If the hardest rains in 50 years and a fire occurred at the same time, where would the rainwater and fire water end up? These kinds of questions were considered by the planners of the energy company Vantaa Energy and the consulting and engineering company Pöyry, as the drawings for the future waste power plant of Vantaa, one of the largest cities in Finland, were on the table.

The goal was to prevent flooding, even in the worst case scenario. An extra challenge had to do with the fact that it had been decided that the waste power plant of Vantaa was going to be constructed in a rockbound hollow.

“It is like a tub surrounded by 10 metres of rock-face. Water is absorbed into the ground so slowly that it is better to collect it all and dispose of it,” says Project Manager Markku Vuorimaa from Vantaa Energy.

The waste power plant was built on the eastern side of Vantaa, where the distance to the river Krapuoja is quite short, but this good salmon river should not be subjected to extra water. It was decided that the stormwater drainage pipeline should be drilled through rock, built over the motorway and into an open ditch where the plant is allowed to discharge one hundred litres of water per second. So floods must be prevented at both ends of the pipeline.

“We decided to use a stormwater detention tank,” says Project Manager Jarmo Salo, responsible for regional piping planning of Pöyry. Pöyry will planned and built the power plant and supervised the whole project from the purchases through its commissioning.

From concrete to plastics

The material for the stormwater detention tank had to be selected first. Salo thinks that old habits would have directed the planners to choose concrete, if the rocky hollow had not defined its own criteria. That is, as the excavation proceeded, it was discovered that the tank had to be dug deep below the groundwater level in order to get a sufficient slope for the stormwater drainage system. Building with concrete was no longer a viable option.

He stated that it would be substantially easier and faster to install a polyethylene tank structure. Two 700-cubic-metre tanks from Uponor Infra were chosen as the stormwater retention tanks.

Strict schedule

The tanks are made of Weholite pipes with internal diameters of three metres and they are one hundred metres long. Their ten-metre ends were assembled in the factory and were brought to Vantaa at the same time as the 20-metre pipes. The pipes were welded together on-site and the tanks were pressure tested by the company staff, after which builders from the contractors Destia anchored them into the excavated pit. The installation was completed in December, and in January 2012 the tanks and the yard were already covered with portable buildings. In principle, the tanks could have been used right away, but the excavation work was still going on. The entire stormwater system was put into operation in March 2012.

Absolutely tight

In addition to the quick installation work, Jarmo Salo required that the tanks be absolutely tight. No groundwater should seep into the tank. Salo also wanted to pay attention to the scenario of dirtier substances getting into the clean rain water, for example along with fire waters.

“Then the pump is stopped and the water in the tank is examined. We can decide if the water is passed to a reservoir for contaminated water or discharged to the open ditch. It is also important that the solids that have managed to pass through the stormwater chambers settle down onto the bottom of the tank.”

“A maintenance man can climb down into the tank and also wash it when needed.” In a normal state, the pumping station always pumps water when the water level exceeds a certain level, and the water will continue via the discharge chamber along the gravity discharge pipe to the open ditch.

The start of a giant project

Markku Vuorimaa regards the completion of the stormwater retention system as a good intermediate point in the huge construction project.

“The system would have been needed already a year ago when a pit for the waste bunker was being excavated, 18 metres downwards from the upper rock surface level,” laughs Vuorimaa.

Retention is becoming more important

Polyethylene stormwater retention tanks are not just solutions for big industrial facilities; they are becoming more and more important and popular in various kinds of logistics centers and commercial centers. The popularity of solutions for stormwater retention is not only explained by heavy rains becoming more common, but also by the expansion of built areas. They are needed when there are large roof surfaces and asphalt areas.