Hydro-dynamics in the Wadden Sea during storm conditions: Analysis of storms of 1 Nov. 2006 and 18 Jan. 2007.

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A1848

Prepared for:

Rijkswaterstaat RIKZ

Hydro-dynamics in the Wadden

Analysis of storms of 1 Nov. 2006

Sea during storm conditions

and 18 Jan. 2007

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postal address PO Box 248 8300 AE, Emmeloord visiting address Voorsterweg 28, Marknesse tel : 0527 24 81 00 e-mail info@alkyon.nl internet

Client

Rijkswaterstaat RIKZ

Title

Hydro-dynamics in the Wadden Sea during storm

conditions

Analysis storms of 1 Nov. 2006 and 18 Jan. 2007.

Abstract For two historical storms the water levels and currents in the Wadden Sea were simulated using the so-called model-train of Rijkswaterstaat. The first storm of 1 Nov. 2006

generated exceptionally high water levels in the Eems-Dollard estuary. The second storm of 18 Jan. 2007 was also scaled up to represent a 1 in 4000 year design storm. In general the simulated water levels agree rather well with the measured ones, except for levels near the Eems-Dollard estuary during the Nov. 2006 storm. The main effect of the storm wind is to increase the current magnitudes. In the case of very strong winds, an overall flow southwest-northeast flow occurs in the Wadden Sea that crosses the shallow areas south of the Wadden islands. The wave computations indicate that the tidal inlets effectively block North Sea waves and that the wave conditions are mostly locally determined and depth limited under storm conditions. The flow conditions in the tidal inlets determine the amount of wave penetration through them.

References RIKZ contract RIKZ1797 (d.d. 9 March 2007) SAP bestelnummer 4500073341

Alkyon Project A1848

Rev. Originator Date Remarks Checked by Approved by

0 G.Ph. van Vledder and J. Adema 18 July 2007 draft 1 G.Ph. van Vledder, J. Adema 20 Sept. 2007 final J.Groeneweg. A.Luijendijk D.P. Hurdle G.Ph. van Vledder

Document Control Contents Status

Report number: A1848R2

Keywords: Wadden Sea, storms, SBW, currents, water levels SWAN, wave conditions, WAQUA

Project number: A1848 File location: a1848r2r1

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Hydro-dynamics Wadden Sea during storms Rev.1: 20 September 2007

file: a1848r2r1 _i

Executive’s summary

In this study the effect of wind on the hydro-dynamics in the Wadden Sea were studied using water circulation models and wave models. To that the historical storms of 1 November 2006 and 18 January 2007 were simulated, in which the wind direction is northwest in the former and west in the latter. For both storms the water levels and currents were computed with the WAQUA model. For both storms water level and current fields were computed based on astronomical and storm wind forcing. For the January 2007 storm water level and current fields were computed also using scaled wind fields such that the so-called 4000-year water level at station Nes was obtained.

The results indicate that storm winds have a strong effect on the flow pattern in the Wadden Sea and that these winds dominate astronomical effects. This results in large scale flows through the Wadden Sea that cross the shallow tidal flats south of the Wadden islands. The wind direction determines the shape of these large-scale patterns. In areas with high water levels, strong currents may occur that move parallel to the dikes.

For all of the above conditions the wave conditions in the Wadden Sea were computed with the SWAN wave model using the measured wave conditions at the stations ELD and SON as offshore boundary conditions and the HIRLAM wind fields as the forcing

condition. For the January 2007 storm an additional run was made with scaled HIRLAM winds to drive the SWAN model using the water level and current fields that were also computed with these scaled winds. In the analysis of the wave model results attention was given to the general pattern of wave conditions in the Wadden Sea for the various storm conditions, their differences, and to the effect of currents on these conditions. The results indicate that the tidal inlets block most of the North Sea waves. The actual water level and current direction determines the amount of wave penetration through the tidal inlets. Currents have a relatively small effect on the significant wave height Hm0, where they do significantly affect the spectral period Tm-10. The results also indicate that during storm conditions the waves are depth-limited in the shallow parts of the Wadden Sea in agreement with similar findings of Young and Verhagen (1996 in Lake George. Along the outer edges of the ebb tidal deltas large wave-induced forces occur, but their effects on the water level in these areas and in the Wadden Sea could not be

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List of tables

2.1 Summary of flow simulation characteristics.

3.1 Summary of boundary conditions for SWAN computations. 3.2 Date and time (CET) of SWAN hindcasts.

3.3 List of figures showing results of SWAN computations at the peak of the storm. 3.4 List of figures showing the difference plots based on SWAN computations at the

peak of the storm.

3.5 List of figures showing the variation of wave conditions along output rays through the Wadden Sea and along the coast near the Amelander Zeegat.

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List of figures

2.1 Outline of the Kuststrook model and detail of the computational grid in the Amelander Zeegat.

2.2 Bathymetry of the Kuststrook model and detail in the Amelander Zeegat. 2.3 HIRLAM wind and pressure fields for the November 2006 storm for 26 October.

2006, 0:00, 6:00, 12:00 and 18:00 hours GMT.

2.4 HIRLAM wind and pressure fields for the November 2006 storm for 27 October. 2006, 0:00, 6:00, 12:00 and 18:00 hours GMT.

2.9 HIRLAM wind and pressure fields for the November 2006 storm for 1 November 2006, 0:00, 6:00, 12:00 and 18:00 hours GMT.

2.10 HIRLAM wind and pressure fields for the January 2007 storm for 8 January 2007, 0:00, 6:00, 12:00 and 18:00 hours GMT.

2.22 Comparison of exceedance water levels and resulting maximum water levels from simulations using different scale factors for re-scaling the wind and pressure fields. 2.23 Output locations of the Kuststrook model.

2.24 Time series of simulated (with and without wind) and measured water levels at the stations Den Helder and Oudeschild for the November 2006 storm.

2.25 Time series of simulated (with and without wind) and measured water levels at the stations Den Oever and Kornwerderzand for the November 2006 storm.

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2.27 Time series of simulated and measured water levels at the stations Nes and Schiermonnikoog for the November 2006 storm.

2.28 Time series of simulated (with and without wind) and measured water levels at the stations Holwerd and Lauwersoog for the November 2006 storm.

2.29 Time series of simulated (with and without wind) and measured water levels at the stations Den Helder and Oudeschild for the January 2007 storm.

2.30 Time series of simulated (with and without wind) and measured water levels at the stations Den Oever and Kornwerderzand for the January 2007 storm.

2.31 Time series of simulated (with and without wind) and measured water levels at the stations Harlingen and West-Terschelling for the January 2007 storm.

2.32 Time series of simulated (with and without wind) and measured water levels at the stations Nes and Schiermonnikoog for the January 2007 storm.

2.33 Time series of simulated (with and without wind) and measured water levels at the stations Holwerd and Lauwersoog for the January 2007 storm.

2.34 Spatial variation of simulated (with and without wind) water levels in the Wadden Sea for 1 Nov. 2006, 0.00 hours.

2.37 Spatial variation of current magnitude and direction in the Wadden Sea based on simulations with and without wind for 1 Nov. 2006, 0:00 hours.

2.38 Spatial variation of current magnitude and direction in the tidal inlet of Ameland based on simulations with and without wind for 1 Nov. 2006, 0:00 hours.

2.43 Spatial variation of simulated (with and without wind) water levels in the Wadden Sea for 18 Jan. 2007, 14.00 hours.

2.46 Spatial variation of current magnitude and direction in the Wadden Sea based on simulations with and without wind for 18 Jan. 2007, 14:00 hours.

2.47 Spatial variation of current magnitude and direction in the tidal inlet of Ameland based on simulations with and without wind for 18 Jan. 2007, 20:00 hours. 2.48 Spatial variation of current magnitude and direction in the Wadden Sea based on

simulations with and without wind for 19 Jan. 2007, 3:00 hours.

simulations with and without wind for 18 Jan. 2007, 20:00 hours.

2.51 Spatial variation of current magnitude and direction in the tidal inlet of Ameland based on simulations with and without wind for 19 Jan. 2007, 3:00 hours.

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2.53 Spatial variation of simulated (with scaled and without wind) water levels in the Wadden Sea for 18 Jan. 2007, 20.00 hours.

2.54 Spatial variation of simulated (with scaled and without wind) water levels in the Wadden Sea for 19 Jan. 2007, 3.00 hours.

2.55 Spatial variation of current magnitude and direction in the Wadden Sea based on simulations with scaled and without wind for 18 Jan. 2007, 14:00 hours.

2.56 Spatial variation of current magnitude and direction in the tidal inlet of Ameland based on simulations with scaled and without wind for 18 Jan. 2007 20:00 hours. 2.57 Spatial variation of current magnitude and direction in the Wadden Sea based on

simulations with scaled and without wind for 19 Jan. 2007, 3:00 hours.

simulations with scaled and without wind for 18 Jan. 2007, 20:00 hours.

2.60 Spatial variation of current magnitude and direction in the tidal inlet of Ameland based on simulations with scaled and without wind for 19 Jan. 2007, 3:00 hours. 3.1 Outline of WAQUA grid and SWAN computational grids NS2 and AZG3A and

location of the wave buoys SON and ELD.

3.2 Bathymetry Wadden Sea on the NS2 grid and location of output ray through Wadden Sea.

3.3 Variation of significant wave height, peak period, mean wave direction and directional spreading during the November 2006 storm at the buoys locations SON en ELD. The black vertical lines correspond to the time instants of the SWAN

computations.

3.4 Variation of significant wave height, peak period, mean wave direction and directional spreading during the January 2007 storm at the buoys locations SON en ELD. The black vertical lines correspond to the time instants of the SWAN

computations.

3.5 Variation of water level and current speed and direction for 1 Nov. 2006, 5:00 hours on grid NS2, based on astronomical forcing.

3.6 Variation of water level and current speed and direction for 1 Nov. 2006, 5:00 hours on grid NS2, based on storm wind and astronomical forcing.

3.7 Variation of significant wave height H_m0, spectral period T_m-1,0 and mean wave direction for 1 Nov. 2006, 5:00 hours. Computed on grid NS2 using astronomically generated water levels and currents.

3.8 Variation of wave height over depth ratio Hm0/d and dimensionless water depth kd for 1 Nov. 2006, 5:00 hours. Computed on grid NS2 using astronomically generated water levels and currents.

3.9 Variation of mean wave length and wave induced force for 1 Nov. 2006, 5:00 hours. Computed on grid NS2 using astronomically generated water levels and currents.

3.10 Variation of significant wave height H_m0, spectral period T_m-1,0 and mean wave direction for 1 Nov. 2006, 5:00 hours. Computed on grid NS2 using storm wind generated water levels and currents.

3.11 Variation of wave height over depth ratio Hm0/d and dimensionless water depth kd for 1 Nov. 2006, 5:00 hours. Computed on grid NS2 using storm wind generated water levels and currents.

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3.13 Variation of the difference in significant wave height H_m0 and spectral period T_m-1,0 for 1 Nov. 2006, 5:00 hours computed on grid NS2 based on storm wind and astronomical water level and current fields.

3.14 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using

astronomically generated water levels and currents.

3.15 Variation of wave height over depth ratio Hm0/d and dimensionless water depth kd for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using astronomically

generated water levels and currents.

3.16 Variation of mean wave length and wave induced force for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using astronomically generated water levels and currents.

3.17 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using storm wind generated water levels and currents.

3.18 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using storm wind generated water levels and currents.

3.19 Variation of mean wave length and wave induced wave force for 1 Nov. 2006, 5:00 hours. Computed on grid AZG3A using storm wind generated water levels and currents.

3.20 Variation of the difference in significant wave height Hm0 and spectral period Tm-1,0 for 1 Nov. 2006, 5:00 hours computed on grid AZG3A based on storm wind and astronomical water levels and currents.

3.21 Comparison of wave and wind parameters and current speed along a ray through the Wadden Sea in grid NS2 for 1 Nov. 2006, 5:00 hours. Based on SWAN

computations using astronomical and storm wind generated water levels and currents.

3.22 Comparison of wave and wind parameters and current speed along ray 17 in grid AZG3A for 1 Nov. 2006, 5:00 hours. Based on SWAN computations using

astronomical and storm wind generated water levels and currents.

3.23 Variation of water level and current speed and direction for 18 Jan. 2007, 20:00 hours on grid NS2, based on astronomical forcing.

3.24 Variation of water level and current speed and direction for 18 Jan. 2007, 20:00 hours on grid NS2, based on storm winds and astronomical forcing.

3.25 Variation of water level and current speed and direction for 18 Jan. 2007, 20:00 hours on grid NS2, based on scaled storm winds and astronomical forcing. 3.26 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave

direction for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using astronomically generated water levels and currents.

3.27 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using astronomically

3.28 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using astronomically generated water levels and currents.

3.29 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using storm wind generated water levels and currents.

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3.31 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using storm wind generated water levels and currents.

3.32 Variation of the difference in significant wave height Hm0 and spectral period Tm-1,0 for 18 Jan. 2007, 20:00 hours computed on grid NS2 based on storm wind and astronomical water levels and current.

3.33 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated water levels and currents.

3.34 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated water levels and currents.

3.35 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated water levels and currents.

3.36 Variation of the difference in significant wave height H_m0 and spectral period T_m-1,0 for 18 Jan. 2007, 20:00 hours computed on grid NS2 based on scaled storm wind and storm wind water levels and currents.

3.37 Variation of significant wave height H_m0, spectral period T_m-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated waves, water levels and currents.

3.38 Variation of wave height over depth ratio Hm0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated waves, water levels and currents.

3.39 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid NS2 using scaled storm wind generated waves, water levels and currents.

3.40 Variation of the difference in significant wave height Hm0 and spectral period Tm-1,0 for 18 Jan. 2007, 20:00 hours computed on grid NS2 based on scaled storm wind and storm wind waves, water levels and currents.

3.41 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using astronomically generated water levels and currents.

3.42 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using astronomically generated water levels and currents.

3.43 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using astronomically generated water levels and currents.

3.44 Variation of significant wave height H_m0, spectral period T_m-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using storm wind generated water levels and currents.

3.45 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using storm wind

3.46 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using storm wind generated water levels and currents.

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3.48 Variation of significant wave height H_m0, spectral period T_m-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated water levels and currents.

3.49 Variation of wave height over depth ratio Hm0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated water levels and currents.

3.50 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated water levels and currents.

3.51 Variation of the difference in significant wave height H_m0 and spectral period T_m-1,0 for 18 Jan. 2007, 20:00 hours computed on grid AZG3A based on scaled storm wind and storm wind water levels and currents.

3.52 Variation of significant wave height Hm0, spectral period Tm-1,0 and mean wave direction for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated waves, water levels and currents.

3.53 Variation of wave height over depth ratio H_m0/d and dimensionless water depth kd for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated waves, water levels and currents.

3.54 Variation of mean wave length and wave induced forces for 18 Jan. 2007, 20:00 hours. Computed on grid AZG3A using scaled storm wind generated waves, water levels and currents.

3.55 Variation of the difference in significant wave height Hm0 and spectral period Tm-1,0 for 18 Jan. 2007, 20:00 hours computed on grid AZG3A based on scaled storm wind and storm wind waves, water levels and currents.

3.56 Comparison of wave and wind parameters and current speed along the ray through the Wadden Sea on in grid NS2 for 18 Jan. 2007, 20:00 hours. Based on

SWAN computations using astronomical, storm wind, scaled storm wind generated

waves, water levels and currents.

3.57 Comparison of wave and wind parameters and current speed along ray 17 in grid AZG3A for 18 Jan. 2007, 20:00 hours. Based on SWAN computations using

astronomical, storm wind, scaled storm wind generated waves, water levels and currents.

3.58 Relative effect of currents on the significant wave height H_m0 and spectral period T_m-10 for 18 Jan. 2007, 20:00 hours. Based on SWAN a computation on grid NS2 for

the storm wind condition ‘w’.

3.59 Relative effect of currents on the significant wave height Hm0 and spectral period Tm-10 for 18 Jan. 2007, 20:00 hours. Based on SWAN a computation on grid NS2 for the storm wind condition ‘w’.

3.60 Relative effect of currents on the significant wave height Hm0 and spectral period T_m-10 for 18 Jan. 2007, 20:00 hours. Based on SWAN a computation on grid NS2 for the scaled storm wind condition ‘s’.

3.61 Relative effect of currents on the significant wave height H_m0 and spectral period T_m-10 for 18 Jan. 2007, 20:00 hours. Based on SWAN a computation on grid NS2 for

the storm wind and wave condition ‘x’.

3.62 Comparison of (dimensionless) wave parameters along the output ray through the Wadden Sea on in grid NS2 for 1 Nov. 2006, 5:00 hours. Based on SWAN

computations using astronomical and storm wind generated waves, water levels and currents.

3.63 Comparison of (dimensionless) wave parameters along the output ray through the Wadden Sea on in grid NS2 for 18 Jan. 2007, 20:00 hours. Based on SWAN

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3.64 Evolution of the non-dimensional energy ε as a function non-dimensional fetch for specific values of non-dimensional depth δ. (From Young and Verhagen, 1999, figure 12), [Included in text].

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1 Introduction

According to the Dutch “Flood Defences Bill” (Wet op de Waterkering, 1996), the effectiveness of the primary water defences in the Netherlands must be assessed at five year intervals (2001, 2006, 2011, etc.). The assessment must evaluate whether they offer the protection level defined by the Hydraulic Boundary Conditions (HBC) and in the “Regulation on Safety Assessment” (Voorschrift op Toetsen op Veiligheid, VTV, 2004). These HBC need to be reconfirmed or, if necessary, redefined every five years by the Minister of Transport, Public Works and Water Management.

The spectral wave model SWAN (Booij et al., 1999) plays a key role in the derivation of the Hydraulic Boundary Conditions (HBC) for the primary sea defences of the

Netherlands. Since uncertainty remains with respect to the reliability of SWAN for application to the geographically complex area of the Wadden Sea, a number of

activities have been initiated under the project SBW Wadden Sea to devise a strategy for the improvement of the model.

In recent years, various measurement campaigns in the Wadden Sea have been performed, producing a large amount of wave buoy data. Unfortunately, only a small fraction of these data has been analyzed to address the performance of SWAN in this

area. In addition, concerns exist about the amount of long-period (10 to 20 seconds) swell waves that can penetrate into the Wadden Sea. For these reasons it is not yet possible to derive reliable and validated boundary conditions using SWAN in the Wadden Sea.

The problem outlined above was the immediate trigger for the project “Strength of and Loads on Water Defences: Natural boundary conditions Wadden Sea” (SBW Wadden Sea). Within the framework of the SBW project WL|Delft Hydraulics was requested to formulate a Plan of Action with a strategy for answering the primary objective: “How to derive reliable Hydraulic Boundary Conditions for the year 2011 for the Wadden Sea area?” (WL, 2006).

This Plan of Action is part of the project “Strength of and Loads on Flood Defences” (SBW, 2005). The purpose of the Plan of Action is to improve the quality of the models and methods used to derive the HBC such that authorities and other experts have sufficient confidence to use them for the five-year assessment. The ways this is being achieved include:

− Conducting measurements in the project SBW - Field measurements;

− Developing methods and models for translation of the so-called deep-water statistics to statistics valid near the water defences and

One of the uncertainties in the derivation of the HBC comprises of the role of currents on the wave conditions during extreme storms and the method in which current effects need to be included. Alkyon (2001a) performed a series of computations using WAQUA

and SWAN in the Wadden Sea. These results showed that currents affect the wave

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and Ameland. These results suggest that the flow pattern during severe storms might be different from the flow pattern during milder conditions.

Hindcast studies with SWAN in the tidal inlet of Ameland (WL and Alkyon, 2007) show that inclusion of currents and spatially varying water levels improve the performance of

SWAN considerably, at least at the measurement locations in the area around the tidal inlet. The hindcast results also indicated that current effects on the wave conditions decrease with increasing distance from the tidal channels. Near the dikes current effects are rather small. Unfortunately, no validation of the SWAN model has yet) been

performed for the simulation of current effects on the wave conditions near the dikes or in severe storms.

In view of the situation sketched above, an activity was defined in the Plan of Action to study the current and wave conditions during extreme storms. The purpose of this activity is to investigate the role of wind on the currents and water levels in the Wadden Sea and the resulting effects on the wave conditions.

This was realized in two steps. The first step comprised of applying the WAQUA model to

compute the current and water levels for two historical storms using two types of forcing: only astronomical forcing and one with astronomical and wind forcing. In addition, a hypothetical design storm was constructed to simulate the water circulation under extreme conditions. The second step consists of simulating wave conditions with the SWAN wave model for both storms. For each storm the same wind field was used to drive the wave model, whereas different current and water level fields were used to to isolate the effect of (wind-induced) currents on the wave conditions.

From the viewpoint of deriving HBC it is of interest to get insight into the following questions:

• What are the differences in water levels and flow patterns in the Wadden Sea during ‘normal’ storms and design storms?

• What is the role of wind on the water circulation in the Wadden Sea, especially during extreme conditions?

• Where are the currents in the Wadden Sea more or less parallel to the dikes of the Wadden islands and the main land?

• Is the local flow pattern over shallow banks different from the overall flow in the Wadden Sea?

• At which locations affect currents the wave conditions near the dikes?

• Are current induced modulations in wave loading on the dike confined in space and time?

• At which locations occur wave induced currents and do they play a role in the water circulation in the Wadden Sea?

• Is there a need to account for currents effects in the HBC?

• Is it possible that wave tunnelling effects may enhance the penetration of ‘long’ swell waves through the tidal inlets during storm conditions?

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To study the effect of different wind induced water level and current fields on the wave conditions in the Wadden Sea and to answer the wave related questions, SWAN wave

model simulations were performed with a model covering the whole Wadden Sea and a detailed model covering the area around the tidal inlet of Ameland. The wave modelling activities are described in Chapter 3.

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2 Flow

modelling

2.1 Introduction

Three types of storms had been proposed for this study: The historical storm of 8 February 2004; the exceptional storm of 1 Nov. 2006 and an artificial design storm. The February 2004 storm had been selected because it is characterised by north-westerly wind and wave directions which probably give significant wave penetration through the tidal inlets. The November 2006 storm was selected because it generated exceptionally high water levels in the Eems-Dollard estuary. Moreover, since no wave measurements are available from this storm, it was of interest to reconstruct these wave conditions. Unfortunately, no wind fields were available at the start of this project for the February 2004 storm. Instead, the historical storm of 18 January 2007 was selected to investigate the water circulation in the Wadden Sea. Originally, it was intended to generate an artificial storm representing a design storm. However, experience at Rijkswaterstaat revealed that the best way of constructing such a storm is to rescale the wind and pressure fields of a historical storm such that it generates the required water level or wave condition at some location. In view of this experience it was decided to use the Jan. 2007 storm wind and pressure fields for this purpose. These deviations from the original scope of work were made in consultation with Ap van Dongeren of WL|Delft Hydraulics.

The storm of 1 November 2006

On 1 November 2006 a severe storm occurred on the North Sea generating record high water levels near the town of Delfzijl in the Eems-Dollard estuary, despite the fact that the wind speeds were not extreme. A fast moving low-pressure centre moved in

eastward direction over the North Sea. On 31 October 2006, the wind direction was from the southwest with wind speeds of 15 m/s. As the storm moved over the North Sea, the wind direction turned to more northerly directions and the wind speed increased to about 23 m/s, as measured at Huibertgat. At the peak of the storm the wind direction was from the northwest. Wind speed and pressure fields are shown in the Figures 2.3 to 2.9 at 6 hour time intervals.

A preliminary analysis by RWS (2007) indicated that the timing (spring tide) and path of this storm where conducive to the generation of relatively high water levels in the Eems-Dollard estuary. A very local low-pressure area moving along with the storm

strengthened this effect. Unfortunately, no wave measurements in the tidal inlet of Ameland were performed. Therefore, reconstruction of these conditions is desirable.

The 18 January 2007 storm

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2.2 Model

setup

The simulations were performed with the ‘model-train’ of Rijkswaterstaat, consisting of the CSM, ZUNO (ZUidelijke NOordzee) and Kuststrook model. The CSM model covers the whole North Sea. The ZUNO model is nested in the CSM model and covers the southern North Sea. Nested in the ZUNO model is the Kuststrook model. The resolution of the CSM model is approximately 10 km, ZUNO has a resolution of about 1500 m in the North Sea near the Wadden Sea, whereas the Kuststrook Fijn model has a resolution of

approximately 500 m. Details of the grid definition and other characteristic of these models are described in, e.g. Alkyon (2001b). The models from the model-train were used without additional calibrations for this study. In fact, RIKZ calibrated these models to obtain reliable water levels along the Dutch coast and in the Wadden Sea for a wide range of conditions. Therefore, time series comparisons were made between measured and simulation water levels for North Sea stations. Similarly, no verification tests were performed for astronomical situations.

The outline and bathymetry of the Kuststrook model are shown in the Figures 2.1 and 2.2. This figure also contains a detail of the grid and bathymetry around the tidal inlet of Ameland. The bathymetry of the Kuststrook model is based on nearshore depth

sounding from 1999. Further into the North Sea depth sounding from the ‘Dienst Hydrografie voor de Noordzee’ were used, supplemented with information from a digital terrain model for the North Sea from 1990.

The Kuststrook model was developed for Rijkswaterstaat to predict the water circulation along the Dutch coast and in the Wadden Sea. The Kuststrook model used in this study was taken off the shelf, no tuning was deemed necessary since Rijkswaterstaat already had done this. The spatial resolution of the Kuststrook model was deemed sufficient for the purposes of this study.

This report contains many figures in which land boundaries are plotted as a background line. It is noted that this line is only indicative of the location of NAP 0 m line. In some locations, especially around the eastern tip of Terschelling, these land boundaries are outdated.

For the present study two storm periods were simulated using CET as the reference time. The duration of the simulations are summarized in Table 2.1.

Astronomical November 2006 26 October – 2 November

with wind Astronomical with wind January 2007 8 – 20 January

using scaled wind and pressure fields

Table 2.1: Summary of flow simulations characteristics.

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were provided in GMT, they were shifted one hour; to obtain boundary conditions in CET.

The operational system uses Kalman filtering to better match model results with

available measurements. Rijkswaterstaat performed this filtering. For the re-scaled storm no measurements exist and Kalman filtering could not be applied. Therefore, for this storm, the model train (CSM – Zuno – Kuststrook) was applied to obtain the necessary boundary conditions needed for the nesting of the various models. The CSM and Zuno model were run in astronomical mode as well as with scaled wind and pressure fields. The CSM provided time series of water levels along the boundaries of the Zuno model. The boundary conditions for the Kuststrook model were constructed from astronomical components and the computed setup, by subtracting the results of the astronomical Zuno-simulation from the results of the simulation with wind. This method is frequently applied, since the Zuno-model is capable of accurately computing the wind-induced setup, but predicts the tidal flow less accurately. The desired degree of accuracy is obtained by combining the setup from the ZUNO-model with tidal constituents at the boundaries of the Kuststrook model . This method is also referred to as the astro-correction.

The HIRLAM wind- and pressure fields were provided by Rijkswaterstaat at 3 hour intervals. The spatial resolution of these fields is λ=0.125° and ϕ=0.083°, which roughly corresponds to a resolution of 11 km in the southern North Sea. The Figures 2.3 through 2.9 show these fields at time intervals of 6 hours for the November 2006 storm. The contours represent the pressure isobars at intervals of 10 mBar and the arrows represent the direction to which the wind is blowing. The length of each arrow is proportional to the wind speed. The Figures 2.10 through 2.21 show the wind and pressure fields of the January 2007 storm.

The HIRLAM wind data were interpolated to the grids of the circulation models using linear interpolation in time, and bi-linear interpolation in space for each velocity component separately.

The Kuststrook model has not been run with the default wind stress formulation. Instead of a constant drag coefficient C_d, the Charnock formulation that is also used in CSM and Zuno has been applied. This is consistent with the approach followed in RWS (2007) describing the evaluation of the storm of November 2006 (referred to in Dutch as the Allerheiligenvloed 2006). In this way all three circulation models consistently use the same wind drag formulation.

2.3 Up-scaling of wind and pressure fields

The purpose of up-scaling the wind and pressure fields is to obtain a so-called 1/4000-year storm with which the flow and wave conditions in the Wadden Sea can be

determined. Strictly speaking, such a storm cannot be defined, since a return value can only be attributed to one scalar parameter, whereas each storm is described by a very large number of parameters. To avoid this problem, we defined a storm that generates hydrodynamic parameters, such as the significant wave height, wave period or water level that corresponds to the 1/4000-year condition.

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island of Ameland, matches the desired exceedance level. In practise this was achieved by multiplying the wind and pressure fields with a constant factor, which is equal for both the wind and pressure fields to retain the baroclynic balance. The appropriate scale factor was determined on the basis of a number of trial runs with the model-train using scale factors from 1 to 2 with steps of 0.1. The proper scale factor was determined by comparing the flow model results with the exceedance levels of water levels in the Wadden Sea as described SDU (2005); the yellow booklet ‘Getijtafels voor Nederland’ (SDU, 2006). The result of this analysis is summarized in Figure 2.22.

The upper panel in Fig. 2.22 shows the various design water levels as a function of frequency of occurrence for a number of measurement stations in the Wadden Sea. The black dashed line refers to the 1/4000-year norm. The lower panel shows the results of the simulations for the various water levels at the same measurement stations. The black dashed line refers to the water level corresponding to the 1/4000-year condition.

In general a factor 1.4 seems most appropriate to obtain the desired water level at station Nes. Along the Afsluitdijk, however, a factor of 1.3 better resembles the desired value. These differences are due to the fact it is practically impossible to determine a storm generating the desired water levels at all stations. This is to be expected as the extreme conditions at different locations will in practice be generated by different storms. Possibly, another re-scaled storm might provide a better match for more measurement stations. However, such an analysis is beyond the scope of this research project.

2.4 Results of simulations

2.4.1 Introduction

The results of the simulation are presented in the form of time series of water levels at 10 of measurement stations in the Wadden Sea, contour plots of the spatial variation of the water level in the Wadden Sea, and scaled arrow plots showing the current

magnitude and direction of the simulated currents. These plots are based on the results of the Kuststrook-fijn model at a number of stations shown in Figure 2.23.

2.4.2 November 2006 storm

Time series of simulated water levels for the November 2006 storm are presented in the Figures 2.24 through 2.28. The figures show that the wind –induced water level reached a value of 2 m near Den Helder and more than 3 m near Lauwersoog. The agreement is best for the westerly stations, whereas the peak is under-predicted at Lauwersoog. The latter is consistent with the findings of RWS (2007), which suggest that some small-scale features in the wind field that were not included in the HIRLAM wind fields (cf. RWS, 2006) are responsible for this mismatch. Despite this mismatch, the result of the simulations agrees rather well in the other areas of the Wadden Sea.

For the November 2006 storm the simulated water levels in the Wadden Sea are shown in the Figures 2.34 through 2.36 for the time instants 1 Nov. 0:00 hours, 1 Nov. 5:00 hours and 1 Nov. 9:00 hours. The results show that the storm winds cause the highest water levels in the Eems-Dollard estuary, close to NAP +4 m.

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currents, whereas the directions remain more or less the same as for the astronomical situation. At the peak of the storm at 1 Nov. 5:00 hours (Fig. 2.39 and 2.40) the current pattern remains more or less the same in the western part of the Wadden Sea, except for an increase in current speed. In the eastern part of the Wadden Sea a strong west to east flow occurs. This large-scale feature crosses the shallow areas south of the Wadden islands and it moves more or less parallel to the dikes of the islands and the main land. South of Terschelling, the current speeds are small. A remarkable feature is the strong increase in the speed of the ebb-current in the Amelander Zeegat.

After the peak of the storm (Fig. 2.41 and 2.42), the Eems-Dollard estuary empties rather fast, resulting in very strong currents through tidal inlets between Schiermonnikoog and Borkum. These strong currents are due to the large storage capacity of the Dollard estuary. The pattern of the current field during the storm resembles the pattern of the astronomical current field, except for the difference in magnitude of the current speeds.

2.4.3 January 2007 storm

For the January 2007 storms time series of simulated water levels are presented in the Figures 2.29 through 2.33. Results are presented for the situation with pure astronomical forcing, with storm wind forcing and for the scaled storm winds. The agreement

between the simulation with storm wind and the measurements is rather good, the prediction error is less than 0.1 m at most stations. The scaled storm winds cause an increase in water level of about 2 m in most stations, compared to the actual storm winds. The results also indicate that the peak water level is reached about one hour earlier. As required, the water level at Nes reaches the desired value for the scaled storm. For the January 2007 storm, two sets of figures with the spatial distribution of water levels and currents are given. One set showing the comparison of results for the situation without wind and with the storm wind. The second set contains the results for the astronomical situation and for the scaled storm winds. Results are presented for the time instants 18 Jan. 14:00 hours, 18 Jan. 20:00 hours and 19 Jan. 3:00 hours.

The Figures 2.43 to 2.45 show the spatial variation of the water levels in the Wadden Sea for the January storm using the astronomical forcing and with actual storm winds. The results show that the highest water levels occur near Harlingen and that the water levels are higher along the dikes of the main land than at the North Sea. The corresponding current magnitudes and direction are shown in the Figures 2.46 through 2.51. Prior to the peak of the storm (Figs. 2.46 and 2.47), the effect of wind is to generate a strong west to east current through the Wadden Sea. Water enters the Wadden Sea through the Marsdiep and leaves the Wadden Sea through the Amelander Zeegat and Friesche Zeegat. The main difference with the astronomical situation is the strong flow over shallow parts south of the Wadden islands. This large scale flow moves parallel to the dikes of the islands and the main land. Another difference is the reversal of the flow direction on the North Sea. Figure 2.47 shows a detail of these differences near the Amelander Zeegat.

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After the peak of the storm (Fig. 2.50 and 2.51) the flow patterns of the astronomical situation and the situation with wind are almost equal. A main difference is the location of the area on the North Sea where the flow changes direction. For the astronomical situation this area is west of Vlieland, whereas for the storm wind this area is north of Ameland. This is due to the very strong winds, which dominate astronomical forcing. In addition, a strong southwest-northeast flow occurs in the western part of the Wadden Sea, which crosses the ‘wantij’ areas. Thus the flow does not follow the tidal channels but moves with the wind.

Scaled storm

For the scaled January 2006 storm, simulated water levels in the Wadden Sea are shown in the Figures 2.52 through 2.54. For the scaled storm no comparison of water levels is possible against measurements. Only a comparison of model results is possible. Similar to the actual storm winds the highest water levels are found near Harlingen, but almost equally high water levels are now found also in the Eems-Dollard estuary. The winds cause higher water levels in the Wadden Sea than on the North Sea. This is a large-scale feature, such that almost no variation occurs in water levels around the smaller Wadden islands of Vlieland, Ameland and Schiermonnikoog.

The effect of these extra strong winds on the flow patterns (Fig. 2.55 – 2.60) shows that prior to and during the peak of the storm a strong overall southwest-northeast current occurs in the whole Wadden Sea which crosses the ‘wantij’ areas south of the islands. After the peak of the storm, the flow pattern in the Wadden Sea and tidal inlets is similar to the pattern during the ‘normal’ storm, whereas on the North Sea the flow direction is still opposite to the direction for the situation without wind. Another feature is the reversal of the flood current to an ebb current in the tidal inlet of Ameland during this storm. This is a local effect that fits well in the overall flow pattern in the Wadden Sea.

Further information about the hydro-dynamic conditions in the Wadden Sea is presented for three time instants per storm (full information at 30 minute time intervals will be made available on DVD), before, near and after the peak of the storm. In each plot, results are presented for the situation without and with wind.

2.5 Discussion

of

results

The results of WAQUA simulations indicate that storm winds are able to change the water levels and current patterns in the Wadden Sea considerably. The main effect is that currents are generated in the direction of the wind, such that a large scale flow may occur through the Wadden Sea. In certain situations this flow moves over the tidal channels and shallow tidal flats. In the deeper parts the (depth-averaged) current speeds are relatively low, whereas over the shallow parts the current speeds are relatively high. It is found that the wind direction determines the large-scale flow pattern. For the north-westerly storm wind (as in the Nov. 2007 storm) the wind may create different flow directions in the Wadden Sea such that the current direction is to the south-west in the western part of the Wadden Sea and to the north-east in the eastern part of the

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3 Wave

modelling

3.1 Introduction

In the previous chapter a description was given of the effect of storm winds on the flow conditions in the Wadden Sea. This was realized by performing WAQUA computations for the storms of 1 November 2006 and 18 January 2007 for forcing conditions. For the November 2006 storm the WAQUA model was run in astronomical mode and one including the actual storm winds and astronomical forcing. The January 2007 storm was also simulated using a scaled wind field such that the water level at station Nes reached the 1 in 4000 year design condition. The general characteristics of these storms are described in Chapter 2.

The purpose of the present activity is to assess the effect of storm wind-induced currents on the wave conditions along the dikes in the Wadden Sea. To that end six storm events were hindcast with different combinations of environmental conditions. The

characteristics of the corresponding environmental conditions, comprising of current fields, water level fields and wind fields, are summarized in Table 3.1.

For the November 2006 storm, current and water level fields were generated for the astronomical conditions (wind turned off) and for the historical wind conditions

(including astronomical forcing). For the January 2007 storm, the current and water level fields were also computed for a scaled wind field, representing the 4000 year storm condition.

To isolate the effect of the different (wind induced) current and water level fields on the wave conditions in the Wadden Sea, a series of wave computations was performed which differ only in one aspect. For the November 2006 storm the difference comprises the current and water level fields, one set based on astronomical forcing ‘a’ and one set based on forcing with the actual storm wind and astronomical forcing ‘w’. For both conditions the wave model was driven by the actual storm winds and measured wave boundary conditions at the stations ELD and SON. Thus, the only difference between the cases ‘w’ and ‘a’ are the underlying water level and current fields.

Similarly, the wave model computations for the January 2007 storm used two different set of water level and current fields, viz. ‘a’ and ‘w’. In addition, scaled storm winds (‘s’) were used to simulate a 4000-year water level at station NES. For this condition, the wave model was driven by the actual storm wind and wave boundary conditions. Thus, the only difference between the cases ‘s’ and ‘w’ are the underlying water level and current fields. Finally, an additional case ‘x’ was defined in which the wave model is driven also by the scaled storm winds and scaled wave boundary conditions. Thus, the only difference between the cases ‘x’ and ‘s’ are the wind speed and wave boundary conditions.

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Code name

Currents and water levels Wave boundary conditions Applied to storms of Run id. A Astronomical forcing, no wind

Buoy data and HIRLAM winds

2006, 2007 2a, 3a

W HIRLAM winds and

astronomical forcing

2006, 2007 2w, 3w

S Scaled HIRLAM winds

(factor 1.4) and astronomical forcing

2007 3s

X Scaled HIRLAM winds

(factor 1.4) and astronomical forcing

Scaled buoy data to match 1 in 4000 conditions and scaled

HIRLAM winds

2007 3x

Table 3.1: Summary of boundary conditions for SWAN computations. The last column

refers to the identification code of SWAN computation.

3.2 Model

set-up

3.2.1 Computational grids

In the present study two non-uniform grids were used to compute the wave conditions in the Wadden Sea and the area around the tidal inlet of Ameland. These grids were derived from the curvi-linear Kuststrook model that was used for the WAQUA

computations. The outer grid NS2 covers almost the whole Wadden Sea and part of the North Sea. The outer boundary of this grid is chosen such that it coincides with the wave buoys SON and ELD, enabling the specification of proper wave boundary conditions. Nested in the NS2 grid is the non-uniform grid AZG3A. This grid is identical to the one used in the hindcast study of storms in the tidal inlet of Ameland (WL&Alkyon, 2007). An overview of the Kuststrook Fijn grid is shown in Figure 3.1 together with the outlines of the SWAN grids NS2 and AZG3A and the locations of the wave buoys that provide the wave boundary conditions. The bathymetry of the Wadden Sea on the NS2 grid is shown in Figure 3.2. This figure also contains the location of an output ray along which wave model results are compared for different sets of computations.

3.2.2 Wind fields

The HIRLAM wind fields used for the flow computations were provided by RIKZ on geographical coordinates with resolution of λ=0.083° and ϕ = 0.125°, whereas the SWAN

computations are performed in the RD-coordinate system. This implied that the HIRLAM wind fields needed to be converted from the geographical coordinate system to the RD-coordinate system. The resolution of the HIRLAM data is approximately 11 km, whereas the resolution of the SWAN model varied from 60 m in the AZG3A grid to about 1500 m in the NS2 grid. The conversion to the SWAN computational grid was performed in two steps. In the first step the HIRLAM data were converted to a regular grid with a resolution of 5 km covering the whole computational domain. In the second step the wind data on the 5 km grid were interpolated to the non-uniform computational grid. This step was performed within the SWAN model.

The conversion from geographical to RD-coordinates was performed in two steps. In the first step the points on the regular 5 km grid model were transformed to geographical coordinates. In the second step, the u- and v- components of the wind field were interpolated from the regular geographical grid of the HIRLAM model to the

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the time instants for which a SWAN computation was performed. The interpolation was performed on the wind speed components separately. The interpolation was linear in time and bi-linear in space.

3.2.3 Currents and water levels

The currents and water levels for both storms were computed with the ‘Model-train’ of Rijkswaterstaat consisting of CSM, ZUNO and the Kuststrook Fijn. The resolution of the CSM model is approximately 10 km, ZUNO has a resolution of about 1500 m in the North Sea near the Wadden Sea, whereas the Kuststrook Fijn model has a resolution of

approximately 500 m. For the present study the SWAN model used only the results of the Kuststrook Fijn model. This required an interpolation of current velocities and water levels from the curvi-linear Kuststrook Fijn grid to both SWAN model grids. This

conversion was performed using the same method as used in the hindcast study of WL & Alkyon (2007). In this study the conversion was performed for the grids NS2 and AZG3A. One of the key elements in this conversion was the treatment of the dry falling points that occur around the Wadden islands and along the coast of the main land. Details of this procedure are described in WL & Alkyon (2007).

3.2.4 Wave boundary conditions

Wave boundary conditions are provided along the outer boundary of the NS2 grid, see Figure 3.1. These data are available as time series of integral parameters and wave spectra at the wave buoys SON and ELD. For the purpose of this study, a sensitivity study of effect of wind on currents and waves, it was deemed sufficient to use only the integral wave parameters to specify the wave boundary conditions. Here, a JONSWAP spectral shape was specified with a peak enhancement factor of γ=3.3. The directional spreading was taken from the buoy measurements.

The time variation of the significant wave height Hm0, peak period Tp, mean wave

direction θ and the directional spreading σ as measured by the directional wave riders at SON and ELD are presented in the Figures 3.3 and 3.4. The black vertical bars in these figures indicate to the time instants for which the SWAN computations were performed.

3.2.5 Model computations

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Sequence number Date and time (CET)

2.1 1 Nov. 2006, 0:00 hours 2.2 1 Nov. 2006, 5:00 hours 2.3 1 Nov. 2006, 9:00 hours 3.1 18 Jan. 2007, 14:00 hours 3.2 18 Jan. 2007, 20:00 hours 3.3 19 Jan. 2007, 3:00 hours Table 3.2: Date and time (CET) of SWAN hindcasts

3.3 Results

of

SWAN

computations

The results of the SWAN computations are presented in the form of coloured contour

plots of the wave conditions in the Wadden Sea, differences plots of these wave conditions and line plots showing the wave conditions along an output ray in front of the Frisian mainland.

Three types of plots showing the wave conditions are shown. The first type includes the spatial distribution of the significant wave height H_m0 and spectral period T_m-1,0, together with the mean wave direction θ , which is plotted as an arrow scaled with the wave height. The second type of plots shows the wave height over depth ratio H_m0/d and the dimensionless water depth kd, in which the wave number k is obtained from the mean wave length L according to k=2π/L. The third type of plot shows the mean wavelength and wave induced force per unit surface area (computed from the gradient of the radiation stress). The latter type of plot is useful to identify areas with wave growth respectively the areas were wave-driven currents may be generated. For practical reasons only the results for the peak of the storms (viz. 2.2 and 3.2) are included in this report. This results in 36 figures showing the wave conditions in the Wadden Sea (2 grids, 6 environmental conditions (2a, 2w, 3a, 3w, 3s, 3x) each containing 3 sets of wave parameters). The list of plots showing these conditions is shown in Table 3.3.

Difference plots were only made for the significant wave height Hm0 and spectral period Tm-10. For all storm combinations 8 difference plots were made at the peak of the storm (2 grids, and 4 possible differences:

• [2w-2a] and [3w-3a] showing the effect of wind driven water levels and currents on the wave conditions;

• [3s-3w], showing the effect of ‘scaled’ water levels and currents on the wave conditions;

• [3x-3s], showing the effect of scaled wind and scaled wave boundary conditions on the wave conditions.

The list of difference plots is given in Table 3.4.

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The table summarizes the figures showing the results of the SWAN computations for the

peak of the storm.

Figure number Grid Condition Parameters

3.6 NS2 2a H_m0, T_m-1,0 3.7 NS2 2a H_m0/d, kd 3.8 NS2 2a Len, Force 3.9 NS2 2w Hm0, Tm-1,0 3.10 NS2 2w Hm0/d, kd 3.11 NS2 2w Len, Force 3.14 AZG3A 2a Hm0, Tm-1,0 3.15 AZG3A 2a H_m0/d, kd

3.16 AZG3A 2a Len, Force

3.17 AZG3A 2w H_m0, T_m-1,0

3.18 AZG3A 2w H_m0/d, kd

3.19 AZG3A 2w Len, Force

3.26 NS2 3a Hm0, Tm-1,0 3.27 NS2 3a Hm0/d, kd 3.28 NS2 3a Len, Force 3.29 NS2 3w Hm0, Tm-1,0 3.30 NS2 3w H_m0/d, kd 3.31 NS2 3w Len, Force 3.33 NS2 3s H_m0, T_m-1,0 3.34 NS2 3s Hm0/d, kd 3.35 NS2 3s Len, Force 3.37 NS2 3x Hm0, Tm-1,0 3.38 NS2 3x Hm0/d, kd 3.39 NS2 3x Len, Force 3.41 AZG3A 3a H_m0, T_m-1,0 3.42 AZG3A 3a H_m0/d, kd

3.43 AZG3A 3a Len, Force

3.44 AZG3A 3w Hm0, Tm-1,0

3.45 AZG3A 3w Hm0/d, kd

3.46 AZG3A 3w Len, Force

3.48 AZG3A 3s Hm0, Tm-1,0

3.49 AZG3A 3s H_m0/d, kd

3.50 AZG3A 3s Len, Force

3.52 AZG3A 3x H_m0, T_m-1,0

3.53 AZG3A 3x H_m0/d, kd

3.54 AZG3A 3x Len, Force

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Figure number Grid Difference code

3.13 NS2 2w-2a 3.20 AZG3A 2w-2a 3.32 NS2 3w-3a 3.36 NS2 3s-3w 3.40 NS2 3x-3s 3.47 AZG3A 3w-3a 3.51 AZG3A 3s-3w 3.55 AZG3A 3x-3s

Table 3.4: List of figures showing the difference plots based on SWAN computation at the

peak of the storm.

Figure number Date and time Output grid Code of difference

3.21 1 Nov. 2006, 5:00 hours NS2 2w-2a

3.22 1 Nov. 2006, 5:00 hours AZG3A 2w-2a

3.56 18 Jan. 2007, 20:00 hours NS2 3w-3a, 3s-3w,3x-3s

3.57 18 Jan. 2007, 20:00 hours AZG3A 3w-3a, 3s-3w, 3x-3s

Table 3.5: List of figures showing the variation of wave conditions along output rays through the Wadden Sea and along the coast near the Amelander Zeegat

3.4 Results

3.4.1 The storm of 1 November 2006

For this storm the wave conditions in the Wadden Sea were computed using two different water levels and current fields. One based on astronomical forcing and based on storm wind forcing. The corresponding water level and current field at the peak of the storm (1 Nov. 2006, 5:00 hours) are shown in the Figures 3.5 and 3.6. Significant differences between the two situations are the much higher water levels in the Wadden Sea with levels higher than 3 m in the eastern part. It is also found that the current velocities are higher for the storm condition ‘w’ than for the astronomical condition ‘a’. Higher current speeds are found in the Marsdiep and in the eastern part of the Wadden Sea. The highest current velocities occur in the tidal inlets around Texel. For both situations the smallest current velocities occur in an area south of Terschelling. West of Terschelling the currents are generally directed to the southwest whereas east of

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respect to the incoming wave direction influences the amount of penetration. The results also indicate that wave growth takes place in the western part of the Wadden Sea. The variation of the wave height over depth ratio (Fig. 3.8) indicates that in most areas the waves are depth-limited. The highest values of the wave height over depth ratio are found on outer edges of the ebb-tidal deltas north of the tidal inlets. The variation of the dimensionless water depths (kd) indicates that the lowest values (kd≈0.8) are found on the outer edge of the ebb-tidal delta, whereas much larger values (kd>1.6) are found in the Wadden Sea. This indicates that strong non-linear effects are confined to the ebb-tidal deltas. It is also found that the highest waves in the Wadden Sea occur in the deeper parts. This can be seen by inspecting the variation of the significant wave height Hm0 and spectral period Tm-1,0 (Fig. 3.7) and the mean wave length L (Fig. 3.8). Closer to the dikes of the main land, the waves decrease again. The lower panel of Fig. 3.9 indicates that the areas with wave-induced forces are confined to small narrow strips on the outer edges of the ebb-tidal delta.

The results for the actual storm condition ‘w’ on grid NS2 (Figs. 3.10, 3.11 and 3.12) give a similar picture of the spatial variations of the wave conditions in the Wadden Sea to that for the astronomical situation (Fig. 3.7, 3.8 and 3.9). The main difference is that the wave conditions are more severe since the water levels are higher than for the

astronomical case. Comparison of the dimensionless quantities (Fig. 3.11 versus 3.8) reveals only small differences. This implies that in both cases (‘w’ versus ‘a’), the wave conditions are depth-limited. Almost no differences exist between the patterns of the wave induced forces.

The differences in wave conditions are better seen in the difference plots of the wave height and wave period. The comparison between the cases ‘w’ and ‘a’ on the NS2 grid is shown in Fig. 3.13. They reveal that the largest increases in wave heights occur on the North Sea side of the Wadden islands and along the dikes of the main land. On the lee side of the island the wave heights are very similar, indicating the dominance of local wave generation, which is not yet depth limited. The results also indicate that more wave penetration occurs for the storm situation in the Amelander Zeegat, the Friesche Zeegat and the area east of Schiermonnikoog. These increases are mainly due to the higher water levels in these areas compared to the astronomical situation (see Fig. 3.5 and 3.6).

The difference in spectral period Tm-1,0 between the two conditions (Fig. 3.13, lower panel) shows a different picture than for the wave heights. The areas with a relative large increase in wave periods are more confined to the area around the tidal inlets. In addition, almost no variation in spectral period occurs on the North Sea side of the Wadden islands. A striking phenomenon is the strong increase in (absolute) wave periods in the tidal inlets. These higher wave periods are most noticeable in the Friesche Zeegat and the area east of Schiermonnikoog. Another feature is that the wave periods are generally higher in the Wadden Sea (typical increase of 1-2 s) for the storm situation ‘w’ than for the astronomical situation ‘a’.