noun
flood bypass; floodway; discharge channel; diversion channel
Civil engineering term for a channel or waterway built to divert excess water, especially during floods.
大雨のとき、放水路がなければ町は水浸しになる。
During heavy rain, the town would be flooded without the diversion channel.
この放水路は川の氾濫を防ぐために作られた。
This floodway was built to prevent the river from overflowing.
水路 is a broad term for any waterway or canal; 放水路 is a specific type designed for flood control.
Compound of 放 (release, discharge) and 水路 (waterway, channel).