The municipality has already experienced both floods and landslides, and large parts of the area of the two streams are below marine limit. NGI was engaged to conduct an initial investigation and to survey safety and risks associated with natural hazards such as:

  • Flooding
  • Landslides and debris flows
  • Quick clay slides

NGI was was provided with information about the municipality's drainage network as well as input about areas planned for new developments in the vicinity of the streams, which were included for assessment. An area of concern was if these new developments could lead to higher risk of run-off downstream in case of extreme rainfall or run-off.

Sikkerhet og risiko langs bekkedrag i byen Halden kommune 1 2000x1000

Picture showing a path which has been diminished by erosion. The ravine appears as a flood plain. 

Based on this, NGI has conducted several types of investigations:

  • Hydrological analysis and modelling of flood and surface water
  • GIS analyses and assessments based on available data
  • Surveys along both streams to identify critical points
  • Assessment of historical events
  • Assessment of the effects of climate change

The assessments were submitted in the form of a report and associated maps adapted for use in the municipal area planning. For each natural hazard, the most detailed type of map possible was produced, given the available material and scope of the fieldwork. The hazard maps show areas of potential danger but also where it was not possible to calculate probability. The hazard maps show where the various natural hazards can occur and how likely they are to occur.

The following maps were produced by NGI:

  • Hazard map - flooding
  • Areas of concern - landslides
  • Areas of concern - quick clay slides
  • Consequence map - landslides
  • Consequence map - quick clay slides
  • Total consideration zones for all natural hazards

The maps can be further used by the municipality to:

  • Define consideration zones with associated provisions in the municipal area planning
  • Prioritize further surveys to safeguard residents in connection with emergency preparedness and comprehensive risk and vulnerability analysis in accordance with the Civil Protection Act
  • Prioritize areas for development, for example by avoiding development in areas of potential danger or prioritizing areas involving least risk 
  • Identify the need for further and more detailed investigations in continued planning work (regulation plans and building issues)

NGI welcomes many municipalities such as Halden in taking precautionary measures by mapping safety and risk associated with natural hazards in challenging areas. The Planning and Building Act requires that municipal plans must show considerations and restrictions that are important for land use.

By assessing safety and risk associated with natural hazards, the municipality possesses the necessary knowledge to avoid development in hazardous areas, secure existing buildings against natural hazards, initiate other measures to prevent damage from natural hazards and improve preparedness related to natural hazards and extreme weather. The fact that the municipality is aware of challenging natural hazards is of prime importance in making the decisions that recognise climate change.