The Background of the Glomma and Lågen Floods in June 1995
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In the spring of 1995 the snow remained unusually late and was even increased in large parts of the Glomma and Lågen catchment area. At the end of April 1995 snow storage accumulated to 130-150 persent of normal.

From May 8th and a week forward the temperature dropped to far below the seasonal average.But as figure 2 shows later it suddenly rose 10 degrees centigrades.

In a press release from May 9th 1995 it says, "A delay in the melting of the snow may result in rising temperatures during the thaw and may increase the danger of severe floods in the snowiest areas. Also heavy precipitation during the thaw will increase the risk of flooding."

Geographical information

The Glomma is 617 km long and the longest river in Norway. It rises in the Roerosvidda and has its outfall at Frederiksstad. It catches precipitation from 13% of the total area of Norway.

16% of the water has been subject to regulation.

The annual precipitation varies between 30 cm and more than 100 cm in the mountains.


Figure 1.

The Glomma and Laagen catchment area. Changes in the snow storage where the Glomma runs into the Østerdalen Valley from April 24th to May 22nd 1995 measured by using NOAA  (AVHRR).
Source: NVE - Norway Water and Energyoffice 1995

The satellite based weather service contributed to an early warning of a possible flood event and a close monitoring of the snow lying during the melting period.

Already in the early spring satellite observation had revealed large amounts of snow in south eastern Norway.

The white areas indicate where reliable measurements were prevented by clouds.

Blue indicates areas of total snowmelt since April and pink indicates partial melting.

Red indicates accumulation i.e. snowfall in more elevated areas.

The Glomma
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Figure 2.
Source: Glommen's and Laagen's Water Management Association, GLB 1995

On May 28th it was announced that the Glomma near Elverum and Gudbransdalslaagen had reached medium flood level and the water level was rising fast. The water continued to rise in the days to follow.

Figure 3 Glomma river at Elverum
Source: Glommen's and Laagen's Water Management Association, GLB 1995

In the evening on May 31st at Elverum the flood reached the highest level so far this century

On June 2nd the water level was 70 cm above the 1934 floods.

The low-lying areas along the Glomma had already then been flooded for some time. Many thousand hectares were flooded and many households had to be evacuated.

The Gudbranddalen Valley

Figure 4
44 Powerstation and 26 Reservoir
Source: Glommen's and Laagen's Water Management Association, GLB 1995

Figure 6
The Glomma and Laagen catchment areas stretch from Røros in the north east to Frederikstad in the south.

Source: Glommen's and Laagen's Water Management Association, GLB 1995

Figure 5. The graph shows the long term predictions elaborated on May 16th for Lake Mjøsa
Source: Glommen's and Laagen's Water Management Association, GLB 1995

The water in Lake Mjøsa, Norway's largest lake, rose quickly from June 1st by approximately 2 cm/hour for several days. Consequently measures were taken to offer protection against the floods.

Protection dams were built to prevent the water from reaching the lowest-lying buildings.

On June 11th the floods reached the highest level at 7.94 m. measured at Hamar.

Figure 7. Floods in Lake Mjøsa at Hamar. Water level 0.00 m corresponds to 117.69m. Highest water level was 125.63m. on June 11th, 1995.

Source: Glommen's and Laagen's Water Management Association, GLB 1995

On April 7th the GLB asked the Norwegian Water Resources and Energy Administration, NVE, for permission to pre-release water extraordinarily from one reservoir, Lake Osen, and later again on May 8th from Lake Mjøsa.

Lake Øyeren
Figure 8.The effect of flood reducing measures in Lake Oyeren
Source: Glommen's and Laagen's Water Management Association, GLB 1995
Protection dams were also built at Lillestrøm all the way through the built-up area to keep out the flooding waters of the Øyeren.

After the floods of 1967 50 million NKK were spent on establishing safeguards at Øyeren.

The mouth of the Øyeren was enlarged by blasting and two old bypass tunnels were reconditioned with floodgates to facilitate the release of another 800 m3/s

During the 1995 floods the Øyeren was drained maximally.

The NVE opened the bypass tunnels already on May 31st and on June 2nd, when the water level in the Øyeren approached 6 m.

The total water-lowering effect of these measures turned out to be 2.25 m.

As a result of this the built-up area around the Øyeren was saved from heavy flood damages.

FLISA
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Satellite data
The satellite data from 1995 were registered by the SPOT satellite on June 27th. The recording is multispectral (XS) and was made after the satellite had been programmed to record the floods in Østlandet in 1995 for mapping purposes.

In the registration the sensors in the satellite measure the reflected radiation of several bands (channels) within the visible and infrared light ranges. The smallest unit of measurement is 20m.

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Image processing
After geometrical correction and GIS adjustment the satellite data go through a comprehensive digital image processing (made by Statens kartverk, Miljøenheten i Arendal).
Green, red and near-infrared light are colour-coded to enhance vegetation and land use features. Especially the information from the infrared radiation is important as the vegetation shows the greatest variation in the reflection from this range of the spectre, which is invisible to the human eye.

The segment shown here is colour-coded to give a survey of the most important landscape features and the colours make it easier to understand the contents of the image. Near-natural colours have been aimed at in the coding.

. Nature, landscape formations and land use in the area:

The Flisa region is in the lower Østerdalen, where the valley is wide and flat with loess sedimentation from rivers and glacial streams.

In the segment can be seen sand, fine sand and silt deposits from former flood events. The deposits form terraces at several levels above the present river bed. Traces of former river beds are distinct in the image with the characteristic meanders or the oxbow lakes seen in connection with the feeder stream (Flisa) on the east side of the Glomma. Light sections are seen in the rivers indicating accumulation of sand on the inside of the meander bends.

The major part of the area is under cultivation, and corn production dominates. Potato-growing and grazing are quite common, too. The natural vegetation consists of coniferous trees, mainly pine, in the dry areas and birch and other deciduous trees on the slopes, along the rivers and on the terraces where the soil humidity is higher. In the cultivated area farm buildings and scattered houses are found. Parts of the town of Flisa can be seen at the top of the image.

. Interpretation of flood damages.
To gain a better understanding of the features of the area the use of map data is recommended. a 1 : 5 000 scale map will give much information about local landscape features in a smaller area, whereas a 1 : 50 000 scale map will give a survey of a larger area.
In the segment from the satellite image from June 27th the water volume of the river is only somewhat larger than usual for the
season and the river runs in its usual bed again. During the floods it flooded a large area and formed new riverbeds or relapsed into old ones that had long been dried out and cultivated. The damages were caused by erosion of the soil in cultivated fields and the destruction of the infrastructure and buildings. Whole houses were seen to be torn off their bases and carried downstream or smashed to matchwood.

The most prevalent damage, which covers the larger area in the image and is very visible, is the destruction of the natural vegetation and the soil in cultivated fields and the ensuing deposits of sand and gravel or the thick silt deposits covering the area. Cultivation and production will be destroyed or impaired for a long period of time. These areas are seen as grey/white colours indicating vegetation-free areas. The position and delimitation of these areas also clearly indicate that they are flood damages. Areas of continued high water levels will be seen as darker blue-green colours.

Areas of cultivated fields, which were not damaged by the flood, may have the same colours as the damaged areas if they had no green vegetation at the time of recording (newly plowed e.g.). However, such fields will normally be distinctly squared and be surrounded by cultivated fields (seen as orange). Fields lying above, but near the flood line may be damaged, too, by the high level of water and "drowning" of the root system. Potatoes e.g. are quite sensitive, whereas grass may be flooded for some days without being seriously damaged. One of the lessons after the floods was that the natural vegetation such as forest, brushwood and grassland were most resistant to damaging and erosion from the floods and should be used more in the future in the flood-endangered areas.

Generally speaking, the extent of damage is calculated on the basis of the remaining intact vegetation and cultivation in the area. Cultivated fields that have not been damaged are seen as orange colours and forest areas are seen as green. Built-up areas and roads look like the damaged areas, but are identified on the basis of structure and patterns.

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