Historic meander reconnected during Upper Wensum river restoration
PUBLISHED: 07:23 24 September 2016 | UPDATED: 07:23 24 September 2016
After 200 dry years, a forgotten meander of the Wensum has been reconnected to the main channel as part of a £700,000 project to restore the protected river’s traditional form – while retaining its vital drainage functions.
In centuries gone by, the approach to land drainage was much simpler – cut a deep, straight channel to carry water away as quickly as possible.
But these days a greater understanding of hydrology, flood risk, contaminants and conservation has made it a much more complex calculation.
And this is why so much effort has gone into diverting a 125m upstream section of the River Wensum back to its original winding course – at least 200 years after the channel was artificially straightened.
The Norfolk Rivers Internal Drainage Board (IDB) has started the second phase of the Upper Wensum River Restoration Project, which will see 2km of the internationally-protected chalk stream habitat restored through the Raynham Estate near Fakenham.
It involves installing berms, pools and riffles to reinvigorate natural processes and drive ecological recovery on the waterway, which has a raft of environmental designations including Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC).
But it has also included the excavation and re-connection of a dry, redundant meander, known as a paleochannel, which was identified near the headwaters of the river, between the villages of East and West Raynham.
Matthew Philpot is project engineer for the Norfolk Rivers IDB, which is working on the project with engineering consultants Atkins and contractor Five Rivers.
He said: “What we are looking at now is more sympathetic ways of managing the water. By going away from the traditional ‘wide and deep’ philosophy, it helps to speed up the flow, keeping silt in suspension for longer and reducing the maintenance burden.
“Historically the emphasis has been on draining land quickly and efficiently, and the thinking on the best way to do that was with a wide straight channel. That straight channel with nothing in it is a really efficient way of moving water quickly. But if it is too wide it chokes with vegetation and fills up with sediment and then we would have to come back and dredge it.
“Also, we have a legal duty to improve water courses under the EU Water Framework Directive. This is one river where the status was failing, which meant we were allowed to bid for funds for this project.”
With £700,000 of funding from Defra, the Wensum restoration project will continue into 2017, eventually restoring more than 7km of chalk river habitat.
Ian Morrissey is the principal environmental consultant for Atkins, which designed the scheme at Raynham. He said: “The river has SPA and SSSI protection so there are various obligations on statutory authorities to improve these watercourses in line with those designations. That is the driver for the work here to improve the morphology and ecology of the watercourse.
“A lot of work has been concentrated on the main river, which the Environment Agency is responsible for, so it is nice that Matthew has been able to secure funding for this headwaters project.
“Here we are on a completely straight channel which would have been dug for land drainage. Looking at the old maps it is difficult to say exactly when, but this work would have bene done at least 200 years ago.
“When we were doing our walk-over survey five years ago we picked up the historic paleochannel – you could see it as a shallow depression meandering its way through the woodland, so we decided we would take the opportunity to realign the channel back to the original course of the Wensum.
“We have gone from 125m of straight channel to 250m of meandering channel, so we have doubled the length of it. We are going to be putting gravels in to provide faster, shallower sections, which are good for invertebrates and fish spawning. And we will provide deeper pools so there are refuges for aquatic species to go to during low-flow events.
“Chalk streams are a very rare habitat at a national and international scale, and it is that rarity that means the Wensum houses an assemblage of species you don’t find all over the country. It is a very fragile and potentially vulnerable eco-system.”
Mr Morrissey said the renovation could also help alleviate flood risk further down the valley.
“The straight channel takes water from Point A to Point B quickly, so if there is a build-up of water it can quickly flood Point B,” he said. “By having this slower and more meandering course, the water is held up here rather than flooding somewhere in Norwich.”
Viscount Raynham, chief executive of the Raynham Estate, said he was very pleased to see the historic channel re-established, and he would wait to see if the change in water flow meant any change to flood risk on the estate’s land.
“It is lovely to bring the old channel back alive,” he said. “When these channels were first made, the thinking on land drainage was very different. It has improved a lot now.
“We are waiting to see what effects it will have. There might be a risk of some slightly higher flooding, but we are hoping it will remain within the old flood plain, which we have not cropped. But we will see what happens.
“I think farmers play a vital role in the stewardship of the land and we will continue to do so. It is great that this will be a part of that, particularly on the river system itself – it will be fantastic to see better breeding grounds for fish and wildlife within the river. It will help the sustainability of the river and will only be of benefit.”
After the work to restore the river channel at Raynham, the area will be left to re-vegetate naturally, with the redundant section of straight channel filled in with spoil from digging out the new channel, so nothing is moved off site.
Before the gravel “plug” was placed to divert the water flow, the project team rescued about 120 trout, as well as bullheads, sticklebacks and stone loaches, from the water, so they could be moved upstream away from the disruption.
They were caught using electro-fishing methods, which involves creating an electrical current between a submerged cathode and anode to attract fish and temporarily stun them so they can be netted.
Once the restored meander has matured, it is hoped it will become home to important chalk-stream species such as brown trout, brook lamprey and potentially white-clawed crayfish.
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