weirs

A weir is a barrier placed across a river or channel to regulate flow, raise the water level upstream, and measure discharge. Weirs are widely used in water resources engineering for gauging stream flow, controlling irrigation diversions, managing flood risk, and providing upstream storage or head for hydropower or navigation. They can create a stable, controlled drop in energy and often serve as the starting point for further hydraulic structures.

Weirs come in several forms. Common types include sharp-crested rectangular weirs, which provide relatively precise discharge measurements in unobstructed, uniform approach flows; broad-crested weirs that span a wider crest; and notch weirs, such as triangular or V-notch designs, used for accurate measurements at low to moderate discharges. Submerged weirs operate with the crest under water and require different interpretation of discharge. Some weirs include end contractions or wing walls to modify discharge characteristics and approach flow.

Operation and interpretation rely on the relationship between upstream head (the water level above the crest) and discharge. The discharge is a function of crest geometry, weir type, and flow conditions; in general, discharge increases with head and is influenced by the weir’s coefficient of discharge. Accurate measurements require a still, unobstructed approach flow and properly placed staff gauges or sensors.

Design and maintenance considerations include sedimentation, debris buildup, vandalism, fish passage, and safety. Weirs are typically constructed of concrete, metal, or timber and must be inspected and maintained to preserve accuracy and structural integrity. Environmental impacts and downstream effects are also considered in modern planning.