Capt Claret and Worrals - There won't be subsidence in the runway sub-base... the preparatory work and the substantial compaction levels when the runway was put down, would ensure that no subsidence would have taken place.
Remember, a runway has to take serious impact at speed when a heavy touches down a little too heavily... so runway compaction is at substantial compaction levels, to serious depths (around 600-900mm).
After a flood event like this, the serious problem, is separation of the bond between the runway base material, and the seal. You've seen it on the damaged roads on the news shots.
The bitumen lifts away from the road-base in a sheet, or in big bubbles.
This means the damaged sections have to be cut out... the base re-worked with new road-base material, the base re-compacted, and new seal (bitumen or hot-mix) laid down.
A close visual inspection generally reveals damage. Unfortunately, hot-mix or bitumen will only resist rainwater to a modest depth, and water that drains off... that doesn't hang around.
When a sealed surface is immersed in a depth of water that just lays there, and doesn't drain, the water finds its way into the area under the seal (mostly via small cracks) and commences to separate the bond between the seal and the base.
A deep layer of hot mix (50mm) is more impervious to water ingress, than a few coats of bitumen seal.
This breakdown of the seal bond only becomes evident after the surface is trafficked (if not picked up by visual inspection), and the bitumen or hot mix proceeds to break up, under the impact of rolling wheels.