Hydraulically efficient channels are designed to minimize the energy losses due to friction and turbulence, and to ensure a smooth and uniform flow of water. One of the conditions for a hydraulically efficient channel is to have a minimum wetted perimeter.

Explanation:
1. Definition of Wetted Perimeter
The wetted perimeter of a channel is the length of the channel that is in contact with the flowing water. It includes the bottom and the sides of the channel. A larger wetted perimeter means more surface area is in contact with the water, resulting in higher frictional losses.

2. Relationship with Efficiency
A channel with a minimum wetted perimeter will have a smaller surface area in contact with the flowing water. This reduces the frictional losses and improves the hydraulic efficiency of the channel. A smaller wetted perimeter also means a smaller cross-sectional area, which can help maintain a constant flow velocity.

3. Importance of Constant Velocity
Maintaining a constant velocity is crucial for hydraulically efficient channels. A constant velocity ensures a uniform flow and minimizes turbulence. Turbulence in a channel can increase energy losses and result in uneven flow patterns. Therefore, minimizing the wetted perimeter helps maintain a constant velocity and improves the overall efficiency of the channel.

4. Other Considerations
While a minimum wetted perimeter is important for hydraulic efficiency, other factors also need to be considered in channel design. These include the slope of the channel, the cross-sectional shape, and the flow rate. The slope affects the velocity and flow pattern, while the cross-sectional shape determines the flow resistance. The flow rate determines the required channel dimensions to accommodate the desired flow.

Conclusion:
In summary, a condition for a hydraulically efficient channel is to have a minimum wetted perimeter. This reduces frictional losses, helps maintain a constant velocity, and ensures a smooth and uniform flow of water. However, other factors such as slope, cross-sectional shape, and flow rate also need to be considered in channel design to achieve optimal efficiency.