Floods in Norway

The most common flood generating processes in Norway are rain and snowmelt, often combined with high soil moisture contents. The large rivers in the eastern part of Norway react slowly because of their catchment size and large altitude intervals. Here snowmelt floods in spring are most common, although autumn rain floods also occur. In the western part of Norway large floods in the late autumn or early winter are common. In this region ‘westerly’ air flow patterns can bring large amounts of moisture and heat over land masses with a subsequent lifting of the air masses result in condensation, rain and snowmelt. The large rivers in the northernmost county of Norway, Finnmark, have very marked snowmelt floods. This region has only small altitude differences within river basins such that snowmelt occurs simultaneously in the whole basin. Autumn rain floods in this region are rare. In the central part of Norway both spring, autumn and winter floods may occur. The largest floods are most often caused by a combination of rain and snowmelt.

Short-term, intense rainfall can result in large floods in small catchments. Floods are often aggravated by frozen soils or other impermeable surfaces such as are found in urban areas. Tributaries can also cut new channels relative to the original river course, and this can result in large flood damages when it occurs in densely populated areas. 

A special type of flood related to glaciers, is the so-called “jökulhlaup” (glacier outburst) floods. These are a result of lakes dammed by glaciers that are emptied abruptly. There are also floods caused by the breach of ice-dams across rivers. These floods are of short duration, but are intense and often devastating. Ice-dams can lead to flooding even when the river discharge is not particularly large.

Flood analyses and R&D
Flood estimates are needed for a number of purposes in water resources management such as the design of dams, flood controls, bridges and for flood inundation mapping. The best information on flood magnitudes that are likely to occur in the future is obtained from observed streamflow records. Observed data are used both for calibration of conceptual rainfall-runoff models and for flood frequency analysis.

Climate change will most likely alter flood regimes in Norway.  Research at NVE is focusing on how climate change will affect flooding in the future and further on how projected changes will influence, for example, areas at risk of flooding and dam safety.

NVE is participating in the COST action ES0901 FLOODFREQ