Wednesday, October 20

Erosion of a stream was the main cause of the Gjerdrum landslide

GJERDRUM (Aftenposten): The committee’s experts believe that work should continue on who was responsible for rectifying the situation in the stream.

The Gjerdrum committee presents the first interim report on the causes of the rapid clay landslide in Ask. The committee is chaired by former Storting politician and Nord-Trøndelag County Governor Inge Ryan. Photo: Ole Berg-Rusten / NTB

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

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, Inge Ryan said at the press conference.

In addition, he says that the conditions were a steep slope and unstable soil conditions.

The landslide took away 30 homes in the Nystulia residential area. According to the expert committee, Nystulia was the last affected area.

Tistilbekken runs from the center of Ask, through a culvert past the golf course near the Nystulia residential area, under Provincial Highway 120 and into Tangeelva.

The landslide area viewed from north to south on September 27, 2021. The landslide on December 30, 2020 passed through provincial highway 120 and came to a stop for the first time 800 meters on the other side. The county road reopened in early September.

The researchers also point to the scarcity and lack of security over time. The creek should never have been paved, and it broke from the pipes in which it was installed. This caused a lot of erosion in the fast clay area.

Triggering cause of precipitation

– The landslide was triggered by a wet fall with a lot of precipitation, committee leader Inge Ryan said at Wednesday’s press conference.

The fall of 2020 was the wettest since 2000. High water flow in the days leading up to the landslide may have loosened the clay shells at the edge of the creek, says committee leader Ryan.

But the committee believes that the precipitation itself cannot explain that the landslide went away. Ryan pointed out that the landslide could have happened just as easily in 2000. Then there were similar conditions, but with even more precipitation.

The reason the landslide was triggered in 2020 and not 20 years earlier, the committee explains by erosion in the time before the fateful January night. This weakened stability.

The Minister of Oil and Energy, Tina Bru, pointed out that the explanation of the landslide is complex and that more work must be done with causal explanations.

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 there, which to some extent indicated a map of danger zones. The landslide incident and investigations carried out on the ground have confirmed this. However, it 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 December 2020 landslide was the result of erosion at Tistilbekken over several years that 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

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. Changes to the terrain in various rounds and urbanization, that is, the development of the site, helped to strengthen the process.

In the past, the roots of trees and shrubs helped keep fast clay stable. With the development of the area, much of this was removed.

But the heavy rains that arrived in late fall 2020 were, according to the committee, a decisive reason why the landslide was triggered.

– Could have been avoided

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

Thus, the avalanche took a total of more than 30 housing units, which headed west of provincial highway 120, near the Gjerdrum nursing home. Parts of the town of Ask in Gjerdrum were evacuated.

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

But the Gjerdrum committee has not concluded if anyone was responsible for this, or who it was in that case.

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

– Could the landslide have been prevented?

– It’s always easy to see this in hindsight. But if one had known it could have been a disaster and implemented the measures that we discovered through a thorough review, with creek paving and that sort of thing, then it could have been avoided, Ryan says.

In addition, it says that the municipality could coordinate the security work, without concluding that it was the responsibility of the municipality.

– When tragedies like this happen, we must learn from what happened. We must ensure that it does not happen again. Therefore, I greatly appreciate the painstaking work that the committee has done in a short time, says Oil and Energy Minister Tina Bru.

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. The municipality received warnings about the erosion of the stream as early as 2008.

NGI noted that stability towards the top of Tistilbekken was not satisfactory. NGI also noted that there was a need for erosion protection in the creek.

The report describes that measures were implemented to improve stability. The stream was erosion proof. In general, the correct evaluations were made.

But not enough was done.

– The municipality of Gjerdrum has many danger zones for fast clay. It is clear from the processing of the planning and construction case that the municipality has a conscious relationship with this risk in planning and construction cases, the report says.

Furthermore, he states that despite this, it may appear that knowledge about the risk of rapid clay landslides and the consequences of previous landslides at Romerike was not sufficiently linked to the erosion warnings.

– It seems to the commission that the municipality understood that there was a risk, but not the degree and possible consequences of this risk, so the measures were delayed, he says.

Known theory

Tistilbekken’s lack of security 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 the surface water from much of the Ask center in Gjerdrum ended up 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 in 2008 that he strongly warned against Nystulia development. He had personally discovered erosion in a stream below the residential area.

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.

Gjerdrum Avalanche:

Leave a Reply

Your email address will not be published. Required fields are marked *