A low dam or barrier constructed across a channel with a precisely shaped crest over which water flows, allowing discharge to be calculated from the measured water depth upstream. Weirs are primary flow measurement structures for small to medium streams.
A weir is an engineered structure built across an open channel with a precisely shaped notch or crest that creates a predictable relationship between the upstream water depth (head) and the discharge flowing over the structure. Common weir types include sharp-crested weirs (thin plate with V-notch, rectangular, or Cipolletti trapezoidal openings), broad-crested weirs, and compound weirs that combine different notch shapes for accurate measurement across a wide flow range. The V-notch (triangular) weir is preferred for low flows due to its sensitivity, while rectangular weirs handle higher flow rates. Discharge is calculated using well-established hydraulic equations; for example, the Francis formula for a rectangular weir: Q = 1.84(L - 0.2nH)H^(3/2), where L is crest length, n is the number of end contractions, and H is the head above the crest. Weirs provide highly accurate flow measurements (typically 2-5% uncertainty) when properly installed and maintained, making them standard flow measurement structures at wastewater treatment plants, irrigation canals, small watersheds, and research catchments. Installation requirements include a level crest, adequate approach channel conditions (subcritical flow, uniform velocity distribution), free discharge conditions (no submergence), and regular sediment removal from the upstream pool. Flumes (Parshall, H-flume, cutthroat) serve similar purposes but with less head loss and better sediment passage characteristics.