Understanding Canal Escapes and Weir Type

Canal escapes are crucial hydraulic structures built along irrigation canals. Their primary function is to discharge surplus water from the canal. This surplus water might occur due to various reasons like heavy rainfall in the command area, sudden closure of outlets downstream, or errors in regulating the headworks supply. Discharging excess water prevents the canal from overtopping its banks, which could lead to breaches and damage.

Different types of canal escapes are designed based on factors like the required discharge capacity, available head, and economic considerations. Let's look at some common types:

• Sluice type escape: This type typically consists of an opening controlled by a gate, usually positioned below the canal's full supply level (FSL). Water is released by opening the gate.
• Weir type escape: This type features a weir wall built across a channel leading away from the canal. Water flows over the crest of this weir.
• Regulator type escape: Similar to sluice gates, these use gates to control the flow, often designed as a side channel regulator.
• Orifice type escape: Utilizes an orifice (an opening with a specific shape) for discharging water, often operating under pressure.

Characteristics of Weir Type Canal Escape

In a Weir type canal escape, the design is such that the crest of the weir wall is intentionally kept at a specific level relative to the canal's Full Supply Level (FSL).

When the crest of the weir wall is kept exactly at the Reduced Level (R.L.) equal to the canal FSL, water will begin to flow over the weir as soon as the water level in the canal reaches the FSL or slightly exceeds it. This provides a simple and automatic mechanism for discharging surplus water without requiring manual operation of gates, as would be needed in a sluice or regulator type escape.

This specific characteristic - the weir crest being at canal FSL - is the defining feature mentioned in the question, pointing directly to the Weir type escape.

Comparing Escape Types based on Crest Level

Let's compare the typical characteristics related to the crest or sill level for some escape types:

Based on this comparison and the description in the question, the type of canal escape where the crest of the weir wall is kept at the R.L. equal to the canal FSL is the Weir type.

Revision Table: Canal Escape Summary

Additional Information on Canal Design & FSL

The Full Supply Level (FSL) is a critical parameter in canal design. It represents the designed maximum water level in the canal when it is carrying its design discharge.

• Importance of FSL: FSL determines the required height of the canal banks (freeboard is added above FSL), the command area that can be irrigated by gravity, and the design levels of structures like bridges, culverts, and escapes.
• Purpose of Escapes: Escapes are safety valves for the canal system. They protect the canal from damage caused by excessive water levels.
• Location of Escapes: Escapes are typically located at the tail end of the canal, before significant drops, or at points where excess water can be safely diverted into a natural drain or another channel.
• Design Considerations: The design of a canal escape involves calculating the maximum surplus discharge expected and designing the structure (weir dimensions, gate size, etc.) to handle this flow while preventing the canal level from rising excessively above FSL. The chosen type (weir, sluice, etc.) depends on site conditions, required precision of control, and maintenance considerations.