💡 Detailed Explanation
"～を余儀なくされる" means to be forced into a situation where one has no choice but to do something, usually against their will, due to circumstances. In this context, the external factor of heavy rain forced the action of an "emergency discharge," making this the most appropriate choice. "をものともせず" means to defy difficulties. "を禁じ得ず" means to be unable to suppress an emotion. "をよそに" means to ignore or disregard something. None of these fit the context.