What type of design allows water to flow horizontally through a long tank or basin?

The design that allows water to flow horizontally through a long tank or basin is the rectangular basin. This configuration is specifically advantageous for maintaining a uniform flow of water along its length, which is beneficial for various applications such as sedimentation, filtration, or treatment processes.

In a rectangular basin, the extended horizontal surface area and consistent depth enable effective control of flow velocity, which promotes even distribution of water across the basin. This helps facilitate processes where uniform flow is essential, such as sedimentation where particles have a longer residence time to settle or when treating water to ensure adequate contact time with treatment agents.

Other designs like circular or triangular basins may not effectively maintain horizontal flow throughout their length due to their geometric properties, which can lead to variations in flow velocity or areas where water may stagnate. Rectangular basins are, therefore, the preferred choice for applications that require controlled horizontal flow.