CLASSIFICATION OF SPILLWAYS BASED ON PURPOSE

CLASSIFICATION OF SPILLWAYS BASED ON PURPOSE

Main (or service) spillway: 
A main (or service) spillway is the spillway designed to pass a prefixed or the design flood. This spillway is necessary for all dams and in most of the dams, it is the only spillway. Therefore, in general terms, the spillway means the main spillway.

Auxiliary spillway: 
In some dams, where the site conditions are favorable, an auxiliary spillway is usually constructed in conjunction with a main spillway. In such a case, the main spillway is usually designed to pass floods which are likely to occur more frequently. When the floods exceed the designed capacity of the main spillway, the auxiliary spillway comes into operation and the total flood is passed by both the spillways. In that case, the capacity of the main spillway is kept less than that required for the design flood. An auxiliary spillway cannot be provided alone without the main spillway. The crest of the auxiliary spillway is kept higher than that of the main spillway. The auxiliary spillway therefore, comes into operation only after the flood for which the main spillway is designed   is exceeded. As already mentioned, the capacity of the main spil1way when an auxiliary spillway is also provided is kept less than that required for the design flood. Thus the total spillway capacity is equal to the sum of the capacities of the main and auxiliary spillways, Therefore,

Q = Q_m + Q_a

  Where,

 Q is the design flood,

 Q_m is the capacity of the main spillway and,

 Q_a is capacity of the auxiliary spillway.

 If no auxiliary spillway is provided then,

Q=Qm

Emergency spillway: An emergency spillway is sometimes provided in addition to the main spillway. It comes into operation only during an emergency which may arise at any time during the life of the dam. Thus an emergency spillway is an additional safety valve of the dam. The emergency may arise when such conditions occur that have not been anticipated and considered in the design of the main spillway. Some of the conditions which may lead to emergency are as follows:

        i.            When the actual flood exceeds the design flood.

      ii.            When there is an enforced shutdown of the outlets.

    iii.            When there is a malfunctioning of spillway gates.

    iv.            When there is damage or failure of some part of the main spillway

      v.            When a high flood occurs before the previous flood has been evacuated by the main spillway.

An emergency spillway is usually provided in a saddle or a depression along the reservoir rim or by excavating a channel through an abutment or a ridge. Because an emergency spillway is not required to function under normal reservoir operations, its crest is placed at or slightly above the design maximum water level in the reservoir. Thus an encroachment on the minimum free board is usually permitted for the design of an emergency spillway. The emergency spillway is generally in the form of a fuse plug or a breaching section which is washed out as soon as the water level in the reservoir reaches a predetermined elevation. The breaching section is sometimes called fuse plug spillway. Because a fuse plug is also sometimes provided as an auxiliary spillway, the following differences between the auxiliary and emergency spillways should be noted.