This is the 5001st most frequent Russian word.
ливневой
"Ливневой" means "stormwater" or "related to heavy rain or downpour."
Here, 'ливневой' describes a rain that is torrential.
Ливневой дождь начался внезапно.
The heavy rain began suddenly.
In this context, 'ливневой' modifies 'канализацию' referring to infrastructure designed for heavy rain.
Для предотвращения затоплений необходимо установить ливневую канализацию.
To prevent flooding, a storm drain system needs to be installed.
Here, 'ливневой' is used metaphorically to describe darkness as being intense as during a downpour.