It is beacuse there is no proper drainage to drain water
if the water is drained only there wont be rain
so they must have proper drainage system to drain water

As the city lies on the banks of river, there would be fertile land which the people would use to cultivate. As the city has poor drainage system the rainwater during monsoon must be blocked and due to this blockage the water must settle on the land and the place must be a bit developed due to which the water must not able to seek in the grounds as the land must be cemented. So at the time of monsoon there must be fear of flood.