Process of Flushing Sand Accumulated under Gates on a Dam by Sluice Outlet

In some new diversion dams, sand and gravel were accumulated under gates by floods. But occasionally accumulated sand is flushed by sluice outlet at closing gates after flood. Analysis of this process by two methods (field surveys and flume experiments) clarified as follows:(1) To flush accmulated sand, the difference of water levels at upstream and downstream of a gate should be five times as large as the maximum size of sand or gravel.(2) In a dam on a sand-bed river, flushing sand by sluice outlet is so easy to occur that controllable gates are easily closed after flood.(3) In a dam on a gravel-bed river, gravel flushing is difficult. Therefore, even if gravel can be flushed, gates will get damaged by collision of gravel.