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Wetlands reduce flood risks in Aarhus

Wetlands reduce flood risks in Aarhus

In Aarhus water from heavy rainfall is being channelled to a new lake. The lake also reduces nitrogen leaching from surrounding agriculture. The city is now seeking to extend the project and create another wetland.

Climate change implies more intense cloudbursts and rising sea levels, making the provision of flood protection for low-lying and densely populated areas increasingly urgent. An inexpensive and intelligent solution to these problems is to use natural areas as a "buffer" to hold water during and after extreme rainfall events and at high tide. Egåedal is an example of this type of water body.


The valley of Egå

Close to Aarhus, the second largest city in Denmark, the southern part of the Egådal is densely populated, low-lying and at risk of flooding under increased precipitation.
 

 

From the 1950s, the area was the object of extensive drainage to gain more land for agricultural purposes. Two pumping stations were established at the old natural steam, Egå, in order to pump water out of the wetland and into a new channel that carried it to Aarhus Bay. After this, intensive farming dominated the Egå valley.

 

Today the stream has been partly restored to its natural state and represents an example of adaptation to climate change that has had multiple benefits.

 

A lake is created

When it was realised that intensive farming and leaching of nitrogen in particular had negative consequences on animal and plant life. the Danish parliament adopted the Action Plan for the Aquatic Environment II in 1998. Its purpose was to reduce nitrogen leachate to the aquatic environment. One of the means to achieve this was to restore original streams and wetlands.

 

Thus began a collaborative project involving the municipality of Aarhus and other related institutions with the aim of applying the Action Plan for Aquatic Environment II in the Egå valley. Chosen areas were expropriated and a dam was established. In 2006 the old stream dykes were removed and about half of the previously reclaimed area was flooded. The result was Egå Engsø, a lake of 115 ha. with a mean depth of approximately one metre and 1.7 metre its deepest. The water level can be raised by about one metre depending on seasonal and rainfall conditions.

Figure 1: Photograph of the established Egå Engsø.

 

Egå Engsø affords both space and time for achieving a water balance

A dynamic flow model for the area has been produced by the company Orbicon. The model shows that the lake has reduced the flood risk of the densely populated areas in the lower part of the river valley and along Egå. This is because Egå Engsø acts as a holding lake, slowing runoff when there are heavy or prolonged rainfall events. This helps reduce high water levels downstream, but also extends the period during which the water table is slightly raised.


Egådalen remains at risk of flooding

Egådalen is a characteristic landscape element, and is very low-lying in comparison with the surrounding terrain. Figure 1 shows a terrain model illustrating the extent of the Egå valley and the low-lying areas.

Figure 2: Terrain model of Egådal catchment area. The former area of reclaimed land is outlined with a white line. The colours on the map show terrain elevation, with blue lowest and red highest. The restored lake from 2006 appears turquoise in the low-lying area.

 

The low-lying areas in the bottom of Egådal near Risskov are at risk of flooding, either through intense runoff from uplands or at high tide in Aarhus Bay. Currently, the area is protected by levees and a pumping station at the Egå mouth in Aarhus Bay.


The lake's holding capacity is reduced if the water level in the lake is already high at the onset of a heavy rainfall event. In 2007 rainfall during January, February and March was significant and lakes, streams and ground water were at maximum capacity. Furthermore early March saw a large amount of snowmelt, which exacerbated the danger. This resulted in the highest sustained water flow rates in 10 years and the Egå was close to bursting its banks. Emergency response units from all over Denmark were called to the area to prevent a flood that could have had major economic consequences. The event highlighted the need for further preventive measures.


An investigation in 2008 recommended the incorporation of the remaining reclaimed land in the Egå valley to enhance the holding capacity and reduce flood risk from extreme rainfall events.


Adaptation potential for a new wetland

The Aarhus municipal plan has designated the remaining reclaimed land areas as "possible future wetlands". This is an excellent basis on which to plan the establishment of a new wetland.


Figure 2 shows the proposed wetland area marked with red line. The proposed area is located immediately downstream of the current Egå Engsø and will thus enhance the storage effect.

Figure 2: the yellow area is Vejlby-Egå Enge reclaimed land and the red line marks the boundary for a further proposed wetland, called Hede Enge (heath meadows).

A new wetland will serve several purposes, including:

  • reduction in nitrogen leaching into the Bay of Aarhus
  • further reduction of the risk of flooding in the lower part of the Egå system through attentuation
  • improvement in the natural environment by ensuring environmentally-conscious land management
  • improvement in recreational opportunities in conjunction with Egå Engsø.


The new wetland is therefore unique as a climate change adaptation project. Calculations show that it could hold large volumes of water from a downpour lasting half to one day in a controlled flood zone, thereby reducing the risk of flooding of many homes along the lower Egå.


Regulations

Because of an EIA-screening prior to the project, Engsø did not require an environmental impact assessment (EIA) before the project could be launched in 2006. It seems likely therefore that flooding the adjacent area will not require an EIA.


The actual execution is relatively simple, but requires appropriate coordination to ensure that the many interests and concerns are taken into account. Before the project can be initiated, it must be approved in accordance with relevant Danish environmental acts.


As the area is part of an existing land reclamation area, rules and agreements must be established for the administration of the project. The project in Egå Engsø and the plans to extend the wetlands with a new lake therefore require that a sharing of costs between private landowners and government be considered. 

 

 

Costs
A large portion of the costs relate to compensation for affected landowners for expropriation of land. The cost of constructing Egå Engsø in 2006 was DKK 20 million, VAT excluded. The cost of the proposed project at Hede Enge is estimated at approximately DKK 25 million, of which 80 per cent is for compensation to landowners.

 

Similar reclaimed land can be found at many other locations in Denmark, and the work in Aarhus can therefore serve as a pilot project in terms of climate adaptation in the water sector.