How hurricanes damage highways:
the "weir-flow-damage mechanism"

This photograph shows damage to a coastal road after a hurricane that is somewhat paradoxical. The pavement is partially undermined on the side away from the ocean! This photo was taken near the Florida/Alabama border after Hurricane Ivan, but this scenario is typical and has been seen at other places in the southeastern US after other hurricanes.

The likely damage mechanism is water flowing across the road as the storm surge rises. The roadway pavement acts like a broad-crested weir and flow down the back shoulder of the road is likely high-velocity, super-critical flow down to a hydraulic jump. The high velocities scour the sand shoulders and undermine the pavement. Essentially, the roadway is elevated above the adjacent ground acting like a dam to the storm surge with this back side acting like a spillway. This scour mechanism may be exacerbated by broken waves periodically pumping water across the roadway.

We are calling this the "weir-flow-damage mechanism."
To see some preliminary results of some full-scale laboratory experiments of this failure mechanism, click here.
To see some recommended design countermeasures for coastal roads subjected to severe overwash, click here.

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