Building a river: lessons from Phase 1.

Having just finished the first phase of our WEG-funded project, I’m taking stock of how it unfolded, to better inform the next, much bigger phase of works scheduled for next year. I divided this 1600 metre re-meandering project into two partly because it felt like too much to deliver in one go and partly because I felt we would learn a lot from doing a short phase first, with time to gather thoughts over the winter.

It was touch and go in the summer whether we would get started at all in 2019, but I’m glad we did because we learned a lot.

Ground conditions were a challenge: we were working on a peaty floodplain, where the gravel was up to 80 cm below the surface and the groundwater half that. That kind of ground can take very little traffic before it breaks up. The site is a SSSI, so keeping it in good condition was paramount.

Groundwater was also a challenge. Although we cut ‘in the dry’, the channel would fill with groundwater overnight: even in late September, with groundwater levels at their annual low-point.

In some ways it might have been better to start at the downstream end, in order to cut the channel in such a way that it drained as we carried on upstream. But that would have presented another set of problems, not least that (for now) we were returning the new channel into an impounded and raised section of old channel: this backed up water level would have flooded back up the new channel and given us even more of a water ingress headache.

So we started at the upstream end. For the first week the weather remained bright and the dryish ground held up very well … so long as the tracked dumper took straight lines back and forth with the spoil. This first section of spoil was placed along the edge of the existing river channel, ready to be pushed in at the end when we made the cut through.

But then as we worked downstream we came into a more fragile, peaty part of the floodplain. In addition from a certain point on we had to start taking the spoil off site and this across and off the floodplain.

We worked out a methodology so that the digger always worked within the confines of the channel it was cutting. This meant that we could keep the surrounding ground intact, but it also meant we got just the one pass at cutting the channel to the exactly correct levels for the pools and riffles.

We brought the dumper up the line of the new as yet undug channel and filled it by turning the digger a slightly laborious 180 degrees, one bucket at a time.

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This worked okay for about a day or two. But where the dumper tried to follow the meandering course of the as yet undug channel it began to cut the ground in the tighter turns: tracked vehicles turn by going faster on one side than the other which creates a shearing effect on the ground. Eventually the floodplain surface broke apart and after that the dumper started to churn the ground up , to the point that it almost got stuck on a few occasions.

So we decided to take the peat away first and excavate down to the hard gravel which we could use as a roadway. This meant the digger had to “hay-make” the peat into accumulating piles and roll it on out of the site to a spot where the dumper could run in straight lines to and from the floodplain.

This seemed to proceed reasonably, if long-windedly well, but groundwater would seep in up to the surface of the gravel, so what was exposed as a dry gravel road on the day we cut down to it, had become a soggy, muddy road by the following morning.

When we then cut through that gravel to the desired bed level, the channel immediately began to fill with water. Consequently we had to leave coffer dams of gravel when we started each morning to keep the overnight infill out of the new day’s dig.

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And thus we proceeded, day by day, until the weather broke and things got really tricky. Once it started to rain the ground because even more hazardous. After one, long and very wet weekend we returned to find our new channel absolutely brimful, like an infinity pool.

We tried pumping the water out, but this was a hiding to nothing really. The pump kept blocking, or sucking in air and stopping altogether. Somehow, by hook and by crook we managed to get to the bottom end and to hay-make the material back out to the one spot the dumper could reach without having to turn.

This main drag to and from the river became a real mess, but it was, at least, the only mess we made.

After all that the cut through was the simplest job of all. We cut a small channel from the existing river in to the new one, waited for the existing channel to drain down a bit and then carefully laid a large tree across the old channel. Building against this edge we created a land bridge across the old channel and then rolled on down from here filling in and levelling as we went.

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On the way out we placed a goodly number of branches and ranks from trees that had been cleared a couple of years before. These will be pinned in place to add some diversity and grit to the new and still somewhat bare river channel.

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I’m happy to report that trout, being curious creatures, didn’t take long to move in. Macrophytes, shards of starwort and ranunculus particularly, have already caught up on some of the stones. And I watched a kingfisher follow the new channel as we dug it.

I am now planning the fine details on the next phase.

If anyone would like to come and see the works do drop me a line. If a few people are interested we could arrange a field day and maybe a small workshop looking at some of the planning and delivery issues.

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