STORMS: A CHALLENGE OF KNOWLEDGE
Papili Sonia , Thomas Wever , Yves Dupont
1
2
3
(1)Belgian Navy-RCMG: VSW-MWU, Marinebasis Zeebrugge, Graaf Jansdijk 1, 8380 Zeebrugge, Belgium. Sonia.Papili@ugent.be
(2) Wehrtechnische Dienststelle für Schiffe, Marinewaffen, Maritime Technologie und Forschung, WTD 71 Berliner Str. 115, D-24340 Eckernförde, Germany. Thomas Waver@bwb.org
(3) Belgian Navy: Marinebasis Zeebrugge, Graaf Jansdijk 1, 8380 Zeebrugge, Belgium. Yves.Dupont@mil.be
Introduction
Dealing with storms is always a fascinating matter. They are uncontrollable and
destructive. It is not always possible to get real data measurements during a storm
due to large magnitudes involved. Having recording instruments deployed on the
seafloor and properly working, gave the rare opportunity to observe and analyze
the storm impact directly on the sea-floor.
Area of investigation
This paper presents the effects of two storms on a measuring object deployed on
the seafloor in shallow water. The experimental area has a water depth range
between 7 to 12 meters, depending on tides. Two storms passed the experiment
site during a three months experiment directed towards a better understanding
of the sand mobility in a highly dynamic environment.
Storm 1
1.6
Hc r for sedi m ent s with fi ne gr anu lo me try
Hc r for sedi m ent s with me di um gr anu mo me try
Hc r for sedi m ent s with coar se g ra nul o met ry
4
me asur ed H
2
a
700
800
900
1000
1100
1200
12000
m
8000
4000
80
0
b
60
600
40
700
800
900
1000
1100
20
12000
1200
water depth
6m
8m
10m
12m
1.2
current speed (m/s)
bu rial v olu me (%)
wa ve en erg y (J /m ²)
l
0.4
16000
0
16000
0.8
n
0.4
0
600
700
800
900
ID number
1000
1100
1200
8000
2
3
4
date
5
6
7
4000
c
0
600
Measuring instrument
0.8
600
100
The experimental area named Wandelaar
(blue circle in figure 1) is located at 12 km
distance from the port of Zeebrugge in the
vicinity of the main navigation channel on
the Belgian part of the North Sea. It is a
sandy shallow water area characterized by
the presence of small to large dunes (sensu
Ashley, 1990), of up to 2 m in height.
1.2
0
0
Fig. 1: extract from a grain size map on the BCS. (Verfaillie et
al., 2006)
current speed (m/s)
6
wave energy (J/m²)
w av e h ei ght (m)
8
700
800
2
900
ID n um ber
3
da te
4
1 0 00
5
1 1 00
6
Fig. 4: storm in October 2008: comparison
between measured curves of current speed (l
and n) and calculated curve of wave energy
(m). Curve l represents current speed measured
for superficial layer of water depth, curve n for
deep water.
1 2 00
7
Fig. 3: storm in October 2008. (a): comparison between measured curve of real wave height (red) and calculated
curves of critical wave height (different tone of grey); (b): curve of burial volume; (c): curve of wave energy.
The hydrological and meteorological data were analyzed to evaluate the two storms affecting the
area during the experiment. The threshold necessary to initiate movement of sand grains on the
seafloor was calculated. The threshold orbital velocity for sediment motion is used to calculate the
corresponding required critical wave height.
The calculated curves were compared with the measured wave height, percentage of sediment
volume around the object, and current measurements.
Fig. 2: BRM: experimental instrument
Storm 2
The Burial Recording Mine (BRM) is an
experimental
instrument
recording
presence or absense of sediment at
programmed time recording. With its 3 rings
of 24 led bridges (sensors) equally spaced
on its sides and its centre detects the
sediment height surrounding itself once is
deployed on the sea-floor. It has a lenght of
1.70 m, diameter of 0.47m and weight in air
of 500 Kg. Accelerometers inside the object
monitor the pitch and the roll and their
variation in time.
Fig. 5: storm in November 2008: comparison
between measured curves of current speed (l
and n) and calculated curve of wave energy
(m). Curve l represents current speed measured
for superficial layer of water depth, curve n for
deep water.
burial volume (%)
current magnitude (m/s)
1.2
0.8
l
0.4
a
80
b
60
40
20
0
16000
16400
16800
17200
16000
12000
m
8000
4000
0
16000
16400
16800
17200
1.2
Wave energy (J/m²)
16000
wave energy (J/m²)
2
100
0
current magnitude (m/s)
4
0
1.6
Conclusion
During both storms wave affecting the
seafloor caused reduced burial (i.e.,
sediment coverage) of the BRM. After the
storms increased burial of 60% (October)
and 80% (November) was observed. The
high storm waves eroded sediment in the
vicinity of the BRM and created scour
holes at both ends. Once the scour holes
merged to form one, the BRM rests on a
small sediment cone. Upon collapse of the
cone the BRM rolls into the bigger scour
hole. During the sedimentation phase this
new deeper hole is filled and causes a
higher percentage of buried volume.
wave height (m)
6
12000
8000
c
4000
0
0.8
n
0.4
5400
5600
ID number
5800
6000
24
26
0
16000
16400
16800
17200
20
22
date
Fig. 6: storm in November 2008. (a): comparison between measured curve of real wave height (red) and calculated
curves of critical wave height (different tone of grey); (b): curve of burial volume; (c): curve of wave energy.
References
Ashley, G.M. (1990). Classification of large-scale subacqueous bedforms: a new look at an old problem. J. Sedim.Petrol., 60, 1,p. 160-172
Verfaillie E., Van Lancker V;, Van Meirvenne M. (2006). Multivariate geostatistics for the predictive modelling of the surficial sand distribution in
shelf seas. Continental Shelf Research, 26, p. 2454-2468
Acknowledgements
Current data by Ministery of the Flemish Community, Maritime Services, Coastal Division/Hydrography
Hydro-meteo data by 'IVA MDK - afdeling Kust - Meetnet Vlaamse Banken'
I would like to thank The Belgian Navy, founder of this research, Prof. Dr. M. De Batist and Dr. Vera Van Lancker, promotor and co-promotor of
the related PhD research; Ralf Ludher, from the German Navy; all my colleagues from the Belgian Navy and from Ghent University.