Landslide risks in a changing
climate – The Nors River valley
Part 1: Map report and summary of results
SGI Publication 18-1E
Linköping 2015
Publication 18-1E Cite as:
Bergdahl, K, Odén, K (2015). Landslide risks in a changing climate – The Nors River valley. Part 1: Map report and summary of results. Swedish Geotechnical Institute, Publication 18-1E, Linköping.
Diary number: 1303-0243 Project number: 15053 m.fl.
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Swedish Geotechnical Institute Information Service
SE-581 93 Linköping, Sweden Phone: +46 13-20 18 04 E-mail: info@swedgeo.se
Download this publication as a PDF-document at www.swedgeo.se
Landslide risks in a changing
climate – The Nors River valley
Part 1: Map report and summary of results
Karin Bergdahl Karin Odén
SGI Publication 18-1E
Preface
Society needs to adapt to the ongoing climate change. A large number of existing buildings and infrastructure need to be adapted to cope with changes in precipitation and water flows and rising sea levels. In addition, society will have to take account of climate change and its implications in the design of new buildings and infrastructure. Adaptation measures are complex as they involve a number of different subject areas, sectors and uncertainties over the long term based on knowledge that is constantly updated as climate research rapid-ly. Effective climate adaptation requires not only management information and decision support that is flexi-ble, multidisciplinary and takes account of local variations, but also possibilities to coordinate various actions at the regional level.
Since 2009 SGI has allocated funds from the governmental appropriations 1:10 “Climate Change Adapta-tion” to, for example, mapping of landslide risks, method development and utilization of materials from the mappings.
Making use of material from the Göta River Valley investigation (Göta älvutredningen or GÄU) (SGI 2012) which ran between 2009 and 2012, SGI subsequently identified and prioritised additional watercourses for landslide risk mapping (Bergdahl et al in 2013). The Nors River valley (further referred to in this report by the by the Swedish name, Norsälven) is the first river valley mapping after the GÄU, and has served as a pilot area for the development of a simplified methodology for landslide risk mapping. The investigation along Norsälven builds on the methodology that was developed within the GÄU and aims to provide a suffi-cient basis which can suggest the need for further analysis in municipalities’ and county administrative boards’ work with adaptation.
The investigation results and conclusions are presented in the present report "Landslide risks in a changing climate – The Nors River valley", consisting of three parts:
Part 1 –Map report and summary of results, contains a summary of the governmental mission and
documents how the material can be used in climate adaptation work in municipalities and counties. It presents an account of landslide risks in maps. The maps contain both landslide risks for today's condi-tions and estimated sensitivities to climate impacts along the river valley.
Part 2 – Method for mapping contains a description of the survey methodology, inventories, surveys,
calculations and analyses.
Part 3 – Advanced annexes, contains detailed descriptions of the methodology used for analysis of
erosion and impact assessment.
The work has mainly been carried out by employees at SGI and organized as a core mission for project man-agement as well as a number of sub-projects for method development, analysis and investigation. The work was led by a senior management team comprising Karin Bergdahl (main project leader), Karen Odén (deputy project leader), Gunnel Göransson and Hjördis Löfroth. Charlotte Cederbom has been internal client. In addi-tion to the above, Rebecca Bertilsson, Daniel Elm, Åsa Jönsson, Ramona Kiilsgaard, Godefroid Nday-ikengurukiye and Stefan Turesson have been sub-project leaders. GIS analysis and GIS applications have been made by Mats Öberg, Jim Hedfors and Godefroid Ndayikengurukiye. Bo Lind has been internal re-viewer. A total of 30 SGI employees contributed to the work, see further in Chapter 2.2 of Part 2 Method for mapping.
Other governmental authorities and research institutes have cooperated in the research, including the Geolog-ical Survey of Sweden (SGU), the Swedish Civil Contingencies Agency (MSB) and the Swedish Meteoro-logical and HydroMeteoro-logical Institute (SMHI), the municipalities of Kil and Karlstad, the Värmland County Administrative Board and the Swedish Transport Administration. A number of consultants have also been used for the preparation of background material (field and laboratory investigations), documentation and calculations.
Approval for publication was given by the Head of SGI’s Land use planning and climate adaptation depart-ment, Charlotte Cederbom.
This is a translation from the Swedish original and the SGI may not be claimed responsible for any misunder-standing or error that may occur from the translation.
SGI Publication 18-1E
Contents
Preface... 5
Landslide risk analysis along the Nors River valley – in brief ... 9
1.
Background ... 11
1.1 Scope and limitations ... 11
2.
Landslide risks in Nors River valley ... 13
2.1 Summary of the results of the risk assessment/mapping ... 13
2.2 Presentation of results in maps ... 18
2.3 Probability, consequences, and risk ... 21
3.
Reducing landslide risk ... 26
3.1 Urban planning and building ... 26
3.2 Detailed investigations ... 27
3.3 Climate adaptation measures ... 27
References ... 29
Appendices
Overview of map charts (Bladindelning) Probability maps (Sannolikhetskartor) Consequence maps (Konsekvenskartor) Landslide risk maps (Skredriskkartor)
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SGI Publication 18-1E
Landslide risk analysis along the Nors
River valley – in brief
The landslide risk analysis along the Nors River valley (Norsälven in Swedish) has result-ed in a comprehensive overview of the risk of landslides in the present and future climate, for built-up as well as yet undeveloped land and areas of vital infrastructure.
A highlandslide risklevel, ie where the probability oflandslidesis considerable andwherethe consequence ofa landslidewould be largeforexisting buildingsand infrastructure occurs in two percent ofthelandslide riskclassedsurfaces alongNorsälven. The areasarein the South sectionat Edsvalla, Höglunda/Trossnäs, Norsbron andVålberginKarlstad.Affectedmajorroad and rail fa-cilities are found atEdsvalla, Skårebol, Väsby andHannäs. Mediumlandslide riskleveliscommon closest tothe riverbanksalong most of theriver(12.5percent oftheclassifiedsurface)where the probability oflandslidesis significantbutexisting buildingsandvital infrastructureare lacking.A heavylandslide inthese areascould causesecondaryimpacts likeflooding of buildingsand infra-structureupstream and downstream, which is not takeninto accountin this analysis. The main part ofthe study areahas a lowrisk level(85.5 percent ofthelandslide riskclassedsurface).
The sensitivity toclimate changein a futureclimate is deemed greatestin the Southand Middle sectionsof the area andalong alimited portion in the North section.
Identifiedareas of highlandslide risklevelshouldbe further investigatedin more detailfor any potentialaction. To reducethe risk of landslidesalong the river,measures canbe taken toreduce theprobability and/orconsequences. Measures to reduce probability are usuallyphysicalin the form ofexcavations, press embankmentsand erosion control.Measures to reduce consequences canbe, for example, to move thebuildings and facilities. There are several examples along the riverwhereproperties havebeen redeemedasa measure employed by municipalities.
Norsälvenis the firstriver valley which SGI has mapped since the Göta River valley investigation, andthe investigationhas served asa pilot projectfor the development ofa simplifiedmethodology forlandslide riskmapping.Methodological development hasbeen carried out in order to reducethe costs of investigationand simplifythe interpretation ofthe maps, thereby increasingthe societal relevance and usability ofthe results.The methodology forriskmapping that hasbeen developedis applicableforlandslide riskmappingalong other streams and river valleys. Boththe methodology forclassification of probabilityand consequencevaluations can beused individually; the method-ologyfor consequenceclassificationcan also be usedfor mapping ofother naturalevents.
On our web site there is a map application where probability, consequences and risk of landslide maps along the river are shown, as well as additional spatial information not presented in this report.
www.swedgeo.se
SGI Publication 18-1E
1.
Background
Building further on the Göta River valley investigation (GÄU 2009-2011), SGI has developed meth-ods for mapping the risks for landslides. These methmeth-ods can also be applied for landslide risk mapping in other geographical areas
Norsälven is the first of a number of subsequent identified river valleys for which the mapping of landslide risk has been planned. Thus Norsälven serves as a pilot area and was presented as a priority in SGI’s budget for 2013-2015. There it is described as, "An investigation of a more comprehensive nature that can be used for planning and decision-making by county administrative boards and munic-ipalities in their adaptation work at regional and local level".
With an overall picture of all areas prone to landslides along the current water course it is possible to make a better substantiated assessment of which areas require more detailed geotechnical investiga-tions and where geotechnical adaptation measures provide the most public benefit and are most cost-effective. In addition, the municipalities concerned receive a more complete basis for probability and societal consequences of landslides in built-up areas and in areas where new buildings are planned.
The purpose of landslide risk mapping is to produce a comprehensive map of landslide risks along the current river valley. The map presents the distribution of the risk levels of landslide and probability and consequences (in pairs), as well as the impact of climate change in a 100-year perspective. The landslide risk map can be used as a basis for planning decisions on climate adaptation measures at the municipal level in their comprehensive plans.
The methods utilized in the Göta River valley (GÄU) have been applied as far as possible. Method development has, however, been necessary to reduce the investigative costs and to simplify the inter-pretation of the maps without leading to a substantial impairment of the usability. Account has been taken of evaluation and comments that arrived after the GÄU. Among other things, this has served to simplify the results and make them more understandable.
Landslide risk mapping contributes to considerable societal benefits by providing material to:
avoid/mitigate the consequences of landslides.
reduce the likelihood of landslides,
support environmental quality objectives good built environment (and good non-toxic envi-ronment),
provide input to planning for adaptation.
1.1 Scope and limitations
The investigation of Norsälven extends from the Lake Lower Fryken outlet in Kil municipality to Norsälven’s estuary in Lake Vänern in the Karlstad municipality, see Figure 1.1. In total, this means a distance of about 30 km, equivalent to 60 km shoreline. The investigated area's width is limited to approximately 600 metres from the shoreline, in some cases a shorter distance when delimitation against solid ground could be made. The area has been divided into sections South, Middle and North. The sub-division is made primarily on the basis of diverse geological conditions but also for practical
Sweden has many areas prone to landslides. Areas along watercourses that flow through loose soil layers are often more vulnerable than others. In such areas the effects of climate change can also become apparent, for example, with increased water flow that causes in-creased erosion and deterioration of stability in soil layers.
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reasons, as these sections are divided by two hydropower plants at Frykfors and Edsvalla. Investiga-tion of landslide risks in tributaries or deep valleys that lie within the area of investigaInvestiga-tion has not been done explicitly, but with assessment based on the results of the calculations along the river.
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2.
Landslide risks in Nors River valley
2.1 Summary of the results of the risk assessment/mapping
In broad terms, the investigation shows that the probability of landslides along Norsälven is defi-nite in much of the South section and parts of the Middle section. In the North section the probabil-ity is distinct all along the south western bank of the river, but otherwise mainly low.The impact is the greatest in the South section where communities of Vålberg and Edsvalla are located, as well as by the major road and rail facilities that exist within the whole area of investiga-tion.
Areas of high landslide risk are relatively limited along the river (2% of the assessed surface). These areas are found in Edsvalla, Höglunda/Trossnäs, as well as Norsbron and in Vålberg and primarily closest to the river. Along the greater part of the river valley a medium risk level is as-sessed and usually closest to the river banks (13% of the asas-sessed surface). The main part of the investigation area has a low level of risk (86% of the assessed surface).
The sensitivity to a future climate change is highest in the South section, the lower part of the Mid-dle section and a limited part of the North section.
Below the results of the different sections are presented in more detail.
2.1.1 The North section - from Lower Fryken to the Frykfors
hydroelectric plant
The North section generally has a distinct probability of landslides (category S4) along a narrow section closest to the shoreline. The probability decreases rapidly with increasing distance from the river. Quick clay has been found in a few sections with flatter slope on the south side of the river (km 25/600 and 26/100). The north shore between Fryken inlet and km 24/800, however, has a low probability of landslides (category S2).
Consequences along the route are generally mild. Sparsely scattered settlements are found as well as a few roads. The railway running in a north-south direction and crossing the river at Väsby has the highest impact class, as well as the Frykfors hydroelectric power station. Out on the headland north of Hannäs there is an object with risk class registered according to the Swedish Method of Surveying Contaminated sites or so-called MIFO 2 areas.
A section along the railway line on the eastern side of Väsby peninsula has a high level of risk. Along the southern shore of Lake Fryken and Gunnita there is a medium risk level. Moreover, the risk is predominantly low in this part of the North section.
Landslide risk is defined here as a combination of the probability of a landslide and the con-sequences of the landslide. The developed probabilities for landslides and associated conse-quences are combined in a risk matrix. The risk is classified in three levels; high, medium and low, with corresponding colors red, orange and yellow in the maps of Landslide risks.
The survey represents a general level of probability and consequences, where a qualitative assessment of the consequences has been carried out and, therefore, a qualitative evaluation of the landslide risk.
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Climate impact
The climate impact has been assessed as low for large parts of the area except between 23/200-25/700 where it has been assessed as moderate to high.
Figure 2.1 shows an overview of the map of the landslide risks in the North section.
Figure 2.1 Overview of the map of landslide risks in the North section. (© SGI, Lantmäteriet,
Geo-datasamverkan).
2.1.2 The Middle section - from Frykfors hydroelectric plant to Edsvalla
hydroelectric plant
The northern part of the area, between km 20/200 and Frykfors power plant on the river's western side and between km 18/400 and Frykfors power station on the eastern side has the probability of landslide classified as distinct (class S4) closest to the shoreline. The probability decreases relative-ly quickrelative-ly with increasing distance from the river. In the area between km 16/600 and 20/200 on the western side and between 15/900 and 18/400 on the eastern side, the probability of landslides is negligible (class S1). Further south, between about 13/000 and 16/600 on the western side and be-tween 13/000 and 15/900 on the eastern side, the probability of landslide is distinct to definite (class S4 and S5). High and steep slopes, combined with the presence of high sensitive clay and quick clay means that large progressive landslides could occur in the area. In the area south of it, between Edsvalla power plant and about 13/000, the probability of landslides is negligible. Several large and small landslide scars are found along the entire distance between the Frykfors and Eds-valla power plants.
Consequences along the Middle section are generally mild as this area is characterised by very sparsely dispersed settlements. Around Edsvalla the settlement pattern becomes denser. The Eds-valla hydroelectric plant is also situated there with the highest consequence class. At EdsEds-valla there is a railway bridge over the river, as well as in the northern part of the Middle section. Route 61
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High levels of risk exist only where the railway crosses the river north of Skårebol. Along some stretches of the river, the risk is moderate despite the mild consequences, due to the high probabil-ity class.
Climate impact
Climate impact with respect to increased probability of landslides is considered low in the far south, high between 12/800 and 16/600, then low for a short distance, becoming moderate in the far north.
Figure 2.2 shows an overview of the map of the landslide risks in the Middle section.
Figure 2.2 Overview of the map of landslide risks inMiddle section. (© SGI, Lantmäteriet,
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2.1.3 The South section - from Edsvalla hydroelectric plant to Lake Vänern
In large parts of the area probability of landslide is definite (class S5) closest to the river. The northern part, from km 4/400 and north, also encompasses a wider area from the shoreline. This is because highly sensitive and quick clay is present along the distance, which means that larger pro-gressive landslides may occur. Several landslide scars are also found in this area. Along the short sub-sections where enhancement measures have been carried out the probability of landslides is negligible.The southern part of Norsälven contains the greatest consequences of all the three different areas. The consequences are generally larger, especially on the western side of the river. There is urban development in the south of Edsvalla, in the surroundings of Norsbron and in Vålberg. In Vålberg the railway passes the area along the river. E18 passes over the river in the far south. In the south there are also three MIFO objects (Method of Surveying Contaminated Sites) registered as risk class 1, including two in Vålberg and one in Edsvalla.
The risk level varies within the South section between a low, medium and high. Seen from the en-tire investigation area along Norsälven, most parts with a high risk level are found in area South section. The higher levels of risk are primarily found closest to the river, and reduce to lower levels of risk at greater distances from the rivers. Areas with a high level of risk are among others, the northern part of Vålberg, around Norsbron, at Höglunda, Trossnäs and Edsvalla.
Climate impact
Climate impacts with respect to increased probability of landslides are considered large in most parts of the area. Along a distance south of the Edsvalla hydroelectric plant, the climate impact is estimated as moderate to small.
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Figure 2.3 Overview of the map of landslide risks in theSouth section. (© SGI, Lantmäteriet,
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2.2 Presentation of results in maps
The investigation’s results are reported in three different map series at the scale 1: 15 000 (A4) in the maps annexed hereto. The maps are divided into probability maps, consequences maps and landslide risk maps. See sheet classification as described below.
Probability maps (Sannolikhetskartor)
The probability of landslides along Norsälven is reported in the probability maps (Sheet 1-10).
Consequence maps (Konsekvenskartor)
Consequences of landslides along Norsälven are reported in the impact maps (Sheet 1-10).
Landslide risk maps (Skredriskkartor)
Landslide risks in various areas along the river are reported in the risk maps (Sheet 1-10).
In Figure 2.4 the meaning of the different symbols are described as well as designations appearing in the legend on the risk maps. The cumulative risk (of probability and consequence) is reported at three levels: low, medium and high risk level.
Risk level
Area with low (låg) landslide risk. No special investigation is required for existing buildings and facilities.. New buildings and developments require stability investigation.
Area with medium (medel) landslide risk. Existing buildings and structures should be checked by detailed stability stud-ies. New buildings and developments require stability inves-tigation and possibly action is to be taken.
Area with high (hög) landslide risk. Need of action for exist-ing buildexist-ings and structures should be clarified with de-tailed stability study. New constructions require dede-tailed investigation of stability and likely measures to improve stability conditions.
Climate impact *)
Low (liten) sensitivity to climatic influences. Climate
change generally implicates no change in probability class.
Moderate (måttlig) sensitivity to climatic influences.
Climate change causes the probability class to increase by one level in the relevant sections along the river.
High (stor) sensitivity to climatic influences. Climate
change causes the probability class to increase by one to two levels in the relevant sections along the river. *) For areas with the highest probability class (S5) probability class cannot increase. In these areas, however, even a small influence caused by climate change means that land-slides may occur.
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2.2.1 Using the maps
The overall consistency of a landslide depends on the size of the landslide, which is illustrated in Figure 2.5. The most common scenario is that a landslide begins at the riverside and evolves back-ward to different degrees depending on soil mass properties and the topography of the area. Where quick clay is present the landslide could be extensive. The aggregated consequences are not shown in the maps of consequences, but must be assessed in each individual case.
Figure 2.5 Illustration of how the consequences of a landslide depend on the size and expanse of the slide.
Figure 2.6 shows the mapped historical landslide areas for part of the South section with various map backgrounds. This is given as an example of how to assess probable landslide expanse and get a rough idea of the size of the impact.
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Figure 2.6 shows views of previous landslide expanses (pink line) within the territory of the South section in
various maps. The sizes of the landslides are approximately 200 m long and 100 m deep on the western side, and 350 m long and 150 m deep on the eastern side (the southern landslide scar). © SGI, SGU, LM,
geo-SGI Publication 18-1E
2.3 Probability, consequences, and risk
2.3.1 Probability of landslide
Security against landslide, also called stability, is usually expressed as the ratio between the resist-ing and drivresist-ing forces in a slope. This relationship is called the safety factor. The resistresist-ing force is composed primarily of the strength of the soil but also stabilizing forces, such as the water column in a watercourse or added resisting forces in the form of soil and stone fill, in the lower part of the slope. The driving forces are generated by the soil’s gravity and the loads on the ground in the form of buildings, material storages or other loads.
In order to obtain as good a description of reality as possible, the traditional calculation of safety factors have been complemented by an assessment of the probability of landslides, taking into ac-count the uncertainty that exists in the input parameters. Stability is analysed using parameters which is given a variation that describes their uncertainty. The variation is determined in each case by using experience from similar areas as well as with statistics from surveys and other investiga-tions. Some parameters change over time, and/or as a result of climate change, which means that the calculations must be made both for the current and future situations.
The probability of landslides has been divided into five classes, S1-S5, see table 2.1. The bounda-ries between the different probability classes have been based on European and Swedish standards commonly used for design of buildings. Classes have been selected so that probability class S5 means poorer conditions than the worst class that may be accepted for temporary structures, while probability class S1 means better stability than the requirements for common buildings (Berggren et al, 2011, GÄU sub-report 28). The probability in areas between the calculated sections has been estimated based on their geotechnical and geometric conditions in relation to the conditions and results in the calculated sections.
Table 2.1 Classification of probability of landslides
Probability class Landslide Probability Relative failure probability
S1 Negligable Pf < 310 -6 S2 Low 310-6 ≤ Pf < 110 -4 S3 Some 110-4 ≤ Pf < 310-3 S4 Distinct 310-3 ≤ Pf < 110-1 S5 Definite Pf ≥ 110-1
2.3.2 Consequences of landslide
In parallel with the calculation of the probability of landslides, the consequences of landslides along the river have also been assessed. The consequences for the buildings and the transport routes in the area have been valued on the basis of four quality criteria; life, environment, economy and social importance. Landslides in the river valley may affect many people and important social func-tions. An exhaustive description, and, above all, economic valuation of all possible consequences has not been carried out. A landslide also means suffering, sorrow or discomfort for many people and thus the overall consequence of a landslide can include various implications for which it is difficult to make a systematic valuation.
Impact of a changing climate has not been assessed in the consequence analysis since such infor-mation is unavailable. Planning decisions undertaken are assumed to take into account the existing conditions as well as conditions in a changing climate. The produced probability map can be used
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as a guide to show where geotechnical conditions in particular should be taken into account in ur-ban planning.
The consequences have been divided into five consequence classes, K1-K5. Layers of data with the various consequences have since been divided into 10 x 10 m squares and combined so that the greatest value within each box has determined the consequence class. This ties in with classifica-tion from previous landslide risk mapping and corresponds to a gradual increase in the implicaclassifica-tions of classes, see table 2.2.
Table 2.2 Classification of consequences of landslides
Consequence class
Consequences of
lands-lides Description of Consequences
K1 Mild
No persons are injured or killed. No environmentally hazardous activity/enterprises affected and little environ-mental risk. Small economic losses. Loss of social func-tion with very little social significance.
Other/Minor roads (minor road, tractor road, footpath, running tracks, hiking trail, cable car, ferry route)
K2 Large
A few people are injured or killed. No environmentally hazardous activities/enterprises affected and medium environmental risk. Medium-sized economic losses. Loss of social function with low social significance.
Public road class III (width < 5 m); Drive/neighbourhood road.
K3 Very large
Number of wounded or dead persons that corresponds to the number of people in a smaller dwelling with several homes. Environmentally hazardous activities suffer with serious consequences for the environment. Large eco-nomic losses. Loss of important social function. Public roads class II (width 5-7 m); Thoroughfare; Road under construction
K4 Extremely large
Number of wounded or dead people that corresponds to the number of people at a larger school, apartment build-ings or major railway station. Environmentally hazardous activity involving extremely serious consequences for the environment. Extremely large financial losses. Loss of important social function.
Public road separate carriageways; Public road in (width > 7 m)
K5 Catastrophic
Number of wounded or dead people that corresponds to the number of people in an indoor arena (thousands people, high density). Environmentally hazardous activity involving catastrophic consequences for the environment. Catastrophically large economic losses that distinguishes itself from most of the economic losses. Loss of very important social function.
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2.3.3 Classification of landslide risk
The descriptions above imply that all parts in the investigation area are assigned a probability class and a consequence class. The combinations of these two classes make pairs of values that describe a risk class. Classification can also be illustrated in a matrix to show how various risk classes relate to one another, see Figure 2.7.
S5
Conside-rable S5/K5 Considerable probability of landslide with a catastrophic consequenceS4
DistinctS3
SomeS2
Low S2/K2 Low proba-bility of landslide with a large consequenseS1
NegligableK1
MildK2
LargeK3
Very largeK4
Extremely largeK5
Cata-strophicFigure 2.7 Matrix of risk classes
2.3.4 Landslide risk levels
In order to simplify the risk assessment the risk categories developed have been grouped into three risk levels consisting of a number of classes that correspond to similar landslide risks, see Figure 2.8.
SGI Publication 18-1E Probability classes S5 S5/K1 S5/K2 S5/K3 S5/K4 S5/K5 S4 S4/K1 S4/K2 S4/K3 S4/K4 S4/K5 S3 S3/K1 S3/K2 S3/K3 S3/K4 S3/K5 S2 S2/K1 S2/K2 S2/K3 S2/K4 S2/K5 S1 S1/K1 S1/K2 S1/K3 S1/K4 S1/K5 K1 K2 K3 K4 K5 Consequence classes
Figure 2.8 Matrix of risk levels.
Risk levels are expressed as low, medium and high risk, see Figure 2.4 for a more detailed description of levels.
area with a low landslide risk area with a medium landslide risk area with a high landslide risk
2.3.5 Risk of progressive landslides and secondary effects
In areas of high sensitive clay/quick clay, progressive landslides may occur where large areas may suffer from continuous landslide. Such an event means that secondary consequences in addition to the losses and damages to the land may occur. Examples of secondary effects that may occur are impoundments of the river (and its tributaries) and tidal waves of different magnitudes depending on the volume of the sliding mass. These secondary consequences may, if the affected area is suffi-ciently large, exceed the primary consequences in the area. The secondary consequences cannot be predicted with enough certainty, and therefore have not been addressed in the inquiry.
2.3.6 Climate impact
Anticipated climate change involves increased future flows in the river, implying increased erosion on the river's slopes and bottoms, as well as higher groundwater and pore water pressure. This has an impact on the probability of landslides because of slope geometry change within large parts of the area of investigation. This climate impact is expressed in three classes: low, moderate and high impact. Climate impacts are reported on the landslide risk maps with different dashed patterns in the river area.
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Depending on the class of an area, climate change will result in a probability class increase of one to two levels within some sections along the river. In areas with the highest probability class (S5), even a small influence caused by climate change can mean that landslides may occur and thus a high impact level.
2.3.7 Digital background material
In the investigation a large number of external supporting data has been collected and used. Multi-ple datasets have been acquired via SGI's participation in “Geodatasamverkan” or geodata collabo-ration, for example, concerning different maps, charts and the national elevation database. During the investigation a lot of new data and results have also been produced. When gathering back-ground material, the digital material has been added in databases. The investigation has sought to collect data in a GIS format. The datasets have been managed in a GIS environment where the ref-erence systems, SWEREF99TM in plan, and RH2000 in height has been used. The work has used ESRI ArcGIS, QGIS, and Web-based GIS applications.
Final results in the form of landslide risk classes, probability and consequence classes are made available in a Web-based GIS application that is open to external users. In the map pane selected supporting data will also be shown which may be useful when using the survey's results. Support-ing external data appears in the so-called WMS version from the data producer.
Figure 2.9 shows an excerpt from the Web-map application developed in the investigation.
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3.
Reducing landslide risk
Below are summary recommendations for the investigation area along Norsälven::
Identified areas of high landslide risk level should be investigated further for possible actions.
A zone with restrictions such as that established by the municipality of Karlstad may be a good way to verify activities that may affect stability. It may be relevant to review the extent of the zone.
When investigating possible actions, it is important to take into account the presence of quick clay, which affects the extension of possible landslides.
A review of existing erosion control should be carried out within the South sector, with regard to the maintenance and additional measures.
3.1 Urban planning and building
The landslide risk map has a resolution that is suitable for comprehensive planning. It is possible and recommended to consider this in connection with other risk areas for natural disasters, such as flood risks.
For detailed planning and building permits, more detailed geotechnical investigations should be made, taking into account the buildings and the facilities that the planning/building permit will allow. This action should be taken to prevent an increase in the probability of landslides within the current area.
It is also important to consider that the level of risk may be increased as consequences increase, that is to say, if the land is exploited with buildings or facilities. Other changes in land use may also play a role in the landslide risk level. For example, the neglected maintenance of land drainage due to land use change could lead to a build-up of water pressure in the ground that may increase the likelihood of landslides.
Landslide prevention measures are an important part of the work with urban planning, both in view of the current climate and with regard to climate change over the longer term. The land-slide risk maps give a picture of where there are sensitive areas to pursue and investigate in more detail by clarifying the geotechnical conditions, as well as the consequences that could arise because of a landslide. To do this, one must take account of a "probable" landslide ex-tent because the total consequence of a landslide is the sum of the effects within the probable landslide extent. The maps also provide an indication of the areas likely to be affected most in a changed climate and worsening stability conditions.
The results of this investigation can be used to influence the siting and construction of new buildings and activities so that landslide risks can be prevented. To reduce the risk of land-slides along the river, measures to reduce the probability and/or consequences could be tak-en. Measures to reduce the probability of landslide are usually physical in the form of excava-tions, supporting fill or erosion control. Measures to reduce consequences can be, for exam-ple, moving buildings and facilities. Along Norsälven several examples are found where re-demption of property has been used as an action for reducing landslide risks.
SGI Publication 18-1E
3.2 Detailed investigations
Geotechnical stability investigations tend to be made with multi-stage increasing levels of detail. In Swedish documents such as IEG 4:2010 and Skredkommissionen report 3: 95 for planning and IEG 6:2008, rev 1 for design and construction, information is found about regulations and recommenda-tions for geotechnical stability investigarecommenda-tions according to Swedish standards.
3.3 Climate adaptation measures
Before making decisions on the so-called risk-mitigation measures, a quantitative cost-benefit analysis should be done where the costs of a measure are weighed against the benefits. Then the probability of landslides and the consequences can be investigated in more detail for a defined area to see if the action should aim to reduce the probability of landslide and/or the consequences. Phys-ical measures to reduce the probability of landslide in the form of excavation, supporting fill, ero-sion control, soil reinforcement, etc. are often costly. Options that reduce the consequences can sometimes be more effective. Such alternatives may include the redemption of property/demolition of building on a property or restrictions in the use of land.
Preventive measures against natural disasters can sometimes counteract each other’s’ purposes. For example, a flood prevention dike causes a load of soil layers that may impair the stability of the area. It is therefore advantageous to coordinate different planned climate adaptation measures.
In Figures 3.1 and 3.2 examples of stability-enhancing measures are shown, enacted as well as possible measures.
SGI Publication 18-1E
UTFLACKNING AV SLÄNT
Figure 3.2 Examples of various measures to improve slope stability. Illustrations from Skredkommissionens
Report 5: 95.
Supporting fill
Excavation
Flattening of the slope
Reinforcement by lime-cement columns
Soil-nailing Erosion control
Replacement of erod-ed soil masses
SGI Publication 18-1E
References
Bergdahl, K, Cederbom, C, & Göransson, G 2013, Prioritering av områden för skredriskanalys,
Klimatanpassningsanslag 2013, Statens geotekniska institut, SGI Publikation 6, Linköping.
Berggren, B, Alén, C, Bengtsson P-E & Falemo, S 2011, Metodbeskrivning sannolikhet för skred:
kvantitativ beräkningsmodell. Statens geotekniska institut, Göta älvutredningen, GÄU
Del-rapport 28, Linköping.
IEG 2008, Tillämpningsdokument, EN 1997-1 Kapitel 11 och 12, Slänter och bankar, Implemente-ringskommissionen för Europastandarder inom Geoteknik, rapport 6:2008, Rev 1, Stock-holm.
IEG 2010, Tillståndbedömning/klassificering av naturliga slänter och slänter med befintlig
bebyg-gelse och anläggningar, Vägledning för tillämpning av Skredkommissionens rapporter 3:95 och 2:96 (delar av), Implementeringskommissionen för Europastandarder inom Geoteknik,
rapport 4:2010, Stockholm.
SGI 2012, Skredrisker i Göta älvdalen i ett förändrat klimat, Statens geotekniska institut, Linkö-ping. Also available in English version.
Skredkommissionen 1995, Anvisningar för släntstabilitetsutredningar, Skredkommissionen, Rap-port 3:95, Linköping.
Skredkommissionen 1996, Anvisningar för stabilitetsutredningar, Information, Skredkommission-en, Rapport 5:95, Linköping.
Appendices
Overview of map charts
Probability maps 1-10 (Sannolikhetskartor)
Consequence maps 1-10 (Konsekvenskartor)
Landslide risk maps 1-10 (Skredriskkartor)
2/000 1/800 1/600 1/400 1/200 1/000 0/800 0/600 0/400 0/200 0/000
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Sannolikhetsklass 5 Påtaglig 4 Tydlig 3 Viss 2 Låg 1 Försumbar Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) Sannolikhetskarta
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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2 1:15 000 (A4) Konsekvenskarta8/800 8/600 8/400 8/200 8/000 7/800 7/600 7/400 7/200 7/000 6/800 6/600 6/400 6/200 6/000
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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2 1:15 000 (A4) Konsekvenskarta15/600 15/400 15/200 15/000 14/800 14/600 14/400 14/200 14/000 13/800 13/600 13/400 13/200 13/000
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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2 1:15 000 (A4) Konsekvenskarta27/200 27/000 26/800 26/600 26/400 26/200 26/000 25/800 25/600 25/400 25/200 25/000 24/800 24/600 24/400 24/200 24/000 23/800 23/600 23/400 23/200 23/000
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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2 1:15 000 (A4) Konsekvenskarta^
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Skredrisker i ett förändrat klimat - Norsälven Konsekvensklass 5 Katastrofal 4 Extremt stor 3 Mycket stor 2 Stor 1 Lindrig
MIFO riskklass 1-2 (länsstyrelsen)
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2 1:15 000 (A4) Konsekvenskarta2/000 1/800 1/600 1/400 1/200 1/000 0/800 0/600 0/400 0/200 0/000
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
S1/ K1 S5/ K1 S1/ K4 S1/ K3 S1/ K2 S1/ K5 S4/ K1 S3/ K1 S5/ K5 S2/ K1 S5/ K2 S4/ K2 S5/ K4 S4/ K5 S3/ K5 S2/ K3 S2/ K5 S3/ K2 9/800 9/600 9/400 9/200 9/000 8/800 8/600 13/000 12/800 12/600 12/400 12/200 12/000 11/800 11/600 11/400 11/200 11/000 10/800 10/600 10/400 10/200 10/000
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
S1/ K1 S4/ K1 S1/ K4 S2/ K1 S1/ K2 S3/ K1 S1/ K5 S4/ K5 S3/ K5 S2/ K5 S1/ K3 S4/ K4 S2/ K4 21/600 21/400 21/200 21/000 20/800 20/600 20/400 20/200 20/000 19/800 19/600 19/400 19/200 19/000 18/800
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
S1/ K1 S2/ K1 S4/ K1 S1/ K2 S1/ K4 S4/ K5 S1/ K5 S3/ K1 S1/ K3 S2/ K4 S3/ K4 S2/ K5 S4/ K2 S3/ K2 S4/ K4 S2/ K2 S3/ K5 S2/ K3 S4/ K3 27/200 27/000 26/800 26/600 26/400 26/200 26/000 25/800 25/600 25/400 25/200 25/000 24/800 24/600 24/200 24/000 23/800 23/600 23/400 23/200 23/000 22/800 22/600 22/400 22/200 22/000 21/800 21/600
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
S1/ K1 S2/ K1 S4/ K1 S1/ K2 S4/ K5 S1/ K5 S1/ K3 S1/ K4 S3/ K1 S2/ K4 S3/ K4 S2/ K5 S2/ K2 S4/ K4 S3/ K5 S2/ K3 S4/ K3 27/200 27/000 26/800 26/600 26/400 26/200 26/000 25/800 25/600 25/400 25/200 25/000 24/800 24/600 24/400 24/200 24/000 23/800 23/600 23/400 23/200 23/000
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
S1/ K1 S2/ K1 S4/ K1 S1/ K2 S1/ K4 S4/ K5 S1/ K3 S1/ K5 S3/ K1 S2/ K3 S4/ K3 S2/ K5 S2/ K2 S2/ K4 S3/ K5 28/400 28/200 28/000 27/800 27/600 27/400 27/200 27/000 26/800 26/600 26/400 26/200 26/000 25/800 25/600 25/400 25/200 25/000 24/800 24/600 24/400
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Skredrisker i ett förändrat klimat - Norsälven Risknivå HÖG MEDEL LÅG Fast mark Klimatpåverkan LITEN MÅTTLIG STOR 1:15 000 (A4) S1/K1 Sannolikhetsklass/Konsekvensklass Skredriskkarta
Swedish Geotechnical Institute
SE-581 93 Linköping, Sweden Phone:+46 13 20 18 00 E-mail: sgi@swedgeo.se
