Your parsimonious EC members are treading on dangerous ground. If they willfully ignore professional advice and that results in damage to common property (or, indeed, private property) even if they had EC insurance it would probably be invalidated by their negligence.
You and the other affected neighbours need to get an official report done by a qualified hydraulic engineer then take your EC through Fair Trading and the CTTT where it will be explained to them that they have an unlimited legal responsibility to properly maintain common property.
If you're feeling bold you could even take them to the Supreme Court on the grounds that you want damages (as well as having the problem fixed) and the CTTT doesn't award damages.
Also, if your sinking fund doesn't have a spare $2k to fix this properly, you may be in even bigger trouble than just flooding.