Proposed electricity generation developments: peat landslide hazard best practice guide

Scoping

3.1 Background

Peat landslide hazard and risk assessments ( PLHRAs) are generally delivered as part of the Environmental Impact Assessment ( EIA) process, often as a technical annex to the geology chapter. The need for an assessment is typically determined at the scoping phase of the EIA, whereby a scoping opinion is offered by the competent authority indicating that peat stability should be one of the matters covered in environmental information.

This guidance considers that PLHRAs should be a requirement where there is peat within the application boundary of a proposed development site. This short section considers information developers might use to determine whether a PLHRA report is likely to be required.

3.2 Peat

3.2.1 Presence of peat

The presence of peat at a proposed development site can be determined in a number of ways:

(i) By direct observation of peat during a site walkover;

(ii) By reference to published geology or soil maps; and

(iii) By intrusive investigation ( e.g. coring).

In Scotland, organic soils are defined (Scottish Government et al., 2014) in the following way:

• Peaty (or organo-mineral soil): a soil with a surface organic layer less than 0.5m deep;
• Peat: a soil with a surface organic layer greater than 0.5m deep which has an organic matter content of more than 60%;
• Deep peat: a peat soil with a surface organic layer greater than 1.0m deep (Bruneau and Johnson, 2014).

Depending on the geomorphology of the site ( e.g. presence of gullies, haggs, incised streams), it may or may not be possible to verify the presence of peat by direct observation. Organic soils as thin as 0.35m are capable of accommodating species typical of blanket bogs (such as Sphagnum species) which would often be assumed to indicate the presence of peat of a reasonable thickness. In these cases, intrusive investigation by hand augering can be used to provide verification of the presence of peat.

Superficial geology maps or soil maps should normally provide information on the presence or absence of peat, although these can be out-of-date or inappropriate for local scales and peat cover can be present even where not indicated.

3.2.2 Evidence of peat instability

The presence of past peat landslides or pre-failure indicators (see Section 2.2.4) indicates the need for detailed assessment of peat stability, even if slopes are generally gentle and minimal peat depth information has been obtained at the scoping stage. While assessment of these features is not expected at the scoping stage, their locations should be noted and the features investigated in further detail during the detailed site assessment.

3.3 Slope

In the previous iteration of the guidance, the need for a PLHRA report was determined as a function of the presence of peat and the severity of slopes within the application boundary. The threshold slope was considered to be 2° such that sites with peat cover but with very gentle gradients under 2° did not necessarily require submission of PLHRA reports. This reflected reports of peat landslides in blanket bogs, which were limited to slopes >2°. However, Boylan et al., 2008 note that failures on raised bogs occur on slopes of less than 2°. Accordingly, in this revised guidance, a discrimination is made between raised bogs and blanket bogs as to whether the slope threshold is applied.

3.3.1 Blanket bogs

In blanket bogs, which typically mantle hillslopes, PLHRAs should be undertaken where slopes exceed 2° reflecting published data on peat landslide source slopes for blanket bogs (Evans and Warburton, 2007; Boylan et al., 2008).

3.3.2 Raised bogs

In raised bogs (which typically occur on very gentle terrain), PLHRAs should be undertaken, reflecting published data (Boylan et al., 2008) which indicates the occurrence of peat landslides on very low gradients in raised bog environments.