Rain water Vs bridges

Building of dams is done to store water for irrigation purpose and for hydro power generation which is quite expensive but has end returns. On the other hand in river beds water stoppers are used to store water during rainy season in order to increase ground water level. The later though may not be as expensive like dams but end result is only preservation of ground water.

On the other hand we build bridges across river bed and the primary aim is for transportation. Why can't we combine the idea of water stoppers / dams while building bridges. We normally build curved surface for the water to pass through instead raise the bottom to say 10 or 20 feet.

This will act as a small dam to store water may be in the rainy season but still it will in reducing the ground salinity.


Can this be made mandatory for all bridges that will be constructed now on.

Any takers?