A weir system in which the downstream water level of the weir nappe is higher than the crest is called _____.
Submerged
Understanding Weir Systems and Water Levels
Weirs are hydraulic structures that are commonly used in rivers, canals, and other open channels to measure or control the flow of water. They work by obstructing the flow, causing the water to back up and flow over a shaped crest. The relationship between the water level upstream and downstream of the weir is crucial in classifying its flow conditions.
Analyzing Weir Flow Conditions
The question describes a specific condition where the downstream water level of the weir nappe (the sheet of water flowing over the crest) is higher than the weir crest itself. This situation affects the flow over the weir because the high downstream water level restricts the free fall of the water nappe. This type of flow is known as submerged flow.
- Submerged Weir: In a submerged weir, the downstream water level is high enough to influence the flow over the crest. The nappe is not able to fall freely and is "submerged" by the downstream water. This reduces the discharge coefficient compared to free flow conditions.
- Free Flow Weir: In contrast, a free flow weir occurs when the downstream water level is low enough that it does not impede the flow over the crest. The water nappe falls freely into the downstream channel.
Evaluating the Options
Let's look at the provided options based on this understanding:
- Submerged: This term directly matches the description given in the question, where the downstream water level is higher than the crest and affects the flow over the weir nappe.
- Overflowing: This is a general term indicating water is flowing over the weir, but it doesn't specify the downstream condition relative to the crest. Both free flow and submerged flow weirs are overflowing.
- Broad-crested: This refers to the shape of the weir crest, meaning it has a significant width in the direction of flow. It describes the weir's geometry, not the flow condition based on downstream water level.
- Cipoletti: A Cipoletti weir is a specific type of contracted trapezoidal notch weir used for flow measurement. This describes the type of weir notch, not the flow condition based on downstream water level.
Based on the definition and the analysis of the options, the weir system where the downstream water level is higher than the crest and affects the nappe is called a submerged weir.
Important Questions from Open Channel Flow
- A trapezoidal channel carries $5\text{ m}^3\text{.s}^{-1}$ of water under uniform flow condition. The channel has a bottom width of $2\text{ m}$, side slope of $2:1$ (horizontal: vertical), and a bed slope of $1\%$. If the Manning roughness coefficient is $0.03$, the conveyance of the channel, in $\text{m}^3\text{.s}^{-1}$, is
- Two rectangular channels, A and B, join to form a large rectangular channel, C. Each rectangular channel is lined with the same material and has the same bottom slope. The bottom widths of Channels A and B are $2\text{ m}$ and $1\text{ m}$, respectively. If the flow depth in each channel is $2\text{ m}$, the bottom width of Channel C (in m) is ________. (Rounded off to two decimal places)
- Water is flowing at the rate of 15 m$^3$/s through a rectangular channel of 5 m width.
If the acceleration due to gravity ($g$) is 9.81 m/s$^2$, the critical velocity for the flow is ______ m/s (rounded off to three decimal places). A Hydraulic Jump is said to be Oscillating Jump if its Froude number is ________.
The Francis formula for the discharge over cippoletti weir is
