WEIRS

  • 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.

Sheet pile at end (Khosla Curves)

 

  • Depending upon the design features and available construction materials gravity weirs or simply, weirs can be further subdivide into the following three categories.
  1. Vertical drop weirs.
  2. Rockfill weirs.
  3. Concrete weirs with sloping glacis.

Fig. 15.2. Vertical drop weir.

1. Vertical drop weirs.

  • 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.

Fig. 15.3. (Plate 15.3 Exit gradient) (Khosla Curve).