What is a Broad Crested Weir?
A broad crested weir is a flat-topped hydraulic structure placed across an open channel to measure or control the flow of water. Unlike sharp crested weirs that have a thin knife-edge over which water spills, a broad crested weir has a horizontal crest surface long enough that the flow passing over it becomes approximately parallel to the crest. This parallel flow condition simplifies the hydraulic analysis and makes the structure well suited for flow measurement.
Broad crested weirs are widely used in irrigation systems, river monitoring stations, flood control channels, and wastewater treatment plants. Their robust construction makes them more durable than sharp crested weirs in field conditions where debris, sediment, and varying water levels are common. Many concrete spillways on low-head dams are effectively broad crested weirs.
The key to using a broad crested weir for flow measurement is understanding the relationship between the water level upstream of the weir, the flow depth over the crest, and the resulting volumetric discharge.
The Flow Rate Formula
The volumetric flow rate over a broad crested weir is calculated as:
[\text{Q} = C_{d} \times h_{2} \times b \times \sqrt{2g(h_{1} - h_{2})}]
Where:
- Q is the volumetric flow rate (ft³/s or m³/s).
- C_d is the discharge coefficient (dimensionless, typically 0.30 to 0.60).
- h₁ is the upstream head -- the water depth measured from the channel bottom to the upstream water surface.
- h₂ is the head over the weir crest -- the depth of water flowing over the top of the weir.
- b is the width of the weir perpendicular to the flow direction.
- g is the acceleration due to gravity (32.174 ft/s² or 9.81 m/s²).
The term under the square root represents the energy available to drive flow over the weir. The discharge coefficient corrects the theoretical flow rate for real-world losses including friction, turbulence, and flow contraction.
Calculation Example
Calculate the flow rate for a broad crested weir with the following parameters:
- Discharge coefficient (C_d): 0.50
- Upstream head (h₁): 15 ft
- Head over weir crest (h₂): 5 ft
- Weir width (b): 40 ft
Step 1: Calculate the energy term.
[\text{2g}(h_{1} - h_{2}) = 2 \times 32.174 \times (15 - 5) = 643.48]
Step 2: Take the square root.
[\sqrt{643.48} \approx 25.37 \text{ ft/s}]
Step 3: Multiply all terms.
[\text{Q} = 0.50 \times 5 \times 40 \times 25.37]
[\text{Q} = 2{,}537 \text{ ft}^{3}\text{/s}]
The flow rate through the weir is approximately 2,537 cubic feet per second.
Discharge Coefficient Reference
| Weir Condition | Typical C_d Range |
|---|---|
| Well-rounded upstream edge | 0.50 - 0.60 |
| Square upstream edge | 0.40 - 0.50 |
| Rough or damaged crest | 0.30 - 0.40 |
| Submerged downstream | 0.25 - 0.35 |
Factors Affecting Flow Rate
Several factors influence the accuracy of the flow rate calculation:
- Crest length. The crest must be long enough relative to the head for the flow to become parallel. A ratio of crest length to head of at least 2:1 is generally required. If the crest is too short, the weir behaves more like a sharp crested weir and a different formula should be used.
- Upstream approach conditions. The formula assumes a uniform approach velocity. If the channel upstream is curved, obstructed, or significantly wider than the weir, the velocity distribution is uneven and the discharge coefficient must be adjusted.
- Downstream submergence. When the tailwater level rises above the weir crest, the weir becomes submerged and the flow rate decreases. The standard formula applies only to free-flow conditions where the downstream water level does not affect the discharge.
- Surface roughness. A smooth concrete crest produces higher discharge coefficients than a rough, deteriorated surface. Algae growth, sediment deposits, and damage from debris all reduce the effective discharge coefficient over time.
Broad Crested vs. Sharp Crested Weirs
The two most common weir types used for flow measurement are broad crested and sharp crested (thin-plate) weirs. Each has distinct advantages that make it better suited for certain applications.
A sharp crested weir has a thin crest plate, typically less than 2 millimetres thick at the edge, over which the water sheet (nappe) springs free and falls into the downstream pool. Because the nappe separates cleanly from the crest, the hydraulic behaviour is well understood and the theoretical discharge equations are highly accurate. Sharp crested weirs offer the best precision for low to moderate flows and are the standard choice in laboratories, small irrigation channels, and environmental monitoring stations where accuracy is the priority.
A broad crested weir has a flat crest long enough in the flow direction for the water to become parallel to the surface. The minimum ratio of crest length to head for parallel flow is typically 2:1 to 3:1. Broad crested weirs sacrifice some theoretical precision for practical durability. Because the crest is wide and robust, it withstands debris impacts, sediment loading, and high-velocity flows that would damage or destroy a thin plate. This makes broad crested weirs the preferred choice for rivers, flood channels, spillways, and field irrigation systems.
When to Use Each Type
| Criterion | Sharp Crested | Broad Crested |
|---|---|---|
| Accuracy | Higher (well-defined nappe) | Moderate (depends on calibration) |
| Durability | Low (thin plate bends easily) | High (solid concrete or masonry) |
| Debris tolerance | Low | High |
| Best for | Labs, small channels, monitoring | Rivers, spillways, field systems |
| Head range | Low to moderate | Moderate to high |
| Maintenance | Frequent edge inspection | Periodic crest inspection |
In many field installations, the choice is straightforward: if the weir must survive floods, debris, and years of exposure without maintenance, a broad crested weir is the only practical option. If maximum accuracy is needed in a controlled environment, a sharp crested weir is preferred.
Some installations combine both approaches by using a sharp crested weir insert mounted on a broad crested structure, gaining the accuracy benefits of the thin plate with the structural support of the broad crest. This hybrid design is common in research-grade river monitoring stations.
Comparing Weir Types
Broad crested weirs are one of several weir types used in hydraulic engineering. Each type has specific advantages depending on the application:
Sharp crested weirs have a thin metal or plastic crest plate over which water spills freely. They provide highly accurate flow measurements at low to moderate flow rates and are the standard choice for laboratory flumes and small-scale field installations. However, they are fragile, easily damaged by debris, and unsuitable for channels carrying sediment or floating material.
V-notch (triangular) weirs use a triangular opening cut into a sharp crested plate. The V shape makes them extremely sensitive to small changes in flow, making them ideal for measuring low discharges. A 90-degree V-notch weir can accurately measure flows as low as a few litres per second. They are widely used in environmental monitoring and industrial wastewater measurement.
Cipolletti (trapezoidal) weirs have a trapezoidal opening with side slopes of 1 horizontal to 4 vertical. The sloped sides compensate for the end-contraction effect that reduces flow through rectangular openings, simplifying the discharge calculation. They are common in irrigation systems where a simple, self-correcting measurement is valuable.
The broad crested weir excels where durability and high-capacity flow measurement are priorities. Its massive construction resists damage from floods, debris, and ice. It can handle much larger flow rates than sharp crested or V-notch weirs, and its flat crest is easier to inspect and maintain. The trade-off is lower accuracy at very low flows, where the head over the crest becomes too small to measure reliably.
Environmental and Regulatory Considerations
Weirs affect the ecology of a waterway. They create a barrier to fish passage, alter sediment transport, and change the upstream and downstream flow regime. Regulatory agencies in many jurisdictions require environmental impact assessments before installing a permanent weir, even for measurement purposes.
Fish-friendly weir designs incorporate low-flow notches, baffled fish passes, or removable crest sections that allow fish to migrate during spawning seasons. In environmentally sensitive streams, non-intrusive flow measurement methods such as acoustic Doppler instruments or dilution gauging may be preferred over physical weirs.
Sediment accumulation upstream of the weir is another practical concern. Over time, deposited sediment raises the channel bed, effectively reducing the upstream head and underestimating the true flow rate. Regular surveying of the upstream channel bed and periodic sediment removal maintain measurement accuracy.
Practical Tips for Weir Flow Measurement
- Calibrate the discharge coefficient. The most accurate results come from field calibration, where the actual flow is measured independently (using a current meter or dilution gauging) and compared to the calculated value. Adjust the discharge coefficient until the formula matches the observed flow.
- Maintain the crest. Inspect the weir crest regularly for damage, sediment buildup, and biological growth. Even small changes to the crest profile can shift the discharge coefficient significantly.
- Install a stilling well. Measure the upstream head using a stilling well connected to the channel by a small pipe. The stilling well dampens wave action and turbulence, providing a stable water level reading.
- Check for submergence. If the downstream water level approaches the crest elevation, measure the tailwater depth and apply a submergence correction factor. Flow measurement under submerged conditions is inherently less accurate.