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Sediment ponds

Within WP2, two sediment ponds were constructed in 2010 and 2011: one at the Molenbeek in Erpe-Mere and one at the Vondelbeek on the border of Dendermonde and Lebbeke.

Molenbeek sediment pond in the existing controlled flood area (GOG) in Erpe-Mere.

Photo 9: Sediment pond MolenbeekEvery year, 3,300 tons of sediment is transported in the Molenbeek. This sediment comes from both household discharges and from soil erosion. The sediment is deposited in the watercourse and in the flood areas. In the long term it thus reduces the water transport capacity and the storage capacity, which indirectly leads to an increased chance of flooding. Sediment ponds force the silt to settle in strategically chosen places as a result of which the clearance costs are significantly reduced. Furthermore, we prevent contaminated sediment being deposited in nature area and that the silt has to be removed in vulnerable areas by making the sediment settle locally and upstream of contamination sources. A sediment pond was created to catch a large part of the silt in the Molenbeek. The sediment pond has a volume of 6,000 m³.

It has been calculated that the sediment pond has an efficiency of 50%, so that some 260 m³ of silt will be caught in the sediment pond.

Sediment pond in the existing GOG in Lebbeke/Dendermonde

Photo 10: VondelbeekIn the past, Dendermonde and Lebbeke regularly had to deal with flooding. When there was a lot of rainwater drainage, the Vondelbeek and its tributaries were oversaturated and became unpredictable, with all the subsequent risks of flooding. The VMM therefore created a flooding basin.

Each year, some 600 tons of sediment are transported via the Vondelbeek. To prevent siltation in the flooding basin – and thus a reduced long-term storage capacity – a sediment pond was created at the weir. The efficiency is 90% and that means that the pond has a capacity of 4,200 m³ of silt.

Most of the silt in the Vondelbeek originates from erosion. However, a substantial part (approx. 15%) also comes from household sources (e.g.  Lebbeke) and a minor discharge from the Affligem brewery. Due to the controlled capture of the contaminated silt components, they contaminate the rest of the watercourse less.

Additional advantages of sediment ponds:

  • clearance of whole stretches of watercourses has a negative ecological impact:
    • disruption of the habitat during clearance (physical, noise)
    • CO2 emissions from the clearance machinery
  • excessive siltation is generally considered to be negative for certain species such as the bullhead pike and the European brook lamprey. These species are no longer found in the Molenbeek-Erpe-Mere and the Vondelbeek. Siltation in many watercourses is greater than the natural situation due to more erosion from upstream fields.