Eleven metres of mud reveals 8,000 years of flood history

When a river is in flood, 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, explains Pål Ringkjøb Nielsen, Associate Professor at the Department of Earth Science, University of Bergen.

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, causing 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.

Yet the 2014 flood was far from the largest flood Voss has experienced.

Historical records describe a flood in the 17th century during which the water level in Lake Vangsvatnet may have risen by as much as eleven metres. According to accounts, the water reached approximately one metre up the wall of Vangskyrkja.

- That is an enormous amount of water. The entire town centre must almost have been submerged, says Pål.

The impact of the 2014 flood highlighted the need for knowledge to improve future flood preparedness in Voss and across Western Norway, and it led to the launch of the research project Hordaflom. As part of this project, an article was published this summer on the findings from Lake Vangsvatnet.

- The Vangsvatnet study is ultimately the result of an event that affected the community quite severely. 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, Pål explains.

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.

The lake is particularly well suited for this type of research. Voss has extensive historical records of flooding and a long hydrological observation series from 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. As soon as heavy flooding begins, the lake reacts very quickly, leading to inundation that rapidly affects all development along the shoreline.

The lowest part of the sediment core dates back approximately 8,000 years. Researchers determined the age of 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. In the upper part of the core, we therefore have a very good overview. 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.

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, particularly over the past 500 years, temperatures were lower and snow accumulation in the mountains was greater. This led to more large spring floods caused by snowmelt.

Researchers believe that the major flood of the 17th century was most likely one of these snowmelt floods. 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 Pål.

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 a greater share of 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 Pål.

Climate and human intervention interact

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 floodwaters 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 Pål.

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 leave the lake more rapidly, meaning that 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, proposals have also been discussed to divert some floodwater through tunnels directly to the Hardangerfjord.

Such solutions would be extremely expensive and could conflict with nature conservation objectives 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 Pål.

Could we see an autumn flood on the scale of the 17th century?

Researchers use the sediment core to understand the past, not to make their own future projections. 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 flood of the 17th century.

The consequences, however, would be very different from those of several hundred years ago.

- The floods have happened before. But today we live around the lake in a completely different way, and we have far more infrastructure that could be affected. We are probably not fully prepared for what might happen, says Pål.

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. In the valley floor, 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 Pål.