WAVE retention basins prove their value immediately
Flooding has often been a problem along the Molenbeek in Zandbergen on a number of occasions. As part of the WAVE project, the Flemish Environmental Agency (VMM) therefore constructed two controlled flooding areas ("GOGs") in the town of Geraardsbergen (GOG Molenbeek) and the municipality of Lierde (GOG Ophasseltbeek). The retention basins proved their value even before they had actually been finished. Because work was still going on and the electricity had not yet been connected up, the two constructions had been closed off manually when there was heavy rainfall in December 2011. This intervention prevented local residents being affected by large-scale flooding.
Intensive communication is worth it
Constructing the two controlled flooding areas will make it possible to retain some 350,000 m3 (i.e. 350 million litres) of water temporarily, thus significantly reducing the likelihood of local problems due to flooding. Local residents were therefore an important discussion partner for the Flemish Environmental Agency, and an information meeting was held right at the start of planning. Further meetings for residents were held during each new phase of the project, or information was provided by means of leaflets or an info newspaper. It was also possible for residents to contact members of the project staff at any time.
This intensive communication - which was inspired by the way our English project partner tackles things - was really worth it because it enabled us to deal with potential objections early on, so that the necessary permits could quickly be issued. The latter was naturally also due to the high level of political support for the project. The mayors of Geraardsbergen and Lierde and the Flemish Minister for Environment, Nature and Culture openly expressed their support for the project at a number of events.
In contrast to other projects, the VMM did not contract out much of the work to external organisations; instead, its own staff often dealt with any problems. Even where acquisition of the necessary land was concerned, each landowner was approached individually so as to make the best possible agreements. This personal approach meant that the land could be purchased quicker and work could get started without delay.
Constructing the controlled flooding areas required a number of innovative techniques. At the site where the culvert was to be built, the ground was not solid enough and a lot of settling was expected. The settling needed to have ceased before the various concrete structures could be built. The VMM therefore installed "pre-loading masses", in other words a thick layer of soil whose weight would in itself cause the ground to settle. That process was further accelerated by installing sand piles, which ensured faster drainage. The settling was measured by installing a plate under the mass which was connected to a monitoring point on the mass. This made it possible to keep close track of how settling was proceeding, and when it had taken place sufficiently for construction work to start.
Local residents relieved
Actual construction commenced in May 2011 and was finished in July 2012. Optimum use was made of the capacity of the Molenbeek GOG by constructing a dyke around the infrastructure within the GOG and raising the level of a road. The soft ground of the road embankment was reinforced by liming it.
The controlled flooding areas were officially inaugurated on 3 September 2012 in the presence of the Flemish Minister for Environment and the Mayor of Geraardsbergen. This was just in time to deal with the heavy rain that fell in this area at the end of December 2012. Numerous local residents came to admire the effect of the controlled flooding areas, and said they were very relieved that there were no problems dues to excess water. No better complement is possible!