There are lots of challenges when it comes to managing lowland wet grassland, the biggest and most fundamental is control of water levels, if you get water control right then everything else becomes much easier. Waders that breed on wet grassland, in particular, Lapwings, require fields with short vegetation and shallow pools with exposed bare mud around the edges. Holding water in the fields through the breeding season and then removing it in the summer for grazing and cutting of vegetation is the biggest challenge we face at Ynys-hir. To do it we use adjustable pipe dams and drop board sluices. Currently a lot of these control points are not big enough to move water effectively off site in late summer to draw down the water table and allow access for agricultural vehicles. If we can’t get on and control the rush which is a consequence of the high water levels then we end up with a monoculture which is too dense and long for waders to consider breeding in.
So, this week we set about fitting a new sluice at a key point on the reserve, we replaced a 150mm pipe dam with a large box sluice attached to a 600mm twinwall pipe. Below shows the difference in size between the two pipes.
The key to installing any new feature like this is to know how the hydrology of the site works, prior to installing it I took the level of the two pipes where water leaves the site, if I match the bottom level of the sluice to this height then it means I can lose the maximum amount of water from the ditch. Any higher then I could be left with water I can’t get rid of and any lower and I could pull water in from elsewhere. The reason for the big diameter pipe is that I don’t want the size of the pipe to be the limiting factor for the amount of water I can remove in a short space of time after the breeding season.
One of the major challenges on the day is that when working in peat it can be hard to set a solid base, for this we used stone and sandbags filled with concrete. We then lowered the sluice into place and attached the pipe onto the back.
Above is the sluice in position, prior to lowering it in I had taken the level where it is sitting, making sure it is at exactly the same level as the outflow on the Afon Ddu.
Finally, we built up the sides with more concrete bags and then removed the coffer dams we had built to drain the ditch and let the water flow back in.
This is the finished sluice in position, at the moment we are letting water flow through it but come spring we can put boards in the front and effectively it will become a dam, backing water up in the ditch and raising the water table in the field. This will mean that in the breeding season the Lapwings have their nice wet field and when I want to get on with a tractor I don’t sink!
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