Soft stuff stronger than it seems!

Now that the storm surge in the wave flume has subsided, the winds are picking up to gale force outside of the flume building… But we have been indoors, inspecting the marsh surface. A number of bare patches give the marsh a rather sad appearance, but the soil has held together amazingly well, as we suspected from our underwater window view. So it is looking as though both plants and soil were stronger than we thought. Though the former gave way to the power of the waves first, the strength of the plants was impressive! Next step: mowing. We should then discover how much longer the soil will hold together without plant cover, when we, once again, throw increasingly powerful waves at it.

Broken plants and 'brushed' surface: the front of the marsh after our 'storm surge'

Broken plants and ‘brushed’ surface: the front of the marsh after our ‘storm surge’

Erosion feature on marsh surface

Erosion feature on marsh surface

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The storm surge is here!

imageWe have reached the point in our tests where water is as deep and waves as high as can be expected during a storm surge on the North Sea coast! Nail-biting stuff, as we stand and watch the waves break over the marsh with such ferocity that a person would be swept off their feet in an instant…. What this has done to our marsh will be revealed when we let the water out  – so keep an eye out on our blog!

Meanwhile, more of the plants float to the surface – with us having to risk getting our feet wet while fishing them out so that our wave gauges continue to be fully operational.

Storm wave crashing at the front edge of the marsh

Storm wave crashing at the front edge of the marsh

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Plants are due a health check

With a rather clever instrument, it is possible to measure how ‘healthy’ our plants still are. The PAM (pulse-amplitude-modulation) instrument is used to record the state of the plants’ photosynthesis system. Whether a plant photosynthesises well or not so well depends on the stress it is exposed to. While we are becoming increasingly stressed humans as the experiment is going on, our plants in the flume seem to be surprisingly ‘chilled out’ – little sign of stress so far, compared to our ‘control treatments’ and in as far as the PAM measurements are able to indicate.

Plants in our control box (light treatment without waves) undergo their health check

Plants in our control box (light treatment without waves) undergo their health check

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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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