from becoming ice-bound. In some cases, bell-mouth spillways are gate-controlled. The spillway at Hungry Horse Dam (pictured), in Montana, U.S., the highest morning glory structure in the world, is controlled by a ring gate. One of the most well-known of these spillways is the one in Covão dos Conchos reservoir lake, in Portugal, which is constructed to look like a natural formation; a video of this went viral on the Internet in early 2016. The largest bell-mouth spillway is in Geehi Dam, in New South Wales, Australia, measuring in diameter at the lake's surface. Siphon spillway A siphon makes use of the difference in the height between the intake and the outlet to create a pressure difference needed to remove excess water. Siphons, however, require priming or the removal of air in the bend for them to function, and most siphon spillways are designed with a system that makes use of water to remove the air and automatically prime the siphon. One such design is the volute siphon, which makes use of water forced into a spiral vortex by volutes or fins on a funnel that draw air out of the system. The priming happens automatically when the water level rises above the inlets used to drive the priming process. Other types Other spillway types include an ogee crest, which over-tops a dam, a side channel that wraps around the topography of a dam, and a labyrinth, which uses a zig-zag design to increase the sill length for a thinner design and increased discharge. Also, a drop inlet, which resembles an intake for a hydroelectric power plant, transfers water from behind the dam directly through tunnels to the river downstream. Design considerations One parameter of spillway design is the largest flood it is designed to handle. The structures must safely withstand the appropriate spillway design flood (SDF), sometimes called the inflow design flood (IDF). The magnitude of the SDF may be set by dam safety guidelines, based on the size of the structure and the potential loss of human life or property downstream. The magnitude of the flood is sometimes expressed as a return period. A 100-year recurrence interval is the flood magnitude expected to be exceeded on the average of once in 100 years. This parameter may also be expressed as an exceedance frequency with a 1% chance of being exceeded in any given year. The volume of water