A new sluice with an associated pump unit to prevent flooding of Aarhus city centre stood complete at the end of 2015.
The sluice is located in the centre of Aarhus where the river Aarhus Å empties into the bay, close to the city's new waterfront community and arts centre, DOKK1. The project was adopted by Aarhus City Council on 22 September 2010, when the Council granted DKK 46 million to the project. Construction work began in 2013 and took place concurrently with the construction of DOKK1, which was convenient as the city centre had to be cordoned off for excavation work in connection with both construction projects. The sluice is a collaborative project between the local utilities company, Aarhus Vand, and the City of Aarhus. The ALECTIA consultancy firm was the consulting engineer on the project.
The Aarhus city centre is situated in a river valley. Therefore, parts of the low-lying city are very vulnerable to high water levels in the bay and the river.
In 2007, parts of the city were flooded twice, in connection with a storm over the bay and, a few months later, due to a cloudburst event which caused the watercourses to swell with water. "We ran both events in a computer simulation model and saw that, had the events happened a few hours apart instead of a few months apart, all of Aarhus city centre would have been flooded. We therefore had to come up with a solution to protect the city in the future," explained Mogens Bjørn Nielsen, who is head of department at Nature and the Environment at the City of Aarhus.
In future, a sluice and pump unit at the mouth of Aarhus Å will protect central parts of Aarhus against flooding. The project also involves a new protective barrier running some way into the city from the new community and arts centre, DOKK1.
The sluice is open during normal water levels. During raised sea levels, the sluice gate is closed to prevent the water in the bay from flowing up the river. The gates close automatically when the water level exceeds 1.4 meters above normal. The sluice gates will probably be closed around 25-30 hours annually and no more than eight hours at a time. The periods during which the gates are kept closed will be as short as possible to allow fish in the river to migrate between the river and the sea. The gates will also be closed during raised water levels in the river, and six built-in pumps will make sure that the excess water in the river is pumped out into the sea, preventing the river from overflowing its banks.
The pumps can move a total of 18,000 litres of water per second, corresponding to 65,000 cubic metres of water an hour. If a cloudburst occurs when there is already raised water levels in the river, the pumps at the sluice will pump river water out into the bay, which means the river can serve as a cloudburst reservoir. In order to provide further protection for the low-lying parts of the city during high water levels, the two streets Havnegade and Europaplads, as well as the area around the sluice itself, have been raised. The streets form a 300-meter long, invisible dyke along the river.
The citizens of Aarhus are now considerably more well-protected against flooding. At the same time, this alternative solution has saved the city a considerable amount of money.
The total project costs were DKK 46 million. A traditional solution, e.g. involving underground retention basins to retain the water, would have amounted to more than DKK 500 million. The water level in the bay can now reach as high as 2.5 metres above mean sea level without causing flooding in Aarhus. The highest water level ever observed in the harbour was 1.8 meters. Aarhus Å now serves as a long water reservoir, and the water level in the river can be controlled as required. As a result, the risk of damage to buildings and furnishings in connection with flooding has been reduced considerably. As a part of the project, the final stretch of Aarhus Å is being cleared so that canoes and kayaks can pass unhindered under the bridges.
The City of Aarhus has granted DKK 46 million to the sluice project. The sluice is owned by the municipality, which is also responsible for its maintenance. The municipally owned utility company, Aarhus Vand, is in charge of operations.
Initially, the City of Aarhus paid for the construction costs and took out a loan on behalf of the project. Aarhus Vand is financing the municipality's instalments and interest on the loan through the wastewater tax levied on citizens.
Citizens, the business community and public authorities have had a common interest in developing the climate-change adaptation project to protect Aarhus city centre from future flooding.
"No one has complained, on the contrary. But then again, we have saved everyone half a billion DKK," said Mogens Bjørn Nielsen, head of department at Nature and the Environment at the City of Aarhus. Flood protection has been improved, e.g. for: - Private enterprises and residential properties situated in a 40-hectare or so, area at the centre of the city. - Properties situated along Aarhus Å, from Brabrand Sø (a lake) to the harbour. - A large number of allotment gardens near Brabrand Sø and Årslev Engsø (a lake). - All technical installations such as power lines, bridges, roads and sewers in the area. - Future development projects throughout the river valley. The consultancy firm ALECTIA supplied risk analyses, requirement specifications and detailed project planning. The firm was also responsible for tendering procedures and for financial management of the project, and it designed the hydraulic assumptions for the project.
Due to the architectural design, the entire pump unit had to be constructed below ground.
"That was something of a challenge. The plant takes up the space of several houses," said Mogens Bjørn Nielsen, head of department at Nature and the Environment at the City of Aarhus. The pump unit is only visible via access covers and a total of four inlet and outlet gates. The building and construction phase also saw a series of practical and traffic-related challenges. However, according to Mogens Bjørn Nielsen, these were not beyond what could be expected in connection with excavation work in the centre of a big city.