## 3 Results
### Intertidal Bedforms  
#### 3.2.1 Intertidal Bedforms in the Upper Island Flats of the East Frisian Islands  
In large areas of the upper back-barrier tidal flats of the East Frisian islands, the sediment surface forms a pattern of periodic crests and troughs thus creating bedform fields of considerable size. The troughs are covered with water throughout the whole time of emergence, therefore the bedforms are clearly reproduced by TerraSAR-X imagery and they can be detected in the whole set of images (acquired from 2009 to 2016) and in the upper island flat of each of the East Frisian islands. In Figure 5, an overview of the bedform fields of the East Frisian islands is given, it shows the bedforms directly adjoining the southern island's shores are generally oriented in a north-easterly direction, but especially in the lower flats also cross-profiles can appear. The dimensions and the exact orientations may vary from island to island. In the study area at Norderney the bedform positions and their dynamics were examined in detail. The photograph (Figure 6a) gives an impression of their appearance in the field. The bedforms imaged by TerraSAR-X and the vectorized classification result for the water-covered troughs are given in Figure 6b,c. The results of the survey are described in detail by Ref. [19], who demonstrate that visual trough detection as well as results from unsupervised ISODATA classification of textural parameters from TerraSAR-X data are in good accordance with the in situ measurements of the bedforms. Spatio-temporal GIS-analysis of trough positions extracted from a time-series of TerraSAR-X images then revealed a shifting of the bedforms in an easterly direction during the study period from 2009-2015. This general bedform shift is demonstrated for the years 2012-2015 in Figure 7. The western trough edges are highlighted because in situ measurements as well as TerraSAR-X data reproducing variable states of water-cover indicate an asymmetry of the bedforms leading to steeper western trough edges and smoother eastern edges. Therefore the waterlines of the western edges proved to be a better indicator for the trough positions even with a slightly varying amount of residual water on the exposed flats due to environmental conditions or tidal state. Figure 4: Relocation of tidal channels imaged by TerraSAR-X: (**a**) location of study area (rectangle) in the tidal flats of Norderney (SL of 30/11/2012); (**b**) channel course in 2009 (HRS of 21/07/2009, 111 cm \(<\) NHN, ascending orbit); (**c**) shifted channel course in 2012 (red lines) compared to course of 2009 (blue lines) (SL of 30/11/2012, 129 cm \(<\) NHN, descending orbit). Figure 5: Bedforms in the upper flats of the East Frisian islands imaged by TerraSAR-X: (**a**) Juist (SM of 05/04/2011, desc. ); (**b**) Borkum (SM of 05/04/2011, desc. ); (**c**) Norderney (HRS of 02/06/2011, asc. ); (**d**) Baltrum (SL of 07/06/2012, desc. ); (**e**) Langeoog (HRS of 21/06/2016, desc. ); (**f**) cross-patterns, detail of (**g**); (**g**) Spiekeroog (SL of 17/05/2011, asc. ); (**h**) cross-patterns, detail of (**i**);(**i**) Wangerooge (SL of 15/10/2012, asc.). HRS = High resolution Spotlight, SL = Spotlight, SM = Stripmap acquisition mode of TerraSAR-X, asc. = ascending orbit, desc. = descending orbit. Figure 6: Intertidal bedforms at Norderney: (**a**) photography of intertidal bedforms in the test area (26/03/2014); (**b**) image section: test area in the flats of Norderney (HRS of 02/06/2011, asc. ); (**c**) Trough extraction result (white lines) from the same TerraSAR-X data. The high frequency of TerraSAR-X data acquisition also enabled us to study the bedform positions in the course of the year and in connection with the effects of storm events. [PERSON] et al. [19] showed that the trough positions extracted from TerraSAR-X data generally remained stable from late winter to late summer and a shift to the east regularly occurred during winter. The change from the summer to the winter situation in 2013 provides a good insight into the shifting forces.