1. Unless the drainage system is improved, the streets will be flooded in the rainy season.

if+not=unless

Unless drainage systems are improved, streets will be flooded in the rainy season.

(If drainage systems are not improved, streets will be flooded in the rainy season.)

Unless drainage systems are improved, streets will NOT be flooded in the rainy season.

(If drainage systems are not improved, streets will NOT be flooded in the rainy season.)

The second sentence does not make any sense if only the speaker doesn't mean some systems that help to flood the city)