FHVI technology is being used at a growing number of locations in Apeldoorn. One such example is the street Gentiaanstraat in the Zevenhuizen neighbourhood. This proved to be an ideal location for a FHVI test case.
Several small squares in the street were already disconnected from the drainage system a few years ago. Shallow, vertical infiltration was used for this purpose. The idea was that the water would seep into the subsoil after 24 hours but, recently, the system has not functioned properly, and the water did not drain away for days. Flooding arose due to contamination of the wells. No traffic crosses the squares, but they are surrounded by large trees. Therefore, the blockage was probably due to organic particles such as pollen and leaf remains.
The Gentiaanstraat is located in the east of Apeldoorn, which is at the edge of the push moraine, in the transition area to the river landscape of the IJssel. This means that higher groundwater levels and a less permeable subsoil are found here than on the west side of Apeldoorn, where the FHVI wells have been drilled up until now.
FHVI in de Gentiaanstraat: two birds with one stone
The municipality of Apeldoorn chose the Gentiaanstraat as an FHVI location because of the flooding and the type of subsoil in this street. Here, the FHVI technology can be tested on a more clayey subsoil, and the existing flooding problem can be tackled at the same time. Several parking areas next to the squares have also been included and, in doing so, additional hardened surface has been detached from the mixed drainage system.
As the test drilling revealed that this location has only half the drainage capacity of locations with a more sandy subsoil, it was decided to position two FHVI wells close to each other. To increase the drainage capacity in the area, two pipes, one 12 metres and the other 17 metres in length, were sunk deep and within a distance of a metre from each other. Furthermore, at each square, a second FHVI system was placed about 40 metres away. If one of the wells becomes overloaded, then the water can flow via the street into the vortex of the other infiltration well.
The initial results of these new wells are positive: so far, there have been no more complaints about flooding. However, one filter well became blocked quite quickly, but that proved to be caused by an oil leak on the street. The problem was solved by replacing the filter membrane in the well. More important still: due to the prefilter, the contamination did not enter the FHVI well.
Fifty new FHVI wells are planned in Apeldoorn this year. The intended lifespan of these is several decades, but that will only be possible if the water flows through them quickly enough. For this reason, the prefilter was first of all tested, and a two-year monitoring plan was drawn up to check whether the flow rate remains high enough. With this approach, the blockage of an infiltration point like in the Gentiaanstraat should largely become a problem of the past.