Hindcast of 1st Nov 2006 Storm in the Norderneyer Inlet: Wave & Flow boundary conditions

the Norderneyer Inlet

Wave & Flow boundary conditions

Lydia Lim Sin Hwei

Report

June 2008

Hindcast of 1st Nov 2006 Storm

in the Norderneyer Inlet

Title Hindcast of 1st Nov 2006 Storm in the Norderneyer Inlet

Abstract

In the framework of the SBW (Strength and Loading of Water Defenses) project which Deltares has been commissioned to execute for Rijkswaterstaat – Waterdienst, Rijkswaterstaat has requested a hindcast of the 1st November 2006 storm as part of the International Data Exchange Project, in this case an exchange between Netherlands and Germany.

This storm is very interesting for the SBW (Strength and Loading of Coastal Structures) project, the objective of which is to develop reliable tools to determine hydraulic boundary conditions in the Wadden Sea. One of these tools is the SWAN wave transformation model, the reliability and performance of which needs to be validated using large measured storms.

During the 1 November 2006 storm high water levels and high waves occurred both in the Dutch and German parts of the Wadden Sea. Since the geography of the two areas is similar, it is worthwhile investigating whether SWAN can reproduce the wave conditions in the Norderneyer Inlet.

FSK Norderney will perform the hindcast study. However, since wave data offshore is not available during the time of the storm, Deltares is given the responsibility of determining the offshore boundary conditions for both the wave model and the flow model, which FSK Norderney will use as input for the hindcast of the tidal inlet.

The performed hindcast provides the required offshore boundary conditions for the future wave and flow computations. Where measurements are available, the computed results compare well with the measurements. There are no wave measurements available outside the inlet to validate the hindcasts, but the results seem to be correct in a qualitative sense. One of the findings in this study is that the computed water levels

underpredicted the peak of the measured water levels, if 6-hr interval windfields were used. Application of 3-hr interval windfields, obtained from the official 6hr intervals interlaced with predicted 3hr windfields, improved the results dramatically.

References RWS Waterdienst overeenkomst WD-4968/4500121262

Raamovereenkomst WD-4924 betreffende `Specialistische adviezen van de Stichting Deltares t.b.v. het Ministerie van Verkeer en Waterstaat'

Ver Author Date Remarks Review Approved by

1.0 Lydia Lim Sin Hwei 29/04/2008 draft J. Groeneweg M.R.A. van Gent

2.0 Lydia Lim Sin Hwei 12/06/2008 final J. Groeneweg M.R.A. van Gent

3.0 Lydia Lim Sin Hwei 9/7/2008 final J. Groeneweg M.R.A. van Gent

Project number H5107.43

Keywords Wadden Sea, SBW, boundary conditions, SWAN, Delft3D, hindcast

Number of pages 29

Classification None

Contents

1 Introduction ...1

1.1 SBW project background and problem statement...1

1.2 Problem statement of present study...2

1.3 Objectives of this study...3

1.4 Approach and restrictions...3

1.5 Outline of the report ...4

2 Description of the 1 November storm...5

2.1 Storm track...5

3 Flow model...7

3.1 Introduction ...7

3.2 Description of Nordemeyer Seegat ...7

3.3 Storm instants...7 3.4 Model schematization ...8 3.4.1 Grids ...8 3.5 Input data ...10 3.5.1 Wind ...10 3.5.2 Water level ...12 3.5.3 Bathymetry ...13 3.5.4 Bottom roughness ...14 3.5.5 Definition of output ...15

3.6 Results and analysis...16

3.6.1 Introduction...16

3.6.2 Fields ...17

3.6.3 Measured and modelled water level comparison ...17

3.7 Discussion and conclusions ...18

4 Wave model ...19 4.1 Introduction ...19 4.2 Model schematization ...19 4.2.1 Grids ...19 4.2.2 Input data ...21 4.2.3 Boundary conditions...21 4.2.4 Model settings ...22 4.2.5 Definition of output ...22

4.3 Results and analysis...22

4.3.1 Introduction...22

4.3.2 Storm development ...23

4.3.4 Wave spectra at the peak of the storm ...23

5 Conclusions and recommendations...24 6 References ...25

List of Figures

In text:

Figure 1.1 Aerial View of the study area of Norderney and locations of water level

measurements ...2

Figure 2.1 Pressure field at 31 Oct 2006 12 UTC (Coordinated Universal Time)...5

Figure 2.2 Pressure field at 31 Oct 2006 18 UTC (Coordinated Universal Time)...6

Figure 2.3 Pressure field at 01 Nov 2006 06 UTC (Coordinated Universal Time) ....6

Figure 3.1 Water level time series at Norderney Riff...8

Figure 3.2 CSM grid in spherical co-ordinates ...9

Figure 3.3 Curvilinear nested flow grid in Delft3D ...9

Figure 3.4 HIRLAM wind fields for 1 Nov 2006, 0:00 to 6:00 am ...11

Figure 3.5 Locations of offshore boundary conditions and discharges ...13

Figure 3.6 Interpolated bathymetry for the Flow Model in Delft3D ...14

Figure 3.7 Spatially-varying Manning roughness used for the nested Flow model.15 Figure 3.8 Location of water level measurements...16

Figure 3.9 Simulation period ...16

Figure 3.10 Water level stations ...18

Figure 4.1 Flow and wave grid...20

Figure 4.2 SWAN grid ...20

Figure 4.3 Depth profile (in m wrt NN) in SWAN computations...21

In Appendix:

Figures 3.11a-b Spatial distributions of water levels computed using Delft3D Flow stand-alone (without influence of waves).

Figures 3.12a-b Spatial distributions of currents computed using Delft3D Flow stand-alone (without influence of waves).

Figures 3.13a-b Comparison of water levels between results computed from from Delft3D Flow stand-alone and Delft3D Flow (2-way coupling) and actual measurements at two locations Norderney Riff and Vogelsand.

Figure 3.14 Comparison using boundary conditions generated by the CSM8 model and using MATROOS water levels.

Figure 4.4 Timeseries of the significant wave height Hm0 and the mean

wave period Tm0,1hindcasts

Figure 4.5 Spatial variation of the hindcasted wave parameters Hm0and

Tm0,1at 04:10am.

Figure 4.6 Measured wave spectra at SON and FINO and the computed wave spectra at Location 14

List of Tables

1 Introduction

1.1 SBW project background and problem statement

In compliance with the Dutch Flood Defences Act ("Wet op de Waterkering, 1995"), the primary coastal structures must be monitored every five years (2001, 2006, 2011, etc.) for the required level of protection. This assessment is based on the Hydraulic Boundary Conditions (HBC) and the Safety Assessment Regulation (VTV: Voorschrift op Toetsen op Veiligheid). These HBC are derived every five years and approved by the Minister of Transport, Public Works and Water Management.

The HBC are used to subject the sea defenses to a stepwise assessment ranging from "simple" to "advanced" tests. During these assessments so-called "knowledge vacuums" are encountered. The result may be that the assessment cannot be completed and sections of the sea defense are labelled "geen oordeel" (no judgement; safety level unknown), which is an undesirable situation. Another possibility may be that sea defenses erroneously pass or fail the assessment.

Because of this problem of a "knowledge vacuum" (kennisleemte) with respect to the assessment of the safety of flood defenses, the overall SBW ("Sterkte en Belasting Waterkeringen”, i.e. Strength and Loading of Water Defenses) project has the following general objective:

"To fill the most important knowledge vacuums in order to achieve a better estimate of the safety of the primary flood defenses against flooding."

As part of this larger project, the subproject SBW-Waddenzee was started in 2006. The starting point is the observation that there is uncertainty concerning the quality of the HBC which are an important input into the assessment, in particular those for the Waddenzee. This is because they were obtained from an inconsistent set of measurements and design values (WL, 2002), while for the rest of the Dutch coast (the closed Holland Coast and the Zeeland Delta) they have been determined with a probabilistic method, in which offshore wave statistics are transformed to nearshore locations. For the latter the wave model SWAN has been applied. Due to the lack of validation data there is insufficient confidence in applying the wave model SWAN to the complex area of the Wadden Sea and to use it to obtain reliable boundary conditions in the Wadden Sea at present. The subproject sets out to determine the general suitability of SWAN in the Wadden Sea and to specify the improvements required to produce reliable HBC in the Wadden Sea.

The objective of the SBW-Waddenzee project is therefore to

"Verify and where possible improve the quality of the models and methods so that in 2011 and beyond better HBC can be calculated for the Wadden Sea. "

The path towards meeting this objective is laid out in a Plan of Action (WL, 2006) which describes a step-by-step approach of performing hindcasts of storm events in the Wadden Sea and other relevant areas, analysis of the results, and sensitivity and uncertainty analyses. Despite recent and ongoing measurement campaigns in the Dutch part of the Wadden Sea, the storm events are scarce, and information about the performance of the wave model in relevant areas is highly valued.

1.2 Problem statement of present study

During the storm of 1 November 2006 some of the highest water levels in years were recorded and witnessed in the Dutch and German Waddensea. This storm was the 5th largest storm that has been measured in the Norderney area since the beginning of the recording of measurements, with a maximum water level of 3.78 m NN1. During the peak of the storm (04:00-05:00 of 1 Nov 2006) narrow wave spectra were observed. These conditions make this a very interesting storm for SBW purposes.

However, due to logistical reasons, this storm was not measured in the Dutch Amelander Zeegat inlet. Wave measurements were carried out inside the tidal inlet of Norderney (Niedersachsen, Germany) by the Forschungsstelle Küste (FSK) situated at the island of Norderney (see Figure 1.1). Due to a malfunctioning of the only buoy seawards of the inlet, there are no measured wave conditions available that can be imposed as offshore wave boundary conditions to SWAN model of the inlet. In addition, water levels and current fields are necessary inputs into the SWAN model which are presently also not available.

.

Figure 1.1 Aerial View of the study area of Norderney and locations of water level measurements

1.3 Objectives of this study

The objective of this study is to perform a larger-domain hindcast of the storm of 1st of November 2006 so as to calculate the boundary conditions that are needed for the wave hindcast in the tidal inlet of Norderney, to be performed by FSK.

The products of the study are the following:

1 Wave conditions at the boundaries of the wave model of FSK; to be used for the hindcast of the tidal inlet of Norderney.

2 Currents and water levels at the boundary of the hydrodynamic model of FSK; to be used for the hindcast of the tidal inlet of Norderney.

3 Documentation of the set-up of the models and the simulations performed.

1.4 Approach and restrictions

At 26 June 2007 RIKZ, WL (both are presently joined in Deltares) and FSK discussed the approach to fulfil the objectives during a meeting at FSK. The general approach taken is to conduct flow modelling in a regional domain using Delft3D and then deriving currents and water levels at the FSK flow model boundary. The results from Delft3D will also be used as input to SWAN to compute wave conditions (including the effects of wave-current interaction) at the FSK wave model boundary.

A line of approach was established during the meeting at 26 June 2007 as follows:

• FSK validates the measured wave data and comes up with an overview of the availability and reliability of the data. The data validation was finished at the end of August 2007;

• Deltares will determine reliable boundary conditions for both the wave computations and the flow computations that has to be performed in due course by FSK. (subject of this report);

• FSK will perform the hindcast and publish the results.

SWAN version 40.51AB, including the bug-fix of the hotfile precision from 4 to 6 digits, is used in this study to derive wave conditions at the FSK wave model boundaries. This version is not an official release yet, but with the increase of precision of data in the hotfile, it leads to higher accuracy for non-stationary runs. For flow-modelling, Delft3D version 3.27.01 is used.

As no additional measurements are available at the time of the study, hence validation by model-measurement comparisons will not be made. However the models have been set up based on the in-depth knowledge and experience gained from past modelling efforts in the same area. Any model-measurement comparisons will form the subject of future studies.

1.5 Outline of the report

Section 2 will give a general description of the storm of 1st November 2006. Section 3 and 4 will discuss the setup and the results of the flow and wave model respectively. Conclusions will be drawn in section 5.

2

Description of the 1 November storm

2.1 Storm track

The storm of 31 October to 1 November 2006 is known by the Dutch name “Allerheiligenvloed 2006” and is described in Storm Surge Report Allerheiligenvloed SR84 (2006, hereafter SR84) and Allerheiligenvloed 2006 (2007). Both reports are in Dutch, but the following section provides some relevant information, translated from these reports.

At 28 October 2006, a wave occurs in the polar front at the Atlantic Ocean. Between 29 October and 30 October, this wave passes south between Iceland and Scotland and transforms to a depression with a central pressure of 985 hPa. At the western side of this low pressure area, the wind force increases to 9 Bft. At 13h00, 31 October 2006 the depression reaches its minimum pressure of 977 hPa and is located close to Bergen in Norway, see Figure 2.1.

Figure 2.1 Pressure field at 31 Oct 2006 12 UTC (Coordinated Universal Time)

In the afternoon, the depression moves to the North of Denmark. West of the centre of the low pressure area, the wind force increases to 10 Bft from north-westerly direction. In the evening of 31 October the Dutch Meteorological Institute (KNMI) issues a Weather Alarm for a severe north-westerly storm. The cold front of the depression reaches the German and Dutch Coast at 8h00. Behind this cold front, a relatively strong westerly wind is blowing (7 Bft.). Around 18h00-19h00, the back-bent occlusion of the depression arrives at the German and Dutch coast, see Figure 2.2. Behind the occlusion, the wind veers to north-westerly direction. At the western flank of the depression, the storm above the eastern part of the North Sea is identified as severe (10 Bft.). The western and southern part of the North Sea experiences somewhat milder wind speeds (7-8 Bft.).

Figure 2.2 Pressure field at 31 Oct 2006 18 UTC (Coordinated Universal Time)

Around midnight, the storm becomes more severe, especially in the Wadden Sea. Locally wind speeds up to 115 km/h are measured above the Dutch departments of Groningen and Friesland. At the German Bight, a wind force of 11 Bft is measured for a short period of time with wind gusts of 120-150 km/h. Around High Water, a pressure trough passes the Dutch Ems-Dollard area, which causes a brief increase in wind speeds and very high water levels. In the course of 1 November 2006, the low pressure area shifts towards the Baltic States and the wind veers further to the north, while gradually decreasing to 8 Bft at the northern part of the German and Dutch Coast (Figure 2.3).

3 Flow

model

3.1 Introduction

FSK requires boundary and initial conditions for their flow model computations in the Norderneyer area. The location of the boundaries of their flow model is shown in Figure 3.3. The goal is to determine the water levels and currents at these boundary locations and within the computational domain. Hence a flow model using Delft3D version 3.27.01 was set up to generate the necessary water levels and current fields needed by the FSK to perform their flow computations. Two approaches for flow modelling were considered here, with and without considering the effect of waves on the flow. The following sections will discuss the details of model setup and the results.

3.2 Description of Nordemeyer Seegat

The Norderneyer Seegat is situated west of the German Norderney Island and east of the Island of Juist in the Wadden Sea; see Figure 1.1. The wave climate offshore of the inlet is characterised by waves originating mainly from Northwest, the direction from which the winds may have very long fetches, and which is also the direction from which the higher waves come. The high waves are mostly dissipated on the ebb-tidal delta, before they enter the shallow Wadden Sea.

3.3 Storm instants

As mentioned in section 1 the northerly storm of 1 Nov 2006 was the 5th largest storm that has been measured in the Norderney area with a maximum water level of approximately 3.78 m NN2, see Figure 3.1 for water level time series at Norderney Riff (refer to Figure 1.1 for measurement location of Norderney Riff).

30-Oct-2006 00:00:00-2 31-Oct-2006 00:00:00 01-Nov-2006 00:00:00 02-Nov-2006 00:00:00 03-Nov-2006 00:00:00 -1 0 1 2 3 4 Date/Time Wat e r e lev at io n (m ) w .r .t . N o rmal Nu ll

Water level at Norderney Riff

maximum water level of 3.78m at 01-Nov-2006 04:24:59

Figure 3.1 Water level time series at Norderney Riff

In the storm of 1 Nov 2006, the conditions at 04:10am (GMT) offshore reached a significant wave height of 8.8m and a peak period of 16.7s at SON and at FINO, a significant wave height of 9.8m and a peak period of 16.7s. The windspeed was about 16 m/s at 04:10AM blowing from North-North-West. (From our communication with FSK, however, it was found that much higher wind speed were measured. This is the (already known) problem that the forecasted windfields were too low around Norderney. For example at Huibertgat, which is to the west of Norderney, it is reported3 that the maximum 3-hourly HIRLAM forecasted wind near the time of the storm were 25m/s and the actual measurements (hourly-averaged) were 23m/s)

3.4 Model schematization

3.4.1 Grids

Two flow grids have been used in this study. The regional flow model (CSM) uses a curvilinear grid based on spherical co-ordinates as shown in Figure 3.2. There is a total of 34773 (201x173) grid points in the domain. The other nested flow grid used in this study was also curvilinear, based on UTM zone 32. For the nested flow model, there is a total of 60382 (266 x 227) grid points in the domain (covering an area of 292 km x 182 km). The grid resolution near the boundary of the FSK hydrodynamic model is of the order of 500-1000 m (See Figure 3.3)

3. From the report “Allerheiligenvloed 2006, Achtergrondverslag van de stormvloed van 1 november 2006”

Figure 3.2 CSM grid in spherical co-ordinates

Figure 3.3 Curvilinear nested flow grid in Delft3D

3.5 Input data

3.5.1 Wind

HIRLAM (High Resolution Limited Area Model) windfields were provided as input to the model. Basically these are spatially interpolated values of HIRLAM mean sea level pressure (pmsl) fields and HIRLAM wind speed components (u10 and v10) on csm8 grid. The analysed wind fields are at 6-hourly interval and forecasting leadtimes have a timestep of 3 hours from 27-10-2006 00:00 to 03-11-2006 00:00 (GMT).

The initial approach is to impose a surface wind and pressure using the 6-hourly analysed wind field based on the rationale that an analysed wind field will be more accurate than a forecasted wind. However due to the fact that the analysed wind fields are at 6-hourly intervals, comparison with the 3-hourly forecasted wind field shows that the 6 hourly wind fields is missing an extreme high wind at the time of the storm which is only captured by the forecasted wind, which significantly improves the storm surge prediction. Hence the final imposed wind field consist of analysed wind fields at 6-hourly interval with intermittent 3-hourly forecasted wind, see Figure 3.4.

Figure 3.4 HIRLAM wind fields for 1 Nov 2006, 0:00 to 6:00 am Analysed

Forecasted

2

General description of storm

The storm of 31 October to 1 November 2006 is known by the Dutch name “Allerheiligenvloed 2006” and is described in Storm Surge Report Allerheiligenvloed SR84 (2006, hereafter SR84) and Allerheiligenvloed 2006 (2007). Both reports are in Dutch, but the following section provides some relevant information, translated from these reports.

At 28 October 2006, a wave occurs in the polar front at the Atlantic Ocean. Between 29 October and 30 October, this wave passes south between Iceland and Scotland and transforms to a depression with a central pressure of 985 hPa. At the western side of this low pressure area, the wind force increases to 9 Bft. At 13h00, 31 October 2006 the depression reaches its minimum pressure of 977 hPa and is located close to Bergen in Norway, see Figure 2.1.

Figure 2.1 Pressure field at 31 Oct 2006 12 UTC (Coordinated Universal Time)

In the afternoon, the depression moves to the North of Denmark. West of the centre of the low pressure area, the wind force increases to 10 Bft from north-westerly direction. In the evening of 31 October the Dutch Meteorological Institute (KNMI) issues a Weather Alarm for a severe north-westerly storm. The cold front of the depression reaches the German and Dutch Coast at 8h00. Behind this cold front, a relatively strong westerly wind is blowing (7 Bft.). Around 18h00-19h00, the back-bent occlusion of the depression arrives at the German and Dutch coast, see Figure 2.2. Behind the occlusion, the wind veers to north-westerly direction. At the western flank of the depression, the storm above the eastern part of the North Sea is identified as severe (10 Bft.). The western and southern part of the North Sea experiences somewhat milder wind speeds (7-8 Bft.).

Figure 2.2 Pressure field at 31 Oct 2006 18 UTC (Coordinated Universal Time)

Around midnight, the storm becomes more severe, especially in the Wadden Sea. Locally wind speeds up to 115 km/h are measured above the Dutch departments of Groningen and Friesland. At the German Bight, a wind force of 11 Bft is measured for a short period of time with wind gusts of 120-150 km/h. Around High Water, a pressure trough passes the Dutch Ems-Dollard area, which causes a brief increase in wind speeds and very high water levels. In the course of 1 November 2006, the low pressure area shifts towards the Baltic States and the wind veers further to the north, while gradually decreasing to 8 Bft at the northern part of the German and Dutch Coast (Figure 2.3).

3 Flow

model

3.1 Introduction

FSK requires boundary and initial conditions for their flow model computations in the Norderneyer area. The location of the boundaries of their flow model is shown in Figure 3.3. The goal is to determine the water levels and currents at these boundary locations and within the computational domain. Hence a flow model using Delft3D version 3.27.01 was set up to generate the necessary water levels and current fields needed by the FSK to perform their flow computations. Two approaches for flow modelling were considered here, with and without considering the effect of waves on the flow. The following sections will discuss the details of model setup and the results.

3.2 Description of Nordemeyer Seegat

The Norderneyer Seegat is situated west of the German Norderney Island and east of the Island of Juist in the Wadden Sea; see Figure 1.1. The wave climate offshore of the inlet is characterised by waves originating mainly from Northwest, the direction from which the winds may have very long fetches, and which is also the direction from which the higher waves come. The high waves are mostly dissipated on the ebb-tidal delta, before they enter the shallow Wadden Sea.

3.3 Storm instants

The northerly storm of 1 Nov 2006 was the 5th largest storm that has been measured in the Norderney area (since record of measurements) with a maximum water level of approximately 3.78 m NN2, see Figure 3.1 for water level time series at Norderney Riff (refer toFigure 1.1 for measurement location of Norderney Riff.)

30-Oct-2006 00:00:00-2 31-Oct-2006 00:00:00 01-Nov-2006 00:00:00 02-Nov-2006 00:00:00 03-Nov-2006 00:00:00 -1 0 1 2 3 4 Date/Time Wat e r e lev at io n (m ) w .r .t . N o rmal Nu ll

Water level at Norderney Riff

maximum water level of 3.78m at 01-Nov-2006 04:24:59

Figure 3.1 Water level time series at Norderney Riff

In the storm of 1 Nov 2006, the conditions at 04:10am (GMT) offshore reached a significant wave height of 8.8m and a peak period of 16.7s at SON and at FINO, a significant wave height of 9.8m and a peak period of 16.7s. The windspeed was about 16 m/s at 04:10AM blowing from North-North-West. (From our communication with FSK, however, it was found that much higher wind speed were measured. This is the (already known) problem that the forecasted windfields were too low around Norderney. For example at Huibertgat, which is to the west of Norderney, it is reported3 that the maximum 3-hourly HIRLAM forecasted wind near the time of the storm were 25m/s and the actual measurements (hourly-averaged) were 23m/s)

3.4 Model schematization

3.4.1 Grids

Two flow grids have been used in this study. The regional flow model (CSM) uses a curvilinear grid based on spherical co-ordinates as shown in Figure 3.2. There is a total of 34773 (201x173) grid points in the domain. The other nested flow grid used in this study was also curvilinear, based on UTM zone 32. For the nested flow model, there is a total of 60382 (266 x 227) grid points in the domain (covering an area of 292 km x 182 km). The grid resolution near the boundary of the FSK hydrodynamic model is of the order of 500-1000 m (See Figure 3.3)

3. From the report “Allerheiligenvloed 2006, Achtergrondverslag van de stormvloed van 1 november 2006”

Figure 3.2 CSM grid in spherical co-ordinates

Figure 3.3 Curvilinear nested flow grid in Delft3D

3.5 Input data

3.5.1 Wind

HIRLAM (High Resolution Limited Area Model) windfields were provided as input to the model. Basically these are spatially interpolated values of HIRLAM mean sea level pressure (pmsl) fields and HIRLAM wind speed components (u10 and v10) on csm8 grid. The analysed wind fields are at 6-hourly interval and forecasting leadtimes have a timestep of 3 hours from 27-10-2006 00:00 to 03-11-2006 00:00 (GMT).

The initial approach is to impose a surface wind and pressure using the 6-hourly analysed wind field based on the rationale that an analysed wind field will be more accurate than a forecasted wind. However due to the fact that the analysed wind fields are at 6-hourly intervals, comparison with the 3-hourly forecasted wind field shows that the 6 hourly wind fields is missing an extreme high wind at the time of the storm which is only captured by the forecasted wind, which significantly improves the storm surge prediction. Hence the final imposed wind field consist of analysed wind fields at 6-hourly interval with intermittent 3-hourly forecasted wind, see Figure 3.4.

Figure 3.4 HIRLAM wind fields for 1 Nov 2006, 0:00 to 6:00 am Analysed

Forecasted

3.5.2 Water level

The boundary conditions of the CSM regional model are defined by tidal constituents, hence no additional water levels are required. The nested flow model requires water level boundaries, of which there are two ways of imposing the boundary conditions of the nested flow model.

Initially the first approach taken was to start the flow modelling from the regional CSM (Continental Shelf Model) so as to generate boundary conditions for the nested flow model shown in Figure 3.3. However preliminary results and comparison with water level measurements shows that by taking this approach, the nested model is still underpredicting the water levels at the time of the storm by a considerable 1m at Norderney Riff (SeeFigure 1.1 for its location).

Figure 3.14 (in Appendix) shows the comparison using boundary conditions generated by the CSM8 model and using MATROOS4 (Multifunctional Access Tool foR Operational Oceandata Services) water levels. Both runs were generated using HIRLAM 6-hourly analysed wind field. By using the nested results from CSM8, the storm surge water levels were under-predicted by 1.1m and by imposing MATROOS water levels on the boundaries, the under-prediction was reduced to 0.7m. The MATROOS water levels were the results from multi-model forecast analysis5. Since they gave a better prediction, they were imposed on the boundaries of the nested flow model.

To account for the fresh water inflow of rivers Ems, Weser, Elbe and Eider, discharges are also imposed at 4 locations inside the flow model, see Figure 3.5. These discharge values were extrapolated from the model set up in 1997 (WL, 1997) and hence were not updated to the time of the storm. But they are expected to have minimal influence on the results.

4. Visit http://matroos/index.html for further information 5. Visit http://matroos/index.html for further information

Figure 3.5 Locations of offshore boundary conditions and discharges

3.5.3 Bathymetry

For regional areas in deeper waters, bathymetry from nautical charts were used and these were supplemented with high resolution detailed surveyed bathymetry in Wadden Sea area updated to around 1997. The composed bathymetry is shown in Figure 3.6

1m3/s

100m3/s 10m3/s

50m3/s Matroos Water levels

Figure 3.6 Interpolated bathymetry for the Flow Model in Delft3D

3.5.4 Bottom roughness

For bottom roughness, the Manning formulation is used with a spatially varying Manning value gradually changing from 0.02 at the deep waters to 0.04 at the shallow waters, see Figure 3.7.

Figure 3.7 Spatially-varying Manning roughness used for the nested Flow model

3.5.5 Definition of output

For verification purposes, time histories of water level and current were generated at the 4 measurement locations shown in Figure 3.8, namely Borkum, Norderney Riff, Alte Weser, Vogelsand, At the boundary of the FSK hydrodynamic model shown in Figure 3.3, water level and current time series were computed.

Spatial variations over the entire flow grid of standard computed quantities (such as depth-averaged velocity, bed shear stress etc.) were also generated as output.

Figure 3.8 Location of water level measurements

3.6 Results and analysis

3.6.1 Introduction

For their hindcasts FSK requires currents and water levels during the period 3 days before and 1 day after the storm. The preferred time resolution of the data is 10 minutes. (personal communication between FSK and Ester Groenendaal).

Hence for flow simulations, the period of simulation has been set from 24-10-2006 00:00 to 02-11-2006 00:00, taking into account that the peak of the storm is at 01-11-2006 04:00. See Figure 3.9 for a schematization of the simulation.

The simulation starts with a spin-up period of three days for which only a background pressure of 101400 N/m2 and no wind is imposed. After the spin-up phase, the HIRLAM wind fields (with 6-hourly analysed fields intermitted with 3-hourly forecasted fields) were imposed.

Figure 3.9 Simulation period

24 Oct 00hr Start 27 Oct 00hr 1 Nov 04hr Time of Storm 2 Nov 00hr End Spin-up, No wind, background pressure

In the following subsections, computed and measured water levels are compared at the four locations where measurements are available, see Figure 3.8, namely Borkum, Norderney Riff, Alte Weser, Vogelsand. Since no current measurements are available, only the computed results of the current velocity are shown.

In addition a comparison between the flow results from Delft3D Flow stand-alone and Delft3D Flow (2-way coupling) will be shown to check the effect of waves on flow computations. For now, we refer to section 4 for the description of the wave model.

3.6.2 Fields

The results of the 1st November 2006 03:00 to 06:00 hindcast are shown in Figures 3.11 a-b and Figures 3.12 a-b (in Appendix), which present spatial distributions of water levels and currents computed using Delft3D Flow stand-alone (without influence of waves).

From the figures we see a strong increase in water level in Eems-Dollard estuary at 04:00 hrs to about 4m and a surge in the Wadden Sea from 03:00 to 04:00 hrs. A strong increase in water level up to about 5m can be observed at 05:00 hrs at the German Bight as the surge propagates eastwards.

Currents were generally flowing in an eastward direction and velocities in the vicinity of Norderney were of the order less than 1m/s. Velocities on the offshore northern side of Norderney were generally higher than the nearshore southern side.

3.6.3 Measured and modelled water level comparison

Figures 3.13a-b (in Appendix) show the comparison of water levels between results computed from from Delft3D Flow stand-alone and Delft3D Flow (2-way coupling) and actual measurements at two locations Norderney Riff and Vogelsand. The Delft3D runs were generated using imposed wind field consisting of analysed wind fields at 6-hourly interval with intermittent 3-hourly forecasted wind (hence a 3-hourly wind field as a result). As can be seen from Figure 3.10, Norderney Riff is at a relatively shallow location (a few metres deep) and Vogelsand is at a relatively deeper location (tens of metres deep). The comparison shows (Figure 3.13a) that at shallow areas, the influence of wave–induced setup is significant. Especially at the peak of the storm, the water levels calculated by Delft3D (2-way) is about 0.4m higher than that calculated by Delft3D stand-alone but over-estimating measurements.

Figure 3.10 Water level stations

At Vogelsand, as shown in Figure 3.13b, both produce very similar results, showing no, or a limited, effect of waves on flow. Both runs show an overestimation of approximately 0.2m, increasing to 0.5m at the peak of the storm.

3.7 Discussion and conclusions

Based on Figure 3.13a, results from Delft3D stand-alone gave an almost exact agreement with the storm surge at the time of the storm at Norderney and since at offshore locations, waves do not affect the flow, it is decided to deliver the results from Delft3D stand-alone computation for boundary conditions to FSK flow model.

4 Wave

model

4.1 Introduction

The wave model in SWAN is set up to generate wave conditions at the boundaries of the FSK wave model as shown in Figure 4.3 (circles represent the location of FSK model boundary). Unlike the FSK flow model, the boundaries of the FSK wave model lie relatively nearshore. Hence the effect of flow has to be taken into account in the wave model. The effect of flow on waves can be taken into account through two approaches of modelling, namely a 1-way or a 2-way coupling. This is done interactively through the Delft3D graphical interface.

The periods of simulation (Delft3D-WAVES) are as follows:

1 way couple - 31 Oct 2006 18:00 to 01 Nov 2006 12:00 2 way couple - 31 Oct 2006 12:00 to 01 Nov 2006 12:00

There is a limitation on the length of simulation period due to the file size limit (4GB) of the Nefis output file of Delft3D. In these simulations results were stored at the client-requested interval of 10 minutes and had a spin-up period of 6 hours.

4.2 Model schematization

4.2.1 Grids

The computational grid used to model the 1st November 2006 storm covers a region of 90 km x 66 km consisting of 450 x 333 grid cells. This is a rectilinear grid with a resolution of 200m x 200m, rotated 57 degrees anticlockwise from the positive x-axis. The northwestern offshore boundary now connects SON and FINO. The wave spectra at these locations are imposed as boundary conditions. See Figure 4.1 for the schematization of the wave grid with respect to the flow grid and see Figure 4.2 for a closer view of the wave grid.

In terms of spectral resolution, a directional resolution of 10 degrees is used and the spectra are discretized in 37 frequencies distributed logarithmically between 0.03 Hz and 1 Hz, which is the optimised spectral resolution based on wave modelling experience in this area.

Figure 4.1 Flow and wave grid

4.2.2 Input data

Bathymetry, water level, currents and wind were read from the communication file produced from Delft3D FLOW. The depth profile used in the SWAN computations is shown in Figure 4.3.

Figure 4.3 Depth profile (in m wrt NN) in SWAN computations

4.2.3 Boundary conditions

The SWAN wave model requires boundary conditions at the eastern and north-western boundaries. Since the only available measurements offshore in the area of interest were from the SON and FINO wave buoys, these measurements were used to construct boundary conditions for the wave model. The actual application of the boundary conditions can be summarised as follows in Table 4.1.

Table 4.1 Summary of boundary conditions imposed in SWAN

No. Boundary Segment6 Boundary condition

1 AB uniformly - SON

2 BC varying - spectral interpolation between SON and FINO

by SWAN

3 CD uniformly - FINO

4 DE uniformly - FINO

6. See Figure 4.2 for the actual location of the boundary segments

4.2.4 Model settings

Current simulations were run using the latest to-be-released version 40.51AB which included the latest bugfixes. Depth-induced wave breaking has been modelled according to Battjes and Janssen (1978) and the JONSWAP formulation for bottom friction (Hasselmann et al., 1973) has been applied, with a bottom friction coefficient set to 0.067 m2s-3.

Also triads have been activated in order to be consistent with Kaiser and Niemeyer (2001). In version 40.51AB the effect of triads and quadruplets are taken into account simultaneously.

Please note that all new features implemented in SWAN after version 40.01 that are not activated by the default settings, such as diffraction, are not accounted for in the present computations, for one exception. In version 40.51AB alternative formulations for the wind generation and whitecapping are available (see Van der Westhuysen et al., 2007), solving part of the problems occurring when modelling combined sea-swell states. In all computations with 40.51AB these formulations have been applied and they are activated with GEN3 WEST.

SWAN was run in non-stationary mode using both 1-way and 2-way coupling. By 1-way coupling, it was meant that the output from flow (in the form of a hydrodynamic communication file containing bathymetry, water level, current and wind) was transferred to SWAN and the flow will influence the SWAN results but not vice versa. In 2-way coupling, both flow and wave simulations are carried out simultaneously and their results will interactively influence each other.

4.2.5 Definition of output

Directional wave spectra are generated at locations inside the SWAN computation grid corresponding to the boundary of the FSK wave model (see Figure 4.3). Spatial variations over the entire grid of a number of standard integral wave parameters, such as significant wave height, mean wave direction, wave period, are also generated as standard output.

4.3 Results and analysis

4.3.1 Introduction

To hindcast the November 2006 storm, 1-way and 2-way couplings with the flow model were considered. The difference in results between 1-way and 2-way coupling will be discussed in Section 4.3.2.

4.3.2 Storm development

Figure 4.4 (in Appendix) shows the timeseries of the significant wave height Hm0 and

the mean wave period Tm01hindcasts and the respective observations of the offshore

buoys. The comparisons show good correlation between the offshore buoy data (at SON and FINO) and hindcasted nearshore timeseries at location 14 (See Figure 4.3).

The hindcasted wave heights nearshore are of about the same order of magnitude as the measurements at SON and about 20% lower than the FINO measurements. From

offshore to nearshore, SWAN, however, predicts quite a strong decrease of the mean wave period of about 25%.

The comparisons of Figure 4.4 show that the results of both hindcasts of 1-way and 2-way coupling are rather similar with differences of less than 2.5% in Hm0. This is

because the nearshore hindcasts are provided at relatively deep water (about 20m), where the wave-induced current and set-up are negligible.

4.3.3 Wave field at the peak of the storm

Based on the offshore buoy measurements we have determined that the peak of the storm occurred at 4:10 a.m. on the 1st of November 2006, which is in accordance with other storm reports in the press.

Figure 4.5 (in Appendix) shows the spatial variation of the hindcasted wave parameters Hm0and Tm01at 04:10am. Note that the white spots in these and other field plots are

locations where the bottom level is higher than the water level (drying).

Figure 4.5 shows significant wave dissipation from offshore to the North side of the barrier islands; offshore wave heights are in excess of 9 m and just offshore of Norderney about below 5 m. The periods decrease from more than 10 s to less than 7s.

4.3.4 Wave spectra at the peak of the storm

November 1, 2006; 04:10 hr

Figure 4.6 (in Appendix) presents the measured wave spectra at SON and FINO and the computed wave spectra at Location 14 (see Figure 4.3 location 14 marked with a cross) using both 1-way and 2-way coupling.

The first conclusion to be drawn from this results is that 1-way and 2-way coupling does not appear to have an effect on the spectra at Location 14 lying on the boundary of the FSK wave model.

Secondly, as already noted in the previous comparisons, there is significant energy loss from the boundaries of the model (SON & FINO) to the boundaries of the FSK wave model due to energy dissipation, most probably due to bottom friction and wave breaking. The reasons for the energy dissipation, however, can only be identified through further investigation, which is beyond the scope of the present study.

5

Conclusions

and

recommendations

In this study we have used the SWAN wave and Delft3D flow models to hindcast the 1st of November 2006 storm and determined boundary conditions for regional models. These flow and wave model will be used by FSK to perform a wave hindcast of the storm within the Norderneyer Seegat.

For the flow model, we have imposed water levels extracted from MATROOS as boundary conditions in Delft3D-Flow and for the wave model, buoy measurements from SON and FINO were used as boundary conditions in SWAN.

The performed hindcast provides the required offshore boundary conditions for the future wave and flow computations. Where measurements are available, the computed results compare well. There are no wave measurements available outside the inlet to validate the hindcasts, but the results seem to be correct in a qualitative sense.

One of the findings in this study is that the computed water levels underpredicted the peak of the measured water levels, if 6-hr interval windfields were used. Application of 3-hr interval wind fields, obtained from the official 6hr intervals interlaced with predicted 3hr wind fields, improved the results dramatically. It is recommended in the future to use these 3hr intervals.

6 References

Battjes, J.A. and J.P.F.M. Janssen (1978). Energy loss and set-up due to breaking of random waves, Proc. 16th Int. Conf. Coastal Engineering, ASCE, 569-587. Hasselmann, K., T.P. Barnett, E. Bouws, H. Carlson, D.E. Cartwright, K. Enke, J.A.

Ewing, H. Gienapp, D.E. Hasselmann, P. Kruseman, A. Meerburg, P. Müller, D.J. Olbers, K. Richter, W. Sell and H. Walden (1973). Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project

(JONSWAP), Dtsch. Hydrogr. Z. Suppl., vol. 12, A8.

Van der Westhuysen, A.J., M. Zijlema and J.A Battjes (2007). Nonlinear saturation-based whitecapping dissipation in SWAN for deep and shallow water. Coastal Engng., 54, 151-170.

WL (1997). Deutsche Bucht and Dithmarschen Bucht : set-up and calibration of tidal flow models. WL|Delft Hydraulics Report H1821, November 1997.

WL (2002). Measurement campaign Wadden Sea. Module: What to measure? WL | Delft Hydraulics report H4174, October 2002.

WL (2006). SBW Plan of Action on the Boundary Conditions for the Waddenzee. WL | Delft Hydraulics report H4750, April 2006.

WL (2006). SBW Storm hindcasts Norderneyer Seegat and Amelander Zeegat. WL|Delft Hydraulics Report H4803.11, August 2006

Water level spatial map

01-nov-06

Upper panel @ 03:00 AM, Bottom panel @ 04:00AM

Delft3D Flow

Norderneyer

Juist

Wadden Sea

x coordinate (m) y coo rdi na te (m) 2.5 3 3.5 4 4.5 5 5.5 x 105 5.85 5.9 5.95 6 6.05 6.1 6.15 0.5 1 1.5 2 2.5 3 3.5 4 4.5 water level (m) 01-Nov-2006 04:00:00 x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 5.5 x 105 5.85 5.9 5.95 6 6.05 6.1 6.15 6.2 x 106 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Water level spatial map

01-nov-06

Upper panel @ 05:00 AM, Bottom panel @ 06:00AM

Delft3D Flow

Norderneyer

Juist

Wadden Sea

x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 5.5 x 105 5.85 5.9 5.95 6 6.05 6.1 6.15 0.5 1 1.5 2 2.5 3 3.5 4 4.5 water level (m) 01-Nov-2006 06:00:00 x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 5.5 x 105 5.85 5.9 5.95 6 6.05 6.1 6.15 6.2 x 106 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Current magnitude and direction spatial map

01-nov-06

Upper panel @ 03:00 AM, Bottom panel @ 04:00AM

Delft3D Flow

Norderneyer

Juist

Wadden Sea

x coordinate (m) y coordi nat e (m) 2.5 3 3.5 4 4.5 5 x 105 5.9 5.95 6 6.05 6.1 6.15 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

depth averaged velocity 01-Nov-2006 04:00:00 x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 x 105 5.9 5.95 6 6.05 6.1 6.15 6.2x 10 6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Current magnitude and direction spatial map

01-nov-06

Upper panel @ 05:00 AM, Bottom panel @ 06:00AM

Delft3D Flow

Norderneyer

Juist

Wadden Sea

x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 x 105 5.9 5.95 6 6.05 6.1 6.15 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

depth averaged velocity 01-Nov-2006 06:00:00 x coordinate (m) y coordi nat e (m ) 2.5 3 3.5 4 4.5 5 x 105 5.9 5.95 6 6.05 6.1 6.15 6.2x 10 6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Water Level time series

Delft3D

comparison between computed and measurements

Norderney

Norderneyer

Juist

Wadden Sea

WL1-Norder

ney

-2 -1 0 1 2 3 4 5 26-10-06 0:00 27-10-06 0:00 28-10-06 0:00 29-10-06 0:00 30-10-06 0:00 31-10-06 0:00 1-11-06 0:00 2-11-06 0:00 GMT tim e water elevat ion( m) Germ an Measur em ent D3D (2-way )-3hour ly wind D3D s tandalone-3hour ly wind peak of s torm

Water Level time series

Delft3D

comparison between computed and measurements

Vogelsand

Norderneyer

Juist

Wadden Sea

WL4-V

ogelsan

d

-2 -1 0 1 2 3 4 5 26-10-06 0: 00 27-10-06 0: 00 28-10-06 0: 00 29-10-06 0: 00 30-10-06 0: 00 31-10-06 0: 00 1-11-06 0: 00 2-11-06 0: 00 3-11-GM T time water elevatio n (m) G erm an Measurem ent D3D (2-way) D3D standal one peak of storm

Water Level time series

Delft3D

comparison of different boundary conditions

Norderney

Norderneyer

Juist

Wadden Sea

WL1-Norderney

-2 -1 0 1 2 3 4 5 23-10-06 0:00 25-10-06 0:00 27-10-06 0:00 29-10-06 0:00 31-10-06 0:00 2-11-06 0:00 GM T time water elevat ion( m) D3D Nested f rom CSM-6hourly wind Germ an Measurem ent D3D Matroos BC-6hourly wi nd

1st November 2006 storm development

Measurements and hindcasts of H_m0 & T_m0,1

Delft3D WAVES Norderney 31-Oct-2006 00: 00:00 31-Oct-2006 12: 00:00 01-Nov -2006 00:00: 00 01-Nov -2006 12:00: 0 2 4 6 8 10 H m0 (m) St orm dev el opm ent from 31/10 00: 00 to 01/ 11 18:00 SON FINO Loc.14(2-w ay coupl e) Loc.14(1-w ay coupl e) 31-Oct-2006 00: 00:00 31-Oct-2006 12: 00:00 01-Nov -2006 00:00: 00 01-Nov -2006 12:00: 2 4 6 8 10 12 14 T m0,1 (s) St orm dev el opm ent from 31/10 00: 00 to 01/ 11 18:00 SON FINO Loc.14(2-w ay coupl e) Loc.14(1-w ay coupl e)

Top panel: Hs at 01−Nov−2006 04:10:00 Bottom panel: T m0,1 at 01−Nov−2006 04:10:00 Delft3D WAVES Norderney 3 3.2 3.4 3.6 3.8 4 4.2 x 105 5.9 5.91 5.92 5.93 5.94 5.95 5.96 5.97 5.98 5.99 6 Easting [m, UTM32] Northing [m, UTM32] 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 3 3.2 3.4 3.6 3.8 4 4.2 x 105 5.9 5.91 5.92 5.93 5.94 5.95 5.96 5.97 5.98 5.99 6 6.01x 10 6 Easting [m, UTM32] Northing [m, UTM32] T m0,1 (s) 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

1st November 2006 storm Measurements and hindcasts

Delft3D WAVES Norderney 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0 20 40 60 80 100 120 140 160 180 Frequency (Hz ) Energy density (m 2 /Hz) SO N 01-11-2006 04:10A M FI NO 01-11-2006 04:09A M Loc.14(2-w ay coupl e) 01-11-2006 04:10A M Loc.14(1-w ay coupl e) 01-11-2006 04:10A M