November 23, 2009

expert reaction to bridge collapsing in Cumbria’s floods

The bridge collapse occurred during severe flooding in the north-west of England, following several days of heavy rainfall.

Prof Roger Falconer, Cardiff University, Fellow of the Royal Academy of Engineering, said:

“The key problem that causes failure of bridges in this situation is scour, or erosion, on the upstream side of the bridge piers. Water flowing down the river creates a vertical vortex that scours out the material at the bottom of the upstream side of the piers, weakening the foundations. This can be difficult to detect on inspection after the storm has receded as sand tends to settle back into the eroded section. After such extreme rainfall and heavy river flow the vortex is much bigger and erosion much deeper, destabilising the bridge to the point of collapse.

“The solution is to protect bridge piers with large blocks or ‘riprap’ on the upstream side to disrupt the vortex. Where bridges need to be replaced then single span bridges will remove the problem, and have the added advantage of enabling flood debris to pass down the river more easily.”