Tag Archives: Lesser Teise

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!

The Three Weirs: Part 1

Author: Nick Hale, Project Officer

An Environment Agency assessment of the Teise and Lesser Teise, two tributaries of the Medway in Kent, indicated there is a problem with wild fish stocks in the river.  Generally there is an absence and/or low abundance of key expected species such as brown trout, chub, predatory perch and pike, roach and stone loach.

In fact, the situation is probably even worse than this assessment indicates as a large proportion of the fish caught here are derived from artificial stocking, masking the figures of the wild population.

So what can be done about it?

One major contributing factor to these poor wild fish populations is the presence of barriers to fish migration: weirs. Weirs fragment the available habitat, prevent fish from reaching spawning grounds and change the geomorphology of the river channel as well. (For a reminder about the pros and cons of weirs, our very own Dr Chris Gardner wrote an excellent article recently on how weirs affect fish communities).

The Medway Catchment Partnership have been looking at barriers to fish migration in a bid to address this on a catchment scale. As a start, last year we successfully removed Harper’s Weir, restoring fish passage to a 3.5 km stretch of the River Teise. However, if you travel downstream, you are confronted by more and more weirs.

The Three Weirs Project looked to tackle a set of three weirs on the Lesser Teise, continuing this great work.

Where are the Three Weirs?

The Three Weirs, named Gatehouse (1), New Lodge (2) and Dairyhouse (3) are located on a 1.2 km stretch of the Lesser Teise near Chainhurst, Kent. All three are concrete weirs, likely originally installed in the mid-20th century for the purposes of agricultural land management.

weirs

What ecological benefits will result from removing the weirs?

The presence of a weir creates an impounding effect whereby the upstream water levels are much higher than normal.  This drowns out any natural features within the channel making it more like a canal than a river.

Impounded channel upstream of Dairyhouse Weir.

Impounded channel upstream of Dairyhouse Weir.

Removing the 3 weirs will increase the hydromorphological diversity (flow and river bed structure) of this stretch of river. Natural processes such as sediment transport and gravel mobilisation will return, exposing a range of original channel features like gravel berms and meanders. For example, the removal of Harpers Weir revealed 16 new riffles.

Ecological benefits will be seen both upstream and downstream of the existing weirs with salmonid and coarse fish being able to migrate through this previously unpassable stretch, improving their ability to carry out their full life cycle and ultimately increasing their chances of survival.

Localised improvements in water quality through faster, low flow conditions will reduce the build-up of ammonia and phosphates and increase dissolved oxygen to support a wider range of invertebrates. To capture these changes, an invertebrate survey has been undertaken by Robert Aquilina at specific locations along the proposed works area.  This process will be repeated in September 2017 to record any changes in diversity and abundance of the invertebrate community living within the channel.

In preparation for full removal, the wooden boards that were fixed to the crest of all 3 weirs were removed in 2015 with help of the Environment Agency. This reduced some of the impounding effect and allowed the channel upstream to adjust and stabilise prior to a complete removal of the structures in the future.

The next step now is to remove the weirs, but you’ll have to wait for part 2 to find out all about that!