Flow through an orifice

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Orifices are simply holes which restrict the flow of water. They may be used at the bottom end of an LID practice to add some flow control:


  • B is the clogging factor (between 0.5 (for matured installation) and 1 (for a new perfectly performing orifice)),
  • Cd is the coefficient of discharge (usually 0.61 for a plate, or 0.8 for a pipe),
  • Ao is the area of the orifice (m2),
  • g is acceleration due to gravity = 9.81 m/s2, and
  • h is the head of water over the centre of the orifice (m).

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