Within the framework of a sustainable exploitation of the mineral resources on the Belgian Continental Shelf, the Belgian government has established a surveillance and monitoring program of the extraction activities and the impact on the environment. The up to date results show a concentration of the activities on the Kwintebank, resulting in a strong but merely local impact of the extractions on the bathymetry and sediments of the seafloor. In the near future the governmental control will evolve to a real-time surveillance and a dynamic and three-dimensional monitoring program to assure the future sustainability of the economically important exploitation of mineral resources

Degrendele, K.

Roche, M.

Open Marine Archive

70
MULTIBEAM (1 EN 2) - Degrendele and Roche
Monitoring The Sand Extraction On The 
Belgian Continental Shelf
Methodology, Results And Expectations
Within the framework of a sustainable exploitation of the mineral resources 
on the Belgian Continental Shelf, the Belgian government has established a 
surveillance and monitoring program of the extraction activities and the impact on the environment. The 
up to date results show a concentration of the activities on the Kwintebank, resulting in a strong but merely 
local impact of the extractions on the bathymetry and sediments of the seafloor. In the near future the 
governmental control will evolve to a real-time surveillance and a dynamic and three-dimensional monitoring 
program to assure the future sustainability of the economically important exploitation of mineral resources.
Legislative and economic framework
Numerous tidal sandbanks characterize the Belgian continen-
tal shelf. These sedimentary bodies stretch out over several 
tens of km. They reach a height ranging between 10 and 20m 
and are separated by parallel swales with a maximum depth 
of 40m. The extraction of sand on tidal sandbanks of the Bel-
gian Continental Shelf started very cautiously in 1976 and is 
inventoried since 1979. The annual extraction has increased 
regularly from 370,000m3 in 1979 to 1,700,000m3 in the mid 
of the 90’s. In 2001 the production exceeded the limit of 
1,900,000m3 (or nearly 3,000,000m3 tons at a mean density 
for sand of 1.5t/m3). The last 3 years, however, the production 
has been around 1,600,000m3.
To assure a sustainable exploitation of the mineral resour-
ces on the Belgian Continental Shelf a legislative framework 
for the regulation of the exploration and exploitation of the 
aggregates was established in 2004 by the General Direction 
Quality and Safety of the FPS Economy. Within this frame-
work, three control zones and one exploration zone are 
defined (Figure 1).
Methodology and results
The governmental control is two-fold:
Monitoring of the extraction activities
-    Each extraction vessel operating in Belgium has to be in 
possession of a register providing all relevant information on each extraction (vessel identification, date and location and 
discharging volume);
-    A black box has to be installed aboard each exploitation vessel, This black box registers parameters entered by the crew 
before each journey (identification of the concession holder and reference number of the journey) and the automatically 
collected data (identification of the vessel; date and hour of the registration; position and speed of the vessel; status of the 
pumps and status of the extraction activity). All data are collected and analysed.
The analysis of the registers and the black-box records of the trailer suction hopper dredgers operating on the Belgian Con-
tinental Shelf reveals that, since the beginning of the extraction in 1976, at least 75% of the total extracted volume originates 
from only one sandbank, the Kwintebank. 
On this sandbank the two most dredged areas are morphologically distinguished by two depressions: one in the central and one 
in the northern part of the bank (Figure 2). In order to limit the impact of the sand extraction on the bathymetry, the Fund for 
Sand Extraction has closed the central depression of the Kwintebank for exploitation in February 2003.
K. Degrendele and M. Roche
Figure 1: Dredging zones on the Belgian Continental Shelf.
Mb-028 Roche   70 10/10/2006   11:34:36 AM
 71
MULTIBEAM (1 EN 2) - Degrendele and Roche
Monitoring of the impact of the extraction on the seabed
In the framework of the follow-up of the marine extractions 
by the Fund for Sand Extraction, the Federal Public Service 
Economy acquired a Kongsberg Maritime 1002S multibeam 
echosounder. This system has been installed aboard the RV 
Belgica in summer 1999. 
The EM1002 provides 111 beams of 2° (athwart) x 3.3° (fore-
aft) width. It works at a nominal frequency of 95kHz with a 
ping-rate of around 4 to 6Hz. The data are real-time cor-
rected for the roll and heave with a Seatex MRU 5 motion 
sensor and for the heading with an Anschütz Standard 20 
gyrocompas. The geographic co-ordinates are provided by a 
Sercel NR103 (1999 until January 2003) and a Thales Aqua-
rius 02 (since January 2003) GPS positioning system with a 
theoretical precision of respectively <5m and 10mm.
The soundings are tide-corrected using the specific M2 tidal 
reduction method for the Belgian coastal zone and refe-
renced to the level of mean lowest low water at springtide 
(MLLWS).
The large amount of gathered data and the full-coverage 
capabilities of such remote system allow the construction of 
accurate terrain models of the seafloor. A global bathymetric 
error (2σ) of 0.35% of depth has been estimated on the basis 
of the variance between the resulting bathymetrical digital 
terrain models of four successive surveys of the same area 
within one tide cycle and on successive measurements on a stable wreck. This global error on the final product, the terrain 
model, is the combination of the independent errors of the EM1002 multibeam echosounder, the auxiliary sensors and the 
draught and tide corrections.
The processing of the backscattered 
acoustic signal through specific software 
packages results in the acoustic carto-
graphy of the seabed (seabed imagery). 
Despite the large amount of along-
track artefacts, acoustic zones can be 
delimited (acoustic classification). The 
resulting maps partially reflect the real 
sedimentary nature of the seabed.
To evaluate the impact of the extrac-
tion in detail, several monitoring zones 
were defined: two in the active control 
zones (central and northern part of the 
Kwintebank) and one on the neighbou-
ring bank to the east, the Middelker-
kebank, situated outside the control 
zones (Figure 3). From November 
1999 until the closure for extraction 
in February 2003 and the subsequent 
post-dredging evolution until June 2005, 
a total of 17 surveys were carried out 
on the central part of the Kwintebank 
(KBMA, Figure 4) and on a reference 
zone situated on the Middelkerkebank 
(R2), outside the dredging area. The 
resulting time series of digital terrain 
models (DTMs) and backscatter strength maps allow a detailed comparison of the bathy-morphological and sedimentary evo-
lution of both monitoring areas.
Figure 2: Superposition of the extraction intensity on the geomorpho-
logical model of the Kwintebank (number of dredging records from 
05/11/1996 to 30/03/2005; each record represents 30 seconds of 
operation by a trailer suction hopper dredger).
Figure 3: Location of the monitoring areas KBMA, KBMB and R2.
Mb-028 Roche   71 10/10/2006   11:34:38 AM
Getting to the bottom of things.
sales@seabed.nl • +31(0)20 636 84 43
Positioning
Sub-bottom
sampling
Dredge & Port
monitoring
SEABED Navigation
Seabed ad   72 12-10-2006   9:23:21
 73
MULTIBEAM (1 EN 2) - Degrendele and Roche
Since the start of multibeam monitoring in 1999, 
a global deepening of 0.5m of the whole KBMA 
monitoring zone is observed until February 2003. 
From that moment on, KBMA was closed for 
extraction and the bathymetrical trend on the 
central depression becomes less negative. The 
evolution of the KBMA can be considered as a 
combination of natural and anthropogenic factors. 
After the subtraction of the extracted volumetric 
component from the total volume evolution of the 
KBMA monitoring area, a close to natural evolu-
tion can be deduced and enables a comparison 
with the volume evolution of the R2 area outside 
the dredging area. This forms, together with the 
much smaller decrease since the closure of the 
zone for extraction and the lack of indications for 
cumulative effects on the bathymetry, a clear evi-
dence of the merely local impact of the extraction. 
At the scale of the KBMA monitoring area, there 
is no indication of a change in natural processes. 
The volume evolution that was observed during 
extraction is reversed nor maintained.
The mapping of the backscatter values suggests a 
clear difference in sediments between the depres-
sion sensu stricto and the rest of the KBMA moni-
toring area. The central depression has a mean 
backscatter value of –24dB, which coincides with 
a backscatter strength mean value for medium to coarse sand. The mean of the backscattered strength values recorded on 
the east side of the KBMA area is up to –27dB, suggesting the dominance of very fine sand in this sub-area. The western side 
of the KBMA shows intermediate backscatter values, and is characterized as medium to coarse sands with grab samples. The 
focus of the extraction industry on this depressed part of the bank is explained by the presence of this medium to coarse sand. 
The backscatter strength values are fairly stable and do not show a clear evolution before or after the cessation of dredging. 
According to these data within the KBMA area, the sedimentary composition of the seabed at the water-sediment interface 
seems to be quite stable. However, the seabed classification indicates a minor tendency towards more fine and homogenous 
sand in the depression after February 2003. A similar stability of the backscatter strength is observed for the R2 monitoring 
area.
Expectations
The monitoring of the extraction activities is a control activity with a delay of a few months at best. The registers and black box 
readings are collected at regular intervals and processed. This constitutes an effective control of the extraction activities, but 
doesn’t allow a real-time surveillance. Furthermore, the extracted volume for each dredging vessel and company is calculated 
based on the extracted volumes listed in the registers. These values are submitted on his word of honour by the exploitation 
vessel’s captain. To establish a real-time monitoring independent of the controlled organisations the black boxes should be used 
to calculate the extracted quantities. The black box data should be collected as close as possible to real time to enable a direct 
response to infractions and a close follow up. For the latter, the use of AIS data can complement the information of the black 
boxes and provide a real time follow up of the extraction vessels and activities.
The monitoring of the seabed can still be improved on a few fundamental points. The important compensations for the draught 
of the vessel and the tidal height can be ameliorated and made available in real-time. Most important however, the use of a 
more adapted tool for the measurements, a very shallow water high frequency echosounder, instead of the by all means non 
ideal solution of a shallow to medium water echosounder, will greatly improve the accuracy, resolution and efficiency of the 
measurements. This will permit the researchers to refine and intensify the study of the impact of the extractions. Due to the 
limitations of the present system the monitoring is confined to small areas. The large-scale monitoring is currently executed by 
comparing the mapped sandbanks with recently sailed transects. With a more efficient system the re-mapping of large structures 
becomes a feasible option.
If the monitoring of the extractions activities can evolve to a near real-time surveillance, a dynamic monitoring of the impact 
becomes possible. The surveillance will enable the adjustment of the monitoring to the up to date information and provide a 
more adaptive, dynamic monitoring.
Figure 4: Detail of the monitoring area KBMA and the delimitation of the central 
depression.
Mb-028 Roche   73 10/10/2006   11:33:32 AM
74
MULTIBEAM (1 EN 2) - Degrendele and Roche
The monitoring of the seabed is at present a 2D evaluation of the surface of the seafloor. A real 3D evaluation of the mineral 
resources and present sedimentary bodies is necessary to establish the impact on the available volumes of sand and gravel and to 
develop a sustainable exploitation. From this point of view the Fund for Sand Extraction has launched a new project to develop 
and optimise techniques to transgress to a full 3D monitoring and cartography of the Belgian Continental Shelf.
Acknowledgements
The Fund for Sand Extraction wishes to thank the crew of the R.V. Belgica for their assistance during the surveys. The Manage-
ment Unit of the Mathematical Model of the North Sea and the Scheldt Estuary (MUMM) is thanked for the provision of ship 
time on board the Belgica.
Koen Degrendele, Federal Public Service Economy - Fund for Sand Extraction, 1000 Brussels, Belgium
e-mail: Koen.Degrendele@mineco.fgov.be
Marc Roche, Federal Public Service Economy - Fund for Sand Extraction, 1000 Brussels, Belgium
e-mail: Marc.Roche@mineco.fgov.be
Mb-028 Roche   74 10/10/2006   11:33:34 AM


Monitoring the sand extraction on the Belgian Continental Shelf: methodology, results and expectations

Abstract

Similar works

Full text

Available Versions

Open Marine Archive