July 21: Double Dutch dikes
As sea water levels are expected to rise, there comes a limit to how high and broad dikes can be made. Henk Jan Verhagen from the Delft University of Technology investigates if lower double dikes might do as well.
Dikes are high in the Netherlands. After the 1953 flooding of the southwest of the Netherlands, an impressive programme has been started to bring all dikes at a safe level. The so-called Delta works also included dynamic closure of the waterways to the North Sea.
A safe level for the dikes means that the berms of the embankments are brought to a level that spring flood plus wind set-up might reach once in 4,000 years. The top of the dike may be five to six meters higher, depending on the possible run up from waves at the dike.

In the perspective of climate change, sea water levels are expected to rise and storms might be stronger, creating the need for even higher and broader dikes. This begs the question of how far you can or should carry this strategy. The European ComCoast project has been set up to search for alternatives for dike enforcements.
The alternative Henk Jan Verhagen is studying has its roots in the past. In 18th century Zeeland, on the south coast of the island of Schouwen a double dike system has been created as a backup system. Because of the strong tidal currents south of the coast, the primary dike was liable to erosion. So, a second dike was constructed some hundred meters inland. This location, called Flaauwers Inlaag, now has a wetland in between the two dikes, functioning as a bird sanctuary.
When there is such a second dike in place, the primary dike may be lower than otherwise required. In the case of waves coming over the dike, the sea water will then be retained in a lake between the two dikes.
For this concept to work, it is essential to know whether the primary dike will not be damaged by the overtopping waves. Verhagen therefore conducted experiments with a large installation deliberatly flushing water over the dike to see how the inside of the dike would be affected. The results were encouraging. A grass covered dike with a one in three slope can handle 50 liters per second per meter dike with no problem.
However if there is a sudden cinch such as a horizontal tarmac road at the inside of the dike, or if there are mole dug channels close to poles in the dike, these are the places were the descending flow of water might do structural damage, possibly endangering the integrity of the dike from the inside.

The double dike alternative has limited value, Verhagen admits. It is not cheaper to construct a double dike than enforcing the existing primary one. However, if there is a second dike present, the primary dike can be less enforced than otherwise required. If at least the primary dike is resistant to overtopping waves, and if the space between the dikes is large enough to contain the quantity of water that might come over the primary dike under extreme conditions.
Verhagen also participates in a Vietnamese dike enforcement project, supported by the Dutch ministry of development. He has collaborated in overtopping experiments of Vietnamese dikes last spring. But with the typhoon season coming up, the Vietnamese agency has now stopped the experiments in order not to endanger the dikes at the moment they are needed most.
The Vietnamese situation differs from the Dutch insofar that they want to investigate if a double dike system can be made cheaper than a single large one. The safety requirements in that case are less stringent than a one in 4,000 years event. “In the Netherlands, safety is the primary concern”, says Verhagen, “costs come second.”