What is the primary difference between a barrage and a weir?

A weir is a high-crest structure where ponding is mainly done by the raised crest itself. A barrage is a gated structure with a low crest, where ponding is almost entirely controlled by the gates, allowing for better flow and silt regulation (Clause 3).

Which theory should be used for uplift pressure calculations?

Khosla's theory of independent variables is recommended for accurate design of impervious floors against uplift pressure, as it accounts for the effect of piles and floor thickness (Clause 7.4.2).

What is the significance of the exit gradient?

The exit gradient at the downstream end of the impervious floor must be kept below a safe limit for the soil type to prevent the phenomenon of piping, which can lead to foundation failure (Clause 7.4.2.2).

What is the minimum recommended freeboard?

A minimum freeboard of 1.0 m to 1.5 m should be provided for abutments, guide banks and afflux bunds over the maximum flood level (Clause 6.3.2).

IS 10432

: 1982

Criteria for Design of Barrages and Weirs

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This standard provides criteria for the hydraulic and structural design of barrages and weirs. It covers aspects from site investigation and data collection to the detailed design of components like the weir profile, impervious floor, and energy dissipation devices, based on established theories like Bligh's and Khosla's.

Lays down criteria for the hydraulic and structural design of barrages and weirs across rivers for diversion of water.

Quick Reference Values

Minimum freeboard above pond level1.0 m

Minimum freeboard above max flood level for guide banks1.0 to 1.5 m

Factor of safety against sliding (static condition)1.5

Factor of safety against overturning (static condition)1.5

Safe exit gradient for coarse sand0.17 to 0.20 (1/6 to 1/5)

Bligh's creep coefficient for coarse-grained sand12

Lacey's silt factor 'f' for standard silt1.0

Key Formulas

Lacey's Scour Depth: R = 1.35 * (q^2 / f)^(1/3)

Bligh's Creep Length: L = C * H_L

Uplift Pressure (Khosla's Theory): P = γ_w * h_residual

Practical Notes

Khosla's theory is generally preferred over Bligh's creep theory for designing impervious floors in major structures as it provides a more rational and safer assessment of uplift pressures.

Accurate estimation of the design flood (typically 100-year or higher), afflux, and scour depth are the most critical inputs for a safe and economical design.

The design of the stilling basin and other energy dissipation works downstream is crucial to prevent undermining of the structure's foundation due to scour.