As a flood control area with controlled reduced tide (FCA-CRT), Bergenmeersen is expected to make a significant contribution to natural quality in the Scheldt estuary. Via a sophisticated sluice construction with combined inlets and outlets above each other, a reduced tide is introduced into the polder, while retaining the spring/neap tide variation. The purpose: a functional mud flat and marsh ecosystem. A monitoring programme was developed to examine whether these nature objectives are actually being achieved and to make adjustments where necessary

Maris, T.

Peeters, P.

Open Marine Archive

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As a flood control area with controlled reduced tide (FCA-CRT), 
Bergenmeersen is expected to make a significant contribution to 
natural quality in the Scheldt estuary. Via a sophisticated sluice 
construction with combined inlets and outlets above each other, a 
reduced tide is introduced into the polder, while retaining the spring/
neap tide variation. The purpose: a functional mud flat and marsh 
ecosystem. A monitoring programme was developed to examine 
whether these nature objectives are actually being achieved and to 
make adjustments where necessary.
Authors:  Tom Maris (University of Antwerp) and Patrik Peeters (Flanders Hydraulics  
Research) 
8.1 MONEOS: a coordinating  
monitoring programme
A coordinating monitoring programme,  
MONEOS, was drawn up for the Scheldt es-
tuary. This programme describes three types 
of monitoring: system monitoring, research 
monitoring and project monitoring.
 
•	 System monitoring is a long-running 
programme that monitors all the basic 
parameters for the smooth operation of 
the Scheldt ecosystem. This includes, 
among other things, standard measure-
ments of the water quality in the Scheldt. 
System monitoring also covers water 
levels and the safety-related functioning 
of the FCAs in the Scheldt basin, such as 
Bergenmeersen.
•	 Research monitoring encompasses 
the detailed monitoring of pilot projects 
to gain greater insight. The Lippenbroek 
is one such research monitoring project, 
in which knowledge is gathered for the 
development of other FCA-CRTs, of 
which Bergenmeersen is one.
•	 Project monitoring. During the first 
three years after the area starts up, a pro-
ject monitoring programme examines the 
extent to which the area is meeting the 
set expectations and whether it is evolv-
ing in the right direction. Keeping a finger 
on the pulse allows possible problems to 
be detected earlier. This makes adaptive 
management and adjustment possible. 
Project monitoring is the best tool for this. 
The MONEOS report describes project 
monitoring as monitoring that is carried 
out – in addition to regular system moni-
toring – to measure certain parameters 
with an increased frequency, limited in 
time and space. After a period of three 
years, an evaluation is carried out. If eve-
rything is progressing as desired, project 
 8. MONITORING
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monitoring can be run down and the area 
included in the regular system monitoring.
8.2 Most suitable tide
Accurately setting the most suitable tide is 
vitally important to the successful develop-
ment of an FCA-CRT. After all, for estuarine 
nature, that tide is the driving force behind 
ecological development. The volume of wa-
ter (and suspended material) exchanged, 
the flooding frequency, duration and height, 
will be decisive for the development of the 
area. The success of the Bergenmeersen 
project therefore depends entirely on set-
ting the right tide. Initial monitoring activities 
focus entirely on setting the tide, during 
which the desired final picture and the pre-
vailing preconditions are taken into consid-
eration. As an extra framework condition for 
Bergenmeersen, special attention is being 
paid to the sandfly issue.
First, an improved digital height model 
(DHM) needs to be created. Based on that 
DHM, the water heights and flooded areas 
can be estimated for different inflow vol-
umes. The inflow and outflow volumes for 
average, neap and spring tides on the Upper 
Sea Scheldt were calculated by Flanders 
Hydraulics Research when determining the 
sluice dimensions.
The sluice configuration that followed from 
the modelling study forms the starting 
point for further refining the heights of the 
stop-logs. Specifically, experience from the 
Lippenbroek shows that minor discrepan-
cies between model and reality, for example 
in the DHM, can produce differences in 
flooded area. The inflowing water volume 
can be adjusted by raising or lowering 
stop-logs, and therefore also the flooded 
area. Particular attention is paid here to the 
differences in flooded area between spring 
and neap tide. These differences determine 
the frequency of flooding and are decisive 
for the development of a diverse ecosys-
tem.
As well as setting the right tide, it is vital that 
sedimentation is steered in the right direc-
tion. More water means more sediment 
flowing in. The most suitable tide will be a 
compromise between a tide that is suf-
ficient for nature development and a tide 
that limits sedimentation as much as pos-
sible. Specifically for Bergenmeersen, mud 
flat zones near the residential area must 
also be avoided to combat sandflies. The 
sluice configuration of an FCA-CRT is there-
Figure 8.1. SET measurement
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fore not static. It forms part of the manage-
ment with which the various objectives of 
the area will be aligned with each other over 
the coming years.
Producing a good digital height model
A good, up-to-date digital height model 
(DHM) is needed. In the land study, the 
morphology (also depth) of the channel 
system is also surveyed. This is necessary 
to accurately monitor silting-up in the chan-
nels during the start-up phase. The land 
surveys are carried out using an RTK (real-
time kinematic) GPS to obtain an accurate 
picture of the topography. Such an RTK-GPS 
determines its position through ten satel-
lites plus correction factors continually sent 
through by a reference station. This allows 
the position to be determined accurately to 
within a few centimetres.
Multiple level measurements are needed to 
monitor the tide during the start-up phase, 
but also to manage the sluices at a later 
stage according to the development of the 
area. In Bergenmeersen, the water level will 
be recorded every 10 minutes at 5 locations 
during the start-up phase. One gauge will be 
located near the sluice complex. The others 
will be spread over the site. This allows the 
duration, height and frequency of flooding 
to be determined in large parts of the area, 
which in turn enables slow-draining parts 
and parts with a high flood duration to be 
properly mapped out. This is extremely 
important for monitoring sandflies, for 
example.
SET measuring set-ups (surface elevation 
table) are recommended for monitor-
ing sedimentation. Five SET set-ups are 
installed. A SET set-up allows changes in the 
bottom level as a result of sedimentation 
or erosion to be accurately measured (to 
within 1 mm). The set-up is useful during the 
start-up phase, but especially to continue 
to track sedimentation closely in the years 
that follow. A SET set-up consists of a fixed 
pole in the ground. To this is attached a 
level movable arm. Using ten or so verti-
cal rods, the distance from the bottom of 
the marsh to this arm is measured. The 
distance measurement is repeated with 
the arm in 4 different directions. The SET 
measurement is repeated every 2 months. 
This allows the slightest change in bottom 
level around the SET pole to be accurately 
recorded. To avoid changes in the relief at 
the SET through walking on it, 2 benches are 
used. By carrying out measurements from 
on these benches it is possible to avoid 
disturbing the soil.
8.3 Starting up the  
flood area
Before the sluices are opened when the 
flood area starts, the threshold levels in 
these sluices must be selected. Determin-
ing the correct threshold level requires at-
tention. The tide must be set as favourably 
as possible to be able to achieve the objec-
tive of the FCA-CRT as much as possible: a 
functional intertidal area that contributes 
to the operation of the Scheldt ecosystem. 
The area must also be managed sustain-
ably and must not cause any nuisances for 
local residents (sandflies). Finally, the safety 
function as an FCA must never be jeopard-
ised. All of which imposes clear precondi-
tions for the (reduced) tide.
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With the new DHM and the information from 
the preparatory study and the model output 
(based on previous and new sluice calcu-
lations), an estimate can be made of the 
flooded areas for different tidal levels in the 
Upper Sea Scheldt. Based on this, an initial 
stop-log configuration is chosen. Stop-logs 
are inserted to the desired level and the 
sluices are opened for a few tides. The tide is 
monitored and evaluated at various places in 
the polder. Does the tide meet the require-
ments? Where is adjustment needed? Is 
there any significant sedimentation? With 
the knowledge from measurements taken 
with this first stop-log configuration, a new 
configuration is drawn up. This is repeated 
several times in an iterative process to reach 
the most suitable configuration.
As soon as a potentially suitable configura-
tion has been obtained, the newly set tide will 
be monitored closely for a few spring/neap 
tide cycles. Considerable attention will also 
be paid to sedimentation and erosion pro-
cesses. Based on detailed measurements, a 
forecast will be made for the morphological 
evolution of the area. Detailed surveys of the 
thalweg of creeks, of a number of fixed ditch-
es and SET measurements are designed to 
help map out sedimentation and erosion. A 
number of targeted thirteen-hour measure-
ments are also carried out, paying particular 
attention to the inflow and outflow of floating 
material to then draw up water and sediment 
balances. If necessary, the sluice configura-
tion is further adjusted on the basis of these 
insights.
After the start-up phase there are two pos-
sibilities: either a fixed tide is chosen that is 
maintained without being changed, or a tide 
is chosen that is adapted over time.
8.3.1 Unchanged sluice  
configuration 
As soon as a favourable tide is found during 
the start-up phase, the tide is set and main-
tained. That is the option that was applied 
in the Lippenbroek. There, the stop-log 
configuration remained unchanged from 
the start of the flood area. However, choos-
ing unchanged sluice parameters does not 
mean that changes to the sluices are never 
required. Through the action of the tide, the 
morphology of the area will change. This 
has an influence on the tide. Minor changes 
to the sluices may therefore be required 
to maintain the same flooding frequencies 
despite a changing morphology.
8.3.2 Adaptive management
In a new FCA-CRT it may be desirable to 
allow more tidal influence at the onset. As a 
result, morphodynamic processes such as 
sedimentation and erosion can play a bigger 
role. Thus, the area may evolve more quickly 
towards the desired mud flat and marsh 
area. Creeks may be formed more quickly, 
depressions may be filled in more quickly, 
and compact polder clay may be covered 
more quickly with a layer of typical marsh 
sediment rich in benthic life. After such an 
initial period of strong tidal action, as a con-
sequence of which the area has undergone 
a first series of rapid morphological changes, 
the tide can be modified. After all, maintain-
ing a tidal travel that is too large can lead to 
excessive silting-up of the area in the long 
term. The tide must then be further reduced 
to a typical controlled reduced tide.
Because of the sandfly problem in Bergen-
meersen, water-saturated mud must be 
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avoided near the ring dyke. It is therefore 
better to avoid an overly strong tidal action 
in the initial phase. However, increased wa-
ter levels during average or spring tides do 
not lead in and of themselves to more mud 
flat zones near the ring dyke. It is therefore 
worth considering setting the tide a little 
stronger at the start, to promote faster 
creek development. After all, if the drainage 
of certain parts of the flood area is not run-
ning smoothly enough at the start because 
of the lack of a well-developed, branched 
system of creeks, this could encourage the 
proliferation of sandflies.
 
8.4 Project monitoring  
in the initial phase
On the one hand, starting up the FCA-CRT 
requires detailed monitoring of the tide, to 
be able to choose the desired stop-log con-
figuration. On the other hand, monitoring 
of sedimentation and erosion, vegetation, 
nutrient exchange, development of benthic 
life and fish is recommended. After all, the 
flood area has multiple nature objectives. 
Among other things, specific habitats with 
associated species are sought. Ecological 
functions are also linked to the area, such as 
nutrient cycling (see Chapter 4).
Below you will find an overview of all moni-
toring efforts.
Sedimentation and erosion
Four times a year, the soil level is monitored 
by means of five SET set-ups. Twice a year 
the development of creeks is monitored, as 
well as a few ditches, with Total Station  
and/or RTK-GPS. 
Vegetation
Three times a year, permanent squares are 
measured by the five SET set-ups. In addi-
tion, a vegetation map covering the whole 
area is produced each year.
 
Benthos
Twice a year the benthos is sampled in at 
least five locations (certainly the five SET 
locations). Benthos sampling is useful for 
monitoring the quality of the intertidal area 
as a suitable habitat.
 
Thirteen-hour campaigns
To monitor the area’s functionality in terms 
of water quality, there are four thirteen-
hour campaigns each year. During such 
a campaign, the water quality is sampled 
every hour for a complete tidal cycle. The 
following parameters are determined: 
floating materials, chlorophyll a, biochemi-
cal oxygen demand, nitrogen (ammonium, 
nitrate, nitrite, Kjeldahl nitrogen), phospho-
rus (phosphate and total phosphorus) and 
silicon (dissolved and biogenic). Oxygen, 
pH, conductivity, temperature and turbid-
ity are continually monitored during the 
campaigns. These parameters will also be 
continually monitored for a few days before 
and after each campaign.
Based on the thirteen-hour campaigns, 
mass balances will be produced for the 
various substances. This allows the area’s 
contribution to general water quality in the 
Scheldt to be estimated. This is intended to 
show whether the objectives set within the 
framework of European nature objectives 
are being achieved.
 
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Fish monitoring
During the project monitoring phase, the ex-
tent to which the area is being used by fish 
will be monitored. This monitoring is carried 
out by the Research Institute for Nature and 
Forest (INBO).
In April 2009 the baseline assessment was 
carried out. Fish were caught in the central 
channel in Bergenmeersen, one of the few 
water-retaining channels. At the time, only 
two species were caught: three-spined 
stickleback and ten-spined stickleback.
In May 2013, approximately one month after 
the area became operational, fish were 
caught electrically in the creek. Two traps 
were also placed for a period of 24 hours. 
A total of 9 species were caught. The pres-
ence of flounder larvae (on average  
2.3 cm) indicates that this species is using 
the area as nursery habitat (incubator 
function). Several European smelt larvae 
were also caught. Table 8.1 shows the results 
(number of individuals per species). Clearly 
the pioneer species Prussian carp, stone 
moroko and common roach have very 
quickly found their way to the new area. The 
presence of European chub is remarkable. 
This species was caught only once in the 
last 10 years in the Sea Scheldt.
Figure 8.2. Young flounder
21/05/2013 22/05/2013 22/05/2013
Electrical Shoot trap Net trap
Common roach 0 7 0
Stone moroko 2 12 2
Flounder (larvae) 165 1 0
Three-spined stickleback 79 130 13
Prussian carp 0 2 1
European chub 1 0 0
Common rudd 0 1 0
European smelt (larvae) 3 0 0
Ten-spined stickleback 84 10 1
Species 6 7 4
Number of individuals 331 163 17
Chinese crab 1 3 14
Table 8.1. Results of fish monitoring in Bergenmeersen (May 2013)
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Bird monitoring
The agricultural area that Bergenmeersen 
was until recently was home to only a 
limited number of summer bird species. 
An inventory in 2008 recorded several ter-
ritories of reed birds (Eurasian reed warbler, 
bluethroat and common reed bunting) 
and meadow and farmland birds (northern 
lapwing and grey partridge).
From the start of the work in April 2012, and 
certainly in the spring of 2013, the work zone 
attracted many waders (such as common 
redshanks, spotted redshanks, common 
greenshanks, common snipes and little 
ringed plovers) and a few waterfowl (such 
as northern pintails). In the first few weeks 
after the FCA-CRT became operational, the 
area also became very attractive to (transi-
tory) waders and waterfowl. In addition to 
the species mentioned above, there were 
also black-tailed godwits, whimbrels, pied 
avocets, ruffs, red knots, common ringed 
plovers, wood sandpipers, northern shovel-
lers, garganeys, black terns and little gulls.
The first inventory of summer birds in 
Bergenmeersen (23 May 2013) recorded 
breeding attempts by northern lapwings 
and little ringed plovers. The first regional 
breeding attempt by common redshanks 
was also recorded, the first “new” summer 
bird after the CRT had been operational for 
just one month.
Changes in the (summer) bird community 
will be monitored by the Agency for Nature 
and Forest over the coming years.
Flow rates through the sluices
By measuring flow rates and sediment at 
the inlet and outlet sluices, water and sedi-
ment balances can be drawn up and the 
amount of sediment that remains in the 
area estimated. The remaining quantities of 
sediment can be converted into sedimen-
tation heights using sediment densities 
measured in situ and then compared with 
SET measurements.
Monitoring for sandflies
Because the area is close to habitation, a 
suitable programme for monitoring sand-
flies is advisable. No traps will be used. On 
the one hand, there is no evidence of the 
need to catch sandflies. On the other hand, 
the use of traps for monitoring draws too 
much attention to a problem that may not 
even exist. The suitable habitat is therefore 
monitored, linked to monitoring of sandfly 
larvae. This allows the potential of the area 
for sandflies to be estimated. Measures 
against sandflies, such as the raised ridge 
against the ring dyke, and measures for 
tide and drainage can also be evaluated. 
This allows better planning and more ef-
ficient management of future FCA-CRTs. If 
there are problems in Bergenmeersen, the 
measurements will help adapt manage-
ment better.
8.5  References
•	 MONEOS.	Geïntegreerde	monitoring	
van	het	Schelde-estuarium.	(Integra-
ted monitoring of the Scheldt estuary.)  
P. Meire et al. (2008), ECOBE Report 08-
R-113. University of Antwerp / www.vliz.
be/imisdocs/publications/136078.pdf
21/05/2013 22/05/2013 22/05/2013
Electrical Shoot trap Net trap
Common roach 0 7 0
Stone moroko 2 12 2
Flounder (larvae) 165 1 0
Three-spined stickleback 79 130 13
Prussian carp 0 2 1
European chub 1 0 0
Common rudd 0 1 0
European smelt (larvae) 3 0 0
Ten-spined stickleback 84 10 1
Species 6 7 4
Number of individuals 331 163 17
Chinese crab 1 3 14


Monitoring

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Monitoring

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