A silt control device consisting of a number of rectangular tunnels - provided parallel to the axis of head regulator and terminating near the under sluiced weir - to allow the clearer water to flow through the head regulator is called:

Question

Prepp Team · Accepted Answer

Understanding Silt Control in Irrigation Channels

Silt is a major challenge in irrigation systems, particularly where rivers carry a heavy sediment load. When water is diverted from a river into a canal through a head regulator, silt can enter the canal and cause problems like reducing the canal's carrying capacity, silting up reservoirs, and damaging pumps and machinery. To prevent or reduce the entry of silt into the main canal, various silt control devices are used near the head regulator.

Identifying the Silt Control Device

The question describes a specific type of silt control device located near the head regulator. Let's break down the key features mentioned:

It consists of a number of rectangular tunnels.
These tunnels are provided parallel to the axis of the head regulator.
They terminate near the under sluiced weir.
Their purpose is to allow clearer water to flow through the head regulator while diverting silt-laden water.

Based on these characteristics, we can evaluate the given options.

Analyzing the Options

Let's look at each option in the context of the description provided:

Silt excluder: This device is constructed at the bed of the river, upstream of the head regulator. It consists of tunnels that run parallel to the regulator. Silt-laden water, which tends to flow along the bed, enters these tunnels and is guided back to the river through the under sluices, while the clearer water near the surface flows into the head regulator. This description perfectly matches the details given in the question.
Silt tunnel: This is a general term and not the specific name for the described structure. While silt excluders use tunnels, "silt tunnel" itself doesn't define the device's function or location as precisely as the question requires.
Under sluice: Under sluices (or scouring sluices) are openings in the weir structure near the head regulator. They help in maintaining a clear approach channel to the head regulator and passing sediment downstream, but they are part of the weir/barrage structure itself, not the separate device of tunnels designed to draw off silt from the water approaching the head regulator.
Silt ejector: A silt ejector (or silt extractor) is a device constructed in the canal itself, *after* the head regulator. Its purpose is to remove silt that has already entered the canal. The question describes a device that prevents silt entry at the head regulator, not one that removes it from the canal. Therefore, a silt ejector is not the correct answer.

Conclusion

The device described, with its rectangular tunnels parallel to the head regulator axis and located upstream near the under sluices to prevent silt entry, is known as a silt excluder.

Revision Table: Silt Control Devices

Device	Location	Function
Silt Excluder	River bed, upstream of head regulator	Prevents silt from entering the canal by diverting silt-laden water back to the river via under sluices. Works on the principle of excluding silt from the water before it enters the head regulator.
Silt Ejector	Canal bed, downstream of head regulator	Removes silt that has already entered the canal by extracting it through tunnels or other structures. Works on the principle of ejecting silt that has entered the canal.
Under Sluices (Scouring Sluices)	Part of the weir/barrage structure near the head regulator	Help in maintaining a clear approach channel for the head regulator and passing sediment downstream during floods or flushing operations. They are part of the diversion structure itself, aiding the operation of the silt excluder.

Additional Information on Silt Excluders

Silt excluders are very effective in controlling silt entry, especially when the river carries coarse to medium sand. They are designed based on hydraulic principles to utilize the difference in velocity and sediment concentration at different depths of flow. The bed layers, which carry a higher concentration of silt, are drawn into the tunnels of the excluder and discharged back into the river downstream of the weir through the under sluices.

Key design considerations for a silt excluder include:

The number, size, and shape of the tunnels.
The level of the tunnel inlets relative to the head regulator sill.
The amount of water to be discharged through the tunnels (pocket waterway).
The velocity of flow through the tunnels to ensure effective silt transport.

Proper operation of the under sluices is crucial for the effective functioning of a silt excluder, as they provide the escape route for the silt-laden water collected by the excluder tunnels.