Measurement report tidal inlet Ameland for the storm season 2004-2005

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cg\05449\1356 16 December 2005

Measurement report tidal inlet Ameland for the

storm season 2004-2005

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Document title Measurement report tidal inlet Ameland for the storm season 2004-2005

Short document title Measurement report tidal inlet Ameland Status Final Report

Date 16 December 2005 Project name Tidal inlet Ameland Project number 1356

Client RWS RIKZ Reference cg\05449\1356

Authors C. Gautier, M.J.G. van den Boomgaard Checked by M.N. Ruijter Schiehaven 13G 3024 EC Rotterdam P.O.Box 91 3000 AB Rotterdam The Netherlands T +31 - 10 - 467 13 61 F +31 - 10 - 467 45 59 E info@svasek.com I www.svasek.com

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Final Report 16 December 2005

cg\05449\1356

CONTENTS

1 INTRODUCTION 1

1.1 General 1

1.2 Background 1

1.3 Wave measurements in a tidal inlet 1

1.4 Objective 2 1.5 Set up of report 2 1.6 Organization 3 2 MEASUREMENTS 5 2.1 Introduction 5 2.2 Measurement locations 5 2.3 Wave measurements 8 2.3.1 General 8 2.3.2 Non-directional waverider 8 2.3.3 Directional Waverider 8 2.3.4 Data-handling 8

2.4 Data files and parameters 9

2.5 Measuring period and registration density 9

3 ANALYSIS OF THE WAVE MEASUREMENTS 13

4 INFLUENCE SAMPLING FREQUENCY ON THE RELIABILITY OF THE

MEASUREMENTS 15

4.1 Introduction 15

4.2 Theory of the sampling frequency of buoys 15 4.3 Comparison non-directional and directional waverider 17

4.3.1 Available data 17

4.3.2 Spectra comparison non-directional and directional waverider 18 4.3.3 Comparison wave parameters waverider and directional

waverider 21 4.4 Influence high frequent energy 22

5 STORM EVENTS 27

5.1 Introduction 27

5.2 Storm period 1 – 3 January 2005 27

5.2.1 General 27

5.2.2 Wind and water level measurements 28

5.2.3 Wave measurement 29

5.3 Storm period 7 – 9 January 2005 31

5.3.1 General 31

5.3.2 Wind and water level measurements 32

5.3.3 Wave measurements 32

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6 CONCLUSIONS ANS RECOMMENDATIONS 36

6.1 Conclusions 36

6.2 Recommendations 37

LITERATURE 38

APPENDICES 39

Appendix 2.1 Description of the data files... A1 Appendix 2.2A Availability of the wave parameters Hm0,Tm02, Tm-10, Th0, Th010 for each

location... A3 Appendix 2.2B Availablility of the wave spectra (Czz10 and Czz10_M), water level, wind

velocity, wind direction for each location... A4 Appendix 3.1A Time series of the wave height Hm0 and the wave direction Th0 of the

locations AZB1* and AZB2* for the period 1/12/04 tot 1/3/05 ... A5 Appendix 3.1B Time series of the wave height Hm0 and the wave direction Th0 of the

locations AZB3*, AZB4* and AZB5* for the period 1/12/04 - 1/3/05... A6 Appendix 3.1C Time series of the wave height Hm0 and the wave direction Th0 of the

locations AZB1* and AZB2* for the period 1/3/05 - 1/5/05... A7 Appendix 3.1D Time series of the wave height Hm0 and the wave direction Th0 of the

locations AZB3*, AZB4* and AZB5* for the period 1/3/05 - 1/5/05... A8 Appendix 3.2A Time series of the wave periods Tm02 en Tm-10 of the locations AZB1* and AZB2* for the period 1/12/03 tot 1/3/05 ... A9 Appendix 3.2B Time series of the wave periods Tm02 en Tm-10 of the locations AZB3*,

AZB4* and AZB5* for the period 1/12/03 tot 1/3/05 ... A10 Appendix 3.2C Time series of the wave periods Tm02 en Tm-10 of the locations AZB1* and

AZB2* for the period 1/3/05 - 1/5/05 ... A11 Appendix 3.2D Time series of the wave periods Tm02 en Tm-10 of the locations AZB3*,

AZB4* and AZB5* for the period 1/3/05 - 1/5/05... A12 Appendix 3.3A Time series of the wind parameters of the locations F3 and Lauwersoog for

the period 1/12/04 tot 1/3/05... A13 Appendix 3.3B Time series of the wind parameters of the locations F3 and Lauwersoog for

the period 1/3/05 - 1/5/05 ... A14 Appendix 3.4 Time series of the water level for the survey period 2004/2005... A15 Appendix 3.5 Pivot tables wave height Hm0 versus wave period Tm02 for the number 1

locations ... A16 Appendix 3.6 Pivot tables wave height Hm0 versus wave direction Th0 for the number 1

locations ... A17 Appendix 3.7 Pivot table wind velocity U10 versus wind direction Th0 for Lauwersoog. A18 Appendix 4.1 Description of the calculation of the spectral parameters from the spectra up to 1 Hz... A19 Appendix 4.2 Spectra 5 januari 2005 1:30 of the locations 3 en 4 ... A20 Appendix 4.3 Spectra 13 februari 2005 11:00 of location 3... A21 Appendix 4.4 Spectra 15 Februari 2005 16:00 of location 4... A21 Appendix 4.5 Spectra 16 februari 2005 3:00 of location 3... A21 Appendix 4.5 Spectra 16 februari 2005 3:00 of location 3... A22 Appendix 4.6 Spectra 22 februari 2005 14:20 of location 4... A22 Appendix 4.7 Spectra 16 maart 2005 19:50 of location 3... A23 Appendix 4.8 Spectra 6 april 2005 19:20 of the locations 3 and 4 ... A24 Appendix 4.9 Spectra 15 april 2005 22:30 of location 4 ... A25

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cg\05449\1356 Appendix 4.10 Power spectral density of AZB32 for 4 januari 2005 om 00:30 uur... A26 Appendix 4.11 Difference between Hs (0 - 0.5 Hz) and Hs (0 – 1 Hz) ... A27 Appendix 4.12 Scatterplots of HS versus Tm02 (0-0.5 Hz) and Tm02 (0-1 Hz) ... A28 Appendix 5.1A Power spectral density of 2 jan 2005 at 7, 9 en 11h ... A29 Appendix 5.1B Power spectral density of 2 jan 2005 at 13,15 en 17h ... A30 Appendix 5.2A Power spectral density of 8 jan 2005 at 12,14 en16h ... A31 Appendix 5.2B Power spectral density of 8 jan 2005 at 18,20 and 22h... A32

LIST OF FIGURES

Figure 1.1: Overview Tidal inlet system ... 2

Figure 2.1: Wave measurement locations tidal inlet of Ameland 2004-2005... 6

Figure 2.2: The used wind and water level measurement locations... 7

Figure 4.1: Power spectral density of 0.4 m, 0.7 m and 0.9 m diameter buoy. The upper arrows indicate the cut-off frequency (Source: ref 3). ... 16

Figure 4.2: Influence sampling frequency on the measured wave ... 17

Figure 4.3: Differences in the measured wave height between the locations AZB31-AZB32 and AZB41-AZB42 (based on the frequency range of 0 -0.5 Hz) ... 22

Figure 5.1: Weather chart 2-1-2005 (Source: ref 7) ... 27

Figure 5.2: Time series of the wind, water level and wave conditions of 1-3 January 2005 . 28 Figure 5.3: Time series of Hm0 and Tm-10 of all the measurement locations of 1-3 January 2005. ... 29

Figure 5.5: Time series of the wind, water level and wave conditions of 7 - 9 January 2005 32 Figure 5.6: Time series of Hm0 and Tm-10 for all the measurement locations of 7-9 January 2005. ... 33

LIST OF TABLES Table 2.1: Overview wave measurement locations (type, sample frequency, coordinates and depth) ... 6

Table 2.2: Overview water level and wind measurement locations ... 7

Table 2.3: Availability of the different parameters for all the measurement locations during the survey period 2004/2005 ... 10

Table 3.1: Characteristics of the wave height Hmo and the wave period Tm02 ... 13

Table 4.1 Cut-off frequency ... 16

Table 4.2: Data files relevant for the comparison between the different buoy types... 18

Table 4.3: Measured Hm0 en Tm02 at 5/1/2005 1:30 at main locations 3 and 4 ... 19

Table 4.4: Measured Hm0 en Tm02 at 13/2/2005 1:30 at main location 3 ... 19

Table 4.7: Measures Hm0 en Tm02 at 22/2/2005 14:20 at main location 4 ... 20

Table 4.9: Measured Hm0 en Tm02 at 6/4/2005 19:20 at main locations 3 and 4 ... 21

Table 4.11: Difference Hm0 between the locations AZB31-AZB32 en AZB41-AZB42 ... 22

Table 4.12: The average wave height up to 0.5 Hz and up to 1.0 Hz, and the absolute differences... 23

Table 4.13: The mean differences of the wave height up to 0.5 Hz and up to 1.0 Hz, absolute and relative... 24

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cg\05449\1356 Table 4.14: The average wave period to 0.5 Hz and up to 1.0 Hz, and the absolute

differences... 24 Table 4.15: The mean differences of the wave period up to 0.5 Hz and up to 1.0 Hz, absolute

and relative... 25 Table 5.1: Wave height Hm0 and wave period Tm-10 belonging to the spectra of 2 January 2005 (Appendix 5.1) ... 30 Table 5.2: Wave height Hm0 and wave period Tm-10 belonging to the spectra of 8 January

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Measurement report tidal inlet Ameland cg\05449\1356

16 December 2005

1 INTRODUCTION 1.1 General

This report contains the analysis of the measurements by ten wave buoys near the tidal inlet of Ameland during the storm season 2004-2005 (1 December 2004 - 1 May 2005). This report has been made by Svašek Hydraulics, on request of RWS RIKZ.

1.2 Background

Seawalls and dikes protect the Netherlands against flooding from the North Sea, the main rivers, the Markermeer and the IJsselmeer. By law ('Wet op de Waterkering') these dikes and sea defences must be tested every five years to check whether they can guarantee the prescribed safety for the Hydraulic Conditions in terms of waterlevel and waves.

The critical wave conditions which are required for this testing are assessed on the basis of long term (25 year) wave measurements at five permanent measuring locations on deep water. A translation of the wave conditions in deep water to the wave conditions in shallow water (just in front of the dikes) are made by means of the numerical wave model SWAN.

In order to assess reliable wave conditions, a reliable wave model is required. For the verification of the present wave modelling, observations near the coast are required.

1.3 Wave measurements in a tidal inlet

Wave measurements are performed on different locations along the closed Dutch coast (near Petten) and in estuaries (in the Wester- and Oosterschelde).

A hypothesis is that the waves in the Wadden Sea at high water levels (storm surge) are possiblystrongly influenced by waves coming from the North Sea, through the tidal inlet (see Figure 1.1). Since the end of 2003 wave measurements are carried out in the Wadden Sea as well. These wave measurements are carried out in order to gain insight in the development of the wave in the Wadden Sea and to tune and validate numerical wave models (like SWAN).

Hordijk, 2004 [ref 6] reported the wave measurements during the storm season of 2003-2004. A conclusion of this report was that the penetration of the waves from the North Sea into the Wadden Sea indeed strongly depends on the water level.

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16 December 2005 Figure 1.1: Overview Tidal inlet system

1.4 Objective

The objective of the survey campaign in the tidal inlet of Ameland can be defined as:

To monitor the development of the waves coming from the relatively deep North Sea, trough the tidal inlet, into the Wadden Sea during storm events. The main interest is the possible penetration of long waves with a wave period of 10 to 15 seconds, coming from the northwest direction. These waves could have a substantial impact on the wave run up and overtopping at the dikes along the Wadden Sea.

The objective of this report can be divided in three parts:

1) To give an overview of the wave measurements in the tidal inlet of Ameland for the storm season 2004-2005.

2) To compare the results of the directional waveriders (sampling with a frequency of 1.28 Hz) with the results of the non-directional waveriders (sampling with a frequency of 2.56 Hz).

3) To select two storm periods and describe the monitoring results for these periods in detail.

1.5 Set up of report

First, an overview of the measurement locations and measurement periods is given. Subsequently the instruments and the availability of the data files are described (see chapter 2). All data is in digital format and provided by RWS RIKZ.

BUITENDELTA ZEEGAT VLOEDKOM WADDEN-EILAND EILAND NOORDZEE NORTH SEA OUTER DELTA TIDAL INLET ISLAND ISLAND INNER DELTA

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16 December 2005 A brief analysis of the wave measurements for the survey period 2004/2005 is given in chapter 3. The tables and graphs present among others time series of the parameters and the exceedence frequencies of the values of the different parameters. The tables and graphs are used for a quality check of the data and to obtain an overview of the results of the wave measurements of this survey period.

It was not the intention of this analysis to adjust the received data files (reparation, validation, interpolation and suchlike). A first analysis of the data on validation is carried out by the “Meetdienst Noord-Holland” of RWS. In spite of this, it is still possible that the data sets can have incompletenesses.

This survey period two types of buoys are used. The main reason of the use of a second type of wave buoy is the higher sampling frequency of the second buoy, which is

important in order to measure short waves accurately [ref 9]. The influence of the higher sampling frequency on the accuracy of the measurements is analysed in chapter 4. Besides, two interesting storm events within the survey period are selected. In chapter 5 the two selected storms are discussed in more detail.

Finally in chapter 6, conclusions are drawn and recommendations are given.

1.6 Organization

The survey campaign in the tidal inlet of Ameland is part of the project “Strenght and loads Sea Defences (SWB)” of Rijkswaterstaat in the Netherlands. The wave measurements of the SWB project are done by order of RWS RIKZ.

The realization, the management, the maintenance and the data-handling (collect, compute and the storage of the data) of the survey campaign in the tidal inlet of Ameland are performed by Directorate North Sea (DNZ) and Directorate North Netherlands (DNN) of Rijkwaterstaat in the Netherlands. The co-ordination of these activities is with DNZ.

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16 December 2005

2 MEASUREMENTS 2.1 Introduction

This report focuses on the wave measurements in the tidal inlet of Ameland during the storm season 2004-2005 (1 December 2004 – 1 May 2005). In the analysis of the wave measurement wind and water level measurements are used as well.

This chapter contains a general description of the measurement locations, the measurement instruments, the measurement periods and the parameters.

2.2 Measurement locations

Wave measurements

At the end of 2003 the wave buoys were situated along a transect starting at the North Sea through the tidal inlet of Ameland into the Wadden Sea. The buoys were situated at five main locations:

1) Location 1 at the North Sea.

2) Location 2 on the sea side of the outer delta.

3) Location 3 in the tidal gorge, at the head of Ameland. 4) Location 4 and 5 in the Wadden Sea (flood tidal delta).

Environment limitations (like, closed areas, shipping routes, suppletion activities, wrecks, etc.) and the technical limitations of the instruments (like, the water depth, the slope of the bottom with respect to the anchorage) were taken into consideration when determining the buoy locations.

In the previous survey period (2003-2004) a back-up buoy was placed at main location 1 (which is a very important location in order to compose hydraulic boundary conditions for potential hindcasts) to minimize the risk of lacking data (for example by mechanical failure of a buoy). This survey period (2204-2005) a back- up buoy is not placed at main location 1 only, but at every main measurement location, resulting in the use of ten wave buoys in survey period 2004-2005. The nomenclature of the wave buoys is as follows: AZBhb. Hereby h stands for the main measurement location (so, 1, 2,…5) and b for buoy 1 or buoy 2 at this main location.

This survey period a second type of wave buoy was added to the measurement campaign in the tidal inlet of Ameland (“Amelander Zeegat”) namely the non-directional waverider. The main reason for using a second buoy type is the higher sampling frequency of the non-directional waverider (which is important for measuring short waves). At the measurement locations positioned further into the tidal inlet of Ameland, the local waves (<2 seconds) will form an important part of the spectrum. For this reason the non-directional waveriders are placed at these locations. Paragraph 2.3 examines the two buoy types further.

Figure 2.1 shows the positions of the ten waveriders. The coordinates of the waveriders, the type of waverider and the local depths can be found in Table 2.1.

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16 December 2005 Figure 2.1: Wave measurement locations tidal inlet of Ameland 2004-2005

Location Buoy Coordinates (RD) Depth [m] Code Type f [Hz] X [m] Y [m] 1 AZB11 DWR 1.28 161240 613520 NAP - 19m AZB12 DWR 1.28 164990 614010 NAP - 19.5m 2 AZB21 DWR 1.28 167200 610400 NAP - 11m AZB22 DWR 1.28 167610 610400 NAP - 11.3m 3 AZB31 DWR 1.28 169380 607320 NAP - 9.2m AZB32 WR 2.56 169450 607110 NAP - 9.1m 4 AZB41 DWR 1.28 171340 604400 NAP - 16.7m AZB42 WR 2.56 171500 604250 NAP - 17.6m 5 AZB51 WR 2.56 174290 601500 NAP – 6.9m AZB52 WR 2.56 175600 600820 NAP - 13.4m

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16 December 2005 Wind- and water level measurements

In addition to the wave measurements, water level and wind measurements are used to get a complete impression of the conditions during the monitoring period.

The positions of the used water level and wind measurement locations are presented in Figure 2.2.

Figure 2.2: The used wind and water level measurement locations

The coordinates of these locations are given in Table 2.2.

Coordinates (RD)

Parameter X [m] Y [m] NES Water level 179810 604920 F3 Wind velocity and wind direction 112889 763066

Lauwersoog Wind velocity and wind direction 208850 602790 Texelhors Wind velocity and wind direction 112130 557109

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16 December 2005

2.3 Wave measurements

2.3.1 General

In the measurement campaign in the tidal inlet of Ameland two kinds of wave buoys are used, the waverider and the non-directional waverider. In the previous survey period (2003-2004) only directional waveriders were used. The directional waveriders are used at locations where the information about the wave direction is important. The sampling frequency of the directional waverider (1.28 Hz) is less appropriate to measure high frequent energy [ref 6]. For this reason the non-directional waveriders with a higher sampling frequency (2.56 Hz) are added at the end of 2004. These non-directional waveriders are placed at the locations situated further into the tidal inlet of Ameland, because of the expectation that at these locations local waves (<2s) will form an important part of the wave energy spectrum (see Table 2.1 and Figure 2.2). 2.3.2 Non-directional waverider

The non-directional waverider is a spherical, 0.7 m diameter, buoy in which the sensors, the electronics, the transmitter and the batteries are placed. The buoy consists of a stabilized platform with an accelerometer and tracks the acceleration in vertical direction caused by the wave forces on the buoy. From this the change of height of the sea surface is calculated from which the wave characteristics can be determined. This signal is transferred by a HF-link to a receiver onshore. The non-directional waveriders used in this measurement campaign are produced by Datawell. The sampling frequency of these waveriders is 2.56 Hz. These waveriders can not measure the wave direction. [ref1]

2.3.3 Directional Waverider

De Datawell directional waverider is a spherical, 0.9m diameter, buoy which measures wave height and wave direction. This spherical buoy is closed and inside the sensors, the electronics, the transmitter and the batteries are placed. The buoy consists of a stabilized horizontal platform with an accelerometer for tracking the acceleration in the vertical direction. Furthermore a compass, an accelerometer for tracking the

acceleration in the horizontal direction and pitch- and roll sensors are installed in the buoy. With these measured accelerations of the buoy movements in vertical direction and in the compass directions the wave height and wave direction are determined. The waverider transmits the data though a radio contact to a receiver onshore. The sampling frequency of this waverider (1.28 Hz) is smaller than the sampling frequency of the non-directional waverider. The non-directional waveriders used in this measurement campaign are produced by Datawell. [ref 2]

2.3.4 Data-handling

De signals of the waveriders are transmitted through the lighthouse of Ameland and collected at a location in Ferwerd. From here the data is processed and saved by the RMI applications of “Meetnet Noordzee”. The RMI processing is based on SWAP (Standard Wave Analysis Package). This is a standard analysis module within

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16 December 2005 Rijkswaterstaat for processing of wave sensor data [Hoekstra, 1994 [ref 5]]. In this process of analysing first of all the data are automatically checked on unrealistic values and stagnations, followed by the calculation of the wave parameters and spectra. The results are stored in the operational database of “Meetnet Noordzee” and the results are saved in a yearly database and in DONAR as well. DONAR is a long-term database of measurement data of Rijkswaterstaat. Before the data is put in DONAR (seven days after the actual measurement) the data is visually controlled on the interconsistency by DNZ.

2.4 Data files and parameters

In this report the following parameters are used: Waves

• Hm0 The average significant wave height of the 10 mHz power spectral

density of 30 – 500 mHz.

• Tm02 The average wave period, defined as m0/m2 of the10 mHz power

spectral density of 30 – 500 mHz.

• Tm-1,0 The average wave period, defined as m-1/m0 of the power spectral density

of 30 – 500 mHz.

• Th0 The average wave direction, regard to the north, of the power spectral

density of 30 – 500 mHz.

• Th010 Main spectral wave direction regard to the north of 30- 500 Hz.

• Czz10 Power spectral density of 0 – 500 mHz .

• Czz10 Power spectral density of 0 – 1000 mHz

Water level

• H10 10-min average water level regard to NAP. Wind

• WS10 10-min average wind velocity measured at 10 m height. • WR10 10-min average wind direction measured at 10 m height. A short description of the data files can be found in Appendix 2.1.

2.5 Measuring period and registration density

This report addresses to the survey period of 1 December 2004 up to 1 May 2005. Not all the used measurement instruments have operated properly during the whole survey period. Table 2.3 presents the registration density, defined as the ratio between the number of available validated observations to and the maximum number of observations in the whole survey period (1 December 2004 up to 1 May 2005 (with ∆t=10 min) = 21744 observations).

The time convention in all the data files is MET (Mid-European-Time (which is the same as the Dutch “wintertime”) and the time step in each data file is ten minutes.

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16 December 2005

Waves

Start End Time step Registration density

AZB11 Czz, Hm0, Tm02,Tm-10,Th0, Th010 2 december 2004 30 april 2005 10 min. 98%

AZB12 Czz, Hm0, Tm02, Tm-10, Th0, Th010 1 december 2004 30 april 2005 10 min. 79%

AZB21 Czz, Hm0, Tm02, Tm-10,Th0, Th010 1 february 2005 30 april 2005 10 min. 38%

AZB22 Czz, Hm0, Tm02, Tm-10, Th0, Th010 1 february 2005 30 april 2005 10 min. 59%

AZB32 Czz, Hm0, Tm02, Tm-10 2 december 2004 30 april 2005 10 min. 82%

AZB42 Czz, Hm0, Tm02, Tm-10 28 december 2004 30 april 2005 10 min. 82%

AZB51 Czz, Hm0, Tm02 2 december 2004 30 april 2005 10 min. 86%

Tm-10 1 february 2005 30 april 2005 10 min. 58%

AZB52 Czz, Hm0, Tm02, Tm-10 2 december 2004 30 april 2005 10 min. 86% Wind

F-3 Wind velocity and wind direction 1 december 2004 30 april 2005 10 min. 92% Lauwersoog Wind velocity and wind direction 1 december 2004 30 april 2005 10 min. 93% Texelhors Wind velocity and wind direction 1 december 2004 30 april 2005 10 min. 93%

Water level

NES Water level 1 december 2004 30 april 2005 10 min. 100%

Table 2.3: Availability of the different parameters for all the measurement locations during the survey period 2004/2005

Table 2.3 shows that the water level data (measured at NES) are available for the whole survey period (registration density = 100%). The wind measurements contain only a few interruptions which is expressed by the high registration density (92-93%). There is a bigger variation in the performance of the wave buoys. Conspicuously is the low registration density of the waveriders AZB21 and AZB22 (38% resp. 59%). It should be mentioned that the different registration density of the different wave parameters of buoy AZB51 is unexpected. The cause of the absence of the data of the wave parameter Tm-10 for the period 2 December to 1 February cannot be accessed to this moment. At a few measurement locations the causes of a lower availability of the data are discussed in the progress reports of the project “Wave measurement campaign in the tidal inlet of Ameland” [ref 10, 11, 12]:

• There was a problem in the data communication in the period December 2004-January 2005, resulting in a disturbance of the receiving signal of the wave buoys. Location AZB32 was the most sensitive location because the deflection of the transmission frequency of this buoy is the biggest. This problem causes some failure of buoy AZB32 in February as well.

• On March 30 between 11:30 and 12:30 a power cut occurred. This electricity failure was caused by excavations of KPN, in which an electricity cable was damaged. Due to the power cut onshore all the wave buoys data could not be processed in this period.

• The waverider AZB21 is lost since 12 March, resulting in missing data. In the second half of April a new buoy is placed at the same location.

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16 December 2005 • From 22 March to 30 March 2005 the buoy at the location AZB12 had some

problems.

The two failures of the wave buoys AZB12 and AZB21 mentioned above, were probably caused by the fact that the anchoring of these buoys was destroyed by the propeller of a passing ship.

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16 December 2005

3 ANALYSIS OF THE WAVE MEASUREMENTS

In order to get an impression of the variations of the measured parameters time series of the different parameters for the whole survey period 2004/2005 are plotted. These time series can be found in the following appendices:

Appendices 3.1A, 3.1B, 3.1C en 3.1D: Time series of the wave height Hm0 and

the wave direction Th0 for all the

measurement locations.

Appendices 3.2A, 3.2B, 3.2C en 3.2D: Time series of the wave periods Tm02 and

Tm-10 for all the measurement locations.

Appendix 3.3A en 3.3B: Time series of the wind direction and wind velocity for the measurement locations F-3 and Lauwersoog.

Appendix 3.4: Time series of the water level measured at NES.

On the basis of the time series, two storm events are selected and analysed more closely in chapter five.

For all measurements and for each location the mean and the values of the exceedence possibilities of 0.1%, 1%, 5%, 10% en 50% are determined for two wave parameters (Hm0 and Tm02). For this determination the 10-minutes mean values are used. The

maximum value of the parameter Hm0 is calculated as well. The results are presented in

Table 3.1.

The exceedence frequency is expressed as a percentage. Since the survey period counts 151 days (December, January, February, March and April) 1% of the time corresponds with 1.51 days of the survey period. Noted that these exceedence

frequencies are based on the short survey period (December 2004- April 2005), so they can be used for this period only.

Hm0 [cm] Tm02 [s]

Max Mean Exceedence frequency Max Mean Exceedence frequency [cm] [cm] 0.1% 1% 5% 10% 50% [s] [s] 0.1% 1% 5% 10% 50% AZB11 631 160 558 490 383 321 136 8.8 5.0 8.2 7.7 7.1 6.7 4.9 AZB12 580 177 529 480 389 331 154 8.3 5.1 8.0 7.7 7.1 6.7 5.1 AZB21 243 85 228 206 162 144 78 7.3 4.2 7.1 6.1 5.5 5.1 4.1 AZB22 232 81 221 196 157 141 76 7.8 4.1 7.2 6.2 5.5 5.1 4.0 AZB31 243 70 214 189 151 130 63 7.6 4.1 6.9 6.1 5.4 5.1 3.9 AZB32 224 64 207 172 142 122 57 7.5 4.1 7.0 6.0 5.4 5.1 4.0 AZB41 154 28 127 99 70 57 24 6.8 3.5 6.2 5.7 5.2 4.8 3.3 AZB42 141 27 125 95 67 54 22 11.8 3.6 7.8 6.3 5.4 5.0 3.3 AZB51 128 20 117 90 60 45 14 10.9 2.9 7.0 5.7 4.5 3.7 2.6 AZB52 134 19 106 83 58 46 13 12.3 3.1 9.6 6.8 5.2 4.5 2.7 Table 3.1: Characteristics of the wave height Hmo and the wave period Tm02

The wave height Hm0 decreases as the distance of the measurement location to the

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16 December 2005 situated the most into the tidal inlet of Ameland (AZB52) is only twelve percent of the average measured wave height at the North Sea location (AZB11). Also the wave height Hm0 at AZB11 which is exceeded 0.1% of the time is approximately 20% lower compared

to location AZB52.

For the wave period Tm02 it is less unequivocal, but generally it turned out (see Table

3.1) that the wave period Tm02 also decreases as the distance of the location to the

North sea increases. The average wave period measured at AZB52 is only 38% of the average wave period measured at location AZB11. Expectations are that the long waves refract to the slopes of the channel after which dissipation on the slopes of the channel occurs resulting in a low penetration of these long waves into the tidal inlet of Ameland and only short waves will reach location AZB52. Besides, local waves will be generated with short wave periods because of the short fetch.

Various pivot tables are created in order to show the relation between the different parameters. These pivot tables can be found in the following appendices:

Appendix 3.5: Pivot tables of the wave height Hm0 versus the wave period Tm02

for the survey period 2004/2005 (December - April) for all the number one locations.

Appendix 3.6: Pivot tables of the wave height Hm0 versus wave direction Th0 for

this survey period for all the number one locations (except for main location 5, because at this location only non-directional waveriders are used).

Appendix 3.7: Pivot table of the wind velocity versus wind direction of this survey period measured at Lauwersoog.

The pivot tables present a lot of data in compact form. In this way data anomalies can be retrieved, but in these data sets it hardly happens. Furthermore it shows for example that almost 100% of the waves at location AZB51 are lower than 0.5 m and that

approximately 2/3 part of the waves measured at AZB51 have a wave period between 2 and 3 seconds. At location AZB11 the waves are 20% of the time higher than 2.5 m and the belonging wave periods are always bigger than 5 seconds.

The pivot tables of the wave height versus the wave direction (Appendix 3.6) show that most of the time the waves are coming from the northwest quadrant (N, NW and W). Only 4% of the time the waves were coming from the sectors east, southeast, south and southwest. With these wave directions the wave height at AZB11 is height lower then 2 m. At all measurement locations the dominant wave direction is northwest, except at main location 2 where the dominant wave direction is north. With waves coming from north location AZB41 is sheltered by Ameland resulting in very limited north sector. Appendix 3.7 shows that 26 % of the time the wind was coming from southwest and 80% of the time the wind velocity was lower than 10 m/s. Wind velocities higher than 15 m/s were coming mainly from southwest to northwest. Half of the time the wind was coming from south, southwest and west.

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16 December 2005

4 INFLUENCE SAMPLING FREQUENCY ON THE RELIABILITY OF THE

MEASUREMENTS 4.1 Introduction

In the survey period 2003/2004 only directional waveriders with a sampling frequency of 1.28 Hz were used. In Roskam, 2004 [ref 9] it is concluded that the sampling frequency of 1.28 Hz is probably too low for measurements in the Wadden Sea. This is because in the Wadden Sea the locally generated short waves can form an important part of the spectrum. In the previous measurement report of the tidal inlet of Ameland (2003-2004) [ref 6] it was recommended to determine the spectra with an upper limit of 1Hz only. For this reason four waveriders with a sampling frequency of 2.56 Hz are added this survey period. At the main locations 3 and 4 a directional waverider and a non-directional waverider are situated close to each other. The other two non-directional waveriders are used at location 5 and those are situated much more apart of each other (see Figure 2.1 and Table 2.1).

The difference between the two kinds of buoy and especially the influence of the high frequent energy on the accuracy of the measured waves is treated in this chapter. First a theoretical consideration of the sample frequency is given in paragraph 4.2. Paragraph 4.3. gives the comparison of the results of the measurements between the waverider and the directional waverider. The influence of the higher sampling frequency is shown in paragraph 4.4.

4.2 Theory of the sampling frequency of buoys

An orbital wave measuring buoy should follow the motion of the water particles at the surface. However, due to the finite size of the buoy it will not be able to follow waves of sufficiently small wavelengths. The limiting frequency where the buoy just can follow the motion of the water particles at the surface is called the cut-off frequency and depends on the buoy diameter.

Besides, the high frequent waves can be near the resonance frequency of the buoy resulting in an oscillation of the wave buoy which is much higher than the amplitude of the waves (order 200%) [ref 4]. This problem can slightly be resolved with a correction based on the transfer function. This correction can be executed by the SWAP module [ref 5]. The data used in this report is executed by SWAP and therefore the used data is corrected for these resonance effects.

Theoretically, the high-frequency end of a wave spectrum follows an f-5 law, where f is the wave frequency. A double-logarithmic plot of power spectral density (PSD) against frequency should therefore show a descending line. The line continues as long as the buoy is able to follow the waves, i.e. up to the cut-off frequency. Subsequently a

deviation of the measured wave spectra from the empirical line appears, see Figure 4.1. This Figure concerns wave measurements on the North Sea in 15 m of water with Hs about 0.75 m and a peak period about 5 s, both based on 4 hour trail with three wave buoys all with different diameters [ref 3]. The deviation of the missing contributions to the integral bounded by the empirical line and the measured wave spectra are 0.8%, 1.0% en 1.3% of the wave height for buoys with a diameter of 0.4 m, 0.7 m and 0.9 m. If the

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16 December 2005 wave height increases the deviation diminishes. For Hs is 2 m the values would read

0.2%, 0.3% en 0.4%.

Figure 4.1: Power spectral density of 0.4 m, 0.7 m and 0.9 m diameter buoy. The upper arrows indicate the cut-off frequency (Source: ref 3).

The manufacturer Datawell operates with a cut-off frequency for the wave buoys with a diameter of 0.4 m, 0.7 m en 0.9 m of respectively 1.00 Hz, 0.88 Hz en 0.78 Hz, see Table 4.1.

Table 4.1 Cut-off frequency

Besides the size of the buoy and the resonance frequency, the sampling frequency determines the accuracy of the registration of the movement of the water surface as well. The buoy measures short waves more accurate when the sampling frequency is higher.

As an example a time series of a sine curve (blue line) with a wave period of 1 second is presented in Figure 4.2. Within this figure two fictive measured waves, with a different sampling frequency, are plotted. This figure shows how the measured waves will be transformed with a decreasing sampling frequency. The original wave with a wave period of 1 second is measured with the sampling frequency of 2.56 Hz as a wave with a wave period of ca 1 second and by the sampling frequency of 1.28 Hz as a wave with a wave period of ca 3.5 seconds. With a smaller sample frequency less points are

sampled so that short waves can not be measured but are shifted to larger periods. The

General Tidal inlet of Ameland

buoy diameter [m] cut-off frequency [Hz] Buoy Sampling frequency [Hz]

0.4 1.00 - -

0.7 0.88 wave rider 2.56

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16 December 2005 energy at the higher frequencies is not fully lost when using the lower sample frequency but it is - wrongly - attributed to the lower frequencies. So the sample frequency has more influence on wave period than on wave height. If high frequency energy is present, the directional waverider overestimates the wave period.

Figure 4.2: Influence sampling frequency on the measured wave

For measurements with sample frequency 1.28 Hz a reliable upper limit of the wave spectrum is 0.5 Hz. Unfortunately, a sample frequency of 2.56 Hz does not automatically stretch the upper limit to 1.0 Hz. The buoy diameter and cut-off frequency reduce this upper limit to 0.88 Hz.

4.3 Comparison non-directional and directional waverider

4.3.1 Available data

In Table 4.2 the relevant data for the comparison of the two buoy types is given. The received data files “Hm0.txt” and “Tm02.txt" are based on the frequency range of 0.03 – 0.5 Hz, even for the waveriders with a larger frequency range. Besides, the wave height Hm0 and the wave period Tm02 have also been calculated based on the data files of the

spectra up to 1 Hz. The calculated wave heights are generally slightly larger than the wave heights from the received data files.

NB 1: Because the frequency range of the spectra of the non-directional waveriders continues to 1 Hz we use the data up to 1.0 Hz, although the cut-off frequency is 0.88 Hz. Although the energy between 0.88 Hz and 1.00 Hz will be

underestimated, but it is always better to include this small amount of high frequency energy than omitting it entirely.

NB 2: In this report sometimes a frequency range starting from 0 Hz is mentioned and sometimes from 0.03 Hz. This does not make any difference because in the preceding data processing the measured energy between 0 and 0.03 Hz is already filtered off.

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 Tijd [s] gol fhoo gt e [ m ] inwinfrequentie 1.28Hz inwinfrequentie 2.56Hz

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16 December 2005

Sample frequency Hm0.txt Tm02.txt spectrum Czz Spectrum Czz 0.03-0.5 Hz 0.03-0.5 Hz 0-0.5 Hz 0-1.0 Hz AZB31 1.28 Hz x x x AZB32 2.56 Hz x x X AZB41 1.28 Hz x x x AZB42 2.56 Hz x x X AZB51 2.56 Hz x x X AZB52 2.56 Hz x x X

Table 4.2: Data files relevant for the comparison between the different buoy types

Direct comparison of the measuring data of the waverider and the directional waverider does only make sense if both buoys are situated close to each other with more or less the same wave field. Both on location 3 and location 4, the two buoys have a mutual distance of circa 200 m and lay in more or less equal water depth.

4.3.2 Spectra comparison non-directional and directional waverider

In order to make a comparison between the wave buoys with different sampling frequencies, this paragraph examines the spectra of the wave buoys at various moments. It is assumed that the directional waverider makes use of an anti-aliasing filter.

The moments are selected by searching for relatively small wave periods. The wave height must not be too small. In addition we have searched for small and big differences between the spectra. On request of RWS RIKZ some moments during the February storm (12-16 February) are considered as well.

The following moments are selected for the spectral comparison of the different sampling frequencies:

5 January 2005 1:30 for main locations 3 and 4

In Appendix 4.2 the spectra of the main locations 3 and 4 on 5 January 2005 1:30 are shown. There is a difference (converted more than 10 % in wave height) between the spectra of main location 3, not in the tail of the spectra, but between the 0 and 0.5 Hz. This implies that the difference between the spectra is not always a result of the different sampling frequency. The difference can be found in the measured wave height at the locations both based on the frequency range of 0- 0.5 Hz as well (108 cm at AZB31 and 91 cm at AZB32). Although the high frequency energy (>0.5 Hz) is small, there is a difference in wave period (Tm02 of AZB31 is 3.8 s and of AZB32 3.3 s).

At location 4, the non-directional waverider with the higher sampling frequency shows relatively much energy at frequencies above 0.5 Hz, which is not measured by the directional waverider with the smaller sampling frequency. Because of the extra amount of high frequent energy the wave height measured by the non-directional waverider is 26 cm while the directional waverider measures a wave height of 21 cm. Besides, the high frequent energy provides a reduction of the wave period from 3.3 seconds to 2 seconds (see Table 4.3).

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16 December 2005 Location 0-0.5 Hz 0-1 Hz Hm0 [cm] Tm02 [s] Hm0 [cm] Tm02 [s] AZB31 108 3.8 - - AZB32 91 3.7 93 3.3 AZB41 21 3.3 - - AZB42 20 3.3 26 2.0

Table 4.3: Measured Hm0 en Tm02 at 5/1/2005 1:30 at main locations 3 and 4

13 February 2005 11:00 of main location 3 (February storm)

Appendix 4.3 presents the spectra of main location 3 on 13 February 2005 at 11:00. This moment is selected because there is a big difference between the spectra in the frequency range of 0-0.5 Hz. The largest deviation between the spectra can be found in the low frequency band (0.1-0.2 Hz) which represents the longer waves. For example at 0.1 Hz (10 seconds) the amount of energy at AZB31 is twice the amount of energy at AZB32. Apparently the differences between the different buoy types are not limited to the high frequency energy tail only. The difference between the measured wave height of both buoys is approximately 20% (Hm0 =198 cm at AZB31 and 162 cm at AZB32).

Furthermore, the selected spectra indicate that despite the only small amount of high frequent energy, a difference in the measured wave periods exists (at AZB31 4.5 s and at AZB32 3.9 s).

Location 0-0.5 Hz 0-1 Hz

Hm0 [cm] Tm02 [s] Hm0 [cm] Tm02 [s]

AZB31 198 4.8 - - AZB32 162 4.5 166 3.9

Table 4.4: Measured Hm0 en Tm02 at 13/2/2005 1:30 at main location 3

15 February 2005 16:00 of main location 4 (February storm)

The spectra of main location 4 on 15 February 2005 at 16:00 can be found in Appendix 4.4. This moment is selected because of the similarity in the low frequent part of the wave spectra of the waverider and the directional waverider, but in the high frequent part of the spectra a substantial energy is measured by the waverider only. The measured wave height Hm0 and wave period Tm02 at both locations (based on the frequency range

of 0-0.5 Hz) correspond well with each other (29 cm resp. 28 cm). Above 0.5 Hz a second peak in the spectrum of AZB 42 at 0.72 Hz could be noticed. The waverider seems to prove its advantage here since the spectra below 0.5 Hz are similar, but the high frequency energy is measured by the waverider only. This is clearly visible in the measured wave height at AZB42 based on the frequency range of 0 to 1 Hz (The wave height is more than 50 % higher and the wave period is approximately 50 % lower, see table 4.5).

AZB41 29 4.3 - - AZB42 28 4.3 33 2.2

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16 December 2005 16 February 2005 3:00 of main location 3 (February storm)

In Appendix 4.5 the spectra of main location 3 on 16 February 2005 at 3:00 are

presented. Noticeable with the comparison of the wave spectra of main location 3 is that the waverider and the directional waverider have measured almost the same spectrum which can be seen in the measured wave heights and wave periods as well (see Table 4.6). A small tail can be found in the spectrum AZB32 (the existing high energy at AZB32 with frequencies higher than 0.5 Hz), but the energy is too small to cause considerable differences with respect to the measured values in the range of 0-0.5 Hz (see Table 4.9).

AZB31 108 4.4 - - AZB32 109 4.6 111 4.2

22 February 2005 14:20 for main location 4

The spectra of main location 4 on 22 February 2005 at 14:20 can be found in Appendix 4.6. This moment is selected because of the high peak at the high frequent part of the spectrum measured by the waverider at AZB42 in comparison with the energy at the lower frequencies. This implies the existence of mainly short waves (see Table 4.7). The difference in the measured wave height Hm0 at this location based on the frequency

range of 0-0.5 Hz and 0-1 Hz is 9 cm. The wave energy measured at the locations AZB41 and AZB42 up to 0.5 Hz is almost similar (see Appendix 4.6 and Table 4.7). For this selected moment the higher sampling frequency is a profit, if measuring short waves is part of the interest. Note however that these small wave periods are close to the ranges with less reliable data (see Paragraph 4.2).

AZB41 14 3.1 - - AZB42 13 2.9 22 1.6

Table 4.7: Measures Hm0 en Tm02 at 22/2/2005 14:20 at main location 4

16 March 2005 19:50 for main location 3

Appendix 4.7 shows the spectra of main location 3 of 16 March 2005 at 19:50. This moment has been selected because of the existence of high frequency energy. This high frequent energy can not be measured by the directional waverider. This results in a lower measured wave height and a higher measured wave period in comparison with the measured values of AZB32 based on the frequency range 0-1 Hz.

AZB31 50 2.9 - - AZB32 46 2.9 56 2.1

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16 December 2005 6 April 2005 19:20 for main locations 3 en 4

In Appendix 4.8 the spectra of main location 3 and 4 of 6 April 2005 at 19:20 are given. This moment is selected to show that there are moments where the spectra of the waverider and the directional waverider are almost similar. The directional waverider hardly misses energy because there is hardly any high frequent energy. Even “left” of 0.5 Hz the spectra look similar. Logically the measured wave parameters have almost the same values (see Table 4.9).

Location 0-0.5 Hz 0-1 Hz Hm0 [cm] Tm02 [s] Hm0 [cm] Tm02 [s] AZB31 94 3.3 - - AZB32 94 3.3 95 3.2 AZB41 65 2.8 - - AZB42 67 2.8 71 2.5

Table 4.9: Measured Hm0 en Tm02 at 6/4/2005 19:20 at main locations 3 and 4

15 April 2005 22:30 for main location 4

The spectra of main location 4 of 15 April 2005 of 22:30 can be found in Appendix 4.9. With the comparison of the spectra it appeared that the measured wave height of the direction waverider is too low, which is caused by the presence of high frequency energy. This energy was only measured by the waverider resulting in a higher measured wave height and a lower measured wave period at location AZB42, based on the frequency range of 0-1 Hz. Location 0-0.5 Hz 0-1 Hz Hm0 [cm] Tm02 [s] Hm0 [cm] Tm02 [s] AZB41 22 2.7 - - AZB42 21 2.6 34 1.8

Summary

The directional waverider has a smaller sample frequency than the waverider and therefore misses sometimes high frequency wave energy. As a result, wave height is underestimated and wave period is overestimated (especially Tm02 which is sensitive to

higher frequencies). High frequency energy does occur at location 3 and 4 but by far not all the time. Moreover the amount of high frequency energy is most of the time small. However, the differences in spectra are not limited to the higher frequencies only (>0.5 Hz). Even in the range between 0 and 0.5 Hz, significant differences between both types of buoys do occur. In general the directional waverider measures more energy than the non-directional waverider. In the examples of chapter 4 this mainly (but not always) occurs for higher waves. In the calmer periods the spectra till 0.5 Hz do agree better. 4.3.3 Comparison wave parameters waverider and directional waverider

Figure 4.3 shows significant differences in the measured wave height. In Table 4.11 the mean, the maximum positive and maximum negative differences between the measured wave heights of the wave buoys AZB31-AZB32 en AZB41-AZB42 are presented.

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16 December 2005 Maximum positive difference [cm] Total number difference>0 [-] Maximum negative difference [cm] Total number difference<0 [-] Mean difference [cm] Hm0AZB31 – Hm0AZB32 46 15712 -27 1199 6 Hm0AZB31 – Hm0AZB32 24 12659 -15 1869 2 Table 4.11: Difference Hm0 between the locations AZB31-AZB32 en AZB41-AZB42

Table 4.11 shows that the directional waverider measures higher waves in comparison with the non-directional waverider. This is not due to the cut-off frequency since the wave heights were based on the frequency range 0.03 - 0.5 Hz for all locations. Possibly, it is caused by local depth effects. At location 3 the differences tend to be larger than at location 4, but the waves at location 3 are larger too.

Figure 4.3: Differences in the measured wave height between the locations AZB31-AZB32 and AZB41-AZB42 (based on the frequency range of 0 -0.5 Hz)

4.4 Influence high frequent energy

Control of the cut-off frequency of the waveriders at Ameland

As a check, we compared the cut-off frequency of one of the measured waverider spectra with the situation drawn in Figure 4.1. Appendix 4.10 presents a double-logarithmic plot of the wave spectrum of AZB32 on January 4th at 0:30 including the empirical line (f-5). This plot shows as from 0.8 Hz a deviation of the measured wave spectrum from the empirical line, which corresponds with the cut-off frequency of a waverider with a diameter of 0.7 m (see Table 4.1).

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16 December 2005 Influence high frequent energy on the wave height

The difference between the measured wave heights based on the frequency range of 0-0.5 Hz (the Hm0 in the received data files) and based on the frequency range of 0-1 Hz

(the calculated Hm0) is investigated for the non-directional waveriders (with a sampling

frequency of 2.56 Hz). The first mentioned wave height is what one should measure more or less if a directional waverider with a sampling frequency of 1.28 Hz would have been used.

The lines in Appendix 4.11 present the differences between the measured wave heights (0-0.5 Hz) and the calculated wave heights (0-1 Hz) of the four non-directional

waveriders. Only 1 out of 100 values is plotted in order to improve the overview. The dots symbolise the received wave heights (0-0.5 Hz). It can be concluded that the differences (delta) increase as the measurement location is positioned further down into the tidal inlet of Ameland, while the wave heights decreases.

In Table 4.12 the mean and maximum values of the differences are given. It is

interesting to see that the mean differences at the locations with smaller waves (location 4 and 5) are larger than at location 3 (with higher waves), even in absolute numbers. This corresponds with the hypothesis that the wave heights decrease as the

measurement location is positioned further down into the tidal inlet of Ameland (locations 4 and 5), and there is more high frequent energy (paragraph 2.3 [ref 6]).

Mean Hm0 [cm] Mean difference [cm] Maximum difference [cm] frequency range 0 - 0.5 Hz frequency range 0 - 1.0 Hz AZB32 64 67 3 26 AZB42 27 31 4 26 AZB51 20 27 7 39 AZB52 20 26 6 30

Table 4.12: The average wave height up to 0.5 Hz and up to 1.0 Hz, and the absolute differences.

In order to see if the mean differences between the wave heights based on the whole frequency range (0-1 Hz) and based on only half of the frequency range (0-0.5 Hz) depend on the wave height a division of wave heights (below and above 0.5 m) has been made, see Table 4.13. Table 4.13 presents both the relative (percentage of the wave height based on 0 -0.5 Hz) and absolute values (centimetres). It turns out that the differences are larger for smaller wave heights, even in absolute centimetres. In order to prevent very large percentages, wave heights smaller than 0.1 m, are left aside.

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16 December 2005 Mean Hm0 [cm] Mean difference (absolute) [cm] Mean difference (in terms of percentage) [%] Mean Difference (absolute) [cm] Mean difference (in terms of percentage) [%] frequency range 0 - 0.5 Hz frequency range 0 - 1.0 Hz For 0.1<Hs<0.5 For Hs>0.5 m AZB32 64 67 3 12% 3 3% AZB42 27 31 4 21% 3 5% AZB51 20 27 8 43% 4 7% AZB52 20 26 6 35% 5 8%

Table 4.13: The mean differences of the wave height up to 0.5 Hz and up to 1.0 Hz, absolute and relative

Hordijk, 2004 [ref 6] already noticed that deviations as a result of using the energy up to 0.5 Hz increase as the wave height decreases. In view of the objective of this

measurement campaign this aspect is less relevant.

Influence high frequent energy on the measured wave period

In line with Table 4.12 and 4.13, also for the wave periods it has been examined what the differences are if Tm02 would be based on wave energy till 0.5 Hz or till 1.0 Hz, see Table 4.14 and 4.15.

As expected, the mean wave period based on the frequency range of 0-1 Hz is smaller than the mean wave period based on the frequency range of 0 – 0.5 Hz. The differences can increase up to 6 seconds (this occurs for very small wave heights only). For waves with a wave height less than 0.5 m the mean difference lies between 0.6 s and 1.0 s, depending on the location, see Table 4.15. In general higher waves (>0.5 m) have longer wave periods and the mean difference is smaller, between 0.3 and 0.6 seconds. It turns out that the difference in wave height increases as the location is situatuated more south in the tidal inlet of Ameland. The biggest difference in wave periods can be found at location 3, see Table 4.15.

Mean wave period Tm02

[s] mean difference Tm02 Tm02 (0-0.5Hz) -Tm02 (0-1Hz) [s] maximum difference Tm02 Tm02 (0-0.5Hz) -Tm02 (0-1Hz) [s] frequency range 0 - 0.5 Hz frequency range 0 - 1.0 Hz AZB32 4.1 3.4 0.8 5.9 AZB42 3.6 2.7 0.9 6.3 AZB51 2.9 2.0 0.9 6.6 AZB52 3.1 2.2 0.9 6.7

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16 December 2005 Mean period Tm02 [s] Mean difference Tm02 (absolute) [s] Mean difference Tm02 (in terms of percentage) [%] Mean difference Tm02 (absolute) [s] Mean difference Tm02 (in terms of percentage) [%] frequency range 0 - 0.5 Hz frequency range 0 - 1.0 Hz for 0.1<Hs<0.5 for Hs>0.5 m AZB32 4.1 3.4 1.0 25 0.6 13 AZB42 3.6 2.7 0.9 25 0.3 10 AZB51 2.9 2.0 0.6 22 0.3 12 AZB52 3.1 2.2 0.6 21 0.4 13

Table 4.15: The mean differences of the wave period up to 0.5 Hz and up to 1.0 Hz, absolute and relative

The scatter plots of the wave height versus the wave period show clearly that the waveriders with a higher sampling frequency (black points) measure smaller wave periods than the directional waveriders (yellow points). It appears that the cloud of one type of wave buoy is shifted in wave period from the other type of wave buoy ("vertical" in the figure). A shift in wave height ("horizontal" in the figure) is not visible.

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16 December 2005

5 STORM EVENTS

5.1 Introduction

Based on the wave, wind and water level conditions (see Appendices 3.1A, 3.1B, 3.2A, 3.2B, 3.3 and 3.4) two storm events within the survey period are selected and are discussed in this chapter in more detail. With the selection of these storm events the preference of RWS RIKZ to select a northwest and a southwest storm is taken in consideration, but eventually there is chosen for the combination of a high water level with a high wind velocity and the presence of considerable wave heights. Also, it has been checked whether most of the wave buoys had operated properly during selected storms. Finally, in mutual agreement with RWS RIKZ, the following two storm events have been selected:

1. 1 January 2005 – 3 January 2005

In this storm event the wind direction was almost continuously from the west and the wind velocity was considerable (15 m/s measured at Lauwersoog). The water level measured at NES reached its maximum at NAP +2.23 m. The highest wave height of the whole survey period 2004-2005 is measured in this storm period, namely 6.31 m at location AZB11 (see Appendix 3.1A).

2. 7 January 2005 – 9 January 2005

During this storm period the wind was veering from SSW to WSW in which the highest wind velocity of the whole survey period 2004-2005 was measured (32 m/s measured at location F3). The wind velocity at Lauwersoog was about 15 to 20 m/s. The water level measured at NES reached its maximum at NAP + 2.29 m.

In this chapter the two storms are analysed more closely. Paragraph 5.2 gives a

description of the storm period 1 - 3 January 2005. The storm period 7 - 9 January 2005 is examined further in paragraph 5.3.

5.2 Storm period 1 – 3 January 2005

5.2.1 General

De SVSD has described this storm period in close cooperation with KNMI [ref 7]: Between the high-pressure area west of

Spain and a low trough near Iceland a secondary depression moved from Scotland to South Scandinavia (see Figure 5.1). At New Year’s Eve the accompanying cold front crossed the Dutch coast. After passage of the cold front the wind veered to the west and increased to stormy wind (8 Bft) in the southern part of the North Sea to gale force (9 Bft) above the Wadden.

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16 December 2005 During January 2th_{, the wind subsided gradually in the southern part of the North Sea to}

high wind (7 Bft); during the whole day of January 2th westerly gale was present above the Wadden. Just in the night of 2 to 3 January the wind decreases in importance to powerful wind (6 Bft) at Monday morning. The accompanying cold front passed at New Year’s evening the Dutch coast. The westerly gale caused some considerable set up in particular along the northern coast area. During several high tides a set up of 50 cm (at Vlissingen) to 154 cm (at Harlingen) occurred. The measured set up at Harlingen occurs on average 1.5 times a year.

Figure 5.2 shows the measured wind, water level and wave conditions of the storm period 1 January 2005- 3 January 2005, near the tidal inlet of Ameland.

Figure 5.2: Time series of the wind, water level and wave conditions of 1-3 January 2005

5.2.2 Wind and water level measurements

The wind conditions are presented in Figure 5.2 and concern the 10-minutes average wind velocities and wind directions measured at Lauwersoog (see Figure 2.2). The conditions of the water level concern the 10-minutes average water level at the

measuring station NES, located at the Wadden Sea side of Ameland (see Figure 2.2). Figure 5.2 shows that the wind on January 1th_at_{12:00 was coming from SSW and the}

wind velocity is around 6 m/s. At the end of the day the wind was veering to the west and the wind velocity increases to approximately 16 m/s. On January 2th at 0:50 the wind velocity achieved its maximum of this storm period of magnitude 19 m/s. During January 2th_{the wind velocity decreases little bit and the wind direction was holding west.}

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16 December 2005 Just during the day of January 3th_{the wind velocity decreased in importance to a}

powerful wind (of magnitude 12m/s).

Figure 5.2 shows a considerable set up. The water level attains its maximum during the second high water of January 2th and measures NAP +2.23 m.

5.2.3 Wave measurement Time series

In Figure 5.3 the development of the wave height Hm0 and the wave period Tm-10 is

shown graphically for the different measurement locations.

During this storm period eight of the ten wave buoys has operated well. Unfortunately the directional waveriders AZB21 and AZB22 did not register any wave data during this storm period, the cause is unknown.

Figure 5.3: Time series of Hm0 and Tm-10 of all the measurement locations of 1-3 January 2005.

Figure 5.3 shows that the wave height Hm0 at location AZB11 round midnight of January

2th_{to 7:30 increases strongly from 2.5 m to 6.31 m, the highest H}

m0 measured in the

whole survey period 2004-2005. On first thoughts the wave heights measured at AZB12 and AZB11 (situated at approximately 900 m of each other and with comparable depth) seems similar. Nevertheless occasionally the difference in wave height at these

locations is more than 1 meter. The wave periods at the North Sea locations (AZB11 and AZB12) increase in the night of January 1th to January 2th – comparable with the development of the wave height Hm0 – from 6 to 9 seconds.

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16 December 2005 The wave height reduces significantly by the wave breaking on the shallow outer delta (approx NAP -4 m [ref 6]), this is clearly visible in the lower measured wave height Hm0

at the locations AZB31 and AZB32 (2.27 m resp. 2.16 m). A further reduction in the wave height takes place as the measurement location is positioned further down into the tidal inlet of Ameland (see Figure 5.3). Figure 5.3 shows that the reduction of the wave height along the measurement line is involved with a reduction of the wave period. In the development of the wave height the high and low tides are clearly visible.

Wave spectra

The spectra of the storm event of January 2nd 7:00 -17:00 (with a time step of 2 hours) of the AZB*2-buoys at all main measurement locations can be found in Appendices 5.1A and 5.1B. First the spectra of all five wave buoys at a certain moment are plotted in one figure. For comparison, the scaling is constant. Subsequently, under this figure, only the spectra of locations 2 - 5 are plotted, because the amount of energy at these locations has reduced significantly compared with main location 1. The frequency range runs up to 1 Hz in order to make the amount of energy between 0.5 and 1 Hz visible in

connection with the two types of wave buoys with a different sampling frequency (see chapter 4).

The values of the wave height Hm0, wave period Tm-10 and the water level for the six

moments (shown in Appendix 5.1A and 5.1B) for all the measurement locations are presented in Table 5.1. 2 January 2005 t=7:00 (water level NAP +45 cm) t=9:00 (water level NAP +45 cm) t=11:00 (water level NAP +164 cm) t=13:00 (water level NAP +223 cm) t=15:00 (water level NAP +195 cm) t=17:00 (water level NAP +134 cm) Hm0 [cm] Tm-10 [s] Hm0 [cm] Tm-10 [s] Hm0 [cm] Tm-10 [s] Hm0 [cm] Tm-10 [s] Hm0 [cm] Tm-10 [s] Hm0 [cm] Tm-10 [s] AZB11 _{529 9 572 9 520 9 456 9 454 9 457 9} AZB12 _{499 9 507 9 480 9 470 9 405 9 446 9} AZB21 _{- - - - - - - -} AZB22 _{- - - - - - - -} AZB31 _{117 6 138 6 227 7 201 8 182 8 163 7} AZB32 _{119 6 152 6 208 7 181 7 161 7 144 7} AZB41 _{73 4}_{72 4 79 4 117 4 97 4 80 4} AZB42 _{64 4}_{69 4 72 4 111 4 88 4 73 4} AZB51 _{69 -}_{63 - 50 - 89 - 87 - 71 -} AZB52 _{68 3}_{64 3 40 3 89 3 91 3 65 3}

Table 5.1: Wave height Hm0 and wave period Tm-10 belonging to the spectra of 2 January 2005 (Appendix 5.1)

The spectrum of the outside wave buoy has at all the six moments almost the same shape. The peak lies around the 10 seconds.

Wave breaking on the shallow outer delta causes dissipation of energy resulting in a significant reduction of the wave energy between the main locations 1 and 3. In addition It is expected that mainly the longer waves (they feel the seabed) refract to the sides of the channel (after which they dissipate on to the sides of the channel) and therefore they do not enter far into the tidal inlet of Ameland.

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16 December 2005 The spectra of buoy AZB12 become smaller which means that the wave height

decreases in the course of the time too. The wave height at the other locations does not show this reduction, but - on the contrary - the waves increase in course of time at t=11:00 and t=13:00. This is directly connected with the water level which reaches its maximum at approximately 13:00. At higher water levels (see Appendix 5.1A and 5.1B at t=11:00, t=13:00, t=15:00) the low frequency peak of the North Sea waves can somewhat penetrate to location 3. Even at location 4 the small low frequent peak at 0.1 Hz is visible at these moments.

The wave spectra of the main locations 4 and 5 contain significantly less energy in regard to the other locations. The peak at the low frequencies has disappeared completely in the spectrum of main location 5. During high water levels at t=13:00 and t=15:00 the spectra of main locations 4 and 5 contain significantly more wave energy, mainly near 0.3 Hz (order 3 s), than during the low water levels. This is probably due to depth related dissipation processes (wave breaking and bottom friction). In order to find out whether the waves are influenced by the tidal currents, it might by useful to include current measurements.

During this storm period the amount of energy belonging to the frequencies above 0.5 Hz is very small at all the measurement locations. This implies that the upper limit of 0.5 Hz is sufficient.

5.3 Storm period 7 – 9 January 2005

5.3.1 General

SVSD has described this storm period in close cooperation with KNMI [ref 8]: A secondary depression of a low

pressure area near Iceland moved very fast lowering over the North Sea to South-Scandinavia. After passage of the accompanying cold front a very heavy storm blew up over the Northern North Sea. During the height of the storm, wind speeds with hurricane power (12 Bft) were measured on the Northern North Sea during some time. In the southern part of the North Sea wind speeds didn’t exceed gale force (9 Bft). During the

afternoon of January 8th, a very heavy Figure 5.4: Weather chart of 8 January 2005 south western gale (11 Bft) was blowing (Source: ref 8)

over the Wadden Sea for several hours.

During the afternoon and evening, the wind subsided gradually to a hard west-south western wind (7 Bft).

This very heavy south western gale caused a considerable set up in particular along the northern coastal area. During several high tides a set up of 47 cm (at Vlissingen) to 142 cm (at Delfzijl) was observed. On average the measured set up at Delftzijl occurs two

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16 December 2005 times a year. The wind, water level and wave conditions of the storm period 7 January – 9 January 2005 are shown in Figure 5.5.

Figure 5.5: Time series of the wind, water level and wave conditions of 7 - 9 January 2005

5.3.2 Wind and water level measurements

The wind conditions shown in Figure 5.5 concern the 10-minutes average wind velocity and wind direction measured at Lauwersoog (see Figure 2.2). The water level conditions concern the 10-minutes average water level measured at NES, which is situated at the side of the Wadden Sea of Ameland (see Figure 2.2).

During the night of January 7 th to 8th 2005 the wind velocity starts to increase from 13 m/s to 20 m/s at the end of the morning. Afterwards the wind velocity subsided and in the evening the wind velocity is below 10 m/s. The wind direction was SSW but from 9am it veered round to WSW. During this storm period the highest wind velocity of this whole survey period was measured at F3 (approx 32 m/s).

Figure 5.5 shows a considerable set up of the water level. The maximum water level of NAP +2.29 m is measured during the second high tide of January 8th_.

5.3.3 Wave measurements Time series

In Figure 5.6 the development of the wave height Hm0 and the wave period Tm-10 is