The Generation of Severe Surges in the Dutch Wadden Sea

SSC2010-95
POSTER 82
The generation of severe surges in the Dutch Wadden Sea
part C
part B
part D
Insights from data, hindcasting and numerical experiments
G. Lipari, J. Adema
part A
Alkyon Hydraulic Consultancy and Research, The Netherlands
The studies shown here investigate the relationships between severe (wind)storms and severe surges in a semi-enclosed basin with multiple inlets
Historical records indicate that extreme surges, in contrast with open coast, have not always been generated by storms with extreme wind speeds
Unsteady North Sea storms veering with increasing speed cause(d) water accumulation within the Wadden Sea resulting in propagating wind-driven surges
Identifying the most hazardous storms requires consideration of the interplay of flow fields, weather systems, geographic outline and bathymetry
A2. Storms and coastal protection in
the Dutch Wadden Sea (DWS)
On the one hand, had the wind-driven peak coincided with high tide in
1990, the total water level could have exceeded 500 cm (100+ years;
sum c + d) against 483 cm measured in 2006. On the other hand,
reverse the argument: it was the high tide to make the 2006 surge
more dangerous than that of 1990.
23
Vlieland
1
a
b
c
483
351
351
500
27 Feb 1990
1 Nov 2006
400
400
water level
wind surge
Water levels (cm)
200
425
288
312
24
421
284
287
3
1995 (10/1)
12
403
290
294
4
1990 (27/2)
15
393
226
352
126
1981 (24/11)
16
389
259
300
41
1983 (2/2)
17
381
217
301
84
200
There are 5 main barrier islands, 5 tidal inlets and a wide entrance to a
deep estuary. Distances in km are indicated
The geographic difference between the western and eastern DWS is
substantial (shape, extension, openness to North Sea waters)
Top right: Numerical flow simulations are run in a suite of nested
C. Hindcasting
The hourly snapshots (left) show the 2006 surge building up in the
western Wadden Sea owing to a unfavourable sequence of net
influxes through the inlets. A growing wedge of water moves
eastward between the barrier islands and the mainland (follow the
orange band marking the 3+ m water level). Eventually this water is
‘diverted’ into the Eems estuary by the well-timed wind rotation (see
right). This is the time of the peak surge. The increasing wind speed
could act on growing water masses effectively.
32
28
24
w ind speeds
-200
2
-2
-1
0
1
2
Days counted from day of the peak
32
32
360
4
YR1 2006
-4
180
12
8
90
270
21.8 m/s
20
16
180
12
8
90
4
4
0
0
2
-3
-2
YR12 1995
-1
YR15 1990
YR16 1981
0
1
YR17 1983 (Texelhors wind)
2
3
4
Days counted from day of the peak
360
0
0
-2
-1
0
1
The 1990 storm was mostly westerly: the speed built up quickly to 27
m/s and kept steady around 19 m/s for 24+ hours.
The 2006 storm grew much more gradually to a lower peak speed
(22 m/s) and dimmed rapidly, while the direction veered from SW to
N.
Were the winds of 1990 and 2006 similar to each other? The answer
is thus no, those wind patterns were different. Yet both surge peaks
(above) did occur near the time of peak wind speed and during veer.
This is investigated in the panel on the right.
Peak time in estuary

Conclusions, conjectures and vision
D. Numerical experiments
The local wind history at Huibertgat
was used as blueprint for a uniform
wind forcing all over the Wadden
Sea. The real storm was thus deprived
of spatial details. This is shown here
for the 1990 storm.
We argue that all storms that:
• cause water accumulation between the islands and the mainland
• push this water against the coast
can generate severe surges in the Dutch Wadden Sea (DWS). The
complex geography makes simplified approaches ineffective.
The flow simulations bear little
resemblance with the measured surge
at Delfzijl (below). The computed
surge grows approaching a sort of
steady state, while the real surge
developed much earlier.
Severe surges are the outcome of active flow dynamics inside and
around the DWS. Temporal and spatial changes are both important.
Veering winds so timed to maximize the net influx through the tidal
inlets may give the “perfect storms” of the (eastern) DWS. Winddriven surges are then naturally aggravated by bad timing with tide.
(And they also are coupled with wave actions, neglected here.)
315
From historical data, the wind-driven surges of 1990 and 2006 were
very close. Yet the storm of 2006 was 30% milder in terms of wind
speed and wider in terms of veering sector than in 1990. We see that
there is no unique worst storm, and the time shift with tide makes the
ultimate difference. The direct link between peak winds and peak
surges is questionable. Several more parameters may be relevant.
500
270
period of application of (manipulated) uniform wind field
27 Feb 1990
400
225
180
135
90
2
Days counted from day of the peak
Alkyon HRC is a trading name of ARCADIS Nederland BV
www.arcadis.nl www.alkyon.nl www.arcadis.de
w ind directions
Direction (deg nautical)
16
YR9 1994
0
360
24
Wind speed (m/s)
20
Direction (deg nautical)
270
12
8
28

16
1 No v 2006
27.2 m/s
24
20

Water levels (cm)
1
Days counted from day of the peak
Insights from the hindcasts
• water is the carrier of a surge hazard (nice and simple)
• the dynamical interaction of atmospheric forcing, bed resistance and topographic steering
redistributes this hazard over the basin
• surge-generating storms are (at least) those which maximize the growth of water mass
contained in the basin and are strong enough to push it against the coast

Caveat: the interaction of surge and tide
may be nonlinear, at least locally
0
-200
Hindcasts produce the bigger picture of the spatial and temporal
evolution of past events. The flow is driven by reconstructed
synoptic wind fields based on measured values. We simulated the
flow numerically ( Tools and Data below) neglecting the coupling
with waves for the moment.
Winds veering from SW to N over the Netherlands are the feature of
an important class of North Sea storms ( B2 lower left). In the
DWS such winds will first push water through the westernmost inlet
(facing W) and then through the others farther east (facing NW).

Insights from the table
• Low tide hides surge-generating storms
• Missing high tide can be seen as a
hazard attached to the storm
• Severe storms create severe tide-free
surges
100
The plots below show corresponding wind speeds and directions at
Texelhors and Huibertgat (T and H in the map – thicker symbols for H).
The histories at opposing sides of the DWS are similar.
Wind speed (m/s)
gauges (among others) referred to in this poster
Columns in the table [cm]
a: the storm’s peak water level (measured)
b: the wind-driven surge at peak (a minus
tide)
c: the storm’s largest wind-driven surge
d: the estimated overhead if peak surge
and high tide had happened at the
same time (c-b)
-100
GL made (v5). JA, GvB, GvV reviewed 8.9.10. DH 17.9.10
Top left: Red dots are wind gauges and dark blue dots are water-level
d
9
-100
1
Afsluitdijk
0
11
0
0
Dollard flats
Eastern Dutch
Wadden Sea
models covering the waters from the continental shelf to the Dutch
coast
2007 (9/11)
6-hour average w ind speed (m/s)
100
10
Left: Bathymetry of the DWS: range -40 m (channels) to 14 m (dunes)
1994 (28/4)
6-hour average wind direction (deg nautical)
Water levels (cm)
water levels
wind surge
300
Days counted from day of the peak
Western Dutch
Wadden Sea
30
4
We looked at the six storms of 1980-2007
( B1 above). The measured wind speeds
and directions at Huibertgat (station H in
the map) were shifted in time so as to refer
to the day of the largest surge. 6-hour
averages were calculated for convenience
of representation. Look up the plots
below.
500
3
Eems River
T
The storms of 1990 and 2006 had the same capacity of raising the
waters at Delfzijl (B1 above; D in the map). The histories of total
water levels and wind-driven surge are shown below.
-1
D
Texel
What may six surge-generating storms
have had in common?
-2
7.5
2.5
B2. Were the storms of 1990 and 2006 similar?
27 Feb 1990
10
7
Coastal defenses withstand total water levels (and wave actions: oral
presentation SSC2010-134 by G. van Vledder et al.). Yet the storm
surge is generated in the first place by the atmospheric forcing. We
subtracted the tide from measurements (simplistic perhaps but needed
– column b) and looked at the histories of the associated tide-free
levels (column c;  B2 below). The storms that generated the largest
wind-driven surge were those of 1990 and of 2006.
28
24
23
YR
rank
2006 (1/11)
We selected six historical storms in 1980-2007 that gave the largest
water level of the year at D. Look up the table on the right (YR=year
record): the 1 Nov 2006 surge is the largest ever on record, with a
return period of 50+ years (for the total water level).
0
10
The link between state-of-the-art practice and
coastal protection is a requirement of the national
law.
The station at Huibergat (H on the map) measures the wind off the
Eems-Dollard estuary since 1980. Inside the estuary, the gauge at
Delfzijl (D on the map) measures the water levels since 1877.
-1
Ameland
Dikes ensure protection of life and assets on the
low-lying coast of the DWS. The safety assessment
of the dikes; urgency of adaptations; size of
investments; and, ultimately, confidence in the dikes
all rely on the understanding of surge generation.
B1. Insights from historical records
-2
3
Terschelling
Two other prominent features are the 30-km long
closure of the Afsluitdijk in the western part, and
the Eems-Dollard estuary shared with Germany in
the eastern part.
Further information available from
www.waddensea-worldheritage.org
www.waddensea-secretariat.org
www.waddenvereniging.nl
www.seaonscreen.org
www.waddenzee.nl
300
23
Schiermonnikoog
The Dutch Wadden Sea forms the westernmost
reach of the entire basin, shown in the maps on the
right. It is ~3400 km2 and is lined by five main
barrier islands (Texel, Vlieland, Terschelling,
Ameland, Schiermonnikoog).
Tidal channels, up to 40 m deep, run into the inner
basin, while wide intertidal flats, wetlands and salt
marshes are successively exposed and flooded
during the semidiurnal ebb-flood cycle.
Waddenzee in Dutch indeed means ‘the sea of the
shoals’.
© ARCADIS 2010. Prepared for the Storm Surge Congress 2010. Hamburg, 13-17 September. (Last edited 17.9)
6
H
North Sea
10
The Wadden Sea is a ‘leaky lagoon’ between a strip
of barrier islands and the Dutch, German and Danish
mainland in the North Sea. It is listed as natural
World Heritage Site for its exceptional scenic,
biological and ecological value. Every year up to 12
million migrating birds forage in the Wadden Sea.
Fisheries, tourism, and gas and salt extraction are the
main economic resources.
10
A1. The Wadden Sea (sea of shoals)
300
200
100
45
YR1 2006
YR9 1994
YR12 1995
YR15 1990
YR16 1981
0
1
Hindcast forced with uniform
winds
YR17 1983 (Texelhors wind)
0
0
-4
-3
-2
-1
2
3
Measurements minus tide
4
Days counted from day of the peak
Insights from the six storms’ histories
All wind speeds increased (top) and all
wind directions veered from S-SW to NWN (bottom). This is a common pattern in
the North Sea (extra-tropical cyclones), but
also a surge-generating one in the DWS.
Caveat: relatively steady storms can also be
hydraulically severe: the storm of 4 Jan
1976 ranks as 6th YR surge at Delfzijl
-100
-2
-1
0
1
2
Days counted from day of the peak
Hence, in one way or another, the
surge generation in the DWS is an
unsteady and non-uniform process
(and this applies to both air and
water). We thus confirm the
importance of dynamics, which has
already shaped the past severe surges
(C, above)
We hope that this information will help
• unravel the processes of surge generation in semi-enclosed basins
• inspire regulations for a state-of-the-art, reliable coastal protection
We encourage a multidisciplinary study approach for:
• identifying the weather forcing that maximizes the adverse
hydraulic response inside the DWS in a physically-based way
• connecting the probability of occurrence of such severe storms
with the regulatory safety standards of coastal defenses.
Acknowledgements
Tools and data
Team
These results were produced in two “no-regret” studies
commissioned in 2008-9 to Alkyon Hydraulic Consultancy
& Research by Deltares on behalf of the Centre for Water
Management of Rijkswaterstaat (an office of the Dutch
Ministry for Transport, Public Works and Water
Management) within the framework of the research
program WTI 2011 (Wettelijke Toets Instumentarium).
Both projects were guided by J Groeneweg of Deltares
www.deltares.nl www.rws.nl
SIMONA is the suite of hydrodynamic numerical solvers
supporting governmental decision-making
WAQUA is the solver for the unsteady 2D depthaveraged free-surface equations in SIMONA that we used
Rijkswaterstaat maintains SIMONA and controls the
quality of input data and settings
KNMI the Dutch Royal Met Office provided the wind and
pressure fields
www.helpdeskwater.nl www.knmi.nl
J Adema, J Cleveringa, O Koop, G Lipari and G van
Vledder contributed to Alkyon’s Simulation studies for
storm winds, flow fields and wave climate in the Wadden
Sea (Report A2108, Nov ‘08) and Viability study of a
prototype windstorm for the Wadden Sea surges (Report
A2239, May ‘09)
Contact
Giordano Lipari [email protected] +31 527 248 101
Mark Mainz [email protected] +49 221 890 0657