The entrance of the cave. Culvert-type sluices are used for sluices with higher water levels and smaller orifice sizes. The water flow through the sluice is controlled by the low water level. The entrance to the sluice is a culvert and a gate is installed at the entrance. It is also called a closed sluice or a culvert-type sluice. Culverts are small structures designed to allow surface water flow to cross a highway.