5 Support and instruments for reduced vulnerability 5.1 Mapping of floods, landslides and erosion There are areas in Sweden which, because of their location and topography and the nature of the soil represent risk areas for landslides and flooding. Natural disasters of this kind entail high costs for society as well as risks to human life, but can be prevented to a reasonable degree. The Swedish Parliament (Riksdag) in 1986 passed a government bill on preventive measures against landslides and other natural disasters (1985/86:150). The Swedish Rescue Services Agency was given responsibility for general stability mapping and for sharing out government grants between preven- tive measures in developed areas. Since 1998 the Swedish Rescue Services Agency has also carried out general flood mapping along parts of the larger Swedish watercourses in accordance with tasks identified for it in the Agency's appropriation directions. 5.1.1 Existing flood and stability mapping and inventory of shore erosion Surveying areas at risk of floods, landslides and erosion entails work in several stages − general mapping, detailed mapping, where necessary in-depth mapping of stability and preventive measures. The general mapping carried out by the Swedish Rescue Services Agency identifies areas that do not have satisfactory safety and where detailed investigations should be carried out. The detailed investigations may show that conditions are satisfactory or provide the basis for continued investigation or planning of preventive measures and applications for government grants. Detailed investi- 529 Support and instruments for reduced vulnerability SOU 2007:60 gations at present are the responsibility of the municipality or property owner. Flood mapping The purpose of general flood mapping is to broadly identify areas at risk of flooding along watercourses. The maps provide support in the operational planning and action plans of the rescue services and serve as a basis for the municipality’s overall planning. They are used in the county administrative board’s review of the munici- palities’ public planning. Around 10 per cent of Swedish water- courses have been prioritised by the Swedish Rescue Services Agency for general mapping, which is equivalent to 10 000 kilo- metres. To date around 8000 kilometres have been mapped in 56 watercourses in 154 municipalities, see Figure 5.1. 530 SOU 2007:60 Support and instruments for reduced vulnerability Figure 5.1 Watercourses with general mapping (2007) Source: Swedish Rescue Services Agency, 2007. Swedish Rescue Services Agency general flood mapping Mapped Mapping in progress Phase 1 to be mapped Phase 2 to be mapped Phase 3 to be mapped The mapping is done for the estimated 100-year flow and design flow. The 100-year flow is estimated on the basis of statistical analysis of observed water discharge series and occurs with a risk of 1 in 100 for each year during a 100-year period. The probability of the flow occurring during a 100-year period is 63 per cent. The design flow is calculated on the basis of the guidelines of the 531 Support and instruments for reduced vulnerability SOU 2007:60 Design Flood Committee for the dimensional design of risk cate- gory I dams, when the most unfavourable conditions are combined, see section 4.2.2. The watercourse is described using dam and bridge drawings, the characteristics of the watercourse and the surrounding topo- graphy. The water level is calculated in two sections across the watercourse using a hydraulic model and is interpolated forward along the watercourse. The area of flooding is calculated with the aid of the digital elevation database of the National Land Survey of Sweden. GIS (geographical information system) is used for mapping. More detailed calculations of water levels and a more accurate description of topography are required in the detailed planning of building development. The elevation database of the National Land Survey of Sweden is based on elevation data every 50 metres and a mean error in elevation of 2.5 metres. The deviation may be positive or negative. The database is intended for overall planning. More detailed elevation data are required for more accurate calcula- tions and mapping. The mapping is passed by the Swedish Rescue Services Agency to the affected municipalities and county administrative boards in report and digital form. The digital map layers can be linked to other suitable maps which show flood risks for instance for building construction and roads. The map layers can also be linked to various coordinate registers which indicate risk areas for land- slides, environmentally hazardous activity etc. The municipalities are advised by the Swedish Rescue Services Agency if necessary to draw up an action plan stating which areas should be the subject of detailed investigation and any preventive measures. Stability mapping In support of the work of municipalities and county administrative boards on risk management, the Swedish Rescue Service Agency has been responsible since 1987 for carrying out general mapping of the stability conditions of the ground in areas with building development on clay and silt slopes. The aim is to identify areas which by the general mapping method cannot be regarded as stable and to provide a basis on which to decide where detailed investi- gations should be carried out. Stability mapping was performed in 532 SOU 2007:60 Support and instruments for reduced vulnerability 151 municipalities in 2006. Three municipalities had already been mapped with respect to ravines and slopes in moraine and coarse sedimentary deposits. Completed stability mapping is handed over to affected municipalities and county administrative boards in both map format and in digital form as map layers. General stability mapping is carried out in two stages – a preli- minary study and a main study. The preliminary study is performed very broadly in consultation with the municipality with the aim of defining areas to be dealt with in the main study. The main study is carried out in two stages and is an ongoing nationwide activity which is gradually being performed one municipality at a time. The first stage entails distinguishing special areas with and without the necessary conditions for landslides on the basis of soil type, topo- graphy, proximity to watercourses and places where groundwater emerges. The second stage involves distinguishing areas on the basis of field studies and calculations that cannot be classified as satisfactorily stable or where the stability conditions cannot be clarified. A detailed geotechnical investigation should be carried out for these areas. The main study does not provide precise answers on the risk of landslides. Prior to the adoption of detailed development plans and building permits there is a need for at least a detailed geotechnical investigation to establish the stability of the ground. Figure 5.2 shows which municipalities have been the subject of stability mapping and what stages of mapping have been carried out. 533 Support and instruments for reduced vulnerability SOU 2007:60 Figure 5.2 Total number of municipalities subject to stability mapping in preliminary study/main study (2007). Some form of preli- minary study/main study has been carried out in the munici- palities shown in colour Source: Swedish Rescue Services Agency, 2007b. Inventory of shore erosion The Swedish Geotechnical Institute (SGI) is conducting an inven- tory of the extent of shore erosion in Swedish in collaboration with affected municipalities. The purpose of the inventory is to obtain an overview of where shore erosion occurs along the country’s coasts and on the shores and banks of lakes and watercourses. The inventory to date has been carried out for all coastal stretches including adjoining large and small islands and the larger lakes. Watercourses will be the subject of a subsequent inventory. The inventory may form the basis for a first overall assessment of the extent of shore erosion. Supplementary studies are needed to determine risk areas, the need for reinforcement measures and as a basis for physical planning. Known erosion conditions and the factors predisposing to erosion are reported for each municipality. Known erosion areas 534 SOU 2007:60 Support and instruments for reduced vulnerability are based on data from the municipalities with a description of geological conditions, erosion status and the extent and type of any protection against erosion. In addition to this, the inventory includes a survey of factors predisposing to shore erosion on the basis of geological conditions. The inventory is being gradually published on the SGI website. 5.1.2 Use of mapping in the municipal planning process Under the commission of inquiry we have arranged for a survey of ways in which municipalities take account of floods and landslides in the municipal planning process, how they deal with the Swedish Rescue Services Agency’s general mapping and to what extent they do their own mapping. The questionnaire has been sent to all municipalities, in other words both to those which have been mapped by the Swedish Rescue Services Agency and those which have not. The questionnaire was returned by 136 municipalities. More than half the respondents have suffered damage as a result of flooding, almost half as a consequence of landslips/landslides and around a quarter as a result of both. The results are briefly described below; see also Annex B15. Flooding Of the 136 respondents, around 80 per cent have taken account of flooding risks in their planning, particularly in detailed deve- lopment plans but also in the comprehensive land-use plan. More than half have not done any mapping of their own. The reason for this has been adequate knowledge of local conditions and circum- stances, no development taking place in areas at risk, lack of resources, inadequate data etc. Fifty-eight municipalities state there is general flood mapping from the Swedish Rescue Services Agency, which should be regarded as a low number as the SRSA had mapped watercourses affecting 154 municipalities in 2006.