What is the recommended side slope for a canal in cutting?

For most soils in cutting, a slope of 1.5H:1V is recommended. For rock, it can be steeper, from 0.25H:1V to 0.5H:1V (Table 1).

How is the required freeboard determined?

Freeboard depends on the canal discharge. For discharges less than 10 cumecs, it is typically 0.5 m to 0.6 m, increasing to 0.9 m for discharges over 150 cumecs (Table 3).

What is a typical Manning's 'n' value for a concrete lined canal?

For straight reaches with a good concrete finish, 'n' is typically 0.014 to 0.016. For canals with average maintenance, a value of 0.018 is commonly used (Table 2).

What is the purpose of drainage behind the canal lining?

To relieve the external hydrostatic pressure (uplift) on the lining that can occur due to high groundwater levels, preventing cracking and failure of the lining (Clause 8).

IS 9456

: 1980

Criteria for Design of Lined Canals

CurrentSpecializedCode of PracticeWater Resources · Irrigation and Canal Structures

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This standard provides criteria for the hydraulic and structural design of lined irrigation canals. It covers the selection of canal cross-section, determination of flow parameters using Manning's formula, design of lining thickness, and requirements for freeboard and drainage arrangements.

Provides criteria for the hydraulic and structural design of various types of lined canals.

Quick Reference Values

Typical Manning's 'n' for Cement Concrete Lining0.015 - 0.018

Typical side slope for canals in cutting (ordinary soils)1.5H : 1V

Maximum permissible velocity for concrete lining2.7 m/s

Minimum freeboard for canals with <10 cumecs discharge0.5 m

Minimum lining thickness (in-situ concrete)75 mm for subgrade with processed gravels

Key Formulas

V = (1/n) * R^(2/3) * S^(1/2) — Manning's formula for velocity

Q = A * V — Discharge formula

Practical Notes

The selection of Manning's roughness coefficient 'n' is critical and should be based on the expected quality and maintenance of the finished lining surface, not just the material type.

Proper drainage arrangements behind the lining are crucial to prevent damage from external hydrostatic (uplift) pressure, especially in areas with high groundwater tables or heavy rainfall.

This is a foundational but old code; modern designs often supplement it with advanced hydraulic modeling and materials like geomembranes for which other specialized guidelines should be consulted.