Hindcast tidal inlet of Ameland storms: January and March 2007

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Document title Hindcast tidal inlet of Ameland storms January and March 2007

Document short title Hindcast tidal inlet of Ameland 2007 Status Final Report

Date 14 december 2007

Project name Hindcast Amelander zeegat 2007 Project number 9S8833.A0

Client WL | Delft Hydraulics

Dr. J. Groeneweg Reference 9S8833.A0/R0002/JLANS/SSOM/Rott George Hintzenweg 85 P.O. Box 8520 Rotterdam 3009 AM The Netherlands +31 (0)10 443 36 66 Telephone +31 (0)10 443 36 88 Fax info@rotterdam.royalhaskoning.com E-mail www.royalhaskoning.com Internet Arnhem 09122561 CoC

Drafted by Joost Lansen, Sjaak Jacobse, Maarten Kluyver, Erik Arnold

Cooperation of Bram Roskam Checked by Keming Hu

Date/initials check 13-12-2007 ………. Approved by Sjaak Jacobse

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SUMMARY

During the storm season from October 2006 untill April 2007 several severe storms occurred at the North Sea. Three storm periods are selected to study the realiability of the SWAN wave model by means of a hindcast.

The objective of this hindcast is to gain insight into the performance of the SWAN model, especially under storm conditions for the Hydraulic Boundary Conditions with particular emphasis upon the strengths and weaknesses of the model.

The three selected stormperiods (11 and 12 January 2007, 17 and 18 January 2007 and 17 and 18 March 2007) can be described as regular western storms with wind directions varying from the southwest to the northwest and with moderate waterlevels. For these storm periods, measurements from twelve locations using the measurement network at the Tidal inlet of Ameland are used.

At the start of this study, the measured wave conditions are validated agian by a consistency check. In order to prevent a selection of spurious measurements for the comparison of measured and modelled waves, all suspicious measurements are

excluded from the hindcast selection. The selection of hindcasting moments is based on three different hypotheses. These hypotheses were formulated using the expected behaviour of SWAN during wind growth at shallow water, current conditions and triad interactions. In totally 31 moments are simulated (representing one third of the storms) with the current version of SWAN.

The analysis of the differences between the modelled and measured wave parameters and wave spectra shows that for typical Wadden Sea conditions (wind growth over shallow water) SWAN seriously underestimates the significant wave heights. The wave period is well simulated by SWAN. The largest underestimation is observed at the buoy locations at the shallow locations. In the table below the averaged deviation (sigma factor) of SWAN in relation to the measurements is given for different areas.

Area Deviation Hm0 [%] Deviation Tm-1,0 [%]

All buoys 1 _{-15 %} _{-7 %}

Ebb tidal delta -16 % -10 %

Buoys in tidal channels -10 % -9 %

Inner Shoals -21% -6 %

Based on the three different hypotheses the following findings are presented:

1.) For depth limited wind growth situations SWAN demonstrates a considerable underestimation of the significant wave height.

2.) Tidal currents strongly affect the waves in tidal channels. For opposing currents SWAN overestimates the spectral energy around the peak frequency.

Conversely, with following currents SWAN dissipates too much energy.

3.) Triad interactions affect the spectral shape at the ebb tidal delta. A second high frequency peak is observed in the SWAN simulations. At locations with an important role of triad interactions SWAN is underestimating the mean wave period.

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CONTENTS

Page

1 INTRODUCTION AND BACKGROUND 1

-1.1 Coastal safety the importance of reliable models 1

-1.2 Project background 1

-1.3 Objectives 2

-1.4 Project organization 2

-2 ANALYSIS OF THE AVAILABLE STORMS 3

-2.1 The measurement network 3

-2.2 Severe storms in January till March 2007 4

-2.2.1 Characteristics of the storm of 11 and 12 January 2007 4

-2.2.2 Characteristics of the storm of 18 and 19 January 2007 7

-2.2.3 Characteristics of the storm of 18 and 19 March 2007 9

-3 VALIDATION FIELD MEASUREMENTS 11

-3.1 Validation of consistency of measured wave parameters 11

-3.2 Assessment correlation and outliers in wave parameters 11

-3.3 Results of validation 13

-3.4 Second opinion data validation 16

-3.4.1 Questions about reliability of data processing with WAVES2006 16

-3.4.2 Conclusions regard to wave processing 16

-3.4.3 Conclusions regard to buoy locations 17

-3.4.4 Influence on hindcast study 17

-4 HINDCAST SET-UP 19

-4.1 Introduction 19

-4.2 Literature 19

-4.3 Hypotheses 20

-4.4 Selection of hindcast moments 22

-4.5 Hindcast method 27

-4.5.1 SWAN version 27

-4.5.2 Choice of grids and determination of wave boundary conditions 27

-4.5.3 Numerical settings 30

-4.5.4 Physical settings 30

-4.5.5 Bathymetry 31

-4.5.6 Water levels and currents 31

-4.5.7 Wind speed 32

-4.5.8 Output locations 34

-4.6 Pre- and post processing 36

-4.7 Methods of analysis 38

-5 IMPLEMENTATION AND TESTING OF MODEL RUNS 41

-5.1 Convergence behaviour and boundary effects 41

-5.1.1 Convergence at buoy locations 41

-5.1.2 Boundary effects 43

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-6 STATISTICAL ANALYSIS 47

-6.1 Introduction 47

-6.2 Performance of SWAN at buoy locations 47

-6.3 Performance of SWAN at the characteristic tidal inlet areas 49

-6.3.1 Analysis for characteristic areas 49

-6.3.2 Observed behaviour of SWAN 53

-6.3.3 Sumarizing the performance of SWAN at the characteristic tidal

inlet areas 54

-6.4 Performance of SWAN for the hypotheses 54

-6.4.1 General 54

-6.4.2 Wind growth for depth limited situations 54

-6.4.3 Under opposing and following tidal currents 63

-6.4.4 At triad dominated locations 71

-7 DISCUSSION 81

-7.1 Findings 81

-7.2 Discussion 82

-7.3 Relevance for deriving new Hydraulic Boundary Conditions 83

-7.4 Recommendations 83

-Appendices

A Measurement locations Wadden Sea

B Buoy Wave Parameters

C1 Consistency checks of wave parameters

C2 Waqua Currents

D SWAN INPUTS

E Visualisation of computed wave fields

F Scatterplots wave parameters

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1 INTRODUCTION AND BACKGROUND

1.1 Coastal safety the importance of reliable models

Dikes and dunes are protecting the low lying areas of the Dutch coast against flooding by the sea. In order to guarantee the coastal safety, a five yearly inspection is

prescribed in the Dutch Act on Water Defences (in Dutch: Wet op de Waterkeringen). This periodic inspection is carried out by the responsible Water Boards based on legal regulation that prescribed methods and extreme conditions. The hydraulic conditions for the inspection are given in the Hydraulic Boundary Conditions (in Dutch; Hydraulische Randvoorwaarden).

It goes without saying that these Hydraulic Boundary Conditions (hereafter HBC) should be accurate and reliable. Unreliable HBC may be result in an unrealistic image of our safety. Too conservative HBC will result in high costs of required strengthening of coastal defences. Most HBC consist, in addition to water level conditions, of wave conditions computed with numerical models. The used numerical wave model for coastal regions is SWAN. In this case, the reliability of the HBC is strongly correlated

with the reliability and performance of the SWAN2_model.

One of the principal methods to determine the reliability of the wave model is to validate the model with real data. For such a case, representative measurements of storm conditions in the area of interest are required. Hindcasting is a method to simulate wave conditions using a numerical model and validate the performance of that model by comparing the modelled results with the measured wave conditions.

1.2 Project background

Around 2002, Rijkswaterstaat has initiated a large scale project to improve the quality of future HBC and to increase the knowledge about failure mechanisms. This project was called “Strength and Loading on Coastal Structures” (in Dutch Sterkte en Belasting Waterkeringen; SBW). Special interest was given to physically complicated areas with a lack of good measurement data. For most coastal systems wave measurements for a period of minimal 10 years are available including a few severe storm conditions. However, especially in the Wadden Sea, wave measurements for severe storms were missing.

To fill in that gap, Rijkswaterstaat initiated a measurement campaign for the tidal inlet of Ameland in 2003. Since the year 2003 different moderate storms were measured of which most were used to check the reliability of SWAN. In the last few years several hindcasts were carried out in the framework of the SBW-project in order to quantify the reliability of the SWAN model in the Wadden Sea (WL, 2006; Royal Haskoning, 2006; WL, 2007a; WL, 2007b, Alkyon, 2007a; Alkyon, 2007b). Most storms used for the hindcasts can be characterized as mild. Making use of the knowledge and experience of other engineering firms, WL|Delft Hydraulics has asked Royal Haskoning to participate in the project. This participation provides a good opportunity for sharing knowledge and experience in the field of hindcasting.

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1.3 Objectives

The objective of performing hindcasts of selected storms in the Amelander Zeegat is threefold:

1. To gain insight into the performance of SWAN;

2. To gain insight into the strong and weak points of the most recent version of

SWAN and;

3. To gain insight into the effect the required model input has on the model results

in the Wadden Sea.

A better understanding of the performance of the SWAN wave model and a potential improved version should provide more reliable Hydraulic Boundary Conditions for the primary sea defenses along the Wadden Sea coast. Especifically for the primary sea defences of the Departments Groningen and Friesland, according to the strategy of SBW.

The current hindcast differs from previous hindcasts. In contrast to previously

considered storm events (WL, 2006), the number of considered events is increased as well as the variety of considered conditions. This leads to a more homogeneous set of conditions. The first hindcast in the Wadden Sea by WL (2006) showed that certain aspects within the approach could be improved, e.g. the determination of offshore wave boundary conditions. Later hindcasts have resulted in a better representation of the area by a new numerical model grid (see e.g. Royal Haskoning, 2006). The present

approach, as developed in the most recent hindcast (WL, 2007a) is believed to lead to optimum reliable SWAN results which imply that the fairest conclusions about the performance of SWAN can be drawn.

1.4 Project organization

WL|Delft Hydraulics coordinates the SBW Wadden Sea project, as a part of the overall SBW project. Royal Haskoning will function as subcontractor of WL|Delft Hydraulics and carries out the hindcast study as a subproject of the SBW Wadden Sea project. Royal Haskoning carries out the analysis of the storm measurements, the computations for the hindcast and the analysis of the performance of SWAN.

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2 ANALYSIS OF THE AVAILABLE STORMS

2.1 The measurement network

Periodically, Rijkswaterstaat evaluates the wave measurement network. This network consists out of a series of wave measuring devices (wave buoys among others) in the Dutch coastal waters. The evaluation is based upon the analysis of measurements and the results of the Hindcast studies. In 2006 Rijkswaterstaat decided to expand the present network in the Amelander tidal inlet by an additional set of buoys. The measurement network in the Dutch part of the Wadden Sea nowadays consists of around twenty wave measurement locations. Twelve buoys are located at the

Amelander tidal inlet which can be devided in a western and eastern branch. In table 2-1 and figure 2-1 the network of the Amelander tidal inlet is illustrated.

Table 2-1 Position of measurements (DWR = Directional Waverider; WR=Waterider)

Nr Network

code Since Buoy type Diameter X-Coordinates Y-Coordinates Bottom Depth

[dd.mm.yy] [cm] [m. RD] [m. RD] [m. +NAP] 1 AZB11 27.11.06 DWR 90 160997 616011 -18.25 2 AZB21 27.11.06 DWR 90 167302 610610 -9.70 3 AZB31 27.11.06 DWR 90 167750 607205 -3.00 4 AZB41 28.11.06 DWR 90 168792 600498 -1.00 5 AZB51 28.11.06 WR 70 168003 596498 -1.00 6 AZB61 28.11.06 WR 70 167501 592499 -0.80 7 AZB12 27.11.06 DWR 90 173008 617306 -21.60 8 AZB22 27.11.06 WR 90 170990 612006 -4.20 9 AZB32 23.01.07 DWR 90 169480 607108 -10.60 10 AZB42 23.11.06 DWR 90 171367 604176 -18.10 11 AZB52 23.11.06 DWR 90 175494 600768 -13.00 12 AZB62 23.11.06 WR 70 180498 598627 -1.00

Figure 2-1 Westerly and easterly branch of the network

West

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The two most offshore (Directional Wave Rider) buoys are located at relatively deep coastal waters, outside the ebb tidal delta. Over the ebb tidal delta, two buoys measure the effect of the shallow areas on wave propagation into the tidal inlet. Deeper into the mouth of the tidal inlet, two buoys measure the wave penetration into the Wadden Sea. From thereon, the network is devided into a western and eastern branche. The buoys that are part of the western branch are intended to measure the local generation of waves in the Wadden Sea and propagation over the shallow flats. The eastern branch is mainly intended to measure wave penetration of North Sea waves into the Wadden Sea through the main channel towards the mainland.

In addition to wave measurements within the Dutch coastal waters, wind speed, wind direction and water levels are measured continuously. In Annex A, the positions of the wind and water level instruments are visualised. An additional note is made with respect to the wave buoys; the directional wave riders measure with a sampling rate of 1,28 Hz. The directional wave riders measure with 2,56 Hz. Form this point of view, the non-directional wave riders are more suitable to measure short waves.

2.2 Severe storms in January till March 2007

The storm season of 2006-2007 in the north of the Netherlands, can be characterised as rough in comparison to previous storm seasons. After a severe north-westerly storm on

November 1st_{2006, which caused extreme high water levels at Delfzijl, three westerly}

storms occurred in the following periods:

• 11th and 12th January 2007;

• 18th and 19th January 2007;

• 17th till 21st March 2007.

2.2.1 Characteristics of the storm of 11 and 12 January 2007

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Figure 2-2 Wind speed and wind direction (left) and air pressure (right) above the North Sea for

11-01-2007 20:00u

Figures 2-3 and 2-4 shows the observed (and time averaged) wind at three locations along the Wadden Sea; Hoorn, Huibertgat and Terschelling-West. The vertical lines shows the selected hindcast moments for three hypotheses (see chapter 4).

00:000 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 5 10 15 20 25 30 Date 11012007 to 12012007(dd/mm) Windspeed [m/s] Hoorn Hoorn fit Lauwersoog Lauwersoog fit Huibertgat Huibertgat fit Hindcast moments

Figure 2-3 Measured wind speed at 10 meter height for 11-12 Januari 2007

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00:000 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 50 100 150 200 250 300 350 Date 11012007 to 12012007(dd/mm) Wind Direction [ o ] Hoorn Hoorn fit Lauwersoog Lauwersoog fit Huibertgat Huibertgat fit Hindcast moments

Figure 2-4 Measured wind direction at 10 meter height for 11-12 Januari 2007

Figure 2-5 shows the measured waterlevels at Nes, Huibertgat and Terschelling-West and shows the selected moments per hypotheses (Please note that some hindcast moments are coupled to multiple hypotheses, so lines can overly). The maximum of the surge reaches values of +2.1 m NAP at Harlingen and +2.5 m NAP at Delfzijl (source: storm flood note SVSD). At the water level instrument of Nes, located at the lee side of Ameland, the surge reaches values of over +3 m NAP.

00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 −100 −50 0 50 100 150 200 250 300 350 Date 11012007 to 12012007(dd/mm)

Water level [cm] NAP

Water level Nes Water level Huibertgat Water level West Terschelling Hindcast Moments

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2.2.2 Characteristics of the storm of 18 and 19 January 2007

Only a short week after the storm of 11 and 12 January, a new large scale depression was generated over the Northern Atlantic. This low pressure area followed a track over the British Islands via Denmark to the Baltic Region. Over the North Sea, the passage of

this depression caused on January 18th_{, a severe south westerly storm with wind forces}

up to 10 Beaufort. At the peak of the storm, wind gusts reached hurricane magnitudes of 12 Beaufort. Figure 2-6 shows the predicted wind field and a satellite image of the cloud cover.

18-01-2007 13:00u

Later in that afternoon, the wind changed to westerly directions over the North Sea and in the evening the wind direction changed to northwest. During the whole day wind forces over the North Sea varied around 10 Beaufort. After the passage of the so-called back-bent-occlusion, the wind speed dropped down instantly. At the Wadden Sea this reduction took place just before a new high tide period, so that the water levels did not reach extreme values. Figures 2-7 and 2-8 shows the measured wind speed and direction above the Wadden Sea at 18 and 19 January.

Figure 2-7 Measured wind speed at 18-19 January

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Figure 2-8 Measured wind direction at 18-19 January

Along the Dutch coast, this westerly storm caused a surge of at maximum 1.7 meter at the coastline of Holland and 2.2 meter in the Wadden Sea near Delfzijl (source: storm flood note SVSD). Despite the high wind speeds, surges did not reach extreme levels. Figure 2-9 shows the measured water levels in the Wadden Sea. The maximum observed water level at Nes was still below NAP +3 meter.

00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 −100 −50 0 50 100 150 200 250 300 Date 18012007 to 19012007(dd/mm)

Figure 2-9 Measured Water levels 18 till 19 January

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2.2.3 Characteristics of the storm of 18 and 19 March 2007

On March 17th, a depression propagated from the North (Iceland) to the South of

Norway. Over the northern part of the North Sea, the pressure dropped to 955

hectopascal. During the afternoon of March 18th_{, a storm developed over the Northern}

part of the North Sea with wind forces in storm category up to 10 Bft. from North-westerly conditions. Close to the Netherlands, the wind direction was merely North-westerly orientated and wind forces reached 9Bft. The wind speed dropped down during the night

of 18th-19th March and the direction changed to North (see wind and pressure field at

figure 2-10).

18-03-2007 19:00u

Above land the wind speed was significantly lower than over the northern North Sea. Figure 2-11 shows the observed wind at three locations on the Wadden Sea (Hoorn at Terschelling, Lauwersoog and Huibertsgat). The maximum wind speed was around 20 meter per second. Note that the wind direction at the top of the storm was very stable around 270 degrees (figures 2-11 and 2-12).

Figure 2-11 Measured wind speed at 18 and 19 March 2007

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Figure 2-12 Measured wind direction at 18 and 19 March 2007

Especially the storm surge was significant during this storm, due to the wind conditions favourable for surge in the Wadden Sea estuary. It was neaptide at the time. Still, water levels reached significant values up to 3.78 m + NAP at Delfzijl. This means a storm surge at the Eems-Dollard of 2.2 meter. At the measurement site the water level reached values around 3.20 m + NAP. Figure 2-13 shows the observed water levels at Nes, Lauwersoog and Terschelling.

00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 −150 −100 −50 0 50 100 150 200 250 300 350 Date 18032007 to 19032007(dd/mm)

Figure 2-13 Measured Water levels 18 and 19 March (Please note that some hindcast moments are

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3 VALIDATION FIELD MEASUREMENTS

3.1 Validation of consistency of measured wave parameters

A first validation of the wave buoy measurements in the Amelander Zeegat is performed by the RMI-validation of the measurement network. This first, somewhat broad,

validation is followed up by a second validation carried out by the WAVES2006 suite. WAVES2006 is a software package developed by RIKZ. This validation is a

computerised analysis of the signal recorded by the instruments. Abnormal instrument behaviour or irrational recordings are corrected or removed from the signal by

WAVES2006.

However, whether the resulting measurements are realistic according to the real

physical conditions at site at the time of recording is not validated by WAVES2006. In addition, any correlation between the buoy measurements is not performed. It was noted in previous studies, that although the wave instrument data passed the WAVES2006 test, the signal could still be out of line with physical conditions expected at the site. E.g. unrealistic long waves caused by a too low measuring frequency will not be detected by the validation of WAVES2006.

The wave data validated by RMI and WAVES2006 have been provided by RIKZ, Rijkswaterstaat (owner of RMI, WAVES2006) for the use in the present project. To improve the validity of the used measurement data and to increase the reliability of the provided data set, a correlation study is performed with the recorded wave parameters. Main principal for this correlation check is the known fact that the chance that all wave parameters at one time step are wrong is much smaller than the chance that one parameter on one location is showing spurious or outlying values at that time. Additional spurious signals which could decrease the reliability of the outcome of the statistical analysis of this hindcast are possible to be removed by relating measurements with physical conditions.

3.2 Assessment correlation and outliers in wave parameters

The first validation of the data is performed by WAVES2006, which checked the data for reliability of the signals. It checked for staggers, spikes, solitary values, data gaps and dummy values. If more than 90% of the available data is approved by WAVES2006, wave parameters are computed, if less, no parameters are available. As a result of this procedure, the data of the wave buoys is not continuous in time. At certain periods gaps are seen in the time series of the wave parameters. However, it seems that the

procedure is not sufficiently rigid to remove all the unreliable measurement data from the signal. An overview of the wave heights, wave periods and directions that passed the WAVES2006 test is presented in annex B. In order to identify remaining doubtful measurements, the following procedure has been followed:

1. Visualising all wave parameters;

2. Visualising the relation of H1/3, T1/3 and wave steepness between the eastern and western branch of buoy series;

3. Visualising the relations between the measurement locations along the transect line from the North Sea to the Wadden Sea;

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5. Validating the range of the parameters Hm0 and Tm02 (see table 3-1); 6. Validating the standard deviation of wave parameters Hm0 and Tm02 in time.

For this validation we compute the standard deviation of three successive records (see table 3-1);

7. Validating the consistency of wave parameters of the west and east branch (i.e. the difference between 011 and 012 etc.);

8. Accumulating the number of errors found per location and time record. Ad 2.) Annex C shows the relation of wave parameters between the two branches.

For each location, the wave parameters H1/3 and T1/3 of the locations at the western ray is divided by the wave parameters of the accompanying locations in the eastern ray (e.g. AZB021 / AZB022). Besides H1/3 and T1/3, the wave

steepness (based on H1/3/(1.56*T1/3)2_{to discard the influence of the water}

depth) is analysed. This type of validation is illustrating the difference between both branches. Inconsistency is hard to prove but the following can be stated:

• Especially the measurements of AZB022 are apparently not very

consistent with AZB021. AZB022 is located over shallow water depth east of the ebb tidal delta. AZB021 is located at the exposed side of the ebb tidal delta in deeper water. Most of the time, both wave height and wave period of AZB022 proves to be much higher than AZB021. Also the wave signal of AZB022 looks very spiky;

• For the fetch limited locations (AZB042 and AZB052) the wave heights

at the eastern branch (in channel) are significantly higher in comparison with the western branch (on tidal flats);

• At the most inner locations unrealistic high values for T1/3 were

observed in relation with wave heights below 20 centimetres. Ad 3.) Annex C also shows the relations of wave parameters between the

measurement locations of the same measurement branch. For each location in transect, the wave parameters H1/3 and T1/3 are divided by its value at the previous location in the transect running from the North Sea into the Wadden Sea (e.g. AZB021 / AZB011). This type of validation is checking the deviation between two consecutive measurement locations. Normally the wave

parameters representing wave energy show a declining trend from the North Sea to the dikes of the Wadden Sea. This declining trend is also observated in the measurements

Ad 4-7.) For the validation of the wave parameters the settings of table 3-1 are used to identify suspicious values in wave height Hm0 and wave period Tm02. Doing this, very small wind waves (which were inaccurate measured by a buoy) will be excluded. The defined parameters for outliers are not based on the physical maximum range and variability, but are based on the purpose of the hindcast. The values of table 3-1 mark the interesting periods in the storm

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Table 3-1 Parameter settings validation of measured signals Upper range Hm0 Lower limit Hm0 Upper range Tm02 Lower limit Tm02 Std. dev. Hm0 Std. dev. Tm02 Deviation3_{value Hm0} With regard to Deviation value Tm02 With regard to

Besides the described consistency checks all wave spectra and time series were visualised. Also animations of the transformation of wave spectra have been made. This data can be found on the accompanying data disk.

3.3 Results of validation

After carrying out the validation, the results were summarized for each measured record. For most locations the validation procedure proved the good quality of the

measurements; more than 90% of data was approved by the validation and can be assumed as suitable for a hindcast study. In table 3-2 per measurement location the number of identified suspicious, spurious and valid records is summarized. In table 3-3 the definition of suspicious and spurious is given as follows:

Suspicious record One check (per record) was resulting in a disapproval of the consistency; Spurious record More than one check (per record) was resulting in a disapproval of the

consistency;

Table 3-2 Results of validation

Range Hm0 Range Tm02 Std. dev. Hm0 Std. dev. Tm02 Deviation Hm0 Deviation Tm02 Missing values Suspicious records Spurious records Valid records Valid records [%] 3

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Findings:

• Both outer buoys AZB011 and AZB012 are functioning very well. Almost 100%

of time there are measurements available. For only a few moments the data can be called suspicious. Overall for 98% of time valid data is available;

• For westerly storms it is reasonable that the measured waves at AZB022 differ

from AZB021. Surprisingly the measured waves at AZB022 (east) were most of the time significantly higher than AZB021. Also the wave period (T1/3) is approximately 20% higher than AZB021. Besides that, the quality of the total signal of AZB022 is often bad. In many cases the signal was rejected by WAVES2006 (resulting in a missing parameter);

• Measurements from location AZB032 are only available for the last storm period;

• Waves at AZB041, AZB042, AZB051, AZB052, AZB061 and AZB062 were

sometimes depth-limited. Especially in combination with low waves the results (Hm0, H1/3, T1/3 and Tm02) do not look consistent. Often low wave heights were seen combined with unrealisticly long wave lengths at those locations (> 6 seconds). It is reasonable that the measurement technique of a (Directional) Waverider with a diameter of 90 centimetre is becoming unreliable for those very low waves. This mismatch between sampling frequency and the characteristics of the wave signal is resulting in an inaccurate signal. Above wave heights of 25 centimetres the measurements look much more consistent;

• Most traced unreliable values are measurements with low wind speeds and low

water levels before or after the storm peak. At some moments, unreliable values occurred shortly after the highest wave load. This possibly indicates that shallow water wave buoys could not be functioning in heavy wave conditions. Breaking waves might be the cause of these problems.

In order to use only the reliable wave measurements in the hindcast analysis, four different quality labels were defined which were given in table 3-3.

Table 3-3 Definition status codes wave validation

Status code Description

-9 No data available;

0 Wave heights and wave periods look consistent and can be characterised as valid;

1 Wave heights and wave periods look suspicious. The status code one is given if one check (per record) was resulting in a disapproval of the consistency;

2 Wave heights and wave periods look spurious. The status code two is given if more than one check (per record) was resulting in a disapproval of the consistency.

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011 012 021 022 031 032 041 042 051 052 061 062

no data valid suspicious spurious

12 -0 1-07 1 6: 00 12 -0 1-07 2 0: 00 12 -0 1-07 0 :0 0 12 -0 1-07 4 :0 0 12 -0 1-07 8 :0 0 12 -0 1-07 1 2: 00 11 -0 1-07 8 :0 0 11 -0 1-07 1 2: 00 11 -0 1-07 1 6: 00 11 -0 1-07 2 0: 00 11 -0 1-07 0 :0 0 11 -0 1-07 4 :0 0

Validation AZB011-AZB062 from: 11-1 2007 to 12-1 2007

Figure 3-1 Data quality 11 and 12 January 2007

011 012 021 022 031 032 041 042 051 052 061 062

19 -0 1-07 8 :0 0 19 -0 1-07 1 2: 00 19 -0 1-07 1 6: 00 19 -0 1-07 2 0: 00 18 -0 1-07 1 6: 00 18 -0 1-07 2 0: 00 19 -0 1-07 0 :0 0 19 -0 1-07 4 :0 0 18 -0 1-07 0 :0 0 18 -0 1-07 4 :0 0 18 -0 1-07 8 :0 0 18 -0 1-07 1 2: 00

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011 012 021 022 031 032 041 042 051 052 061 062

19 -0 3-07 8 :0 0 19 -0 3-07 1 2: 00 19 -0 3-07 1 6: 00 19 -0 3-07 2 0: 00 18 -0 3-07 1 6: 00 18 -0 3-07 2 0: 00 19 -0 3-07 0 :0 0 19 -0 3-07 4 :0 0 18 -0 3-07 0 :0 0 18 -0 3-07 4 :0 0 18 -0 3-07 8 :0 0 18 -0 3-07 1 2: 00

Figure 3-3 Data quality 18 and 19 March 2007

3.4 Second opinion data validation

3.4.1 Questions about reliability of data processing with WAVES2006

Based on the outcome of above described validation and consistency checks some questions have been raised by the reliability of the processing of the wave

measurements by WAVES2006. Comparing the two different methods to process energy density spectra of directional wave rider signals learns that there is an unacceptable difference in the processed spectra resulting in differences in wave parameters of more than 10%.

Complementary to the parameter validation a second opinion of the data processing with WAVES 2006 had been carried out by mr. B. Roskam [ref. Roskam, 2007]. In his validation he reproduced the data processing of WAVES2006 with the authentic Fortran routines (based on the RWS-SWAP module). After that differences in parameters and energy density were used to criticize the data processing method.

3.4.2 Conclusions with regard to wave processing

The two different methods to process wave spectra, i.e. directional processing based on the three signals of a directional wave buoy and the non-directional processing based on one signal, can result in small differences in wave spectra and wave parameters. In general by the directional processing the three available signals are validated

synchronously. Detected errors in one of the three signals will result in a rejection of the three signals for that time record.

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number of the degrees of freedom of the data signal is less than 24 WAVES2006 is overwriting some data records of the previous data block in the non-directional analysis. Roskam concluded that the non-directional processing of WAVES2006 (spectra called GS-spectra) is implemented incorrectly.

3.4.3 Conclusions with regard to buoy locations

For all measurement days the wave signal of buoy AZB022 is showing huge wave phenomenas with amplitudes of more than 10 metre. Given the local depth at the outer delta (around NAP – 4 metre) those “waves” are indicating a problem in the hardware of

the buoy. Earlier for measurement location SON4_{was concluded that buoys on relatively}

shallow water with heavy breaking waves can result in an incorrect registration of the vertical acceleration instrument.

A short period after the storm days of 11 and 12 January the number of spikes and staggers in the signal of buoy AZB031 is increased. This indicates that the buoy was damaged at the storm period. The measurements after 12 January look unreliable. The peak frequency of the wave spectra at the very shallow buoy locations AZB051, AZB061 and AZB062 exceeds 0.5 Hz several times. For those conditions the used measurement technique becomes inaccurate.

3.4.4 Influence on hindcast study

After the supplementary validation new wave spectra were processed based on the authentic data processing module. Those new spectra are used as wave boundary conditions for the hindcast and for the analysis of the hindcast. Additionally the following choices have been made:

1.) The measurements of AZB022 are not considered in the hindcast;

2.) The measurements of AZB031 for the storms of 18-19 January and 18-19 March are not considered either.

Thanks to the applied consistency checks (table 3-1) most other possible unreliable measurements were already excluded from the dataset. Further information from the supplementary validation will not result in another selection of valid data as presented in figure 3-1, 3-2 and 3-3.

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4 HINDCAST SET-UP

4.1 Introduction

Most hindcast studies start with a random selection of interesting storm moments. Selection criteria can be quasi stationarity, wind peaks or water level peaks. Most times such approach of a hincast study will result in general conclusions and will not answer the questions “when” and “why” there will be differences between model and

measuments.

Starting point of the hindcast is a selection of hypotheses, covering the main subjects that influence the performance of SWAN compared to wave measurements. The selection of hindcast moments will be coupled to these hypotheses. Doing so, the significance of the selection criteria can be further increased. Next to this, conclusions on the results of the hindcast study in relation to the performance of SWAN can be drawn more easily. The selection of hypotheses is based upon:

• A literature study by reviewing reports of previous hindcasts (see 4.2);

• An expert meeting with (senior) wave modelling experts (see 4.3).

o Dr. ir. A.J. (André) van der Westhuysen.

o Dr. ir. L.H. (Leo) Holthuijsen.

o Dr. ir. G. Ph. (Gerbrant) van Vledder.

The review of literature reveals most important issues. The expert meeting is used to discuss among experts some findings of the review of previous hindcast reports and literature. In addition to this, pre-selected hypotheses are elaborated. This will furthermore improve the setting and scope of the hindcast.

4.2 Literature

The topics discussed in this section only reflect the performance of SWAN in storm conditions in relation to wave measurements, and not the hindcast method (such as convergence behaviour) or any assumptions (for instance constant bottom profile for 3

storms). The intrinsic sensitivity of wave propagation to any conditions as currents,

bathymetry etc. is not under review either. The following literature was reviewed:

• Analysis SWAN hindcast tidal inlet of Ameland Storm events of 8 Febr. 2004

and 2, 8 Jan. 2005 (Alkyon, 2007a);

• Analysis SWAN hindcast Storms of tidal inlet of Ameland 17 December 2005

and 9 February 2006 (Alkyon, 2007b)

• Storm hindcasts for Wadden Sea: Hindcasts in inlet systems of Ameland,

Norderney and Lunenburg. WL | Delft Hydraulics Rapport H4918.20 (WL, 2007a);

• SBW Wadden Sea, Gevoeligheidsanalyse fysica. WL | Delft Hydraulics Rapport

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4.3 Hypotheses

A literature study reveals the most important hindcast subjects of the SWAN wave model, which have been addressed to affect the performance of SWAN. The following topics have been addressed;

• The wave conditions in the tidal inlet and in the tidal basin of the Amelander

Zeegat are insensitive to the boundary conditions of the model, or better insensitive to offshore boundary conditions. Apparently, the ebb tidal delta strongly works as a filter for North Sea waves. This hypothesis has been supported by several sensitivity runs and is supported by measurements, in which the decrease of low frequency wave energy in the tidal inlet is clearly observed (see WL (2007b)). This is more evident in the wave height (depth limited conditions) than it is for the wave period. In other words, some low frequency wave energy penetrates into the inlet, but is not significant compared to locally generated waves by wind. In the transition zone over the ebb tidal delta however, SWAN does not predict spectral peaks correctly in the tidal inlet (see Alkyon (2007b)). Low frequency peaks develop in SWAN over the ebb tidal delta due to triad processes, which is not physically realistic. Moreover, intrusion of long period wave energy largely depends on the water depth over the ebb tidal delta; hence the inclusion of wave set-up is important. The use of the WAQUA nowcast (actual simulation of storm) implicitly takes wave set-up into account.

• The influence of currents on the model results of SWAN is significant. The

performance of SWAN under current conditions is in general good, as long as no strong dissipation takes place. The results of SWAN correlate fairly well to wave measurements in situations when currents travel in the same direction as the wave propagation in the main tidal inlet (Flood situation in the Amelander Zeegat), see WL (2007a). The performance of SWAN in counter current situations (Ebb situation) is far less accurate; also see Alkyon (2007a). One reason for this might be that the frequency shift of spectral components (which is a function of the frequency itself) is correlated to the shear of the flow in the water column. However, by comparing the results of SWAN computations with a depth-averaged current and with a representative current, WL (2007c) did not find evidence for the effect of a sheared current. More likely is the

underestimation of wave dissipation by any of the white-capping formulations in SWAN. Additional dissipation by means of a bore-like formulation (e.g.

formulation by Battjes-Janssen), as proposed by Ris (1997) and applied in WL (2007c), would be preferable.

• Current effects on the wave height only seem to be present in areas where

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spreading seem to be better modelled in ebb conditions. In addition to current effects, also the dissipation in the tidal channels could result in differences in modelled and measured wave direction.

• It was concluded in WL (2007b) that according to the results of SWAN, energy

saturated conditions occur at the shallow areas along the Frisian coast during storm conditions. Wave heights seem to be under-predicted and wave periods seem to be under-predicted. The shallow water growth limit is suggested to be underestimated by SWAN De Waal (2002). The latter is not concluded from the ONR test-bed case in Lake George. Whether the wave period will increase sufficiently in depth limited situations is not known.

• In Alkyon (2007a) and Alkyon (2007b), it is suggested that the initial wave

growth is not well modelled in SWAN. As a result, in the lee site of the islands, the steepness is too high, and as a result of this, wave periods are under predicted at inner buoy locations and waves heights are over-predicted at those locations. However, it is very difficult to test this hypothesis with the current buoy network configuration because the wave climate in the inner part of the Wadden Sea is very inhomogenious.

• It is suggested that over the ebb tidal delta in the transition zone of offshore

wave conditions to local wave conditions, where bi-modal spectra occur, SWAN predicts the spectral shape not correctly. As a result the wave period seems to be under-predicted; SWAN seems to exaggerate the influence of triads (WL, 2007a). In addition, deactivating the triads seems to results in better agreements of model results with measurements over the ebb tidal delta and over shallow areas in the tidal inlet (see WL, 2007a). In the tidal basin itself, triads are of less relative importance compared to other physical processes, although on the shallow areas in the surf zone in front of the dykes, it is important again. It is suggested to include triads in SWAN 40.51 with bugfix a.

• The breaker parameter is at the moment applied using a constant formulation. It

has been shown however (see Holthuijsen and Booij (2006)), that applying a dynamic breaker parameter, as a function of characteristics of the breaker zone, improves the performance of SWAN significantly.

Based on the literature study and the expert meeting, the following 3 subjects have been selected for this hindcast study;

1. What is the performance of SWAN in situations with strong counter and strong following currents?

The selection of hindcast moments will be based on high-velocity counter- and following currents. It is preferable to select hindcast moments with low wind velocities and high frequency waves, which would imply most influence comes from orbital motions by wave propagation. In addition, the influence of wind driven currents is reduced as much as possible. This implies moments are outside the peak of the storm. Also moments with no currents will be selected as a reference.

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Hindcast moments will be selected for which high waves over the ebb tidal delta are observed in the measurements. To study the performance of SWAN at locations where triads play an important role, it is recommendated to select moments at (or even close after) the peak of the storm for buoy locations AZB21, ABZ31 and AZB32 in relation to other buoy locations.

3. What is the performance of SWAN in depth limited situations?

Hindcast moments will be selected looking at long periods of constant wind speed over long fetches.

Inner buoy locations AZB41, AZB42, AZB51, AZB52, AZB61 and AZB62 are of interest. The ratio of wave height / wave length over depth is reviewed.

4.4 Selection of hindcast moments

The selection of hindcast moments is based on the hypotheses as described in section 4.2. These hypotheses each require different selection criteria. In addition, only

moments have been selected for which the reliability of the buoy data is optimum. For this purpose the following conditions have been reviewed for selection of the hindcast moments;

Hypothesis Currents

For selection of hindcast moments for this hypothesis, the current direction and current speed (following from the ambient current field) has been set next to the measured wave direction at buoy locations in the tidal channels, AZB21, AZB32, AZB42 and AZB52. Doing so, hindcastmoment have been selected for which the wave direction is either opposing or following the tidal current direction. In addition to this, hindcast moments have been selected for this hypothesis, outside the peak of the storm. This in order to reduce the influence of wind driven currents.

Hypothesis Triads

The performance of the triad formulation should be best seen at moments at which large waves propagate onto the ebb tidal delta. For this purposes, the wave height at the outer buoy locations AZB11 and AZB12 has been reviewed. To reduce the influence of local wind growth, moments outside the peak of the storm have been selected. To reduce the influence of currents, the current speed at the buoy locations, following from the ambient current field, has been reviewed as well.

Hypothesis depth limited situation

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The following moments have been selected;

Table 4-1 Selected hindcast moments

Hypothesis 1 Currents Hypothesis Triads Hypothesis Wind depth limited

1/11/2007 4:00 1/11/2007 12:00 1/11/2007 13:00 1/11/2007 15:00 1/11/2007 22:40 1/11/2007 14:00 1/11/2007 16:00 1/11/2007 20:40 1/11/2007 22:00 1/11/2007 16:40 1/11/2007 21:20 1/11/2007 22:40 1/12/2007 2:00 1/18/2007 17:20 1/18/2007 12:20 1/12/2007 5:00 1/18/2007 18:00 1/18/2007 14:00 1/12/2007 8:00 1/18/2007 20:40 1/18/2007 17:20 1/18/2007 18:40 1/18/2007 21:20 1/18/2007 20:40 1/19/2007 7:40 3/18/2007 07:40 3/18/2007 10:00 1/19/2007 12:00 3/18/2007 09:20 3/18/2007 14:40 3/18/2007 7:40 3/18/2007 14:40 3/18/2007 15:40 3/18/2007 9:20 3/18/2007 17:00 3/18/2007 16:00 3/18/2007 19:20 3/18/2007 17:00

The moments to be hindcasted are given in the figures of the wind speed and wind direction, and in the figures of the water level in sections 2.2.1 to 2.2.3. The hypotheses can be coupled to buoy locations;

• Hypothesis Currents; The buoy locations in the main tidal channels AZB21,

AZB32, AZB42 and AZB52 are reviewed in the analysis and are compared to the other buoy locations.

• Hypothesis Triads; the outer buoy locations AZB21, ABZ31 and AZB32 (if

available) over the ebb tidal delta are reviewed in the analysis compared to the other buoy locations. This also holds for the inner buoy locations AZB61 and AZB62 over the most inner shoals.

• Hypothesis Depth Limited wind growth;The model results at buoy locations,

which are located in line with long fetches over the Wadden Sea, are compared in the analysis to the other buoy locations. Distinction is made between buoys over deeper waters (AZB 42 and AZB52) and over shallow waters (AZB41 AZB51 AZB61 and AZB62);

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Table 4-2 Overview hypotheses at hindcast moments and buoy locations of hypothesis currents No current points 011 012 021 022 031 032 041 042 051 052 061 062 No current points 011 012 021 022 031 032 041 042 051 052 061 062 No current points 011 012 021 022 031 032 041 042 051 052 061 062 19 /0 3/ 07 4 :0 0 19 /0 1/ 07 8 :0 0 Flood currents

no data Ebb currents Flood currents

18 /0 1/ 07 2 0: 00 19 /0 1/ 07 0 :0 0 19 /0 1/ 07 4 :0 0 19 /0 3/ 07 2 0: 00

Hypothesis currents AZB011-AZB062 from: 18-3 2007 to 19-3 2007 no data Ebb currents

18 /0 3/ 07 0 :0 0 18 /0 3/ 07 4 :0 0 18 /0 3/ 07 8 :0 0 18 /0 3/ 07 1 2: 00 18 /0 3/ 07 1 6: 00 18 /0 3/ 07 2 0: 00 19 /0 3/ 07 0 :0 0 12 /0 1/ 07 1 2: 00 12 /0 1/ 07 1 6: 00 12 /0 1/ 07 2 0: 00 19 /0 3/ 07 8 :0 0 19 /0 3/ 07 1 2: 00 19 /0 3/ 07 1 6: 00 18 /0 1/ 07 0 :0 0 18 /0 1/ 07 4 :0 0 18 /0 1/ 07 8 :0 0 18 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 6: 00 19 /0 1/ 07 2 0: 00 18 /0 1/ 07 1 6: 00 12 /0 1/ 07 8 :0 0 11 /0 1/ 07 0 :0 0 11 /0 1/ 07 4 :0 0

Hypothesis currents AZB011-AZB062 from: 11-1 2007 to 12-1 2007

Hypothesis currents AZB011-AZB062 from: 18-1 2007 to 19-1 2007

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Table 4-3 Overview of hindcast moments and buoy locations of hypothesis triads No triad points 011 012 021 022 031 032 041 042 051 052 061 062 No triad points 011 012 021 022 031 032 041 042 051 052 061 062 No triad points 011 012 021 022 031 032 041 042 051 052 061 062 11 /0 1/ 07 0 :0 0 11 /0 1/ 07 4 :0 0

Hypothesis triads AZB011-AZB062 from: 11-1 2007 to 12-1 2007

Hypothesis triads AZB011-AZB062 from: 18-1 2007 to 19-1 2007

11 /0 1/ 07 8 :0 0 11 /0 1/ 07 1 2: 00 11 /0 1/ 07 1 6: 00 11 /0 1/ 07 2 0: 00 12 /0 1/ 07 0 :0 0 12 /0 1/ 07 4 :0 0 18 /0 1/ 07 0 :0 0 18 /0 1/ 07 4 :0 0 18 /0 1/ 07 8 :0 0 18 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 6: 00 19 /0 1/ 07 2 0: 00 18 /0 1/ 07 1 6: 00 12 /0 1/ 07 8 :0 0 18 /0 3/ 07 1 6: 00 18 /0 3/ 07 2 0: 00 19 /0 3/ 07 0 :0 0 12 /0 1/ 07 1 2: 00 12 /0 1/ 07 1 6: 00 12 /0 1/ 07 2 0: 00 19 /0 3/ 07 8 :0 0 19 /0 3/ 07 1 2: 00 19 /0 3/ 07 1 6: 00 19 /0 1/ 07 0 :0 0 19 /0 1/ 07 4 :0 0 19 /0 3/ 07 2 0: 00

Hypothesis triads AZB011-AZB062 from: 18-3 2007 to 19-3 2007 no data Outer Triads

18 /0 3/ 07 0 :0 0 18 /0 3/ 07 4 :0 0 18 /0 3/ 07 8 :0 0 18 /0 3/ 07 1 2: 00 19 /0 3/ 07 4 :0 0 19 /0 1/ 07 8 :0 0 Inner Triads

no data Outer Triads Inner Triads

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Table 4-4 Overview of hypothesis at hindcast moments and buoy locations of hypothesis wind growth No wind growth 011 012 021 022 031 032 041 042 051 052 061 062 No wind growth 011 012 021 022 031 032 041 042 051 052 061 062 No wind growth 011 012 021 022 031 032 041 042 051 052 061 062 11 /0 1/ 07 0 :0 0 11 /0 1/ 07 4 :0 0

Hypothesis windgrowth AZB011-AZB062 from: 11-1 2007 to 12-1 2007

Hypothesis windgrowth AZB011-AZB062 from: 18-1 2007 to 19-1 2007

11 /0 1/ 07 8 :0 0 11 /0 1/ 07 1 2: 00 11 /0 1/ 07 1 6: 00 11 /0 1/ 07 2 0: 00 12 /0 1/ 07 0 :0 0 12 /0 1/ 07 4 :0 0 18 /0 1/ 07 0 :0 0 18 /0 1/ 07 4 :0 0 18 /0 1/ 07 8 :0 0 18 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 2: 00 19 /0 1/ 07 1 6: 00 19 /0 1/ 07 2 0: 00 18 /0 1/ 07 1 6: 00 12 /0 1/ 07 8 :0 0 18 /0 3/ 07 1 6: 00 18 /0 3/ 07 2 0: 00 19 /0 3/ 07 0 :0 0 12 /0 1/ 07 1 2: 00 12 /0 1/ 07 1 6: 00 12 /0 1/ 07 2 0: 00 19 /0 3/ 07 8 :0 0 19 /0 3/ 07 1 2: 00 19 /0 3/ 07 1 6: 00 19 /0 1/ 07 0 :0 0 19 /0 1/ 07 4 :0 0 19 /0 3/ 07 2 0: 00

Hypothesis windgrowth AZB011-AZB062 from: 18-3 2007 to 19-3 2007 no data Shallow water

18 /0 3/ 07 0 :0 0 18 /0 3/ 07 4 :0 0 18 /0 3/ 07 8 :0 0 18 /0 3/ 07 1 2: 00 19 /0 3/ 07 4 :0 0 19 /0 1/ 07 8 :0 0 Deep water

no data Shallow water Deep water

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4.5 Hindcast method

Following the recommendations of WL (2007a) and the hindcast method applied by Alkyon (2007a), the hindcast method is described in the following paragraphs. For a revision of adaptations and changes compared to previous hindcasts reference is made to WL (2007a).

4.5.1 SWAN version

This hindcast uses the present version of SWAN, 40.51 with bug fix A5, as advised by

the Hydraulic Review Team. The objective of this hindcast is to qualitatively assess the SWAN model performance, using the latest version. In the future, it is possible that an improved version of SWAN will be used to determine the Hydraulic Boundary conditions (HBC 2011/2016), assumed that the performance of the SWAN model in the Wadden Sea is satisfactory. These boundary conditions will subsequently be used for input in safety considerations of the Dutch coast line. For the hindcast, SWAN is run in

stationary mode. For more details, see Booij et al. (1999) or the newly released SWAN

user manual (Delft University of Technology website of Fluid Mechanics6₎

4.5.2 Choice of grids and determination of wave boundary conditions

The hindcast is performed using 2 grids; the first curvilinear grid (the red one; called GridCL; NS2 from WL (2007b)) covers a large part of the Wadden Sea (based on the ‘Kuststrook’ model). The second (called azg3a; blue one) is a detailed curvilinear grid over the tidal inlet.

Figure 4-1: GridCL used for the simulations (every fourth line is shown). GridCL in green, outline of

grid AZG3A in blue. Labelled dots indicate buoys used for the boundary conditions.

5_{Bug fix A refers to patch A applied to the SWAN 40.51 version, released on 04-01-2007} 6

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The numerical attributes of these computational grids are summarized in Table 4-5.

Table 4-5 Numerical characteristics of the SWAN computational grids

Name Nx Ny Percentage active points

GridCL 391 161 79

AZG3A 286 412 75

Following the method applied in Haskoning (2006), the wave boundary conditions for the AZG3A grid are obtained from the wave buoys AZB11 and AZB12. These buoys provide only the boundary conditions along the northern boundary of grid AZG3A. The remaining boundary conditions are obtained from the output of the simulations performed on the grid GridCL.

For the simulations on the GridCL the measured spectra of the wave buoys SON and ELD are used. Figure 4-2 illustrates the origin of boundary conditions for GridCL. The wave buoy information of buoy ELD is used for the boundary conditions of the western boundary and a part of the northern boundary. Boundary conditions are imposed in the form of 1D spectra at the purple squares with grey labels in the figure below. The information of buoy SON is used for the eastern part of the northern boundary. Between SON and ELD SWAN interpolates the spectra. At ten locations (triangles in the figure below) output is generated which is subsequently used as input for the calculations on the smaller grid AZG3A.

Figure 4-2 Locations where boundary conditions are imposed at GridCL (purple squares with grey

labels). Output locations of spectral information indicated with triangles. GridCL in green (every fourth line is shown), outline of grid AZG3A in blue

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northeast corner the AZB12 spectrum is imposed. Between the buoys AZB11 and AZB12 information from both buoys is used by linear interpolation.

Figure 4-3 Locations where boundary conditions are imposed at the AZG3A grid (purple squares

and triangles with grey labels). GridCL in green (every fourth line is shown), outline of grid AZG3A in blue

The Northern wave boundary condition of the AZG3A grid is specified as a directional wave spectrum derived from buoy measurements of buoy AZB11 and AZB12. The boundary condition at the east and west side of the grid is derived from the GridCL grid and is specified in 2D spectra.

As mentioned above, these spectra originate from two sources; viz. computed output of the grid GridCL and measured spectra obtained from the buoys AZB11 and AZB12. The representations of measured and computed spectral information differ. The measured

spectra are provided as energy density, mean wave direction (

θ

) and directional

spreading (

σ

), each as a function of frequency. These frequencies are linearly

distributed in the interval 0.01 Hz – 0.5 Hz. The SWAN 2D spectra are given as a function of frequency and direction, where the frequencies are geometrically distributed in the interval 0.03 Hz – 1.0 Hz and direction in intervals of 10° over the full directional circle.

The measured spectra are corrected before they are applied as boundary condition. This correction step consists out of the following actions:

• Time averaging of the spectra around the hindcast moment.

0.25*(T-1)+0.5*T+0.25*(T+1);

• The mean wave direction and directional spreading is usually unreliable for

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These 1D-spectra are substituted in the data file with all 1D-spectra along the boundary points of the AZG3A grid. Along the northern boundary of the domain west of AZB11 the transformed AZB11 spectrum is imposed. East of AZB12 the transformed AZB12

spectrum is used. For the area between the buoys AZB11 and AZB12 SWAN interpolates the components of the 1D-spectra.

4.5.3 Numerical settings

The frequency range is set to 0.03 Hz – 1.0 Hz and f =0.1f. The directional space is

discretized with 36 bins of 10° over the full directional circle. Following the

recommendations of Alkyon (2007a,b) the curvature-based convergence criterion is imposed;

NUM STOPC 0.00 0.01 0.001 99.5 STAT mxitst=80

This command sets the maximum number of iterations steps to 80. SWAN stops the iteration process before this maximum number if the following conditions are satisfied in at least 99.5% of all wet grid points.

1. The relative change in local significant wave height from one iteration step to the next is less than 0.01

01 .

0 )

,

(

)

,

(

)

,

(

1 0 1 0 0

<

−

− −

j

i

H

j

i

H

j

i

H

n m n m n m

2. The curvature of the iteration of Hm0 is less than 0.001.

1 2 3 0 0 0 0 0 ( ) 0.001, 3,4,... 2 n n n n m m m m n m H H H H n H − − − − + + < = 4.5.4 Physical settings

Following WL (2006) and Haskoning (2006) the physical settings are identical to

previous storm hindcasts, with the exception of the settings for the triads. Following HRT (2006), the current default settings for the proportionality coefficient (TRFAC set to 0.05 instead of 0.1) and the maximum frequency for triad computations (CUTFR set to 2.5 instead of 2.2) are used, resulting in the following settings:

GEN3 WESTH QUAD

TRIAD TRFAC=0.05 CUTFR=2.5 BREAKING 1 0.73

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4.5.5 Bathymetry

The bathymetry of the tidal inlet of Ameland and surrounding area was obtained from depth soundings performed by RIKZ in the period 1999 through 2006. These soundings were processed by RIKZ to obtain a digital representation on regular grids with a typical resolution of 20 m. The depth values for the grid GridCL were provided together with the grid information of the Kustfijn-V4 model. For the grid AZG3A the bathymetry was obtained from various sources, in order to cover the complete domain of these grids. The following sources were used:

• Source 1 Tidal inlet of Ameland from 2004;

• Source 2 Data from a small coastal strip along the ‘kwelder’ of the Frisian

coast on a 5 m grid based on laser altimetry measurements in 2006;

• Source 3 Data for the complete Wadden Sea from 1999;

• Source 4 Tidal inlet of Vlieland from 2004;

• Source 5 Bathymetry of the Kustfijn-V4 model;

• Source 6 Additional survey data Tidal inlet of Ameland (Oct. 2007).

In WL (2007a) the bathymetry for the AZG3A computational grids was generated hierarchically from the sources above, so that for each grid point the most recent bathymetrical data was used (details explained in WL 2007a). For this study the same bathymetry as used in WL (2007a) is used. Additional the available depth surveys from 2007 were used to complement the bathymetry.

4.5.6 Water levels and currents

Water level and current fields are obtained from 2DH hydraulic simulations carried out with the WAQUA model. The WAQUA model has been calibrated on measured water levels. However, due to the fact that the model is based on equations of continuity, the currents should implicitly be reliable at a large scale given the fact that the water levels have been calibrated correctly to measurements. This does not imply that very locally, the currents at the buoy locations could not deviate from calculated currents, and hence some unreliability is still present. A procedure has been developed by Alkon (describerd in WL, 2007a) to remove spurious values in the current field in shallow areas and adapt the water level near dry points.

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Figure 4-4 Current velocity and current direction in tidal inlet of Ameland

4.5.7 Wind speed

Previously performed hindcast studies show that the sensitivity of the model performance to a varying wind field mainly is affected if sheltering effects occur, for instance in Northwesterly storm conditions behind the Wadden Sea islands. This effect is limited to a small area. The hindcast moments of 2007 all occurred with Southwesterly to Northwesterly wind conditions, and hence much sheltering of waves in the tidal inlet of Ameland is not to be expected, and wind pertains over long fetches.

As results of this consideration, the selection of wind speed is based on the

measurements at 3 wind locations, Hoorn, Huibertgat and Lauwersoog. The fetch of the hindcast moments is in most cases located over the Wadden Sea. Hence, it is likely that the measurement at Hoorn is most suitable for present hindcast moment purposes. To reduce the scatter in wind speed measurements, the hindcast wind speed is a weighed average over three wind measurement stations.

In addition to the variation in space, also quite some variation over time is observed in the wind signals. As a result, especially during the peak of the storm, a considerable variation in the signal can be seen. In addition to this, the development of the wave field also depends on the history of the wind field.

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The following weighed average is therefore used, based on 2 foregoing wind measurements.

This representative wind speed and wind direction is uniform applied in the SWAN model. In table 4-6 the deriving of representative wind speeds is illustrated.

Table 4-6 Wind and waterlevels for selected hindcast moments

Hindcast moment Wind speed

[m./s.] Wind dir [°N] Water level (Nes) [cm.] Water level (W- Tersch.) [cm.] RepU10 U10(T) U10(T-1) U10(T-2) Repdir

1/11/2007 4:00 11.88 11.97 11.53 12.3 237 136 80 1/11/2007 12:00 19.12 19.41 18.75 18.99 224 70 72 1/11/2007 13:00 19.46 19.58 19.39 19.23 228 96 80 1/11/2007 14:00 16.95 16.86 16.16 18.74 273 101 82 1/11/2007 15:00 13.46 13.27 13.23 14.49 257 122 73 1/11/2007 16:00 14.84 14.56 15.03 15.28 265 106 61 1/11/2007 16:40 14.81 14.54 15.03 15.17 264 95 46 1/11/2007 20:40 18.17 18.43 18.2 17.36 268 44 44 1/11/2007 21:20 18.7 19.19 18.25 18.14 271 63 71 1/11/2007 22:00 17.91 18.07 17.65 17.95 275 93 102 1/11/2007 22:40 18.83 19.57 18.01 18.21 279 129 130 1/12/2007 2:00 15.52 14.44 16.56 16.64 283 306 269 1/12/2007 5:00 14.33 - 14.58 13.85 281 226 166 1/12/2007 8:00 10.77 - - 10.77 271 128 88 1/18/2007 12:20 21.07 20.84 21.31 21.27 233 82 56 1/18/2007 14:00 20.24 20.41 19.42 21.3 263 60 39 1/18/2007 17:20 20.3 21.38 19.27 19.12 267 143 145 1/18/2007 18:00 20.06 19.99 20.07 20.24 268 182 169 1/18/2007 18:40 19.91 20.4 19.49 19.27 269 224 197 1/18/2007 20:40 18.85 18.66 19.16 18.81 274 281 250 1/18/2007 21:20 18.19 18.27 18.2 17.96 274 291 248 1/19/2007 7:40 13.09 13.08 13.32 12.64 271 145 137 1/19/2007 12:00 14.27 14.52 14.2 13.64 272 136 70 3/18/2007 07:40 14.78 15.88 13.61 13.82 274 110 113 3/18/2007 09:20 13.77 14.64 12.42 13.79 275 176 125 3/18/2007 10:00 13.79 14.28 13.15 13.62 279 169 121 3/18/2007 14:40 18.1 18.54 17.26 18.41 266 67 27 3/18/2007 15:40 17.91 17.57 18.65 17.48 271 63 65 3/18/2007 16:00 18.67 19.06 18.66 17.57 270 64 87 3/18/2007 17:00 17.07 16.88 16.85 18.08 268 117 141 3/18/2007 19:20 16.32 16.67 16.1 15.7 268 299 265 U10;representative = 0,5*(U10;T) + 0,33*(U10;T-1) + 0.17*(U10;T-2)

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4.5.8 Output locations

The output locations are defined along rays, following the tidal inlet and in addition following paths over the buoy locations. In addition, rays have been defined following east west directions. To investigate the windgrowth in SWAN over shallow areas, and considering the windspeeds and wave directions of the storms to be hindcasted, these rays are of interest. Finally, several rays have been defined from the lee side of the island to the innermost buoy locations, to assess the initial wave growth formulations. Grid output

The user can define output locations in SWAN, at which the program will generate either 1D or 2D spectral files, spectral parameters (e.g. Hm0, Tm01, etc) or physical parameters (Current velocity, Depth, etc.). The most convenient way is to request most of the

parameters at all computational grid points (using BLOCK ‘COMPGRID’)7_.

The following parameters are executed at all grid points;

Table 4-7 Overview of block output parameters

Parameter Description

XP YP Coordinates of grid point in x,y coordinates

DEPTH Water depth

HSIGN Significant wave height calculated from the zero-th order spectral moment

RTP Relative peak period of the spectrum

TPS Relative smoothed peak period

TM01 Mean absolute wave period from zero-th and first order moments

TM02 Mean absolute wave period from zero-th and second order moments

TMM10 Mean absolute wave period calculated from the minus first and zero-th order moment

DIR Mean wave direction

DSPR Directional spreading

WLEN Wave length

DHSIGN Difference in Hs between two final iterations DRTM01 Difference in Tm01 between two final iterations

WATLEV Water level

VEL Current Velocity

DISSIP Wave energy dissipation

Point output

Objective of this hindcast is to compare simulated waves with observed waves at the buoy locations. The SWAN output is requested at all the buoy locations. As mentioned in paragraph 2.1, the calculations and the buoy observations are sensitive to small fluctuations in time (see for instance the scatter in observed spectra) and space (the location of a wave buoy is not exactly fixed and the depth variation could influence the

7_{SWAN does not allow infinite output commands; therefore some irrelevant output blocks were omitted. For}

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result if the buoy is in more shallow waters than assumed). To analyse these fluctuations, also output locations around the wave buoy locations are requested. Another objective of the hindcast is to analyse the model behaviour in the tidal inlet of Ameland. Several output locations have been identified at which output is generated for analyses of especially numerical performance parameters. These points have also been added to the test output locations. Figure 4-5 visualises the output locations

geographically.

The parameters requested in the output in tables are identical to the block output files. For convenience, STEEPNESS is also requested at the point output. For this hindcast, the steepness is calculated from the averaged wave length and the wave height.

Figure 4-5 Output points Red point indicate buoy locations 2007

Ray output

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Figure 4-6 Output indicate rays 2007 Hotfile

The user can request a so called hotfile, which is nothing more than the 2D Spectra at all computational grid points. The file contains all the information at the final iteration of the calculation. The hotfile can be used later on, either to extract additional information, or to restart the calculation.

4.6 Pre- and post processing

As explained in the previous section each hindcasted storm moment is modelled using calculations on two different grids. The directory structure is set-up analogously, one folder for every storm with subfolders for each grid. The file handling for the SWAN run procedure (input) as well as the output are structured by means of unique file ID’s. These file ID’s relate to the hindcast storm moments and the grid definition. For

instance, the swan input file for the hindcasted storm at 10hr, 18th January 2007, is

located in the folder 20070118_1000\GridCL\ and named

20070118_1000GridCL.swn. The water level and current input data can be found in