Spillway need; chute, shaft; ski-jump conditions

Why a spillway is provided

A reservoir cannot store unlimited flood water. A spillway passes the excess flood safely to the river downstream, so the dam is not overtopped or damaged (this matters most for an earth or rockfill dam, which would be breached by overtopping).

Conditions for a chute (trough) spillway

• The dam is of earth or rockfill type, so the spillway cannot be built over the dam body; it is a separate structure on a side abutment.
• The terrain has a gentle saddle or side-hill, so a long open channel on a firm foundation is economical.
• The foundation is good (rock or stiff soil), and the chute slope is steeper than the critical slope so that the flow stays supercritical.
• Large floods and wide valley sections, where the channel can easily be widened.

 reservoir  control weir
 ~~~~~~~~~~\___
              \___  chute (steep open channel)
                  \___  stilling basin
                       \______ river

Conditions for a shaft spillway

• Narrow valley with steep rock abutments, so there is no room for a chute or an overflow section.
• A diversion tunnel (built during construction) can be reused as the discharge tunnel, saving cost.
• Moderate floods, sound rock for the shaft and tunnel, and little floating debris (or trash booms are provided).

  ~~~~\____/~~~~
        |  |  vertical shaft
        |  |
        \__|____ horizontal tunnel to the river

Ski-jump energy dissipator

A ski-jump (flip) bucket is provided below a spillway when:

• the tail water depth is too low (much less than the sequent depth), so a hydraulic jump cannot form at the toe without a very deep and costly stilling basin;
• the dam is high and the water arrives with a large head and velocity, so the jet can be thrown well clear of the dam toe;
• the rock downstream is sound and erosion-resistant (massive, not heavily jointed), so the scour hole formed by the jet does not endanger the dam;
• the jet can land far enough from the dam, in a deep plunge pool or a wide river, where its energy is dissipated in the air and in the pool.