There are two main objectives of the flood forecast service in Norway:
Long term: Provide information on and make initiatives against situations that can increase the probability of flood;
Short term: Issue flood warnings in time, so that efforts can be made to reduce potential damage.
The flood forecasting procedure
The flood forecasting procedure consists of three main components:
- Analysis of the present situation (state)
- Calculation of streamflow and water level in the future (forecast), and
- Issuing streamflow forecasts and, if necessary, flood warnings (action).
How to predict streamflow
To obtain an overview of the streamflow situation, data from about 175 gauging stations are transferred automatically to NVE every morning and afternoon, or more frequently if needed. Additional streamflow data can be obtained from hydropower companies and local observers. Synoptic temperature and precipitation data from about 80 stations are sent directly to the computers at NVE from the Norwegian Meteorological Institute twice a day. These data are applied to update the hydrological models.
In addition, met.no supplies NVE with snow depth measurements. Information about the snow’s water equivalent is obtained from NVE’s snow-pillows, and from measurements made by hydropower companies. Image data from satellite sensors is used to derive information about the changing distribution of snow during the period of accumulation and, in particular, ablation.
The streamflow forecasts are based on model calculations. A rather simple conceptual rainfall-runoff model with a time-step of one day, the HBV-model, has been calibrated for about 80 basins. Temperature and precipitation forecasts six days ahead are used as input to the models. Based on the HBV-model results, qualitative streamflow forecasts are made regularly twice a week and issued on Internet and as teletext.
Apart from conceptual modelling, routing models for lakes and hydraulic modelling (MIKE-11) of water levels for specific river stretches are used for quantitative forecasts if needed. If the results for one or more basins indicate flood conditions, flood warnings are issued.
The warning levels depend on the recurrence frequency (probability) of the predicted streamflow as follows:
- Flood, the flow in one or more rivers is expected to exceed a recurrence frequency of 5 years;
- Major flood, the flow in one or more rivers is expected to exceed the 50-year flood.
Issuing of flood warnings
The warnings are, in addition to being issued on Internet and as teletext, distributed by fax to the Emergency division at the county administration (County governor) in the counties exposed to the actual flood. The County governor is requested to confirm the warning within one hour. The County governor evaluates the need for forwarding the warning to the municipalities. The Directorate of Public Roads and met.no also receive flood warnings with the same request for confirmation. In addition, a number of institutions, including NVE’s regional offices, hydropower companies and media, receive the warnings, but these are not instructed to confirm.
The watercourses in Norway have very different characteristics. Moreover, the extent of damage caused by floods varies considerably between the different regions of the country, according to e.g. the extent of development in these regions. Therefore, both NVE and the local authorities (counties and municipalities) are responsible for reducing the risk of damage caused by flood. NVE’s responsibility is to distribute flood warnings in time so that efforts can be made by the local emergency service to lessen the risk for damage.
NVE is also responsible for informing the local emergency services about both the possibilities and the limitations of the flood forecasting and warning system. The responsibility of the local emergency service is to consider whether a flood will cause damage in their region, and if so, to make precautionary efforts, such as informing the public and distributing the flood warning to other flood exposed units.