Field surveys

Dunes are investigated on Spiekeroog at various locations. To that regard measurements were conducted from November 2021 throughout December 2022 focusing on Ammophilia Arenaria, the predominant white dune vegetation. Amongst others the canopy height was assessed, as well as horizontal density and culm thickness. Furthermore, representative vegetation samples were gathered for further analysis at the nearby environemntal laboratory of the national park house Wittbülten on Spiekeroog. This prevented unecessary long transportation trips for the samples, potentially skewing laboratory results due to wilting and aging of the samples. Subterranian sample material was gathered consisting of roots and rhizomes, which had to be evaluated at a laboratory at the Technical University of Braunschweig due to testing standards.

Measurements were conducted monthly to map the seasonal development of vegetation properties first time covering a whole year. Focus area was a single detached white dune on the northern coast of Spiekeroog exposed to direct wave and wind attack from due north. The dune was segmented horizontally into 10 sectors to systematically acquired samples and data. Furthermore, on the western tip of the island a contineous white dune was investigated. A 50 m long segment was surveyed on the sea and land side. In conrast to the detached dune, this line dune was segmented into 5 vertical areas along its cross profile for systematic data acquisition.

Additionally to vegetation samples, sediment samples were acquired and evaluated to obtain sieve curves and map potential influences through weathering. Deployed soil sensors logged various soil parameters contineously over the year to provide contextual data. Surface scans were periodically conduced using LIDAR technology as well as terrestrial laser scanners. Surface and terrain data will allow for morphological analysis. Most of the data is currently being processed.

Physical modelling

In order to deduce influences of dune vegetation onto soil erosion under storm surge conditions physical laboratory experiments were carried out. These were conducted in a wave flume at the TU Braunschweig spanning 90 m in length, 2 m in width and 1 m in depth. Initial tests investigated the influence of horizontal vegetation density onto erosion patterns. By means of LIDAR scans and photogrammetry temporally and spatially highly resolved erosion dynamics were mapped. In addition on a smaller scale in a flume 9 m long and 0,3 by 0,3  m in crosssection initial experiments for mapping the influence of subterranian plant material (roots) onto erosion resistance were conducted. Both sets used live batch plants, previously grown in a green house for several months. Furthermore, first experiments were carried out with vegetation surrogate models based on various fiber materials. Data is currently being processed and evaluated.

Numerical modelling

Utilizing the field survey and experimental data collected for Ammophilia Arenaria, surrogate and dune reinforcement experiments were conducted in the experimental lab to mimick root impact. Data aquired during the tests serves calibration data for setting up numerical models. Computer models allow for much easier changing of parameters regarding soil composition or vegetation properties above and below ground compared to physical laboratory experiments. Thus acquired field and lab data serves as calibration and validation data for simulations.