Acacia Hall – River Darent Restoration

Having completed most of the river works on the River Darent in Central Park, Dartford in January 2020, we now have a bit of time to tell you what we got up to!

The River Darent splits immediately upstream of Dartford Central Park.

The western channel meanders through the park and past Acacia Hall weir before disappearing under an old ballroom building and the A226 before continuing through Dartford Town Centre. The eastern channel acts as flood relief channel and, prior to the restoration project, took the majority of low to medium flows.

Project Objectives

Our main objectives were to enhance and naturalise the river channel and to reconnect park users with the river.

These meant:

  • Removing Acacia Hall weir to remove the impoundment and provide fish passage.
  • Removing the concrete riverbed and banks (where possible) to naturalise the channel.
  • Desilting and narrowing the channel to an appropriate width.
  • Regrading the channel bed to a more natural gradient, including introducing pool-riffle sequences.
  • Tree works to open up the dense canopy and improve light availability to the channel
  • Introducing and securing large woody material in the channel to kickstart natural processes and improve habitat.
  • Regrading the banks to improve public access and increase areas for marginal vegetation to thrive.

Project delivery: the fish rescue

The first task was for Coleman & James Ltd to install a temporary dam at the upstream end of the western channel so that work on both project sites could be undertaken in the dry…. well that was the plan!

As soon as the water levels began to drop, we could get started on the fish rescue with Martin from Moore & Moore Carp. Starting downstream, we battled our way through the knee deep silt, armed with an electrofishing setup that allowed us to net and store fish in a large holding tank on the back of Martin’s vehicle. The fish were then safely relocated to a few nice spots upstream of the project site.

With quite a lot of ground to cover, it took us 2 days to complete the fish rescue. We rescued over 1000 fish, many of which were eels, which was fantastic to see. The nine other species found were roach, dace, chub, tench, gudgeon, minnow, perch, pike and bullhead.

A temporary dam for the Acacia Hall weir removal project

Project delivery: Silt Removal & Channel Narrowing

Next up was dealing with the thousands of cubic metres of silt that had built up over the years since the weir had been installed. We brought in Kenward Groundworks to provide the excavators and dumpers and luckily they sent us Nev, their No 1 operator (well that’s what he told us anyway). This was also the first project for Miguel and Harry, our new Assistant Project Officers. They certainly got ‘stuck in’.

Generally speaking, the existing channel width needed to be narrowed by around a third so that a suitable depth could be maintained as well as higher water velocities to keep the majority of the river bed clear of silt deposition.

Next we installed some locally sourced chestnut posts and hazel/chestnut faggot bundles to provide a retaining revetment. This was lined with a double layer of coir (coconut) matting which helped to keep the silt from oozing back out into the channel.

In total this took us a couple of weeks to complete as the silt needed plenty of time to dry out – a process called “dewatering”. This makes it easier to work with and stops it sliding back in over the top of the revetment.

We managed to reuse all of the old silt from the channel to create the new gently sloping banks which was a relief as removing any of this type of material from site becomes VERY expensive.

Clearing a path through deep silt for the Acacia Hall project © South East Rivers Trust

Channel Regrading & Creation of Pools & Riffles

The pre-project topographic surveys had shown some large ‘humps’ in the elevation of the river bed which were ‘impounding’ the upstream section of the project site.  This was slowing the flow and causing siltation. In previous summers, this whole section was choked by reeds which almost completely blocked the channel, resulting in more flow being diverted down the eastern channel.

Our next task was therefore to regrade the river bed through this section to a more natural gradient. Although we wanted to get a more constant gradient over the whole 600m stretch, we did excavate some deep pools on the outsides of the bends to create some localised variations and refuges for larger fish.

Everything seemed to be going really well until our luck turned and we had a period of VERY wet weather. At this point our “dry” working area suddenly became increasingly less dry with small upwellings and springs seeping through the bed and banks of the channel.

Long shifts working in the mud and soaking rain were remedied with regular evening meals at Dartford Dosa House so we could ‘fuel up’ for the following day’s work.

Regrading the river bed generated huge amounts of lovely clean Darent river gravels, ranging from a few millimetres up to 35cm. This was stockpiled wherever we could find space so we could use it later on. We were very lucky that all this high quality material was already on site – again this would have cost thousands of pounds to import from off site with a huge carbon footprint.

Channel upstream of Acacia Hall weir choked with reeds © South East Rivers Trust

Tree Works & LWM installation- Root Wads & Habitat Creation

During the pre-project walkover surveys it was clear that the dense tree canopy along the river corridor heavily over shaded this section of the river, and that the channel and banks would particularly benefit from the removal of some self-seeded sycamores.

The team from Chaffin Works felled a number of trees over their two days on site and also pollarded a number of willows, both to increase the light and prolong the life of the trees.

The large trunks were used to create a range of habitat structures and deflectors that will increase the diversity of flows and the species that will eventually call the river their home.

A number of the root plates from these trees were excavated and were added to a large number of old tree stumps that were generated from the Acacia Hall redevelopment project that was going on at the same time.

These lovely lumps of tangled roots formed a key part of the designs for this project, as they will prevent the start of the newly installed banks from being eroded in higher flows. A deeper pool was excavated in front of each group of root plates, and a deflector was positioned to divert the flow directly at them. This will prevent the pools from silting up and will make a lovely holding spots – refuges – for fish.

This is definitely one of our favourite parts of our job as you get to be creative, but it can take a while to work out how each piece will best sit on the bed, and how to fix it securely. Normally we have a running river to show us how the shape and direction the deflector is pointing affects the different flow types, but in a dry channel this was a bit more challenging.

We kept as much of the low overhanging trees and other vegetation as we could, which at times meant we had to track our machinery back out of the channel and along the bank to get to the other side. This was definitely worth it as it will look fantastic once the flowing water is restored and the overhead cover will give the aquatic wildlife protection from predators.

Backwater Creation – Gravel Reuse

Another really rewarding part of the project was restoring the backwater in the upper section of the site. The bed elevations of the original backwater had been set against the old water levels, so we needed to readjust them for the new designs.  After clearing away the encroaching willows and removing the silt, we excavated the bed and widened the entrance. We also installed a large woody deflector in the adjacent channel which will prevent it from silting up so quickly in the future.

This generated another huge pile of beautiful gravels that we used to cover the post and faggot revetments and really naturalise the new banks. The gravels were also used to create a low flow channel which will help to narrow and maintain a minimum depth of water during times of drought. Both of these features will be perfect structures for marginal plants.

A restored backwater

What’s Next?

There is still the small matter of removing the weir, demolishing the ballroom, installing a control structure and re-naturalising the final section of channel before the temporary dams can be removed and flows returned to the restored channel.

After all the hard work from the project partners to get to this point, we can’t wait to see the final result!

We will be back onsite as soon as we can to plant up the banks with a range of native plant species.

Many thanks to the team at Kenward Groundworks. They supplied us with lovely new machinery and a great operator (Neville) whose advice, experience and skill is key to how good everything looked once we left site.

We have also really enjoyed working with all the passionate people within the Acacia Hall project team which included CBEC Ecoengineering Ltd, Dartford Borough Council, Campbell Reith, Environment Agency, Coleman & James Ltd, Edenvale Young, TPI Ltd, and North West Kent Countyside Partnership.

Extra thanks must go to all the project funders which included the Environment Agency, Dartford Borough Council and the Veolia Environmental Trust.

We’ll be working together with the Darent Catchment Partnership and a wide range of other organisations to continue to improve the River Darent for people and wildlife, and make it more resilient to the challenges it faces.

Acacia weir that has been removed © South East Rivers Trust