Dimensioning of retention basin in a protected nature area in Aalestrup

Student thesis: Master thesis (including HD thesis)

  • Rikke Birkemose
  • Johan Hybertz Lassen
4. term, Water and Environment, Master (Master Programme)
This project is based on a problem from Aalestrup where a detention pond for surface water must be built in connection with the transformation of the old combined sewer system to a separate sewer system. Due to the topography of the area the detention pond must be built in a protected area adjacent to the stream Simested Å. Since the detention pond must be built in a protected area there is no room to construct the detention pond with a sufficient volume.

COWI A/S has suggested to construct a detention pond with a volume of 1/3 of the required. During heavy rain events the detention pond will overflow, leading to an irrigation of the surrounding meadowland. This irrigation is estimated to result in a more humid environment for plant life, resulting in increased biodiversity.

In this project, further work to optimize the solution proposed by COWI A/S will be conducted. By means of a 2D hydrodynamic surface model, different detention pond designs are studied in order to find a solution that results in irrigation of a large area while also ensuring that most of the irrigation water infiltrates in the meadowland. The best option in terms of optimizing irrigation is considered to be a weir located at the northeast corner of the detention pond and to smooth out a ditch east of the detention pond.

The surface water content of heavy metals and nutrients is studied, and it is concluded that the removal efficiency of the detention pond must be as high as possible. By means of a particle transport model, the removal efficiency of suspended solids is studied for the chosen detention pond design. Here it is concluded that the detention pond will remove 100 % of the SS and that the in- and outlet should be placed in the southern end of the detention pond. The result may however be different if a 3D model, and wind effects are taken into account, which was deselected due to lack of time.

For the chosen detention pond design it is examined if the irrigation leads to erosion of the bottom in the meadowland. Model calculations show that there are only very limited problems with erosion.
LanguageDanish
Publication date6 Jun 2012
Number of pages88
ID: 63615258