Weir

Definition

A weir is a barrier or structure built across a river or stream to control water flow, often allowing for the measurement of water levels and flow rates. Weirs can be used in various hydraulic applications, including irrigation, flood control, and water quality management, by creating a controlled environment for water to flow over or through them.

5 Must Know Facts For Your Next Test

1. Weirs can be classified into different types such as sharp-crested, broad-crested, and overflow weirs, each serving specific purposes based on their design.
2. The flow over a weir can be analyzed using various equations, with the most common being the equation developed by Francis, which relates the flow rate to the head of water above the weir crest.
3. Weirs can help stabilize stream flows and mitigate erosion by maintaining consistent water levels upstream and controlling sediment transport.
4. In addition to flow measurement, weirs can also enhance aquatic habitats by creating pools for fish and other wildlife.
5. The design and placement of weirs are critical; they must consider factors such as upstream and downstream conditions, including existing infrastructure and environmental impacts.

Review Questions

• How do different types of weirs affect flow measurement in open channels?
  • Different types of weirs, like sharp-crested and broad-crested, influence flow measurement by altering the relationship between the hydraulic head and flow rate. Sharp-crested weirs are often more accurate for low flows due to their ability to create a defined crest for measuring water height. In contrast, broad-crested weirs are better suited for larger flows as they allow for smoother transitions and can handle higher volumes of water without causing excessive turbulence.
• Discuss how weirs contribute to managing water quality in rivers and streams.
  • Weirs play an important role in managing water quality by allowing for controlled flow conditions that can aid in sedimentation processes. By slowing down water as it passes over a weir, particulates can settle out before the water continues downstream. Additionally, weirs can create habitats that promote biological filtration by allowing aquatic plants and organisms to thrive, which further enhances the natural purification processes occurring in the water.
• Evaluate the potential environmental impacts of installing a weir on a natural river system.
  • Installing a weir on a natural river system can significantly impact the ecosystem by altering flow regimes, sediment transport, and aquatic habitats. While weirs can help with flood control and irrigation, they may also impede fish migration and disrupt existing habitats. It's essential to evaluate these environmental impacts carefully and implement measures such as fish ladders or bypass systems to mitigate negative effects while still achieving the desired benefits of the structure.

Related terms

Flow Measurement: The process of quantifying the flow of water in open channels, often using devices such as weirs or flumes.

Hydraulic Head: The height of water above a reference point, which is crucial in determining the energy available for flow in a system, especially in relation to weirs.

Spillway: A structure designed to release excess water from a dam or levee, similar to a weir but typically built to handle larger volumes of water.