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Heavy machinery

 

Such large experiments are not possible without heavy machinery. First of all, there is the roughly 1000 horse power wave maker which enables us to produce our artificial storm floods. But we are also grateful for the two portal cranes which transport the pallets into the flume, lift our measuring equipment in and out every day and make for a great commute to work.

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The forklift truck that transports pallets outside the flume almost looks small compared to these heavy weights, but is equally important. For all these machines insurance companies insist on special training and they can thus only be operated by the FZK team. But there is one impressive machine that we run ourselves.

The ‘Instron’ is a multipurpose device which we use to measure stiffness of the plant stems using a three-point-bending test. During this test, a bar pushes down onto the middle of a piece of plant stem and the Instron measures how hard it needs to push to achieve a defined distance downwards. By testing a subset of plants at the beginning and end of each week, we will get a feel for any changes in stiffness that may have been caused by the four storm wave conditions we expose the plants to each week in the wave flume. Additionally, it will help us to interpret any response differences between plant species that we observe after each test run.

It needs a fair amount of samples to detect any differences between species in all the natural scatter. And because we only have a limited amount of seedlings in the flume, we brought some spares which were grown under the same conditions to produce sufficient data for robust statistical analysis and to keep the Instron busy.

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Taking care of the young

This project would not be possible without all the highly motivated young people in the team who do most of the counting and measuring and weeding and cutting. And while it is hard work, particularly in this heat, it is safe to say that none of them is regretting getting involved so far. After all, it is a great opportunity to work in an international and interdisciplinary team and a fantastic experience to work in such a large and unique facility. They will all get an amazing dataset out of this project which will keep them busy over the next few months and will form the basis for their dissertations and theses.

But we are also thinking about the young plants, which often grow on the very forefront of salt marshes. In this exposed location and without sheltering older plants around them their life is harsh, which is why we dedicated a complete experimental zone to them. Here we perform the same measurements than in all other zones, but because the seedlings stand separated from each other, it is much easier to see how the different plant species respond differently to the wave forcing.

The seedlings that we use have been grown from seeds this spring just like seeds would germinate in nature. However, out there a precondition of germination is that the seeds stay in one place long enough to germinate and take root. We therefore are also addressing the question how seeds actually survive storm waves on the sediment surface. And since brown salt marsh seeds are really hard to see on brown salt marsh soil, we use mimics that have a similar shape and weight but are much better to see. They may appear like remnants of a party, but they have been placed with great care!

 

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RESIST in the media

Into the second week of the experiments, we are slowly getting accustomed to having the plants in the flume and the number of photos we take of the setup each day has gone down to a manageable number. It seems that we are not the only ones fascinated by this experiment and its purpose…

…as a total of ten journalists and photographers followed our invitation for a press day last week taking the opportunity to ask questions about the experiment, the previous one we did five years ago here in the large wave flume, and the functions and services of salt marshes in general.

We then opened the flume floor to the cameras while our daily measurements were underway. It made an exciting afternoon for both us scientists who were suddenly facing a TV camera and the camera man who had never travelled by crane before. And it was certainly worth it. The local newspapers reported on our study both on Friday and Saturday last week and we made it into the regional TV programme. The feature on the national radio will be available for download until 23. February 2019, so if your German is not yet up to scratch to understand all of our press coverage, there is still time to catch up.

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More than scratching the surface

Besides the plant pallets on the raised platform, we also install a row of soil cores at the back of the flume each week. To do so, the FZK team especially designed  and built the steel ‘flower pots’ that take the cores and expose their vertical side to the approaching waves. To save us time during flume filling every morning, we drain the water only to a height just below the cores. Given the water temperatures above 20 degrees and air tempereratures even higher, nobody minds this high water level when checking on the cores.

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We use the cores to look at the effect of storm wave attack on exposed vertical sections of sediment representing marsh cliffs. Each week we install six 40cm deep cores with faces exposed to the waves. The cores are from two sites in the UK with contrasting sedimentology – one is sandy, while the other is finer and more cohesive. The cores, extracted as part of the NERC funded RESIST-UK project, have been scanned using a micro-CT scanner so that we can look at their internal structure: lamination, pore spaces, root networks and so forth. We hope that we will then be able to map the erosion that we observe under storm conditions to specific structural characteristics within the sediment. Given that the area where erosion may take place on these cores is vertical, the SET tables cannot be applied. But the SFM method applies here just as well and will give us detailed information how the cores responded to the wave conditions.

During the coming years of RESIST-UK we will also tie these characteristics to the surface vegetation and sediment characteristics that we can map from drones in the field. We will thereby build a link between features that we can see from the air and the vulnerability of the underlying sediments to storm waves, allowing us to make spatial predictions of marsh edge stability that complement the findings for the marsh surfaces that come from the rest of the experiment.

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Upping the force

During the course of the week, we have been constantly increasing the wave conditions that our plants get exposed to: the last wave spectrum of the week finally also contained some waves that spectacularly broke and washed white water across our setup. We were surprised how little biomass came floating up to the surface, but now cannot wait to see what these conditions did to the plants and soil.

The SET measurements that we perform (see previous blog entry) are crucial for documenting soil elevation changes, but they only give us data at certain points within each pallet. To complement these, we are using structure from motion (SFM) to produce 3-dimensional models of the pallets for every day. SFM requires us to take lots of photos (about 200 per row of pallets), from lots of angles, ensuring that we have plenty of overlap between them. The software then identifies matching points in multiple photographs and triangulates their positions in space to build very high-resolution (mm) 3D models. We can then use these models, combined with ground control points, to ensure accuracy, to look at changes in soil volume across entire pallets and to investigate how erosional features change over time in their geometry. We can validate these measurements against the SET measurements amongst others. The SFM approach isn’t only useful for the soil surface, we can also use it to look at plant bio-volumes and characteristics. Check out how clearly you can see the individual plants in the model and how many photos were necessary to produce this model:

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Everything is SET in place

It has to be said, having the plants in the flume simply looks beautiful. And on the first day, when the water very slowly rose so as not to disturb the sediment surface while flooding the pallets, we could not stop admiring them and taking pictures of them until we almost got wet feet.

Once the flume was filled to the desired height, we switched on the instruments, started the waves and hoped for the best. The water was exceptionally clear for GWK standards, but still not clear enough to see in detail what was going on at the bed while the waves were running. The slow and careful draining process then meant even more waiting time until we could finally see the plants and surface again and started the first post-experimental measurements in high spirits.

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One of the core sets of measurements that will be performed everyday are SET measurements. SET stands for sediment erosion table and it will give us detailed information on how the surface elevation of each pallet changes as a result of a wave run. The different plant heights and pallet configurations require slightly different setups in order not to bend the plants, but the method is always the same: A table or bar of a defined height is placed across the pallet which has a line of holes it is. We then feed pins through these holes until they just touch the sediment surface, fix them in position and read their height with graph paper. While an individual reading will not tell us a lot, comparison between the daily measurements will tell us exactly how the surface has changed. To ensure that we always measure in exactly the same locations, the FZK team had prepared tables whose feet precisely fit into holes in the metal bars that hold down the pallets. The fact that they thought about these holes when they designed the metal bars shows once more the team’s fantastic skills when it comes to designing experimental setups.

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