................................

109

11.4.1

Variant 1

................................
................................
................................
.............

109

11.5

Discussion

................................
................................
................................
...................

110



13


11.5.1

Densi
ty currents
................................
................................
................................
...

110

11.5.2

Horizontal exchange
................................
................................
.............................

110

11.5.3

Tidal filling

................................
................................
................................
...........

111

11.5.4

Total

................................
................................
................................
...................

112


12

Conclusion
, Discussion & Recommendation

................................
................................
......

114


13

Literature

................................
................................
................................
.......................

118


Appendix A


Map Port of Rotterdam

................................
................................
..........................

122

Appendix B


Dredge Atlas Botlek

................................
................................
...............................

128

Appendix C


Dredge data 1995
-
2010, Botlek Area

................................
................................
......

130

Appendix D


Assumptions and approximations D3D

................................
................................
...

132

Appendix E


Cross sections mouth Botlek

................................
................................
...................

134

E.1


Cross section A

................................
................................
................................
...............

134

E.2


Cross section B

................................
................................
................................
...............

134

E.3


Cross section C

................................
................................
................................
...............

135

Appendix F


Used depths

for script

................................
................................
............................

136

Appendix G


Transformation
σ to z
-
plane

................................
................................
...................

138

Appendix H


Part of Matlab script

................................
................................
..............................

140

Appendix I


Observation points.

................................
................................
................................

148

Appendix J


Validation flow velocities

................................
................................
........................

150

Appendix K


Depth averaged velocity profile
-

CDW


Variant 1

................................
..................

158

Appendix L


Depth averaged velocity profile
-

CDW


Variant 2a
................................
.................

162

Appendix M


Depth averaged ve
locity profile
-

CDW


Variant 2b

................................
...............

166

Appendix N


Depth averaged velocity profile
-

CDW


Variant 3

................................
.................

170

Appendix O


Depth averaged velocity profile
-

Hole in dam


Variant 1

................................
.......

174

Appendix P


Depth averaged velocity profile
-

Hole in dam


Variant 2

................................
.......

178

Appendix Q


Depth averaged velocity profile
-

Filling underwater dam


Variant 1

......................

182

Appendix R


Horizontal flow pattern around Botlek mouth

................................
.........................

186

Appendix S


Total flow through cross sections

................................
................................
............

192

Appendix T


Decomposed flow through cross sections

................................
................................

198






14


L
IST OF FIGURES


Figure 1.1


Area.

Figure 1.2


Volume maintenance dredging a year in the Botlek

area (source: Database PoR).

Figure 1.3


Current dredging philosophy.

Figure 2.1


Rijn
-
Maas estuarine system and boundaries.

Figure 2.2


Mean tide at Hoe
k van Holland at mean discharge (Getijtafel 1991.0).

Figure 2.3


Spring, neap and mean tide at Hoek van Holland. Lobith discharge 2200 m3/s
(Stroomatlas v3).

Figure 3.1


Classification of
estuaries on the basis of vertical structure of salinity.

Figure 3.2


Salt wedge estuary.

Figure 3.3


Partially mixed estuary.

Figure 3.4


Well mixed estu
ary.

Figure 3.5


Sediment classification (Wentworth, 1922).

Figure 3.6


Sediment transport.

Figure 3.7


Schematic representation of the governing
processes between suspension layer and fluid
mud layer. A two layer approach (Winterwerp, 2010).

Figure 3.8


Schematic description of mud transports (Winterwerp & van Kesteren, 2004).

Figur
e 3.9


Lagrangian sketch of harbour siltation under typical hydrodynamic conditions (from: de
Nijs).

Figure 3.10


Siltation trap at entrance Europoort/Maasvlakte.

Figure 3.11


Siltation t
rap at the Botlek entrance.

Figure 3.12


Placement of the test silt screen.

Figure 3.13


Design low silt screen.

Figure 3.14


The mixing layer in the
harbour entrance.

Figure 3.15


Horizontal entrainment mechanism.

Figure 3.16


Tidal filling and emptying.

Figure 3.17


Sketch exchange harbour basin


riv
er.

Figure 3.18


Currents in a tidal harbour a) ebb, b) flood.

Figure 3.19


Schematisation the components and the combinations of the three.

Figure 3.20


Row of poles and the corresponding velocity profile.

Figure 3.21


Current deflecting wall with sills.

Figure 3.22


Flow pattern. Without and with CDW.

Figure 3.23


Silt trap.

Figure 3.24


Egss of Thijsse.

Figure 3.25


Pneumatic barrier.

Figure 4.1


Dredged m
3

per m
2

a year. According data from 1995
-
2010
. (Databaste PoR, 2011).

Figure 4.2


Costs per m
3
. According data from 2010. (Database PoR, 2011).

Figure 5.1


Model validation and calibration.

Figure 6.1



SIMONA and D3D model.

Figure 6.2


Paths from centre mouth Botlek.

Figure 6.3


Estimation tidal excursion (W
-
E).

Figure 6.4


Estimation tidal excursion
(W
-

SE).

Figure 6.5


Large and small model.



15


Figure 6.6


Boundary conditions large and small model.

Figure 6.7


Model B: Coarse overall, fine local. Shade
s show the depth contour (Note: legend scale
not fixed).

Figure 6.8


Relation resolution
-

computation time.

Figure 6.9


Courant numbers model B.

Figure 7.
1


Hypothetical cross section harbour mouth.

Figure 7.2


Horizontal and vertical circulation.

Figure 7.3


The three cross sections and corresponding observation points at the Botlek
mouth.

Figure 7.4


Scheme to determine the relative contribution of horizontal and vertical circulation.

Figure 7.5


Initial salinity field (in ppt’s).

Figure 7.6


Flow pattern at
-
6 H w.r.t. HW Botlek.

Figure 7.7


Flow pattern at
-
5 H w.r.t. HW Botlek.

Figure 7.8


Flow pattern at
-
4 H w.r.t. HW Botlek.

F
igure 7.9


Flow pattern at
-
3 H w.r.t. HW Botlek.

Figure 7.10


Flow pattern at
-
2 H w.r.t. HW Botlek.

Figure 7.11


Flow pattern at
-
1 H w.r.t. HW Botlek.

Figure 7.12


Flow pattern at HW Botlek.

Figure 7.13


Flow pattern at +1 H w.r.t. HW Botlek.

Figure 7.14


Flow pattern at +2 H w.r.t. HW Botlek.

Figure 7.1
5


Flow pattern at +3 H w.r.t. HW Botlek.

Figure 7.16


Flow pattern at +4 H w.r.t. HW Botlek.

Figure 7.17


Flow pattern at +5 H w.r.t. HW Botlek.

Figure 7
.18


Flow pattern at +6 H w.r.t. HW Botlek.

Figure 7.19


Vortex at the Botlek mouth.

Figure 8.1


From two to three components.

Figure 8.2


Water level of

a tide according the model (from LW Botlek to LW Botlek).

Figure 8.3


Discharge through cross section B according the model (from LW Botlek to LW Botlek).
Positive is harbour outflow, negative is harbour inflow.

Figure 8.4


Distribution components over time (Cross
-
section A).

Figure 8.5


Distribution components over time (Cross
-
section B).

Figure 8.6


Distribution componen
ts over time (Cross
-
section C).

Figure 8.7


Contribution of the three components to the horizontal and vertical circulation.

Figure 9.1


Measuring stations of the de Nijs’ survey on the 11
th

of April 2006 (De Nijs, 2010).

Figure 9.2


The recorded water level, salinity and suspended particulate matter (top to bottom) at
station 2 on the 11
th

of April 2006. Open circles are near bead measurements and dots are

near
surface measurements (De Nijs, 2010).

Figure 9.3


Combined results: exchange flows (cross section B) and sediment concentration (De Nijs,
2010).

Figure 9.4


Goal 1: lowering of the e
xchange flows in general.

Figure 9.5


Goal 2: Low flow rates at times of high concentrations.

Figure 10.1


Mismatch cell boundaries and CDW.

Figure 10.2


No smooth boundaries possible.

Figure 10.3


CDW


Variant 1.

Figure 10.4


CDW


Variant 2a.

Figure 10.5


CDW


Variant 2b.



16


Figure 10.6


CDW


Variant 3.

Figure 10.7


Hole in dam


Variant 1.

Figure 10.8


Hole in dam


Variant 2.

Figure 10.9


Fil
ling underwater dam


Variant 1.

Figure 11.1


Distribution components over time (Cross
-
section B).

Figure 11.2


CDW


Variant 1. Exchange mechanisms before and after, cross section B.

Figure 11.3


CDW


Variant 1. Lay
-
out.

Figure 11.4


CDW


Variant 2a. Exchange mechanisms before and after, cross section B.

Figure 11.5


CDW


Variant 2a. La
y
-
out.

Figure 11.6


CDW


Variant 2b. Exchange mechanisms before and after, cross section B.

Figure 11.7


CDW


Variant 2b. Lay
-
out.

Figure 11.8


CDW


Va
riant 3. Exchange mechanisms before and after, cross section B.

Figure 11.9


CDW


Variant 3. Lay
-
out.

Figure 11.10


Hole in dam


Variant 1.
Exchange mechanisms before and after, cross se
ction B.

Figure 11.11


Hole in dam


Variant 1.
Lay
-
out.

Figure 11.12


Hole in dam


Variant 2.
Exchange mechanisms before and after, cross section B.

Figure 11.13


Hole in dam


Variant 2.
Lay
-
out.

Figure 11.14


Filling underwater dam


Variant 1.
Exchange mechanisms before and after, cross
section B.

Figure 11.15


Filling underwater d
am


Variant 1.
Lay
-
out.

Figure 12.1


Steps to be taken.

Figure 0.1


Dredge atlas.





17






18


L
IST OF TABLES


Table 1
-
1


Research
questions.

Table 3
-
1


Narrowing harbour entrance. Advantages and disadvantages.

Table 3
-
2


Row of poles. Advantages and disadvantages.

Table 3
-
3


CDW.
Advantages and disadvantages.

Table 3
-
4


Sill. Advantages and disadvantages.

Table 3
-
5


Silt trap. Advantages and disadvantages.

Table 3
-
6


Egss of Thijss
e. Advantages and disadvantages.

Table 3
-
7


Pneumatic barrier. Advantages and disadvantages.

Table 6
-
1


Coarse and Fine model.

Table 6
-
2


Computation time

/ simulated time for different combinations. (*Single precision).

Table 7
-
1


SIMONA Boundary conditions (provided by the Port of Rotterdam).

Table 7
-
2


Simulations with and without salini
ty at different time steps.

Table 7
-
3


Delft3D Parameters (* only required if salinity is turned on).

Table 7
-
4


Comparing different time steps.

Table 7
-
5


Comparing different time steps and analysing the spin
-
up time.

Table 7
-
6


Initial values at several locations.

Table 7
-
7


Average circulation at cross section A.

Table 7
-
8


Average circulation at the three cross sections using lowest time step.

Table 8
-
1


Distribution and cell & time averaged discharge for the three components at section A.

Table 8
-
2


Distribution and cell & time averaged discharge for the three components at section B.

Table 8
-
3


Distribution and cell & time averaged discharge for the three components at secti
on C.

Table 11
-
1


CDW


Variant 1. Values before and after.

Table 11
-
2


CDW


Variant 2a. Values before and after.

Table 11
-
3


CDW


Variant 2b. Values be
fore and after.

Table 11
-
4


CDW


Variant 3. Values before and after.

Table 11
-
5


Hole in dam


Variant 1. Values before and after.

Table 11
-
6


Hole in
dam


Variant 1. Values before and after.

Table 11
-
7


Filling underwater dam


Variant 1. Values before and after.

Table 11
-
8


Effects of measures on density currents.

Table 11
-
9


Effects of measures on the horizontal exchange.

Table 11
-
10


Effects of measures on tidal filling.

Table 11
-
11


Effects of measures on total exchange flow.





19






20


1

I
NTRODUCTION


1.1

Problem description


1.1.1

General


Siltation of
harbour basins

and navigation channels is a serious problem in the port of Rotterdam as
well in many other harbours all over the world. Due to siltation
,

these
basins

and channels require
frequent maintenance dredging to guarantee safe navigational depths. The costs associated with
these dredging activities are quite high.


The maritime sector and the vessel size grew rapidly in the last few decades. The current scale
increase requires deeper channels and harbours. As this increase is expected to continue in the
future, the required navigation
al

depth increases as well. Intuitively, this will lead to even more
necessary maintenance dredging. All over the world (port
-
) a
uthorities are trying to reduce
sedimentation in order to decrease the dredging costs. This is rather very difficult to achieve as
siltation is a very complex phenomena and hard to influence. Many possible solutions are proposed
by experts to reduce the se
dimentation of harbours. However some solutions require huge
investments while their applicability
and suitability
in a particular harbour is not fully proven.
Therefore most authorities are not willing to take any risk and adhere to regular maintenance
dr
edging.


1.1.2

Botlek Harbour


The siltation problem in Rotterdam started in the
period from 1957 till 1974 when the port was
expa
nded by the construction of the Botlek, Europoort and Maasvlakte. In the meantime the
channels Eurogeul, Maasgeul, Maasmond, Caland
-

and Beerkanaal
,

and the
harbour
basins where
deepened to receive larger vessels (MKO, 1987). This development had some serious negative side
effects. The morphological and hydrological balance of the system was dist
urbed
. As a result of
this
disturbance
,
nature wants to go back to its equilibrium situation and therefore sedimentation and/or
erosion occurs. In the case of Rotterdam it is mainly s
edimentation

and not erosion, due to the deep
maintenance depths. An additional effect of the
disturbance

is that

salt water intrudes further in the
estuary system which in turn can affect the ecology and the
fresh

water supply.


To keep the channels and harbours in Rotterdam navigable, Rijkswaterstaat and the Port of
Rotterdam are dredging approximately 15 million m
3

per year
: a
bout two
-
third by Rijkswaterstaat
,

and one
-
third of the amount by the Port of Rotterdam. While Rijkswaterstaat is responsible for the
main waterways (Maasmond, Maasgeul, Nieuwe Waterweg and Nieuwe Maas, Oude Maas), the Port
of Rotterdam mainly

takes care of the harbour basins and their entrances. The fact that two parties
are responsible for the maintenance of the contract depth instead of one, makes the implementation
of a possible solution
a bit

more complicated. The more stakeholders involve
d, the harder it is to


21


compromise. Most sedimentation within the maintenance area of the Port of Rotterdam occurs in
the Botlek (
Figure
1
.
1
). At this m
oment between 1.
2

and
2.5

million m
3
/year is dredged in the Botlek
Harbour according
to
data from 1995 until now (
Figure
1
.
2
). The cubic meters can be

equated

to kilos
if the density of the dredged material is known. Mostly this density is
estimated

as it is not known a
priori.



Figure
1
.
1



Area
.




Figure
1
.
2



Volume maintenance dredging a year in the Botlek area (source: Database PoR).



22


1.2

Current approach


The current handling of the siltation problem uses a corrective approach as shown in
Figure
1
.
3
. The
bathymetry is measured at regular intervals by a survey boat. The data is then compared with the
required depths, the so called contract de
pths which can be found in the ‘dredg
ing atlas’

(see page
128
)
. Subtracting t
hese figures indicates the areas

where dredging is required.




Figure
1
.
3



Current dredging p
hilosophy
.





23


1.3

Research goal and

questions


It is a task to keep the (maintenance dredging) costs in the Port of Rotterdam as low as possible to
compete with other ports.
Reducing maintenance dredging costs

is in line with the goal of t
he Port of
Rotterdam to be the most competitive, innovative and sustainable port in the world. As mentioned
before
,

most of these maintenance dredging cost for the Port of Rotterdam Authority is due to the
siltation in the Botlek Harbour. The topic of the
thesis is:



Sedimentation in the Botlek Harbour



A research into
driving water exchange mechanisms

.


Although the current approach
described in the previous
section

more or less works
, the question
arises whether there are solutions that are more cost
-
effective. Intuitively one would opt for
a

preventive approach instead of
a

corrective

one,

namely

preventing the sediment from coming

into
the harbour. This method is described by in PIA
NC as KSO: Keep Sediment Out (PIANC 2006). With
respect to this method the corresponding thesis question could be: “How to prevent sediment
intruding the Botlek Harb
our?” Before a thorough study can be

made to answer this question, we
should consider

wheth
er this is actually desirable. Preventing the siltation of the Botlek Harbour,
while siltation rates elsewhere increase is definitely not the solution. This is simply shifting the
problem from one harbour to another one. This can only be attractive if the
overall maintenance
dredging costs decreases. It must be taken into account that every proposed solution must be seen in
the system’s perspective and the main objective must kept in mind:
reducing the dredging costs
.


There are two other strategies t
hat ca
n be implemented, as regards to
sediment
s
: Keep Sediment
Moving or Keep Sediment Navigable. A research question could then be “How to keep sediment
moving?” or “How
to accept sediment such that the ships can sail through it”.


Up till now

sediment itself i
s considered as the main problem of the high dredging costs. However
,

there are many factors than can influence the dredging costs. Perhaps the current dredging pro
cess
could be optimised. It is wise

to get an overview of all possible solutions in the begi
nning stage of the
study. When
,

during the study
,

more is known about the hydrodynamic and morphological
conditions of Rotterdam harbour area and the dredging processes, some solutions will

prove

to be
more attractive than other
s
. The main question of t
his

thesis can be summarized as follows:



How can the

siltation

in the Botlek
be reduced, in order to reduce the total maintenance dredging
cost for the P
ort
o
f
R
otterdam

?




It must be kept in mind that the overall main goal is to decrease the costs. Only

solutions which are in
a large extent cost effective for the Port of Rotterdam and more or less also for other parties, are
accept
able
. First of all a research
as to

the possible cause of the sedimentation in general

must be
carried out
. Secondly
we have
to find

out
why siltation rates are
high

in the Botlek Harbour
.
In order
to answer these questions, it is wise to analyse the
available data. This will give insight in
whether

a
trend

can be identified
in the
volumes

of maintenance dredging
. If a pattern is discovered
we
investigate

how this trend
has
developed. This might or might not help to find an appropriate
solution.



24


Before brainstorming about the possible solutions for the given problem, it has to be investigated
what
measures

have b
een applied in the past
. From this lessons can be learned, which can be very
useful when thinking about new possible solutions. Especially because every harbour is unique and
has its own conditions.
Also

more lessons can be learnt from experiences in other

harbours all over
the world. A large variety of solutions exist, because different strategies can be applied. The question
arises
which strategy is the most suitable one in this case
.


Possible solutions and their
impacts on
siltation must be explored
. Th
is forms the core of the thesis
which at best results in an innovative,
cost effective

and sust
ainable solution. All these sub
-
questions
will lead towards the final answer of the main question. The topic is quite broad and it is easy to stray
from the subj
ect. Therefore the main question is kept in mind all the time during the thesis.

Table
1
-
1

summarises the research questions.



Main Question


How ca
n the siltation in the Botlek

be reduced, in order to
reduce the total maintenance dredging cost for the P
ort
o
f
Rotterdam

?

Sub
-
question 1

“ What are the main causes of siltation
H

in g敮敲al anT
獰散ifically

in the Botlek Area ? “

Sub
-
qu敳瑩on 2

“ Can a

瑲敮T b攠r散ogniz敤 in 瑨攠volume
s

of main瑥Wanc攠
Tr敤ging

? ”

Sub
-
qu敳瑩on 3

“ Wh
ich

m敡獵r敳e hav攠b敥n unT敲瑡k敮 in 瑨攠pa獴 anT
how
敦f散瑩v攠e敲攠瑨敳攠m敡獵r敳e
? “

Sub
-
qu敳瑩on 4

“ Wh
ich

獴ra瑥Wy i猠瑨攠mo獴 獵i瑡bl攠
in orT敲
瑯 T散r敡獥s瑨攠
main瑥Wanc攠 Tr敤ging co獴
s

? “

Sub
-
qu敳瑩on 5

“ What are possible cost
-
敦f散瑩v攠eoluW
ion猠for 瑨攠por琠of
Ro瑴敲TamH

T in par瑩cular 瑨攠Bo瑬敫 ar敡H

siltation problems ? “

Table
1
-
1



Research questions.



25






26


2

S
YSTEM


2.1

System boundaries


It was already clear in an early stage of the research that the problem should be solved in a system’s
perspective and not only in the Botlek Harbour area. The Rotterdam
Waterway

and its harbours
form

part of a water system
that is being influenced by tide and river discharges from the Rhine and
Meuse. However infrastructural changes, dredging policies and the “memory
-
effect” of the system
are also important parameters.


The wa
ter body can be classified as
estuarine

and

inclu
des the harbours and fair
ways of Rotterdam.
The figures below show
s

the Rhine
-
Meuse estuarine system

and its boundaries

(
Figure
2
.
1
)
.

The up
-
estuary boarders are approximately located at Hagestein (Lek), Tiel (Waal) and Lith (Maas). At the
other side the down
-
estuary borders are Hoek van Holland and the Haringvliet sluices. From 1970 on
these sluices regulate the distribution of fresh w
ater over the Rotterdam Waterway (in Dutch:
Nieuwe Waterweg). Mainly fine and medium sand can be found at the bed of the Rotterdam
Waterway. Mostly sand is dredged in the fairways, while in the harbours typically mud is dredged.




Figure
2
.
1



Rijn
-
Maas
estuarine system

and boundaries.



2.2

Site
Conditions


The hydrodynamics in the system is mainly determined by:

-

Tide

-

River discharges

-

Density differences

-

Wind effects

(set
-
up/set
-
down)

-

The Haringvliet sluices

programme



27



The tide, river discharges and the Haringvliet sluice programme form the most important boundary
conditions. Normally the tide at Hoek van Holland at the down
-
stream boundary is used to
determine the tide. This tide is semi
-
diurnal and has a me
an tidal range of ± 1.75 m and ± 2.0 m at
spring and ± 1.2 m at neap (see
Figure
2
.
2

and
Figure
2
.
3
).



Figure
2
.
2



Mean tide at Hoek van Holland at mean discharge (Getijtafel 1991.0)
.



Figure
2
.
3



Spring, neap and mean tide at Hoek van Holland
. Lobith discharge 2200 m3/s (Stroomatlas v3)
.


The estuary’s fresh water is suppl
ied by the river Rhine and the Meuse
. The distribution of the fresh
water over the Rotterdam
Waterway
is regulated by the Harin
gvliet sluices according the discharge
programme LPH ’84. When the Rhine disch
arge is between 1700 and 3900 m
3
/s
,

about 1500
m
3
/s
is
discharged through the Maasmond (Nieuwe Waterweg and Hartelkanaal). For Rhine discharges
lower than 1700
m
3
/s
,
the Haringvl
iet sluices are closed and all fresh water is discharged through the
Rotterdam Waterway.





28


3

L
ITERATURE REVIEW


3.1

Estuarine hydrodynamics


Estuaries can be classified according to either geomorphology or salinity (Nielsen, 200
9
). As can be
seen in
Figure
3
.
1
,
based on

salinity
,

three types can be distinguished: stratified estuary (sharp
interface), partly mixed estuary (gradually change

in salin
ity

vertical
ly
) and a well
-
mixed estuary
(strong vertical mixing).



Figure
3
.
1



Classification of estuaries on the basis of vertical structure of salinity.


The tide that travels into the estuar
y

is modifi
ed by the shoreline and by shallow water.
T
he tidal
range
at

Hoek van Holland

can be
categorized as mesotidal.


There are typically three zones in an estuary: an outer zone where the salinity is close to that of the
open sea, a middle zone where there is r
apid change in gradient and an upper/riverine zone, where
water may be fresh throughout the tide. Even though there is only 2% in different in density between
fresh (river) and salt water (sea), the horizontal and vertical gradients causes water circulatio
n which
i
n turn results in trapping of sediment
particles. The currents are

variable both in time and space. The
three types

of estuaries
are

described

on

the next page (Dyer 2001).




2
9




Figure
3
.
2



Salt wedge estuary.


In salt wedge estuaries the river water tends to
flow out on top of the denser sea water that rest
almost stationary on the sea bed. A wedge of
salt water is formed, penetrating toward the
head of the estuary. This salt wedge has a
sharp
salinity interface at the upper surface: a
halocline. Between the two layers there is
friction. The velocity shear between the almost
stationary salt wedge and rapidly flowing surface
layer produce small internal waves. As these
waves break, entrainm
ent is caused: some of the
salty water goes to the upper layer. Salt wedge
estuaries have almost fresh water on the surface
throughout, and almost pure salt water near the
bed.




Figure
3
.
3



Partially mixed estuary
.


In partially mixed estuaries current velocities
near the bed are large, producing turbulence
and thus mixing of the water column. The mixing
is a two
-
way process: mixing fresher water
downward and salty water upward. The salt
i
ntrusion is now a much more dynamic feature.
During flood, the surface water

travels faster up
the estuary

than the near bed water. Therefore
the salinity difference between bed en surface is
minimised. Turbulent mixing wil
l be dominant
here. During ebb

e
ntrainment will be dominant
as the fresher water is carried over the salty near
bottom water, which cause stratification. This
process is the so called tidal straining.




Figure
3
.
4



Well mixed estuary.


Turbulent mixing is active throughout the water
column in well mixed estuaries. Yet, there can be
lateral differences across the estuary in mean
flow velocity and salinity. The currents on one
side of the channel can be flood
-
dominated,
while those at the
other si
de are ebb
-
dominated.




30


3.2

Sediment transport and morphology


The Rotterdam harbours and waterways are part of the Rhine
-
Meuse estuary as was described
before. The river transports sediment to the Rotterdam area. Also from the ocean side sediment
part
icles are transported to the system. To understand the siltation problem better is good to have
some background knowledge on sediment and its transport. Changes in the morphology are
consequences of gradients in net sediment transport rates.
T
he interactio
n between
water

and
sediment is very complex an poorly understood.


3.2.1

Sediment properties


Depending on particle size a distinction can be made between silt and clay, sand, gravel and cobbles.
Clay particles are small and have therefore a large surface area
compared to their volume. This
chemically active area leads to cohesive characteristics. Sand on the contrary does not stick together
and thus is called non
-
cohesive. There is no clear boundary between cohesive sediments and non
-
cohesive sediments. The def
inition is usually site specific.
According Wentworth (
Figure
3
.
5
)
sediment particles below
±
60 µm

can be classified as mud. Mud can be further classified as silt or
clay depending on the particle size. Particles between
60 µm

and 2 mm are classified as san
d and are
clearly non
-
cohesive.

The properties of

cohesive
sediment are significantly different from the
properties of non
-
cohesive sediment
. M
any

studies were done on non
-
cohesive sediment, while on
the contrary less is known about cohesive sediment tran
sport.

The shortcomings of most current
cohesive sediment transport models are caused by insufficient description of relevant physical
processes themselves rather than by their numerical implementation.



Figure
3
.
5



Sediment classification (Wentworth, 1922).



31



3.2.2

Non
-
cohesive
sediment
properties


The first parameter of sediment is actually mentioned already: the grain size. Besides grain size,
other properties (of grain or bulk) are important for
the understanding of sediment processes: e.g.
grain density, fall velocity, angle of repose, porosity and concentration. Most of the sand consist of
quartz with
a mass density of 2650 kg/m
3
.
The fall velocity depends on the grain characteristics and
the fl
uid characteristics. In high concentration mixtures, this velocity in reduced by the presence of
other particles. This is the so called hindered settling.


3.2.3

Motion and transport


Incipient motion is important in the study of sediment transport. It is diffic
ult to define precisely at
what flow condition a sediment particle will move, because of its stochastic nature. Sediment is
transported when the shear stress on the grains is large enough. To assess the initiation of motion,
various forces on an individual

grain have to be taken into account: drag, lift and gravity. When an
equilibrium is considered, from proportionality the Shield parameter can be derived. From
experiments the Shields curve can be made,
which

is only valid for non
-
cohesive sediment and
uni
form flow on a flat bed. Despite
this fact
, Shield
s

is still mentioned because in is used in many
practic
al sediment transport formulas.


Sediment transport is defined as the movement of sediment through a well
-
defined plane. The mass
balance contains the
transport in both x and y
-
direction as well as the bed level.

Accretion occurs

If
t
he incoming sediment is more than the outgoing sediment
, and

for the other way around

there is
erosion
.

There are different transport modes: bed load transport and suspended

load transport. The bed load
transport is the transport of sediment in
a thin layer just above the bed.

Particles that are
transported without contact with the bed contribute to suspended load transport.


Due to the complexity of flow and sediment conditi
ons, engineers should select appropriate formulas
under different flow and sediment conditions. The different methods can give huge differences in
answers.



Figure
3
.
6



Sediment transport
.






32


3.2.4

Cohesive sedi
ment

and fluid mud


Cohesive sediment
is

a c
oncern in many waterways and is

often
closely related to water quality
issues.
At locations not very much exposed to the influence of waves and current (low energetic
conditions), often a net deposition of mud is

observed.
Several processes are important when
considering

cohesive sediment transport.


Classical transport models focus on the movement of fine particles in the water column. However,
mud can be present in three manifestations:

-

s
uspended

in the water co
lumn

-

aggregated near the bottom

-

solid at the bottom


Sediment particles present in the water colum
n

settles under the influence of gravity, as the density
of the particles exceeds the water density.

The settling velocity
s
v

of a spherical particle can be
calculated from the Stokes
-
law given the diameter
p
d
, particle density
p

, water density
w

and
water viscosity
w

.






2
(Re 1)
18
p w p
s
w
gd
v
 


 

(
0
.
1
)


An equilibrium concentration profile can be calculated from the balance betwee
n downward particle
convection (by settling) and upward particle diffusion (by concentration gradients). When the
cohesive sediment particles collide the
y

tend to aggregate.

In
the
models
,

aggregation in often
indirectly considered by the change in settling velocity. Small particles form large
r flocs by cohesion
(fluculation
). The floc structure and the inter
-
particle forces determine the settling velocity.
However, aggregate growth is enha
nced by an increase in particle concentration. Therefore the
settling velocity will also increase with concentration. At higher concentrations, generally near the
bed, hindered settling is observed. This also influences the settling velocity. Several e
mpir
i
cal
relationships are found for

settling velocity.


As a result of settling, bed deposition occurs. This process is most efficient a low turbulence
intensities, for example during slack tide.

After deposition the sediment tends to get more
compacted as it

is slowly ‘buried’ by later deposits. This process

is also known as consolidation:
the
self
-
weight of the particles expels the pore water and forces the particles closer together.

During
consolidation, the particles that were fluid
-
supported before deposi
tion, become gradually
supported by the grain matrix. An effective stress (difference between total and pore water pressure)
develops. Outflow of water from the bed is an important part of the process. Consolidation continues
until the pore water pressure
is the same as the hydrostatic pressure. The time needed for
consolidation strongly depends on the permeability (linear

relationship
) and the thickness of the
sediment layer (quadratic

relationship
).




33



Figure
3
.
7



Schematic representation of the governing processes between suspen
sion layer and fluid mud layer. A two
layer approach (Winterwerp, 2010).


A fluid mud layer exist in a highly concentrated suspension of fine grained sediment, in which settling
is
hindered, but which has not formed an interconnected matrix of bonds, strong enough to
eliminate the potential for mobility. Processes involved in fluid mud transport are schematically
indicated in
Figure
3
.
7
.
Fluid mud can either be generated by failure of loosely packed cohesive
sediment beds, or by deposition (if the flux of settling particles exceeds the consolidation rate). Fluid
mud of the first form has a concen
tration close to the original bed concentration, whereas the mud
concentration of the second form has a lower concentration. The typical density of fluid mud is
between 1040 to 1200 kg/m
3
. The concentrations vary from
few
ten
ths

to a few hundred g/l
.


When

fluid mud is left at rest it will consolidate. However when fluid mud is agitated (e.g. by currents
or waves), the consolidation process will slow down. The transport of fluid mud may result in very
high transport rates compared to the water column sedime
nt transport. The sediment concentration
in fluid mud is in the order of a few hundered kg/m
3
, whereas the concentration in the water column
is in the order of a few tenths kg/m
3
.



Figure
3
.
8



Schematic de
scription of mud transports (Winterwerp & van Kesteren, 2004).





34


3.3

Turbidity maximum


A couple of years ago field measurements were carried out by De Nijs
. He

studied the process
influencing the siltation in the Botlek Harbour.

This section shows the results

found by him (De Nijs,
2009 & 2010).


In the first study, it was
found

that the limit of salt water intrusion
indeed
existed

in the Botlek area.
The turbidity maximum is also more or less

located here
. The
density exchange

was found to be the
dominant cause for the transport of suspended particulate matter into the harbour. The survey data
also suggested that the Botlek Harbour basin has a 100% trapping efficiency. Salinity
-
induced density
gradients control the transport an
d trapping of sedimentation
in
the estuary close to the harbour
entrance, the sediment exchange between the estuary and the harbour,

and the trapping in the
basin.


Furthermore it was shown that the turbidity maximum is maintained by trapping of fluvial se
diment
at the head of the salt wedge. This trapping process is associated with the raining out of fluvial
suspended sediment from the upper fresher part of the water column, into the layer below the
halocline. The baroclinic shear flows and change in mixin
g characteristic are the dominant
mechanisms. According
to
De Nijs, the turbidity maximum is independent of bed
-
based supply of
mud. The freshwater discharge supplies sediment and ensures that the turbidity maximum is
maintained. Relative motion between sa
lt water and suspended particulate matter occurs because of
lags due to re
-
suspension. Sediment follows complex pathways and the trapping and transport
mechanisms are three
-
dimensional. To sum up, the salt induced baroclinic and buoyancy structure
keep the

fluvial sediment in the estuary through their effects on currents and turbulent dampin
g. The
length of the salt wedge
controls the trapping probability of fluvial suspended matter. The amount of
the suspended matter is mainly determined by the length of t
he salt intrusion and
to a lesser extent
by gravit
ational flow and tidal pumping.

Furthermore, de Nijs found that during flood tide saline and
turbid water bifurcate at the junction into the Nie
u
we Maas and Oude Maas and are advected back
into the Rotterdam Waterway during ebb tide, but at different phases of the tide.
Figure
3
.
9

shows
patches indicating elev
ated suspended concentrations advancing and retreating with the head of the
salt wedge on a semi
-
diurnal timescale.



Figure
3
.
9



Lagrangian sketch of harbour siltation under typical hydrodynamic conditions

(from: de Nijs)
.




35


3.4

Reducing costs


As mentioned before, the amount of maintenance dredging

has

increased since the sixties as the
port expanded and the navigation depth had to be maintained. This can be seen in seen in the figure
below.



Figuur
3
-
1



Amount of maintenance dredging
in Rotterdam
1918
-
1986 (MKO, 1987)
.

The authorities responsible for the dredging, Rijkswaterstaat and Port of Rotterdam, realised that
something must happen as the costs due to
maintenance dredging increased. Therefore a project
was launched by both parties to decrease the costs: “
Reducing Costs Ma
intenance Dredging” (in
Dutch: ‘
Minimalise
ring Kosten Onderhoudbaggerwerk’
)
. The main goal of this project was to come up
with concret
e solutions to decrease the maintenance dredging
costs. The costs were calculated

as
follows:


K = Q x P x F


K

=

costs maintenance dredging in a year

Q

=

the yearly siltation amount in tons

P

=

effort needed to remove, transport and store 1 ton of sedim
ent

F

=

price for this effort


By use of

this formula,
there are three sub goals
:

-

Increase the insight in siltation in order to come with concrete proposals to decrease the
sedimentation.

-

Increase the insight with respect to the most efficient ways to
remove, transport and store
sediment.

-

Increase insight in cost factors in order to pay as less a
s

possible for the efforts.






36


3.5

Measures

applied

in the past


Although the MKO
-
projects date

from a time when even less was known about siltation, some good
re
sults were achieved. For example
:
a more effective approach for

dredging processes, lower
dredging prices due to the market approach, better exchange of equipment, transparency and much
more

achievements
. Howe
ver the main purpose was to devise

concrete sol
utions to solve the
siltation problem.


3.5.1

Silt
ation

trap


The silt
ation

trap was designed to trap the sediment as much as possible at one location. The
advantage was that silt was not heavily spread out in the region, but instead concentrated at the
location

of the trap. As a result
,

siltation in the hinterland area is minimised. A silt trap leads to a
smaller sailing distance for the dredgers. Also the dredging conditions
turned out to be

much better.
Furthermore
, a better

distribution of dredging capacity a
ll over the year can be achieved.


As can be seen in
Figure
3
.
10
,

the siltation trap was located at the entrance of the
Europoort/Maasvlakte I. This trap ha
d a depth
of 2 meter below the bed level and a capacity of
3000
000
m
3
.


Also
in the central channel in the
Botlek a siltation trap was designed (
Figure
3
.
11
). Theoretically not

much was known about the trapping of sediment over here.
However
, also here it was noticed that
the dredging activities were more efficient.



Figure
3
.
10



Siltation trap at entrance Europoort/Maasvlakte.




37



Figure
3
.
11



Siltation trap at the Botlek entrance
.


3.5.2

Silt screen


According
to
the
MKO
project team
,

there were three concrete solutions to keep the sediment out of
the harbour: a water screen, a pneumatic screen or a closed screen. Early studies showed that the
water screen was not feasible. The pneumatic screen seemed to be a viable solution to keep th
e
sediment out. However from calculations it appeared that lot of energy was needed to supply the
needed air discharge. The current maintenance dredging approach would be more cost effective
than this solution.


The closed screen remained as the last concr
ete option. The most important requirement was that it
should not hinder the shipping. Based on the criteria chance of success, relia
bility, strength, repair
costs and
payback several options were selected out of dozens. Initially
,

most atte
ntion was paid
to

the horizontal movable screen that could slide over a rail. However this option was left
out
as the
ship anchors could damage the rail, which would mean that the screen is not movable anymore.
Another requirement was added: the screen should consist of
pieces which would
suffer least
possible
damage due to

the anchors
. Also
,

the propeller of the ship should not
be damaged. This
resulted in a ‘patchwork’

principle. A flexible silt screen was placed in the Botlek as an experiment
(
Figure
3
.
11


for the exact location). Unfortunately
,

the interaction between the propeller and the
screen was so intense that many repa
irs

had to be made. So also this solution was not the ri
ght one.
The movable screen was again investigated, but this time
it was made
movable in the vertical

direction
. To gain insight in the frequency of damage, first a very low screen of
two meter

was built
over a width of 4.5 m (
see
Figure
3
.
12

for the placement and
Figure
3
.
13

for the design
). Apparently
this did not
function well
since

th
ere is no

silt sc
reen in the Botlek area at present
.

Furthermore an underwater d
am

is still there
,

near the Botlek entrance. The idea was to make the
entrance narrower and thus reducing siltation.




38



Figure
3
.
12



Placement of the test silt screen.




Figure
3
.
13



Design low silt screen.






39


3.6

Minimising harbour siltation


3.6.1

Strategies


All over the world authorities tried and are still
trying to sol
ve the sedimentation problem. T
he
current knowledge is insufficient to apply sediment reducing measures optimally. The low number of
applications in practice makes it even harder. Therefore
a lot of research was

done by Sva
š
ek, SRE,
Deltares and HKV for example,
to
gain

insight in the problem and to finall
y come with concrete
solutions.


A PIANC working group also studied the siltation problem for a long time. They classified all proposed
solutions in different categories, ea
ch presenting a strategy. The successful methods according PIANC
can be classified in six groups:

-

Keep Sediment Moving & mainly passive

-

Keep Sediment Moving & passive

-

Keep Sediment Moving & active

-

Keep Sediment Out & active

-

Keep Sediment Out &
passive

-

Keep Sediment Navigable & passive/active


Some methods are well
-
established and widely applied, w
hile other methods are less

established and
need further elaboration.


3.6.2

Exchange mechanisms


Several mechanisms can induce an exchange of matter betwe
en a river/channel and a harbour basin.
Three main flow mechanisms can be distinguished (Langendoen, 1992):


-

Turbulent exchange

Exchange due to velocity difference between river and harbour flow.

-

Tidal filling

Exchange due to net flow through the harbour e
ntrance.

-

Density currents

Exchange flow driven by a density difference between the river water and the water in the
harbour.


It is important to describe the exchange flows as there is a relationship between the mechanisms and
harbour siltation. In some po
rts one, or more mechanisms can be dominant, while in other ports all
of the mechanisms play a significant role. There are other mechanisms that produce exchange, for
example wind and shipping. However these mechanism play are less important compared to th
e
three main mechanisms.




40


3.6.2.1

Turbulent exchange


This type of mechanism exist mainly at harbours that are located at rivers. Since the current
velocities in the river are higher than relative calm harbour basis, a mixing layer originates between
the river and

the harbour (
Figure
3
.
14
). In this layer mass and momentum is being transferred
between the bas
in and the river.
In the upstream corner wave
-
like disturbances, turbul
ent vortexes
(eddies) are developed and grow further in the downstream direction.

Following the conservation of
mass, the separating streamline at the upstream corner of the harbour entrance is directed in the
harbour. This is true if the assumption is mad
e that the river current is constant outside the mixing
layer and higher than the velocity in the mixing layer. This is generally the case. The water in the
mixing layer flows partly into the river and partly into the harbour at the stagnation point. The
a
mount of river and harbour water that is entrained, determines the location of the stagnation point.
Higher entrainment, results in a wider mixing layer and the location of the stagnation point further
into the harbour. The geometry of the harbour is also
a very important factor.


The entrainment of harbour water into the mixing zone and the supply of water the other way
around at the downstream side wall,
causes

a circulating flow. These flows, the so called eddies can
be seen in
Figure
3
.
15
. The existence of the eddies is also the reason why generated vortexes enter
the mixing zone. Depending on the harbour geometry one primary or more secondary eddies can
occur.


The

mixing layer and the eddies are both important phenomena for the transport of matter from the
river to the basin and in the basin itself. Entrainment processes take care of the exchange of matter
between the river and harbour. The eddies (especially the s
econdary) cause siltation in the centre as
the water velocities are small there. To decrease the exchange
,

the
development and

intensity of the
vortexes must be limited.


Several formulas have been

developed to calculate the silt flux through a harbour e
nt
rance. However
there is an un
certai
nty in the answer as it involves an

exchange coefficient which is hard to
determine a priori. According the formulas the siltation can be reduced by lessening the cross area,
lowering the exchange coefficient, lowering th
e river velocity or lowering the river sediment
concentration.



Figure
3
.
14



The mixing layer in the harbour entrance
.




41





Figure
3
.
15



Horizontal entrainm
ent mechanism
.


3.6.2.2

Tidal filling


Due to variations in water level at the river, a net flow through the entrance can occur. This can also
be caused by withdrawal or discharge of water from/to the basin. The most common cause of net
flow through the entran
ce i
s the water level variation

due to tide. The filling of the basin occurs
during flood when the water level increases, while during ebb the decrease of water level results in
emptying (
Figure
3
.
16
). The one
-
dimensional shallow water equations can be used to compute the
tidal motion in a harbour.


Continuity


0
Q
B
t x

 
 
 

(
0
.
2
)


Momentum


2
2
0
Q Q
Q Q g
gA
t x A x C AR

 
  
   
 
  
 

(
0
.
3
)


Here

is the water level,
Q
is the flow rate,
A
the harbour cross section,
x