Skip to main content

Spillway types suitable for Nepalese rivers

Short answer

Five spillway types: ogee, chute, side channel, shaft (morning glory), siphon. For Nepal's steep, sediment-laden rivers with high monsoon floods and narrow valleys, a gated ogee spillway on a concrete dam/weir with large undersluices is most suitable. It provides high flood capacity, passes sediment and debris, enables reservoir flushing, suits rocky foundations, and integrates with stilling basins or ski-jump buckets. Chute spillways are used for earth/rockfill dams; siphon and shaft types are unsuitable due to debris clogging.

List out any five types of spillway used in hydropower projects in the dam reservoir system. Explain briefly the most suitable type of spillway in the context of Nepalese river site.

Five types of spillways

  1. Ogee (overflow) spillway – crest shaped to the lower nappe; on the dam body itself.
  2. Chute (trough) spillway – short crest followed by a steep open channel to the river.
  3. Side channel spillway – crest parallel to a trough that leads to a chute or tunnel.
  4. Shaft (morning glory) spillway – funnel crest, vertical shaft and horizontal tunnel.
  5. Siphon spillway – inverted-U closed duct that primes automatically. (Others: free overfall, tunnel/conduit, labyrinth, stepped spillways.)

Most suitable type for Nepalese river sites

Nepalese rivers are steep, carry very high monsoon floods (often 10–50 times the dry-season flow), and transport huge loads of sediment, boulders, trees and debris. Valleys are narrow and deep, and many sites have sound rock but fragile slopes and high seismicity.

For these conditions a gated ogee (overflow) spillway on a concrete dam/weir, combined with large undersluices (low-level orifice spillways) is generally the most suitable choice, as used at the Kali Gandaki A, Middle Marsyangdi and Upper Tamakoshi headworks.

Reasons:

  1. Large flood capacity: an ogee crest with radial gates can pass very large floods with a small rise in reservoir level, and it fits in a narrow gorge.
  2. Passes sediment and debris: wide open gated bays and low-level sluices can be fully opened during floods to flush sediment and floating debris through the reservoir; enclosed types (siphon, shaft) choke with debris and logs.
  3. Reservoir sediment flushing: low-level gates allow drawdown flushing, essential for keeping storage of small reservoirs in Nepal.
  4. Concrete structure on rock: suitable for concrete gravity dams and barrages built on the rocky foundations of Himalayan gorges.
  5. Energy dissipation: the ogee profile is easily combined with a stilling basin, ski-jump (flip) bucket or roller bucket, suited to the high tailwater variation.

Where an earth or rockfill dam is built, a chute spillway on one abutment, or a side channel spillway in a narrow valley, is preferred, because water cannot flow over the dam body. Siphon and shaft spillways are generally not suitable in Nepal because of floating debris and sediment.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Hydro Power chapter 6 questions →