Question

how was the drainage of earliest city planned

Answer 1

The Indus civilization had an elaborate sanitary and drainage system, the hallmark of ancient Indus cities., The Authorities maintained a highly efficient drainage system. 

Each house had horizontal and vertical drains. 
There were underground drains for the streets. 
These drains were covered by stone slabs. 
The soak pits were made of bricks. 
The house drains were connected with road drains.

Each and every house had a connection with the main drain. These even had inspection holes for maintenance. The conduits to the main drains running through the middle of the streets below pavement level and covered with flat stones and sturdy tile bricks. The covered drain was connected to the larger sewerage outlets which finally led the dirty water outside the populated areas. The urban plan found in these cities included the world’s first urban sanitation systems. The elaborate brick-lined drainage system for the removal of rainwater is of unparalleled engineering skill.

According to D. D. Kosambi, the drainage plans of the Indus cities definitely establish the separate identity or independent character of the Indus civilization. No ancient civilization before the Roman civilization had such an advanced drainage and sanitation system.