Due to the location of the standard air intake on ALL 406s they seem to be very liable to sucking up water, even with low levels of flooding. Water does not compress like an air/fuel mixture and the likely result is bent conrods or even a cracked block.
DO NOT DRIVE AT SPEED - this probably makes the situation worse as the car can build up a bow wave.
DO NOT GUN THE ENGINE - what are you, nuts?

You have been warned!
See:
viewtopic.php?f=17&t=8799&p=83098&hilit=+drill#p83098
viewtopic.php?f=5&t=3769&start=15&hilit=drill
viewtopic.php?f=5&t=18355&p=182463&hili ... pe#p182463
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A swift How To - how to remove the link pipe from below the air filter (note - this is on my 12-2003 built 2.0 HDi 110 so I'm not sure if all 406s are the same):
Tools required - your hands and possibly a torch.
Time required - 5 minutes (maybe more, depending on your tolerance to rain dripping down your neck)
Start off by moving the air filter box out of the way. It's held in place by a little clip underneath, push this tang towards the back of the car and pull the front of the air filter box upwards. Be careful not to overstretch the MAF sensor wires:

The top of the pipe is slotted into the air filter box bracket, pull this little clip down and slide the pipe towards the front of the car:

The pipe will now need disconnecting from the feed pipe where it comes through the inner wing. It's reluctant but it will come, just try and keep it straight:

The pipe removed:

Once it's out you need to replace the air filter box. Obviously it now lacks the lover clip but if you slot it back on the holder it's not going to go anywhere:

The holes in the bottom of my pipe - enough to stop the engine drawing in water? No idea:

Possibly not a permanent solution but it might get someone out of trouble.