New Lakes Retain Rainwater in the Town of Tommerup Stationsby on Funen

In the Municipality of Assens on Funen, three new retention ponds are keeping the town of Tommerup Stationsby dry-shod, even after persistent and heavy rainfalls. The three lakes - together known as Tallerupsøerne - are the largest climate adaptation project to date on Western Funen. The lakes are interconnected via meandering channels, and during heavy rainfall the three lakes become one large lake able to retain as much as 22,000 cubic metres of water. When the lakes fill up with water, the water is led on via a controlled pipe system into the Brende Å watercourse. The lakes were established on a 7-hectare land area acquired for the project by the local public utility company, Assens Forsyning A/S. The land was already gently undulating. The project entailed digging the hollows deeper and adding the soil dug up to the hilly parts, thus accentuating the undulating terrain. The local inhabitants of the area have been actively involved in creating the new landscape and recreational area surrounding the lakes. The lakes were designed to withstand a five-year rainfall event but have proven capable of withstanding more.

• Status: The lakes immediately proved their worth

  Construction work had barely finished when, in May 2021, 110 millimetres of rain fell on the area in just one month. The lakes came into action immediately. None of the nearby houses and gardens experienced flooding, as had been the case during previous rainfall events.

  
  Tallerupsøerne is one in a series of climate adaptation projects in and around the town of Tommerup Stationsby.

  
  It took less than three years from when the municipality and the local utility company agreed on the idea to when work on the lakes was completed in the summer of 2021.

  
  The project moved ahead when the previous owner of the area agreed to sell the seven hectares to the utility company.

  
  Via the neighbourhood council, the local inhabitants have had a leading role in the design of the recreational area surrounding the lakes.

  
  The neighbourhood council obtained funding for this part of the project from several foundations and the recreational part of the area will be complete in 2022.

  
  The technical installations in conjunction with the lakes have an expected lifetime of 50 years.

• Background: Flood events and more intense rain

  Tommerup Stationsby has had problems coping with increasing volumes of rainwater in parts of the town due to climate change. The town is also under pressure from drainage water from a large, forested area south of the town.

  
  The catchment area of the three lakes incudes the town and approx. 300 square metres of hilly landscape. The town is prone to flooding from drainage water from a large, forested area south of the town.

  
  The water drained from the forest increases the water load on the lower-lying town by a factor of 15.

   

  Before the new lakes were established, there was frequent flooding of houses and gardens.

  
  A study also revealed that the ecological status of the local Brende Å watercourse was unfavourable. Among other things, this was due to the large volumes of drainage water and surface water runoff passing directly from the town to the watercourse without being detained or treated.

  
  The local public utility company must be able to cope with a five-year rainfall event and detain the rainwater before it is led into Brende Å.

   

  Photo: Assens Kommune

• The Solution: The lakes retain, treat and delay rainwater

  The project capitalised on the already undulating landscape. By accentuating the existing valleys and hills, the project established three lakes to detain and treat the rainwater from large parts of the town.

  
  The local utility company excavated three existing hollows on a 7-hectare land area to make them deeper. A total of 54,000 cubic metres of soil was excavated and then added to the existing hills.

  
  The climate adaptation project turned the three hollows into "the retention ponds at Tallerup", where the terrain slopes slightly from the first, southernmost pond to the northernmost pond.

  
  The lakes retain surface and rooftop runoff from parts of the town through a separate sewer system. They also retain drainage water and rainwater from a production forest located on higher ground outside of the town.

  
  The water runs from the first, southernmost lake via a meandering channel to the middle lake and further onwards via a winding channel to the northernmost lake. From the northern lake, the water is led via a culverted waterway into the Brende Å watercourse.

   

  The system includes sand and oil traps installed in the lakes. This is to treat the water before it is led into Brende Å.

  
  The three lakes are one metre deep and contain approx.
  4,400 cubic metres of water. During heavy downpours, the water level increases considerably. Together the lakes can hold as much as 22,000 cubic metres of water.

  
  During intense or long rainfall, the three lakes spill over and become one large lake. In these situations, a "dry lake" located close to the three lakes will also fill up with water.

  
  The "dry lake" was established as a way of visualising the impact of weather and climate on the surroundings for local inhabitants, school students, etc.

  
  The rainwater runs freely from one lake to the next. Because of the large volumes of drainage water that also need to pass through the lakes, it was decided to establish a stormwater outlet structure fitted with holes at various levels and of varying size.

  
  At a later stage, the structure will be fitted with sluices to regulate the amount of water that is led from the lakes into Brende Å.

  
  All three lakes and the culverted waterway leading into Brende Å will also be fitted with water-level data loggers.

  
  With the final system is in place, the rainwater that used to flow uncontrolled and untreated into Brende Å will be detained, treated and controlled in its flow towards the watercourse.

• Benefits and added value: A varied landscape, more nature and recreational opportunities

  Many local inhabitants showed great interest in the project as it unfolded. They contributed ideas for the design of the new recreational area. And an event held in the summer of 2021 was well attended.

  
  The ambition of the municipality and the local utility company is not only to make Tommerup Stationsby resilient to greater volumes of rainfall. They also want to make the town a more attractive place to live.

  
  The current project will therefore also establish a new and varied landscape close to the town, with recreational opportunities and native plant species.

  
  Living fences will be established. Sheep will be put out to graze. Playgrounds and picnic and resting areas will be established, along with outdoor fitness equipment, hiking paths and a climate pavilion.

  
  All of this has been decided by the local inhabitants, who have been helping enthusiastically in the project.

  
  Furthermore, the Municipality of Assens also believes the project will contribute to improving the ecological status of the Brende Å watercourse.

  
  Usually, soil excavated when establishing retention ponds must be disposed of elsewhere. In this case the soil was redistributed to reshape the existing landscape and establish the recreational areas. This was a win-win for everyone involved because it reduced the construction costs.

• Financing: The public utility company, Assens Forsyning A/S, provided most of the investment

  The construction work was the largest cost in the project, and this was paid by Assens Forsyning A/S. The Municipality of Assens will pay for tree planting. Private foundations will cover the expense of establishing the recreational opportunities.

  
  The total cost of the retention ponds at Tallerup is DKK 20 million.

  
  Assens Forsyning A/S has paid most of this amount. The cost of constructing the lakes, including buying the land parcel, carrying out the excavation work and establishing the stormwater pipes leading to the lakes, came to DKK 14-15 million.

  
  The Municipality of Assens has paid DKK 400,000 for its share of the municipal stormwater management plan, for the landscape and planting plan, and for planting trees in the area.

  
  The neighbourhood council obtained DKK 1.5 million in funding from Nordea-fonden, the More Play funds and the Danish Outdoor Council, amongst others. The funding will be used to establish the recreational elements in the project.

• The process and stakeholders: Textbook example of how to engage the local community

  The Municipality of Assens, Assens Forsyning A/S and the neighbourhood council (Tommeruppernes Lokalråd) worked closely together throughout the project from the planning phase to the establishment phase. Local citizens were also involved throughout the process.

   

  The project is a step in the Municipality of Assens' stormwater management plan for Tommerup Stationsby.

  
  The plan was prepared collectively between the municipality and Assens Forsyning A/S and was adopted by the city council in autumn 2019. Assens Forsyning A/S subsequently bought the land for the project.

  
  Prior to startup of the project, a hydraulic analysis was performed for the Brende Å watercourse, and the project was subjected to environmental assessment screening as required by law. Furthermore, permission was obtained for controlled discharge into Brende Å.

  
  The local inhabitants were involved in designing the recreational elements from the beginning. The involvement was through workshops that allowed everyone to contribute their ideas and wishes.

  
  The municipality financed a visualisation of the area based on the input from the workshops, and the neighbourhood council used this visualisation in its fundraising efforts.

• Barriers: The project had only minor technical difficulties

  A technical obstacle concerning the retention ponds at Tallerup was overcome after an inspirational tip from a speaker from another public utility company at a meeting in the Danish Water and Wastewater Association. After this, there were no other obstacles in the project.

  
  "There will always be bumps along the way when several parties work together on the same project," said Gunilla Ørbech, part of the climate secretariat at the Municipality of Assens.

  
  However, Gunilla Ørbech does not believe there have been any serious bumps in this project.

  
  The hardest problem was purely technical. It concerned controlling discharges of rainwater and drainage water and ensuring enough retention time in the lakes to allow for proper treatment of the water.

  
  The technical problem was solved with inspiration from another public utility company, Vejle Spildevand A/S, at a meeting of the Danish Water and Wastewater Association. As a result, the stormwater outlet structure from Tommerup Stationsby was fitted with holes at various levels and of varying size.