What is the difference between an aqueduct and a syphon aqueduct?

In an aqueduct, the canal bed is above the drainage's High Flood Level (HFL). In a syphon aqueduct, the drainage HFL is above the canal bed, forcing the drainage water to flow under pressure through the barrels.

How is the waterway (width) for the drainage decided?

It's typically based on the regime width of the drain (e.g., using Lacey's formula Pw = 4.75 * sqrt(Q)) but constricted to economize, ensuring the resulting afflux (heading up) remains within permissible limits (Clause 5.2).

What is the minimum recommended freeboard in the canal portion of an aqueduct?

A minimum freeboard of 0.5 m is generally provided above the canal's Full Supply Level (FSL) in the trough section (Clause 5.3.4).

What major loads are considered in the structural design?

Dead load, live load (water weight), hydrostatic pressure, earth pressure, uplift pressure, buoyancy, seismic forces, wind forces, and temperature effects are all considered in various combinations (Clause 6).

IS 10431

: 1982

Criteria for Design of Aqueducts and Other Cross Drainage Works

CurrentSpecializedCode of PracticeBIMWater Resources · Irrigation and Canal Structures

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This standard provides criteria for the hydraulic and structural design of cross drainage works, such as aqueducts, syphon aqueducts, superpassages, and canal syphons. It details the selection process for the appropriate structure type, hydraulic design principles including waterway and head loss calculations, and structural design considerations for various loads and components.

Provides criteria for the hydraulic and structural design of aqueducts and other cross-drainage structures for canals.

Quick Reference Values

Minimum freeboard in canal trough0.5 m to 0.75 m above FSL

Maximum afflux (heading-up) permitted0.3 m, generally

Permissible exit gradient for fine sand (Khosla's)1/6 to 1/7

Angle of expansion transition for canal22.5 degrees (1 in 2.5)

Angle of contraction transition for canal11.25 degrees (1 in 5)

Coefficient for head loss at entry0.5 for un-shaped entry

Coefficient for head loss at exit1.0 for abrupt exit

Key Formulas

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

Lacey's Wetted Perimeter: Pw = 4.75 * sqrt(Q)

Khosla's Exit Gradient: GE = (H/d) * (1 / (π * sqrt(λ)))

Head loss due to friction (Manning's): hf = (n^2 * V^2 * L) / R^(4/3)

Practical Notes

Accurate determination of the drainage's High Flood Level (HFL) and design discharge is the most critical input for the entire design.

The design of transitions (inlet/outlet wings) is crucial for minimizing hydraulic losses and preventing scour.

Foundations must be designed against scour and uplift pressure, often requiring deep piles/wells and cutoff walls, based on subsoil investigation.