Weirs - Types & Components

Definition:

1. A solid obstruction put across river to raise its water level and divert water into canal (low head structure)

2. Vertical drop wall or crest wall

3. Upstream, downstream cut off wall at the ends of impervious floor

4. Launching apron for prevention of scour

5. Graduated inverted filter on downstream surface floor end to relieve the uplift pressure.

Types of Weirs

The two main types of weirs are:

1. Gravity weir
2. Non Gravity weir

Gravity weirs:

Uplift pressure due to the seepage of water below the floor is resisted by the weight of floor.

Its further types are:

1. Vertical drop weir
2. Masonry or concrete slope weir
3. Dry stone slope weir
4. Parabolic weir

Explanation:

1. Vertical drop weir
   • Vertical drop weir or crest wall
   • Upstream and downstream cut off wall at the end of impervious floor.
   • Launching apron for scouring prevention
   • Graded inverted filter at downstream floor end to relieve the uplift pressure.
2. Masonry or concrete slope weir
   • Suitable for soft sandy foundation
   • Generally used where the difference in weir crest and downstream river is limited to 3m.
   • Hydraulic jump is formed on sloping crest.
3. Dry stone slope weir
   • Body wall or weir wall
   • Upstream and downstream rock fill laid in form of glacis, with few intervening care walls.
4. Parabolic weir
   • Similar to spillway section of a dam
   • Body wall designed as low dam.
   • Cistern to dissipate energy

Location of Weirs

• A weir should be located in a stable part of the river where the river is unlikely to change its course.
• The weir has to be built high enough to fulfill command requirements. During high floods, the river could overtop its embankments and change its course. Therefore, a location with firm, well defined banks should be selected for the construction of the weir.
• Where possible, the site should have good bed conditions, such as rock outcrops.
• Alternatively, the weir should be kept as low as possible.