Preliminary Flood Risk Assessment in Norway 07 2011 REPORT 1415_report_floodrisk.indd 3 03.11.11 14.08 Preliminary Flood Risk Assessment in Norway An example of a methodology based on a GIS-approach Norwegian Water Resources and Energy Directorate 20111 Report no. 7/2011 Preliminary Flood Risk Assessment in Norway Norwegian Water Resources and Energy Directorate Published by: (NVE) Ivar. O. Peereboom, Oddrun S. Waagø, Marianne Authors: Myhre Print: Norwegian Water Resources and Energy Directorate Number printed: 50 Front-page photo: Hallvard Berg, Flooded area in Steinkjer in 2006. ISSN: 1502-3540 ISBN: 978-82-410-0747-7 Abstract: A preliminary flood risk assessment can be carried out by making susceptibility maps that aim to identify potential high risk areas for more detailed hazard and risk mapping. This method is based on a GIS-analysis and simple hydrological parameters. By undertaking a simple consequence analysis and aggregating the results to areas with a significant risk, maps showing the potential spatial distribution of high risk areas can be created to aid the decision making process of pointing out the areas vulnerable to significant flood risk. Key words: Susceptibility mapping, application of GIS, Flood Directive, PFRA, SAWA Norwegian Water Resources and Energy Directorate Middelthunsgate 29 P.O. 5091 Majorstua 0301 OSLO Telephone: 22 95 95 95 Fax: 22 95 90 00 Internet: www.nve.no 2 October 2011 Contents Preface ................................................................................................. 4 1 Introduction ................................................................................... 5 1.1 Previous flood risk management in Norway ........................... 5 1.2 Requirements of the Flood Directive ...................................... 6 1.3 Relevant conditions in Norway ............................................... 6 2 Components of a preliminary flood risk assessment ................ 7 2.1 Preliminary risk methodology ................................................. 7 2.2 Criteria for a preliminary risk assessment method ................. 7 2.3 Flood types ............................................................................. 8 2.4 Risk indicators ........................................................................ 9 2.4.1 Human health ................................................................. 9 2.4.2 Economic activity ......................................................... 10 2.4.3 The environment .......................................................... 12 2.4.4 Cultural heritage ........................................................... 12 3 Preliminary flood risk assessment ........................................... 13 3.1 Methodology for flood susceptibility mapping ....................... 13 3.2 Preliminary risk analysis ....................................................... 15 4 Results ......................................................................................... 21 4.1 Significant flood risk in sparsely populated countries ........... 21 4.2 Threshold for risk of significance .......................................... 21 4.3 Uncertainties ........................................................................ 24 4.4 Historical floods .................................................................... 25 5 Discussion .................................................................................. 26 6 Conclusions ................................................................................ 30 References......................................................................................... 31 Appendix ........................................................................................... 32 3 1 Introduction According to the European Flood Directive, (article 4.1) all member states and members of the EEA Agreement are committed to undertake a preliminary flood risk assessment for all river basin districts. The nationwide preliminary flood risk assessment for Norway will be conducted by the Norwegian Water Resources and Energy Directorate (NVE). NVE is subordinate to the Ministry of Petroleum and Energy (OED) and is responsible for administration of the nation’s water and energy resources. NVE plays a central role in preventing and mitigating damages from floods and other natural hazards. NVE has been given a central role in the implementation of the Flood Directive (FD) in Norway. This report describes a method for conducting a preliminary flood risk assessment developed for use in Norway. It describes the methods benefits, disadvantages and uncertainties. Most Norwegian river basins are relatively small by European standards, and the topography of Norway is mountainous in many areas. These two factors make it feasible to conduct a preliminary analysis of susceptibility to flood hazard based on topography and hydrology alone, thus postponing hydraulic analyses to subsequent, more detailed mapping. 1.1 Previous flood risk management in Norway Norway experienced a widespread and damaging flood in 1995. An Official Norwegian Report (NoU 1996:16), followed by a government White Paper (no. 42 1996-1997 – Measures against floods) was produced as a result of this flood. The White Paper no. 42 has come to be regarded as a national action plan for Norway with respect to flood mitigation. The White paper no. 42 led to a focus on land use planning as a measure for better flood risk management. The White paper states that ‘the most important measure to reduce flood damage in the future is to improve land use planning in flood prone areas’. In response to this, NVE developed guidelines for land use planning in areas at risk of flooding. The following assessments are now recommended for different aspects of planning: • Municipal plan: potential hazard should be identified • Zoning plan: the actual hazard should be described and the risk quantified • Building case: a satisfactory level of safety must be documented To support municipalities in implementing these guidelines, detailed flood inundation maps have been produced for more than 150 areas with the highest assumed flood risk. The prioritizing of the mapped areas is based on suggestions from municipalities, expert judgement combined with recordings of historical damaging floods. The detailed flood inundation maps show inundated areas for floods with different return periods. The discharge for different return periods is calculated by means of frequency analysis based on data from existing stream flow gauging stations. Water 5 levels are subsequently calculated by means of hydraulic modelling. Historical knowledge from e.g. water lines is used to calibrate the models. GIS is used to calculate the flood extent. The flood inundation maps provide detailed information to be used in both land use planning, applications for building permits and prioritizing protection measures. In other words, the flood inundation maps meet the requirements for flood hazard maps (art. 6) in the Flood Directive. 1.2 Requirements of the Flood Directive According to the Flood Directive, a preliminary flood risk assessment should be based on available or readily derivable information and include climate change impacts on flood occurrences. The assessment shall, according to article 4 paragraph 2, include at least: • Maps of the river basin district at the appropriate scale including the borders of the river basins, sub-basin and, where existing, coastal areas, showing topography and land use. • A description of the floods which have occurred in the past and which had significant adverse impacts on human health, the environment, cultural heritage and economic activity and for which the likelihood of similar future events is still relevant, including their flood extent and conveyance routes and an assessment of the adverse impacts they have entailed; • A description of the significant floods which have occurred in the past, where significant adverse consequences of similar future events might be envisaged. Depending on the specific need of the member state, more information can be included according to article 4.2.d. For international basins, the necessary information should be exchanged between the countries. The preliminary flood risk assessment should be completed by the 22nd of December 2011. 1.3 Relevant conditions in Norway In addition to being a very mountainous country with rather small river basins, Norway is sparsely populated. Making flood inundation maps covering all the flood prone areas regardless of population density will be both time and money consuming without giving much benefit. To identify a useful threshold of significant potential flood risk will therefore be of interest. Another characteristic is that a great number of the rivers and creeks are regulated for hydro power purposes. Regulation often decreases the occurrence of floods up to a certain volume. Taking into account that observations of floods cover only the last 50- 150 hundred years and that land use along many rivers has turned from agriculture to developed areas more vulnerable to water, floods exceeding the regulation storage capacity may cause damages in areas that previously were unaffected. These factors suggest that historical floods alone will not be a good indicator for assessing future flood risks. We therefore decided to approach the task by a more predictive model. 6 2 Components of a preliminary flood risk assessment 2.1 Preliminary risk methodology For this preliminary risk assessment we consider the potential maximum