Not to dwell on the flooding thing...

Yet another condo block is going in near our apt. This will be located very near a tiny klong which is of no use other than drainage. Virtually on top of it. Ive seen many just as close as well...

We were not in Thailand last year during the big floods, traveling in SW USA. Despite being sort of nearish a tiny water outlet (400m), we were told our bldg had little in any flooding.

My questions:

When flooding occured in BKK did the klongs provide faster drainage OR

Increased flooding due to spillover/overflow, but areas drained very slowly as water flooded areas into lowpoints and stood, did not drain.

Or possibly both?

So is living near a tiny klong a plus or minus?

I would personally not worry to much about flooding. Not much you can do about it and a small klong does not really make a difference unless it is connected to a bigger waterway really close.

I would worry about the smell and pollution. These small klongs often have stillstanding water and they collect a lot of unwanted stuff over the years.

In answer to the OP's questions, you may want to research the government's actions following last year's flood; the government implemented a significant dredging program to clean waterways; the initial survey discovered many embarrassing errors such as some properties having been built without planning permission actually blocking the flow of water in some cases. Depending on the success of this government's actions, any future flood may have a different impact than last year's one.

Seeing that the 2011 floods were from rains up north rather than in Bangkok (which is where I assume you are), most of the flooding seen in Bangkok and surrounding provinces were due to the overflow of klongs and rivers.

During particularly heavy rain, the klong can be both good and bad, draining the water from surrounding streets until it gets too full and then flows back. But you could be a fair way from a klong and the heavy rain could flood the local area until it eventually drains off.

Bottom line: if you're worried about flooding, live somewhere where it doesn't rain so much.