23.11 : Design Example: Design of an Irrigation Channel

Trapezoidal channels are widely used in irrigation systems due to their cost-effectiveness and efficiency in conveying water. Trapezoidal channels feature a flat bottom and sloping sides, making them stable and easier to construct compared to other shapes. The bottom width and side slope ratio are determined based on the required flow capacity and site conditions. The side slope is kept gentle for unlined channels to prevent soil erosion.

Hydraulic parameters in channel design include the flow rate (Q), hydraulic radius (R_h), and channel slope (S_o). The hydraulic radius plays a critical role in determining flow velocity and resistance and is given by:

The longitudinal slope is selected based on terrain conditions to ensure sufficient gravitational force for water movement without causing excessive erosion.

Manning’s equation is a fundamental tool used in channel design to calculate the flow velocity and discharge. It relates the flow parameters as follows:

Here, n is Manning’s roughness coefficient, representing the resistance the channel surface offers. For unlined channels, n typically accounts for natural materials like soil or grass. The value of k is one if SI units are used.

The design process involves selecting appropriate dimensions and solving Manning’s equation iteratively to determine the required flow depth. This ensures that the channel meets the discharge requirements while maintaining a velocity that minimizes erosion and deposition. By carefully balancing geometric and hydraulic factors, trapezoidal channels offer an efficient and sustainable solution for water delivery in agricultural and other water management systems.