Wave attenuation over coastal salt marshes under storm surge conditions
Iris Möller, Matthias Kudella, Franziska Rupprecht, Tom Spencer, Maike Paul,
Bregje K. vanWesenbeeck, GuidoWolters, Kai Jensen, Tjeerd J. Bouma,
Martin Miranda-Lange and Stefan Schimmels
Nature Geoscience: http://dx.doi.org/10.1038/ngeo2251
And we have several further papers in preparation, so watch this space!
Only a month after completion of our experiment in Hannover, the team of the Cambridge Coastal Research Unit of the Department of Geography find themselves surveying the aftermath of a real storm surge that wrought havoc around the UK east coast on the 5th and 6th of December. There is evidence, that this event was larger than the 1953 surge in which over 300 people lost their lives in the UK and many more in the Netherlands (see our survey results write up http://www.cam.ac.uk/research/news/cambridge-researchers-learn-lessons-from-recent-storm-surge and our piece in Nature that is linked from there.
This means that the marshes of Norfolk most likely experienced precisely those kinds of conditions that we simulated in this experiment, and the evidence for high marsh surface / soil stability, but loss of vegetation through plant stem breakage is there in places:
Vegetation removal in front of sea wall after the 5th December surge, Norfolk (photo: I Moller)
Apologies to all, who have been following this blog, for a rather late round-up of events in the final stages of the experiment. There is no other excuse than last-minute computer glitches and exhausted scientists! So, now that we are all back in our respective institutes and the plants have well and truly disappeared (helped by the Hamburg team expertly mowing the marsh for our final wave tests!), we can report that the experiment has been a great success – and that means that we now have rather a lot of data (over 6000 data/image files have been generated).
More data… (measuring stem densities after wave impact and before mowing)
All of this now needs to be processed and analysed to extract our various messages, for scientists and engineers, coastal managers and anyone who is interested.
For now, here’s just a small summary of what we have learnt and a taste of what is to come once we have analysed our data:
- It can be done! Marsh blocks, when excavated like this, positioned in the wave tank, and hand-patched together with care do not float away when submerged to up to 2m water depth above the soil. The pressures generated by the simulated storm waves, too, did not manage to destroy our reassembled saltmarsh!
But at the end… the mud is still there – after all it has been through!
Storm wave crashing at the front edge of the mowed marsh
- Mice can swim (and survive when rescued by expert crane and net operators!), fish can make it into the flume (and out again), and so can toads… 🙂
- Waves, even those generated in storm surge conditions, do not easily destroy a marsh – at least not by eroding the soil surface (and we will be able to say a lot more on this when we have had a closer look at the data!).
- Wave energy thresholds do exist (we will have the detail!) beyond which marsh plants do become damaged and break (again, detail to come!)
More floating plants – destroyed by waves and gathered here for drying and weighing
- There really are wave energy and water depth thresholds that control whether (and by how much) marsh plants reduce wave energy… (again, more on this when the number crunching has finished!) – a key finding for the coastal protection people out there, particularly in the face of sea level rise and other coastal challenges!
And, finally, and most importantly, none of this would have been possible without funding from the European Union under the Hydralab IV scheme and without the amazing support we received on a day-to-day basis by the people on the ground, at the Grosser Wellen-Kanal (GWK) in Hannover: the driver of the fork lift truck, the electricians, the detritus-fishing-net builder, the crane drivers, the giant hydraulic ‘cake-slice’ operator, the mouse rescue team, the cabling and data acquisition experts, the coffee suppliers, and the cake bakers. Thank you all – you know who you are!
Remains just one more thing to point out: This blog will stay ‘live’ and we will post further news as and when it becomes available while we publish our findings (though be patient while we crunch our way through the mountain of data!) – keep following us, comment on the blog, and get in touch if you would like us to send you copies of publications!
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’
Erosion feature on marsh surface
We 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
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
… 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…)…
Still small waves…
… and peaceful underwater scenes…
Watch the plants sway in the gentle waves (little do they know what awaits them…):