Explain the effect of the construction of Weir or Barrage on the regime of the river?

Normally weirs and barrages are constructed to enable a continuous supply of water into an off taking canal. We will understand the effect on the regime of river in two parts:

1. As soon as the obstruction is created in the form of weirs and barrages, the velocity of flow reduces, slope of water flattens up to some distance behind the obstruction, resulting in ponding of water in the upstream side and creating an afflux.

Since the velocity of flow reduces upstream, silt carrying capacity reduces and leads to deposition of silt on river bed creating shoals and islands.

The water travels downstream with increased velocity of flow with high demand for silt carrying capacity which is met by erosion on the downstream bed of river causing lowering of bed in downstream side also known as retrogression.

1. The above process continues for a number of years until river in upstream starts regaining its original slope by extending the afflux further upstream of river. After regaining of slope, a stage is reached when the upstream side of obstruction will not absorb further silt. The sediment goes downstream with more sediment than the silt carrying capacity of downstream side of flow. Hence, deposition of sediment takes place in the downstream side and consequently reclamation of the original bed levels.

The affect of the construction of weir or barrage on the regime of the river are as follows :-

– The velocity of flow of water reduces.
– Sedimentation can be occured which makes the height of the flow constant.
– Fishes and other water animals migrations can be restricted.
– Increases the flow height for a short duration.
– Because of the barrier the particles settle down near it.

Effect of Weir construction on the regime of the river: Weir is a solid obstruction which is put across a river to raise the water level.

It's very necessary to avoid any adverse effect on the environment of construction of weir on the environment, to study the fallout of.

The immediate effect of the construction of weir is an increase in water level upstream. The increase in water level is not uniform but it varies from place to place. The velocity of water also varies from place to place. The variation in flow velocity is again not uniform and hence the variation in the capacity to transport gives rise to a pattern of initial erosion and sedimentation along the river. Sedimentation over a relatively long distance occurs upstream of the intervention. Punctuated local erosion occurs an erosion advances downstream as a rarefaction wave.

Thank you.

Weir is a barrier built-in river to change the depth of flow. It is a barrier that alters the floor height of the river. It is also used to control the velocity of flow during high discharge.

weir is also called a small-dam to store water in a small area.

Effects of wier on the regime of the river:

1. In short duration- increase the flow height near the barrier.
2. In long term – sedimentation occurs which makes the height of flow constant.
3. Reduce the velocity of flow.
4. Because of the barrier, colloidal particles settle at bottoms surface of the river.
5. Restricted migration of fishes.

Following are some types of the wier

1. Compound
2. Broad crested
3. V notch
4. Polynomial

The effect of the construction of Weir or Barrage on the regime of the river are given below,

that a weir is constructed across the river at a certain location

(A). The immediate effect is an increase in water levels upstream

(B). This increase is not uniform but varies from place to place.

As a consequence, flow velocities differ from place to place too. The variations in flow velocity and, hence, the variations in the capacity to transport sediment give rise to a pattern of initial erosion and sedimentation along the river . Sedimentation over a relatively long distance occurs upstream of the intervention, whereas punctuated local erosion occurs downstream. This erosion advances downstream as a rarefaction wave.

Eventually, in the long run, the river reaches a new morphological equilibrium without further trends of erosion or sedimentation . Upstream bed and water levels have become higher than at the start of the intervention. Downstream, the river resumes its original shape by sedimentation after an intermediate period of erosion.

The longitudinal profiles in the diagrams provide a simplified picture. They do not include the response of channel width, bed sediment composition or vegetation. Nonetheless, they offer the key to understanding the relation between local pressures or measures and their effects far upstream and downstream