More than 10 tests successfully run…

… before we spot a plume of sediment obscuring part of one of the video windows… Waves were still small (30cm) for the water depth (2m), but after checking our data on the central monitor in the control room, we also discover that one of the drag sensors appears to have failed right at the start of one of our tests. We decided to take a break and lower the water level to inspect the test section – but it all looks remarkably as we had left it, so on we go: more tests with ever higher waves in both 1m water depth (a well known experience for the plants) and 2m water depth (here comes the storm surge condition…)…

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Still small waves…

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… and peaceful underwater scenes…

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Watch the plants sway in the gentle waves (little do they know what awaits them…):

http://www.youtube.com/watch?v=pI8W2OiNCAQ&feature=youtu.be

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Swimming and skimming

We have now gone through a full week of experimental runs with two ‘exposure’ days on which the water level was lowered to let the plants breathe and to let us work – i.e. carry out our measurements on the plants and the soil surface. But before each time we drain the tank, we have decided to go fishing…. for all the debris and plant matter that has floated to the top during the intervening wave runs. We hope that the amount of material fished out might relate to the intensity of the waves that we have sent over the plants: the more energetic the waves, the more plant matter should float to the surface…

Our fishing net...

Our fishing net…

For the purpose of skimming all that is swimming, we use a net suspended on two ropes that is rolled along the flume with two cradles on either side of the tank. All of the material collected in the net is then left to dry, so we can establish its dry weight and compare it with previous ‘fishings’. It all makes the flume smell as good as the seaside!

Dried marsh debris - a lovely smell!

Dried marsh debris – a lovely smell!

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Do marsh plants ‘feel the force’?!?

We want to know what force the individual plants ‘feel’ when they are rocked by the backward and forward motion of the waves. In order to measure this, we attach specimen of real marsh plants to small metal holders and then mounting the holders onto an instrument that measures the force used to push it in one direction. The trickiest part is the attachment of the plant stems to the holders, and after trying everything from cable ties to rubber, straws to glue, we have managed it. The information we have recovered so far is fascinating and clearly shows the increase and decrease of force on the stems with the motion of the waves. We also attached ‘mimics’ of real plants, strips of plastic of varying thicknesses and thus varying flexibility, so that we can compare the force experienced by these artificial plants with the force experienced by the real ones – something that will help future researchers to study the effect of plants without having to harvest real specimen.

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If you squint, you can see some transparent mimics to either side of the plants.

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Waves, fish, and toad…

A tough day trying to get the video hooked up with the data acquisition system prior to starting our first proper wave tests…. The marsh, with 2m of water over the soil surface, looked remarkably peaceful through the underwater cameras – and while the first gentle waves were sent over the marsh section, we were watching from the control room via the video system…  Plant stems began to sway gently, remarkably, even in such deep water depth and with very small and short waves. The data showed little sign of dissipation of wave energy yet – as expected and intended. A big sigh of relief. By gradually increasing wave energy, we are literally ‘inching’ our way towards the threshold between waves moving freely across the ‘rough’ bed and the bed causing them to lose energy (and thus to act as a natural sea defence).

The first gentle waves travel across the unsuspecting plants...

The first gentle waves travel across the unsuspecting plants…

Transfixed by the swaying motion of the tall stems of our marsh plants, we spot a leaf floating backwards and forwards past the underwater window and camera.. or, no! It has an eye! A fish in our ‘aquarium’!!

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‘It’s a fish!!!’

Clearly, the pipes that fill the tank with water from the Mittellandkanal, are big enough to allow fish to pass through…

Then, we spot a toad bobbing up and down in the gentle motion of the waves on the surface of the water and then diving down again, before we can even begin to think of a rescue mission… There should be plenty of food to catch under water and in the marsh, when the plants are given a day to breathe again, before we simulate some less comfortable wave conditions….

Toad in a tank...

Toad in a tank…

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Marsh survives first flooding and die Mäuse do so too!…

Everyone watched intently as we submerged the constructed saltmarsh for the first time… It was amazing to see the water trickling onto the marsh ‘platform’ from both ends and there were several remarks along the lines of “This looks just like a real marsh surface when the tide comes in!”. It was critical to regulate the water inflow carefully, so as to reduce any potential difference in the water level at either end of the marsh that could have resulted in damaging currents and it worked! Slowly but surely the water made its way onto the marsh and along the gravel ‘fill’ in the gap at the very edge on one side of the flume. A simple process, but one that we have now been waiting for with a lot of anticipation for a very long time!

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… and the marsh is under…

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Water is starting to fill the marsh surface…

What we were not sure about was how the air trapped inside the sand base and the soil surface would eventually make its way to the surface… but it was soon clear that there were enough small holes and root channels in the clay to allow a steady escaping of air. Some of the bubbling action became fierce enough to start resembling a whirlpool-type jet, getting us very worried that the force of the bubbles could break up the overlying clay. But the bubbling eventually subsided, the marsh stayed put, but….

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Mr Mouse rescued

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…and reunited with one of the three other fellow mice…

…as the flume starts to fill, up goes the cry ‘mouse!’ And there it is, frantically swimming across the saltmarsh section. The ‘basket’ is quickly deployed and the little ball of fur expertly fished out. And then we see two more… Once in a cardboard box, the babies all look remarkably well, if a little damp,  and we soon feel confident enough to take them outside and release them into the woodland around the flume. Then it is back to work, excitement over…  Or is it? 15 minutes later, we hear ‘maus!’ again and the rescue efforts swing into action once more. This one, surely from the same nest but having been in the water that much longer, looks in a bad way. But we get some heat into it under one of the saltmarsh lamps and gradually it starts to twitch, and then raise itself on its little paws, and then open its eyes, and finally we feel able to take this one outside too where it scuttles off into the undergrowth.  We all feel good – 4 lives saved!

Next step: the first pre-wave measurements…

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Humps and bumps on the salt marsh

It is important to get a picture of the vegetation cover and surface topography (the humps and the bumps) of the saltmarsh section before the wave runs begin because both these factors influence wave transmission.  To do this, the team first expertly lowers a specially built frame across the start of the saltmarsh section, keying it onto 4 reference pipes at the sides of the flume. Then it is time to take a couple of laser scans of the whole saltmarsh section, with the frame, from a camera located high on the gantry above the flume. The frame has a metal bar mounted along one edge across the flume and through this we lower a series of 20 pins onto the marsh surface, carefully feeling the soil surface through the vegetation canopy.  We measure the height of each pin above the marsh surface relative to the horizontal bar. This gives us a cross-flume profile of the surface elevation which we then repeat at 5 further platform positions down the saltmarsh section. We can pick out the horizontal bar on the laser scans so hopefully, if the image processing goes well, we should be able to produce a map of the vegetation canopy, and a good idea of the underlying soil surface, for the saltmarsh. To track saltmarsh surface change, we plan to repeat this exercise each time we drain the flume after a series of wave runs.

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Watch the time-lapse movie!

Thanks to James Tempest for putting this one together! One hour in reality equates to one second in the movie….

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200 square meters complete!

After two weeks of hard work, the saltmarsh is complete! All the seams and all joints with the wall of the flume on the one side and the dividing plastic sheet on the other side have been tightly packed with clay where gaps remained, so that the whole 200 square meters form a connected platform. Along one side of the flume, very coarse gravel has been placed to allow a tight fit while creating a surface with a roughness most likely comparable to the vegetated part, so that wave crests travel uniformly across the whole section.

The final preparations for the first submersion are now underway – and everyone here is holding their breath…

Vegetation complete!

Shortly before completion of the vegetated section…

... and, from the back, complete with gravel along side and back.

… and, from the back, complete with gravel along side and back, and sprinklers to keep it all hydrated….

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Creepy crawlies… and MICE!

The last few days have been marked by a smooth operation of moving, placing, and muddy sticking… but also by red ants, spiders, worms, beetles, and various other creepy crawlies that have emerged from the vegetation pallets. A species rich assemblage overall! The most surprising creatures were two mice that had clearly felt that the slightly drier Elymus soil was perfect as a home…

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Mr Mouse emerges to find himself in a rather different environment...

Mr Mouse emerges to find himself in a rather different environment…

First one of them emerged from the corner of the marsh turf, and then the other… Amazingly, we managed to catch both of them, but them in one of our empty clay container buckets and transport them out of the flume into the long grass next to the flume building – a happy ending for the mice couple. Could have been worse for them…!

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We have reached half way!!

The team are now ‘well oiled’. Pallets are being inspected as they are teased apart from the ‘bulk’ of pallets sitting outside in the ‘storage area’, then labelled and moved sequentially into the flume, where they are carefully pushed onto the board, carried by crane, and positioned in their correct place after being ‘patched up’ with clay where gaps become obvious. So the blocks (and everything in them) has an easy, undisturbed, transition, while the team work hard…

Yippee! Row 17 is placed - half way!

Yippee! Row 17 is placed – half way!

… and towards the end of the week, we finally reached half way point. The team is now working so well together that everyone is hopeful to achieve completion of all 34 rows of saltmarsh blocks by the middle of the following week.

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