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Upland water management, techniques 1: Causeway construction
Back in 1986 Lyn and I achieved one of our core goals and managed to purchase a scrap of land, Tir penrhos Isaf, in "wet Wales", near Dolgellau. According to the National Atlas of Wales this land received about 2.5 metres of rainfall per year yet even then I was thinking that at some point in the future I would have to deal with drought. My weather diaries over the next two decades validated this concern and direct experience over the last few years have confirmed it emphatically. Periods of intense drought in the once wet uplands are inevitable. This will have profound effects on everyone living in Britain.
[Overview of upland water manangement]
I began a practical experiment in increasing water storage in the soils on our holding in 1995 through the construction of a simple causeway. I proceeded carefully following much thought, setting up a small scale trial followed by long observation before extending the experiment in the light of knowledge gained through the small scale trial.
Background and site description
The site of the first trial is approximately two thirds of an acres on a very mild slope. It was slightly sunken in relation to the surrounding land and may well have been a small peat bog in the past with the peat being removed for fuel, leaving the depression. An attempt had then been made by previous occupants to turn this shallow basin into sheep grazing by cutting a crude herring bone drainage ditch pattern into it to encourage the fast removal of water.
This was not particularly successful. The basin was included in a larger field whose straight line boundaries reflected a concern with the cost and ease of fencing rather than actual land types or topography. Regular grazing by sheep, cattle and horses meant that the shallow depression was almost always a muddy corner.
Our initial approach was to gradually re-fence according to land type and topography. This took the depression out of our regular grazing scheme and allowed for far more control and thus less poaching. The drainage ditches continued to flow.
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Causeway construction. First trial.
My first tentative intervention came in 1995, coincident with re-fencing a nearby paddock which resulted in the need to deal with a quantity of half rotten softwood stakes. I laid some of these lengthways in one of the drainage ditches until they were level with the surface then added more on either side to create a simple causeway.
I chose to start roughly at the centre of the depression, my reasoning being as follows; if I started at the "top" any effect "upslope" of the causeway would be difficult to detect and besides, this was drier land and quite good grazing anyway. If I had started at the "bottom" I was concerned that too much water may build up and possibly break through the causeway with consequent erosion. As rivers appear to start in the middle and grow both up and downstream simultaneously, I thought this might be appropriate here.
The first rotten stake causeway extended for about four metres. Spaces between the stakes were filled with soil from minor excavations elsewhere on the site and weedings, prunings and the like. I then watched what happened.
The water backed up in the ditch to the level of the causeway but remained as open water for less than a season as various plants took advantage of the new niche and filled it. It looked a bit like a dam at first but unlike a dam it had no outlet channel and was full of leaks or rather, seeps. In "normal" conditions water seeped evenly throughout the whole length of the causeway and in heavy rainfall, as the causeway is more or less level, it flowed evenly over the whole length without building up any great speed or forming a channel.
Interestingly, the causeway seemed to slow down the flow of water on both the upslope and downslope sides, rather like a shelter belt slowing air movement and creating shelter on both sides. This was noticeable by the fact that a build up of silt occurred not just on the upslope side, as one would expect, but also on the downslope side. Rather than water flowing through the original drainage channel, the flow was now spread out along the whole length of the causeway. This new flow formed many small puddles and hanging pools below the causeway that appeared to be suspended, hanging over and around the original ditch, giving a stepped appearance.
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Observation and Consolidation.
I kept an eye on the trial causeway over the next five years, taking special care to check it out in extreme conditions. It appeared to be stable and showed no signs of collapsing or being swept away in catastrophic flooding. I occasionally added another layer of scrap timber to it and any spare organic material to raise its level. By about 2000 the water level immediately upslope of the causeway was approximately 1 metre above the original ditch level which was still just visible on the lower side.
I had planted some flag iris on the upslope side together with bog bean and spearwort. All these had done very well and together with native volunteers like meadow sweet and water mint had created a mass of luxuriant vegetation. The occasional addition of ducks to the system meant that small areas of water remained open for most of the year. Various dragonfly species also took advantage of the site and could be seen regularly patrolling the area.
Encouraged by the feeling that nothing too drastic or unwanted had happened, I added two more causeways, one above the first one and one below and sat back to watch for a few more years.
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By 2005 the whole appearance of the site had changed from muddy, sheep grazed lawn to a dense mass of foliage. This included the species I have mentioned already plus melinnea, sedges, angelica and a number of self seeded willow, birch and even a scots pine that continues to grow, apparently quite happily, in a foot of water.
During the autumn and winter of 2005-06 I bolstered my courage and completed the full run of each causeway so that they crossed the depression completely from one side to the other in gentle curves. I coppiced the willow on the site and used the larger diameters for firewood. The smaller pieces, the brash, I cut into lengths that were the same width as the causeway and laid them down either loose or tied in bundles as fascines. Again, I consolidated the causeways by tipping barrow loads of weeds onto the surface and treading them in. This operation was pretty much completed by the spring of 2006.
Once again I could observe the water above the causeways being pushed out along their lengths and seeping slowly through, depositing material on both sides.
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There are a number of important and useful consequences to this technique that I have outlined below.
This was the initial reason for the trial. The bulk of the water storage is in the soil rather than on the surface as in a pond, although mini-ponds are present as well. The surface of the first trial causeway is now approximately 1.3 metres above the water level in the original ditch. This represents a considerable increase in water storage.
As I have in effect raised the water table, this altered the through-flow of water surrounding the site. The grazing above it has become damper and on one side it is noticeably wetter. This effect needs to be considered carefully before implementing the technique.
2006 has seen the worst drought conditions on the site in the last twenty years although everything has still remained green. Surface water disappeared for the first time about a week before the drought ended. However, any surface water is very shallow and the mud that was revealed remained wet. As this is essentially a technique to increase water storage in soils, the disappearance of surface water does not represent a great reduction in its value.
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Wetlands and bogs are often difficult to get into (or out of). The causeways provide a relatively easy means of access. After browsing through old copies of Current Archaeology I found reports of several Neolithic causeways or tracks through wetlands that resembled what I was doing. This was most satisfying.
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The access means it is possible to begin gardening on either side of the causeway. My use of weeds in the construction process meant that a lot of stuff inadvertently got planted, such as black currants which have done very well, comfrey, cabbage, kale and strawberries. Obviously much more work could be done here; the causeways provide a wetter edge and a (slightly) drier edge offering a variety of microclimates to plant species.
Much of the material for the causeways was willow and in spring 2006 a lot of this rooted. As the causeway is walked fairly regularly it sprouted predominantly from the edges, creating in places almost two green walls to guide the way. This can be harvested regularly for fascines and added to the surface so the causeway grows the materials required for its own maintenance or to make more causeways.
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The system functions as a filter. Any flow of water is now spread slowly through the whole area, rather than channelled rapidly through a narrow ditch. The whole area is a dense mass of foliage. Water leaving the site, below the bottom causeway, always runs clear. I have not done any detailed chemical analysis but it would seem obvious that there is great potential here for cleaning water.
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During the summer, vegetation is so lush, even in drought conditions, that the actual topography of the site is concealed but in the winter when things die back, a bit, it becomes easier to see what is happening. From the lowest causeway looking upslope the site is taking on the appearance of terracing with each causeway marking the step up of the next terrace. It looks really interesting as the luxuriant vegetation gives it a wild, natural feel while the level causeways introduce a subtle sense of a more human, actively gardened order.
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The causeway is basically biomass, largely woody material and the waterlogged conditions effectively slow decay to a standstill. This has obvious wider implications for both carbon sequestration and water management if we imagine large scale causeways constructed in suitable upland locations.
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There may be legal implications in using this technique, depending on where it is carried out, which should be born in mind. You are legally obliged to inform the Environment Agency of any actions involving waterways including ditches. Consult them first.
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To my mind and that of ecologist, botanist and biology friends (experts in their fields) the site represents a huge increase in diversity and habitat from the muddy sheep grazed lawn it once was. However, as with making any intervention in landscapes, anyone considering trying the technique should think first very carefully about the original habitat and the consequences of raising the water table on existing species. Advice from experts should be sought first where necessary.
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It would be useful if others could try this technique in small scale trials in order to increase our understanding of possible consequences. It seems to me safest to initially consider employing it in non-sensitive sites, ie. not in existing wetland, marsh or bogs but rather where the landform is conducive to creating new wetland systems. I would imagine that natural depressions, dried up stream sites or areas that experience surface water run off would be appropriate to begin with.
I can imagine much larger implementations, particularly in the uplands, in combination with swales and the like that would form part of a broadscale, holistic strategy for water management and carbon sequestration but there are many questions in my mind that would need to be answered first. For example, if the water level of an existing sphagnum bog is increased, will the sphagnum grow to fill the space all will other species take over? Or, if we raise the water table in one area, will that water flow through the soils in unexpected ways and appear in areas where we might not really want it?
There are tremendous possibilities here and much to be done.
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