A lot of our measurements take place in the afternoon, when the flume is drained and we can access the platform to check on the plants and soil. But we also have a suite of instruments installed in the flume to record conditions during the wave runs.
A very essential set of instruments are the wave gauges which we use to record the incoming waves and also assess how waves changed due to breaking from the front to the back of the flume. Furthermore, some of these wave gauges report back their data directly to the wave maker which then is able to directly correct for any potential wave reflection and thus makes the cleanest possible waves for us. The wave gauges measure water level variations but what we are really interested in, are the conditions at the bed. That is why we also installed instruments that measure the flow velocities close to the bed. Acoustic Doppler velocimeters (ADVs) send out a very high frequency acoustic signal that is reflected from particles in the water. The Doppler effect causes the frequency of this signal to change depending on the particle’s speed and the instrument is capable of calculating this velocity from the sent and received signal. This data will tell us what flow conditions the plants and soil encounter as it is these conditions that drive the changes we observe each afternoon.
This flow induces drag forces on the plants which cause them to sway back and forth and may even damage them. To assess these forces, we installed an additional set of instruments, so called drag sensors. The force measurements require the plants to be cut from their roots and fixed onto the sensors. And since even hardy salt marsh plants do not last very long when they get cut off, we attach new plants every morning before the water floods the flume. Here the spare seedlings that we brought for the Instron come in handy as they give us enough fresh plant material for each day of experiments. Each sensor has a seedling of a different plant species attached to it and we measure its dimensions carefully before they get attached to the sensor.
Comparison between the plants and their drag data will help us to answer questions such as: Up to which wave forcing does each of the species still withstand the forces and when do the stems break? How are the forces related to shape and other plant properties? Especially the latter can only be answered if we combine information from the drag sensors, the ADV and the Instron, which once more shows how important it is that the different disciplines involved in this project work closely together. The fact that this interdisciplinary cooperation works really well makes this fantastic experiment very special and really fun to work on.