You can get a fairly comprehensive view of the Broadway/Westlink flooding from the perspective of
the contractor a local road fan Wesley Johnston from their his website.
16 August 2008
Broadway roundabout is unique among the five underpasses on the Westlink in that two live rivers run beside it - the Clowney Water a few metres to the west, and the Blackstaff adjacent to the east.
The problem is that these rivers are underground and therefore have a fixed capacity. Despite the construction of a large overflow chamber under Broadway roundabout, the fact that the design has now failed so disastrously just six weeks after opening suggests fundamental design flaws in terms of its ability to handle this type of heavy and persistent rain, that has become more frequent in recent years.
The author of the 17 August report comments:
I spoke to some contractors on the site. They insisted that there was no design fault with the underpass and that it had simply been overwhelmed by an extreme weather event. Certainly it would be hard to conceive of a pumping system or overflow chamber that could hold back 20 million gallons of water.
Still, with climate change now a reality, weather events like this are no longer uncommon I do feel it is valid to ask whether the design is at least "too optimistic" in terms of the amount of water likely to come down the Clowney and Blackstaff Rivers.
When the Broadway roundabout last flooded (December 2007) it was caused by the same river overflowing at the same spot. Only the fact that the underpass had not been excavated prevented a flood of this scale.
It could be that this is not the first time we see the Broadway underpass submerged in water and the ensuing chaos.