The New Slussen project and reconstruction is vital for the entire Stockholm region around Lake Mälaren. The New Slussen will both protect key infrastructure from flooding, and secure the drinking water basin for two million people.

The project will redesign and retrofit the existing 80-year-old structure. Intended to last for another 100 years, the new facility incorporates elements of climate resilience and flexibility by design. The foundations of the New Slussen facility will be oversized for current conditions in order to bear the heavier loads from rising sea levels and the higher flood gates that are projected to become necessary from 2050 onwards.

Description

Forming the central junction between north and south Stockholm, Slussen is a strategic infrastructure asset with regard to transport, clean water provision and flood protection for Stockholm and the Mälardalen region. Slussen’s lock allows boats to transit between Lake Mälaren and the bay leading to the Baltic Sea (Saltsjön). The lock also acts as one of the gates between freshwater (Mälaren) and saltwater areas (Baltic Sea), thereby playing a key role in the provision of clean water. Mälaren is the main drinking water source in the region, supplying over 2 million people. When the lake’s waters rise, due to spring snow melting, for example, the lock’s gates can also be opened to discharge excess water and prevent flooding.

Mälaren is the third largest lake in Sweden, with about 5% of Sweden’s area within the watershed. The lake stretches 120 kilometres from east to west and holds around 14.3 billion cubic metres of water.

The city of Stockholm has authorisation to regulate the water level of Lake Mälaren through the opening and closing of six locks in Stockholm and the City of Södertälje.

The project aims to achieve three main objectives and functions at the site.

Transportation: Slussen is Sweden’s second largest public transport hub. New Slussen will create more lanes for buses and bikes, and fewer for cars.

Quality of life: The development also includes new green and commercial spaces to make the lock more attractive to locals and visitors.

But the main objective is Flood and Water Management: By building larger channels, the New Slussen project should allow the lock to release nearly five times more water from Lake Mälaren into the Baltic Sea, up to 1,400 m³/s. Coupled with a new water regulation plan for the lake and a responsive system of hydrological monitoring and forecasting, the increase in drainage capacity reduces the risk of flooding in Stockholm and its region.

The main climate-related risk taken into consideration for the design of New Slussen is flooding from Lake Mälaren, given the rise in sea level and the lake’s low drainage capacity. Several vulnerability studies have quantitatively assessed current and future flood risk around the lake.

The towns surrounding Lake Mälaren are highly vulnerable to flooding, according to analysis by the Swedish Meteorological and Hydrological Institute (SMHI). Currently, the mean water level difference between the lake and the sea is less than 70 cm.

What is the innovation/policy/project/technology? How does it work?

The Stockholm area is subjected to two opposing forces: rising water levels in the Baltic Sea related to climate change, and a geological process known as ‘land uplift’ causing its land to rise.

The New Slussen lock has been designed for what has been deemed an acceptable level of risk. The level of risk New Slussen can withstand from the Lake Mälaren and the Baltic Sea bay is:

• a 1-in-10,000 year inflow of freshwater into the lake (from extreme rainfall and rapid snow melt, for instance) combined with a rise in the mean water level of the Baltic Sea by the year 2100;

• a 1-in-300 year event of maximum high tide position in the Baltic Sea in combination with the highest, but not the most probable, projected sea level rise due to climate change over the next 100 years (0.5m), and a local rise in sea level due to strong wind.

This means that the New Slussen facility can cope with a sea level that is over two metres higher than today’s average water level in the Baltic Sea bay (Saltsjön). The project includes three main measures which will make the asset, the City of Stockholm and the Mälardalen region, more resilient to climate change:

• Addressing the current and future flood risk due to climate change around the Lake Mälaren by increasing the lock’s drainage capacity from 300 m3/s to 1,400 m3/s by widening and deepening its existing channels. Overall, New Slussen more than doubles the total discharge capacity of Lake Mälaren.

• Building in climate adaption and flexibility in the project’s design to accommodate higher flood risk levels, by ‘oversizing’ the foundations for the whole facility and to support the heavier water loads from the sea and the higher flood gates that will become necessary from 2050 onwards.

• Developing a new water regulation plan to account for climate change impacts. In addition, this plan integrates a new system of hydrological monitoring and forecasting, led by SMHI, to allow the regulation of water levels in the lake to be more responsive.

Next steps

If the sea level of the Baltic Sea outside Stockholm continues to rise above half a metre, there will be a need for more actions to be taken on a larger regional scale. One such action could be to install barriers outside the city of Stockholm in the Stockholm archipelago.

This would address future unacceptable flooding risks and could be a possible future complement to increased discharge capacity.