Spiekeroog. A day of the Wilhelmshaven Excursion (KOL) Report by Michaela Erni, Reto Grischott, Marcel Nägeli and Gregor Zographos

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1 A day of the Wilhelmshaven Excursion (KOL) Report by Michaela Erni, Reto Grischott, Marcel Nägeli and Gregor Zographos D-Erdw, ETH Zurich, Switzerland 2010

2 1. Introduction is one of the seven East Frisian barrier islands, built after the Platen- Hypothesis of Brackhausen (1969) (Fig.1 and 2). According to this theory, the island results of the combination of current, tides and wind. Periodically flooded sand bars evolved to sand dunes, which mark the initial state of the island. Fig.1: Seven islands with (second from right). (North-oriented picture of GoogleEarth) has an extension of approximately 2.5 km width and 10 km length. Nearly the whole island is a national park and belongs since June 2009, as a part of the Wadden Sea, to the UNESCO world heritage. (Reference: Fig.2: Island seen from the SW. The sand bar in the left side is moving regularly. 1

3 1.1. Tidal conditions The flood reaches the islands first in the Western parts and continues counter clockwise in Eastern direction due to the coastal shape of the Northern Sea. The mesotidal milieu has a large influence on the build-up of such islands and they are not stable by natural conditions in general. As a result of decreasing sediment transport with decreasing water velocity, the biggest islands are situated in the West Island tidal behaviour In the west of Sturmeck we can observe a bidirectional drainage system. There is a geographical separation of inflow and outflow, where a main channel between two smaller side channels is situated (Fig.3). Fig.3: Two flood channels (red) and one main ebb channel (blue) drainage the tidal flats south of (North-oriented picture of GoogleEarth) The flood occurs in these two minor channels, which are shallower than the main channel. This results in a stronger and shorter timed inflow whereas the ebb outflow is slightly longer and weaker. The channels are separated by sandy side bars. Those mark the zone with zero velocity where sediment particles can settle down. In the side channels, where the flood comes in, sediment is imported, which is lost through the main channel by ebb flow, normally after a time period of two decades. During storms it is also possible to have a major sediment inflow in the main channel. The separating side bars allow no real exchange of water during normal conditions anymore. Due to the lower energy behind the protecting sand bars, more and more muddy stuff with smaller grain size gets deposited. Due to the flood direction, the sediments are redistributed around the island and a migration in southern direction is assumed. 2. Island architecture In the North the island is limited by a wide and flat sand beach. The centre is dominated by sand dunes which are already overgrown by several generations of plants, whereas the southern part is marked by salt marshes Sand beach in the North The northern part of the island is dominated by wide intertidal flood plains which are completely flat on larger scale. However, having a closer look, different sediment 2

4 structures can be observed, which are typical for the conditions of the water-land interaction of this tidal regime on the barrier islands Sand ripples In Fig.4 we can observe asymmetrical wave ripples. Whereas wave ripples are symmetric in general, asymmetric features are explained by interaction with a current. However, a flood does not act like a current as it is a continuous increase in sea level. Thus, the origin of the asymmetry is due to the wind. In shallow water depth (few cm) the wind can influence the ripple architecture. To create this shape, the wind direction has to be regular (coming from northern direction in our case) and therefore we observe a 2D- architecture, originating from the wind. Fig.4: Slightly asymmetrical ripples. In slightly deeper water, the ripples become more symmetrical. When digging a little trench profile, herringbone shaped cross-bedding features below those ripples can be observed (Fig.5). These cross beddings were coarser than the fine grained parallel laminations. The beach facies was interpreted as a mixture between Trough and Landward Slope and is located in the lower backshore. Fig.5: Blue drawings indicate herringbone structure. 3

5 Overwash At some points, overwash structures could be observed. As shown in Fig. 6 the ripples are cut by fan deposits and slight erosion. Sediments, especially coarser grains, are usually deposited behind the ripple tops, due to the energy decrease occurring in the depressions. Fig.6: Small-scaled alluvial fans with the coarsest grains at the toe of the fan Initial microbial structures In some artificial basins where the milieu is protected by external forces, like winds or currents, signs of microbial life can be found (Fig.7). As there is not sufficient water supply, the microbial mats accumulate in depressions and build features which can be seen in their initial phase. In times of less water, they get dry and just some desiccated spots remain. Most of these microbial spots are covered by little particles, which were caught in the organic structures. Fig.7: Artificial basin with few desiccated spots. 4

6 Reduced zones One additional element which can be found in the sand beach region is fully evolved microbial mats. They can be recognised by a green surface layer consisting of cyan bacteria. The second layer is characterised by purple sulphur bacteria and the further down we go, the sediment colour turns from purple orange to black, representing the sulphate reducing bacteria (Fig.8 and 9). These sulphate bacteria produce black patches in the ground, indicating the reducing conditions which results in the dark colouring of the ground further down. Fig.8: The black colored zone in the dug hole indicates a reduced milieu. Fig.9: Schematical layers after Gerd Channel feeder system Tidal creeks which are cutting through the borderline of the beach are created as the water is released out of the flood plains sand package and goes back into the sea (Fig.10). This water, originating from the last flood event, returns due to the altitude difference. Tidal 5

7 creeks should not be mixed up with tidal channels, which have oscillating flow directions with the tides. Fig.10: Mini tidal creeks Bubble sand Bubble sand is a feature, caused by foam which gets incorporated into the sand/water mixture and is preserved during deposition (Fig.11). The fast overlaying of the next sediment fraction makes preservation possible without outgassing. It s a purely mechanical feature. Fig.11: Bubble sand holes Grain size distribution Over the whole beach profile the energy of the water flow controls the grain size distribution. During the flood current, the coarser grains are deposited at the top of the beach area. As the back flow has much less energy, flat grains and the shells cannot be transported back to the sea. The grain fraction still remaining in suspension and the rounded small particles are the only ones, which can flow back into the sea. The result is a grain size distribution, where shells and large flat grains mark the highest transgressive 6

8 borderline of the flood. In general, a flood plain contains sand particles as the smallest fraction and all smaller grains are transported back into the sea Berms In coastal systems, a berm is a raised ridge of pebbles or sand found at high tide or storm tide marks on a beach (Fig.12). A high energy system like high tide, storm tide or even may be shore currents erode the beach and leave a lowered beach level. Therefore, different levels of energy impact on the beach can be observed: The storm tide will be preserved until the next storm tide occurs and wipes out the high tide berms. Fig.12: Berme types at the north coast of (Google Earth) 2.2. Salt marshes in the South The salt marshes are divided into two basic areas. A lower salt dominated area and an upper salt tolerating habitat can clearly be distinguished (Fig.13). The lower salt marsh is regularly flooded by the tides and only organisms with high salt toleration can survive (Fig.14). The morphology is dominated by tidal channels or tidal creeks which are filled during normal tidal events and the habitat is dominated by Saliconia. In contrary, the upper marshes are only flooded during storm floods and extreme events. The organisms there are only able to deal with a small amount of saltwater. A separation of those two parts can clearly be seen by the vegetation change (Fig.13). 7

9 Fig.13: Red line indicates the somewhat boundary of upper and lower saltmarshes (GoogleEarth). Fig.14: Lower salt marshes with drainage creek. 2.3 Sand dunes The island basically consists of overgrown sand dunes which can be seen with less vegetation on the outer parts of the island. By forming the sand dunes, water has no function anymore as it is a purely aeolian process. The wind transports the sand from the beach to the interior of the island. This region is called supratidal because the tides do not have any influence anymore. Unstable dunes without a vegetation cover would prograde into the island and destroy most habitats (explained in chapter 3). In Fig.15, a crosssection from an island model, evolving from the beach, over primary dunes with pioneer plants to fully vegetation covered brown dunes can be seen. The older the dunes, the more central within the island they are located and the more evolved plants are growing on them. This change in vegetation is not only due to the protection from other dunes nearer to the beach but also because the influence of the saltwater diminishes and a bigger variety of freshwater loving plants can establish. 8

10 Fig.15: Formation form beach sand to vegetation covered dunes from (Ellenberg, 1986) 3. Anthropogenic influence and perspectives 3.1 Stabilisation of the dunes As discussed under 2.3, the sand dunes are built mostly under natural conditions. However, human impact stops the progradation of the dunes by pulling wooden sticks into these already existant primary dunes (Fig.16). The dune stabilised by these wooden sticks prevents the population of from storm floods and a first stage of vegetation starts growing once the dynamic behaviour of the dune is stopped (which enhances the stability of the dune even more). Due to the stabilisation of the front dune, the dynamic of the whole island is stopped, as the primary dune does not only protect humans but also protects the more central dunes from aggradations. As a result, the people of do not have to remove their houses from time to time, but can settle down. Fig.16: Sand dunes at Sturmeck stabilized with wooden sticks to prevent the dunes from progradation. Instead, the dunes protect the population from storm floods. 9

11 3.2 Drainage systems On the island of two kinds of drainage systems can be distinguished. One of them is natural, the other is anthropogenic. In case of the natural drainage system, the channels meander as they flow in direction of the sea. In the channels, cross bedding like in rivers can be observed. These characteristics strongly differ from the anthropogenic drainage system called Grüppen. Here, the channels are straight lines occurring in a very regular drainage pattern, flowing in any direction. Under normal conditions (especially during ebbs) these channels are dry. With these Grüppen, more land is reclaimed on the island of. Fig.17: Perpendicular and parallel drainage system for reclaiming more land (GoogleEarth). 3.3 Dyke/ Coast protection Dykes are an artificial technique to protect and stabilize the coasts from fluctuating shorelines induced by tides and currents. Dykes have been built for many centuries, which show the big importance of this technique. A dyke is an artificially filled up dam along the shore line (Fig.18). A modern dyke consists of a core-dyke with sand surrounded with a caulking material, especially towards the water side. Dykes have weakly inclined slopes towards the water side to break high waves and steeply inclined slopes at the land side. The top of the dyke exhibits mostly a road for observing the dyke and also for reinforcement measurements. Five possible factors endangering the dyke have to be considered (Fig.18): 1. High waves breaking on the dyke front, pressing water in the upper part of the dyke causes sudden breakthroughs of the dyke crest. 2. Springs of seepage water entering out of the dyke enabling additional erosion with the breakthroughs of high waves. 3. Settlement of the sandy-pebbly base of the dyke may trigger sudden slides on the land slide. 4. Humidity penetration of the clay layer could cause outflow of the sandy layers. 5. Thixotropy behavior of sand core of the dyke causing internally slides and settlements. 10

12 Fig.18: Possible points of danger for a dyke. (Wikipedia.org) 4. References Literatures: Brackhausen (1969) from Streif, H. 1994: Das Ostfriesische Küstengebiet. - Sammlung Geologischer Führer; Nr. 57; S Pictures: Fig.1, 3, 12, 13, 17 are pictures of the island modified from Google Earth. Fig.15 is from Ellenberg, H. 1986: Vegetation Mitteleuropas mit den Alpen. - Ulmer, Stuttgart (4.Aufl.). Fig.18 is from ahrenpunkte_1.jpg&filetimestamp= , Internet: pdf,