Hydraulic Engineering

Protection of the overflow dyke (Kruibeke - Bazel - Rupelmonde, Belgium)

The Need

In Flanders, flooding caused a lot of misery in the past. In 1953, several villages in the Scheldt basin were virtually completely flooded, as well in the South of the Netherlands as in the north of Flanders. The breaches in the dikes were caused by the swollen Scheldt, incapable of draining off the spring tide and storm tide in time. In 1976, the village Ruisbroek was fully flooded after a new breach in the dikes.

The 1977 Sigma Plan offers defence against storm surges in the Sea Scheldt basin, caused by storms on the North Sea. The number of storm tides has greatly increased in the last few years. Because of the climate changes, the near future does not look very bright either. This calls for appropriate measures.

The Sigma Plan consists of:

• the elevation and reinforcement of no less than 512 km of dikes and dams;
• a storm surge barrier in Oosterweel, indefinitely postponed because of the limited benefits;
• the construction of flood control areas, uninhabited flood planes that can collect part of the Scheldt water during storm surges.

The concept is very simple. In case of storm tide, the water flows over the lowered river dike into the flood area. The water is stopped by a higher ring dike. A higher ring dike protects the inhabitants of the hinterland. At low tide, the water flows back into the river through the outlet sluices.

There was one more problem: when no precautions would be taken, the overflow dike would wash away at each flooding. That is why DEC was awarded the construction of the overflow dike and the protection with fibrous open stone asphalt.

The Solution

Fibrous open stone asphalt or GOSA® is a material extremely suitable for this kind of dike protection. A dike protected with GOSA® meets these conditions:

• high reformation capacity: transformations in the dike body are followed;
• jointless: prevents erosion and under currents;
• relatively low roughness: prevents additional turbulence of the overflowing water;
• porous for water: prevents under currents;
• sufficiently open: ground water can flow out without creating over pressures;
• sufficiently strong: resistant to erosion and shifting of the cover;
• durable;
• porous for air: prevents shifting of the cover due to air pockets in the dike body.

The Works

Construction of the temporary damming dike
To protect the hinterland from storm surges during the works, a temporary damming dike was built in front of the Scheldt dike, using soil from the stock piles. The 21st of March 2008, the Scheldt level actually did raise higher than the level of the overflow dike that was to be built.

Profiling Works of the Scheldt dike
Over a total length of 8,000 meters, the Scheldt dike is profiled into an overflow dike. First, the Scheldt dike is consciously lowered from 8.35 TAW to 6.80 TAW. Then, the slope on the land side is decreased from 12/4 to 20/4.

Protection of the overflow dike with fibrous open stone asphalt
Then, 321,000 m² of the land side of the overflow dike is covered with GOSA®, a durable erosion protection layer.

Finishing of the overflow dike
To give the dike a green character, the GOSA® is finally covered with a layer of top soil (thickness 7 cm) and sown with a special grass seed mixture. Because of the open structure of the GOSA®, the vegetation can easily attach to the overflow dike.