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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