What historical data from Tokyo could have helped in the development of tokyo's new flood protection system?

This historical data could have helped with the development of Tokyo’s new flood protection system:

historical frequency of flooding

amount of flooding during each storm

computer models forecasting the worst possible flooding

Explanation:

The frequency of flooding can help Tokyo measure when the next flood will be.

The amount of flooding can help Tokyo determine how much water will come during the flood.

And computer models forecasting the worst possible flood can help reach out to people and tell whether they need to evacuate or what  they should plan for.    

the answer is na (sodium)

the maximum mass of nh3 is nh3 in grams