Appurtenant Features Of Dams

Appurtenances are structures and equipment on a project site, other than the dam itself. They include, but are not limited to, such facilities as intake towers, powerhouse structures, tunnels, canals, penstocks, low-level outlets, surge tanks and towers, gate hoist mechanisms and their supporting struc tures, and all critical water control and release facilities. Also included are mechanical and electrical control and standby power supply equipment located in the powerhouse or in the remote control centres.

In previous sections of this chapter the characteristics and configurations of basic types of dams have been outlined, but no consideration was given to the various appurtenant features that enable use of a dam and the reservoir behind it for their intended purposes. In this section mention is made of various features that are incorporated into the designs of dams for control of flow of water impounded in the reservoir through or outside of a dam. Design of many of these features requires intensive prior investigations of hydrology, topography, and subsurface geology of the dam site. Following is a tabulation and description of several kinds of appurtenant features included in the construction of dams.

Coffer dams usually are temporary structures built upstream from a dam to divert stream flow around the excavation for a dam. In valleys of steep profile diversion commonly is accomplished by a tunnel or tunnels in the walls of the valley. Commonly the diversion tunnels are put to further use to control flow from reservoir either for drainage of the reservoir or for flow under pressure into a hydroelectric generating plant. In valleys of low profile diversion is by tunnels, canals, or by conduits which subsequently are buried by the dam. It is not unusual in embankment dams to incorporate the coffer dam into the larger embankment structure comprising the designed dam.

Tunnels in bedrock outside of dams serve a variety of purposes. Flow through them is controlled by valves external to the dam or in valve chambers or vaults within the dam or in bedrock outside of the dam. Tunnels for control of the water level in the reservoir are commonly called gravity tunnel and serve a principal function in diverting water to some point downstream from the dam. Tunnels that transmit water under pressure to elevate the water to a higher level than the intake of the tunnel or to generate hydroelectric power are called pressure tunnel and usually require considerable competency in the rock through which they are constructed . Valves control the flow of water through tunnels and penstocks. In many large dams the valves are installed in underground vaults or chambers to which access is gained downstream from the dam.

Many dams are constructed to generate hydroelectric power. The powerhouse is located at, or in the vicinity of, the toe of a dam or at some distance downstream. Flow of water into the power house is controlled by valves upstream from the dam, within the dam downstream, or in valve vaults excavated in rock outside of the dam.

A sluice is a passage through the dam itself for lowering the water level of the reservoir. Pipes for conducting water to the power turbines are called penstocks. The flow of water from intake towers through spillways, sluices, and penstocks is regulated by control gates.

A Spillway is designed to contain and control overflow of reservoir water when the reservoir is full. Spillways are, or should be, designed to accommodate flows during maximum flood stage so as to prevent damage to the dam and appurtenant features. Their size and location with respect to the dam is determined by the size and type of dam, local topography geology and a careful review of the history of stream flow at the site of the dam. Water may pass over the crest of the dam itself, or near the dam in chutes, tunnels, or shafts. Overflow of embankment dams outside of a spillway can have especially disastrous consequences so that safety usually requires a spillway capable of containing at least a hundred-year flood. Spillways are located within or on the downstream face of a dam outside of the dam on one side or the other, or within the reservoir, where water spills into a " glory hole" and passes through a shaft and tunnel or tunnels in the abutment of the dam.

Gates are devices installed in the tops of spillways to control the flow of water over the spillway.

Levees are artificial riverbanks constructed high enough to prevent flooding. A dam across a river intended to permit flow, once a certain depth of water has been reached, may be called a barrage. A small dam that forms a millpond or fishpond is called a weir.

0 0

Post a comment