Hollow dike protects against super storms

A new dike needs to be built along the coast of Katwijk, offering new opportunities to combine safety with leisure, business and sightseeing.

Combining safety with leisure, business and scenery. (Artist's impression: DP6 architectuurstudio Delft)

Many residents of Katwijk, a small coastal town between Den Haag and Leiden, are now solely protected from floodwaters by a meagre row of small dunes. There is however a dike that protects the hinterland, but it runs right through the middle of Katwijk. A new dike therefore needs to be built to protect all residents from super storms.

The government’s initial plan was to allow nature to do part of the work. By dumping huge quantities of sand a little ways off the coast, the dunes would regain strength naturally as the sand washes ashore. But big dunes would ruin the nice view of the sea from Katwijk’s boulevard, so the local municipality came with another idea: a dike hidden within a smaller dune. The cost of realizing this plan, which demands less sand nourishment, is estimated to be approximately 32 million euros.

Researchers from the think tank ‘multikering’ (a word play signifying multifunctional dike) however elaborated on this idea and now believe they can do better, while at the same time saving the government and municipality more than half of initial total cost estimate.

Researchers from TU Delft, TNO, Rotterdam’s municipal engineering department, CUR building and infrastructure, and other agencies, suggest using 15 to 20 meter deep concrete diaphragm (water-blocking) walls, which are frequently used for tunnel constructions and quay walls. Seen from above their creation looks like a standard dike, but in fact it is hollow and accommodates car parks on the landside and cafes and restaurants on the seaside.

“With sand nourishment you can do a lot and it’s cheap,” says PhD student Wim Kanning (faculty of Civil Engineering & Geosciences), who is involved in the project. “Yet in places like Katwijk where hard structures are so close to the sea and the row of dunes so thin, you must continually add sand to counter erosion, which makes it very expensive.”

Kanning and his colleagues, Jarit de Gijt (CEG faculty) and Ton Siemerink (CUR), believe diaphragm walls are extremely useful in such circumstances. “Unfortunately, water boards and Rijkswaterstaat are very conservative,” says De Gijt. “They want to be able to inspect dikes by strolling over them,” Siemerink adds. “And the problem with diaphragms is that you can’t see them from the outside.”

The three researchers, who also assert that the condition of the concrete diaphragms can easily be monitored with sensors, will present their concept on 1 November, during the ‘battle of concepts’ in Katwijk.