- Weir is a solid obstruction, put across the river to store water on its U/S.
- The store water is divert to the off-taking canal.
- Depending upon the criterion of design, the weirs may be gravity type or non-gravity type.
- The gravity weir is the weir in which uplift pressure below the weir due to seepage is fully resist by the self weight of the weir.
- In the case of non-gravity weirs the thickness of the floor is kept relatively small and uplift pressure is largely resist by the bending action of the reinforc concrete floor.
- Depending upon the design features and available construction materials gravity weirs or simply, weirs can be further subdivide into the following three categories.
- Vertical drop weirs.
- Rockfill weirs.
- Concrete weirs with sloping glacis.
- This weir consists of a vertical drop wall or crest wall, rectangular or nearly rectangular in shape.
- The weir may have gates at the crest.
- Pucca impervious floor is provid below the crest wall and also for substantial lengths both on U/S and down stream side.
- Cut off piles are also provid both at ends of U/S and D/S pucca floors.
- To guard against the scouring, launching aprons are provid, both at D/S and U/S ends of the pucca floors.
- A grad, invert filter is also provid on D/S side, in continuation of D/S launching apron, to relieve the uplift pressure of seeping water.
- This weir can be use over any type of foundation soil.
2. Rock fill weir.
- The body of this weir is made by a combination of masonry walls and stone boulder filling.
- The main weir wall is construct in the require height.
- A few more masonry walls, also call core walls, are construct from masonry at the suitable interval of the D/S side of the main weir wall.
- The intervening space between masonry walls is fill by hand pack stone boulders.
- Boulder packing and masonry walls are arrang in such a way that sloping glacis both on U/S and D/S are form.
- D/S slope is generally made very flat.
- It requires a very large amount of stone boulders and such is restrict only to the places where the stone is abundantly available.
3. Concrete weir with sloping glacis.
- The design of this weir is of recent origin, and its design is done according to Khosla’s theory.
- This weir consists of a sloping concrete slab.
- The parts of the slab slope downwards both on U/S and D/S sides.
- Deep cut-off piles are driven at the ends of U/S and D/S pucca aprons.
- Sometimes, even intermediate cut-off piles are also provid.
- Hydraulic jump is form at the D/S sloping glacis to dissipate the energy of gliding water.
- This weir is found suitable for soft sandy foundations.
- This weir is mostly use when difference in weir crest and D/S river bed is limit to about 3 m.
4. Parabolic weirs.
- It is similar to the spillway section of a dam.
- The Weir wall is made parabolic and is design a low dam.
- A cistern is develop on the D/ S side to dissipate the energy of flowing water.
- U/S and D/S protection works are provid in a similar way explain for vertical drop or sloping glacis weirs.
- This weir is not much in use.