Wednesday, October 20

Tistilbekken erosion was the main cause of the Gjerdrum landslide

GJERDRUM (Aftenposten): Researchers from the Gjerdrum committee have concluded that erosion in a stream is the main cause of the landslide in Gjerdrum. The committee believes that further work should be done on who was responsible for rectifying the situation at the creek.

After ten months of work, a committee of experts led by former Storting politician and county governor in Nord-Trøndelag Inge Ryan, at 12 noon on Wednesday presented its conclusion on what caused the landslide in Gjerdrum.

They conclude that the main cause was erosion at Tistilbekken, a stream running through the landslide area.

– The landslide started with one or more small landslides in Tistilbekken, says Inge Ryan at the press conference. Also, it says the conditions were a steep slope.

The researchers also point to poor protection and lack of protection over time, that the creek should never have been paved, that old pipes should have smoked, and that this should have caused a lot of erosion in the fast clay area.

– The landslide was caused by a wet autumn with a lot of precipitation, the committee leader also said at the press conference.

Does not put responsibility

Ten people lost their lives when a large rapid clay landslide occurred at Gjerdrum in Romerike just before New Year 2020.

The avalanche took a total of more than 30 housing units, which headed west of County Highway 120, near the Gjerdrum retirement home. Parts of the town of Ask in Gjerdrum were evacuated.

In the press conference, Inge Ryan says that they believe that knowledge about erosion and warnings beforehand should lead to securing the current.

The Gjerdrum committee has not concluded whether anyone was responsible for rectifying the situation in Tistilbekken and, if so, who they were.

The committee believes that this is an important issue, which needs to be clarified in more detail.

The map below shows the entire area of ​​landslides and nearby streams and drains.

Here’s what the committee writes about why:

Witness observations, physical traces, investigations and calculations confirm that the landslide started on the slope between the Holmen farm (Byvegen 1-5) and Tistilbekken, south of central Ask.

Calculations show that the stability of this slope was very poor. There were large amounts of fast clay here, which to some extent indicated a map of the danger zone, and has been confirmed by both the landslide event and field investigations. However, this is not enough to explain why the landslide occurred in December 2020, as the slope has been in this condition for a long time.

The fact that the landslide occurred in December 2020 is a result of the fact that erosion at Tistilbekken over several years had worsened an already poor stability on the hillside west of Holmen. This created a situation where the slope was no longer robust enough to withstand the effects of the wet fall and winter leading up to Christmas 2020. “

The stream sank itself 2.5 meters

The committee notes that parts of Tistilbekken were channeled in the early 1980s, in connection with agricultural planning.

Since then, the stream has escaped from the pipes, a process that likely began in the 1990s.

Therefore, the stream changed course and sank at the foot of the slope right next to it.

This weakened stability.

The committee writes that the erosion was unusually strong. And that the changes in the terrain, which occurred in several rounds, and the urbanization, that is, the development of the site, helped to strengthen the process.

Heavy rains that arrived in late fall 2020 also contributed to the landslide, the committee says.

Meteorological and hydrological data show that fall 2020 was the wettest season (three-month period) on record in the area since fall 2000, according to the report.

This is the role of the municipality of Gjerdrum

The committee states that the municipality of Gjerdrum has been very aware that there is rapid clay in the area, in connection with the development of Nystulia.

NGI noted that stability towards the top of Tistilbekken was not satisfactory. NGI also noted that there was a need to take protective measures against erosion in the creek.

The report indicates that steps were taken to improve stability. The stream was erosion proof. In general, the correct evaluations were made.

But not enough was done.

Known theory

The unsafety of Tistilbekken, which runs through the landslide site, has been cited as a theory in the past.

Aftenposten wrote on January 7 that several streams in the landslide area could carry away masses of soil. In 2009, the municipality of Gjerdrum warned that this could lead to landslides. Consulting firm Asplan Viak wrote in March 2009 that surface water from much of the center in Gjerdrum, Ask, ended at Brådalsbekken and Tistilbekken. According to Viak, both streams were highly vulnerable to erosion.

In Romerikes Blad and TV 2, hydrologist Steinar Myrabø, with sailing time from Jernbaneverket and Naturvernforbundet, recounted just after the landslide that in 2008 he had strongly warned against the development of Nystulia. He had personally discovered erosion in a stream below the residential area.

Gjerdrum Avalanche:

For a few seconds, this lightened the mood a bit. Police officer Kenneth Cortsen with his dog, Piko (7), who was injured in a search in the area of ​​the landslide. The photo is from January 4.

This was the assignment that was given to the committee:

  • Work diligently to find the cause of the tragedy.
  • Consider measures to strengthen the prevention of such landslides throughout the country.
  • Assess the risk landscape for buildings and the risk of rapid clay landslides in Norway, division of responsibilities, regulations and management practices.
  • The answer to the last two tasks should be available in a separate report. It will probably arrive in January-February 2022.

The Norwegian Directorate of Water Resources and Energy (NVE) will begin this fall inspections to verify the condition along the fast clay zones in Gjerdrum, Nannestad and Ullensaker, according to NTB.

The start of the inspections was on Monday, that is, two days before the presentation of the report. Most of the work will take place in weeks 39 and 42, has NVE notified.

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