Gravity weirs:
Uplift pressure due to the seepage of water below the floor is resisted by the weight of floor.
Its further types are:
- Vertical drop weir
- Masonry or concrete slope weir
- Dry stone slope weir
- Parabolic weir
Explanation:
- 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.
- 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.
- Dry stone slope weir
- Body wall or weir wall
- Upstream and downstream rock fill laid in form of glacis, with few intervening care walls.
- 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.
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