Collapsing bridges

“These floods were of the kind that occurs once every two thousand years”, says bridge expert, Wilfred Molenaar, of the faculty of Civil Engineering & Geosciences’ hydraulic engineering section. Molenaar, who followed the events via internet, is not surprised that some bridges constructed only decades ago collapsed after such heavy rainfall: “One of the recent storm days became the wettest on record. This huge amount of rain alone would have been enough to result in a dangerous flood wave through the river system. Another contributing factor may have been an increasing area of built-up land during the last decennia. Built-up land causes rain water to run off the land into the rivers at a much faster rate. If such a storm had occurred in the past, the water would’ve reached the rivers more gradually.” 

“A once every two thousand years weather event would also cause damage in Holland”, Molenaar says. “In Holland we accept for instance that dikes along the IJssel will fail during extreme events that statistically only happen once every 1,250 years.”

Molenaar thinks that, at the time of their design and construction, a considerable number of the bridges both in England and the Netherlands were not meant to cope with extreme weather events: “Even a century ago bridges were still built to resist the load of storms that people could remember having happened. No statistics came into play.”
Judging from the television images, the bridges in Cumbria seem to have been constructed a fairly long time ago. Maintenance and upgrading of (old) bridges generally is the responsibility of governmental bodies that must  work with tight budgets. The same goes for the Netherlands. “But”, Molenaar adds, “fortunately for good economic transport reasons the vast majority of bridges here have been modernised in the recent decades, or at least they’ve been kept up to standard.”

And the Dutch are also lucky in another way. “In the Netherlands, most bridges, if not all, were built on pile foundations that extend deep into the ground, which is a necessity when building on soft soils”, Molenaar explains. “It looks as if the collapsed bridges in Cumbria had their piers on a shallow foundation. That makes the old English bridges more susceptible to erosion than old Dutch bridges.”

Erosion occurring at the base of the bridges’ arches  probably caused a lot of damage, Molenaar believes: “A bridge collapses when the foundation is eroded away. Moreover, the surface of the piers – the vertical parts of the bridge – that the water pushes against becomes bigger and this creates more momentum.”
Bridges that weren’t high enough to remain above Friday’s rising flood waters had an even more difficult time. Molenaar: “The water level rose so high that at certain places it also pushed against the bridge deck.”

Docent Martijn Wackers is met negen fouten de winnaar van het TB-dictee. Afgelopen dinsdag streden 52 studenten en medewerkers van TBM om de titel van beste speller van 2009. Docent Ivo Bouwmans en student Kasper Kisjes werden met twaalf en veertien fouten respectievelijk tweede en derde. In totaal maakten de deelnemers 1565 fouten. Student Tristan de Wildt was verantwoordelijk voor 72 van deze gemaakte fouten en kreeg hiervoor de poedelprijs. Het dictee werd voorgelezen door nieuwslezer Jeroen Overbeek.