Measuring and tracking suspended sediment on tidal flats and inundated marshland: Relevance for vertical accretion rates and coastal protection measures on the North Frisian Halligen
by Ingo Jürgen Hache
Date of Examination:2020-11-30
Date of issue:2021-04-13
Advisor:Prof. Dr. Hilmar von Eynatten
Referee:Prof. Dr. Hilmar von Eynatten
Referee:Prof. Dr. Volker Thiel
Referee:Prof. Dr. Daniela Sauer
Referee:Prof. Dr. Jonas Kley
Referee:Prof. Dr. Arne J.-Arns
Referee:Dr. Volker Karius
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Abstract
English
Inhabited coastal areas around the world have to deal with sea level rise (SLR) and its likely consequences such as increasing natural hazards like heavy storms, flooding events and coastal lowland loss by drowning over the long-term. The latter threat also affects the so-called Halligen, ten island-like marsh areas in the North Frisian Wadden Sea (southern North Sea), Germany. Like marshes in general, periodic inundations of the Halligen have the potential to mitigate or even hamper the risk of drowning by sedimentation and subsequent surface elevation increase. This natural adaptation capacity is strongly influenced by the availability of suspended sediments in the intertidal flats and the transportation of sediment on supratidal flats where it can settle. Coastal protection measures like dykes and breakwater constructions were implemented in the past to prevent erosion of the Hallig margin but are nowadays suspected to limit the sediment transport towards the marsh surface. The result is that vertical accretion of the supratidal areas is outpaced by SLR. Assessment of suspended sediment availability in tidal flat systems and transport mechanisms is therefore necessary to determine the possible adaptation capacity of the marshland against present and future SLR. The latest research results show that today’s accretion rates on the Halligen are out of balance with present SLR. Lack of knowledge about suspended sediment quantities makes it impossible to answer the main question, whether a limited suspended sediment availability in the intertidal flats or an inefficient sediment transport towards the marsh surface is responsible for the imbalance. The development and usage of an autonomously working turbidity measurement network based on IR backscatter sensors and accompanied by suspended sediment accumulators and sediment traps is central to this thesis and to the framework of the project “LivingCoastLab”. The availability of suspended sediment and its transport around and on the Hallig Langeness is measured and assessed during numerous events that range from moderate to storm surge induced inundations. These measurements revealed insights into strong and systematic suspended sediment availability and its increasing as well as decreasing controlling factors. Furthermore, these measurements revealed insights into sediment transport limitations caused by coastal protection measures and demonstrated the high potential of storm surges in generating vertical accretion rates capable of compensating SLR. However, this high potential is not fully utilized as large sediment quantities are lost before they can increase vertical accretion rates due to a disadvantageous coastal protection management. A detailed hydrodynamic model of Hallig Langeness and surrounding tidal flats is used to investigate the transport of suspended sediment during inundations and to simulate various scenarios with adapted coastal protection measure heights at the Hallig margin. The results quantitatively show the potential to increase the accretion rates by decreasing the transport limiting effects of the coastal protection measures. Furthermore, the positive and negative effects on vertical accretion rates of functional plant traits are investigated in a collaborative study. Finally, a theoretical approach revealed an almost doubling of today’s vertical accretion rates where coastal protection measure heights are decreased and the potential of storm surges are fully utilized. Consequently, the results of these studies lead the way to mitigate the impact of future SLR on the North Frisian Halligen.
Keywords: Suspended particulate matter; Turbidity; Langeness; Hallig; Storm surge; Inundation; Sediment transport; Vertical accretion; Sea level rise; Marshland; Particle tracking; Coastal protection; Sediment accumulation; Biogeomorphology; Plant functional traits; Erosion; Compaction; Coastal squeeze