咸水壓力

I am going to design a flushing system retrofitting to an existing building (~15m tall). Where can I look for the following information which I can write up the proposal to WSD?

1. Location of salt water pipe around the building
2. Pressure of salt water - so that i can determine whether the pressure is enough to support direct feed to the roof tank or a sump tank / pump is required at the ground floor.

Normally, I believe that direct feed to the roof tank should be OK.