Weirs are structures built across a body of water, such as a river or channel, to alter water flow. They act as a barrier, raising the water level upstream. Unlike many dams, weirs are generally smaller, and water typically flows over their crest rather than being controlled by gates or spillways. This design allows them to manage water flow and height for various purposes.
Water Flow Dynamics Over a Weir
As water approaches a weir, its velocity decreases, and the upstream water level, known as the “head,” increases due to the obstruction. This upstream water level above the weir’s crest is a key factor in determining the flow rate.
The water then accelerates as it flows over the weir’s crest, the top edge of the structure. This transition often involves a change from subcritical (slow, tranquil) flow upstream to critical or even supercritical flow (faster, shallower) directly over and immediately downstream of the weir. The sheet of water flowing over the crest is referred to as the nappe.
The height of the water above the crest dictates the volume of water passing over the weir. As the water passes the crest, gravity causes it to accelerate and often drop. This predictable relationship between the upstream water level and the flow rate is how weirs are utilized in water management.
Key Functions of Weirs
Weirs serve multiple purposes in water management. One function is the measurement of water flow rates in open channels. By measuring the water level (head) above the weir’s crest, engineers can calculate the volumetric flow rate using established formulas.
Weirs also control water levels. They can raise the upstream water level, which helps in diverting water for irrigation or maintaining navigable depths for boats. This level control supports agricultural practices by ensuring consistent water supply to irrigation canals.
Weirs contribute to flood control by regulating river flow rates, especially during high discharge periods. They slow down water flow and enable a controlled release downstream, which can reduce flooding. Some weirs can also be integrated into hydropower generation systems, converting the potential energy of flowing water into electricity.
Variations in Weir Design
Weirs are designed with different shapes and profiles for various applications. Sharp-crested weirs, characterized by a thin, sharp edge at the crest, are effective for precise flow measurement. Water flows clear of the crest in these designs, allowing for accurate discharge calculations, especially for smaller flows.
Broad-crested weirs have a wide, horizontal crest over which the water flows. These structures are often used for flow control and are suitable for measuring larger flow rates in canals or rivers. Water flows across the broad crest with relatively uniform conditions, and the design helps to dissipate energy gradually.
V-notch weirs, a type of sharp-crested weir, feature a V-shaped opening in their crest. This design is effective for accurately measuring low flow rates, as the V-shape creates a more sensitive relationship between the water level and discharge at small volumes. The angle of the V-notch can be varied to optimize measurements for specific flow ranges.