This page was printed from https://geosyntheticsmagazine.com

Geosynthetics revive Irish bog

June 1st, 2009 / By: / Environmental, Feature, Geogrids, Geotextiles, Separation

Geosynthetics-lined ‘floating trail’ protects and displays rejuvenated blanket bog at Cuilcagh

Introduction: Bringing a bog back to life

In the late 1980s, the blanket bog on the slopes of Cuilcagh Mountain in County Fermanagh, Northern Ireland, was in bad shape. Years of overgrazing, mechanized peat harvesting, and other mistreatment had damaged the sensitive bog ecosystem. The Fermanagh District Council initiated a decade-plus-long restoration project. A geotextile-lined aggregate path, sometimes called a “floating trail,” allows hikers to enjoy the now-restored bog without damaging it. Similar paths are used all over Ireland, Scotland, and other bog-laden countries.

Background: Ireland’s own Amazon

Cuilcagh Mountain is a brooding, 2,188-ft tabletop mountain with a base of cave-riddled carboniferous limestone, a shale-and-sandstone midsection swaddled in a thick bog blanket, and a cap of millstone grit. On the summit looms a colossal Bronze Age burial cairn. Three rivers — the Sruh Croppa, the Aghinrawn, and the Owenbrean — drain the mountain’s north flank and unite underground, forming the Claddagh River, which flows through an extensive cave system known as the Marble Arch Caves.

Cuilcagh’s blanket bog has been called “Ireland’s own Amazon rain forest,” in reference to the fact that bogs are on par with rain forest in their ecological importance. Among other vital roles, bogs act as natural CO2 sponges, absorbing and sequestering the gas considered primarily responsible for global warming.

As the Fermanagh District Council Web site explains, bogland — also known as peatland or mire — forms when rain-leached nutrients form an impermeable layer in the ground. The ground above, unable to drain, becomes saturated with rain and runoff from higher ground. Plants grow, die, and decay in soggy, acidic, low-oxygen conditions. Over a period of thousands of years, peat accumulates. A bog is born.

The blanket bog on the slopes of Cuilcagh Mountain is one of the most extensive bogs in Northern Ireland. Plants that live within it include deergrass, cross-leaved heath, dwarf willow, starry saxifrage, alpine clubmoss, bog cotton, and (sweet-sounding but carnivorous) sundew. In summer, heather splashes the hills with purple.

A menagerie of animals call the bog home, including butterflies, rare birds such as golden plovers, and whimsically-named bugs like pond skaters and whirligig beetles.

The Problem: Sheep and ‘sausage machines’

In 1989, staff members at the Marble Arch Caves — which were developed as a tourist attraction in 1985 —noticed the underground rivers were acting strangely.

“The caves were flooding much more quickly and to greater heights,” longtime park manager Richard Watson was quoted as saying in a 1999 Irish Times article. “There were obvious safety concerns as well as environmental concerns.”

To get to the bottom of the alarming change in river behavior, the park commissioned a scientific survey from the University of Huddersfield. The survey identified two primary culprits: sheep and “sausage machines.”

Bogs can support small numbers of grazing sheep, but in the case of the Cuilcagh Mountain blanket bog, too many sheep were munching away at an already-degraded landscape. “Sausage machines” are used to harvest peat on an industrial scale for use as fuel and in horticulture. Now, peat harvesting is not necessarily unsustainable. According to the Fermanagh District Council, peat had been hand-cut for fuel on Cuilcagh Mountain for generations. When peat was cut by hand, the layer of living sod on top of the peat was temporarily removed, then replaced. This helped speed the recovery of the bog.

Mechanized peat extraction, widely adopted in the 1980s, did not spare the living, top layer. It was also faster than hand-cutting, so much more peat could be harvested with less effort. Finally, miles of drain trenches were dug in the bog to help dry out the peat prior to extraction.

In other words, overgrazing and mechanical harvesting, together with uncontrolled vegetation burning and off-road quad motorbiking, were eroding the bog’s ability to absorb water. Hence, more rainwater was draining down into the caves.

Restoration: More water, fewer sheep

Restoring the Cuilcagh Mountain bog required a multipronged, multinational effort spanning more than a decade, involving, as Richard Watson put it, “literally hundreds of people.”

Major components of the restoration addressed the different factors threatening the bog. A project funded by EU Life — which supports the conservation of rare, threatened habitats in Europe — set out to come up with a grazing plan that would allow the sheep and the bog to thrive together. According to the Irish Peatland Preservation Council, sheep grazing was banned during the winter months and restricted to a stocking density of 2 ewe/ha. for the rest of the year. This was not enough to let the bog recover. Through experimental plots and other efforts by land managers, it was determined that the maximum stocking density compatible with blanket bog regeneration on Cuilcagh Mountain was 0.5 ewe/ha. Sheep were excluded completely from the most badly degraded areas.

Water levels in the bog had dropped due to the aforementioned drainage and defoliation. To raise the water levels, more than 1,000 small dams were erected in the peat-drying drains. The dams were made of a variety of mostly natural or recycled materials, including peat, wood, hay and straw bales, and forestry “brash.” Areas of the bog were seeded by hand with sphagnum moss.

To protect and display the restored bog, a type of geotextile-lined, aggregate path called a “floating road” or “floating trail” was constructed across several kilometers of the Legnabrocky Trail, which winds its way from the Cuilcagh Mountain Park parking lot to the summit of Cuilcagh.

In June 1999, the county celebrated the opening of the Cuilcagh Mountain Park, a 265-ha (655-acre) reserve within a larger Special Area of Conservation. The area has gained an impressive array of governmental and organizational designations, including Area of Special Scientific Interest, Environmentally Sensitive Area, and Wetland of International Importance.

Restoration and conservation efforts were steered by a joint effort between the Fermanagh District Council, theEnvironment and Heritage Service, the Forest Service, and the Department of Agriculture. Funding for the restoration also came from the European Commission, the National Lottery, and the Environment and Heritage Service.

In 2001, UNESCO recognized the ecological importance of the Cuilcagh blanket bog and surrounding landscape by awarding the Marble Arch Caves and Cuilcagh Mountain Park “European Geopark” status. Global Geopark status followed in 2007.

All signs point to the Cuilcagh Mountain blanket bog continuing to regenerate and thrive, serving as a model restoration project for peatlands worldwide, contributing to the local economy, and enhancing an understanding of the importance of peatlands.

Bog trails: Lay a geotextile magic carpet

If you tried to build a typical aggregate path over a bog, you would quickly run into trouble.

When working with deep peat, it is often impractical, if not darn near impossible, to dig down to firm ground or haul in enough outside material to create a solid base. (Your efforts to bring in large quantities of anything would be greatly hampered by the absence of the road or path you were attempting to build! Said efforts would also likely prove grieviously damaging to the bog itself.)

So trail builders in Ireland, Scotland, and other countries with substantial areas of bog sometimes borrow a technique from road construction to create a geotextile-lined aggregate path, sometimes called a “raft path” or “floating trail,” that both protects the bog it traverses and distributes a user’s weight so that he or she won’t sink.

In a classic geosynthetics separation application, the materials required to build a floating trail over a bog are similar to those required for a standard aggregate path — sub-base, base, binding material, and surfacing material — with the addition of geotextile liner that “floats” the trail. (Geotextile liner is sometimes used in nonfloating aggregate paths to keep the aggregate from mixing with softer subsurface material; this lessens the amount of maintenance required.)

The first step in constructing a floating trail is to make the “path tray” by digging out the path (at least 250mm deep) and making it level. Rocks, roots, and other potential fabric-rippers should be removed. This step is the same as the first step in constructing a regular aggregate path, except that the floating trail’s path tray does not need to reach a solid base.

Next, the geosynthetics raft — made of matting, geogrid, or some combination of the two — is laid in the path tray. Any joins overlap by at least 300mm. The specific choice of geotextile depends on peat depth, path gradient, the soil’s mineral content, and the path’s intended level of use.

Shelby Gonzalez is a freelance writer specializing in environmental issues. She can be reached at shelbygonzalez@gmail.com.

Resources

Construction Standards: Geotextile Aggregate Path on Peat (Scottish National Heritage, 1999) www.snh.org.uk/publications/on-line/heritagemanagement/uplandpathwork/3.2.shtml.

“Wise Use of Mires and Peatlands” (International Peat Society and International Mire Conservation Group, 2007) Free for download at www.imcg.net/docum/wisebook.htm.

“Assessment on Peatlands, Biodiversity and Climate Change” (United Nations Environment Program, 2007) Free for download; Executive summary: www.imcg.net/docum/pcb/Assessment_on_Peatlands_Biodiversity_and_Climate_Change_Executive_Summary.pdf.

“Global Peatland Restoration Handbook” (International Mire Conservation Group, on behalf of UNEP-GEF project on global peatlands preservation) Free for download at www.imcg.net/docum/prm/prm.htm.

“Mire and Peat” (online open-access journal) www.mires-and-peat.net.

Leave a Reply

Your email address will not be published. Required fields are marked *

Comments are moderated and will show up after being approved.