© NERC All rights reserved Landslide susceptibility assessment for risk management. Dr. Claire Foster British Geological Survey Forestry Engineering Group

© NERC All rights reserved

Landslide susceptibility assessment for risk management.

Dr. Claire FosterBritish Geological Survey

Forestry Engineering Group9th September 2009

Cumbria.


Talk Outline

• Slope stability and landslides

• Assessing landslide susceptibility

• Case Study: FCW- Assessing the threat of landsliding on 3rd party assets surrounding FCW managed land.


Landslide is the internationally accepted

term for down slope movement of material under the influence of

gravity.

Landslides: An Introduction


Causes of Landsliding

A slope will remain stable if the shear strength of the material is greater than shear stress (F>1).

A slope becomes unstable when the balance is altered so that shear stress exceeds available shear strength (F<1).

Resisting ForcesDestabilising Stresses

Shear StrengthShear StressFactor of Safety = =


Decrease in Shear Strength

Geological Material• Decreases in shear strength when

saturated (Clay, mudstone)

• Discontinuities-faults, joints, pre-existing shears.

Weathering• Reduction in cohesion and angle

of shearing resistance.

Pore Water Changes• High ground water table

• Extreme rainfall/poor drainage

Increase in Shear Stress

Removal of support• Undercutting by marine river

action• Man made excavations• Seepage erosion

Increased loading• Accumulations of snow or water• Man made tipping

Earth stress• Earthquakes• Heavy plant or traffic

Vs.


Landslides in GB

• Certain areas of Great Britain have a much higher concentration of landslides than others.

• If you want to avoid building near/on a landslide or want to include threats posed by landslide into any safe system of working you need to know where they are located.

• However, some landslides have yet to occur so you also need to know which areas will be susceptible to landsliding.

How do you assess landslide susceptibility?


Landslide Susceptibility Mapping

• A landslide inventory is the first step in building a landslide susceptibility map.

• Landslide inventories provide the basis for analysing the spatial distribution of landsliding and their causal factors.

• Susceptibility can be assessed based on the spatial distribution of causal factors.

Geology Slope angle Quaternary history Aspect Geological discontinuities Proximity to streams


Case Study: Forestry Commission Wales

Driver: Gain an understanding of the potential impacts of landsliding from FCW on 3rd party assets within a defined hazard zone.

Overall Objective of FCW: Creation of a risk register.


Risk Register

• The main product of the overall work programme is a risk register.

• The work carried out by BGS supplied the information to inform the FCW risk register.

• Risk registers are a common risk management tool used to identify, analyse and manage risks.

• Information needed: What the threat is. Where it is. What the likelihood of occurrence is.


Phase One

• Landslide screening using BGS data sets:DigmapGeoSureNational Landslide DatabaseDebris flow study

• Identified forestry blocks where landslides had been recorded as well as those with a high landslide/debris flow potential.

Aim was to determine where landslides have already occurred and where was susceptible to future landsliding.


• Vector Mapping

• Detailed Attributes

• National Coverage

• Multi-layered

• Bedrock

• Superficial

• Mass Movement

• Structural Features

BGS Datasets: DiGMapGB50


BGS Datasets: GeoSure

• Layers include running sand, soluble rocks, compressible and collapsible deposits as well as shrink-swell clay.

• The dataset is compiled by weighting digital geological polygons with ‘landslide factors’ and combining this with a DTM.

Legend

A

B

C

D

E


Algorithm =

BGS Datasets: GeoSure

Slope

Geology

=


Topography © Crown Copyright. All rights reserved.

BGS Datasets: National Landslide Database

• Over 14,000 landslide records.

• Relational Access database and GIS.

• Inherited database that BGS has modified and continues to populate.

• Most comprehensive source of landslide information in GB.


Debris Flow Study

• Availability of debris material

• Hydrogeological conditions

• Land Use

• Proximity of Stream Channels

• Slope Angle

Modified GeoSure methodology which takes into account the different factors that cause debris flows.

These include:

Methodology created in conjunction with TRL Scotland for the Scottish Debris Flow Study.


Results of Phase One

• 653 Landslide records within the hazard zone.

• 367 Mapped landslides within hazard zone.

• 3.5% of the hazard zone was rated as having a high landslide susceptibility.

• 3% of the hazard zone had a high debris flow susceptibility.

Next phase was to define forest block sub-divisions and characterise the landslides in order to assess the likely

impacts of on 3rd party assets.


Phase Two: Defining Sub-divisions

• Identified areas where landslides or landslide susceptibility coincided with 3rd party assets within the hazard zone.

• Proximity to the asset is key.

• A landslide in the middle of a forest is not a hazard, its just a landslide.

Hazard Zone (FCW managed land + 500m)

Landslides

Asset (Road)


Sub-division of forestry blocks

• Forestry blocks were sub-divided into discrete areas.

• For each sub-division which has a landslide (or susceptible area) close to an asset a pro-forma was produced.

Asset (Road)Hazard Zone (FCW managed land + 500m)

Landslides

Sub-division


Phase Two: Pro-forma

Each forest block sub-division has a pro-forma which includes information on:

• Geology

• Aspect

• Geomorphology

• Landslide occurrence

• Landslide susceptibility

• Debris flow susceptibility


How do you distill all the information within the qualitative pro-forma to be able to include it within a semi-

quantitative risk register?


Phase Two: Scoring System

• Each forestry block sub-division was given two numbers (scores) reflecting the information gathered in the pro-forma.

• Each forestry block sub-division got a score for Hazard and a score for Likelihood.

• Each score was based on a classification scheme defined at the beginning of the project to allow for replication between sub-divisions.


Sp

eed

Size

Classification Scheme

• Hazard = Speed x Size of landslide.

• Likelihood = Distance x Impediment Factor.


How to classify Hazard?

• Landslide ‘intensity’ is a measure of the destructiveness of a landslide and is a function of landslide volume and expected velocity.

• Size (volume) and speed (velocity) reflect energy released.

• These two factors are a good representation of the hazard posed by a particular landslide.

5 indicates a very high intensity landslide.

1 indicates a very low intensity landslide


Size


Speed


How to classify Likelihood?

• Likelihood determines whether a landslide occurring on FCW land is likely to impinge upon third party assets within the hazard zone.

• Based on scoring the distance to the asset from the landslide and the presence of intervening impediments.

• Distance travelled will be dependant on the type of landslide.

• Assets differentiated (Roads, Railways and Residential areas).


Likelihood

• Distance was given a score on the likely run-out potential of a certain type of landslide related to the distance from the asset.

• Impediment factors included adverse topography, rivers or shallow slopes.

5 indicates a high probability of the asset

being hit.

1 indicates a remote probability of the asset

being hit.

River

Landslide

Forest Block sub-division

Settlement


Landslide


Settlement

Scenario A-

Landslide could run out 200m

Settlement is 300m away

Likelihood: Unlikely

River

Scenario B-



Likelihood: Highly Probable

Scenario C-



River is in between.

Likelihood: Remote


Landslide

Settlement

River


Likelihood



River is in between.

Likelihood: Remote

Hazard

Shallow translational landslides are likely

Moderate Speed

Travel distance <200m

Intensity: Medium


Summary

• Focus of work was on the threat of landsliding to 3rd party assets within 500m of each forestry block.

• BGS provided data on the distribution of actual landslides and the susceptibility of forestry blocks to landsliding and debris flows.

• The overall aim is to produce a risk register, therefore a numerical output was required.

• Hazard and Likelihood were decided upon in conjunction with the FCW.

• Each forestry block was given a score for Hazard and a score for Likelihood.


Questions?

Documents

© NERC All rights reserved Landslide susceptibility assessment for risk management. Dr. Claire Foster British Geological Survey Forestry Engineering Group