Understanding River Training Structures: Closing Dykes

River training work is a crucial aspect of hydraulic engineering aimed at controlling and regulating river flow to achieve specific objectives. These objectives can include preventing erosion, improving navigation channels, protecting banks, or reclaiming land. Various structures are employed in river training to manipulate the river's flow pattern.

The question asks about structures specifically used to close a particular flow path within a river, essentially blocking it off so that the main flow is directed into another desired channel. Let's examine the options provided:

• Attracting groynes: Groynes (or spurs) are structures built perpendicular or at an angle to the river bank. Attracting groynes are designed to attract the flow towards the groyne head, often to scour a deeper channel or push the main flow away from a specific bank. Their primary function is not to completely close off a flow channel but rather to influence the flow direction and velocity near the bank.
• Submerged dykes: Dykes (or dikes) are embankments or walls. Submerged dykes are built below the water surface. They are used to guide flow, reduce velocities, or create specific flow patterns, but being submerged, they don't fully block off a flow path in the sense of closing it above the water level, which is implied by 'closing a particular flow' for diversion.
• Pitched islands: Pitched islands are artificial islands constructed and protected with pitching (like stones) to stabilize them. They might be used to divide flow or create habitat, but they are not typically the primary structure for completely closing off a specific channel or flow path for diversion purposes.
• Closing dykes: Closing dykes are structures specifically built to block or close off a particular channel or flow path in a river. By closing off an unwanted or less desirable channel, they force the entire river flow to be directed into another, more desirable channel. This is a direct method used in river training to consolidate flow and deepen a specific channel, for example, the main navigation channel.

Based on their function in river training work, structures used to close a particular flow to direct the river flow in some other desired direction are known as Closing dykes. They effectively act as barriers that shut down one flow path, forcing the water into an alternative route.