The Chao Phraya river is supposedly flowing at 4,000 m3/sec at the moment. Lets say the northern floop barriers are breached and there is a net inflow (inflow - outflow) of 1,000 m3/sec to the great Bangkok area. The east-west distance between floodwalls are approximately 30 km.

A 0.5 m deep flood will then propagate:

1,000 m3/s /(30,000 m x 0.5 m) = 240 m/hour ~ 6 km/day

Also the greater Bangkok area is about 1,500 km2, so it will take:

1.5x10^9 x 1 m /1,000 m3/s = 17 days to flood to a level of 1 m.

Of course, lower areas will be hit first and hardest.

Similar it will take a long time before the city dries up again!

Taksin pier is taking water as we speak, nothing major ....yet.

I think the fear is a two prong threat:

1. as you describe

2. with khlongs etc. full, and a high tide coming in, if it rains heavily then the surface water cannot escape. so there could be "flash floods" as it were, that will precede the main flood.

I do like your maths however