There are/were huge massive flood drains in Karachi - which have been closed out (there was a jurassic sized flood drain where the clifton underpass is now). The whole of North Nazimabad's 4 lane highway strech is lined with 6x6ft flood drains on each side with a smaller one running in the middle of the green median. All residential areas of it have gardens and playgrounds placed where naturally water will flow these drains were usually on the corners of an inside corner angled house whose land plot was cut triangular - the rest of the triangle was a massively huge iron grid to stop filth from accumulating in the gardens/playgrounds. Most people have bribed the KBCA and KDA to reclaim the triangle cut to extend their house on it. Thats why some residential areas in the south belt of North Nazimabad get flooded like a river.
The Garden area floodgates are huge which empty into Lyari Naddi. But all are plugged up or even have something built on them. Similar case in the flood control near the Karachi museum, there is parking lot built on top of it.
Corruption to its toll - rather than enforcing proper building permits, some people less worthy than used toilet paper took some bribes and allowed this to go on.