Spillway of Llyn Brianne dam in Wales

A spillway is a structure used to provide for the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. Llyn Brianne is a man made Lake or reservoir in the headwaters of the River Tywi. A dam is a barrier that divides waters. Dams generally serve the primary purpose of retaining water while other structures such as Floodgates, Levees Dike (constructionEmbankmentA levee, levée, dike (or dyke) embankment, floodbank or stopbank is a natural or artificial Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. In contrast, an intake is a structure used to release water on a regular basis for water supply, hydroelectricity generation, etc. Hydroelectricity is electricity generated by Hydropower, ie the production of power through use of the gravitational force of falling water Floodgates and fuse plugs may be designed into spillways to regulate water flow and dam height. Floodgates are adjustable gates used to control water flow in reservoir, River, Stream, or Levee systems A fuse plug is a collapsible dam installed on Spillways in Dams to increase the dam's capacity Other uses of the term "spillway" include bypasses of dams or outlets of a channels used during highwater, and outlet channels carved through natural dams such as moraines. Moraine refers to any glacially formed accumulation of unconsolidated glacial debris (soil and rock which can occur in currently glaciated and formerly glaciated regions such as those

Types

Cross section of a spillway

A spillway is located at the top of the reservoir pool. A reservoir is most broadly a place or hollow vessel where Fluid is kept in Reserve, for later use Dams may also have bottom outlets with valves or gates which may be operated to release flood flow, and a few dams lack overflow spillways and rely entirely on bottom outlets.

There are two types of spillways: controlled and uncontrolled.

A controlled spillway has mechanical structures or gates to regulate the rate of flow. This design allows nearly the full height of the dam to be used for water storage year-round, and flood waters can be released as required by opening one or more gates.

An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway it begins to be released from the reservoir. The rate of discharge is controlled only by the depth of water within the reservoir. All of the storage volume in the reservoir above the spillway crest can be used only for the temporary storage of floodwater, and cannot be used as water supply storage because it is normally empty.

Inverted-bell spillways

Bell-mouth spillway of Ladybower Reservoir in operation. Ladybower Reservoir is a large Y-shaped reservoir, the lowest of three in the Upper Derwent Valley in Derbyshire, England.

Some spillways are designed like an inverted bell so that water can enter all around the perimeter. The inverted bell is a Metaphorical name for Geometric shape that resembles a bell upside down These uncontrolled spillway devices are also called morning glory, plughole, glory hole, or bell-mouth spillways. Morning glory is a common name for over 1000 Species of Flowering plants in the family Convolvulaceae, belonging to the following genera In areas where the surface of the reservoir may freeze, bell-mouth spillways are normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound.

Design considerations

The largest flood that needs be considered in the evaluation of a given project, regardless of whether a spillway is provided; i. e. , a given project should have structures capable of safely passing the appropriate spillway design flood (SDF). A 100-year recurrence interval is the flood magnitude expected to be exceeded on the average of once in 100 years. It may also be expressed as an exceedance frequency with a one per cent chance of being exceeded in any given year.

Safety

Spillway gates may operate suddenly without warning, under remote control. Trespassers within the spillway run the risk of drowning. Spillways are usually fenced and equipped with locked gates to prevent casual trespass within the structure. Warning signs, sirens, and other measures may be in place to warn users of the downstream area of sudden release of water. Operating protocols may require "cracking" a gate to release a small amount of water to warn persons downstream. Sudden closure of a spillway gate may result in stranding of fish.

External links

• More than 100, freely available, published research articles on spillway hydraulics and related topics by Professor Hubert Chanson, Civil Engineering Department, The University of Queensland
• Chris, Fish. The Glory Hole. - information, images, and construction information about the Lake Berryessa glory hole. Lake Berryessa is the largest Lake in Napa County California.

Bell-mouth spillway at Llyn Celyn	Ice-breaking rim on bell-mouth spillway	The huge spillway for the Robert-Bourassa Reservoir, Quebec, Canada, fittingly called the "Giant's Staircase". Llyn Celyn is a large Reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in North Wales. Robert-Bourassa is a hydroelectric generating station on the La Grande River that is part of Hydro-Québec 's James Bay Project.	Spillway gates control the level of Lake Diefenbaker, Saskatchewan, Canada
Lake Texoma, on the border of Oklahoma and Texas, USA, used its emergency spillway for the third time in July 2007. Lake Diefenbaker is a man-made Lake in Southern Saskatchewan, Canada. Saskatchewan (səˈskætʃəwən) is a prairie province in Canada, which has an area of 588276	Water flowing over the spillway of Lake Texoma was a big attraction in July, 2007.	Scrivener Dam, in Canberra, Australia, was engineered to withstand a once-in-5000-years flood event. Scrivener Dam, in Canberra, Australia, was engineered to withstand a once-in-5000 years flood Canberra ( is the capital city of Australia With a population of over 340000 it is Australia's largest inland City. For a topic outline on this subject see List of basic Australia topics.	The spillway for the Rivière des Prairies dam, in the north end of Montreal. The Rivière des Prairies (literally River of the Meadows, sometimes called the Back River in English is a delta channel of the Ottawa River