Spillway definition; siphon and shaft spillway

Definition

A spillway is a hydraulic structure, usually part of a dam, that passes the surplus flood water from the reservoir to the downstream river without damaging the dam or the reservoir.

Siphon spillway

It is a closed conduit shaped like an inverted U, with its crest at the normal reservoir level, its inlet below the water surface and its outlet on the downstream side. Discharge starts when the reservoir rises above the crest: the water spilling over the crest seals the outlet and carries the air out of the bend, and once the air is removed the siphon primes. The conduit then flows full under a head equal to the difference between the reservoir level and the outlet (tail water) level.

        air vent
          |   ___crest
     ~~~~~|__/   \__
 res.     /         \
 ~~~~~~~~/  inlet    \ throat
 ========            \_______
                     outlet (sealed in tail pool)

• Priming is automatic, and the siphon breaks when the reservoir drops (air enters through the vent), so it regulates itself and keeps a nearly constant reservoir level.
• Large capacity in a small space with a small rise in head; no gates are needed.
• Disadvantages: limited capacity for very large floods, clogging by floating debris, and vibration or cavitation when it is not fully primed.

Shaft (morning-glory) spillway

Water spills over a circular overflow crest (a funnel-shaped intake) into a vertical or inclined shaft, and then through a horizontal or sloping tunnel to the river below the dam.

        crest ring
      ~~~~\______/~~~~    reservoir
          |    |
          | shaft |       elbow
          |    |
          \____|_________  ==> tunnel to river

• At low head it acts as a ring weir ($Q\propto H^{3/2}$); at higher head the crest is drowned and it behaves like an orifice or full-pipe flow, so the discharge then rises only slowly.
• Suitable for narrow gorges and steep rock abutments, and where a diversion tunnel can be reused.
• Needs anti-vortex piers and air vents to avoid cavitation and pulsation.