Conditions Downstream of a Dam
The rise in the downstream level in heavy floods and its consequences need careful consideration. Certain spillways alter greatly the shape of the hydrograph downstream of a dam. The discharges from a siphon spillway may have surges and break-ups as priming and depriming occurs. This gives rise to the wave travelling downstream in the river, which may be detrimental to navigation and fishing and may also cause damage to population and developed areas downstream.
Nature and Amount of Solid Materials Brought by the River Trees, floating debris, sediment in suspension, etc, affect the type of spillway to be provided. A siphon spillway cannot be successful if the inflow brings too much floating materials. Where big trees come as floating materials, the chute or ogee spillway remains the common choice.
Apart from the above, each spillway can be shown as having certain specific advantages under particular site conditions. These are listed below which might be helpful to decide which spillway to choose for a particular project.
It is most commonly used with gravity dams. However, it is also used with earth and rockfill dams with a separate gravity structure; the ogee crest can be used as control in almost all types of spillways; and it has got the advantage over other spillways for its high discharging efficiency.
It can be provided on any type of foundation,
It is commonly used with the earth and rockfill dams, and
It becomes economical if earth received from spillway excavation is used in dam construction.
The following factors limit its adaption:
It should normally be avoided on embankments;
Availability of space is essential for keeping the spillway basins away from the dam paving; and
If it is necessary to provide too many bends in the chute because of the topography, its hydraulic performance can be adversely affected.
Side Channel Spillways
This type of spillway is preferred where a long overflow crest is desired in order to limit the intensity of discharge, It is useful where the abutments are steep, and it is useful where the control is desired by the narrow side channel.
The factor limiting its adoption is that this type of spillway is hydraulically less efficient.
Shaft Spillways (Morning Glory Spillway)
This can be adopted very advantageously in dam sites in narrow canyons, and
Minimum discharging capacity is attained at relatively low heads. This characteristic makes the spillway ideal where the maximum spillway outflow is to be limited. This characteristic becomes undesirable where a discharge more than the design capacity is to be passed. So, it can be used as a service spillway in conjunction with an emergency spillway.
The factor limiting its adoption is the difficulty of air-entrainment in a shaft, which may escape in bursts causing an undesirable surging.
Siphon spillways can be used to discharge full capacity discharges, at relatively low heads, and great advantage of this type of spillway is its positive and automatic operation without mechanical devices and moving parts.
The following factors limit the adoption of a siphon spillway:
It is difficult to handle flows materially greater than designed capacity, even if the reservoir head exceeds the design level; Siphon spillways cannot pass debris, ice, etc; There is possibility of clogging of the siphon passage way and breaking of siphon vents with logs and debris; In cold climates, there can be freezing inside the inlet and air vents of the siphon; When sudden surges occur and outflow stops; The structure is subject to heavy vibrations during its operation needing strong foundations; and Siphons cannot be normally used for vacuum heads higher than 8 m and there is danger of cavitation damage.
Overfall or Free Fall Spillway
This is suitable for arch dams or dams with downstream vertical faces; and this is suitable for small drops and for passing any occasional flood.
Tunnel or Conduit Spillway
This type is generally suitable for dams in narrow valleys, where overflow spillways cannot be located without risk and good sites are not available for a saddle spillway. In such cases, diversion tunnels used for construction can be modified to work as tunnel spillways. In case of embankment dams, diversion tunnels used during construction may usefully be adopted. Where there is danger to open channels from snow or rock slides, tunnel spillways are useful.