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:
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.
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.
It has been a busy Monday at GWK with the installation of the first set of plant pallets in the flume, last minute adjustments and the first reference measurements for plant and soil surface parameters. Of course everything took longer than during the trial last week, but given that something like this has never been done before, the detailed planning paid off and it all went smoothly. The pallets were arranged in predefined rows and then graciously hovered (by hoist) into the flume. Once set in their dedicated slots in the concrete platform, they were fixed with metal plates to ensure a smooth transition from the surrounding concrete to the pallet’s soil.
At the end of the day, the five rows of plant pallets, all representing different stages of salt marsh development from individual seedlings, via tussocks and closed canopies to small cliffs, were installed. In addition, some of the rows are also composed of different plant species which are typical for brackish or more saline marshes. And some of the pallets are covered with biodegradable grates designed to provide protection from erosion. This broad spectrum of species and development stages will allow us to determine the soil stabilising potential of salt marshes and results will be applicable to a broad range of brackish and saline marshes in northern Europe.
Although the overall focus of these experiments in on soil stability, we are fully aware that the waves will not simply pass by the plants without any effect. For this reason we are determining plant parameters such as biomass distribution (both in the horizontal and in the vertical) and the angle at which the seedlings protrude from the soil’s surface. These measurements get done prior to the first test and then after each experimental run. It will be exciting to assess how the above ground biomass of plants is changing during the course of our experiments.
We now stand by for the first inundation and the switching on of the wave generator for the first waves…
It does not matter how well thought through everything was beforehand, the last few days before such a massive experiment end up being fairly hectic. The last cameras and calibrated boards arrived and had to be attached to the prepared frames. We will use these to take spatially referenced photographs for later analysis if and how much biomass the plants lost during the experiments. Also, the first set of plant pallets needed final preparations: The last weeds were removed carefully with scissors in order not to disturb the sediment and the pallets that will show a cliff needed to be cut to shape. This removal of soil was also used to collect sediment samples for later analysis. Oh, and they needed more watering of course, which the sky took care of with a refreshing thunderstorm for a change. But since this is not a very reliable method, the FZK team also installed a timed watering system.
A test wave run allowed us to make sure that all instruments are working and make some adjustments where data quality was not yet quite up to scratch. The other thing this pre-test showed, however, was that the heat wave is taking its toll on instruments and the flume’s wave maker with computers slowing down and pumps heating up much more quickly than usual. Especially in this context, it was really great to have had this pre-test and it made us rethink our planned daily routine and restructuring it slightly to run the tests as early in the morning as possible, when air temperatures are still below 30 degrees.
It was a fantastic effort, especially of the FZK team who never ran out of patience and always had solutions for arising problems. But now it is done. The flume looks fantastic and is and ready to take the first set of plant pallets on Monday.
It happened to be the hottest day of the year and warmest night since weather recordings just when our plants were loaded onto trucks and transported from NIOZ in Yerseke all the way to Hannover. NIOZ is the place where the plants had been placed onto the pallets in spring and where they have been nurtured over the last few months. But now it was time to say goodbye and bring them to Hannover in preparation of the experiments which are due to start next week. The Yerseke team watered them well prior to loading and the truck divers were so kind to open the truck doors during their night break. So we were confident that the plants would survive the trip. Yet, the relief was massive when all plants arrived in good shape.
While unloading the trucks, they were sorted into the respective batches ready to be transported into the flume. There is only one thing left to do now, and that is automating the watering system.
Air temperature in the flume peaked just in time for the science team to arrive for the trial run. The last instruments to be installed in the flume arrived and responsibilities and task allocations were discussed. And then we dedicated a full day to trialling the installation of plant pallets in the flume, testing the frames that the FZK team had built to allow standardised photos of the plants after each run and perform sediment erosion measurements at defined points and time the different measurement techniques that we are planning to use. While it felt a bit odd to measure the leaning angle of an imaginary plant and taking photos of empty pallets, it was important to get a feel for how long each measurement takes.
Measurements on the plants and soil can only be performed once the flume is drained and for a flume the size of GWK this takes a while. And to be able to perform one test and finish the associated measurements within a day, we have to be as efficient as possible when it comes to taking the measurements. So based on the information how long individual measurements took, we then discussed the sequence of measurements and whether additional hands can speed up the process or not.
All these trials and discussions made for an intensive two days which was amplified by the ongoing heat wave. But by the time the team left again, we felt we had made good progress and can now make last minute adjustments to frames and methods in final preparation for the experiments.
The heat wave in Germany is in full swing with temperatures exceeding 35 degrees at times. Yet, the FZK team continues to puts concrete blocks and gravel in place to get everything ready in time for the experiments and the instruments are waiting patiently along the side of the flume to be installed, ready to be woken by the data acquisition system.
In the meantime, the housing for the various drag sensors were built in the workshop so they can safely be installed in a gap between the concrete blocks. These drag sensors are designed to measure the forces that act on them in a very high resolution. During our experiments, we will use these sensors to measure how forces on individual plants differ between summer and winter state of a salt marsh. For this comparison, identical pins are mounted on the sensors. These do not look like real plants, but it means that any differences we measure are caused by the surrounding plants and their state of health and are not influenced by the shape of the pins.