Salt Marshes under Extreme Waves

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.

As the team at the wave flume is putting together the last remaining pieces of this awesome construction effort, the science team is preparing to converge on the flume facility in preparation of the first trial run… We want our carefully prepared tidal flat and marsh surfaces in tip-top condition right up to the point at which they will be brought into the flume for our first proper wave run, so we have to do this trial run without them.

To check that the equipment is fully working and that we all know what we are doing is extremely important, though – we cannot afford to take any risks after all this effort by so many people who have worked so hard to get us to this point. Thank you all and here’s to a successful trial run!

Final preparations underway in the large wave flume… our five ‘experimental zones’ are clearly visible across the 5m wide channel – imagine this all filled with water and giant storm waves rolling over the top….

Carefully inserting sampling tube…

While the South-East of the UK has experienced the driest June on record and in the middle of a heat wave, the RESIST team in the UK has been working hard… We are carefully excavating cores from a muddy east coast and a more sandy west coast marsh, before taking them to be scanned, allowing us to view their insides without disturbing so much as a single grain (well, not inside them, anyway)!

Beautiful iron deposits at depth – what causes these?

It’s hot and takes about 1.5 hours per core, but spirits are high, as we have time to discuss how marshes manage to build potentially impressive core stability…. Here’s hoping the sunshine lasts and ours is equally high as the UK core team spends several more hours working hard on the marsh…

Check out our linked NERC ‘RESIST (UK)’ project.

Labelling is key!

As core members of the Hydralab+ RESIST project we met for the first time in person at the FZK’s large wave flume facility (GWK) to discuss the challenge of preparing all of our 75(!) europallet-sized sediment and plant containers to be ready for the experimental runs in August this year. The challenge is enormous, but so is our enthusiasm…

Especially Tjeerd was so fascinated by the flume, its dimensions and possibilities that he kept sneaking away from the meeting and we constantly wondered: Where is Tjeerd?

And on that basis, we believe that we can achieve what we set out to do: to detect the wave energy thresholds beyond which seedlings and mature plants are damaged and/or removed from the tidal flats upon which they have become established during less stressful times. Knowing those thresholds will allow the people who look after our coastal communities, properties, and ecosystems, to plan better for the future. And who knows, perhaps we will also be able to find out how we can support plants during their early life stages, e.g. through placing biodegradable protection on the tidal flat until the plants are strong enough to withstand the highest storm surge…

With the new fresh leaves arising on the trees and flowers blooming everywhere, spring is definitely in the air. This makes the perfect time to plant our tussocks into the sediment boxes outside.

The RESIST gardening team prepared the boxes and arranged the different tussock setups for the various plant species. Finally they put the irrigation system in place.

Now lets grow some marsh species!

A hard-boiled group of RESIST team members went out to the marshes this month to collect salt marsh sods that will build the foundation for the salt marsh patches we will expose to hydrodynamic forces in the flume. Frost does not allow to plant them into the pallet boxes directly, but this step will be completed as soon as possible. Until then they will sit safe and sound all over the glass house.

Once in the pallet boxes, the vegetation will root into the boxes’ soil and expand laterally, providing us with a range of growth stages from freshly developed apical shoots to well established root/rhizome systems.