Vortex and plate orifice sizing for flow control devices: head-discharge curves and minimum aperture checks.
1
Parameters
2
Orifice Sizing
3
Head-Discharge
4
Vortex Control
5
V-Notch Weir
6
Summary
Step 1: Design Parameters
Project Information
Greenfield runoff rate or water quality treatment flow (litres/second)
Depth of water in attenuation tank/basin (metres)
Leave blank if no constraint. Limits orifice size (mm)
Guidance Notes
Discharge Coefficient (Cd)
Sharp-edged Orifice (0.61): Traditional orifice plate with square edges. Common in older SuDS designs.
Short Tube Orifice (0.80): Slightly higher discharge, smoother edge effects.
Vortex Control (0.40-0.50): Flow control device (e.g. Hydro-Brake). Highly resistant to blockage, essential for < 75mm orifices.
Head Measurement
Maximum head (H) is the depth of water above the orifice centrepoint at full attenuation basin capacity. Ensure orifice sits above the invert of the outlet structure to prevent siphoning.
Orifice diameter < 50mm (high blockage risk: vortex control essential)
Sewage or heavily contaminated stormwater (debris risk)
Maintenance access difficult (vortex devices are more self-cleaning)
Advantages of Vortex Control
Self-cleaning action due to vortex flow pattern
Reduced blockage risk even at small equivalent orifice sizes
Stable flow control across wide range of heads
Manufacturer-certified performance
Step 5: V-Notch Weir Sizing (Optional)
V-notch weirs are suitable for low-flow control, water quality treatment volume drawdown, and fine-tuned flow regulation. They provide excellent accuracy at small discharge rates.
Common angles: 30°, 45°, 60°, 90°. Equation: Q = (8/15) × Cd × tan(θ/2) × √(2g) × H^(5/2)