Broad Crested Weir Calculator
Calculate discharge flow rate, head height, or weir length for broad crested weir hydraulic systems
Calculate Broad Crested Weir Parameters
Height of water above the weir crest
Width of the weir opening
Standard Earth gravity: 9.8067 m/s²
Calculation Results
Formula used: Q = C × L × H^(3/2)
Coefficient (C): 1.7046 m^0.5/s
Input parameters: H = 0.000 m, L = 0.000 m
Example Calculation
Dam Overflow Example
Scenario: Calculate discharge for dam spillway
Head height (H): 0.5 m above weir crest
Weir length (L): 2.0 m wide opening
Gravity (g): 9.8067 m/s²
Step-by-Step Solution
1. Calculate coefficient: C = (2/3)^1.5 × √9.8067 = 1.705 m^0.5/s
2. Apply formula: Q = C × L × H^1.5
3. Q = 1.705 × 2.0 × 0.5^1.5 = 1.705 × 2.0 × 0.354
4. Result: Q = 1.205 m³/s
Weir Types Comparison
Broad Crested
L/H = 0.5 to 2.5
Thick weir, steady flow
Sharp Crested
L/H < 0.5
Thin weir, precise flow
Ogee Weir
Curved profile
High efficiency, spillways
Key Parameters
Head Height (H)
Height of water above weir crest. Should be 5-50% of weir length for broad crested classification.
Weir Length (L)
Width of the weir opening perpendicular to flow direction.
Coefficient (C)
Discharge coefficient: C = (2/3)^1.5 × √g ≈ 1.705 m^0.5/s
Applications
Dam spillways and overflow structures
Flow measurement in open channels
Irrigation system flow control
Flood control and water level regulation
Wastewater treatment plant outlets
Understanding Broad Crested Weirs
What is a Broad Crested Weir?
A broad crested weir is a hydraulic structure used to control water flow in rivers, channels, and reservoirs. It consists of a thick, rectangular barrier where the upstream head is between 5% to 50% of the crest length. The water flows over the weir crest and follows the structure's surface before falling like a waterfall.
Key Characteristics
- •Thick weir structure (L/H ratio between 0.5 and 2.5)
- •Stable flow pattern with critical depth over crest
- •Suitable for larger flow rates
- •Relatively insensitive to downstream conditions
Broad Crested Weir Equation
Q = C × L × H^(3/2)
- Q: Discharge (flow rate) in m³/s
- C: Discharge coefficient = (2/3)^1.5 × √g ≈ 1.705 m^0.5/s
- L: Length (width) of weir in meters
- H: Head height above weir crest in meters
- g: Acceleration due to gravity (9.8067 m/s²)
Note: The coefficient C assumes ideal conditions. Real-world applications may require correction factors for approach velocity and weir geometry.
Flow Physics and Behavior
Upstream Approach
Water approaches the weir with subcritical flow. The velocity increases and depth decreases as it approaches the crest due to the Bernoulli principle.
Critical Flow at Crest
At the weir crest, the flow becomes critical (Froude number = 1). This is where the specific energy is minimum for the given discharge.
Downstream Fall
After the crest, the flow accelerates due to gravity, becoming supercritical and forming a free-falling jet or nappe.