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Where to Install a Weir

Proper site selection is important to the success or failure of any weir plate installation. The need for large upstream weir pool and a drop after the weir can make it challenging to find just the right site. Proper weir installation isn’t as simply as throwing a weir plate across a channel.

Upstream of the Weir (Weir Pool Sizing)

  • Without flow condition, the channel upstream of the weir should be straight and of a uniform cross-section for at least 20 times the maximum anticipated head (Hmax).
    • For applications where an underflow baffle is placed upstream of the weir, the approach conditions can be eliminated.  

    • Here the underflow baffle is usually place 1-2 Hmax upstream of the point of measurement. 

  • The flow approaching the weir should be smooth, tranquil, and well distributed across the channel.  
  • The upstream channel should be clear of vegetative growth.
  • For earthen channel installations, the sidewalls and weir pool floor should be consolidated so that flow does not bypass the weir.
  • Flow approaching the weir must be sub-critical and should not exceed 0.5 ft/s [0.15 m/s].  
  • If the flow is super-critical approaching the flume a hydraulic jump must be formed well upstream of the flume or upstream energy absorbers and tranquilizing racks must be used).

  • Should a hydraulic jump need to be formed to slow the flow, it should be forced to occur at least 30 Hmax upstream of the flume. 

Weir LOCATION

  • The crest (flow edge) of the weir must be able to be set 2-3 times Hmax above the floor of the channel.

  • The crest of the weir must be able to be set at least 2-inches [5.08 cm] above the maximum downstream water level.

  • The widest portion of the weir crest at Hmax should be at least 2 times Hmax from the sidewall of the channel (on each side).

  • This distance should extend at least 20 times the maximum anticipated head (Hmax) (per Weir Pool Sizing above).

  • The weir must be set level from side-to-side and from front-to-back.

  • Weirs must be able to be set horizontally / vertically level and centered in the channel.

Downstream of the Weir

  • 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.