Watercourses divert urban water and create natural areas

In Frederikssund, the municipality and utility company are working together on re-establishing the local watercourse, Sillebro Å. By curving the watercourse, the water is delayed and water quality is improved. This also minimises the risk of flooding, and creates a recreational area close to the city.

• Extreme rain in 2007

  Large amounts of untreated rainwater from houses and roads flow into the 7.6-km-long Sillebro Å in the middle of the town of Frederikssund. The water quality in the watercourse has therefore not met the requirements in the latest aquatic environment action plan. Moreover, an extreme downpour in 2007 flooded the basements of several citizens, and the areas surrounding the watercourse were under water. This resulted in insurance companies having to pay several million Danish kroner.

   

  The Municipality of Frederikssund and the utility company Frederikssund Forsyning have initiated a cooperation to re-establish the local watercourses with a more meandering route. The areas surrounding the watercourses can then be used to store the town's water in nature, and the dirty water from the town can be treated before being discharged into the watercourse. This is done by collecting the water in a number of basins retaining rainwater in connection with cloudburst events.

   

  Re-establishing the river valley will also help create recreational value close to the town, with water as an active element in the lives of citizens.

   

  "This is a more attractive solution and much cheaper than building concrete basins," as Jørgen Skaarup, financial and project manager at Frederikssund Forsyning A/S puts it.

• A solution with many benefits

  The solution benefits the municipality as well as the utility company, as the utility company can meet the requirements in the wastewater plan, and the municipality can provide new recreational areas for its citizens. It will also be possible to counteract extensive flooding like that in 2007, because six new flood retention basins will be established along the watercourse in connection with the work to curve the river course. The figure below shows the project area and the course of the new river.

   

  

  The project area for re-establishing Sillebro Å. The blue line marks the new meandering watercourse, and the yellow lines mark the area's newly established path system. The large road north of the project area is Ågade. Source Niras (2011).

  
  Curving the watercourse and the new water storage sites have been carefully incorporated into the topography of the area. By using a digital terrain model, the hollows in the landscape have been identified and used as natural flood retention basins. This is cheaper and, in many ways, a better solution than establishing similar large concrete storages.

   

  Troels Karlog, forester at the Municipality of Frederikssund, is pleased that they have succeeded in cooperating on a project that saves money and benefits citizens.

   

  "This cooperation is an expression of holistic thinking and a break with an ingrained engineering attitude that rainwater has to be led down pipes and discharged into the sea a quickly as possible. However, we want to use the water actively in our town and recreational surroundings. Another advantage is that nearby houses will increase in value, because the price of a house rises when it overlooks water or is located close to a recreational area.

   

  

• Potential as a valuable natural area

  The project follows a growing trend to utilise rainwater resources locally and through this create synergies between effective local rainwater management and the creation of recreational and valuable natural areas.
  Moreover, stakeholders reckon that in the long term the area can become a green corridor which can act as a link between the location's other green areas and through this allow animals and plants to spread.

   

  As stated in the local development plan for the area under the section on habitat corridors:
  "The municipal plan designates areas of biological interest where there either are, or can be created, habitats and opportunity to spread for the wild plant and animal life. Sillebro Ådal has been designated as a habitat corridor where it is important to conserve existing natural values and prevent
  creation of new barriers for animals and plants to spread. The watercourse regulation project will create better living conditions for animals and plants in and around the watercourse...".

   

  One purpose is to create growth conditions for local animal and plant populations, including a healthy and self-sustaining trout population which currently has difficult living conditions due to the current shape of the watercourse.
  One way the municipality wants to regulate plant growth and thereby carry out natural management of the area is by letting sheep and other livestock graze north of the watercourse.

   

  

  
  The project was launched with the laying of wastewater pipes in November 2011, while the curving of the watercourse and the establishment of basins and path systems first began in late 2012. The entire project was completed in August 2013.

• Construction work completed

  In Frederikssund, the machines are working hard in the construction phase of the Sillebro Ådal project. The construction phase started in February 2013 by clearing the areas in which the basins will be established. In addition, the area was cleared to recreate the finely and steeply outlined river valley as an open natural habitat. Despite great efforts to inform the public about the project, the clearing work took many by surprise, and there has since been many questions about the project. Felling trees in the winter presents a dramatic sight, because of the great scars left in the landscape. About 2,500 m³ of trees were cleared, which were turned into wood chips and used in CHP plants.

   

  
  Photo: Troels Karlog

  
  In April 2013, excavators began to recurve the watercourse and dig out forebasins and main basins which are to clean and delay rainwater from the surrounding area. A total of 10 basins will be established over a 2-km-long stretch of the river valley. The basins have been placed so that the watercourse can curve through the river valley and receive rainwater over a longer period to protect the town centre from flooding. Huge amounts of soil have been removed, and the river valley has been humming with activity from heavily loaded dumpers. It has been an ordeal for the many users of the area to walk around without getting their shoes muddy and wet.

  
  In connection with recurving the watercourse, which has become 200 m longer, 300 m³ gravel bed and 200 m³ large stones have been laid to provide optimal conditions for fish and small animals.

  
  The large recreational element in the river valley is also well under way. This is to provide more and better opportunities to move about the area as well as to fish in the gravel pit lake and jog along the paths. A car park has been established, there are new wheelchair-accessible paths with fishing sites, all bridges have been replaced, and new pedestrian bridges have been established, so visitors can experience nature and water at close hand. Moreover, there are benches and tables carved from the trees felled in the river valley.

   

  
  Photo: Troels Karlog