Work Package 3 - Observation & Analysis
The overarching aim of work package 3 “Observation and analysis“ is to measure the physical, oceanographical, biogeochemical, and ecosystem changes in the respective real-world laboratory. As it is a tidally and seasonally influenced system, the implemented observation system has to provide spatially and temporally highly resolved data of the coastal zone including the adjacent water mass. This is a prerequisite to understand the effect of ecosystem-strengthening coastal protection measures.
In this context, the term observation system means the integrated use of sensors deployed permanently or at least for a longer period in the field, but also the short-term use of instruments during dedicated field campaigns. The establishment of the observing systems also involves local stakeholders, whose cooperation and support is essential for successful implementation of online measurement systems.
The obtained data will be assembled to so-called environmental situation pictures that allow an overall assessment of the real-world laboratories. They also allow an evaluation of the variability of the environmental status with respect to seasons or disturbance events (e.g. storm surges). Both observational data and environmental situation pictures are the basis for the overall project, especially work packages 2 and 4.
Tidal inlets served as real world labs
One research area and real world laboratory for GKN is the island of Spiekeroog. The Spiekeroog Coastal Observatory (SCO) already offers a well-developed research infrastructure. The tidal inlets to the west and east of the island, which create a connection between the open North Sea and the Wadden Sea, are suitable for the investigation of groynes as coastal protection measures in the GKN project (Figure 1). Compared to the eastern “Harle” estuary (between Spiekeroog and Wangerooge), the western “Otzumer Balje” estuary (between Langeoog and Spiekeroog) is quite natural and only protected by a few small (< 100 m) groynes at the western end of Spiekeroog. Nevertheless, the ecosystem is (over)characterised by “hard” anthropogenic coastal protection measures (groynes, revetments, lahns, flood walls, etc.).
Figure 1: The inlets Otzumer Balje (A) and Harle (B), to the west and east, respectively, of the Spiekeroog barrier island. There is a massive groyne structure in the Harle (groyne H).
Wangerooge, on the other hand, has an even more developed groyne landscape, with the longest groyne (Groyne H) extending 1460 metres into the Harle estuary. This structure changes the natural flow dynamics of the Seegat and creates a deep channel and scouring along the groyne itself. Since the 1930s, groyne construction on Wangerooge has helped to prevent further erosion of the flood-free beach to the west of Wangerooge and the channel of the Dove Harle from shifting. The construction of Buhne H was not completed after an interruption during the Second World War. However, the groyne is to undergo further structural alterations in the near future. Due to these circumstances of the construction measures and the clear differences in the conditions of the sea lagoons around Spiekeroog, this area is suitable for an investigation as part of the GKN project. A conversion and expansion and the various associated effects of the “new” groyne H on the ecosystem can thus be better recorded and, if necessary, predicted.
In recent years, several measurement campaigns have been carried out with the research boats Navicula, Otzum, Egidora and the research cutter Senckenberg to assess the status quo of the inlets and answer our various scientific questions. For this purpose, transects, a fixed station pattern and permanent measuring stations over a tidal cycle were approached and sampled, as shown in Figure 2 above. Water samples were taken for analytical and biogeochemical analyses and sediments were sampled using grabs. Oceanographic data and samples over a tidal cycle were measured and sampled.
The near-surface sediments, i.e. the loose deposits on the sea floor, are a diverse source of information on transport, deposition and flow processes, as well as the biological diversity in the environment. The sedimentological samples collected during the measurement campaigns are analysed for their grain size distribution and composition. The grain size distribution can be used, for example, to understand transport processes. This is because fine-grained, light material requires a lower flow velocity and energy to be transported. As a result, fine-grained material is usually transported over long distances and partly in the water column. Coarser, heavier material tends to be transported short distances along the seabed.
As some marine organisms, such as mussels and snails, form hard calcareous shells, these are preserved even after their death and thus contribute to sediment formation. Plant remains, faecal pills and the remains of sea urchins, fish and crabs are also deposited in the sediments. The composition of biological and inorganic material, such as quartz and flint grains, provides information about the biological diversity in the immediate surroundings, the input of land and the relative age of the sediment. The degree of rounding of the sediment grains, the rounding of the grains through movement, and the preservation of the shells and remains can indicate the transport distance and length.
The example of the Harle inlet will be used to clarify how long-term coastal protection, in the form of groyne H, affects the sedimentology and morphology (Geßner et al., 2023, in review). A subsequent combination with current investigations provides information on sediment transport. The specialty about the Harle is the semi-diurnal tide, which leads to a regular reversal of the current directions and thus has an influence on the morphology both north and south of the groyne H.
Initial results show that the groyne has a strong influence on the current dynamics and thus also on sediment transport, deposition and erosion areas. For example, there are areas in the direct vicinity of the groyne where erosion occurs due to a changed and intensified current. Areas of sedimentation have also formed due to differences in current velocity. Thus sea channels and sand ridges have formed and separate the two main channels Harle and Dove Harle within the Harle inet. A comparison with the inlet Otzumer Balje shows that these bottom forms are not of natural origin, as only one channel predominates in the Otzumer Balje and this has a subaqueous dune field, i.e. underwater dunes, at its widest point. The sand ridges, which lead to a shallow water depth in the Harle, prevent the formation of such dunes and only small ripples and megaripples can develop.
These initial results lead to further questions that are to be clarified in the course of our project Gute Küste Niedersachsen. These include the seasonal variance of the inlets Otzumer Balje and Harle, respectively, and how storm surge events are reflected in the data. Also a closer look will be taken at the heavy metal pollution and particle transport of the tidal inlets.
Imaging measurements are important for our scientific questions throughout the GKN consortium in order to document and analyze changes. We record coastlines, morphology, topography, geology and vegetation from a bird’s eye view by flying over our real world laboratories. We use the research aircraft of the remote sensing working group at the Jade University of Applied Sciences Wilhelmshaven for this, with the ICBM in Wilhelmshaven coordinating the work. Colleagues at the University of Hanover use the images to create orthophotos, rectified and georeferenced aerial images. These photos then provide information on, for example, flood lines for assumed storm surge scenarios, coastal development, vegetation development, sedimentation or dune migration.