Pilot Sites - Trondheim
The Trondheim Municipality is situated in central Norway on the Trondheim Fjord, 70 km away from the open sea, in Trøndelag County. With around 170.00 inhabitants is Trondheim Norways third biggest city after Oslo and Bergen. The Gulf Stream causes a moderate climate and keeps the harbour during the winter period ice free.
Since 1900 the Norwegian meteorological institute measures continuously temperature and precipitation in the Trøndelag County. The figure below shows the deviation in temperature and precipitation according to normal. Nowadays, normal is the period with the average temperature and precipitation from 1961 until 1990. Since 1985 the temperatures were warmer than normal with a continuously increase. Another temperature peak was observed in the thirties. The precipitation increases and since 1982 the values lie continuously over normal.
Climate scenarios for Trøndelag predict an increase in temperature of 2,5 °C and annual precipitation of 10-20% in 2100. Intensive precipitation sometimes combined with periods of snow melting causes large runoff and flood events in Trondheim. Some historical data from the last main events in the last to decades are summarised in the table. In the beginning of 21st century, the extreme rain storms and resulting flood events concentrate in summer. Consequently the risk of flood in Trondheim expects to be increase in the future.
| Heavy precipitation, large runoff and flood events in Trondheim | |||
| Event | Characteristics for scenarios | Estimated return interval | Consequences |
| 9-10.12.1987 | First 30-40 mm in 20 hours, 20-30 mm in the coming 6 hours; and snow on the ground equivalent to 20 mm. | 10-30 year | Damages on the roads and houses |
| 30-31.03.1997 | 95 mm rain and snow melting in two days. | 40 -50 year | About 100 houses flooded and some roads had to be stopped. One manhole cover was lift at one place due to high pressure in the sewers. |
| 30.01-05.02.1999 | 48 mm in 1 day on 04.02.1999 | 15- 20 year | The flood attacked the whole city. Flood warning on 30 January 1999. Flooding in basements and on roads was registered in large parts of the city, caused million crone of economic damages. |
| 29.07.2007 | Rain from midnight to 7-8.00 in the morning with variable intensity in the whole city. | 100 years | 60 houses were flooded. |
| 13.08.2007 | Intensive rain in 1 hour | > 100 years | Over 100 houses were damaged by floods. |
Flood risk in Trondheim: Trondheim is exposed to three different sources for runoff and flood:
- Flooding from the river Nidelva
Starting form the west-end of the biggest lake in South-Trøndelag the river Nidelva enter after 40 km the city centre of Trondheim and discharges into the Trondheim Fjord. Six hydroelectric power plants are located along the river. Due to the large catchment area of 3.178 km2 the river carries a lot of water. Due to regulations measures which have reduced the discharge about 110 m3/s compared to the normal conditions smaller floods could be prevented. However, the occurrence of huge flood events could not been avoided (NVE, 2001).
- Flooding from the sea during storm events
Gale-force storms in the Trondheim Fjord and spring-tides increases the sea-level up to 50 cm and the water level of the river Nidelva due to backflow.
- Flooding in urban drainage systems
The sewer system in Trondheim consist of roughly 50% combined system built before 1965, 40% separate system and 10% non active separate system. About 100 combined sewer systems pollutes the river and the sea during heavy rain and snow melt. On many places in Trondheim the sewer drainage system is not designed for peak runoff discharge. The insufficient capacity of sewer system leads often to flooding.
Consequences of flooding in Trondheim
Different generic risk and vulnerability analyses which were conduct already in the Trondheim Municipality show an increase risk for flood and flood related consequences. In periods of heavy rain and snow melting the residents in Trondheim have to expect flooding in houses, polluted drinking water, restrictions in transport, power supply and with an increase risk of landslides due to special geological conditions.
Challenges for Trondheim in order to reduce the consequences of floods:
- Better weather forecast
- Advanced risk and vulnerability analyses
- Guidelines for area planning and land use with differentiated safety requirements
- Flood maps
- Safety measures against landslides
The main aspects of relevance to the FloodProBE work programme are:
- Identification and analyses of most vulnerable infrastructure in respect to floods (Task 2.1)