Is there any region of Thailand that has not risk of flooding?
Yes the north, close to the Myanamar border is very mountainous and that chain extends down the Malay peninsula to southern Thailand. These mountains are 500 to 1000 metres, so there IS high ground, but that doesn't mean it's always safe from flooding.
From June to October the south west monsoon brings heavy rain to the whole country. When these downpours occur they can cause localised flooding in any place. You may recall Koh Samui was hit badly last year (it's not so bad this year). Even in the mountains people tend to live in the valleys, so they're affected by rivers rising quickly.
The problem in Bangkok at the moment comes about because the north of Thailand is a huge plateau which is susceptible to flooding (which is why it's a major rice growing area), and the water runs down either the Chao Phraya delta (into Bangkok) or the Mekong. It floods to some extent at this time every year, it's particularly bad this time. But by mid-to-late November it should all have drained away.
It's a terrible tragedy for the Thai people who have been flooded out (and for the 500+ who have lost their lives in the region) and our thoughts are with them. about 5 years ago
Oh, there are many: most of Isan http://thailand.e-travel-to.com/thailand-regions-north-east.htm or Hua Hin (at sea level but not much rain and no big rivers) http://www.hua-hin-travel.com. The North http://thailand.e-travel-to.com/thailand-regions-north.htm especially Mae Hong Son, Pai. about 5 years ago