Question

what is a reason for flooding in chennai.

Answer 1

Tamilnadu is a highly industrialised state and in future it will become a major IT hub surpasssing the Silicon valley. Lets take the case of Old Mahabalipuram Road(OMR ). This stretch has n number of IT companies which came up in the past 5- 10 years. Before which it consisted of forests and lakes. With the govt earmarkings of certain areas as SEZ(Special Economic Zone) , IT companies started cashing in along with realtors buying huge plots of land around SEZ for minimal price including lakes/low lying areas. Apartments /leased office building were erected encroaching lakes and ponds with minimal planning for water outflow. This resulted in water getting clogged and ultimately raising water level. 
Secondly , condition was aggravated when Cherumbakkam reservoirs water were released at a stretch which led to unprecedented increase in water level.

May be Chennai was unaware of the limitations behind development. Well the nature has taught us a lesson and bought us together. 

Answer 2

plotting everywhere where water can be stored is the most important reason.building buildings improperly for one's own wish its another reason.