What will the tunnel look like?

The main tunnel has an area of 21 sq m. It is about 4.5 metres wide and 5 metres high. The tunnel will start to be constructed at level minus 27 metres (i.e. 27 metres below sea level) in Bromma and then follow a falling line towards Sickla, where the level is at minus 43 metres. The tunnel will slope at an angle of 1 ‰, which means a drop of 14 metres over the overall 14 km route of the tunnel. The gradient of the tunnel will allow wastewater to flow all the way to Sickla due to gravity.

At its lowest level, the tunnel will travel beneath the bed of Lake Mälaren, 90 metres below sea level. The wastewater in this section will pass through a number of pipes in the tunnel in order to prevent sedimentation at the lowest level. This will allow the pipes to be flushed clean at regular intervals to prevent deposits and sludge.

The diagram above shows a section of the planned main tunnel where water will flow freely. The tunnel will have a slope of 1 per mille, i.e. one metre per thousand metres, so that gravity causes the water to flow all the way. The tunnel will have a V-shaped bottom to facilitate the flow of water.

The water will be flushed via a pumping station at Smedslätten and flow on beneath Lake Mälaren to Eolshäll with the help of the siphon principle. The tunnel will look different under Lake Mälaren – see the section in the diagram above.

The wastewater in this section will pass through a number of pipes in the tunnel to prevent accumulation of excess amounts of sediment, i.e. to stop deposits becoming stuck at the lowest level, under Mälarpassagen. This will allow the pipes to be flushed clean at regular intervals to prevent deposits and sludge.