Design Example: Design of an Irrigation Channel

The Department of Irrigation is designing an unlined trapezoidal channel to deliver 5.66 cubic meters per second of water to an agricultural project.

The channel has a gentle slope of 0.0008 and is constructed with a side slope ratio of 2 horizontal to 1 vertical to ensure stability.

Manning’s roughness coefficient is taken as 0.025, which accounts for the natural resistance of the unlined, earthen channel material.

With the channel width fixed at 3 meters, the required depth is calculated to achieve the desired flow rate.

Manning’s formula is used to determine the flow depth by relating flow velocity to channel slope, roughness, and hydraulic radius.

The hydraulic radius is calculated by dividing the cross-section area by the wetted perimeter, supporting the required uniform flow conditions.

Solving Manning’s equation with these parameters results in a required depth of 1.15 meters, providing a cross-sectional area of 6.09 square meters.

This design maintains the channel’s discharge requirement, supporting efficient water delivery with balanced flow velocity to prevent erosion and ensure stability.