Broad Crested Weir Calculator
Calculate discharge over a broad-crested weir using Q = C × L × H³/² for open channel flow measurement. Free online hydrology calculator with breakdowns and charts.
About This Calculator
The Broad Crested Weir Calculator helps hydraulic engineers, hydrologists, civil engineers, and students determine the discharge flow rate over a broad-crested weir in open channels. A broad-crested weir is a hydraulic structure with a wide horizontal crest installed across rivers, reservoirs, lakes, and ponds to control water flow, measure volumetric discharge, and maintain water levels. Unlike sharp-crested weirs that produce a free-falling nappe, broad-crested weirs allow water to flow along the crest surface before cascading down, making them ideal for larger flows and continuous field monitoring applications.
The calculator implements the standard broad-crested weir discharge equation Q = C × L × H³/², where the coefficient C = (2/3)³/² × √g derives from the energy equation applied to critical flow conditions over the crest. Under standard gravitational acceleration of 9.8067 m/s², C equals approximately 1.705 m⁰·⁵/s. The weir behaves as broad-crested when the upstream head above the crest is between 5% and 50% of the crest length. Results include the discharge in cubic meters per second, the coefficient value, the average velocity v = Q/(L×H), and a comprehensive breakdown table. The charts visualize the discharge, coefficient, and velocity for quick comparison.
For accurate head measurement, install a stilling well or staff gauge at least 3-4 times the maximum head upstream of the weir to avoid drawdown effects. Ensure the approach channel is straight and uniform for reliable flow conditions.
Regional Notes
Broad-crested weir hydraulics are universal and apply worldwide. The calculator uses SI units (meters, m³/s) standard in engineering practice. In the United States, engineers may use imperial units (feet, ft³/s) — convert using 1 m = 3.281 ft. Broad-crested weirs are used globally: in India for canal flow measurement in irrigation systems (Indira Gandhi Canal, Bhakra Nangal), in the US for stream gauging by the US Geological Survey (USGS), and in the UK for river flow monitoring by the Environment Agency.
Frequently Asked Questions
What is a broad-crested weir in open channel flow?
A broad-crested weir is a hydraulic structure with a wide horizontal crest used to measure and control flow in open channels. It acts as a low-level dam installed across rivers, reservoirs, lakes, and ponds to prevent flooding, monitor water levels, perform flow analysis, and measure volumetric flow rate. When the upstream head above the crest is between 5 to 50% of the crest length, the weir behaves as a broad-crested weir.
How do you calculate discharge over a broad-crested weir?
The discharge over a broad-crested weir is calculated using the equation Q = C × L × H³/², where Q is the flow rate in m³/s, L is the weir length in meters, H is the upstream head above the crest in meters, and C is the broad-crested weir coefficient. The coefficient C = (2/3)³/² × √g, which equals approximately 1.705 m⁰·⁵/s under standard gravity (9.8067 m/s²).
What is the broad-crested weir coefficient and how is it calculated?
The broad-crested weir coefficient C depends on gravitational acceleration and is calculated as C = (2/3)³/² × √g. Under standard Earth gravity of 9.8067 m/s², C equals 1.705 m⁰·⁵/s. This coefficient is derived from the energy equation applied to critical flow over a broad-crested weir. The coefficient may vary slightly with weir geometry and approach velocity conditions.
What is the difference between broad-crested and sharp-crested weirs?
Broad-crested weirs have a wide horizontal crest where water flows along the surface before falling, while sharp-crested weirs have a thin edge that causes the nappe to spring free. Broad-crested weirs are better suited for larger flows and are more robust for field installations. The discharge equation differs: broad-crested uses Q = C × L × H³/², while sharp-crested rectangular weirs use the Francis formula incorporating end contractions and approach velocity.
How do you measure head over a broad-crested weir?
The head H over a broad-crested weir is measured as the vertical distance from the weir crest to the upstream water surface. The measurement point should be located at least 3 to 4 times the maximum head upstream of the weir to avoid drawdown effects. A stilling well or staff gauge is typically used for accurate head measurement, and the reading should be taken when flow conditions are steady.
What are the applications of broad-crested weirs in hydrology?
Broad-crested weirs are widely used in hydrology and hydraulic engineering for stream gauging and flow measurement in rivers and canals, flow control in reservoir outlets and irrigation systems, flood management and water level regulation, sediment transport studies, and as part of dam spillway structures. They are preferred for continuous flow monitoring due to their robust construction and reliable rating curves.
What is the velocity over a broad-crested weir?
The average velocity over a broad-crested weir can be calculated as v = Q / (L × H), where Q is the discharge, L is the weir length, and H is the head. Critical flow conditions occur near the downstream edge of the crest where the Froude number equals 1. The velocity increases as the flow accelerates over the crest, transitioning from subcritical upstream to critical at the crest and supercritical downstream.
What are the design considerations for broad-crested weirs?
Key design considerations for broad-crested weirs include: the crest should be long enough that the upstream head is between 5% and 50% of the crest length; the weir should be perpendicular to flow; the crest should be level and smooth; adequate upstream stilling is needed for accurate head measurement; the weir must be designed to pass design flood without overtopping the banks; and approach channel should be straight and uniform for at least 10 times the maximum head.