Design of Barrage | Design capacity of barrage | Weir

 As per IS 6966-(Part-1) the discharge of  barrage or weir is calculated using below formula:

Calculation of discharge capacity of the barrage

CAUSES OF FAILURE OF WEIRS AND THEIR REMEDIES | DAM | BARRAGE | WEIR

CAUSES OF FAILURE OF BARRAGE AND THEIR REMEDIES | DAM | BARRAGE | WEIR

The weir and barrage failure may take place due to the following reasons

1. Piping or undermining.
2. Rupture of the floor due to uplift.
3. Rupture of the floor due to suction caused by standing wave.
4. Scour at U/S and D/S side of weir floor.

1. Piping or undermining

When water seeps under the base of the weirs founded on the permeable soils, emerges out at the D/S end of the impervious floor of the weir, hydraulic gradient or exit gradient may exceed a certain critical value for the foundation soil, the soil starts boiling at the exit point.

Boiling of the soil indicates lifting of the soil against gravity and it happens only when exist gradient of seeping water is greater than the safe limit for the foundation soil. The soil gets wash out with percolating water.

This process of erosion of soil from below the foundation, progressively, works backward towards the U/S. This process ultimately develops a channel or pipe, below the foundation of the weir and causes failure of the weir.

Remedies:

• Providing deep cut-off piles at U/S and D/S ends.
• Provide sufficient length of the impervious floor so that path of percolation is increased and exit gradient is decreased

2. Rupture of the floor due to uplift

Seeping water through the foundation, exerts uplift pressure, on the floor. The uplift pressure is maximum at the point, just D/S of the weir wall or crest wall, when water is full up to the top of the gates and there is no water on the D/S side.

Hence critical section the floor is just at the D/ S side of the weir’s chest wall. If the thickness of the floor is insufficient, its weight would be inadequate to resist the uplift pressure. This may ultimately lead to bursting of the floor and thus failure of the weir may occur.

Actually, once the floor is burst due to uplift pressure the effective length of seepage is very much reduced which causes a further increase in the exit gradient and consequent failure by piping.

Remedies:

• Provide sufficient length of the impervious floor.

• Provide impervious floor of sufficient thickness.

• Provide a cut-off pile at the U/S end so as to reduce the effect of uplift throughout the foundation.

3. Rupture of the floor due to suction caused by standing wave

Standing waves of hydraulic jump formed at the D/S of the weir cause suction. This suction increases the effect of uplift. If floor thickness is inadequate to sustain the combined effect of uplift and suction, it will fail. Such failures occurred at Marala weir on the Chenab and Rasul weir.

Remedies:

• Provide increased thickness of the floor to resist the additional effect of the standing wave.

• The floor should be laid in one layer of concrete, instead of several, thin layers of masonry.

4. By scour on U/S and D/S of the weir

The beds of alluvial rivers are scoured to considerable depths especially during floods. This scouring may take place on the U/S side also, but it is most likely on the D/S side of pucca floors.

If this scour is allowed unchecked, it will form scour holes underneath the pucca floor. These holes may slowly progress towards the main weir and cause its failure. 

Remedies:

• Providing deep piles at U/S and D/S ends of the pucca floor. The piles should be taken much below the calculated scour depth.
• Suitable lengths of launching aprons, both at U/S and D/S of impervious floors should be provided.