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11 metres of mud reveal 8,000 years of flood history
Knowledge of past floods can help us understand what is possible in a warmer and wetter climate.
In 2017, researchers extracted an eleven-metre-long sediment core from Lake Vangsvatnet in Voss. The core provides a rare glimpse into the flood history of this community in Western Norway.
It consists of layer upon layer of material that has been carried into the lake by rivers and streams over thousands of years.
We spoke with researcher Pål Ringkjøb Nielsen from the University of Bergen's Department of Earth Science about the work of uncovering 8,000 years of flood history.
The lakebed is a natural archive
When a river floods, both its flow velocity and its ability to transport sand, silt, clay, and other material increase. As the water flows into the calmer waters of a lake, this material settles on the bottom, forming a new layer.
In this way, the lake bed functions as a natural archive.
“During a flood, a river can carry far more material than it normally does. When the water reaches Lake Vangsvatnet, the material is deposited on the lake floor as a distinct layer,” Nielsen explains.
A community with a long history of flooding
When a major flood struck Western Norway in the autumn of 2014, water levels in Lake Vangsvatnet rose rapidly. This caused extensive damage throughout Voss.
The flood served as a stark reminder of how vulnerable the community is when large volumes of water accumulate within the river system.
But the 2014 flood was far from the largest flood Voss has experienced.
Historical records describe a flood in the 17th century. During that event, the water level in Lake Vangsvatnet may have risen by as much as eleven metres. The water reportedly reached approximately one metre up the wall of Vang Church.
“That's an enormous amount of water. The entire town centre must almost have been submerged,” says Nielsen.
The impact of the 2014 flood highlighted the need for knowledge to improve future flood preparedness in Voss and across Western Norway.
The flood also led to the launch of the research project Hordaflom.
“People wanted to know more about floods in the past. Is this something that could happen again soon? What do the historical records look like? Is this a common occurrence, or an exceptional event? These were some of the questions being asked,” Nielsen explains.
Lake Vangsvatnet is well suited for research
He explains that if we want to predict the future, we must also look to the past.
To understand how often such floods have occurred and how they have changed over time, researchers have examined sediments from the bottom of Lake Vangsvatnet.
According to Nielsen, the lake is particularly well suited for this type of research. Voss has extensive historical records of flooding and a long series of hydrological measurements from the village of Bulken.
Researchers have a good understanding of how large previous floods were, supported by flood markers on buildings, including a historic mark on Vang Church.
There is also extensive documentation showing how water levels have changed over time.
Another feature that makes Lake Vangsvatnet unique is its rapid response to flooding. The water level rises rapidly, causing flooding that quickly affects buildings and infrastructure along the shoreline.
The lowest part of the sediment core dates back approximately 8,000 years. Researchers have dated the sediments using several geological dating methods.
In the youngest sections of the core, the results can also be compared with historical descriptions and instrumental measurements.
“Lead dating can be used for deposits up to about 200 years old, which corresponds to roughly the upper 40 centimetres of the core. That gives us very precise age control," says Nielsen.
He explains that researchers therefore have a fairly good overview of the uppermost part of the core.
"Here we can see that the different layers correspond closely with historical records. This comparison shows that the sediments capture flood events that we already know about,” the researcher says.
From snowmelt to autumn rain
The sediment core does not only reveal how often floods occurred. It also provides information about the types of floods that dominated during different periods.
During the Little Ice Age, approximately 1300-1800, temperatures were lower and there was more snow in the mountains. This led to more large spring floods caused by snowmelt.
Researchers believe that the major flood of the 17th century was most likely such a snowmelt flood. This distinguishes it from the 2014 flood, which occurred in autumn following heavy rainfall.
“During the Little Ice Age, both snowmelt floods and autumn floods occurred. As a result, we see a greater number of floods during this period,” says Nielsen.
As the climate has warmed, flood patterns have changed.
Less snow in the mountains may mean fewer traditional spring floods. At the same time, the risk of intense autumn floods increases as more precipitation falls as rain.
Researchers also observe an increase in flood frequency in the upper part of the sediment core. The same trend is evident in modern monitoring records.
“A flood that we previously referred to as a 200-year flood may no longer be a 200-year flood. If floods occur more frequently, what was once considered a 200-year event may in practice have become a 50-year event,” says Nielsen.
One solution is to reopen old river channels
Climate alone does not determine how severe the consequences of a flood will be.
Throughout history, people have built homes, roads, bridges, and other infrastructure along rivers. River channels have been straightened or narrowed, and natural side channels have been closed off.
This can leave the water with fewer places to spread when rivers overflow their banks.
“When rivers are channelled and natural pathways are removed, the water becomes more concentrated. In some places, reopening former river channels may be an important part of flood protection measures,” says Nielsen.
Solutions can conflict with other priorities
Lake Vangsvatnet also faces a particular challenge. Its outlet is narrow, limiting how quickly water can drain away. The outlet threshold has previously been modified to allow water to flow more quickly downstream through the river system.
As a result, flood levels in the lake are now considerably lower than they were before the modification.
However, measures taken in one location can shift the problem elsewhere.
If more water is released more quickly from Lake Vangsvatnet, the pressure on Evanger and downstream areas may increase. For this reason, there have also been discussions about whether some of the floodwater could be diverted through tunnels directly towards the Hardangerfjord.
Such solutions would be extremely expensive and could conflict with nature conservation and other interests.
“The question is whether to modify the outlet threshold and move the problem downstream, or whether we must accept that major floods will occur and instead make society better equipped to deal with them,” says Nielsen.
Could we see an autumn flood on the scale of the 17th-century flood?
Researchers use the sediment core to understand the past, not to make their own forecasts of future flooding.
However, knowledge of past floods can help us understand what may be possible in a warmer and wetter climate.
Climate projections indicate that precipitation may increase significantly in parts of Western Norway by the end of the century. If a large amount of rain were to fall over Voss within a short period of time, a future autumn flood could approach the scale of the historic 17th-century flood.
The consequences, however, would be very different from those of several hundred years ago.
“Floods have happened before. But today, we live around the lake in a completely different way. We have far more infrastructure that could be affected. We're probably not fully prepared for what might happen,” says Nielsen.
Following the 2014 flood, several flood protection measures have been implemented in Voss. These may reduce the damage caused by a future flood of similar magnitude. An even larger event, however, could exceed the capacity of today’s flood defences.
The study therefore highlights the need for long-term planning, not only for Voss but for many valley and fjord communities across Norway.
“This is typical of Western Norway. Down in the valley, you may face flooding, while higher up on the valley sides, landslides can be the main hazard. We need to consider the entire landscape and all of its risks as part of a single system,” says Nielsen.
Reference:
Nielsen et al. Seasonal variations in flooding inferred from lake sediments in Western Norway, Climate of the Past, 2026. DOI: 10.5194/cp-22-1181-2026
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