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Where to Install a Weir Box

While Weir Boxes take away many of the normal site location issues associated with Weir Plates, there are some considerations that have to be taken into account when selecting an installation site.

Upstream of the Weir BOX

  • If unbaffled, the Froude number (Fr) for flow entering a Weir Box should not exceed 0.5.
  • If flow spills into the Weir Box, the spilling discharge should be shielded so that flow does not splash into the main weir pool. 


  • The Weir Box must be able to be mounted so that the weir plate is horizontally / vertically level.

  • The mounting surface of the weir box should be sufficiently consolidated that it will not shift or move out of position due to settlement, frost heave, etc.

Downstream of the Weir BOX

  • If the flow out of the Weir Box is piped, the downstream piping should be straight for 5 to 20 pipe diameters (depending upon pipe geometry) to ensure that flow does not back up in to the Weir Box.
  • If flow freely spills off the end of the Weir Box, there must be at least 2-inches [5.08 cm] (and preferably 6-inches [15.24 cm]) of height from the lowermost portion of the weir crest to the maximum downstream water level for the nappe (body of water flowing over the weir) to be aerated.
    • If the nappe is not aerated it will be erratic and cling to the downstream weir face.
    • The downstream channel should be armored (riprap) or otherwise protected so that scour does not occur.
    • The downstream channel must be clear of vegetative growth or the collection of debris so that flow is not restricted.