Wrinkled like a washboard

Around Cortenoever and Voorsterklei, the landscape is wrinkled like a washboard. These wrinkles – so-called ‘kronkelwaarden’ – are shaped by the IJssel river, and unique in the Netherlands. Large-scale excavations for riverbed expansion threatened to destroy this fine relief. To prevent this, BoschSlabbers came up with an inventive plan, making the existing relief a departure point for a broadened river.

In the plan, a water level decrease is realised by the replacement of two dikes, combined with lowered land surfaces, new river gullies and a partly removal of buildings. Another point of departure is the preservation of agricultural use of the area. Existing dikes are lowered, to form a dam that is flooded on average once every 25 years.

Relief-inspired excavation

Both parts of the project area have high geomorphological value: ‘kronkelwaarden’, levees and lowlands. Special attention was given to the integration of the new river gullies in the existing morphological pattern.

For Cortenoever and Voorsterklei, different excavation techniques were studied. In the end, a strategy of ‘relief-inspired excavation’ was chosen, in which excavations follow and expand on existing relief structures as naturally as possible.

Functional and legible

On top of the new dikes, bike paths have been realised. Two new pump houses mark the end of the new river branches along the dike. The pump houses are also designed as pleasant rest places for cyclists.

De Heetkool, a 17^th-century farm mound, may have lost its agricultural function, but it has survived intact and is now a green island, prominently visible in the open landscape of Voorsterklei. In addition, a new mound has been constructed, to give an existing local farm business a new start.

A low dike and a ‘landscape verge’

The new dikes in Cortenoever and Voorsterklei are shaped as low embankments, similar to historical dikes in the area. Their new trajectories follow the higher sand ridges as much as possible, making the actual dikes relatively low. In addition, the dikes and its stabilising verges are landscaped with gradual slopes, so-called ‘landscape verges’. As a result, the lower parts of the dike look like a continuation of the sand ridge on which the dike is constructed. Only the upper parts of the dike have steeper slopes and a narrow crest.