Origin and prediction of seiches in Rotterdam harbour basins

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(2) Origin and prediction of seiches in Rotterdam harbour basins Oorsprong en voorspelling van seiches in Rotterdamse havenbekkens.

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(4) Origin and prediction of seiches in Rotterdam harbour basins. Proefschrift. ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op 10 mei 2004 om 10.30 uur door Martijn Petrus Cornelis DE JONG civiel ingenieur geboren te Haastrecht..

(5) Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. J.A. Battjes. Samenstelling promotiecommissie: Rector Magnificus,. voorzitter. Prof. dr. ir. J.A. Battjes,. Technische Universiteit Delft, promotor. Prof. dr. S. Monserrat,. Universitat de les Illes Balears, Spanje. Prof. dr. E. Deleersnijder,. Université Catholique de Louvain, België. Dr. A. J. van Delden,. Universiteit Utrecht. Prof. ir. H. Ligteringen,. Technische Universiteit Delft. Prof. drs. ir. J.K. Vrijling,. Technische Universiteit Delft. Dr. ir. H. Gerritsen,. WL|Delft Hydraulics. c 2004 by M.P.C. de Jong Copyright Printed by PrintPartners Ipskamp B.V., The Netherlands. ISBN 90-9017925-9 This thesis is also published in the series ‘Communications on Hydraulic and Geotechnical Engineering’ of the Faculty of Civil Engineering and Geosciences, Delft University of Technology, as Report No. 04-2, ISSN 0169-6548..

(6) F. A. Forel (who was the first to scientifically explore the essential nature and the origin of seiches): “... I feel bound to recognize in the phenomenon of seiches the grandest oscillatory movement which man can study on the face of our globe.” “If you will follow and study with me these movements you will find a great charm in the investigation.” Originally from: Forel, Les Seiches, Vagues d’Oscillation (1875), cited in English in: G. H. Darwin, The Tides (1898)..

(7) Front cover shows infrared satellite image (11 January 1995, 10:12 gmt) of atmospheric convection cells as they approach the Dutch coast from the north-west. Source image: Dundee Satellite Receiving Station, Dundee University, Scotland. Small cover images show some of the key harbour elements mentioned in this thesis. Top image shows the harbour mouth; second image shows the storm surge barrier in the Rotterdam Waterway during test closure on 13 September 2003 (river side); third image shows the sea side of the barrier during this test closure and the bottom image shows the Caland Canal at nightfall. Photos by M.P.C. de Jong, 2003..

(8) Abstract Origin and prediction of seiches in Rotterdam harbour basins This thesis focuses on the harbour oscillations that occasionally occur in certain basins of the Port of Rotterdam. Such standing waves, called seiches, need to be taken into account in this harbour area for the design of water protection works (such as dykes) and for the closure management of a movable storm surge barrier, located along the Rotterdam Waterway, which can become susceptible to seiches under specific conditions. Previous studies and experiences indicated that the seiches that occur in the Port of Rotterdam have an atmospheric origin. A literature review indicated that the atmospheric generating mechanisms that were known from other ports do not apply to the Port of Rotterdam. Therefore, this study has been conducted with the main aim to identify the origin of the seiches that occur in this harbour and to eventually arrive at a prediction system for significant seiche episodes. The atmospheric origin of low-frequency seawaves that cause seiches in the Port of Rotterdam was investigated using hydrological and meteorological observations. These observations, combined with weather charts, showed that all significant seiche episodes coincided with the passage of a low pressure area and a cold front. The majority of these events (90%) occurred during the storm season. Following the front passages that coincided with seiche events in the storm season, significant wind speed fluctuations occurred with periods in the order of one hour. The records showed that enhanced low-frequency wave energy at sea in the vicinity of the harbour mouth, as well as the seiche events in the harbour, occur more or less simultaneously with these strong wind speed fluctuations. The oscillatory wind speed changes are due to atmospheric convection cells that arise in an unstable lower atmosphere in the area behind a cold front, where relatively cold air moves over the relatively warm sea surface. It has been shown that the moving system of a cold front and trailing convection cells generates forced low-frequency waves at sea that can cause seiche events inside the harbour. It was found that the generating mechanism that has been identified can excite basins with different natural frequencies. The ratios of seiche amplitudes at different locations inside the vii.

(9) viii harbour have been derived from measurements. Moreover, theoretical amplitude ratios have been determined, based on a parameterised excitation spectrum at the harbour mouth and computed amplification spectra at the considered locations. These amplification spectra have been derived in previous studies from a numerical hydrodynamic model of the harbour. The results confirm the validity of this method. Moreover, this method confirms the validity of the amplitude ratio that has been used for the design of the main storm surge barrier of Rotterdam to within 5%. Numerical simulations were made with a hydrodynamical model of the southern North Sea that was forced by fields of atmospheric pressure and wind obtained from a state-of-theart atmospheric model. The results from these simulations show that the hydrodynamical model does not reproduce the generation of low-frequency waves at the North Sea for the most common situation that can be found during seiche episodes in the Port of Rotterdam, characterised by gradual changes in atmospheric pressure. This is ascribed to the fact that at present it is not possible to simulate meso-scale phenomena such as atmospheric convection cells in these numerical weather prediction models. In the final part of this study, a prediction method based on criteria for the occurrence of convection cells has been tested in a hindcast setting. Results indicate that it has now become possible to predict the occurrence of seiche episodes in the Port of Rotterdam with high accuracy. Eventually, this prediction could be expanded through the use of a neural network. This network can be trained with relevant, quantifiable parameters that have been identified in the present study. This may open the possibility of operational forecasting of seiche-prone conditions, together with expected seiche crest heights. In conclusion, the present study has identified the origin of the majority of the seiches that occur in the Port of Rotterdam. Moreover, a method for the prediction of the occurrence of seiche episodes has been developed. Such seiche prediction method could be used by the port authorities for ship traffic control and, after further development and testing, eventually could be incorporated into the closure-management system of the storm surge barrier in Rotterdam Waterway. Martijn de Jong.

(10) Samenvatting Oorsprong en voorspelling van seiches in Rotterdamse havenbekkens Deze disseratie beschrijft onderzoek naar oscillaties die geregeld optreden in sommige bekkens van de haven van Rotterdam. Zulke staande golven, seiches genaamd, moeten in rekening worden gebracht in deze haven bij het bepalen van ontwerphoogten van dijken, en zij zijn van belang voor de sluitingsstrategie van de beweegbare Stormvloedkering Nieuwe Waterweg, aangezien deze kering in specifieke omstandigheden gevoelig is voor belasting als gevolg van seiches. Voorgaande studies en algemene ervaring geven aan dat seiches in de haven van Rotterdam worden opgewekt door atmosferische fenomenen. Een literatuurstudie liet zien dat de atmosferische opwekkingsmechanismen die verantwoordelijk zijn voor de opwekking van seiches in andere havens niet opgaan voor de haven van Rotterdam. Daarom is deze studie gestart met als hoofddoel het identificeren van het opwekkingmechanisme van de seiches die in deze haven optreden en het mogelijk ontwikkelen van een methode voor het voorspellen van significante seiche-episoden. Hydrologische en meteorologische metingen zijn gebruikt voor het bestuderen van de atmosferische oorsprong van de laagfrequente golven op zee die de seiches veroorzaken in de haven van Rotterdam. Deze metingen, gecombineerd met weerkaarten, lieten zien dat alle significante seiche-episoden samenvielen met de passage van een lagedrukgebied en een koufront. Een ruime meerderheid (90%) van de gevallen trad op in het stormseizoen. Volgend op de frontpassages in het stormseizoen traden tijdsintervallen op met toegenomen windsnelheidsfluctuaties met perioden in de orde van een uur. De meetsignalen lieten zien dat toegenomen laagfrequente golfenergie op zee in de omgeving van de havenmond, en de seiches in de haven, ongeveer gelijktijdig optreden met deze sterke windsnelheidsfluctuaties. Deze oscillerende windsnelheidsveranderingen worden veroorzaakt door atmosferische convectiecellen die optreden in een onstabiele laag in de atmosfeer in het gebied achter een koufront waar relatief koude lucht over het warme zeewater stroomt. Het is aangetoond dat het voortbewegende systeem van koufront en achterliggende convectiecellen geforceerde laagfrequente golven op ix.

(11) x zee opwekt die vervolgens een seiche-epsiode kunnen veroorzaken in de haven. Uit analyse van metingen bleek dat het ge¨ıdentificeerde opwekkingsmechanisme bekkens met verschillende eigenfrequenties aan kan slaan. De verhoudingen van seiche-amplituden op verschillende lokaties in de haven zijn uit metingen afgeleid. Daarnaast zijn theoretische waarden van deze verhoudingen bepaald, op basis van een geparameteriseerd aanbodspectrum aan de havenmond en berekende amplificatiespectra van de beschouwde lokaties. Deze amplificatie spectra zijn in voorgaande studies bepaald op basis van een numeriek hydrodynamisch model van de haven. De resultaten bevestigen de geldigheid van deze methode. Bovendien bevestigt deze methode de geldigheid van de waarde van de amplitudeverhouding die toegepast is voor het ontwerp van de Stormvloedkering Nieuwe Waterweg tot op 5%. Numerieke simulaties zijn gemaakt met een hydrodynamisch model dat werd aangedreven door velden van atmosferische druk en van wind, afkomstig uit een hedendaags meteorologisch model. Uit de resultaten van deze simulaties kan opgemaakt worden dat het hydrodynamische model er niet in slaagt de opwekking van laagfrequente golven op de Noordzee te reproduceren voor een situatie van geleidelijk variërende atmosferische druk (waargenomen tijdens veruit de meeste seiche-episoden). Dit wordt toegeschreven aan het feit dat het op dit moment nog niet mogelijk is om meso-schaal fenomenen zoals atmosferische convectiecellen op een betrouwbare manier te simuleren met numerieke weersvoorspellingsmodellen. In het laatste deel van deze studie is een methode getest die het optreden van deze episoden voorspelt op basis van criteria voor het optreden van atmosferische convectiecellen. In een ‘hindcast’ is deze voorspelmethode getest. Resultaten van deze test geven aan dat op dit moment het optreden van seiches in de haven van Rotterdam met een hoge nauwkeurigheid voorspeld kan worden. Uiteindelijk zou deze voorspelling uitgebreid kunnen worden door middel van een neuraal netwerk. Dit netwerk kan getraind worden met kwantificeerbare parameters die ge¨ıdentificeerd zijn in deze studie. Dit zou het mogelijk kunnen maken om een operationele voorspelling van seiche-episoden op te zetten, inclusief voorspelde seicheamplituden. Concluderend kan gesteld worden dat de huidige studie heeft geleid tot de identificatie van de oorsprong van het overgrote deel van de seiche-episoden die in de haven van Rotterdam optreden. Daarnaast is een methode ontwikkeld voor voorspelling van het optreden van seiche-episoden. Na verdere ontwikkeling en testen kan deze voorspelmethode door de havenautoriteiten worden gebruikt voor het toewijzen van getijvensters aan schepen met een grote diepgang. Bovendien kan deze methode uiteindelijk worden ingebouwd in de sluitingsstrategie van de Stormvloedkering Nieuwe Waterweg. Martijn de Jong.

(12) Contents 1 Introduction 1.1. 1. Seiches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1. 1.1.1. General description of seiches . . . . . . . . . . . . . . . . . . . . . . .. 1. 1.1.2. Origin of the word ‘seiche’ . . . . . . . . . . . . . . . . . . . . . . . . .. 2. 1.1.3. Main seiche locations in the Netherlands . . . . . . . . . . . . . . . . .. 4. 1.1.4. Effects of seiches in the Port of Rotterdam . . . . . . . . . . . . . . .. 5. Previous studies/literature overview . . . . . . . . . . . . . . . . . . . . . . .. 10. 1.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10. 1.2.2. Potential seiche generating mechanisms . . . . . . . . . . . . . . . . .. 10. 1.2.3. Seiche generation by atmospheric phenomena . . . . . . . . . . . . . .. 11. 1.2.4. Relevance to the Port of Rotterdam . . . . . . . . . . . . . . . . . . .. 13. 1.3. Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14. 1.4. Outline of thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14. 1.2. 2 Measurement data and analysis 2.1. 2.2. 15. Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15. 2.1.1. Measurements obtained from fixed platforms . . . . . . . . . . . . . .. 15. 2.1.2. Remote sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18. Data analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19. 2.2.1. Fourier based filtering . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19. 2.2.2. Wavelet analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19. 3 Characteristics of seiche events. 21. 3.1. Two types of seiche events . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 21. 3.2. Shape of variance density spectra . . . . . . . . . . . . . . . . . . . . . . . . .. 23. 3.3. Measured seiche amplitudes at different locations . . . . . . . . . . . . . . . .. 29. 3.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29. 3.3.2. Coinciding seiche events in different basins of the Port of Rotterdam .. 29. xi.

(13) CONTENTS. xii 3.3.3 3.4. 3.5. Amplitude ratios derived from measurements . . . . . . . . . . . . . .. 30. Theoretical seiche amplitudes at different locations . . . . . . . . . . . . . . .. 31. 3.4.1. Method for determining theoretical amplitude ratios . . . . . . . . . .. 31. 3.4.2. Verification of theoretical amplitude ratios . . . . . . . . . . . . . . . .. 32. 3.4.3. Theoretical amplitude ratios for roz and the storm surge barrier . . .. 33. Evaluation of known generating mechanisms . . . . . . . . . . . . . . . . . . .. 34. 4 2D numerical simulations. 37. 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 4.2. 2D hydrodynamical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 4.2.1. Governing equations . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 4.2.2. Numerical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 38. 4.2.3. Grids, time steps, boundary conditions, initial conditions and forcing .. 39. Simulation of seiche events based on artificial fields . . . . . . . . . . . . . . .. 41. 4.3.1. Selection of events . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 41. 4.3.2. Driving forces (frozen pressure and wind fields) . . . . . . . . . . . . .. 41. 4.3.3. Seiche event of 30 October 2000. . . . . . . . . . . . . . . . . . . . . .. 41. 4.3.4. Seiche event of 8 and 9 November 2001 . . . . . . . . . . . . . . . . .. 44. 4.3.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46. Simulation of seiche events based on simulated fields . . . . . . . . . . . . . .. 46. 4.4.1. Selection of events . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 46. 4.4.2. Driving forces (simulated fields of atmospheric pressure and wind) . .. 46. 4.4.3. Seiche event of 30 October 2000. . . . . . . . . . . . . . . . . . . . . .. 47. 4.4.4. Seiche events of 8/9 and 22/23 November 2001 . . . . . . . . . . . . .. 49. 4.4.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 50. 4.3. 4.4. 5 Generation of low-frequency waves and seiches. 53. 5.1. Key observations of wind speed and surface elevation . . . . . . . . . . . . . .. 53. 5.2. Generation of low-frequency seawaves by convection cells . . . . . . . . . . . .. 57. 5.3. Observations of episodes with convection cells . . . . . . . . . . . . . . . . . .. 60. 5.4. Previous studies on mechanism of generation . . . . . . . . . . . . . . . . . .. 64. 5.5. 1D hydrodynamical model with harmonic forcing . . . . . . . . . . . . . . . .. 66. 5.5.1. Model description . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 66. 5.5.2. Harmonic forcing and equilibrium response . . . . . . . . . . . . . . .. 68. 5.5.3. Application to the North Sea . . . . . . . . . . . . . . . . . . . . . . .. 69. 5.6. 1D hydrodynamical model with transient forcing . . . . . . . . . . . . . . . .. 74. 5.7. Convection cells in extreme storm situations . . . . . . . . . . . . . . . . . . .. 74.

(14) CONTENTS 5.8. xiii. Convection cells in other parts of the world . . . . . . . . . . . . . . . . . . .. 6 Prediction of seiche events. 76 79. 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 79. 6.2. Prediction based on monitoring at North Sea . . . . . . . . . . . . . . . . . .. 80. 6.3. Prediction through 2D numerical simulations . . . . . . . . . . . . . . . . . .. 83. 6.4. Prediction of relevant cold front passages . . . . . . . . . . . . . . . . . . . .. 84. 6.5. Prediction of the occurrence of convection cells . . . . . . . . . . . . . . . . .. 88. 6.5.1. Criteria for the occurrence of convection cells . . . . . . . . . . . . . .. 88. 6.5.2. Prediction of seiche events based on the prediction of convection cells. 89. 6.6. Operational prediction of seiche events in Rotterdam . . . . . . . . . . . . . .. 7 Conclusions and recommendations. 95 97. 7.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 97. 7.2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 100. References. 103. A Infrared satellite images. 109. Dankwoord. 117. Curriculum vitae. 119.

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(16) Chapter 1. Introduction 1.1 1.1.1. Seiches General description of seiches. Gravity surface waves occur at sea in a broad frequency band ranging from conventional wind waves, with typical periods of a few seconds, to tidal waves, with typical periods of 12 hours. The generation mechanisms of both wind waves and tides are relatively well known, whereas the generating mechanisms of waves in the ‘spectral valley’ between these more energetic tides and sea/swell are not all known or fully understood. Standing waves in the intermediate frequency band can occur in harbour basins and on lakes. These waves are called seiches. The seiches that occur on lakes can be due to e.g. wind induced set up in case the wind suddenly subsides or changes direction. In coastal harbours, standing waves can arise as a result of incoming waves from sea. Resonance can occur in harbour basins in case the frequency of the incoming waves is near a natural frequency (eigenfrequency) of the basin. In these situations of resonance, the incoming waves are strongly amplified. This thesis focuses on the origin of the seiches that occur in the Port of Rotterdam. In the remaining part of this introductory chapter, the etymological origin of the term ‘seiche’ will be reviewed first, followed by a brief description of the seiches that occur in the two largest harbours along the Dutch coast and an overview of the effects of seiches in Rotterdam harbour. A literature review of potential generating mechanisms is described in Section 1.2, followed by an overview of the aims and the scope of this study in Section 1.3. Finally, the outline of the remaining chapters is described in Section 1.4. 1.

(17) CHAPTER 1. INTRODUCTION. 2. 1.1.2. Origin of the word ‘seiche’. The fact that the term ‘seiche’ originates from Lake Geneva (Switzerland/France) is relatively well known (see e.g. Defant 1961). Forel, who was the first to scientifically explore the essential nature and the origin of standing waves in lakes, studied the seiches that occur in this lake (in the second half of the 19th century and the early years of the 20th century, see e.g. Forel 1892). In one of his works (Forel 1901, page 63), Forel mentions that: ”Diese stehenden Wellen werden von den Anwohnern des Genfer Sees ‘Seiches’ genannt. Es ist dem Verfasser wie dem Uebersetzer nicht gelungen, eine einwurfsfreie Verdeutschung dieses Wortes zu finden. Wir haben uns daher entschlossen, den franz¨ osischen Lokalausdruck auch in die deutsche ... Terminologie aufzunehmen und werden in der Folge diese eigent¨ umlichen stehenden wellen Kurzweg mit dem Namen ‘Seiches’ ... bezeichnen.” (These standing waves are called ‘seiches’ by the people living along the shores of Lake Geneva. The author and the translator did not succeed at translating this word unambiguously into German. Therefore, we decided to include the local French expression in the German technical terms and for brevity refer to these peculiar standing waves from now on as ‘seiches’.). The fact that the local people used this word for the standing wave phenomenon on the lake was not always appreciated by scientific authors. According to Le Grand Robert de la langue Fran¸caise (see Rey 1986, page 673), the use of this name for the phenomenon was controversial. This dictionary refers to an encyclopedia (Encyclopédie, Diderot, Léman, 1765), with the following quotation: ”Un phénomène beaucoup moins rare que nous offre le Lac Léman, est une espèce de flux et reflux qu’on y remarque sous le nom vulgaire et ridicule de seiches ...” (A phenomenon much less rare than provided to us by Lake Geneva, is a type of flow and backward flow that the people observe here under the common (ordinary) and ridiculous name of seiches ...). The reason that this strong disapproval of the use of the word ‘seiche’ appears in an encyclopedia could be that at that time the word ‘seiche’ already had another meaning in French (as is still the case), viz. ‘squid’ or ‘octopus’. According to the French dictionary Grand Larousse (see Guilbert et al. 1977), the use of ‘seiche’ in the meaning of octopus goes back to the 12th and 13th century AD, indicating that this was the original meaning. Bouasse (1924) also mentions that the local people (and skippers in particular) named the phenomenon: ”On connaˆıt les seiches depuis longtemps, en particulier pour le lac de Genève: seiches est le nom que donnent les bateliers du Léman aux oscillations superficielles.

(18) 1.1. SEICHES. 3. assez amples pour être constatées sans instrument.” (Seiches have been known for a long time, in particular for Lake Geneva: seiches is the name that skippers (bargemen or ferrymen) give to these surface oscillations, which are large enough to be observed without instruments.). Unfortunately, no comment is made about why these waves were labelled ‘seiches’ by the skippers on Lake Geneva. In general, the origin of the word ‘seiche’ is believed to be connected to the word ‘dry’ or ‘exposed’. E.g. Wilson (1972) states in the context of standing water waves: “The word ‘seiche’ is believed to have a Latin derivation from ‘siccus’, meaning dry or exposed” (this possibility is also mentioned in Fairbridge 1966). Another version of such an explanation is given in the French dictionary Grand Larousse (see Guilbert et al. 1977, page 5435), together with a reference to a dictionary from the 18th century, where the origin of the word ‘seiche’ is ascribed to a derivation of ‘sèche’ (literally ‘sand bank’ or dry) or of ‘sec’ (dry), somehow connected to “écueils sous l’eau a ` peu de profondeur” (cliffs under water in shallow regions). Several other works refer to a similar origin. However, some authors questioned the validity of this frequently suggested origin. E.g. Darwin (1898, page 17) states: ”The word ‘seiche’ is a purely local one. It has been alleged to be derived from ‘sèche’, but I can see no reason for associating dryness with the phenomenon”. Usually, it is suggested that the the term ‘dry’ (sec or sèche) was linked to seiches through local residents who observed a temporary exposure of certain beach areas along the shores of Lake Geneva. This possibility is confirmed by Forel (1892), who mentioned observations which described that at least one of the harbours along Lake Geneva was completely drained during extreme events. He refers to Fatio de Duiller, who was the first to report the seiches on Lake Geneva (in 1730). Forel cites De Duiller with the following quotation: ”On a vu quelquefois dans cette ville (Geneva) des seiches très remarquables: il s’en fit trois ou quatre le 16 septembre 1660, avant midi, d’environ 5 pieds de hauteur; de sorte que les bateaux qui étaient dans le port y restèrent autant de fois a` sec; mais l’eau revenait s’élevait chaque fois avec beaucoup de promptitude” (Very remarkable seiches are occasionally observed in this city (Geneva): three or four of them occurred on September 16, 1660, before midday, with an approximate height of 5 feet; such that the boats which were in the port rested as many times on the dry bottom; but the water returned and rose each time with much quickness.). One reference has been found that explicitly contradicts the connection between ‘seiche’ and ‘sèche’. The Oxford English Dictionary (see Simpson and Weiner 1989, page 891) states:.

(19) CHAPTER 1. INTRODUCTION. 4. ”Seiche, ... Swiss French, ‘seiche’, perhaps a graphic adoption of ‘seiche’ (German), sinking (of water). Not connected, as is usually stated, with seiche, sèche (French), ‘a portion of the sea-bottom left uncovered at low tide’.” This possibility could be considered unlikely, since the local dialect near Lake Geneva is mainly based on French. However, infrequent German influences are indeed included in this dialect. Moreover, the suggested origin (a German word meaning sinking of water, ‘seicht’ means shallow in German) could help explain why the local residents used this word although it already had the meaning of ‘octopus’ in French.. Present day use of the term ‘seiche’ Seiches occur all over the world. Historically, different local names have been given to these standing wave phenomena in some areas. Examples of local names are: ‘Rissaga’ on Menorca (Spain), ‘Marubbio’ on Sicily (Italy), ‘Seeb¨ ar’ on the Baltic Sea, ‘Abiki’ in Japan and ‘Haling’ in The Netherlands. Regardless of the origin of the word ‘seiche’ (which will probably remain obscure), the choice of Forel not to translate it into another language and to simply adopt it as a technical term has contributed to the fact that the originally local term ‘seiche’ is nowadays used throughout the scientific world as a general term for a variety of standing wave phenomena that occur in lakes, in coastal regions and in harbours all over the world. This is also the case for The Netherlands, where the local Dutch name ‘Haling’ is practically not used anymore. Because of its present preponderance (in The Netherlands and internationally), the word ‘seiche’ is also adopted in this thesis.. 1.1.3. Main seiche locations in the Netherlands. In the Netherlands, seiches were first extensively studied for the Port of IJmuiden, near Amsterdam (Wemelsfelder 1957). The canal from the Port of Amsterdam to the North Sea is closed near the coast by a lock at IJmuiden. Seiches occur in the outer harbour, with typical periods of 10 minutes or 30 minutes (Veldman 2000). The seiches that occur in the Port of IJmuiden have been a well-known phenomenon for decades, especially because of their significant effect they had on lock activities. They could hamper the operation of the lock gates, since a seiche could cause unexpected water level differences between both sides of the gates. The resulting force on the gates could cause them to derail from the bottom guidance system. In more severe cases (amplitude exceeding 0.25 m at the sluice location), the lock activities had to be ceased in order to avoid damage to the.

(20) 1.1. SEICHES. 5. lock gates. Some years ago, the guidance system of the gates has been altered and the critical influence of seiches has been greatly reduced. The second main seiche location along the Dutch west coast is the Port of Rotterdam. As was mentioned in Section 1.1.1, the focus of this thesis is on the seiches that occur in this harbour. The outer Rotterdam harbour area, which is seiche prone, is shown in Figure 1.1. Here, the seiches with the largest crest heights occur in a canal called the Caland Canal, with measured amplitudes up to 0.9 m at the closed end near Rozenburgse Sluis (roz). This semi-closed basin has a length of approximately 20 km and a depth of approximately 20 m. For almost all of the observed seiche events at roz, the dominant frequency is the lowest eigenfrequency (quarter-wavelength mode), equivalent to a period of 90 minutes (somewhat dependent on the water depth which varies with the tide, 85–100 minutes). Relatively large amplitude seiches also occur in the Europe Harbour (location eh, see Figure 1.1), where the dominant eigenperiod is approximately 50 minutes.. Figure 1.1: The Port of Rotterdam. Hydro-meteo observations at Rozenburgse Sluis, Hook of Holland and Rotterdam Airport. Figure 1.2 shows an example of a water level registration at roz including a seiche superposed on the tide, together with a registration at a reference location that is not seiche prone (no closed end), near the city center further inland (south of Rotterdam Airport, indicated in Figure 1.1). This example event (1 and 2 January 1995) is the seiche with the largest amplitude in the database that has been used for this study, spanning 1995–2001.. 1.1.4. Effects of seiches in the Port of Rotterdam. Generally, seiches that occur in harbours can influence shipping activities since they can hamper ship manoeuvring or cause flooding of quays. In the Port of Rotterdam, seiches have diverse effects, some of which are specifically related to this area. For example, they also.

(21) CHAPTER 1. INTRODUCTION. surface elevation (cm). 6. Rotterdam Center Rozenburgse Sluis. 300 200 100 0 −100 0. surface elevation (cm). Surface elevation 1 and 2 January 1995. 400. 100. 6. 12. 18. 0 6 12 18 time (h, GMT) Filtered surface elevation (0.1−2.0 mHz), 1 and 2 January 1995. 0. Rotterdam Center Rozenburgse Sluis. 50 0 −50 −100 0. 6. 12. 18. 0 6 time (h, GMT). 12. 18. 0. Figure 1.2: Top panel: surface elevation records at roz and near Rotterdam center (approximately 40 km inland) for 1 and 2 January 1995. The latter shows the tide dominated signal, whereas the observations at roz also show the seiche superposed on the tide signal; bottom panel: the surface elevation records shown in the top panel, now filtered for the seiche frequency band (0.1 – 2.0 mHz). need to be taken into account for the closure management of a storm surge barrier located in the Rotterdam Waterway, which protects a large part of the harbour and the hinterland against flooding (see Figure 1.1). Here, the most important effects of the seiches in the Port of Rotterdam are described in more detail. Movable storm surge barrier Rotterdam The movable storm surge barrier located along the Rotterdam Waterway (see Figure 1.1) is closed during extreme storm conditions. The closure operation is initialised when the predicted storm surge level and the river discharge are both expected to exceed a certain threshold. Two arc-shaped gates are then first pivoted (horizontally) into the Rotterdam Waterway. After this stage, the barrier is ballasted and the arc-shaped gates are sunk to the bottom of the river, closing the Rotterdam Waterway. During such a storm surge, the net forces acting on the gates are diverted via a steel framework to ball-and-socket joints that are.

(22) 1.1. SEICHES. 7. located on the banks of the river (see Figure 1.3). The deployment of the storm surge barrier creates a new situation since in that case another semi-closed basin is created, north of the Caland Canal (Figure 1.1). Seiches can also occur in this temporary basin, with a dominant eigenperiod of approximately 30 minutes. (In the remaining part of this study this temporary basin will be referred to as the Waterway Basin.) The strength of the design of the movable barrier is that the barrier gates are stored on land except when the barrier is closed. This means that maintenance can be done efficiently, which results in low costs. Moreover, the high intensity ship activity that takes place on the Rotterdam Waterway is not hindered in any way when the barrier is not deployed. However, the ball-and-socket joints that enable the special closure-method (first a horizontal movement, then a vertical movement) are also the Achilles heel of the barrier. Because of specific circumstances that can occur during the deployment of the barrier, the trough of a seiche in the Waterway Basin can cause a critical situation when the water level on the sea side of the barrier drops below the level on the river side. In extreme situations, this could cause the failure of the storm surge barrier since it is primarily designed for protection against high water levels on the sea side. If the net force directed towards the sea side of the barrier becomes too large, this could cause the ball-joints to be pushed out of their sockets, similar to the dislocation of a shoulder.. river side. sea side. Figure 1.3: The storm surge barrier of Rotterdam during a regular situation (solid lines) and deployed during an extreme storm surge (dashed lines). The closure-management software of the barrier is designed to avoid these circumstances as much as possible. For this, the water level is monitored by the Ministry of Public Works on both sides of the barrier for a more or less ad-hoc warning method in which the possibility.

(23) CHAPTER 1. INTRODUCTION. 8. of seiches also plays a role. A system of pumps and valves ensures that the barrier floats to the surface in case the water level on the sea side drops below the level on the river side. This approach is expected to avoid damage to the barrier. However, an actual seiche-prediction system is not available for the closure-management of the Rotterdam storm surge barrier.. Ship manoeuvring The Port of Rotterdam is visited by deep-draught vessels on a daily basis. For these vessels, the Rotterdam Municipal Port Management (ghr) needs to be able to guarantee a certain depth (‘nautical depth’) throughout the harbour. The trough of a seiche could cause ships to run aground because the predicted tidal window is no longer valid due to the seiche movement superposed on the tide. For ship traffic control, the Rotterdam Municipal Port Management uses predictions of water levels inside the harbour. These predicted water levels include the effects of tides and wind induced set up. However, so far, there has been no method to include seiche events in this prediction.. Flooding protection works for urbanised areas The storm surge barrier in Rotterdam Waterway is part of a system of water protection works, including dykes and other barriers, which protects a large section of the province (an area with over one million inhabitants) against flooding. This system of protection works needs to be designed for the additional water level increase that can be caused by the seiches. Present design methods for the height of a dyke involve a standard allowance to account for seiches, which is included deterministically in the design heights of dykes (see e.g. Rijkswaterstaat 2001, page 99).. Flooding of quays Harbour areas, located on the sea side of the water protection works, can be flooded in severe cases when a large amplitude seiche coincides with high tide. In the Port of Rotterdam, relatively low quay levels can be found for example at the closed end of the Caland Canal, near roz. At some locations, the height at the head of the quay is nap +3.5 m (nap stands for normal Amsterdam level, the local reference level, approximately at mean sea level), which slowly increases (1:100) to nap +5.5 m further away from the quay. In the past, this area has flooded at least once. Since a large car terminal is located here, with new cars on the quays awaiting transport to the hinterland, the economic consequences of flooding can be very large..

(24) 1.1. SEICHES. 9. Construction of the Caland Tunnel. A temporary possible effect of seiches was unexpectedly created during the construction of a tunnel near the closed end of the Caland Canal in 2001 and 2002 (near location roz). This tunnel, called the ‘Caland Tunnel’, has been built with large prefab concrete elements, which have been constructed in a dry-dock. These elements (approximate width 35 m, length 110 m) were provided with temporary walls at each end of the tunnel element, which ensured that the tunnel sections could be floated to the construction site. Side channels, perpendicular to the main channel, have been constructed with sheet piles and with horizontal steel support pipes between the top ends of the pilings on both sides of the channel. During the construction phase, the floating elements have been towed into these side channels. In this time interval, the elements would have an expected minimum clearance of 0.25 m below the support pipes. The possibility of a potentially critical situation was identified during the construction phase of the tunnel. The expected maximum water level would be predicted for the days during which the elements were placed in the side channels in order to guarantee a minimum clearance between the top of the tunnel element and the support pipes of the side channel. However, the occurrence of a seiche could not be predicted. In case a large amplitude seiche would occur during this critical stage of the operation, the crest of a seiche could cause the floating tunnel element to push the support pipes out of position, which could cause the collapse of the side channel sheet piling. The sinking operation of the tunnel elements was originally planned to start in December 2001. Unfortunately, December is the month in which statistically most significant seiche episodes occur in the Port of Rotterdam (see Chapter 3). Eventually, the elements were placed without any problem with seiches. This was partly the result of a delay that was brought on by other parts of the tunnel construction. Due to this delay, the elements were not placed in the side channels until June and July of 2002. No large amplitude seiches were found in the archived measurements during these months (1995–2001). This also turned out to be the case for June and July 2002. The delay in the construction of the Caland Tunnel was a fortunate coincidence from a seiche point of view. The concern about seiches proved to be realistic, since a relatively large number of significant seiche episodes (four events, with an average crest height of 0.40 m) occurred in the month during which the sinking operation was originally scheduled to start (December 2001)..

(25) CHAPTER 1. INTRODUCTION. 10. 1.2 1.2.1. Previous studies/literature overview Introduction. The effects described in the previous section indicate that seiches can have a significant influence on certain activities and structures inside a harbour such as the Port of Rotterdam. The response of harbour basins to long waves is relatively well known since analytical descriptions exist for simple harbour layouts (such as Merian’s formula, see e.g. Defant 1961, Vol 2, pp 154–244) and the resonance frequencies of more complex geometries are determined numerically. The generation mechanisms of energy in the seiche frequency band (0.1–2.0 mHz) are not all known or fully understood. The origin of the seiches that occur in the Port of Rotterdam has also never been identified so far. The dominating source may differ from area to area, depending on the occurrence of such mechanisms and the specific geographic situation of the location. This literature review focuses on possible generating mechanisms of energy in the seiche frequency band of the Port of Rotterdam.. 1.2.2. Potential seiche generating mechanisms. A comprehensive review of generating mechanisms and the effects of seiches can be found in e.g. Wilson 1972; Giese and Chapman 1993; Korgen 1995. These mechanisms are wind (through wind set up caused by more or less constant wind speed and wind direction, see e.g. Gravili et al. 2001), internal waves (see e.g. Giese et al. 1982, Giese and Hollander 1987, Chapman and Giese 1990, Giese et al. 1990 and Giese et al. 1998), tsunamis (see e.g. Zelt 1986 and Sinadinovski et al. 2001), surf beat (see e.g. Kostense 1985 and Okihiro et al. 1992) and atmospheric pressure changes (see e.g. Donn 1959, Donn and Balachandran 1969 and Rabinovich and Monserrat 1998). Observations in the past have shown that atmospheric phenomena are the most common cause of the seiches that occur in the Port of Rotterdam. This is supported by the fact that all significant seiche events occur during episodes of rough weather. Moreover, the other potential generating mechanisms mentioned above are either extremely rare near Rotterdam (e.g. tsunamis), or they do not generate energy near the eigenfrequencies of the harbour (e.g. surf beat). The possibility of meteorologically generated seiches in harbours along the Dutch coast has been suggested for a long time (see e.g. Wemelsfelder 1957). However, most previous studies of seiches in the Port of Rotterdam focussed on the phenomena inside the harbour (see e.g. De Looff and Veldman 1994). To study the origin of the seiches in the Port of Rotterdam, observations during two seiche events were analysed in more detail by Veraart (1994). For.

(26) 1.2. PREVIOUS STUDIES/LITERATURE OVERVIEW. 11. these cases, he found a correlation between the time of the passage of a cold front over the southern North Sea and the time of occurrence of increased seiche activity. This hypothesis was further supported by the 1D numerical simulations made by Kirkegaard (1996), which showed good (qualitative) agreement between measured energy levels in the seiche frequency band and the energy levels calculated by simulating the passage of a cold front. Since atmospheric phenomena are the most probable generating mechanism of the longwave energy which causes seiching in the Port of Rotterdam, a more thorough review is made of the literature regarding this type of mechanisms.. 1.2.3. Seiche generation by atmospheric phenomena. The atmospheric generation of seiche events has been studied for harbours and sea areas at numerous locations around the world. Donn (1959) studied an atmospheric disturbance (a sharp pressure increase) that moved across the Great Lakes in North-America. A resonance situation occurred (so called ’Proudman resonance’, see Proudman 1953) because the advection velocity of the disturbance was near the phase speed of the shallow water waves. This initialised a seiche in Lake Erie. Donn and Balachandran (1969) studied atmospheric pressure fluctuations that generated low-frequency seawaves on the Long Island Sound. These pressure fluctuations were ascribed to atmospheric gravity waves. Also in this case, a near resonance situation occurred since the speed of the atmospheric pressure changes was near the phase speed of shallow water waves in this region. Hibiya and Kajiura (1982) analysed the atmospheric origin of a seiche event in a harbour basin in Japan, based on water level measurements and coinciding atmospheric pressure measurements. The event was simulated in a numerical model, which used a ‘frozen’ pressure field translating across the sea towards the harbour. This frozen pressure field was obtained by transforming a measured time series into a spatial distribution with an estimated velocity based on Taylor’s hypothesis. The results showed a long wave which was generated at sea, which caused seiching in the harbour. The simulated amplitudes of these seiches were in good agreement with the measurements. Candela et al. (1999) studied the atmospheric generation of seiches in harbours near the Strait of Sicily, Italy. These seiches were found to coincide with the passage of low pressure systems. However, numerical simulations driven by synthetic low pressure systems (without cold front) did not reproduce the observed low-frequency energy at sea. In contrast to this, simulations with hypothetical atmospheric pressure jumps did produce seiches. These jumps were not measured locally and further observations are needed to investigate whether they actually occur in this area. The generation of seiches in the Baltic Sea was studied by Metzner et al. (2000). Numer-.

(27) CHAPTER 1. INTRODUCTION. 12. ical simulations were made with a shallow water code. The model was forced by atmospheric pressure and wind fields obtained from a meteorological model (of which the resolution is not described in this paper). The overall trend (probably without taking seiches into account) of the water level observations, obtained from satellite tracks and local tide gauges, was reproduced according to these authors. During extreme water levels, the simulated signals underestimated the observations. These discrepancies were ascribed to either a lack of resolution in the hydro-mechanical model or to errors in the atmospheric pressure fields and wind fields. Additional measurements of atmospheric pressure and wind with a resolution of three hours were used to study the passing meteorological systems in more detail. The observed seiches were attributed to the large-scale atmospheric pressure changes that are found during the seiches. Possibly, these changes in atmospheric pressure correspond to passing low pressure systems that coincided with the seiches. Seiches on the Adriatic Sea have been studied by e.g. Cerove˘cki et al. (1997). These seiches (dominant eigenperiods of the Adriatic sea are 11 hours and 22 hours) contribute to the floods that occur in the city of Venice. The main focus of this study was on the duration of the decay of the seiches. The start times of the seiches that were considered in this study were found to coincide with cold front passages. Raicich et al. (1999) described an in depth analysis of one event that occurred in December 1997. During this event a low pressure area passed the northern part of the Adriatic Sea and a cold front passage caused the strong wind from south-eastern direction to quickly subside. This started a seiche, which takes several days to damp out. Recently, Canestrelli et al. (2001) gave an overview of seiche events in the Adriatic Sea. They describe 23 seiche registrations (from 1951 to 2000) together with three-hourly observations of atmospheric pressure, wind speed and wind direction. Moreover, they give a detailed description of the large-scale meteorological situation during these events, based on a number of weather charts per event. Most of these seiches coincide with the passage of a low pressure system. Pasarić and Orlić (2001) studied the floods that have occurred on the North Adriatic coast. They described one event in more detail (November 1987), which was also probably caused by the combination of low atmospheric pressure and wind set up as a result of the passage of a low-pressure system, including a cold front. The most extensive research project regarding the generation of seiche energy by atmospheric phenomena has been conducted for inlets at the Balearic and Kuril Islands (see e.g. Tintoré et al. 1988; Gomis et al. 1993; Rabinovich and Monserrat 1996; Rabinovich and Monserrat 1998). On Menorca, one of the Balearic Islands, large amplitude seiches occur in the summer, during fair weather conditions. Evidence of long waves generated at sea by periodic atmospheric pressure changes was found by analysing measurements obtained near the Balearic Islands (Gomis et al. 1993; Garcies et al. 1996). Surface elevation measurements.

(28) 1.2. PREVIOUS STUDIES/LITERATURE OVERVIEW. 13. at sea showed an increase in energy near the eigenfrequencies of the harbour basin. Furthermore, fluctuations in atmospheric pressure were found during seiche episodes. This indicates that the seiche inside the harbour is initialised at sea, excited by the atmospheric phenomena. A study into the origin of these atmospheric pressure fluctuations has been described in e.g. Monserrat and Thorpe (1996). They described a possible mechanism of atmospheric gravity waves that are trapped in the stratified lower atmosphere. Vidal et al. (2001) showed with numerical simulations, forced by frozen pressure fields based on observed pressure fluctuations, that these seiches are generated by a standing wave at sea between two of the Balearic Islands, which is generated by the fluctuations in atmospheric pressure. In the following section, a review is made of the applicability for the Port of Rotterdam of the aforementioned meteorological generating mechanisms that have been identified for seiches that occur in other harbours in the world.. 1.2.4. Relevance to the Port of Rotterdam. The review of the literature on the generation of seiches indicates that meteorologically generated seiches have been observed at numerous locations all around the world, from relatively small harbour basins and inlets to seas. The relatively small areas are influenced by mesoscale phenomena such as periodic changes (also in summer) or sharp jumps in atmospheric pressure (in some cases caused by cold fronts). Larger sea areas are influenced by complete low pressure areas, giving rise to the so called ‘inverted barometer’ effect that causes a local water level increase. In these cases the amplitude of the seiche can be further increased by an additional wind-induced set up, that can initiate a seiche oscillation in case the wind shows a sudden drop in magnitude or a sudden change in direction. However, the situation for the Port of Rotterdam appears to differ from the circumstances that are described in the literature for other locations. During most seiche events in Rotterdam no sharp changes in atmospheric pressure have been found (see Chapter 3). Moreover, the changes in atmospheric pressure that occur during seiche events in Rotterdam are much weaker (O(0.1 hPa)) than those caused by atmospheric gravity waves (O(1 hPa)), described in the literature for other areas (see e.g. Rabinovich and Monserrat 1998). Furthermore, the correlation between the magnitude of the wind-induced set up at the harbour mouth during rough weather episodes and the corresponding crest heights of the seiches inside the harbour was found to be very weak (see Section 3.5). This indicates that strong winds (relatively constant in direction and magnitude) that cause a set up of the mean water level do not have to occur simultaneously with the phenomena that generate the seiche events that occur in the Port of Rotterdam. This prompted the search for another mechanism through which wave energy is generated at the North Sea in the seiche frequency band for this harbour..

(29) CHAPTER 1. INTRODUCTION. 14. 1.3. Aims. The two primary aims of this study are to identify and analyse the generating mechanisms of the low-frequency waves at sea that cause the seiches inside the Port of Rotterdam, and to develop a method of prediction of seiche episodes in the Port of Rotterdam. Secondary aims are to investigate the energy spectrum of gravity waves at sea in the seiche frequency band (0.1-2.0 mHz) and the dynamics and statistics of the basin response in Rotterdam Harbour to these excitations.. 1.4. Outline of thesis. The outline of the remaining part of this thesis is as follows. Chapter 2 describes the observation locations, the measured signals and data characteristics such as sample intervals. It also includes a description of the data processing techniques. Typical observations of relevant (meteorological) parameters and the characteristics of the seiche events that occur in the Port of Rotterdam are described in Chapter 3. Chapter 4 describes a 2D hydrodynamical model that is forced by both artificial and simulated fields of atmospheric pressure and wind. The generating mechanism that has been identified in this study for the majority of the seiche events that occur in the Port of Rotterdam is described in Chapter 5, together with computational results of a 1D hydrodynamical model of the fundamental principles. Chapter 6 describes a number of methods for the prediction of seiche events in the Port of Rotterdam, including a method that is directly based on the generating mechanism that has been identified. The possibility of using this method for prediction of seiche events on an operational basis in the near future is also discussed. Finally, the conclusions and recommendations of this study are described in Chapter 7..

(30) Chapter 2. Measurement data and analysis 2.1 2.1.1. Data acquisition Measurements obtained from fixed platforms. Surface elevation measurements from distant offshore platforms have been obtained from Auk Alpha (auk) and Platform K13 (k13, see Figure 2.1). These are available as 10 minute average values for 1995, 1996, 1999 and November and December 2001. Before 1999, these 10 minute averages were based on the previous 10 minutes, whereas at present the time intervals are centred around the time stamp. At k13 the surface elevation data are rounded to cm’s before storage. Since 1999, the surface elevation data at auk are rounded to dm’s before storage, which makes these more recent data useless for this study because the amplitudes of the low-frequency waves at the southern North Sea have maximum amplitudes of that order of magnitude. Platform auk is maintained by Shell UK. The reason why these values are processed in this manner is presently unknown. Additional parameters have been obtained from the more distant offshore platforms in order to study the stability of the lower atmosphere during significant seiches inside the Port of Rotterdam (results of this analysis are described in Chapter 6). Sea surface temperature measurements have been obtained from k13 (1995-2001) and auk (1995-2000). These are available as daily values. Meteorological soundings (vertical profiles of air temperature and other meteorological parameters from balloon measurements) are available every 12 hours at Ekofisk platform (eko, see Figure 2.1) during most of the seiche episodes (1995-2001). Atmospheric soundings at De Bilt will be used in case the meteorological soundings at sea are not available during a seiche episode. At this land based observation location, located about 75 km east of Rotterdam (Figure 2.1), meteorological soundings are available every six hours. 15.

(31) CHAPTER 2. MEASUREMENT DATA AND ANALYSIS. 16. Figure 2.1: Locations of the observation platforms at the southern North Sea and the land based location De Bilt. Details on offshore observation locations near harbour mouth in Figure 2.2. The sea surface elevation measurements near the harbour mouth were obtained at offshore locations Europlatform (eur), Lichteiland Goeree (goe) and Meetpost Noordwijk (mpn), located approximately 20 km and 40 km from the Rotterdam harbour mouth (see Figure 2.2). At one or more of these platforms, high-resolution (2–4 Hz) surface elevation measurements (obtained with step gauges and radar altimeters) are available for one long period (five winter months of 1996–1997) and for a few events in 1995, 2000 and 2001. From June 2000, processed (validated) surface elevation values from these sea stations are stored continuously on an operational basis with a sample rate of one minute. Meteorological measurements (atmospheric pressure, wind speed and wind direction) are available at these platforms as ten minute average values (centred around time stamp), stored every 10 minutes for a few events in 1995, 2000 and 2001. Inside the harbour, the Rotterdam Municipal Port Management maintains a network of.

(32) 2.1. DATA ACQUISITION. 17. Figure 2.2: Overview of the Dutch coast near the harbour mouth of the Port of Rotterdam and the measurement locations. over ten locations where surface elevations are measured. In most cases, observations from the Caland Canal (at roz, see Figures 1.1 and 2.2) will be used here to describe the seiches and to analyse their origin. This location has been chosen because the seiches in the Port of Rotterdam are most predominant in the Caland Canal. Additional measurements at the Europe Harbour (eh, see Figure 1.1) will be used to further analyse the seiche characteristics of the harbour. The surface elevation measurements inside the harbour are taken continuously since 1995 with floaters, sampled at a 60 second interval. The seiche-characteristics of the harbour have not been constant during this time interval, since changes have been made to the harbour lay-out during these years. The most radical change was made in November 1997, when a part of the Beerdam (see Figure 1.1) was removed and a new connection between a number of harbour basins was herewith created. Where necessary and possible, these inconsistencies in the data sets will be taken into account. Land based meteorological observations have been obtained for 1 and 2 January 1995 (the seiche with the largest crest height in the available data set, shown in Figure 1.2). These are only available at Valkenburg Airport (va, located about 40 km north of Rotterdam, see Fig-.

(33) CHAPTER 2. MEASUREMENT DATA AND ANALYSIS. 18. ure 2.2), with a sampling interval of 10 minutes (average values of 10 minutes centred around time stamp). Moreover, long term meteorological measurements at Hook of Holland (Figures 1.1 and 2.2), consisting of 10 minute average values, are used to analyse the meteorological situation during all seiche events from August 1995 till December 2000. Wind speed, wind direction, atmospheric pressure, precipitation and air temperature data are obtained continuously since January 2000 as one minute averages at Rotterdam Airport (ra, see Figures 1.1 and 2.2). These data will be used to study relatively recent events in more detail. Moreover, they are used in a hindcast analysis of a prediction method of seiche events (Chapter 6).. 2.1.2. Remote sensing. Besides information obtained at observation locations, aerial information will also be used to analyse (meteorological) circumstances during episodes of increased seiche activity. Weather charts have been obtained in order to study the synoptical situation (cold front passages, low pressure areas) during seiche episodes inside the Port of Rotterdam. The charts are available at 00:00 and 12:00 gmt, each day. Weather charts of successive days (available at 00:00 gmt) during longer time intervals have also been obtained in order to be able to study other potentially seiche-prone situations (1999–2001). Possibly, these charts can be used to explain the absence of seiches with significant amplitudes in these cases. Precipitation radar images have been obtained from the Royal Netherlands Meteorological Institute (knmi) for the days on which a seiche with significant amplitude occurred. These precipitation radar images cover The Netherlands and a large part of the southern North Sea. They are available at a time interval of 15 minutes and can supply additional information regarding direction, speed and intensity of the meteorological disturbances that occurred during episodes of increased seiche activity. Moreover, precipitation images have been obtained for days during the studied time interval on which potentially seiche-prone conditions have been identified that did not coincide with a significant seiche episode inside the harbour. In these cases, the precipitation images can be used to possibly explain the absence of a significant seiche event in the Port of Rotterdam. Finally, satellite images of a thermal infrared frequency range (10.3-11.3 µm) have been obtained for a number of days on which increased seiche activity has been detected inside the harbour. These images are available a few times a day as polar orbiting satellites pass over the area of interest (images have been obtained by the noaa-12 and noaa-16 satellites). Since November 1978 these images are stored in an archive by the University of Dundee (uk). The images (with increased contrast for visibility purposes) will be used to analyse cloud structures over the North Sea, possibly indicating relevant atmospheric phenomena..

(34) 2.2. DATA ANALYSIS METHODS. 2.2 2.2.1. 19. Data analysis methods Fourier based filtering. To inspect the time records of the water level measurements visually, the tidal components and the high-frequency wind waves have been removed from the surface elevation to retain time-series in the seiche frequency band (0.1–2.0 mHz). This filtering of the time signal is achieved by Fourier transforming the Fourier spectrum in the seiche frequency band back to the time domain. The seiche frequency band will be well isolated from the other frequency bands by cosine tapering the original time series (over 10 % both at the start and at the end of the signal) before the Fourier transform.. 2.2.2. Wavelet analysis. To identify possible seiche episodes and the corresponding increased levels of low-frequency energy at sea, a wavelet analysis based on the Morlet wavelet (Morlet et al. 1982) has been applied to the surface elevation data. This analysis technique transforms the original data from the time domain to the time-scale (period) domain. The Morlet wavelet in terms of time domains t, t and period T is given by: ψ(T, t, t ) = eim. t −t T. e−(. t −t 2 ) /2 T. (2.1). in which m is a constant value, which needs to be larger than 5 to ensure zero mean of the wavelet (for this study m = 6 has been used). The wavelet transform x ˜(T, t) of a time series x(t ) is then defined as the inner product of ψ and x(t ): 1 x ˜(T, t) = √ T. +∞ −∞. x(t)ψ ∗ (T, t, t )dt. (2.2). in which the asterisk indicates the complex conjugate (see e.g. Farge 1992 and Mallat 1998 for more comprehensive descriptions of the wavelet transform). The transformation in Equation 2.2 can be used to determine wavelet spectra defined as Wx (T, t) = |˜ x(T, t)|2 . These spectra can be interpreted as wave energy density spectra as function of time and period. Therefore, this technique is especially suited for the detection of fluctuations that come in bursts. Here, it will be used to identify the seiche episodes inside the harbour and the corresponding time intervals of increased levels of low-frequency energy at sea. Seiche episodes inside the harbour are visible in the wavelet spectra as a temporary increase in the energy levels around an eigenperiod of the considered harbour basin. Figure 2.3 shows an example of wavelet analysis results of a month during which a relatively high number.

(35) CHAPTER 2. MEASUREMENT DATA AND ANALYSIS. 20. of seiche episodes has been detected (November 2001). The top panel of the figure shows filtered surface elevation measurements from the Europe Harbour and the bottom panel shows the corresponding wavelet spectrum (normalised by the maximum value that is found in this time interval and period range). The dominant eigenperiod of the Europe Harbour (50 minutes, 3000 s) is indicated in this spectrum. Seiche episodes are found on 8/9, 13/14 and 22/23 November 2001. Relatively high energy levels are also found at other eigenperiods of this basin (21 minutes and 13 minutes, approximately 1250 s and 800 s, respectively). Surface elevation Europe Harbour (0.3−2.0 mHz), November 2001 surface elevation (cm). 30 20 10 0 −10 −20 −30. 5. 10. 15 time (date). 20. 25. 30. Wavelet spectrum, norm. by max. value (204.2 cm2/Hz) 1. period (s). 4000. 0.8. 3000. 0.6 0.4. 2000. 0.2. 1000 5. 10. 15 time (days). 20. 25. 30. 0. Figure 2.3: Example of filtered surface elevation measurements (November 2001) and the corresponding wavelet spectrum. Following the first identification based on wavelet spectra, the time intervals with increased seiche activity will be reviewed in more detail. This includes wavelet analyses of atmospheric pressure and wind velocity observations during these events..

(36) Chapter 3. Characteristics of seiche events 3.1. Two types of seiche events. The available surface elevation measurements at roz (1995–2001) have been filtered for the seiche frequency band (0.1–2.0 mHz). A threshold value has been chosen for the seiche amplitude (crest heights in the filtered signals) in order to be able to select discrete significant seiche episodes, ‘seiche events’, exceeding the background noise in the signal. This threshold value has been chosen based on a visual inspection of the filtered surface elevation signals. A suitable value of the threshold needs to be chosen: a continuous ‘seiche event’ can be found in case the threshold is too low, whereas significant events can be missed in case it is too high. Eventually, each event has been defined here as a period of enhanced seiche activity with amplitudes (crest heights) exceeding 0.25 m at this location. A wavelet analysis was applied to the filtered time series of each of the 84 months in the seven year time interval. The wavelet spectra, combined with the chosen threshold, resulted in the identification of 49 events at roz, generally at the dominant eigenperiod of 90 minutes. Meteorological measurements and a visual inspection of the weather charts show that all these events coincided with low-pressure systems with cold fronts crossing the southern North Sea towards the Dutch coast (preliminary results of this review were previously described in De Jong et al. 2002). However, not all cold fronts moving from sea towards the Dutch coast generated a seiche event. On average, approximately one cold front passes the southern North Sea per week in the rough-weather season and only 7 seiche events occur on average per year. Figure 3.1 shows the number of events per month that have been found for 1995–2001, together with the average crest-to-trough height of these events. For this plot, multiple events that occurred within 48 hours have been included as one event (i.e. one independent seiche event per storm episode, maximum corresponding crest-to-trough height is used). The average crest-to-trough height appears to be relatively constant throughout the year. The distribution 21.

(37) CHAPTER 3. CHARACTERISTICS OF SEICHE EVENTS. 22. of the number of events clearly confirms that the seiches in the Port of Rotterdam are a storm season phenomenon, with significant events occurring mainly in the fall and the winter. No events were identified in the seven year time interval for May till July.. number of events/mean crest−to−trough height (dm). 12. 10. 8. 6. 4. Dec. Nov. Oct. Sep. Aug. Jul. Jun. May. Apr. Mar. Feb. 0. Jan. 2. Figure 3.1: Black bars: number of events per month 1995-2001 (amplitude event exceeds 25 cm); gray bars: monthly mean crest-to-trough height of events (dm).. The registrations of the surface elevation at sea during and just preceding seiche events showed two distinct patterns: either a single, soliton-type wave of elevation, or a burst of oscillations. The former category, only 5 out the 49 events, coincided with thunder storms during the late summer (August-September) and a front passage from south-westerly directions. This resulted in precipitation concentrated more or less along a line, approaching the Dutch coast from the south-west (‘squall line’). The corresponding seiches were relatively weak. The bulk of the events (44) was of the second category, occurring mainly in the storm season (October-April) following a cold front passage from west or north-west over the observation locations. In general, these more common situations coincided with dispersed precipitation from the north-west and resulted in the highest seiche response. Since the generating mechanism of this most common (and most intense) type of seiche events is unknown, this thesis deals exclusively with the events that originate from a burst of oscillations. The distinction of two different types of long-wave episodes at the North Sea has been made before. The situations described above appear to correspond to the two types of long wave events that were identified in surface elevation measurements from the southern North Sea.

(38) 3.2. SHAPE OF VARIANCE DENSITY SPECTRA. 23. by Wemelsfelder (1960). Wemelsfelder describes single surface elevation disturbances (here called solitons), which he called ‘buistoten’ (‘squall-induced pulses’), and irregular surface elevation changes during longer time intervals, which he called ‘bui-oscillaties’ (‘squall-induced oscillations’). Surface elevation measurements obtained at sea (eur) and in the harbour (roz) during an example event of the more common type are shown in Figure 3.2 (7 till 9 November 2001). Typical crest heights at sea in these situations reach 10–20 cm. Figure 3.3 shows surface elevation measurements obtained at these observation locations at sea and inside the harbour during an example event of the, more rare, soliton-type situations (25 till 27 August 2001). During these cases, the crest heights at sea typically reach 30–40 cm. Figures 3.4 and 3.5 show meteorological observations during the two example events. The measurements of the November case (Figure 3.4) indicate that the atmospheric pressure (top panel) changed only gradually during this situation (only relatively weak fluctuations are found superposed on an a general trend). Figure 3.5 shows that for the August case a relatively large, temporary increase is found in atmospheric pressure and wind speed around the time of the front passage (around 15:00 gmt at ra). The generation of the soliton wave that is found coinciding with this type of front passage is ascribed to this temporary increase in atmospheric pressure and wind speed. The seiche events that subsequently occur inside the harbour, with relatively limited amplitudes, are ascribed to a free response to this pulse-like forcing. The most intense events of this soliton-type situation occur mainly during August and September (just prior to the storm season), when a cold front with thunder showers in a squall line passes the southern North Sea after a number of days with high atmospheric pressure and calm weather. It is expected that during these situations the storm surge barrier of Rotterdam will not be deployed. In fact, the high pressure area that is found over the North Sea prior to these cold front passages results in a set-down of the mean sea level during these events (due to the inverted barometer effect).. 3.2. Shape of variance density spectra. A white noise spectrum (a uniform energy distribution) of the surface elevation at sea in the seiche frequency band (0.1–2.0 mHz) has been used in preliminary studies in the context of the design of the storm surge barrier of Rotterdam because the actual shape of the variance density spectrum in that frequency range was unknown at that time. Veraart (1994) first studied the spectra of surface elevation measurements at the southern North Sea in this frequency band. Based on a relatively small number of seiche episodes he found that, on average, the variance density in the seiche frequency band approximately followed a f −1.2 trend. Recently, validated.

(39) CHAPTER 3. CHARACTERISTICS OF SEICHE EVENTS. 24. Surface elevation EUR, 8 and 9 November 2001. surface elevation (cm). 300 200 100 0. −100 0. surface elevation (cm). 20. 6. 0. 6. 12. 18. 0. Filtered surface elevation EUR, 8 and 9 November 2001 (0.1−2.0 mHz). 0 −10. 6. 12. 18. 0. 6. 12. 18. 0. 18. 0. Surface elevation ROZ, 8 and 9 November 2001. 300 surface elevation (cm). 18. 10. −20 0. 200 100 0. −100 0 100 surface elevation (cm). 12. 6. 12. 18. 0. 6. 12. Filtered surface elevation ROZ, 8 and 9 November 2001 (0.1−2.0 mHz). 50 0 −50. −100 0. 6. 12. 18. 0 time (h, MET). 6. 12. 18. 0. Figure 3.2: Example of longer time interval with increased energy in the seiche frequency band, observed during most seiche events at roz. Top panel: measured surface elevation at eur 7 till 9 November 2001; bottom panel: measured surface elevation filtered for the seiche frequency band..

(40) 3.2. SHAPE OF VARIANCE DENSITY SPECTRA. Surface elevation EUR, 26 and 27 August 2001. surface elevation (cm). 100 50 0 −50. −100 0. surface elevation (cm). 30. 6. 18. 0. 6. 12. 18. 0. Filtered surface elevation EUR, 26 and 27 August 2001 (0.1−2.0 mHz). 10 0 −10 6. 12. 18. 0. 6. 12. 18. 0. 18. 0. Surface elevation ROZ, 26 and 27 August 2001. 150 surface elevation (cm). 12. 20. −20 0. 100 50 0 −50. −100 0 40 surface elevation (cm). 25. 6. 12. 18. 0. 6. 12. Filtered surface elevation ROZ, 26 and 27 August 2001 (0.1−2.0 mHz). 20 0 −20 −40 0. 6. 12. 18. 0 time (h, MET). 6. 12. 18. 0. Figure 3.3: Example of soliton-type wave at sea observed during a relatively small number of seiche events. Top panel: measured surface elevation at eur 25 till 27 August 2001; bottom panel: measured surface elevation filtered for the seiche frequency band..

(41) CHAPTER 3. CHARACTERISTICS OF SEICHE EVENTS. atm. pressure (hPa). 26. Meteorological parameters 8 and 9 November 2001. 1040 1020 1000 980 0. 6. ↓. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. Wind speed (m/s). 15 10 5 0 0. wind direction ( °). 180 90 0 270 180. cum. precip. (mm). air temperature ( °C). 90. 0. 15 10 5 0 −5 0 15 10 5 0 0. time (h, MET). Figure 3.4: The meteorological observations at Rotterdam Airport during seiche event of 8 and 9 November 2001. Panels show (top to bottom): atmospheric pressure, wind speed, wind direction, air temperature and cumulative precipitation. Time mark in top panel indicates time of cold front passage over ra..

(42) atm. pressure (hPa). 3.2. SHAPE OF VARIANCE DENSITY SPECTRA. 1020. 27. Meteorological parameters 26 August 2001. 1018 1016 1014 0. 6. 12. 6. ↓. 18. 0. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. 6. 12. 18. 0. Wind speed (m/s). 8 6 4 2 0 0. wind direction ( °). 360 240 120. cum. precip. (mm). air temperature ( °C). 0 0 30 25 20 15 0 6 4 2 0 0. time (h, GMT). Figure 3.5: The meteorological observations at Rotterdam Airport during seiche event of 26 August 2001. Panels show (top to bottom): atmospheric pressure, wind speed, wind direction, air temperature and cumulative precipitation. Time mark in top panel indicates time of cold front passage over ra. Notice the difference in vertical scales compared to Figure 3.4..