As a result of a modification to the existing Middelsgraaf stream, which was made meandering, it was found after a relatively short period of time that too little water from Germany was entering the fishponds and the nature reserve behind via this stream. This was caused by the fact that when the course of the stream was reconstructed, not enough attention was paid to the high permeability of the new bottom of the stream, resulting in water sinking into the sand and gravel layers below.

After a variants study to come up with an affordable, natural and sustainable solution, the Water Board and ViForis opted to fill the summer bed of the Middelsgraaf brook with the impermeable Trisoplast at the most permeable locations in several places over a distance of about 1,600 metres in length.

Global work:

• Apply temporary measures to divert the water above ground by means of pumps from the Water Board.
• Excavate the existing stream bed under new profile.
• After profiling, apply Trisoplast to the new subsoil.
• Applying a geotextile, acting as a Separation, Signalling and Anti-root layer.
• Replace and profile the existing excavated base.

As a result of this procedure, the soil was resealed to 15 cm above the backfilled stream bed. As a result, the stream no longer loses water. Water from the surrounding area can still flow into the stream at a higher groundwater level and, vice versa, infiltrate from the stream into the subsoil.

The advantage is that at lower flows, such as in summer, the fish ponds and the area behind them now receive sufficient water. And in case of high water, water can infiltrate the surrounding area, overtopping over the Trisoplast layer.