A spillway is essentially a safety valve for dam and must be properly designed. It should have adequate capacity to dispose off entire surplus water from upstream to downstream at time of arrival of worst design flood.

Maximum conservation level is nothing but normal pool level which is maximum elevation to which reservoir water surface will rise during normal operating conditions. This is equivalent to elevation of spillway crest or top of spillway gates, such uncontrolled spillways are only guided by available water head.

Straight drop weir or Overfall spillway is a low weir and simple vertical fall type structure. It is simplest type of spillway and can be constructed on small bunds, thin arches, etc. The downstream face of spillway may be kept vertical or slightly inclined.

The spillway can be best built independently of dam where re is a deep narrow gorge with steep banks separated from a flank by a hillock with its level above top of dam. A concrete or an earn dam can be constructed across main valley and a spillway can be constructed independently into saddle under such circumstances.

If at worst condition water rises above maximum reservoir level it will overtop such an embankment. A secondary safety arrangement known as subsidiary dam or emergency spillway or breaching section is generally provided on large dams for disposal of excess water, especially on earn and rock-fill dams.

It is an improvement over free over-fall spillway. It is mostly suitable for concrete gravity dams especially when spillway is located within body of dam in same valley.

The lower nappe of falling jet may leave ogee profile when operating head of spillway is more than design head. This generates negative pressure at point of separation which leads to formation of cavities in water resulting in cavitation.

An ogee spillway is designed for maximum head up to RL of maximum reservoir level; re are almost no chances of operating head exceeding design head. Under rare emergency as in excessive floods, it may become necessary to allow reservoir level to go even above MRL in such case operating head may exceed design head.

Ogee spillway is an improvement over free overflow spillway and is widely used with concrete, arch and buttress dam. The profile is made such that shape of lower nappe of a freely falling jet over a duly ventilated sharp-crested weir.

If head of water over spillway is less than design head, falling jet would adhere to crest of ogee spillway. This causes positive hydrostatic pressure and reby reduces discharge coefficient of weir.