Double danger for Groningen inhabitants

Low-lying houses next to an elevated waterway are in double danger, finds a recent master study by the civil engineering graduate Paul Korswagen.

The scenario is as follows. An earthquake could damage the houses, and also make a levee break and cause a flooding. The water flow would result in slightly higher material losses, but it would be a big killer after the earthquake because the water would severely hinder rescue operations.

Korswagen calculated that the risk of dying for an individual living next to a vulnerable levee is around 8.10^-4 per year. That is 800 times higher than the accepted risk of 10^-6 per year for members of the general public to die of a natural disaster.

Assumptions are inevitable in such studies, and the young engineer motivates these extensively in his Appendix II and throughout the thesis. From reports by KNMI and others, he quotes that an earthquake that is perceived as ‘very strong’ and causes moderate damage has an expected return time of 475 years in Groningen. The peak ground acceleration of this magnitude measures 3,5 m/s² (or 1/3 of the gravitation).

From the acceleration, Korswagen calculated the resulting displacement. Displacement can be translated into the loss of resistance in a masonry house, using a mathematical model, similar to the recent push-over tests at the Faculty of Civil Engineering (CiTG). He thus obtained an estimation of the damage due to the earthquake.

Next, he calculated the expected water flow once the levee would break, and the impact of the water and debris on the weakened house. He found that the in about half of the cases, the extra damage was due to floating debris such as cars. In about 5% of such collisions, the house would collapse as a result.

How can the people living at the foot of the levee be protected? Korswagen mentions some measures in his recommendations. Firstly, strengthen the walls against earthquakes with abutments, connections to the floor slabs or steel braces. Also, the levee itself could be reinforced with, for example, sheet piles. And protection bollards between the houses and the street would keep floating debris away from the walls and thus protect the houses.

• Paul A. Korswagen Eguren, Structural Damage to Masonry Housing due to Earthquake-Flood Multi-hazards, master thesis, December 2016, Supervisor Professor Bas Jonkman (CiTG).