EXECUTIVE SUMMARY: Blanket mire covers about 1.5m hectares of England, Scotland, Northern Ireland and Wales, mostly in the uplands, and is often a dominant part of our landscape. Its altitudinal limit depends on its location with the UK and is found even at sea level in the extreme north and west. It is composed of peat deposits up to several metres thick and represents a significant store of carbon as partially decomposed plant material. It is also an important biodiversity resource because of its plant communities and the animals and plants inhabiting it. It also represents a group of locations in which wind velocities are reliably high, and where the agricultural value is relatively low. The development of wind farms on peat raises a number of issues, some of which are not easy to resolve. This report describes the issues, reviews the available literature and provides guidance at stages from drawing up wind farm proposals, through Environmental Impact assessment requirements, to the provision of effective mitigation where this is possible.
Although peat is a widespread substance its physical properties differ from those with which engineers are more experienced. For example, it is mostly water, relatively light and compressible, but has very low internal cohesion. As a continuous deposit that may have accumulated without interruption over several thousands of years it has a two-layered structure that enables water to flow through its top few tens of centimetres. It is waterlogged below, and the anoxic conditions make it an ideal environment for the preservation of human artefacts and even bodies, and of other biogenic indicators of past human activity and climate.
The living biological (biodiversity) resource is concentrated at and above (birds) the surface where growth can take place, but is dependent on maintaining the hydrological and hydro-chemical conditions arising from the long and uninterrupted accumulation of the peat. The growth of the Sphagnum mosses and cotton sedges, so important in the continued accumulation of peat, can only occur where the rain-fed water table remains within a few centimetres of the peat surface for most of the year.
Changes have taken place over time so that much of the UK’s blanket peat is no longer peat-forming, and is described as degraded. The processes involved in degradation, such as the lowering of the water table and the concentration of surface water flow so that the peat becomes eroded from ever-widening gullies, are incremental, and can lead to complete peat loss in locations such as Holme Moss, West Yorkshire. In such areas the peat can no longer support the specific plant cover which makes up its biodiversity importance; and erosion of the peat results in sedimentation and increased colour (dissolved organic carbon) down stream which have negative impacts on water resources, such as drinking water reservoirs. Much of the UK’s upland peat is degraded. It may retain vestiges of its previous vegetation, or contain replacement plant types characteristic of non-peat environments. The UK BAP has a target to restore 70% of the degraded area to active bog. It is against this background, of a mixed intact and degraded resource, that the potential impact of wind farms on deep peat has been assessed.
Wind farm developments can have impacts at the construction, operational and decommissioning stages. The types of impact are common to all stages, and involve: changes in water levels and flow, and dissection of the peat mass, but the duration and intensity varies. In summary, impacts result from the construction of access roads, the casting of turbine bases, the installation of turbines, drainage works associated with the construction process and operation of the site, ongoing maintenance, and then removal of turbines at decommissioning.
Roads may “float” on the peat surface or be cut and filled to the sub-peat base. They require vegetation to be removed, waste peat to be disposed of, non-peat materials to be introduced, the movement of water over the peat surface and through its layers to be interrupted. They change the balance of water availability to different parts of the peat bog and channel surface flow so that is has a greater risk of initiating, or exacerbating, erosion. The digging of voids to caste turbine bases generates waste peat, introduces alkaline concrete and requires some drainage, as do the tracks. Drainage measures have the potential to lower the water level in the blanket bog, resulting in degradation and oxidation of peat. At sites which have a risk of peat slide, there is the additional risk of catastrophic peat failure and landslide. This can have catastrophic consequences for land and the environment, including water resources and fish populations, downstream. The actions taken in construction and operation of wind farms can add to the risk of peat slide.
Although the impacts on intact and degraded bog are much the same, on a degraded bog there are opportunities for the wind farm construction works to include measures that would improve the condition of the degraded bog, which are not present with an intact bog. On all types of blanket bog, how a wind farms is designed, constructed and operated makes a significant difference to how much the blanket bog is affected. Tracks can be designed to reduce the existing erosive forces, and be engineered so as not to create new ones. The blocking of existing drains and moor-grips can lead to beneficial changes towards “favourable condition”, the index of quality condition used in biodiversity assessments.
The ease with which erosion can be triggered, and the amount of material that can be eroded, increases with the depth of the peat deposit. In general, there are far more risks associated with the development of wind farms on deep peat than on peat less than 0.5m thick, or on the fringes around blanket peat. The imperatives for avoiding development on blanket bog sites are greater for those sites with international and national conservation designations. This leaves the remainder blanket bog resource relatively unprotected. These guidelines are intended to ensure that, where there are choices, wise judgements are made, so that the necessary proportion of the resource remains intact for biodiversity improvement and for atmospheric carbon capture in designated and undesignated sites alike.
8 January 2010
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