Assessing peat landslide risk

Dr Andy Mills (AM Geomorphology Ltd) All Energy 2014 21 - 22nd May

All Energy 2014 21 – 22 May 2014

Introduction

• Lead author of Scottish Government Peat

Landslide Hazard and Risk Assessment

(2007) and forthcoming second edition

• BPG guidance commissioned after:

− well publicised Derrybrien peat failure (October

2003)

− Multiple peat slide events in Dooncarton (Mayo)

and Channerwick (Shetland Islands) in 2004

− Other events had also occurred but were not so

widely reported (Sonnagh Old Wind Farm, Co.

Galway, Corry Mountain Wind Farm, Co. Leitrim)

• 1 large event can be equivalent to all peat

excavated during construction on a large

(50 turbine) wind farm on 0.5 – 1.5m peat

• How can peat stability assessments

(PSAs) be improved?

2

Derrybrien

Channerwick

Sonnagh Old

All Energy 2014 21 – 22 May 2014

Peat landslides in Scotland

• Peat landslides are generally

considered to be rare events

• Not many published examples in

Scotland:

− Shetland, Channerwick (Dykes and Warburton,

2008) and Yell (Veyret & Coque-Delhuille,

1989)

− Hermitage Water (Acreman, 1991)

− Isle of Lewis (Bowes, 1959)

• All reported failures are ‘peat slides’ or

‘peaty debris slides’ in morphology (see

Dykes and Warburton, 2007)

• No failures reported as yet with

morphology equivalent to ‘bog bursts’

(not even Bowes4)

3

Dykes and Warburton, 2007

All Energy 2014 21 – 22 May 2014

Guidance content

4

1 Introduction 1

1.1 Purpose

1.2 Guidance objectives

1.3 Context

1.4 Information requirement

1.5 ECU assessment services

1.6 Developer design team

1.7 Checklist for peat landslide hazard assessment

2 An overview of peat landslide mechanisms 7

2.1 Peat landslide mechanisms

2.2 Controls of peat instability

2.3 Pre-failure indicators of instability

3 Initial Assessment 16

3.1 Overview

3.2 Review of existing site information

3.3 Initial mapping and site modelling

3.4 Site reconnaissance survey

3.5 Review of project status

4 Ground Conditions Assessment 22

4.1 Objectives

4.2 Preparation of a detailed geomorphological / terrain map

4.3 Specification and implementation of a ground investigation

4.4 Laboratory testing schedule

4.5 Site instrumentation and monitoring regimes

4.6 Slope stability assessment

5 Hazard and risk assessment 34

5.1 Overview

5.2 Hazard and risk assessment

5.3 Mitigation

5.4 Post construction and restoration works

6 Further reading & acknowledgements 43

6.1 Further reading

Legislative context

Technical overview

Site reconnaissance

GI

Hazard and risk assessment

Most sites with peat cover will require assessment

All Energy 2014 21 – 22 May 2014

Implications of PSA for wind farms

• If previous instability is highlighted, there may be a perception that a

site is inherently high risk

• If previous instability is overlooked (including in the immediate

vicinity or nearby), report may generate suspicion

• If the report is of poor quality, ECU may reject its findings and

stakeholders may use as a tool for raising objections

• Advantages in conducting PSAs early to ensure peat instability risks

are considered as one of the main site constraints

• Otherwise, infrastructure may be sub-optimally sited with respect to

peat instability risk and be seen as ‘problem’ turbines

• The most costly data inputs (e.g. peat probing, aerial photos, terrain

models) that form the basis of PSA would be of value to wider layout

planning (and would benefit from early acquisition)

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All Energy 2014 21 – 22 May 2014

Improving PSAs – 1. Use of digital datasets

• When BPG written, Scotland wide aerial photography was still

being shot – this is now freely available and relatively cheap to

purchase

• Ground resolution is critical:

− 1.0m: insufficient to identify cracks

− 0.5m: sufficient to identify most features

− 0.25m: optimum for identifying features

• Site wide acquisition is worthwhile, saves time later

• Reconnaissance review (of areas outside application boundary)

can be conducted in Google Earth

• Terrain models are key:

− PSAs assess slope stability, so slope data is critical

− Site wide high resolution digital terrain models are not costly but are critical to

reliable PSAs

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All Energy 2014 21 – 22 May 2014

Improving PSAs – 2. Site geomorphology

• BPG recommends inclusion of a site-wide

geomorphological map:

− Existing slide scars and deposits

− Extent and type of erosion (surface drainage)

− Presence of very wet ground (pool complexes)

− Presence of diffuse (flush) and subsurface drainage

systems (collapsed pipes)

• However:

− Including a geomorphological map is the best way of

summarising site geomorphology

− evidence of instability features (e.g. photos) should be

spatially referenced on the geomorphological map

− peat morphology should be shown in zones

− outlines or points should be used to show the locations

of features

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All Energy 2014 21 – 22 May 2014

Improving PSAs – 3. Spatial scope

• Typical peat debris travel distances:

− runout of debris on hillslope (10s to 100s metres)

− channelised debris (100s metres to kilometres)

− sensitive receptors within several kilometres of the

site boundary may be affected (e.g. SACs)

• There may be useful context outside the

application boundary (e.g. in adjacent

catchments / on nearby hillslopes)

• If the catchment or administrative area has

history of failure, take particular care (peat

failures are often geographically clustered

over time), e.g.

− Shetland (Channerwick, Yell)

− North Pennines (Noon Hill)

− Northern Ireland (Cuilcagh Mountain, Skerry Hill)

8

entrained peat debris

NOT ‘the’ Isle of Lewis bog burst

All Energy 2014 21 – 22 May 2014

Improving PSAs – 4. Consider bog bursts

• ‘Peat slides’ and ‘bog bursts’ are not the

same (see Dykes & Warburton, 2007)

• Peat slides:

− “slab-like shallow translational failures (Hutchinson,

1988) with a shear failure mechanism operating within a

discrete shear plane at the peat-substrate interface,

below this interface, or more rarely within the peat body

(Warburton et al., 2004).

• Bog bursts:

− “particularly fluid failures involving rupture of the peat

blanket surface or margin due to subsurface creep or

swelling, with liquefied basal material expelled through

surface tears followed by settlement of the overlying

mass (Hemingway and Sledge, 1941-46; Bowes, 1960).”

• Differing failure mechanisms, controls

(e.g. slope and peat thickness) and

consequences

9

Bog bursts Peat slides

All Energy 2014 21 – 22 May 2014

Improving PSAs – 5. Assessing peat depth

• In 2011, SEPA published their ‘minimum’

requirements for peat probing for windfarms:

http://www.scotland.gov.uk/Resource/Doc/917/0120462.pdf

- low resolution ‘first pass’ 1 probe per hectare

- once infrastructure planned, 1 probe per 50m along

tracks with parallel 50m offset probes to enable micro-

siting

- probes on a 10m interval grid around turbines (for full

extent of micro-siting buffer, typically 50m radius circle

around centre point)

• Assessments often include a 100m grid, with

additional samples along tracks

• Clarity in modelling / extrapolation important

(particularly with sparse datasets)

10

Infrastructure aligned probing

Ground

penetrating

radar on

survey grid

100m grid (in

line with

SEPA)

All Energy 2014 21 – 22 May 2014

Improving PSAs – 6. Assessing peat depth

11

• Things to ensure with probing:

− the number / spread of probes is representative given the full range of site conditions

− probing covers all potential infrastructure locations, and is not too tightly focused on

one layout (i.e. develop a site-wide ground model)

− that the probe is long enough, avoid ‘>x’ values (not reassuring)

− that peat is characterised (e.g. humification) and that peat depth is ‘proved’, with

some knowledge of peat-substrate contact and substrate character (particularly if

clay)

• Things to ensure with peat models:

− where possible, build a peat model

− ensure there is clarity over how the model was interpolated (approach, limitations)

− ensure data gaps / uncertainties are clearly stated

All Energy 2014 21 – 22 May 2014

Improving PSAs – 7. Utilise the desk study

• There is a wealth of published technical

information about peat instability

• This includes:

− characteristic slopes

− characteristic peat depths

− geomorphological setting

• There is less (but still some) information on

geotechnical parameters

− site specific data available?

− use of published valuables?

• Factor based assessments should refer

back to the desk study:

− are failures really more likely on steeper slopes?

− are peat slides really most likely in the thickest peat?

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All Energy 2014 21 – 22 May 2014

Improving PSAs – 8. Hazard and risk assessment

Risk = Probability (Peat landslide) x Adverse Consequences

• Risk is only assessed where adverse consequences are considered

(environmental, infrastructure, life)

• Clarity of terminology (risk ≠ likelihood in this context)

• Assessments based on stability assessment (factor of safety):

− industry standard for landslide analysis

− parameters are critical (representativeness, reliability)

• Factor based assessments:

− factors should be referenced to controls identified in the desk study (both in choice of

factors and in weighting)

− factor combination should be explicit (e.g. through a worked example

− sensitivity analysis is important (demonstrate which factors are important)

− if slope and peat depth are the critical factors, the data underlying them must be robust

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All Energy 2014 21 – 22 May 2014

• PSA outputs are typically presented as:

• This information must be used and plans adhered to post-consent!

Improving PSAs – 9. Carry PSA findings to construction

14

Hazard / risk

maps

Use in refinement of

layout (within

micrositing tolerances)

Infrastructure

specific

mitigation

Use in infrastructure

specific design (e.g.

drainage design)

Generalised

good practice

Geotechnical

risk registers

ECoW or Geotechnical

Engineer to monitor site

working practices

All Energy 2014 21 – 22 May 2014

Are peat failures really that rare?

• Currently undertaking a UK

wide reconnaissance study to

identify unreported failures

• Bog burst morphology has

been identified in at least two

locations (Lewis, Caithness)

• This suggests that bog bursts

need to be considered in PSA

• The 2nd edition guidance will

also be coming soon4

15

Two

examples

from

Caithness