The last couple of days Izzy and I have been working on repairing the breach in the Wetland Dam. As we have now spent a good couple of days on it, I think it is worthy of a blog post to itself.
Now you would think that Izzy and I would be used to putting in dams after spending the winter at RSPB Forsinard damming up hill drains to create bog pools. There is a slight difference in scale however, between the hill drains at Forsinard and the wetland area at Skálanes.
We are not creating a new dam from scratch as the original dam is still in place. Flood events over the last couple of years have eroded around the side of the original dam. What we are trying to do is secure the right hand section of banking and tie it into the original dam, plugging the gap. Sounds easy when you say it like that, doesn’t it.
I should also mention that we are using reclaimed wood (which was once a footbridge) and making a slight design change to the old overflow pipes which are not working as they should. Anyone who has worked on a reserve knows that it is good to recycle where you can!
Phase One involved the construction of a wooden framework on the right bank to prevent future erosion, with the framework then extending across the gap.
Phase Two saw rock, soil and old turf going into the right hand ‘bastion’ and then into the middle section. There was still through flow via the left hand section, which meant that while the water level in the wetland area was rising slowly it was not stopping us working on the dam.
Phase Three involved sealing the dam so that we could start to fill the left hand section. By the end of yesterday there was a noticeable rise of 10cm from the morning (20cm since we started the project) on the front wall of the dam. Some seepage continued through the base of the dam, but this was negligible and no dam of this construction is 100% watertight. There was a noticeable drop in water level on the lower pool as less water was draining into it and this meant that areas of vegetation and mud were visible. These will make good foraging sites for waders.
Phase Four should have seen us fitting the reclaimed pipes to the dam to provide natural through flow at the maximum ‘normal’ water level. However two things happened over night. Firstly the water level rose faster than expected and it rained (quite a bit!).
I took a walk out to the dam with Bjartur this morning, half expecting to see a gapping hole where our new section of dam should be. To my surprise it was still there, all be it a bit wet. The water level had risen by another 30cm over night and was sitting just below the 50cm mark on our depth marker at the front of the dam. This brought it right to the top of our dam wall and the wave/wind action is still currently blowing water over the top of the new section of dam, so that there is water flowing across the top of it.
Hopefully the rocks, turf and clay soil we added to the core will remain fairly intact, but until the wind dies a bit there is little we can do. Having assessed the level of the water we have actually realised that the dam has worked a little too well in terms of flooding the wetlands. (See photos below)
Plan B – Always have a backup plan! After assessing the water level we have decided that at the first opportunity we will cut sections out of the top plank on the front wall to place the drainage pipes. This should create ‘controlled through flow’ and also drop the water level by around 10cm. Until then, we can only hope the water/wave action does not erode away too much of the work we have already done.
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| End of day 23/04/11 - Note stones in water and banking |
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| Morning of 24/04/11 - Water level has increased by 30cm on last photo |
Watch this space to find out whether the dam holds or not!
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