It is not the excess rainfall, but it is the developments which have taken place for the last 18 years, is causing flooding in the township. Construction of buildings, roads, etc., have reduced the natural seepage of rainwater through the soil profile and enhanced the surface runoff, thereby causing flooding in the area. Now based on the rainfall intensity in the peak rainfall season, you have to workout the runoff amount and design the drainage system.

The existing drainage network should be cleaned regularly and extended to receive the maximum flowrate if possible.

There is much information developed on stormwater management that could be helpful, but there are also typically constraints at trying to mitigate or ameliorate these issues within developed or urban areas.  It may also be appropriate to look at watershed condition and stream function in handling both water and sediment.  Serious plans and designs should be reviewed by licensed civil engineer, landscape architect and / or hydrologist as substantial expense and liability may be involved.

Dear Karma Sherab,

As already stated you need to figure out the runoff quantity. Another thing you need to know the per capita waste produced in the city, its growing population, and its future to design the additional drainage systems. one thing can be tried at every home is to recycle the water and use it for gardening and other less important activities in order to reduce the amount of waste water generation. Effective rain water harvesting can also solve recharge the aquifer and prevent the excess water in drainage.

Proper maintenance of the drains and drainage system may prevent the clogging in the drains.

 A separate system for flood water drain should be designed and practiced for effective magnagement of floods. 

Nowadays UAVs provide very high-resolution images which can be used to construct point clouds. With point clouds, you can develop Digital Surface Model (DSM), digital elevation model (DEM). using these recent datasets you can assess the local drainage patterns of the Town. From drainage patterns you can assess where the drainage needs to be altered so that the drainage-related problems can be fixed. furthermore, you can use VHR images to identify the key drainage structures in the area of interest (AOI) which can be further investigated, how well they are working.