The area of Nordkroken in Vänersborg Municipality, Sweden, is exposed to several flood risks from groundwater, lakes, rivers and runoff from rainfall. As the municipality plans for new developments, their stormwater system needed to be re-designed and be able to cope with the natural hydrology of the area. DHI created a coupled MIKE URBAN (now MIKE+) and MIKE SHE model in order to model the full urban hydrologic cycle and implement an open stormwater system in the area. Our findings showed that the proposed stormwater system efficiently reduced the flood risks from both groundwater and heavy rains. 

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Challenge

Nordkroken is subject to frequent high groundwater levels, leading to flooded houses and little or no infiltration of rainfall, which in turn creates large stormwater runoff volumes. Since Vänersborg Municipality has been urged by regional authorities to construct a new stormwater system, the municipality needed to investigate stormwater systems and geohydrologic conditions, including the interaction between the two.

Solution

To examine the area of Nordkroken, we created a coupled MIKE URBAN (now MIKE+) and MIKE SHE model for the following three scenarios:

  1. Natural conditions with no developments in the area
  2. Current conditions with existing development
  3. Future conditions with current and planned developments

In addition to these three scenarios, we ran simulations for various rainfall events during both dry (summer) and wet (autumn) seasons. With the simulations, we identified the main sources of flooding during different conditions, established a water balance for the area, and tested the efficiency of a future stormwater system. By including a groundwater model, the stormwater system was designed to fit the specific groundwater conditions at the site to ensure safe and efficient management of surface flooding.

Results

Design and verification of future open stormwater system

Safe stormwater system during both wet and dry seasons

Quantifying the contribution of groundwater and stormwater runoff to surface flooding

About the client

Vänersborg Municipality is located at the south end of the biggest lake in Sweden, Lake Vänern. The municipality has high environmental ambitions and was recently appointed ‘Climate Adaptation Municipality of the Year’ in Sweden.

Software used

MIKE URBAN (now MIKE+)
MIKE SHE

Case story - Highlights flyer

Stormwater management from a holistic hydrologic perspective​

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