If crest length is equal to the width of the channel then it is called:
Suppressed weir
Understanding Weirs and Channel Flow
A weir is a barrier constructed across a river, stream, or canal to raise the water level or to measure the flow rate. Different types of weirs are classified based on various features, including their shape, the material they are made from, and importantly, the relationship between the length of the weir's crest and the width of the channel it is placed in.
Suppressed Weirs Explained
The question asks about a specific condition where the crest length of the weir is equal to the width of the channel. This configuration is known as a suppressed weir.
In a suppressed weir, the sides of the weir notch coincide with the sides of the channel. This means that the flow over the weir is essentially spread across the entire width of the channel, and there are no side contractions of the water flow as it passes over the weir crest.
Consider the key characteristic: the crest length is equal to the width of the channel. This direct relationship is the defining feature of a suppressed weir.
Comparing Suppressed and Non-Suppressed (Contracted) Weirs
To better understand suppressed weirs, let's compare them to non-suppressed or contracted weirs:
- Suppressed Weir: The length of the weir crest is equal to the width of the channel. Side contractions are 'suppressed' because the sides of the flow are guided by the channel walls.
- Non-Suppressed (Contracted) Weir: The length of the weir crest is less than the width of the channel. This configuration allows the water flow to contract laterally as it approaches and passes over the weir crest, resulting in 'side contractions'.
| Weir Type | Relationship: Crest Length vs. Channel Width | Side Contractions |
|---|---|---|
| Suppressed Weir | Crest Length = Channel Width | Absent (Suppressed) |
| Non-Suppressed (Contracted) Weir | Crest Length < Channel Width | Present |
Analyzing the Given Options
Let's look at why the other options are not correct based on the definition provided in the question:
- Deformed weir: This is not a standard hydrological or engineering term used to classify weirs based on crest length relative to channel width.
- Suppressed weir: This option perfectly matches the description given in the question where the crest length is equal to the width of the channel.
- Balanced weir: This is not a standard term for classifying weirs based on their physical dimensions relative to the channel.
- Non-suppressed weir: As discussed, a non-suppressed weir (or contracted weir) has a crest length less than the channel width, which is the opposite of the condition described in the question.
Therefore, based on the standard classification of weirs, when the crest length is equal to the width of the channel, it is correctly identified as a suppressed weir.
Revision Table: Key Weir Concepts
| Concept | Definition/Description | Relevance to Question |
|---|---|---|
| Weir | Structure across a channel to control or measure flow. | The main subject. |
| Crest Length | The length of the top edge of the weir that water flows over. | A key parameter in the question. |
| Channel Width | The width of the stream or canal where the weir is installed. | A key parameter in the question. |
| Suppressed Weir | Weir where crest length equals channel width; no side contractions. | The correct answer. |
| Non-Suppressed Weir | Weir where crest length is less than channel width; side contractions occur. | A contrasting type. |
Additional Information on Weir Flow and Contractions
Understanding contractions is important for accurately calculating the flow rate over a weir using formulas like the Francis formula or the Rehbock formula. These formulas often include terms or coefficients that account for the presence or absence of side contractions (as in suppressed weirs) and sometimes bottom contractions (when the crest is significantly above the channel bed).
A suppressed weir simplifies flow calculation because there are no side contractions to consider. However, ensuring that a weir is truly suppressed requires careful construction so that the channel walls extend smoothly up to and past the weir sides, preventing air from getting under the nappe (the sheet of water flowing over the weir) at the sides.
Weirs are fundamental structures in hydraulic engineering for flow measurement, flow control, and raising upstream water levels for irrigation or navigation.
Important Questions from Weirs and Barrages
In which type of weir is the excess energy of overflowing water dissipated by means of a hydraulic jump?
The slopping floor below and in continuation of the raised crest of a weir is known as _________.
In which type of barrier is most of the ponding done by gates and smaller or nil part of it is done by the raised crest?
Which one of the following is the purpose of providing the downstream sheet pile in a barrage?
Discharge over an ogee weir remains the same as that of:
