These strange holes that can be found in dams and reservoirs  are quite frightening to envisage but are also quite fascinating at the same time.  They conjure up nightmares of holes into a bottomless abyss that should fall in, you would never be seen again...well not seen alive  again maybe.

When dams and reservoirs fill up to capacity there is always the risk of the water overflowing, either into the valley below or deluging an adjacent field or village.  The solution to prevent this is the construction of big overflow holes called giant spillways that are built at the same time as the dam or reservoir.

A spillway is basically a giant concrete funnel that is flush with the level of the water so that the water can enter around its perimeter.  They are not regulated by any means, they simply sit awaiting for the water level to rise and then drain it off.

The biggest one in the world is located in the Monticello dam that was constructed between 1953 and 1957 to hold back the waters of Lake Berryessa.

The giant spillway is situated about 200 feet from the dam wall itself and has a diameter of 87 feet narrowing to 27 feet and it can drain off  367500 US Gallons per second.  When the spillway is in operation it is because the level of water in the dam has risen more than 15 feet.

Below at low tide

Below at high tide

There are warning signs located around the dam that prohibit swimming near the spillway, the spillway is also fenced off to stop the bold and inquisitive minded from venturing out for a closer look, marker buoys partition off a large circumference to safeguard small boats, dinghies and swimmers from accidentally getting too near.

When the spillway is draining off water the current is quite strong and if you got within several yards of it then you could get sucked in, and even though the aperture is quite large, there is the likelihood of drowning or getting severely injured before emerging at the other end.

The photo below shows the other end of the spillway with it rushing torrents of overflow dam water.

Bell-mouth spillways have ice breaker arrangements to restrict the spillway from freezing over into solid ice.  The Chaffey Dam in Tamworth, New South Wales in Australia probably has the the best example of this type of spillway.  It is interesting to note that it was also the first of its kind to be used in Australia.

Below is an image of a stepped bell-mouth spillway of Ladybower Reservoir in Derybshire United Kingdom

These spillways are very dangerous and if you do see one on a trip to a dam or a reservoir then remain behind the wire fence.