Raising water levels – 7000 years a bog – 40 years an NNR

Flanders Moss NNR

Flanders Moss recently celebrated its 40th year as a National Nature Reserve, though it has been around for 7000 years. With COP26 due to start shortly and a climate and biodiversity crisis ongoing this anniversary seemed to a good time to look at the history and evolution of Flanders Moss through time. Over this week 5 blog posts will look at the changes of this special place and how it is developing as a nature reserve.

For hundreds of years a huge effort was made to drain the water out of Flanders Moss to help to get rid of it. Much of this effort involved digging ditches to drain water out of the moss but also the moss was used a a reservoir and the water in it was gathered to used with washing cut peat from the peat clearing works away into the river system. In more recent years about 110 hectares of Flanders was ditched to dry it out for the peat to be harvested while another 40 ha was ditched and planted with conifers. In total there was about 60 km of ditches dug across the site and by the 1980s all this drainage work was having a huge impact on the moss by lowering the water table and so changing the surface vegetation. But with Flanders Moss becoming a nature reserve, work started to reverse this damage by slowing the flow of water off the moss through damming the ditches.

A mass of ditches put in to drain Flanders before peat harvesting.

But how do you dam ditches? Ditch damming on Flanders Moss has evolved through trial and error over the last 40 years with techniques and expertise growing all the time.

The first dams constructed were major structures. There were made from oak and elm timbers, heavy wood lengths that took a lot of effort to just get them on site. It then took a team of workers about a day to put each dam in. Gradually more efficient methods using metal or plywood sheets developed. Lighter materials were easier to get far out on the moss. Today dams of all of these techniques can still be found out on the moss, some are still functional but many have failed.

One of the old wooden dams being put in.

A more recent technique that is still used today is using sheets of interlocking corrugated plastic piling. These lengths can be flexibly made into dams of all sizes, they slide together and are banged into the peat using a large rubber hammer. The bigger dams have a supporting beam laid across the front of the dam.

A small recycled plastic piling dam and tools for putting it in

For small ditches with less flow of water, peat itself can be used. It doesn’t involve purchase or transport of materials, small numbers can be dug by hand but it if there are lots to do a specially adapted digger in the hands of a skilled operator does the job better and faster.

Specially adapted diggers putting in peat dams

But it isn’t just as simple as whacking in a dam in a ditch. The work needs to be planned strategically otherwise you can end up flooding your access routes and hydrologically paint yourself into a corner. Reading the peatland to pick the best place to position your dams is also a skill. Peat often cracks parallel to a ditch as the peat of the ditch edge dries and slumps. It has happened that a dam has been put in and the water table starts to rise. The workers step away for a coffee. And then they come back to find the water table lower than it started with the water having risen and found a diversion around a dam through a crack.

A new method to deal with cracks in the peat is deep trench bunding. The methods involves building a wall of compressed, wet peat under the surface of the bog with a small raised bund on the surface. The slows down the flow of surface and subsurface water off the moss, a way of plugging hidden cracks and pipes.

Deep trench bunding diggers in action.

All the techniques have been applied to Flanders over time. It is difficult to estimate just how many dams have been put in but it is into hundreds. But in terms of lengths of ditches about 45 km of ditches have been dammed now. And about 10 km of bunding has been built. It has been a mammoth effort over the years by staff, volunteers and contractors. Much of it is hard labour on difficult terrain and gives me a feeling of affinity with those people who 200 years ago spend so much time and effort digging ditches and clearing peat. Though the objectives are the reverse, drying out the bog compared to making it wetter, but the connection to the bog and toil, and even some of the tools are the same.

A dammed ditch filed with water and getting chocked up with sphagnum
A plastic piling dam blocked up with sphagnum

At the edges of the peat water normally leaks out at the peat / clay interface creating a different sort of wetland – a lagg fen. But in most places at Flanders this has been replaced by a large ditch that catches the water flowing off the moss and keeps the next door farmland dry. However, at Flanders we have been working with neighbouring landowners and have been rewetting the edges of the moss and forming a new lagg fen. By working with landowners about km of bog edge has been made wet again through the used of bunds, sluices and blocking of old subsurface drains.

The building of a bund to create lagg fen.

There is still more work to do but the results of 40 years of effort to restore this great bog are clear to see. Our monitoring of water levels with high tech equipment show that the moss is getting wetter, with the water table stabilising so that it doesn’t fluctuate as much but stays nearer the surface for longer. But you don’t need high tech equipment to see some of the changes when out on the bog itself. Blocked ditches are disappearing underwater. Spreading carpets of sphagnum. Water at the surface everywhere. Flanders Moss is a bog getting back to its watery best.

Flanders Moss in 1992 – note more trees, conifer plantations and dark purple heather.
Flanders Moss is 2015 – note less trees and heather and more green of cotton grass and sphagnum

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1 Response to Raising water levels – 7000 years a bog – 40 years an NNR

  1. Anne says:

    You have painted a fascinating word picture of the hard work and care that has gone in reclaiming the bog so far.

    Liked by 2 people

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