bell-mouth spillway; morning glory spillway; glory hole spillway
Colloquial term for a bell-mouth spillway used in dams. The English gloss 'glory hole spillway' is a technical term and may be unfamiliar to general learners.
A bell-mouth spillway is a facility for releasing water when the reservoir level gets too high.
The bell-mouth spillway of that dam creates a large whirlpool when viewed from above.
Compound of ダム (dam) and 穴 (hole), literally 'dam hole'. The term is a colloquial description of the bell-mouth spillway's appearance.