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«Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften im Fachbereich Geowissenschaften der Universität Bremen vorgelegt von Gerald ...»

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Morphodynamics of Barrier Island Systems

Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften

im Fachbereich Geowissenschaften der Universität Bremen

vorgelegt von

Gerald Herrling

Bremen, 23. Oktober 2014


PD Dr. Christian Winter

Prof. Dr. Tobias Mörz

Tag des Dissertationskolloquiums:

6. März 2015


PD Dr. Christian Winter

Prof. Dr. Tobias Mörz Prof. Dr. Dierk Hebbeln Prof. Dr. Andrés Fernando Osorio Arias Dr. Marius Becker BSc. Markus Benninghoff Gerald Herrling 23.10.2014 MARUM Leobenerstr.

28359 Bremen Erklärung Hiermit versichere ich, dass ich die Arbeit ohne unerlaubte fremde Hilfe angefertigt habe, keine anderen als die von mir angegebenen Quellen und Hilfsmittel benutzt habe und die den benutzten Werken wörtlich oder inhaltlich entnommenen Stellen als solche kenntlich gemacht habe.

Gerald Herrling Bremen, den 23.10.2014 Contents Contents Acknowledgements



Chapter 1: Introduction

1.1. Motivation

1.2. Objectives and research questions

1.3. Thesis outline

Chapter 2: Paper I

2.1. Introduction

2.2. Study area

2.3. Methodology

2.3.1. Modeling system

2.3.2. Morphological acceleration factor

2.3.3. Model nesting and boundary conditions

2.3.4. Model bathymetry

2.3.5. Meteorological forcing

2.3.6. Multiple-grain-size model

2.4. Model validation

2.4.1. Hydrodynamics

2.4.2. Sediment dynamics and morphology

2.4.3. Sedimentology

2.5. Results

2.5.1. Tide-dominated fair-weather conditions

2.5.2. Wave-dominated high-energy storm conditions

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2.7. Conclusions

Chapter 3: Paper II

3.1. Introduction

3.2. Study area

3.3. Methodology

3.3.1. Model grids and wave-current coupling

3.3.2. Wind and wave forcing

3.3.3. Model bathymetry

3.3.4. Sediment grain-size distribution

3.4. Results

3.4.1. Water levels

3.4.2. Current velocity

3.4.3. Morphological changes

3.4.4. Sediment transport pathways

3.5. Discussion

3.6. Conclusions

Chapter 4: Paper III

4.1. Introduction

4.2. Study area

4.3. Methodology

4.3.1. Model grids and open boundary conditions

4.3.2. Wind forcing

4.3.3. Model bathymetry

4.3.4. Model quality

–  –  –

4.4. Results

4.4.1. Tidally-driven residual discharges

4.4.2. Wind effect on residual discharges

4.4.3. Sensitivity study

4.4.4. Tidal transformation along the barrier islands

4.5. Discussion

4.5.1. Tidally-driven residual circulation

4.5.2. Wind-driven residual circulation

–  –  –

4.6. Conclusions

Chapter 5: Concluding remarks and perspectives

–  –  –

Acknowledgements This study is funded by and associated with the research project WIMO (www.wimonordsee.de) and is financed in equal parts by two ministries in Lower Saxony: the Ministry of Environment, Energy and Climate Protection, and the Ministry of Science and Culture. Additional funding for the participation in three international conferences and an international summer school was received through the WIMO project’s support program for young researchers.

I am very thankful to my supervisor, P.D. Dr. Christian Winter, who gave me the chance to freely develop the research questions and approaches of this thesis. I greatly appreciate his steady availability and openness to listening to new ideas and the ability to discuss with him my findings in an open, pleasant and fruitful atmosphere. I am grateful for his encouragement and guidance throughout the manuscript writing.

I would also like to express my gratitude to Prof. Dr. Tobias Mörz, the second reviewer.

The coastal dynamic research group is a team of inspiring young coastal researchers and friends who made the everyday working atmosphere productive and enjoyable. I am especially thankful to Dr. Marius Becker for his openness, feedback and all of the stimulating discussions on a variety of scientific topics.

Finally, I would like to express my warm thanks to my mother who always supported me.

Unfortunately, my father was not able to witness my path into science that is actually so close to his lifetime’s work.

Special thanks go to Anne who indulged my every whim during the last period of my finalizing thesis work.


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Summary Barrier island systems, also referred to as multiple-inlet systems, are coastal environments with shallow, interconnected tidal basins that are fringed by a chain of elongated islands. Their geomorphology and tidal habitats encompass numerous transitional zones between the land and the sea that are rich in species specially adapted to the varying hydrodynamic conditions. Morphodynamics at mixed-energy barrier island coasts have been studied for several decades on the basis of aerial photographs, field observations and numerical or analytical models. A process-based understanding of the morphological response to the driving hydrodynamic forces, however, has still not been achieved.

The aim of this study is to assess the system morphodynamics in response to the interaction of tidallyand wave-induced currents, wind stress and the availability of mobile sediments. The study area is the East Frisian Wadden Sea (Germany), a lagoon-type environment with intertidal flats that are sheltered by seven inhabited barrier islands; it belongs to the Wadden Sea extending along the southern North Sea coast.

A state-of-the-art process-based model is applied as a hindcasting and experimental tool for the evaluation of relevant processes at short term (tidal cycle) to medium term (annual) time scales.

The spatial scales encompass sand shoals (meso-scale) as typical morphological features at ebb-tidal deltas, to the entire system covering the upper shoreface, the barrier islands and the back-barrier basins (large-scale).

The main results of the investigations are shown in three manuscripts:

1. Relevant hydrodynamic drivers of tidal inlet morphology and sedimentology are identified at an exemplary study area, the mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands Spiekeroog and Langeoog. The morphological and sedimentological response to high-energy wave-dominated storm conditions is compared to mid-term tide-dominated fair-weather conditions. A multi-fractional approach with five grain-size fractions allows the explanation of corresponding surface sediment grain-size distributions and the evaluation of residual sediment fluxes in response to distinct hydrodynamic drivers (paper I).

2. The littoral sediment drift along barrier island coasts involves sediment bypassing at tidal inlets. Morphological evolution and sediment fluxes in response to real-time forcing conditions of tides, wind and waves are simulated at two East Frisian tidal inlets for a representative mid-term period (two years). Tidal inlet bypassing is studied by identifying residual transport pathways of three sediment grain-size fractions. Common tidal inlet

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bypassing schemes which were typically assessed on the basis of aerial photographs or sedimentological surveys are discussed in view of the detailed model results (paper II).

3. The residual circulation into and through barrier island systems is primarily driven by the nonlinear interaction of tidal and meteorological forcings. This study is a first attempt to quantify the residual discharges through tidal inlets and across tidal divides between interconnected basins in the East Frisian Wadden Sea by the application of a hydrodynamic model. The response of residual water fluxes to varying boundary conditions is evaluated including representative wind forcing, different bottom frictions and an imposed sea level rise. A substantial surplus of water flows into the multiple-inlet system from the lateral margins, which may cause relevant implications for the accumulation of fine-grained sediments, flushing capacities and nutrient cycles in the Wadden Sea area (paper III).

The main method applied in this study is process-based numerical modeling. Generally, models incorporate uncertainties when natural processes are simplified through numerical schematizations and reduction methods. To justify the applicability of a model to a specific domain, simulated parameters are thus verified against observations, e.g. water levels, wave heights, sediment compositions and morphological changes. The pursued approach to comparatively evaluate model scenarios, e.g. storm versus fair-weather, further improves the confidence in the model as the comparison may cancel out relative errors and uncertainties. It is shown that the two-way-coupling of wave and hydrodynamic models and the consideration of sediment transport of multiple grain-sizes nowadays permits the schematization and simulation of coastal processes in environments that are as complex as mixed-energy tidal inlets. The model outcome, i.e. the reproduction of the interacting natural processes, gets increasingly complex which then made a well-considered differentiation of the imposed forcings and relevant parameters necessary in order to decipher and evaluate the governing processes.

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Zusammenfassung Lagunenartige Küstengewässer sind oftmals gesäumt von langgestreckten Barriereinseln, die der Festlandsküste parallel vorgelagert sind. Gezeitenöffnungen zwischen den Inseln, sogenannte Seegatten, ermöglichen den Wasseraustausch zwischen den flachen Tidebecken und der offenen See.

Ein lateraler Austausch benachbarter Tidebecken kann dabei über die Wattwasserscheiden erfolgen.

Die Geomorphologie dieses zusammenhängenden, marinen Ökosystems umfasst zahlreiche Lebensräume, die sich im Übergangsbereich zwischen Festlandsküste und offener See befinden und reich an Tier- und Pflanzenarten sind, welche sich den besonderen hydrodynamischen Bedingungen angepasst haben. Die Morphodynamik dieses gemischt-energetischen Systems ist vorwiegend durch den Gezeitenstrom und Seegang beeinflusst und ist über Jahrzehnte hinweg erforscht worden. Frühere Studien beschreiben die morphologische Entwicklung anhand von Luftbildaufnahmen sukzessiver morphologischer Zustände oder Feldmessungen sowie durch numerische und analytische Modelluntersuchungen. Ein prozessbasiertes Verständnis der Wirkung unterschiedlicher hydrodynamischer und meteorologischer Einflüsse auf die Morphodynamik dieses Systems wurde jedoch noch nicht erreicht. Das Ziel dieser Studie ist die Analyse und Bewertung der morphodynamischen Prozesse sowie der morphologischen Veränderungen und sedimentologischen Verteilungsmuster unter der Wechselwirkung interagierender Strömungskomponenten gesteuert durch die Gezeiten, Wind und Seegang sowie der Verfügbarkeit mobiler Sedimente. Das Untersuchungsgebiet umfasst das Ostfriesische Wattenmeer, welches durch eine Kette von sieben Barriereinseln von der offenen, südlichen Nordsee getrennt ist.

Ein anerkanntes, prozessbasiertes Modellsystem wird für die Simulation relevanter hydro- und morphodynamischer Prozesse verwendet und ermöglicht die Abschätzung morphologischer Entwicklungen über kurze und mittelfristige Zeiträume, d.h. von der Dauer einer Tide bis hin zu zwei Jahren. Die untersuchten räumlichen Skalen reichen von der Morphologie einer Plate bis hin zum gesamten Untersuchungsgebiet, welches den Vorstrandbereich der Inseln, die Seegatten und die Inselrückseitenwatten umfasst. Die grundlegenden Ergebnisse der Untersuchungen wurden in drei

Manuskripten zusammengestellt:

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