Tag Archives: Weir Removal

Three Weirs: Part 3

Author: Nick Hale, Project Officer

Read Part 1 and Part 2 first!

And so 3 weirs fell!

Gatehouse, Weir 1

gatehouse-1

New Lodge, Weir 2

new-lodge-2

and Dairyhouse, Weir 3

dairyhouse-3

Watch the video of their removal here:

A total of approximately 250 tonnes of concrete and steel was removed from the river.

Removing the three weirs has unimpounded over 1 km of channel. By reconnecting these previously isolated sections of river, just over 3 km of this part of the Lesser Teise is now fully passable for fish!

Increased flow velocity and diversity has already seen the return of natural processes such as erosion and deposition. The mix of deeper and shallower areas will create a range of habitat niches for invertebrates and fish etc.

Directly upstream of the former weirs, the flows have begun to rework the river bed and gravel berms until its natural gradient is restored (as before the weirs were introduced). A previously drowned-out ‘in channel’ meander sequence has exposed gravel riffles, berms and mid channel bars.

These gravels will be continually reworked in a downstream direction, being replaced by new material from upstream. The well-oxygenated gravel riffles will become ideal spawning habitat for both salmonid and coarse fish like brown trout and chub.

A number fish were identified during the works with species including brown trout, minnow, bullhead, stone loach, gudgeon, dace, roach, perch, pike, brook lamprey and barbel.

This stretch of historically dredged river is typically characterised by steep/vertical banks with little or no marginal habitat. This project has exposed an abundance of berms and marginal features that given time will be transformed into a diverse marginal habitat.

Removing the weirs has eliminated the landowners’ responsibility for the upkeep of the structures and saved costs on the twice a year maintenance visits from the EA to maintain the signage and clear blockages in the channel.

What’s next?

We will be monitoring the river over the next few years to record how the project develops. It will be interesting to see how the higher flows over the winter months rework all the gravels.

In the spring we will sow some native marginal plant seed mix on the lower banks of our structures and take some more photos for the next blog.

A BIG thanks to must go to the landowners, Ian Johnstone and the Kent High Weald Partnership and the Environment Agency for all their help!

The Three Weirs: Part 2

Author: Nick Hale, Project Officer

Read Three Weirs part 1 here.

After many months of planning the project we were finally able to start the fun part and get on the riverbank!

The first job was to take delivery of all the machines and equipment such as the 15m long reach excavator and wheeled dumpers.  Access to the site was quite tricky but having a good delivery driver is always good!

1

Mike our excavator driver was raring to go but first we had to install our sediment control features, marker posts to measure changes in water levels/river bed level and also take a few photos for comparison.

It was great to finally get the breaker attached onto the machine and begin breaking out the concrete. The centre of the channel was broken up first, and then the sides.

2

breaking-out-concrete

The weirs were certainly well built! Sheet piling had been installed at the up/downstream ends of each weir to key it into the riverbed and act as the formwork when casting the concrete. Lots of 6mm steel rebar had been used to provide extra strength, and in some places the concrete was almost 1m thick.

Removal of the sheet pilling was quite a challenge at the first weir, with the downstream section being 1.8m in length.

Mike gradually broke up the weirs into manageable pieces, which were loaded into the wheeled dumpers and transported to the collection site. A local company was contracted to collect the waste material using grab lorries. Each weir was made up of about 80 tonnes of concrete which was about 5 grab lorry loads per weir.

concrete

Chestnut faggots were secured with chestnut posts (both locally sourced) to form the line of the new bank.

A marginal shelf of site-won soil was then encased in coir matting behind the faggots and secured to the banks with oak pegs. We will come back to plant these up in the spring.

10 14

With high spirits and with the weather thankfully on our side, we tracked up the bank to do battle with the next two weirs. Although the construction of each was similar, each site posed its own challenges. However one by one they fell, with all three weirs being removed and banks reinstated in a three week period, reinstating flows and passage for the first time in over half a century.

The benefits of the work were made clear before we had even pulled out of the site. We saw numerous fish passing the previous barrier to explore the stretch upstream.

Look out for our third and final blog on the Three Weirs Project to see some before and after photos and the timelapse video!

Rustling up some Riffles

Our Project Officer Rosie has been out on the River Teise undertaking her first weir removal.

Why was this weir an issue?

Harpers Weir formed an impassable barrier to fish passage on the Teise, a tributary of the River Medway in Kent.

Harpers Weir

Weirs such as this impede the movement of fish upstream and downstream, preventing access to other habitats required for a healthy lifecycle.

They also impound the river upstream, slowing the flow of water and resulting in the deposition of silt on the channel bed. In the case of Harpers Weir, this impoundment was observed for several hundred metres upstream.

Impoundment

For all these reasons, we’ve been very keen to remove the weir and restore fish passage to this section.

In 2015, Rosie started the process by removing the wooden boards on top of the weir, reducing the height to see how the river would respond. You can read this blog here, but in summary it was looking good!

Then, in February this year, Rosie and our contractors Amenity Water Management arrived on site to start the full weir removal.

The concrete weir and flanks were broken up using a hydraulic breaker attachment, before being removed with an excavator. The concrete underneath the footbridge immediately upstream was also removed to let the river bed naturally re-grade back upstream (the footings would otherwise create a new barrier after weir removal).

Diggers on Site

Once the weir and bridge footings were removed, the banks where the weir once stood were regraded, using the excavator with a bucket attachment.  The toe of the banks was stabilised using coir geotextile to line the banks secured with wooden pegs, which in turn were held in place with faggots and pinned with untreated chestnut posts secured with high tensile fencing wire.

After Weir Removal

Once this weir was removed, the upstream impoundment disappeared, and a total of 16 new riffles emerged, with pools between them – all great habitat for many species of bugs, fish and water birds!

You can watch the whole project in our time-lapse footage below!

This project was funded through Defra’s Catchment Partnership Action Fund awarded to the Medway Catchment Partnership, focusing on the River Teise within the Medway Management Catchment.