Richard, it's Bangkok and areas to the north which are affected. The international airport has 3.5 meter high flood barriers around it, six pumping stations and reseervoirs and canals to control flooding. The authorities say they are 99.9% sure the airport will not flood. The second airport, Don Muang, does not have these protections and IS floded and closed.
There is no significant flooding in Pattaya (it is the end of the monsoon so there's always some localised flooding especially after heavy downpours).
The reason Bangkok is flooded is because the Chao Phraya river runs through it. The Chao Phraya and the Mekong are the drainage systems fo most of south east Asia.
It's always a good idea to check with your airline, tour provider and hotel just to be on the safe side. You should be fine.
Phil about 6 years ago