Diversion works can be classified under the following two heads:

1. Temporary bunds or spurs.

2. Permanent weirs and barrages.

  • Temporary bunds or spurs are temporary works, which have to be constructed every year after floods.
  • These bunds cannot sustain the assault of floods and get washed away.
  • Immediately after floods, they are again constructed.
  • Temporary bunds can be constructed on small streams only.
  • Such works are generally carried out by the nearby villagers jointly, so as to make arrangements for irrigation for their fields for lean months of flow.
  • Such temporary spurs or bunds can be economically constructed in boulder reaches of the river.
  • However, for important diversion works permanent weirs and barrages have to design and construct.


  • Weir is a solid obstruction, construct across the river.
  • It is used to raise the water level in the river and then divert it into the canal.
  • The weirs can also be used to store surplus flood water, to tide over the shortages likely to occur during lean months.
  • In such a case, the weir is known as a storage weir.
  • The main difference between storage weir and dam is only in regard to the height and duration of storage.
  • Dams store large amounts of water and for longer durations than storage weirs.
  • One more difference is that water in the reservoir never overtops the dam, but in the case of weirs the water can flow over the weir crest. In the case of dams, the surplus flood water is disposed of by D/ S through spillways.


  • Functions of a barrage are the same as those of weirs; the only difference being in the mode of heading up of water.
  • The weirs cause heading up of water by obstructing the flow by themselves, but in the case of barrages, water is heated up with the help of gates.
  • In the case of weirs, the high flood water passes over the crest but in the case of barrages, gates are lifted up and flood water passed D/S.
  • In the case of a barrage, the crest level is maintained at a low water level which is almost in the level of bed level of the river.
  • Because gates can be lifted for passing the flood water and shut down for storing water, control on the water level in the river is better exercised with the help of barrage.
  • However, barrages are much costlier than the weirs.
  • A road bridge is generally constructed over the barrage, with a little additional cost.
  • Hence a barrage maybe used to act as a bridge also.