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Field study and supporting analysis of air curtains and other measures to reduce salinity transport through shipping locks

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This paper presents an overview of a study on salinity intrusion through shiplocks that are located at a saltwater and freshwater interface, and the possible measures that can be taken to reduce this salinity transport. The main focus of the study was to test the effectiveness of several measures against salt intrusion through a shiplock. For this reason a series of field experiments was conducted in the Stevin shiplock in the Afsluitdijk, near Den Oever, the Netherlands, in between the Wadden Sea and the IJsselmeer. The measures tested include an air curtain at both ends of the shiplocks, alone and in combination with a water jet, as well as a rigged sill to reduce the effective depth, and flushing of the lock with fresh water. A new type of air curtain with air injectors was designed and built for this study. Prior to the field experiments, a series of laboratory scale experiments and computer simulations of lock‐exchange flow were conducted to gain insight into the salinity transport process and to support the design of the field study. The study has shown that a significant reduction of salinity intrusion can be attained by using a combination of measures. These findings are relevant for shiplocks located in a saline–freshwater transition zone for which salinity intrusion should be reduced as much as possible. Copyright © 2011 John Wiley & Sons, Ltd.
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FIELD STUDY AND SUPPORTING ANALYSIS OF AIR CURTAINS AND OTHER
MEASURES TO REDUCE SALINITY TRANSPORT THROUGH SHIPPING LOCKS
GEERT KEETELS
1
*, ROB UITTENBOGAARD
1
, JOHN CORNELISSE
1
, NICKI VILLARS
1
and HANS VAN PAGEE
2
1
Deltares, Delft, The Netherlands
2
Rijkswaterstaat, Centre for Water Management, Lelystad, the Netherlands
ABSTRACT
This paper presents an overview of a study on salinity intrusion through shiplocks that are located at a saltwater and freshwater
interface, and the possible measures that can be taken to reduce this salinity transport. The main focus of the study was to test the
effectiveness of several measures against salt intrusion through a shiplock. For this reason a series of eld experiments was
conducted in the Stevin shiplock in the Afsluitdijk, near Den Oever, the Netherlands, in between the Wadden Sea and the IJsselmeer.
The measures tested include an air curtain at both ends of the shiplocks, alone and in combination with a water jet, as well as a rigged
sill to reduce the effective depth, and ushing of the lock with fresh water. A new type of air curtain with air injectors was designed
and built for this study. Prior to the eld experiments, a series of laboratory scale experiments and computer simulations of lock-
exchange ow were conducted to gain insight into the salinity transport process and to support the design of the eld study.
The study has shown that a signicant reduction of salinity intrusion can be attained by using a combination of measures.
These ndings are relevant for shiplocks located in a salinefreshwater transition zone for which salinity intrusion should
be reduced as much as possible. Copyright © 2011 John Wiley & Sons, Ltd.
key words: salt intrusion; air curtain; lock exchange; gravity current; shiplock
Received 11 October 2011; Accepted 11 October 2011
RÉSUMÉ
Ce document présente un aperçu dune étude sur lintrusion de la salinité à travers les écluses maritimes qui sont situées à l
interface eau salée/eau douce, et les mesures possibles susceptibles dêtre prises pour réduire ce type de transport de la salinité.
Lobjectif principal de létude était de tester lefcacité de plusieurs mesures contre lintrusion du sel par une écluse maritime. Pour
cette raison, une série dexpériences a été menée dans lécluse maritime Stevin dans lAfsluitdijk, près de Den Oever, Pays-Bas,
entre la mer des Wadden et lIJsselmeer. Les mesures testées comprennent un rideau dair, seul et en combinaison avec un jet deau,
ainsi quun seuil pour réduire la profondeur efciente, et le rinçage de lécluse avec de leau fraîche. Un nouveau type de rideau
dair avec des injecteurs dair a été conçu et construit pour cette étude.Un rideau dair a été installé aux deux extrémités de lécluse
maritime, avec un système dinjection deaudouce (à côté de la mer des Wadden) et un seuil (sur le côté IJsselmeer). Dans une série
dexpériences il y avait aussi un ux constant deau douce à travers lécluse. Les mesures sur le terrain ont été menées dans la
période avril-mai 2010. Avant les expériences de terrain, une série dexpériences en laboratoire et des simulations par ordinateur
des ux déchange des éclusées, avec et sans mesures de réduction de lintrusion de la salinité, ont été menées pour mieux
comprendre les processus de transport de la salinité et de soutenir la conception de létude sur le terrain.
Létude a montré quune réduction signicative de lintrusion saline peut être obtenue en utilisant une combinaison de
mesures. Ces résultats sont pertinents pour des écluses situées dans une zone de transition saline-eau douce, pour laquelle l
intrusion saline devrait être réduite autant que possible
mots clés: intrusion saline; rideau dair, échanges dans les écluses; courant gravitaire; écluse maritime
INTRODUCTION
Salt intrusion from coastal waters can be a serious threat to
drinking water, industrial process water and agricultural
* Correspondence to: Mr Geert Keetels, Deltares, Delft, the Netherlands.
E-mail: geert.keetels@deltares.nl
Étude sur le terrain et analyse de soutien de rideaux d'air et autres mesures
visant à réduire les transports de salinité par écluse.
IRRIGATION AND DRAINAGE
Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ird.679
Copyright © 2011 John Wiley & Sons, Ltd.
water supply, in particular in tidal river systems with mildly
sloping beds.
To reduce salt intrusion (chloride concentration) it was
proposed around 1960 to apply air curtains in open channels
and at the entrance of shiplocks. A set of experiments by van
der Burgh (1962) in the Kornwerderzand shiplocks at the
Afsluitdijk in the Netherlands demonstrated that it was
indeed possible to reduce salt intrusion signicantly.
A recent issue in the Netherlands as been the proposal to
reconnect the fresh water Volkerak-Zoommeer with the
North Sea. This would result in an unacceptable amount of
salinity transport through the Volkerak shiplocks towards
the Hollandsch Diep and yield too high values of chloride
concentration at the water intake locations for drinking,
agriculture and industrial water supply. Motivated by this
recent problem, the concept of salinity reduction by air cur-
tains was revised in this study by testing several combina-
tions of an air curtain and a (plane) freshwater jet, a sill
and ushing the sluices with fresh water during ebb tide.
Figure 1 gives an illustration of these methods that could
be applied in a shiplock. In addition, an innovative design
of air injectors was tested that yields a closely packed air
curtain and a more evenly distributed air ux over the width
of the lock entrance when compared with the traditional
design of air injector (Figure 2).
DESIGN STRATEGY
The objective is to reduce salt transport through shiplocks to
an acceptable level with a minimum usage of fresh water.
Moreover, salt reduction measures should not cause pro-
blems for the manoeuvring of ships towards and inside the
lock and thus delay the lock cycle. Figure 3 gives an over-
view of the design strategy followed in this study. Starting
points are the ship trafc requirements and the acceptable
chloride concentrations at the water intake points of the
freshwater system. A large-scale model assesses the maxi-
mum acceptable salt ux through the shiplock. By using a
conceptual model for salt transport through a shiplock, we
can compute salinity transport through the shiplock if a reli-
able estimate for the effectiveness of the different measures
(indicated as the salt transmission factor) is available. The
description and validation of this model will be published
elsewhere. The scope of this paper is marked by the dashed
box in Figure 3. The focus is to assess the value and robust-
ness of the salt transmission factor as a function of air and
Figure 1. Illustration of the considered measures to reduce salt intrusion in a shiplock at a freshwater and saltwater interface
Figure 2. New design of air injector for a closely packed air curtain with
evenly distributed air ux over the width of the lock entrance
43AIR CURTAINS AND OTHER MEASURES TO REDUCE SALINITY INTRUSION
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
water ux. This parameter describes the effectiveness of a
measure against salt intrusion. It is the ratio between salt
mass intrusion when a measure is present and salt mass intru-
sion in the absence of any salt reduction measure, i.e. lock-
exchange ow. As a reference time window, we consider
one internal period of an undisturbed lock-exchange ow,
i.e. the time that is required for a gravity current to traverse
the length of the lock twice. This factor is the most critical
and uncertain parameter in the design process. The essential
physical processes of salt intrusion through an air curtain are
not well understood despite the theoretical study of Abraham
et al. (1973). For this reason, it was necessary to conduct a
large number of detailed numerical simulations in combina-
tion with laboratory experiments in a ume tank in advance
of the eld experiments at the Stevin shiplock at Den Oever,
the Netherlands.
SET-UP OF THE LABORATORY EXPERIMENTS
Figure 4 gives an overview of the experimental set-up in the
laboratory. For each experiment in the laboratory ume tank
a large reservoir was lled with salt water and a smaller reser-
voir with fresh water. The water depth was approximately 30
cm in both reservoirs. A gate in the ume separated the two
reservoirs. The air injector and freshwater injector were situ-
ated inside the saltwater reservoir near the gate. The air injec-
tor consisted of a perforated tube with a diameter of about 1
cm. This yields a closely packed air curtain with bubble sizes
ranging from 3 to 5 mm (Figure 5). Fresh water was injected
through an opening of 3 mm over the width of the ume.
The air curtain and freshwater jet were started before the
opening of the gate. Water could leave the ume at the
end of the saltwater compartment in order to prevent an
Figure 3. Overview of the different parts of the overall study. The dashed box indicates the scope of the present paper
Figure 4. Experimental set-up and dimensions
44 G. KEETELS ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
increase in the water level by the injected fresh water during
the experiment. Within a few seconds the gate was opened
and the salt water of the gravity current started to enter the
freshwater compartment while fresh water moved towards
the saltwater compartment. Salinity was measured at 2
locations at 12 positions distributed over the vertical
direction (Vezo). The position of the measurement section
varied per experiment. In some experiments additional
salinity measurements were performed to verify if the total
salt mass computed by averaging over 2 12 vertical
measurement sections is sufciently accurate. About 30
combinations of air and water ux were tested. As a reference
case, a lock-exchange experiment without any measures
against salt intrusion was conducted. Figure 6 demonstrates
the progressing interface between salt and fresh water of a
gravity current that develops in the lock-exchange experiment.
SET-UP OF THE NUMERICAL MODEL
A detailed numerical model of lock-exchange ow was devel-
oped using the computer code Ansys-CFX that covers the entire
experimental set-up of Figure 4. Bubbles are not resolved
individually but the air is considered as a dispersed uid phase
inside a continuous water phase. The air can leave the domain
at the surface by means of a degassing boundary condition. For
each uid phase, a separate set of equations for momentum,
mass and energy is resolved. Empirical relations give the inter-
face forces between the bubbles and the water phase. The most
important interface force for this problem is the drag force. The
bubble radius changes as a function of water depth. Complex
bubble behaviour such as coalescence and break-up was
neglected. These processes can be important at the eld scale
but are not of signicant importance for the present laboratory
experiments. Turbulence inside the continuous phase (water) is
modelled by equations for turbulent kinetic energy and
turbulent energy dissipation. Salinity is considered as an active
scalar transport quantity. An equation of state is used to relate
the salinity with the density of the continuous phase. The
model was validated against velocity measurements and
salinity measurements obtained from several laboratory
experiments and the theoretical formulation for gravity
currents derived by Shin et al. (2004). After the validation of
the model against laboratory experiments, the same model
was applied to simulate salinity intrusion in the Stevin
shiplock. The computed velocities in this model are consistent
with the velocity measurements around air curtains in deep
water by Bulson (1961). The main purpose of the simulation
was to support the design of the eld experiments.
RESULTS OF LABORATORY EXPERIMENTS
AND SUPPORTING SIMULATIONS
Figure 7 shows a computed gravity current in a classic lock
exchange without measures against salt intrusion. The reser-
voir on the right-hand side of the gate contains fresh water
and the reservoir on the left-hand side (partially shown) is
lled with salt water. After the removal of the gate salt water
moves into the reservoir while fresh water ows out. The
propagation speed of the gravity current and layer thickness
are consistent with the theory of Shin et al. (2004). Salt water
does not replace all the fresh water. About 80% of the fresh
water will be replaced by intruding salt water. This well-known
phenomenon can be related to energy dissipation in the
propagating salt wedge, i.e. a part of the potential energy
released in the saltwater compartment is not restored in the
freshwater compartment. Salinity measurements in the
laboratory experiments also reveal this behaviour (not shown).
Figure 8 shows the typical ow pattern and salt intrusion
path observed in the laboratory experiments. At relatively
high air uxes two circulation cells emerge around the air
curtain. Salt water transfers towards the surface layer and
mixes with fresh water. As a result a weak density current
develops at a distance from the air curtain equal to
Figure 5. Closely packed air curtain in the laboratory
Figure 6. Gravity current of an undisturbed lock-exchange experiment in a ume
tank without measures against salt intrusion. The arrow below indicates the
displacement of salt water and the arrow above the displacement of fresh water
45AIR CURTAINS AND OTHER MEASURES TO REDUCE SALINITY INTRUSION
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
approximately twice the water depth. The corresponding nu-
merical simulation is shown in Figure 9.
Figure 10 shows the effect of different combinations of air
ux and water ux on the total salt intrusion. The salt ex-
change is normalized with the total amount of salt contained
by a reservoir that is completely lled with salt water. When
there is no air or water ux (i.e. classic lock exchange) the
air ux parameter is zero and the salt exchange is about
80%, as illustrated in Figure 7. The rst series of experiments
was focused on a high ux of air and the second series of
experiments was conducted to nd an optimal combination
of air ux and water ux. The numerical simulations were
performed in advance of the second series of experiments in
order to provide some guidance for the set-up of these experi-
ments. In both the numerical simulations and experiments, a
route towards optimal salt reductions was found by making
small steps in the air ux and water ux.
SET-UP OF THE FIELD EXPERIMENT AT DEN
OEVER
Figure 11 gives an overview of the set-up of the eld exper-
iment at the Stevin shiplock at Den Oever. An air curtain
was installed at both entrances of the lock. On the Wadden
Sea side there was a plane freshwater jet installed. In a
few experiments a sill was added next to the air curtain on
the IJsselmeer side.
Conductivity meters for measuring salinity were positioned
at ve locations on one side of the lock. At each location there
were ve measurement points in depth. The difference in
water level between the Wadden Sea and IJsselmeer varies
with the tide. Therefore, it was necessary to adjust the vertical
position of the salinity measurements with the tide. Additional
salinity measurements were taken inside the harbours at each
end of the shiplock. An extensive data management system
was developed in order to record all the essential parameters
such as the water levels, valve positions, opening and closure
times of the doors, air ux, water ux, salinities and type and
load of the passing ships.
RESULTS OF THE FIELD EXPERIMENTS AND
SUPPORTING SIMULATIONS
Figure 12 shows the salt transmission factor versus the recip-
rocal of non-dimensional circulation (denition given in the
Appendix) for the experiments at the Stevin sluices and other
eld experiments and laboratory studies. It can be deduced
that if the non-dimensional circulation is larger than approxi-
mately 4 (1/C<4) the salt transmission factor strongly
depends on the reciprocal of non-dimensional circulation
and thus the air and water uxes. This can also be observed
in the data obtained from the air curtain experiments by van
der Burgh (1962) in the Kornwerderzand shiplocks. Our
experiments in the Stevin shiplock with only a plane freshwa-
ter jet can also be found in the unsaturated regime with non-
dimensional circulation larger than 4.
The value of the reciprocal of non-dimensional circulation in
the eldexperimentsattheStevinlockwithanaircurtainand
freshwater jet is between 0.25 and 0.4. The salt transmission
factor is 0.25 0.05 if only an air curtain is applied and 0.15
0.05 if both an air curtain and a freshwater jet are applied.
The results of the eld experiments at the Kornwerderzand
and Stevin locks can be compared with the present laboratory
results and the laboratory experiments on salt intrusion
reduction by saltwater jets performed by Bruyn (1963).
In some experiments in the Stevin shiplock with a small air
ux and relatively large water ux smaller values for the salt
Figure 7. Computed lock-exchange ow with the numerical model. Salinity
distribution at different times 20, 35, 60 and 90 s after the release of the
gate. Black lines indicate the minimal and maximal thickness of the salt
wedge according to the theory of Shin et al. (2004). The contours range
from 0 to 35 ppt. The yellow line marks the original position of the gate
(removed instantaneously)
Air curtain
Figure 8. Typical ow pattern around an air curtain at a salt- and freshwater
transition in the laboratory ume experiment. The circulation cell on the fresh-
water side is shown. Air ux is 92 Nl s
1
at atmospheric pressure, the salinity
difference is 35 ppt, Vezo1 is the rst salinity measurement location
46 G. KEETELS ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
transmission factor were found. Note that the fresh water has
multiple effects, i.e. buoyancy, replacement of salty lock water.
Moreover, the application of a freshwater jet during many se-
quential levelling cycles will reduce the salinity in the harbour
at the salt end of the shiplock. In the laboratory experiments
and supporting numerical simulations the transmission factor
was not smaller than 0.1. Further analysis is necessary to
explain the results of these experiments in the Stevin lock.
Figure 13 shows the typical ow patterns that can occur
around an air curtain on a transition between salt and fresh
water for different values of the non-dimensional circulation
(air ux). At low air uxes a single circulation zone develops
Figure 9. Numerical simulation that corresponds with the laboratory experiment shown in Figure 8. The arrow marks the position of air injector. Salt water is
situated on the left-hand side and fresh water on the right-hand side. Contours range from 0 to 35 ppt.
Figure 10. Relative salt exchange versus the air ux parameter (see Appendix). Experiments at high values of the air ux parameter, numerical simulations and
experiments with modest values for the air ux parameter. GC: gravity current, A: air and W: fresh water. Dashed arrow indicates an increase of air ux and
solid arrow a change in water ux
air curtain and plane fresh
water jet
Wadden Sea
IJssel lake
air curtain and sill
salinity
measurements
air curtain and plane fresh
water jet
Wadden Sea
IJssel lake
air curtain and sill
salinity
measurements
Figure 11. Experimental set-up at the Stevin shiplock, Den Oever, the Netherlands. The shiplock has a length of 150 m, the width is 14 m and the depth varies
by the tide (5 m on average)
47AIR CURTAINS AND OTHER MEASURES TO REDUCE SALINITY INTRUSION
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
around the air curtain at the salt side. Salt water penetrates
through the air curtain at a certain level. At higher values of
the air ux a second circulation zone develops (reduces the
non-dimensional circulation) around the air curtain. Salt water
is transferred to the surface layer and mixes with the fresh
water, which yields a weak density current. Increasing the
air ux further increases a larger circulation zone, but does
not result in a reduction of the salinity intrusion.
Table I summarizes the obtained salt transmission factors
found in the Stevin lock experiments. Note that the obtained
results depend on the particular limitations such as the
maximal amount of fresh water that can be used for ushing
the lock chamber or for the water jet or the allowed sill
height in this study.
CONCLUSION AND DISCUSSION
In this paper, we focus on salt intrusion during the time win-
dow the doors are open on one side of the lock. This is only
a part of the total lock cycle. Using the robust estimates for
the salt transmission factors obtained in this study it is now
possible to estimate the total salt transport per lock cycle as a
function of lock design parameters by a conceptual exchange
Figure 12. Salt transmission factor (n= 1 lock exchange, n= 0 no salt transmission) versus the reciprocal of non-dimensional circulation (C)dened in the
Appendix, for several eld and laboratory experiments. Air ux is dened at atmospheric pressure (Nl s
1
)
1/C=0.19, n=0.3
1/C=0.26, n=0.14
1/C=0.37, n=0.15
Figure 13. Typical ow pattern transitions in the numerical simulation. Contours range from 0 to 35 ppt. The arrows indicate the position of the air injector. Cis
the non-dimensional circulation and nthe salt transmission factor
48 G. KEETELS ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
model. The amount of reduction that can be achieved for the
total lock cycle strongly depends on these parameters.
This study demonstrates that with a closely packed air
curtain in combination with a freshwater jet it is possible to
obtain a salt transmission factor (on one side of the lock) of
0.15. This implies a possible reduction of 85% of the salt mass
transport during the time window the doors are open on one
side of the lock. As a refrerence time window the internal period
of a gravity current that develops in the absence of measures
against salt intrusion has been considered.
If a salt transmission factor (on one side of the lock) of
0.15 is not sufcient to reduce the total salt mass transport
per lock cycle to the desired level, additional measures
against salt intrusion can be considered. For example,
ushing the lock chamber with fresh water or pumping
intruding water back to the harbour on the saltwater side.
Note that these additional measures might yield extra
demands in the amount of fresh water or energy supply.
ACKNOWLEDGEMENTS
WewishtothankDickMastbergenforperformingthelaboratory
experiments and Arno Nolte for helpful comments on this paper.
The study was funded by and carried out with cooperation of
the Dutch Ministry of Infrastructure and the Environment.
CONFLICTS OF INTEREST
None of the authors have any conicts of interest to declare.
REFERENCES
Abraham G, van der Burgh P, de Vos P. 1973. Pneumatic Barriers to Reduce
Salt Intrusion through Locks. Rijkswaterstaat communications No. 17.
Government Publishing Ofce: the Hague, the Netherlands.
Bruyn J. 1963. Waterschermen ter bestrijding van zoutbezwaar van
schutsluizen aan zee. Waterloopkundig Laboratorium. M799: Delft, the
Netherlands (in Dutch).
Bulson PS. 1961. Currents produced by an air curtain in deep water. Dock
and Harbour Authority 42(487): 1520.
Shin JO, Dalziel SB, Linden PF. 2004. Gravity currents produced by lock
exchange. J. Fluid Mech 521:134.
Van der Burgh P. 1962. Proeven met luchtschermen. Rijkswaterstaat Dienst
voor de Waterhuidshouding: the Hague, the Netherlands (in Dutch).
APPENDIX A: DEFINITION OF THE
NON-DIMENSIONAL CIRCULATION
Energy transfer rates
Bulson (1961) derived an expression for the power deliv-
ered by the air curtain to the water. If one assumes that the
air is injected at a pressure that is just sufcient to overcome
the hydrostatic head and that the temperature of a rising air
bubble is constant, the energy transfer from the bubble to
the water is
Pair ¼QSPSln 1 þrgH
PS

in ½W
where ris the density of the water (kg m
3
), gis the grav-
itational acceleration (m s
2
) and Hthe water depth (m) and
Q
S
represents the volume ux (m
3
s
1
) of air at atmospheric
pressure P
S
. For a buoyant jet the power that is given to the
rising water reads:
Pbuoy ¼gH rjet
r

Qjet in ½W
where Q
jet
is the volume ux of jet water m(
3
s¹) and
ris
the density of the ambient water.
Pjet ¼1
2rjetU2
jetQjet in ½W
where U
jet
is the inlet velocity of the water jet. The energy
that is transferred to the water results in both turbulent and
mean ow motion. According to Bulson (1961), the power that
is given to the mean owcanbecomputedbyconsideringthe
current in the surface layer on one side of the air curtain. The
total power that is given to the current in the surface layer is
Pc¼1=2rB
Z
T
0
v3dtin ½W
where Bis the width of the lock and Trepresents the thickness
of the surface layer. In the experiments of Bulson (1961) it is
found that the horizontal velocity at a water depth distance from
the air curtains is almost linear with depth and the thickness of
the layer is approximately H/4. The maximum and average hor-
izontal velocity in the surface layer can than be estimated as
Table I. Summary of the obtained salt transmission factors in the
Stevin lock experiments
Method against
salt intrusion
Salt transmission
factor N
Blockage/
efciency 1N
None (classical
lock-exchange ow)
1.00 0.00
Traditional design of
air injector
0.40 0.60
New air injector 0.25 0.75
Freshwater jet 0.40 0.60
New air injector
+ freshwater jet
0.15 0.85
New air injector + sill 0.20 0.80
New air injector + ushing during
ebb
0.20 0.80
49AIR CURTAINS AND OTHER MEASURES TO REDUCE SALINITY INTRUSION
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
Vmax ¼2
V¼16Pair
rBH

13
=
where the energy transfer ratio is denedby2P
c
=P
air
.Note
that there are two surface currents towards both sides of the air
curtain. Bulson (1961) conducted several experiments with air
cuntains in a dry dock. Based on these data he found the follow-
ing expression for the energy transfer ratio:
¼0:125 1 þH
Hatm

1
:
Non-dimensional circulation
An important non-dimensional circulation for steady (quasi)
two-dimensional vortices is the non-dimensional circulation
C¼Γ
ffiffiffiffiffi
2E
p
where Γis the total circulation around a given curve around
the vortices and Eis the total kinetic energy per unit density.
The circulation cells around an air curtain or water jet are
conned in a rectangular box with aspect ratio 4. Therefore,
the circulation and total kinetic energy can be estimated as
~
Γ¼8HUgrav
~
E¼2
V2H2
where the volume average velocity over the circulation cells
is computed by
V¼1
2
16Ptot
rBH

13
=
where P
tot
=P
air
+P
jet
+P
buoy
is the total power that is given
to the water and
Ugrav ¼1
2ffiffiffiffiffiffiffiffiffiffiffiffi
gΔrH
rs
s
represents the propagation speed of a gravity current in the
absence of dissipation (Shin et al., 2004). The non-dimensional
circulation can be related tot the air ux parameter in the case
where P
tot
=P
air
, which yields
1
C¼
4

13FL
=
where the air ux parameter is dened as
FL¼Qsg
B

1=3
Δr=rgHðÞ
1=2
GEERT KEETELS
Deltares, Delft, The Netherlands
ROB UITTENBOGAARD
Deltares, Delft, The Netherlands
JOHN CORNELISSE
Deltares, Delft, The Netherlands
NICKI VILLARS
Deltares, Delft, The Netherlands
HANS VAN PAGEE
Rijkswaterstaat, Centre for Water Management, Lelystad,
the Netherlands
50 G. KEETELS ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Irrig. and Drain. 60 (Suppl. 1): 4250 (2011)
... was previously used by Keetels et al. (2011), van der Ven et al. (2018 and Oldeman et al. (2020) to characterise the bubble curtain. In analogy to air curtains, we expect two operating regimes to exist for the bubble curtain as well, namely the breakthrough and the curtain-driven regime. ...
... To assess the performance of bubble curtains separating two sides of a channel or a ship lock, the so-called salt transmission factor has been used in the past (Abraham et al. 1973;Keetels et al. 2011;van der Ven et al. 2018;Oldeman et al. 2020). It is defined as ...
... (2004) observed circulation patterns around bubble curtains in their field experiments in lakes that were vertically stratified. Keetels et al. (2011) noted the presence of recirculation cells in their small-scale experiments on bubble curtains separating two sides at different densities but did not study the variation of the horizontal extent of the recirculation cells and how it depends on other parameters of the system. The proportionality coefficient in (5.4) provides a good fit to our measured extents of recirculation cells in small-scale experiments. ...
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Bubble curtains are multiphase line plumes that are used to reduce buoyancy-driven flows between two water zones at different densities. They are similar to air curtains, plane turbulent jets, that are installed in doorways of buildings between two climatically different environments. In this study, we establish a formal analogy between bubble curtains and air curtains and unify the two frameworks for their description that had previously been used. By means of small-scale laboratory experiments conducted in a channel with freshwater and brine solutions, we study how effectively a bubble curtain acts as a separation barrier for a wide range of density differences as well as different air fluxes and water depths. Qualitatively, two regimes of operation of a bubble curtain are identified and we establish the optimum operating conditions on the basis of quantitative measurements and theoretical considerations. We develop a theoretical model to calculate the infiltration flux of dense water across the bubble curtain that is in very good agreement with experimental measurements and yields a theoretical upper limit on the effectiveness of the bubble curtain. We also study the zones of mixed fluid around the bubble curtain, provide a scaling law for their horizontal extent as well as theoretically predict the water density inside these mixed zones. We discuss how the theoretical models derived from our small-scale experiments apply to real-scale bubble curtains that are, for example, used in ship locks.
... Bulson (1961) performed large scale tests with bubble screens as wave dampers in docks, but only measured a very limited amount of vertical and surface velocities. More recently, Deltares has performed field tests for investigating screen performance with pilot setups in the Stevin locks and the Krammer recreational lock ( Keetels et al., 2011;van der Ven et al., 2018 ). Wen and Torrest (1987) have performed several laboratory-scale measurements on line-source bubble plumes for aeration of lakes. ...
... However, the modeling of bubble screens is scarce. Specifically for bubble screens as separator for fresh and salt water systems, a 2D Euler-Euler model has been developed at Deltares as support for specific experimental field tests ( Keetels et al., 2011 ). Next to this, a 3D Euler-Euler model ( van Meerkerk and O'Mahoney, 2015 ) was developed for reproducing scale model measurements. ...
... Also referred to as the salt-leak ratio or salt reduction factor, the STF is defined as the ratio of salt mass intrusion with a mitigating measure present to the salt mass intrusion without any measure, i.e. the undisturbed density current. Originally developed by Abraham et al. (1973) and later also used by Keetels et al. (2011) , Uittenbogaard et al. (2015 and van der Ven et al. (2018) , it is used to describe the effectiveness of a measure for mitigating salt intrusion. Abraham et al. (1973) derived a semi-empirical correlation from large-scale field test results describing the STF as a function of the Froude air number: ...
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In order to mitigate the effects of salt water intrusion at sea locks, bubble screens are installed which act as a barrier between the dense sea water and the fresh water inland. In order to optimize the design of the bubble screen, in this study a state-of-the-art numerical model is developed based on the Euler-Lagrange CFD method which is expanded with a simple salt balance and concentration-density coupling. The model has been validated by means of experimental results on a laboratory-scale bubble screen. The liquid circulation and entrainment have been investigated for two types of bubble injection methods. It is found that the bubble screen is successful as a separator of salt and fresh water in an initial period of τsep=30 seconds but acts more as a mixer at later times due to the swaying of the screen. The rate of the mixing increases with the air flow rate. Two mechanisms of salt intrusion are distinguished; a delayed density current along the bottom and entrained liquid being circulated through the domain back to the screen. An optimum in air flow rate is found at a Froude air number Frair=0.91. Bubble screen behaviour is also checked at the lock-scale using lock-scale geometry and simulations. The amount of salt transmitted agrees well with the large-scale field tests up until the reported Frair numbers but Frair > > 1 need to be tested to check for the optimum as found in the lab-scale tests.
... Further in situ measurements can be found in Uittenbogaard et al. (2015a) and Keetels et al. (2011), who present the application an innovative combination of bubble and water screens at the Stevin Lock (The Netherlands). The dimensions of this shipping lock are 148 m length, 14 m width and 4.7 m depth. ...
... This section presents a selection of such research. Keetels et al (2011) have performed laboratory tests in preparation to the in-situ tests discussed in Paragraph 4.1. A flume approximately 0.30 m depth was used, consisting of a large compartment filled with salt water and a smaller compartment filled with fresh water. ...
... and even 0.15±0.05 when a fresh water screen is used additionally (seeKeetels, 2011). ...
Conference Paper
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Salt intrudes into the freshwater system via shipping locks located at the sea every time these open the lock gate. With increasing lock size and increasing shipping traffic intensity, this issue requires attention. The application of bubble screens along the lock's entrance is one of the available mitigating measures. The effect of such a screen is often expressed as a factor reducing the speed of the lock exchange process: the so-called salt transmission factor, which may reach 0.25, depending on the bubble screen design and operation. The effect on the salt intrusion during successive lockages may be enhanced further when the duration of the doors being open is minimized. This paper presents various methods to determine the effect of bubble screens on salt intrusion, a discussion on the assessment of its design with scale model tests or numerical computations as well as the effect of the salt intrusion on the inland water system. 1. INTRODUCTION Salt intrusion and mitigating measures have been studied extensively in recent years. Shipping locks are a clear example of a location where salt intrusion via surface water would occur if not properly mitigated. The importance of mitigation is related to the required quality of the fresh water, for ecological reasons or due to its use for agriculture and/or drinking water. Recent shipping traffic developments contribute negatively to salt intrusion unless proper mitigating measures are taken. Increasing shipping intensity demands more frequent lockages, with every lockage adding to the amount of salt intrusion through the lock. Furthermore, the enormous size of modern locks means they contain a large amount of salt water that potentially flows towards the inland water bodies. Sea level rise as a result of climate change also leads to higher water levels at sea locks and increasing salt intrusion. Similarly an increasing strain on freshwater resources requires that more attention is given to issues concerning water quality. This can also lead to less water being available for some mitigation measures, such as flushing of water through locks and sluices. The research on salt intrusion through shipping locks has led to computational methods to model the exchange of water in the vicinity of the lock and the (ongoing) development of the coupling with regional models. Furthermore, the studies have increased the knowledge of various salt intrusion mitigating measures-in particular the bubble screen, being the specific topic of this paper. Bubble screens are applied at shipping locks between salt and fresh water bodies, in order to decrease the rate of salt intrusion as a result of the locking process. Various measurements have been performed in the past decades to address the effectiveness of bubble screens in shipping locks. The potential of bubble screens is currently given a renewed assessment due to the developments mentioned above.
... Examples of technical measures are a bubble screen, a water screen and a flushing discharge or a combination of these measures, see Figure 2. These measures are discussed in Keetels et al. (2011) andUittenbogaard et al. (2015). ...
... Salt intrusion mitigating measures in a shipping lock on a salt-fresh water transition. FromKeetels et al. (2011). ...
Conference Paper
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Salt intrusion resulting from lockages is an important aspect to be studied in the design process of shipping locks situated on the interface of salt and fresh water. To this end, Deltares has developed the generic salt intrusion model WANDA-Locks. This paper presents the development of an additional method within WANDA-Locks, allowing more accurate calculations of salt intrusion by simulating the actions of the lock operator more explicitly and allowing the inclusion of the stochastic nature of traffic demand. This method has been applied in a study considering the application of salt intrusion mitigating measures at the Krammer locks in the Netherlands.
... Using electric barrier systems in the ship locks of canals could prevent the inter-basin movement of aquatic nuisance species between different basins [179]. Barriers such as air curtains or bubble screens can be installed in the canal or at lock entrances to mitigate saltwater intrusion [115,180]. ...
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Ship canals, which play a crucial role in facilitating transport, exert significant and long-term impacts on wetland ecosystems. For social and economic development, numerous countries have put forward plans for ship canals. This paper reviews the literature on the ecological effect of ship canals on wetland ecosystems, identifies research gaps, and suggests future research directions. Ship canals typically involve high construction intensity, usually including river regulation, ship locks, and water diversion for navigation. The ecological effects of ship canals on wetlands refer to changing wetland hydrological processes, degrading water quality, eliminating wetland botany, disturbing wetland animals, and increasing ecological threats. The cumulative impact can either alter the trend of ecological succession or degrade biodiversity. Thus, there is a need for further research to elucidate the mechanisms by which canals affect wetland ecosystems, enhance the practices for wetland protection associated with canals, and develop a robust evaluation system for green canal projects.
... setting restrictions to the opening time of locks, which limits the water exchange with the exterior water body. In some locks in the Netherlands, e.g. in those of the North Sea Canal bubble screens are employed, to enhance vertical mixing and in this way reduce salt intrusion (Keetels et al., 2011). Here we showed that when freshwater is available, temporary increasing the discharge can also reduce salt intrusion. ...
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Salinization threatens coastal freshwater bodies, but little is known about this phenomenon in man-made canals. Here, salt intrusion and effective longitudinal dispersion in such canals are investigated, where the Ghent-Terneuzen Canal in Belgium-the Netherlands is used as a prototype example. A calibrated, width-averaged model is employed to quantify the sensitivity of these quantities to forcing conditions. This model performs better than a calibrated, cross-sectionally averaged model with a constant longitudinal dispersion coefficient, because density-driven advection of salt, which turns out to be important in man-made canals, is explicitly resolved. It is found that, in equilibrium, discharge at the upstream boundary is more important than exterior salinity for salt intrusion and effective longitudinal dispersion. Furthermore, the time-dependent salinity response to an increase in freshwater discharge is faster than that to a decrease in discharge. In contrast, the time it takes the system to adjust to a change in the exterior salinity does not depend on the sign of that change. From these results, a parametrization of the effective longitudinal dispersion coefficient is developed, which explicitly accounts for the horizontal salt transport by the density-driven current. A cross-sectionally averaged model that uses this effective longitudinal dispersion coefficient successfully simulates the salt dynamics of the width-averaged model.
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This paper presents research on the impact of a plan to change the salt-fresh water separation system at the Krammer locks in the Netherlands. This lock complex forms the connection for inland navigation between the Eastern Scheldt estuary and the hinterland. The new salt-fresh system will include bubble screens at both lockheads and an additional flushing discharge through the lock chambers and a neighbouring flushing sluice. The interaction between water management, salt management, ecology requirements and navigational requirements has been accounted for with the aid of WANDA-Locks and Delft 3D simulations. The effect of expected sea level rise on the performance of the new proposed new system is also modelled. The modelling efforts show that the new system should be able to keep the salt concentration of the fresh water lake Volkerak-Zoom within the required limits for the duration of the system’s technical lifespan. The modelling approach is able to tune the operation of the lock to the hydraulic conditions of each lockage and within the seasonal restrictions on the water management of the region, giving for each situation the required air discharge for the bubble screens and the flushing discharge. This operation has also been designed to remain safe for the vessels passing through the lock complex.
Article
The intrusion of sea saltwater has destructive effects on freshwater resources as well as hydraulic structures. The first disastrous effect is reducing the volume of available freshwater storage and reducing available surface water quality. Various methods are used to prevent salinity intrusion into upstream river. One of the applied systems in reducing salinity intrusion in the mouth of rivers and especially in the shipping locks is the air bubbles curtain. For proper use of the air bubbles curtain system, it is necessary to determine the effect of different parameters on its performance. Therefore, in the present study, the effect of seawater density and air bubbles discharge on the performance of the air bubbles curtain was studied using a numerical model. The results show that the air bubbles curtain can prevent salinity intrusion by forming a vertical flow. Indeed, the air bubbles curtain's proper performance depends on the air bubbles discharge and the difference between saltwater and freshwater density. In other words, an increase in the density of seawater raises the salinity intrusion force and thus leads to the formation of a clockwise rotational flow upstream of the air bubbles curtain, which in turn intensifies the salinity intrusion into upstream of the air bubbles curtain and reduces the efficiency of the air bubbles curtain. Increasing air bubbles discharge culminates in preventing saltwater intrusion, although there is an optimal discharge that discharges greater than it has insignificant effects on air bubbles curtain performance.
Article
The dynamics of gravity currents are believed to be strongly influenced by dissipation due to turbulence and mixing between the current and the surrounding ambient fluid. This paper describes new theory and experiments on gravity currents produced by lock exchange which suggest that dissipation is unimportant when the Reynolds number is sufficiently high. Although there is mixing, the amount of energy dissipated is small, reducing the current speed by a few percent from the energy-conserving value. Benjamin (J. Fluid Mech. vol. 31, 1968, p. 209) suggests that dissipation is an essential ingredient in gravity current dynamics. We show that dissipation is not important at high Reynolds number, and provide an alternative theory that predicts the current speed and depth based on energy-conserving flow that is in good agreement with experiments. We predict that in a deep ambient the front Froude number is 1, rather than the previously accepted value of 2\sqrt 2. New experiments are reported for this case that support the new theoretical value.
Proeven met luchtschermen
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Van der Burgh P. 1962. Proeven met luchtschermen. Rijkswaterstaat Dienst voor de Waterhuidshouding: the Hague, the Netherlands (in Dutch).
Pneumatic Barriers to Reduce Salt Intrusion through Locks Rijkswaterstaat communications No. 17. Government Publishing Office: the Hague, the Netherlands
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Abraham G, van der Burgh P, de Vos P. 1973. Pneumatic Barriers to Reduce Salt Intrusion through Locks. Rijkswaterstaat communications No. 17. Government Publishing Office: the Hague, the Netherlands. Bruyn J. 1963. Waterschermen ter bestrijding van zoutbezwaar van schutsluizen aan zee. Waterloopkundig Laboratorium. M799: Delft, the Netherlands (in Dutch).
Waterschermen ter bestrijding van zoutbezwaar van schutsluizen aan zee
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Bruyn J. 1963. Waterschermen ter bestrijding van zoutbezwaar van schutsluizen aan zee. Waterloopkundig Laboratorium. M799: Delft, the Netherlands (in Dutch).